Principles of Aeronautics 9781637004203

Table of contents :
Advanced Composite Materials in Aeronautical Engineering
Advanced Composite Materials Repair
Advanced Propulsion
Aerobatics and Flight
Aerodynamics and Flight
Aeroelasticity
Aeronautical Engineering
Aerospace Industry in the United States
Ailerons, Flaps, and Airplane Wings
Air Flight Communication
Air Transportation Industry
Aircraft Icing
Airfoils
Airplane Accident Investigation
Airplane Cockpit
Airplane Guidance Systems
Airplane Maintenance
Airplane Manufacturers
Airplane Propellers
Airplane Radar
Airplane Safety Issues
Animal Flight
Neil Armstrong
Atmospheric Circulation
Autogyros
Autopilot
Aviation and Energy Consumption
Avionics
Avro Arrow
Daniel Bernoulli
Biplanes
Blimps
Boomerangs
Richard Branson
Conservation of Energy
Contrails
Glenn H. Curtiss
Leonardo da Vinci
DC Plane Family
Differential Equations
Dirigibles
Jimmy Doolittle
Amelia Earhart
Federal Aviation Administration (FAA)
First Airplane Flight across the English Channel
First Cross-Channel Balloon Flight
First Flights of Note
First Manned Balloon Flight
Flight Altitude
Flight Balloons
Flight Control Systems
Flight Instrumentation
Flight Landing Procedures
Flight Propulsion
Flight Recorder
Flight Roll and Pitch
Flight Schools
Flight Simulators
Flight Testing
Fluid Dynamics
Flying Wing
Forces of Flight
Steve Fossett
Yuri Gagarin
German Luftwaffe
John Glenn
Glider Planes
Robert H. Goddard
Gravity and Flight
Greenhouse Gases
Heavier-than-air Craft
Helicopters
High-altitude Flight
High-speed Flight
History of Human Flight
Homebuilt and Experimental Aircraft
Hot-Air Balloons
Howard R. Hughes
Human-Powered Flight
Hypersonic Aircraft
Jet Engines
Jet Propulsion Laboratory (JPL)
Johnson Space Center
Landing Gear
Lighter-Than-Air Craft
Otto Lilienthal
Charles A. Lindbergh
ErnstMach
Mach Number
Materials Science
Messerschmitt Aircraft
Military Aircraft
Billy Mitchell
Model Airplanes
Monoplanes
Montgolfier Brothers
National Aeronautics and Space Administration (NASA)
National Advisory Committee for Aeronautics (NACA)
National Transportation Safety Board (NTSB)
Sir Isaac Newton
Paper Airplanes
Parachutes
Plane Rudders
Wiley Post
Pressure
Propulsion Technologies
Ramjets
Eddie Rickenbacker
Rocket Propulsion
Rockets
Rotorcraft
Russian Space Program
Burt Rutan
Scramjet
Alan Shepard
ShockWaves
Igor Sikorsky
Sound Barrier
Space Shuttle
Spacecraft Engineering
Spaceflight
Stabilizers
Stealth Bomber
Supersonic Aircraft
Supersonic Jetliners and Commercial Airfare
Supersonic Jets Invented
Tail Designs
Takeoff Procedures
Taxiing Procedures
Temperature
Valentina Tereshkova
Training and Education of Pilots
Triplanes
Konstantin Tsiolkovsky
Andrei Nikolayevich Tupolev
Turbojets and Turbofans
Turboprops
Types and Structure of Airplanes
Ultralight Aircraft
Unidentified Aerial Phenomena (UAP)
Uninhabited Aerial Vehicles (UAVs)
Jules Verne
Viscosity
Manfred von Richthofen (Red Baron)
Wake Turbulence
Weather Conditions
Wind Shear
Wind Tunnels
Wing Designs
Women and Flight
Wright Brothers’ First Flight
X-Planes (X-1 to X-45)
Chuck Yeager

Citation preview

Principles of Aeronautics

Principles of Aeronautics

Editor Richard M. Renneboog, M.Sc.

SALEM PRESS A Division of EBSCO Information Services, Inc. Ipswich, Massachusetts GREY HOUSE PUBLISHING

Cover photo: Jetlinerimages/iStock. Copyright © 2023, by Salem Press, A Division of EBSCO Information Services, Inc., and Grey House Publishing, Inc. Principles of Aeronautics, published by Grey House Publishing, Inc., Amenia, NY, under exclusive license from EBSCO Information Services, Inc. All rights reserved. No part of this work may be used or reproduced in any manner whatsoever or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without written permission from the copyright owner. For information, contact Grey House Publishing/Salem Press, 4919 Route 22, PO Box 56, Amenia, NY 12501. ¥ The paper used in these volumes conforms to the American National Standard for Permanence of Paper for Printed Library Materials, Z39.48 1992 (R2009). Publisher’s Cataloging-In-Publication Data (Prepared by Parlew Associates, LLC) Names: Renneboog, Richard M., editor. Title: Principles of aeronautics / editor, Richard M. Renneboog, M.Sc. Description: Ipswich, MA : Salem Press, a division of EBSCO Information Services, Inc. ; Amenia, NY : Grey House Publishing, 2023. | Series: [Principles of science]. | Includes bibliographic references and index. | Includes b&w photos and illustrations. Identifiers: ISBN 9781637004203 (hardback) Subjects: LCSH: Aeronautics — Encyclopedias. | Aviation -- Encyclopedias. | Flight — Encyclopedias. | Rocketry — Encyclopedias. | BISAC: SCIENCE / Mechanics / Aerodynamics. | TECHNOLOGY & ENGINEERING / Aeronautics & Astronautics. | TECHNOLOGY & ENGINEERING / Reference. Classification: LCC TL546.7 R46 2023 | DDC 629.13 --dc23

First Printing Printed in the United States of America

Contents Publisher’s Note . . . . . . . . . . . . . . . . . . . . . . . . . . vii Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii Advanced Composite Materials in Aeronautical Engineering . . . . . . . . . . . . . . . . . 1 Advanced Composite Materials Repair . . . . . . . . . 3 Advanced Propulsion . . . . . . . . . . . . . . . . . . . . . . . 6 Aerobatics and Flight . . . . . . . . . . . . . . . . . . . . . . 12 Aerodynamics and Flight . . . . . . . . . . . . . . . . . . . 18 Aeroelasticity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Aeronautical Engineering . . . . . . . . . . . . . . . . . . . 26 Aerospace Industry in the United States . . . . . . . 30 Ailerons, Flaps, and Airplane Wings. . . . . . . . . . . 37 Air Flight Communication . . . . . . . . . . . . . . . . . . 39 Air Transportation Industry . . . . . . . . . . . . . . . . . 44 Aircraft Icing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Airfoils. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Airplane Accident Investigation . . . . . . . . . . . . . . 53 Airplane Cockpit. . . . . . . . . . . . . . . . . . . . . . . . . . 59 Airplane Guidance Systems . . . . . . . . . . . . . . . . . 61 Airplane Maintenance . . . . . . . . . . . . . . . . . . . . . 65 Airplane Manufacturers . . . . . . . . . . . . . . . . . . . . 69 Airplane Propellers . . . . . . . . . . . . . . . . . . . . . . . . 76 Airplane Radar . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Airplane Safety Issues . . . . . . . . . . . . . . . . . . . . . . 85 Animal Flight . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Neil Armstrong . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 Atmospheric Circulation. . . . . . . . . . . . . . . . . . . 100 Autogyros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Autopilot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Aviation and Energy Consumption. . . . . . . . . . . 109 Avionics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Avro Arrow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 Daniel Bernoulli . . . . . . . . . . . . . . . . . . . . . . . . . 121 Biplanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 Blimps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Boomerangs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Richard Branson . . . . . . . . . . . . . . . . . . . . . . . . . 132 Conservation of Energy . . . . . . . . . . . . . . . . . . . 137 Contrails . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 Glenn H. Curtiss . . . . . . . . . . . . . . . . . . . . . . . . . 142

Leonardo da Vinci . . . . . . . . . . . . . . . . . . . . . . . 145 DC Plane Family . . . . . . . . . . . . . . . . . . . . . . . . . 150 Differential Equations . . . . . . . . . . . . . . . . . . . . . 154 Dirigibles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 Jimmy Doolittle . . . . . . . . . . . . . . . . . . . . . . . . . 163 Amelia Earhart . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Federal Aviation Administration (FAA). . . . . . . . 173 First Airplane Flight across the English Channel. . 178 First Cross-Channel Balloon Flight . . . . . . . . . . 182 First Flights of Note . . . . . . . . . . . . . . . . . . . . . . 185 First Manned Balloon Flight. . . . . . . . . . . . . . . . 189 Flight Altitude. . . . . . . . . . . . . . . . . . . . . . . . . . . 192 Flight Balloons . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Flight Control Systems . . . . . . . . . . . . . . . . . . . . 200 Flight Instrumentation . . . . . . . . . . . . . . . . . . . . 203 Flight Landing Procedures . . . . . . . . . . . . . . . . . 208 Flight Propulsion . . . . . . . . . . . . . . . . . . . . . . . . 212 Flight Recorder . . . . . . . . . . . . . . . . . . . . . . . . . . 217 Flight Roll and Pitch . . . . . . . . . . . . . . . . . . . . . . 220 Flight Schools . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 Flight Simulators . . . . . . . . . . . . . . . . . . . . . . . . 228 Flight Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 Fluid Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . 236 Flying Wing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 Forces of Flight . . . . . . . . . . . . . . . . . . . . . . . . . . 242 Steve Fossett . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 Yuri Gagarin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 German Luftwaffe . . . . . . . . . . . . . . . . . . . . . . . . 250 John Glenn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 Glider Planes . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 Robert H. Goddard. . . . . . . . . . . . . . . . . . . . . . . 260 Gravity and Flight. . . . . . . . . . . . . . . . . . . . . . . . 263 Greenhouse Gases. . . . . . . . . . . . . . . . . . . . . . . . 268 Heavier-than-air Craft . . . . . . . . . . . . . . . . . . . . 275 Helicopters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 High-altitude Flight . . . . . . . . . . . . . . . . . . . . . . 287 High-speed Flight. . . . . . . . . . . . . . . . . . . . . . . . 291 Hindenburg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 History of Human Flight . . . . . . . . . . . . . . . . . . 300 Homebuilt and Experimental Aircraft . . . . . . . . 307 Hot-Air Balloons. . . . . . . . . . . . . . . . . . . . . . . . . 313 Howard R. Hughes . . . . . . . . . . . . . . . . . . . . . . . 317

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Human-Powered Flight. . . . . . . . . . . . . . . . . . . . 318 Hypersonic Aircraft. . . . . . . . . . . . . . . . . . . . . . . 323 Jet Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 Jet Propulsion Laboratory (JPL) . . . . . . . . . . . . . 335 Johnson Space Center . . . . . . . . . . . . . . . . . . . . 339 Landing Gear . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 Lighter-Than-Air Craft . . . . . . . . . . . . . . . . . . . . 348 Otto Lilienthal . . . . . . . . . . . . . . . . . . . . . . . . . . 354 Charles A. Lindbergh . . . . . . . . . . . . . . . . . . . . . 355 Ernst Mach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 Mach Number. . . . . . . . . . . . . . . . . . . . . . . . . . . 361 Materials Science . . . . . . . . . . . . . . . . . . . . . . . . 364 Messerschmitt Aircraft . . . . . . . . . . . . . . . . . . . . 369 Military Aircraft . . . . . . . . . . . . . . . . . . . . . . . . . 374 Billy Mitchell. . . . . . . . . . . . . . . . . . . . . . . . . . . . 378 Model Airplanes . . . . . . . . . . . . . . . . . . . . . . . . . 379 Monoplanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 Montgolfier Brothers . . . . . . . . . . . . . . . . . . . . . 384 National Aeronautics and Space Administration (NASA) . . . . . . . . . . . . . . . . . 387 National Advisory Committee for Aeronautics (NACA). . . . . . . . . . . . . . . . . . . . 395 National Transportation Safety Board (NTSB) . . 398 Sir Isaac Newton . . . . . . . . . . . . . . . . . . . . . . . . . 402 Paper Airplanes. . . . . . . . . . . . . . . . . . . . . . . . . . 407 Parachutes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 Plane Rudders. . . . . . . . . . . . . . . . . . . . . . . . . . . 413 Wiley Post . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414 Pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416 Propulsion Technologies. . . . . . . . . . . . . . . . . . . 419 Ramjets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425 Eddie Rickenbacker . . . . . . . . . . . . . . . . . . . . . . 429 Rocket Propulsion . . . . . . . . . . . . . . . . . . . . . . . . 431 Rockets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436 Rotorcraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441 Russian Space Program . . . . . . . . . . . . . . . . . . . 445 Burt Rutan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 Scramjet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455 Alan Shepard . . . . . . . . . . . . . . . . . . . . . . . . . . . 458 Shock Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459 Igor Sikorsky . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 Sound Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . 466

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Space Shuttle . . . . . . . . . . . . . . . . . . . . . . . . . . . 469 Spacecraft Engineering. . . . . . . . . . . . . . . . . . . . 475 Spaceflight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 Stabilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488 Stealth Bomber . . . . . . . . . . . . . . . . . . . . . . . . . . 491 Supersonic Aircraft . . . . . . . . . . . . . . . . . . . . . . . 494 Supersonic Jetliners and Commercial Airfare . . 498 Supersonic Jets Invented . . . . . . . . . . . . . . . . . . 499 Tail Designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503 Takeoff Procedures . . . . . . . . . . . . . . . . . . . . . . . 507 Taxiing Procedures . . . . . . . . . . . . . . . . . . . . . . . 510 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512 Valentina Tereshkova . . . . . . . . . . . . . . . . . . . . . 514 Training and Education of Pilots . . . . . . . . . . . . 519 Triplanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526 Konstantin Tsiolkovsky . . . . . . . . . . . . . . . . . . . . 528 Andrei Nikolayevich Tupolev . . . . . . . . . . . . . . . 529 Turbojets and Turbofans. . . . . . . . . . . . . . . . . . . 530 Turboprops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 534 Types and Structure of Airplanes . . . . . . . . . . . . 537 Ultralight Aircraft . . . . . . . . . . . . . . . . . . . . . . . . 545 Unidentified Aerial Phenomena (UAP) . . . . . . . 548 Uninhabited Aerial Vehicles (UAVs) . . . . . . . . . . 553 Jules Verne . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559 Viscosity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 560 Manfred von Richthofen (Red Baron) . . . . . . . . 564 Wake Turbulence. . . . . . . . . . . . . . . . . . . . . . . . . 567 Weather Conditions . . . . . . . . . . . . . . . . . . . . . . 569 Wind Shear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574 Wind Tunnels . . . . . . . . . . . . . . . . . . . . . . . . . . . 576 Wing Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . 580 Women and Flight . . . . . . . . . . . . . . . . . . . . . . . 584 Wright Brothers’ First Flight. . . . . . . . . . . . . . . . 590 Wright Flyer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593 X-Planes (X-1 to X-45) . . . . . . . . . . . . . . . . . . . . 597 Chuck Yeager . . . . . . . . . . . . . . . . . . . . . . . . . . . 603

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organizations . . . . . . . . . . . . . . . . . . . . . . . . . . . Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . .

607 631 645 647

Publisher’s Note Aeronautics is the next volume in Salem’s Principles of Science series, which includes Microbiology, Energy, Marine Science, Geology, Information Technology, and Fire Science. This new resource explores the art and science of flight. Readers will gain a solid grounding in the history of flight, major events and individuals associated with flying, and the physics that makes flight possible. This work begins with a comprehensive Editor’s Introduction to the topic written by Richard M. Renneboog, M.Sc. Following the Introduction, Principles of Aeronautics includes 155 entries that follow a convenient alphabetical arrangement, making subjects easy to locate. From the earliest legends and cave carvings of human flight to the most recent composite materials for airplanes and rockets, this volume takes readers on a journey through the world-altering discoveries, and at times harrowing tragedies, of humanity’s determined effort to unlock the secrets of flying. Covering all types of aircraft—from balloons and dirigibles to modern airplanes and space vehicles— Principles of Aeronautics delves into the scientific discoveries that enabled progress in mechanized flight as well as fa-

mous firsts, human-powered flight, flight engines and other components, the environmental impact of flight, and women pilots, to name a few topics. Entries begin by specifying related Fields of Study, followed by an Abstract and then a list of Key Concepts summarizing important points; all entries end with a helpful Further Reading section. Numerous photographs and illustrations throughout enhance the categories. This work also includes helpful appendices, including: • Bibliography; • Glossary; • Organizations; • Subject Index Salem Press extends appreciation to all involved in the development and production of this work. Names and affiliations of contributors to this work follow the Editor’s Introduction. Principles of Aeronautics, as well as all Salem Press reference books, is available in print and as an e-book. Please visit www.salempress.com for more information.

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Introduction Flight has truly been the dream of humanity for as long as humanity has had dreams. Few people have not awakened from sleep recalling a dream of flying or floating freely through the air. Ancient peoples watched the birds flying effortlessly or soaring high up in the sky. Perhaps they wondered how birds flew, and divined that their wings were the key to flight. The ancient Greek legend of renowned architect and builder Daedalus and his son Icarus certainly suggests this. The pair were imprisoned by the king of Minos, who had contracted their services, so that no one could learn the secrets of his castle from them. Daedalus observed the seabirds that flew and soared about the castle, and he set about collecting the feathers that fell from them occasionally. Using wax left over from his construction efforts, Daedalus used it to fashion feathered wings for himself and for Icarus. Daedalus and Icarus used these wings to escape from their imprisonment, but Icarus is said to have flown too close to the sun, melting his wings, and he plummeted to his death in the sea. Of course, it is impossible for any human to fly like a bird with a pair of artificial wings, so if there is a grain of truth in the story of Daedalus and Icarus, perhaps it is this: Daedalus could easily have perceived that the wings of birds have a peculiar cross-sectional shape that we understand as an airfoil. He may also have observed that birds rise in the air when they sweep their wings forward and not when they flap them backwards. The backwards sweep gives the birds forward movement but not upward movement. He would certainly have observed that birds remain aloft when soaring with outstretched wings by using rising air currents to push them upward. Movement of air over the airfoil shape of an outstretched wing splits the air in such a way that the air flowing over the upper surface increases in speed, which decreases the pressure above the wing. Air flowing across the lower surface of the wing is therefore of higher pressure and acts to provide an upward force against the wing (this is known as lift).

With this observation of bird flight, Daedalus could have reasoned that he and Icarus could use their properly shaped wings to glide like birds, rather than with the ornithoptic motion of flapping wings. Did Icarus the fly too close to the Sun? Or did he just glide too long in direct sunlight when the clouds parted rather than in their shade, thus allowing the wax of his wings to soften and his wings to fail? It is an old story of unknowable age, and there is even debate among scholars as to whether or not Daedalus and Icarus were actual persons. The consensus is that it took many centuries before the ability for controlled flight in mechanical devices became reality. But did it? Stories thousands of years old, primitive carvings, and ancient legends from all over the world and from all civilizations depict people using flying machines. Renaissance paintings and ancient rock carvings alike show what today would be described as UFOs or other flying machines. An ancient Egyptian hieroglyphic carving clearly depicts what looks for all the world like a modern helicopter and airplane among star symbols. Sacred scriptures tell of airborne vehicles termed "a wheel within a wheel" in the Bible, while Hindu texts thousands of years old speak of flying machines called vimana. While these stories and interpretations are suspect in the jaundiced eye of the present day, they were held as absolute truth in their original times. In our original time—the present day—people do fly using flying machines, a technology that began in earnest early in the twentieth century when the Wright brothers made the first powered flight in a heavier-than-air vehicle. This was founded on their study and development of gliders and kites. A kite simply demonstrates that moving air has the ability to support objects that are much heavier than air. The force of moving air is undeniable, but gliding is not flying so much as just slowly falling over a distance. Based on the aerodynamic principles they learned as part of their experience with gliders and

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kites, the Wright brothers realized that an engine driving a propeller or fan of some kind should be able to make their steerable glider fly farther than it could as an unpowered vehicle. Over the next mere fifty years, powered flight progressed from the Wrights' crude bi-winged powered glider to powered spaceflight, and after fifty more years it promises to carry humanity to other planets. Within those two half centuries an almost unbelievable amount of technical brilliance, ingenuity, and hard work brought about incredible changes. The Wrights understood that powered flight requires a means of propulsion, and they solved that issue by constructing an engine of their own design that would be light enough to be carried as an integral part of their flying machine. Their engine was a small internal combustion engine that could turn a propeller to provide the thrust to push the machine along through the air. As engine technology advanced, so too did the power for propulsion of aircraft. Ever more powerful engines could drive larger and faster aircraft, although the technology was still fairly primitive. Two world wars in the twentieth century played a large role in the development of aircraft. At the beginning of World War I, aircraft were not a considered part of the war effort on either side. The flying machines were little better than the Wright Flyer, and many in fact were built in Wright factories. But aircraft quickly proved valuable in a number of service roles, especially in spotting troop positions and movements, and far more effective than the balloons that were being used for that purpose. This led to a push to develop better aircraft that would be more capable of combat roles. Throughout World War I essentially the only aircraft were biplanes, relatively slow but quick to respond and highly maneuverable. Monoplane designs were also available, but these were not favored by either pilots or military leaders due to their poor performance. The monoplane airframe didn't come into its own until after World War I, as available engines, and hence propulsion, exceeded the ability of biplanes to support their power. At the same time, the aerody-

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namic forces of lift, drag, and thrust became increasingly important for aircraft flying at speed through the air. The strut-and-brace construction of biplanes became a hindrance in regard to both strength of the materials being used and in terms of aerodynamic drag. The only way to improve flyability was to go to the monoplane design and optimize the design of wings as airfoils using Bernoulli's principle. When an airfoil moves through air, its leading edge splits the airflow, directing some of it to pass over the top surface of the airfoil and the rest to pass over the lower surface of the airfoil. According to the Coanda effect, the flow of a fluid in contact with a curved surface will follow the contour of the curved surface, in effect sticking to it as it flows past. Thus, the respective airflows passing over an airfoil closely follow the curved surfaces of the airfoil. The airfoil shape allows the two airflows to reunite as they leave the trailing edge of the airfoil. In between the leading and trailing edges, the upper surface is given a higher profile than the lower surface. Bernoulli's principle affects the two airflows in a specific way, such that the flow of air over the upper surface of the airfoil increases in speed and reduces in pressure relative to the airflow across the lower surface of the airfoil. This difference in pressure is the source of the lift force an aircraft experiences as it moves through the air. The lift force is also directly related to the speed at which the airfoil moves through the air, as determined by the thrust force of propulsion. From the end of World War I in 1918, and over the next twenty-five years, aircraft design and aeronautical engineering maximized the efficiency of thrust and lift to produce propeller-driven aircraft capable of exceeding 800 kilometers per hour. But at those speeds, some parts of an airplane, such as a wing tip or the tips of propeller blades, can exceed the speed of sound. The shock waves this produces caused more than one aircraft to lose lift and fall into an unrecoverable dive. As this phenomenon came to be understood, the speed of sound came to be seen as a speed no aircraft should or could achieve: the sound barrier.

Principles of Aeronautics

In the middle of World War II, a new means of propulsion was applied to aircraft; the jet engine. In a typical internal combustion engine, power is generated by repeated individual explosions of fuel within individual combustion cylinders. These drive pistons connected to a common rotating shaft that turns a propeller. This repetitive operation requires a relatively large number of moving parts and produces a steady vibration against the airframe. But in a jet engine, the combustion of fuel is a continuous process, producing a steady flow of rapidly expanding hot gases from the engine's combustion chamber, or combustor, and out the exhaust vent, providing thrust directly. This is very similar to the manner in which thrust is developed in a rocket engine, although there are many structural differences between them. Since the 1950s, most commercial and military aircraft have been powered by jet engines or turbine engines, even propeller-driven airplanes and rotorcraft. As World War II neared its end in Europe, the German war effort produce two rocket-propelled weapons, the V-1 "buzz bomb" and the V-2 ballistic rocket. The V-1, launched from the German base at Peenemunde, was powered by periodic firing of fuel in a combustion chamber rather than by a continuous flow combustion of fuel. The periodic popping sound from its exhaust as it approached its target gave rise to the name of "buzz bomb," and the sudden cessation of that sound as the engine shut off was the signal for anyone within earshot to seek shelter. The V-2, on the other hand, flew silently. Once launched, it flew on a ballistic trajectory, reaching the peak of that trajectory under powered flight. It then completed its ballistic arc in free fall. People in the target area had no warning sound of its approach. When World War II was over, the V-2 became the foundation of the so-called "Space Race" and the forerunner of all rockets that came after. The fundamental difference between a jet engine and a rocket is simple. Jet engines are supplied with fuel from an external reservoir, but a rocket's fuel is carried with it as an integral part of the rocket's structure. How the fuel is handled and combusted is

Introduction

very different from jet engine to rocket, but the thrust of both is produced and utilized in an essentially identical manner: hot, rapidly expanding gases press against the inside of the combustor and exit through the exhaust, and the vehicle moves in accordance with Newton's law of action and reaction. There is also the law of conservation of momentum at work in both types of engine. Accordingly, the fundamental physics is fairly basic. It is the technical engineering to harness the physics that is the star of the show. Aeronautics is a many-faceted gem of human endeavor. In the eighteenth century, the Montgolfier brothers observed heavier-than-air objects being lifted up by a rising column of smoke in a fireplace. This gave them the idea that given a large enough container with which to capture the hot air, it might be possible to create something that could carry a person into the air. The first hot-air balloon flight took place not long after, and it was proven beyond a doubt that people could fly in this manner. The discovery of "flammable air," which we know as hydrogen gas, and later of helium provided another means of flight. Encased in a suitably large "bag" containing such a gas, flight could last indefinitely without the need to keep a fire lit under a bag, which created a fire hazard. Adding a motor and propellers to such a vehicle resulted in the dirigible (dirigible is a French word meaning directable or steerable, from the verb diriger, to direct or steer). The dirigible was huge, slower than any contemporary airplane, and remarkably stable in even the worst weather. They became one of the first major weapons of war, but this combination of qualities proved to be their downfall. Bags of gas, even those with a supporting structure, can be beaten and folded by a sufficiently strong wind, and their relative slowness made them very easy and big targets for both air and ground fired guns. The airplane was the way to go. In modern aeronautical applications, materials science plays a very important role for both aircraft and spacecraft. Wood, cloth, and paper gave way to lighter silk and nylon fabric for balloons and thin

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aluminum sheet metal for airplanes. Metals gave way to plastics for some components, and metals have been replaced by advanced composite materials in many cases. Advanced composites are recent products, children of the age of polymers. Materials such as polyaramid (Kevlar) and carbon fiber are lighter and stronger than the metals they replace, and they can be formed in any desired shape. Almost all modern aircraft are at least in part constructed of advanced composite materials, from the light panel doors inside an airliner to the rock-hard material of fighter plane wings. In space, the Canadarm used in the NASA space shuttles for material manipulation are constructed of carbon fiber. As one would expect, working with such materials requires special training. The aeronautical engineer has to add "chemist" to the list of expertise required. In the air, all of these things are under the control of the pilot, and they must function flawlessly if catastrophic failures are to be avoided. The pilot has to trust that the aircraft is as it should be, that the ground crews have done their job, that the materials that make up the aircraft are sound, and that the many instruments provided to monitor the aircraft

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and guide the pilot are trustworthy. The job of a pilot, the heart of the air transportation industry, is awe-inspiring, and it comes with heavy responsibility. In this volume we have tried to present a very brief accounting of the various aspects of aeronautics, such as the minutiae of airfoil design, the history of aeronautics, biographical information of a selection of key historical figures, engine design, and aerospace applications. Each of the articles presented herein has been augmented with a set of recent references for further reading, by authors from many parts of the world, all of whom are intimately involved with the world of aeronautics. Various terms and concepts are defined and described at the beginning of each article to assist the reader in understanding the content and context of that particular article, and a glossary of terms is included as an appendix. Understandably, these articles are no more than brief introductions to the topics they address, and it is my hope that they provide an open door to learning more about aeronautics. —Richard M. Renneboog

Contributors Richard Adler University of Michigan, Dearborn

James S. Douglas Independent Scholar

R. Kurt Barnhart Independent Scholar

Robert P. Ellis Northborough Historical Society

Maryanne Barsotti Warren, MI

Victoria Erhart Strayer University

Wendy S. Beckman Independent Scholar

Thomas R. Feller Nashville, TN

Raymond D. Benge Jr. Tarrant County College

David G. Fisher Independent Scholar

Alvin K. Benson Brigham Young University

Richard D. Fitzgerald Onondage Community College

Kenneth H. Brown Northwestern Oklahoma State University

George J. Flynn SUNY-Plattsburgh

Douglas Campbell Independent Scholar

David E. Fogleman Independent Scholar

Roger V. Carlson Jet Propulsion Laboratory

Alan S. Frazier Independent Scholar

P. John Carter Independent Scholar

John C. Fredriksen Independent Scholar

Frederick B. Chary Indiana University Northwest

Yautia Fu Independent Scholar

Monish R. Chatterjee Independent Scholar

Angel G. Fuentes Independent Scholar

Joseph F. Clark III Independent Scholar

K. Fred Gillum Independent Scholar

Douglas Clouatre Mid Plains Community College

Gina Hagler Rare Math

Veronica T. Cote Independent Scholar

Niles R. Holt Independent Scholar

Rafael de la Llave Independent Scholar

W. N. Hubin Independent Scholar

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Thomas Inman Independent Scholar

Chad T. Lower Independent Scholar

Jamey D. Jacob Independent Scholar

James F. Marchman III Independent Scholar

John C. Johnson Independent Scholar

Robert Maxant Independent Scholar

Douglas R. Jordan Independent Scholar

Bernard Mergen Independent Scholar

Maureen Kamph Independent Scholar

Matthew G. McCoy Independent Scholar

Lori Kaye Independent Scholar

Dana P. McDermott Chicago, IL

David Kasserman Rowan University

Randall L. Millstein Independent Scholar

Narayanan M. Komerath Georgia Institute of Technology

Eugene E. Niemi Jr. Independent Scholar

Lillian D. Kozloski Independent Scholar

Cynthia Clark Northrup Independent Scholar

Donald L. Kunz Independent Scholar

Jani Macari Pallis Independent Scholar

Jack Lasky Northeast Editing

Robert J. Paradowski Rochester Institute of Technology

M. Lee Independent Scholar

Shari Parsons Independent Scholar

Denyse Lemaire Independent Scholar

John R. Phillips Purdue University, Calumet

Rowan University Independent Scholar

George R. Plitnik Frostburg State University

Manja Leyk Independent Scholar

Steven J. Ramold Eastern Michigan University

Josué Njock Libii Purdue University

P. S. Ramsey Highland, MI

M. A. K. Lodhi Independent Scholar

Frank J. Regan Independent Scholar

John L. Loth Independent Scholar

Richard M. Renneboog Independent Scholar

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R. Smith Reynolds Independent Scholar

Sonia Sorrell Independent Scholar

Charles W. Rogers Southwestern Oklahoma State University

Polly D. Steenhagen Independent Scholar

Beatriz Martínez Romera Independent Scholar

Cynthia J. W. Svoboda Bridgewater State University

Alison Rowley Independent Scholar

Gregory S. Taylor Independent Scholar

Frank A. Salamone Iona College

Lance Wayne Traub Independent Scholar

Mary Fackler Schiavo Independent Scholar

Mary Ann Turney Independent Scholar

Richard Sheposh Northeast Editing

Janine Ungvarsky Musicare Project NEPA

R. Baird Shuman University of Illinois at Urbana-Champaign

Robert J. Wells Independent Scholar

Sanford S. Singer Independent Scholar

David R. Wilkerson Independent Scholar

Billy R. Smith Jr. Independent Scholar

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A Advanced Composite Materials in Aeronautical Engineering Fields of Study: Physics; Chemistry; Aeronautical engineering; Mechanical engineering; Materials science; Mathematics ABSTRACT Advanced composites are specialty materials used in aircraft manufacture and other applications for their unique structural properties and strengths. Composite materials and composite structural components have been known in a wide variety of applications for literally thousands of years. However, advanced composite materials are a very recent invention that has existed for less than a century. KEY CONCEPTS lay-up: the process of assembling a composite stack according to design parameters thermosetting resin: a mixture of resinous chemicals that undergoes polymerization when heated, forming a solid product that cannot be melted (thermoplastic polymers soften and melt when heated) warp clock: a simple guide diagram that shows the angles at which fabrics are to be aligned when constructing a composite stack WHAT ARE ADVANCED COMPOSITE MATERIALS? Advanced composites are specialty materials used in aircraft manufacture and other applications for their unique structural properties and strengths. Composite materials and composite structural components have been known in a wide variety of applications for

literally thousands of years. However, advanced composite materials are a very recent invention that has existed for less than a century. They are very much a product of the age of plastics. Advanced composite materials in aeronautical applications are carefully constructed and processed in order to create a product of a desired shape, such as an aircraft wing, fuselage part, a hatch cover and other such pieces of a functioning aircraft. The process requires the directed assembly of stacked layers of special fabric that are impregnated with a thermosetting resin. This raw structure is then heated under reduced pressure to solidify the stack into a rock-hard structure. Essentially any desired shape can be formed in this way, from small parts to entire aircraft bodies. Every advanced composite consists of a substrate embedded or encased within a matrix. These are the fabrics and resins that are combined to be consolidated into the final structure. There is a broad variety of fabric materials that can be utilized in constructing a composite material. Many are familiar, with the simplest fabric called glass fiber, which can be had in numerous forms raging from single strand called “rovings” to complex multistrand weaves. in actuality, any kind of cloth fiber can be used as the substrate material for a composite structure—cotton, nylon, wool, hemp, linen, etc. However, advanced composite materials demand fibers of a more substantial nature than common fabrics. Accordingly, glass fibers are the minimum strength fibers used in advanced composites. All other fibers used have higher and different strength factors. Polyaramid fibers, commonly called Kevlar, have very high tensile strength. Carbon fibers are extremely strong and light, and can be readily consolidated into a

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rock-hard material. Boron fiber is much less flexible than carbon fiber, and produces composite structures with high rigidity and stiffness. Another type of fiber that has yet to see broad application in aeronautic applications is similar to glass fiber, but it is produced from basalt, the volcanic rock. Accordingly, there are weight considerations when glass or basalt fiber are used. The matrix materials used in advanced composites are by far the most versatile of materials, by virtue of the fact that they are polymers. Polymeric compounds can be tailored to suit various purposes, making them ideal for use in aerospace applications. One “glaring” exception is the composite material called Glare, a composite of glass fiber sandwiched within thin bonded sheets of aluminum. In many cases, the actual composition of a particular matrix material and the conditions under which it is to be processed into a final composite structure are well-guarded proprietary secrets. CONSTRUCTING AN ADVANCED COMPOSITE STRUCTURE Preparation of a composite structure of any kind is a meticulous process. It begins with planning the choice and lay-up of fabrics according to the designed strength requirements of the finished product. The fabric and its lay-up are engineered to produce a specific strength in the final product. That strength derives from the designed relative orientations of the fabric fibers after the fabric stack has been consolidated. Arranging the stack requires consideration of the type of weave in the fabric and the direction of the fibers in successive layers of fabric. Woven fabrics may have a cross-sectional profile of ridges and hollows that must be made to nest into each other upon consolidation of the stack. Directional orientation of the fibers is arranged according to a “warp clock,” a guide for orienting the layers of fabric. The warp clock directs fabric orientations at different relative angles so that the fabric fibers do

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Principles of Aeronautics

not all run in the same direction. This is required to prevent the structure from warping or twisting after it has been processed. With the fabric set, the stack is infused with matrix resin and placed under reduced pressure so that the resin and fabric are compressed and consolidated into a single mass. At the same time, gases that are evolved in the polymerization reactions as the resin cures are withdrawn and eliminated. Depending on the nature of the resin, a controlled heating program may be used to drive the curing process to completion. When construction of the product is complete, the product is tested for structural integrity and the proper dimensions, and if it is sound the product can be put into service. For large objects, the product is typically formed on a mold and processed in a large, pressurized oven, or autoclave. This is a very important step, requiring the material of the mold to have the same coefficient of thermal expansion as the composite material being formed on the mold. Why is this so important? The composite structure being formed has to conform to precise dimensions, according to its design parameters. During the curing process, the composite material expands on being heated, but when the finished product cools down it must recover its intended dimensions. If the mold does not correspond throughout the entire process, it will force the composite structure to incorrect dimensions or even to internal damage caused by differential expansion and shrinkage of the mold and the structure on it. Delaminations may occur within the structure, which would be the ruination of the workpiece, as well as the loss of a significant investment in time and materials. —Richard M. Renneboog Further Reading Armstrong, Keith, William Cole, Eric Chesman, and Francois Museux. Care and Repair of Advanced Composites. SAE International, 2020.

Principles of Aeronautics

Bafekrpour, Ehsan. Advanced Composite Materials: Properties and Applications. De Gruyter Open, 2017. Dai, Pin Qiang, Wen Zhe Chen, Xing Jun Liu, Yong Lu Chen, and Zheng Yi Jiang. Advanced Composite Materials. Trans Tech Publications Ltd., 2012. Fangueiro, Raul, and Sohel Rana. Advanced Composite Materials for Aerospace Engineering: Processing, Properties and Applications. Elsevier Science, 2016. Mazlan, Norkhairunnisa, S. M. Sapuan, and R. A. Ilyas, editors. Advanced Composites in Aerospace Engineering Applications. Springer Nature, 2022. Tiwari, Ashutosh, Mohammad Rabia Alenezi, and Seong Chan Jun, editors. Advanced Composite Materials. John Wiley & Sons, 2016. See also: Advanced composite materials repair; Aerodynamics and flight; Aeronautical engineering; Air transportation industry; Airplane accident investigation; Airplane maintenance; Fluid dynamics; Materials science; Pressure; Spacecraft engineering; Viscosity

Advanced Composite Materials Repair Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Chemistry; Mathematics ABSTRACT Composite material structures are carefully assembled combinations of layers of fabric embedded in a matrix of cured polymer resin. The method of their fabrication produces materials of exceptional strength and precise dimensions. But the very thing that provides their strength can also be the source of their greatest weakness. KEY CONCEPTS delamination: a separation of the layers of material within the body of a composite structure lay-up: the process of assembling a composite stack according to design parameters void: an opening or hole within the body of a composite structure

Advanced Composite Materials Repair

warp clock: a simple guide diagram that shows the angles at which fabrics are to be aligned when constructing a composite stack Composite material structures are carefully assembled combinations of layers of fabric embedded in a matrix of cured polymer resin. The method of their fabrication produces materials of exceptional strength and precise dimensions. But the very thing that provides their strength can also be the source of their greatest weakness. In the finished product, the layers of fabric must be completely and permanently bonded to each other within their encompassing matrix. This adhesion is the functional aspect that provides the strength of the material. It must be so complete that the many individual components and the matrix amalgamate into one solid material. Failure to achieve this completely makes it inevitable that conditions will exist that could lead to the failure of the material at a critical moment. VOIDS During the consolidation and curing process, the polymerization reactions proceeding within the matrix resin produce by-product gases such as water vapor and hydrogen chloride that must be removed completely. This is achieved by consolidating the resin and fabric stack under vacuum, using the pressure of the atmosphere or the increased pressure inside an autoclave to provide a uniform compressive force. It can, and does, happen that a bubble of by-product gases gets trapped and is unable to be extracted by the vacuum pump being used. It can also happen that the resin does not penetrate the fabric stack uniformly, preventing the necessary adhesion in a particular part of the composite stack. In such cases an empty spot or void remains in the interior of the finished product. These are points of weakness and potential failure of the structure in use. Given that an aircraft may go from a groundlevel temperature of 40ºC or more to a temperature

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at altitude of -40ºC or even colder, the gases trapped in an existing void will respond according to the gas laws relating temperature and pressure. The change in pressure associated with such temperature changes can be sufficient to collapse the composite structure surrounding the void or to increase the size of the void, and perhaps fracture off a section of the surrounding composite. Under load, such as the stress experienced by an aircraft in flight, voids can initiate fracturing in the surrounding composite material that may quickly progress and end in catastrophic failure. It is an essential practice that voids be found through various nondestructive testing methods, and then be corrected or repaired by trained and qualified advanced composites repair specialists. DELAMINATIONS A composite material is produced by laminating layers of fabric together, then consolidating them within a resin matrix. Delaminations occur when the layers of fabric within a composite material come apart from each other without forming a void. This represents the loss of the structural integrity of the composite with concomitant loss of material strength. Delaminations may be considerably larger in area than the area encompassed by a void. The effect of a delamination may be compared to a brick wall in which the bricks in a portion of the wall are sponges instead of bricks. This is where the brick wall, and by analogy the composite with a delamination, will fail. Testing for delaminations is carried out in the same manner as testing for voids. PHYSICAL DAMAGE In functioning aircraft assemblies, composite materials may be subjected to a variety of events that can result in physical damage requiring repair. Impact damage can result from such things as hailstones, bird strikes, lightning strikes, severe turbulence, bullet strikes during military operations, and any num-

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ber of other impact events. Chemical damage can occur as a result of hydraulic line failure, fuel leaks and cargo leaks that may occur, allowing the fluid to contact and infiltrate the composite material. Even a situation in which two materials having different electrical potentials come into contact with each other in some way can cause extensive and expensive physical damage. For example, carbon fiber in a composite material becoming connected in some way to aluminum components either by direct contact or by an indirect connection such as bolts, screws or rivets will result in severe damage as the electrical current resulting from the contact drives oxidation and decomposition of the two materials. In such cases, the damaged areas must be repaired or even removed and replaced. This could, and has, meant the complete gutting of an aircraft in order to replace composite flooring panels. TESTING METHODS Testing of advanced composites to detect voids and delaminations is done using nondestructive testing (NDT) methods. The first method commonly used to quickly test for voids, delaminations and the extent of any internal damage is most often the simple “tap test.” As the name suggests, this nondestructive test is carried out simply by tapping lightly on the surface of the structure with a small tapping hammer that has a hard plastic head. The technique is essentially the same as knocking on a wall inside a house to locate a wall stud. The sound is different at the wall stud compared to the deeper, more resonant sound obtained from the empty space between the wall studs. The sound that is obtained by tapping on a structurally sound composite is very different from the sound produced by tapping on a spot where a void or a delamination exists. Similarly, an area that has been damaged will generally have internal damage radiating outward from the location of the visible damage. This too will produce a different sound in a tap test. A trained and experienced

Principles of Aeronautics

composites technician can often reliably identify and isolate the area to be repaired just by using the tap test. A more reliable test method is to couple tap testing with an electronic acoustic analyzer. The high sensitivity of the acoustic analyzer provides a much more accurate depiction of the extent of the damage to be repaired, and can also generate a hard-copy printout of the area, like an ultrasound image. This higher sensitivity to sound vibrations can detect sound differences that the technician would be unable to hear, thus revealing the true extent of the damage. Electronic methods of NDT have supplanted the tap test in most situations, particularly due to the advances in electronics technology. Electronic devices such as ultrasound, infrared reflectance, X-ray and similar test devices can now be made small enough that they have become portable “shop tools” that facilitate damage analysis in composite structures. The technology is now able to allow a trained technician to capture a detailed three-dimensional rendering of damage to the interior of a composite structure. TRAINING SCHOOLS All composite repairs, especially in regard to aircraft, require a specially trained and qualified technician. Because this is relevant to the air transport industry and the safety of people who fly in those aircraft, there are very stringent regulations and mandates regarding aircraft composite repair, from all of the Federal Aviation Association, Transport Canada, and other national agencies responsible for governance of air transport. Just as air flight communication has been globally standardized to the English language, composite repair in the aircraft industry has been standardized with regard to regulatory governance. All technicians must be trained and certified to the same standard in order to work as composite repair technicians in aerospace, espe-

Advanced Composite Materials Repair

cially with regard to commercial air transport. Accordingly, there are a number of training schools that have been established for the purpose of training aircraft technicians, and others, in the intricacies of advanced composite repair. Some, such as Renaissance Aeronautics Associates (RAA), in London, Ontario, Canada, and Abaris Training, in Reno, Nevada, are privately owned and operated, and serve a worldwide clientele. Large manufacturers such as Boeing and Airbus provide their own training, often as protection for proprietary materials. But all training schools must provide training to the same regulated standards. The training includes comprehensive instruction in the construction and properties of advanced composites, and the methods of repair of advanced composites. The most recent advances in training include computer numerical control (CNC) composites milling and virtual reality repair demonstration, used at RAA and other schools. REPAIR TECHNIQUES When a repair is to be made and the area of the repair has been identified, the first step in the repair is to excise the damaged area so the repair can proceed. If the damage is severe enough, it is generally expedient to simply cut out the damaged area and rebuild the missing composite according to its design standards. This may actually involve replacing the damaged part entirely rather than making a spot repair. For lesser damage it may only be necessary to remove the layers that have actually sustained damage. These are typically scarfed out with air-powered grinders and other tools to create an open area within which the composite will be reconstructed. The reconstruction is not simply a “patch job.” The goal is to have a finished repair that meets all of the original design parameters. This means laying in the same fabric layers in the same prescribed relative orientations as in the original design, infusing them with the appropriate resin, and curing with heat under reduced pressure. The repair

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Principles of Aeronautics

is then tested for integrity, and if it can be certified sound the aircraft can be declared airworthy and put back into service. Such repairs are always fully documented in the aircraft’s service record, and this record can be invaluable if an accident investigation involving that aircraft is undertaken at some later date. Repair of larger components, such an aileron or perhaps a fuselage panel, generally calls for its complete removal and the perhaps manufacture of an entirely new piece. Spot repairs can be cured in place using portable equipment, but large parts will be processed in a suitably large autoclave. These, such as the autoclaves at Boeing and other large aircraft manufacturing facilities, can be large enough to contain an entire aircraft fuselage, although the more usual loading consists of several smaller components to be processed at the same time and under the same conditions. —Richard M. Renneboog Further Reading Armstrong, Keith, William Cole, Eric Chesman, and Francois Museux. Care and Repair of Advanced Composites. SAE International, 2020. Fangueiro, Raul, and Sohel Rana. Advanced Composite Materials for Aerospace Engineering: Processing, Properties and Applications. Elsevier Science, 2016. Jayakrishna, K., M. Rajesh, and Mohamed Thariq Hameed Sultan. Repair of Advanced Composites for Aerospace Applications. CRC Press, 2022. Jefferson, Andrew J., V. Arumugarn, and Hom Nath Dhakal. Repair of Polymer Composites: Methodology, Techniques, and Challenges. Elsevier Science, 2018. Meola, Carosena. Nondestructive Testing in Composite Materials. MDPI AG, 2020. Sierakowski, Robert L. Damage Tolerance in Advanced Composites. CRC Press, 2018. Zhong, Shuncong, and Walter Nsengiyumua. Nondestructive Testing and Evaluation of Fiber-Reinforced Composite Structures. Springer Singapore, 2022. See also: Advanced composite materials in aeronautical engineering; Aerodynamics and flight; Aeronautical engi-

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neering; Air transportation industry; Airplane accident investigation; Airplane maintenance; Fluid dynamics; Materials science; Pressure; Spacecraft engineering; Viscosity

Advanced Propulsion Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Chemical engineering; Mathematics ABSTRACT Advanced propulsion refers to any means for launching or propelling spacecraft beyond the use of traditional chemical rocket engines. If humanity is ever to explore the solar system and access resources beyond Earth, cheaper, faster, and more efficient propulsion systems must be developed. KEY CONCEPTS advanced: indicates a process or mechanism that functions beyond the current standard levels of performance aerospike rocket engine: a rocket engine with an inverted structure such that the exhaust exits rearward along the outside of the engine to provide thrust rather than rearward from the inside of the engine resistojet: an engine that uses electrical resistance to heat exhaust gases that provide thrust as they are ejected scramjet: an engine that uses its supersonic speed to compress air for combustion of hydrogen; it has no moving parts and must be launched from a supersonic carrier before it can fly on its own thrust: the force or pressure exerted by the exhaust gases on the body of an aircraft in the direction of its motion THE FUNDAMENTALS If the space shuttle’s external fuel tank were placed on the pedestal of the Statue of Liberty, it would

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Principles of Aeronautics

stand just taller than Lady Liberty’s torch. At launch, the mass of the space shuttle is about 2,040 tonnes (4.5 million pounds), but it can deliver only 6.5 percent of that mass to low-Earth orbit, and that costs $20,000 per kilogram ($9,100 per pound). For comparison, in 2001, gold sold for about $9,000 per kilogram. To achieve a stable low-Earth orbit, the payload must simultaneously be lifted about 300 kilometers above Earth’s surface and accelerated to a horizontal speed of nearly 8 kilometers per second (about Mach 23, or twenty-three times the speed of sound).

At 111 meters (364 feet), the Saturn V rocket that took the Apollo astronauts to the Moon stood over twice as tall as the space shuttle. In ascending to lowEarth orbit, the Saturn’s first three stages burned for a total of 11.5 minutes, using 75 terajoules (75 x 1012) of chemical energy. That was about 1.5 percent of all of the energy in the world produced from fossil fuel during those 11.5 minutes. Only 6 percent of that energy went into lifting and accelerating the Apollo payload into orbit, while most of the remaining 94 percent was expended on lifting and accelerating the fuel used on the way up.

Space Shuttle Atlantis on a Shuttle Carrier Aircraft. Photo via Wikimedia Commons. [Public domain.]

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There are many plans for more efficient spacecraft. The Venture Star project featured the more efficient aerospike rocket engine. Solar sails, plasma bubbles, gravitational slingshots, the Pegasus spacecraft, and laser- or microwave-launched spacecraft are schemes that leave all or part of the main power source behind and thereby reduce the spacecraft’s mass. Scramjets gather part of their fuel, the oxidizer, in flight, and the hypothetical Bussard interstellar ramjet would gather fusion fuel in flight. Ion drives, plasma drives, and drives using nuclear fission and fusion are all schemes to increase the exhaust velocity of the propellant. AEROSPIKE ENGINES, SCRAMJETS, AND PEGASUS Worldwide, there are a number of projects under way to develop a fully reusable launch vehicle that will orbit payloads for one-tenth the cost of the space shuttle. The X-33 “Venture Star” is a sleek, wedge-shaped craft designed to take off vertically like a rocket and glide to a landing like an airplane. It pioneered the use of lightweight graphite composites in its structure and fuel tanks, and its efficient lifting body shape allowed it to fly with only stubby wings for stabilizers. Although many important technological advances were achieved, development problems led to the withdrawal of support by the National Aeronautics and Space Administration (NASA) in March, 2001, but work continued on the X-33’s Boeing Rocketdyne XRS-2200 aerospike engines. A conventional rocket engine has a combustion chamber that opens into a bell-shaped nozzle. Fuel and oxidizer are mixed and burned in this chamber, and high-speed combustion products escape through the nozzle. For greater efficiency, the pressure of the exhaust plume should match the surrounding air pressure. The aerospike nozzle is V-shaped and is turned inside out: Fuel and oxidizer are mixed in ten combustion chambers (five on each

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side of the V), and the exhaust plume sprays down the outside of the V. Since the outside of the exhaust plume is open to the atmosphere, it automatically blooms outward until it matches the ambient pressure, while the inside of the plume pushes against the V and provides thrust. A scramjet is a supersonic combustion ramjet. Scramjets can be more efficient than rockets because they use oxygen from the air and must carry only the oxygen that they will use in space. A scramjet engine has no moving parts; it uses its supersonic speed (about Mach 10) and internal shape to compress air coming into its engine instead of using the rotating compressor of a normal jet engine. Hydrogen fuel is injected into the airstream in the engine, and the hot combustion gases (mostly water vapor) escape from the rear of the engine to provide thrust. A scramjet must be launched at supersonic speed before it can fly. In June, 2001, a B-52 aircraft lifted an X-43A scramjet mounted on a Pegasus-based rocket 6,000 meters (20,000 feet) into the atmosphere and launched the combination. The rocket was to accelerate to the scramjet’s cruising speed and then release it. Unfortunately, a structural failure occurred shortly after rocket ignition and the mission was terminated. Several nations are working on scramjets. In August, 2001, India announced plans to develop the Avatar, a 25-metric-ton craft believed to be able to cheaply carry a 1-metric-ton payload into a 100-kilometer-high orbit. The Avatar will take off and land like a conventional airliner using a combination of turbofan, ramjet, and scramjet engines fueled with hydrogen. A unique feature is that it is to cruise at Mach 8 for an hour at an altitude of 10 kilometers while it takes in and liquefies 21 metric tons of oxygen before it uses a hydrogen-and-oxygen-fueled rocket to push into space. The Pegasus rocket has placed dozens of satellites into orbit and is the most successful small commercial launch vehicle in the world. The “Stargazer”

Principles of Aeronautics

Lockheed L-1101 aircraft carries Pegasus to a launch point 12 kilometers high, above the densest and most turbulent part of the atmosphere. The three-stage rocket is then released and ignited. It can carry a 450-kilogram payload into low-Earth orbit. SOLAR SAILS AND PLASMA BUBBLES The surface of the Sun is a fearsome place—a seething, turbulent ocean of blinding incandescent gases, incessantly rocked by sonic booms as gigantic gouts of matter race upward through the photosphere. The flood of energy from the Sun tears particles from its outermost part, the solar corona, and constantly drives this sun-stuff into space. This is the solar wind: electrons along with ionized hydrogen and helium atoms streaming outward at an average speed of 400 kilometers per second. Earth’s magnetosphere is the region surrounding the planet that is dominated by its own magnetic field, not the Sun’s. Geophysicist Robert Winglee of the University of Washington realized that the solar wind pushing against Earth’s magnetosphere pushes Earth away from the Sun, except that Earth is far too heavy for this to produce any measurable effect. However, Winglee proposed that if a light spacecraft could generate a large magnetic field, the solar wind would propel the spacecraft. He calls this hypothesis MiniMagnetospheric Plasma Propulsion, or M2P2. Winglee and his colleagues suggest that a 200-kilogram spacecraft (including 50 kilograms of helium) might be built around an electromagnet coil powered by solar cells. Winglee’s group demonstrated that injecting ionized helium into a coil’s magnetic field forces the field to expand like a bubble, becoming a mini-magnetosphere. They calculate that in space this magnetosphere would be 15 to 20 kilometers in radius, and with the solar wind pushing on it, the craft should reach speeds of 50 to 80 kilometers per second after three months. This is

Advanced Propulsion

ten times faster than the speeds previously reached by chemical rockets. Such a craft could reach Saturn in six months instead of the seven years required for the Cassini mission. The mini-magnetosphere is not really spherical. Its shape depends upon its interaction with the solar wind and on the parameters of the coil. To oversimplify, a mini-magnetosphere may be pictured as a flat sheet of paper orbiting the Sun. If the sheet is tilted so that its leading edge is closer to the Sun than its trailing edge, solar wind particles bouncing off of it will push it forward in its orbit and make it go faster and spiral outward from the Sun. Conversely, if the trailing edge is closer to the Sun, solar wind particles will slow it in its orbit, and the Sun’s gravity will pull it inward. Since the magnetosphere is practically without mass, it should be easy to maneuver it by simply rotating the field-generating coil. The concept of propelling a craft with solar sails is similar to M2P2, but these sails are propelled by sunlight, not the solar wind. At the orbit of Earth, the pressure of sunlight is about 9 newtons (2 pounds) per square kilometer. An 820-meter square-rigged sail, named “the clipper” by its designers, is expected to have a mass of about 2,000 kilograms. Carrying a 2,000-kilogram payload, it could travel from Earth to Mars or to the outer planets in about the same time, or less, than a chemical rocket would require. Once solar sail technology is achieved, its use would be cheaper than the use of chemical rockets because it requires much less mass to be lofted into Earth orbit for each mission. Solar sails are also reusable. They can be returned to Earth orbit, but their sunward speed is limited by the relatively weak pull of solar gravity. Energy Science Laboratories of San Diego, California, has developed a novel sail fabric, a very porous mesh of carbon fibers. They have demonstrated that the fabric is light enough to be pushed by laser light, and that it can withstand temperatures of 2,500 degrees

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Celsius. This is important because, someday, solar sails may be given a push by shining a battery of high-intensity lasers on them. ELECTRIC PROPULSION The thrust produced by a rocket depends upon how much reaction mass is ejected per second, and how fast it is ejected. Chemical rockets can deliver a large amount of thrust because they can push out a great deal of mass per second, but ejection speed is limited by the amount of energy released by the chemical reaction of fuel and oxidizer. Electric propulsion engines typically deliver a small thrust with high efficiency since they can handle only a small amount of mass per second, but they can eject it at very high velocities. With a few exceptions, electric propulsion has been commonly employed only in the thrusters used by satellites for station keeping (staying where they are supposed to be). Resistojets use electric resistance to heat propellent gases and thereby increase their ejection speed. They have operated with ammonia, biowastes, hydrazine, and hydrogen. Arcjets ignite an electric arc in the propellant flowing through a rocket nozzle. Arcjets are twice as fuel efficient as chemical thrusters, but ion engines are more efficient yet. The 480-kilogram spacecraft Deep Space 1 (DS-1) is propelled by an ion engine and powered by 2,400 watts from solar arrays. Launched on October 24, 1998, its mission was to test twelve new technologies, including the ion drive and a relatively autonomous navigation system. While DS-1 came within 26 kilometers of asteroid Braille on July 28, 1999, problems kept it from obtaining any closeup images. Its extended mission was to fly through the coma (head) of comet Borrelly on September 22, 2001, when it sampled the materials of the coma and photographed the comet’s nucleus. DS-1’s ion engine uses xenon, a gas 4.5 times heavier than air, for a propellent. Xenon in the engine chamber is bombarded by electrons that ionize

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the xenon. The rear of the chamber is fitted with two wire mesh screens; the first is positively charged, while the second is negatively charged with up to 1,280 volts. Positive xenon atoms passing through the first screen are accelerated to 28 kilometers per second by the voltage on the second screen. The ejection of these ions into space propels the craft forward. Electrons sprayed into the exhaust stream keep the craft from building up a static charge. Although the engine exerts no more force than the weight of a sheet or two of paper, its 82 kilograms of xenon is enough for 6,000 hours of operation and can increase DS-1’s speed by 4 kilometers per second. It is ten times more efficient than a chemical engine with the same weight of fuel. NUCLEAR POWER The great attraction for using nuclear power in space is that nuclear reactions pack millions of times more energy than chemical reactions. While the United States placed a single nuclear reactor in space in 1965, the former Soviet Union has used small nuclear reactors to provide electrical power on dozens of satellites. Both nations have used radioisotope thermoelectric generators (RTGs) that convert the heat from radioactive decay directly into electricity, but neither nation has used nuclear power for propulsion. Since they have no moving parts and are well constructed, RTGs are considered to be relatively safe, but they are not very efficient. However, using electricity from an RTG to power an ion engine in the regions beyond Mars, where solar power is weak, is an attractive possibility. The Nuclear Engine for Rocket Vehicle Application (NERVA) was almost ready for flight testing when the project was canceled in 1972. Under development for a manned mission to Mars, the NERVA engine heated hydrogen by passing it through the reactor core and then expelled it from a rocket nozzle. Uranium carbide fuel elements were coated with carbon and niobium to protect them from corrosion

Principles of Aeronautics

by the hydrogen propellent. The Mars craft would be assembled in Earth orbit and, using nuclear engines, it could travel to Mars, stay for two months, and return to Earth in the space of about one year. A program to develop a nuclear engine code-named Timberwind began in the 1980s and continues under the Space Nuclear Thermal Propulsion (SNTP) Program. Fluidized bed reactors and other advanced reactors that can operate at higher temperatures are being studied since they should be more efficient than the NERVA engine. The most audacious nuclear engine is the nuclear pulse rocket that was the basis of the Orion project, which ended in 1965. The mass of the Orion vehicle was a grandiose 585 tonnes. The rear of the vehicle was connected by shock absorbers to a massive pusher plate. Every few seconds, a small fission bomb with a ten-ton yield was to be dropped out the back end and exploded about 100 meters behind Orion, so that the blast wave would drive Orion forward. About 2,000 bombs would be required for a 250-day round trip to Mars. To prove the concept, a small prototype was successfully launched from the ground with tiny chemical bombs, but international treaties now prohibit nuclear explosions in space, and therefore the Orion project is unlikely to be revived. None of the proposed nuclear engines are very efficient at converting nuclear energy into a means of propulsion, but they are still attractive because of the large amount of energy in nuclear fuel. If the rare artificial element americium-242m could be produced in significant quantities, a much more efficient engine might be constructed. The key is that a thin film of americium-242m can sustain a chain reaction. High-energy fission fragments escape from a thin film and can be directed by magnets out the rear of the craft to provide propulsion. A spacecraft with such an engine might travel to Mars in two weeks instead of the eight to ten months required by chemical rockets.

Advanced Propulsion

TETHERS AND BOLOS Tethers up to 20 kilometers long have already been tested in space. A tether is a cable that can be unreeled from an orbiting craft such as the space shuttle. A mass on the far end of the tether will help keep it stable. The tether may be deployed upward by letting centrifugal force carry it farther from Earth, or it may be deployed downward by letting Earth’s gravity carry it down. If the tether includes a conducting cable, it can be used to convert a satellite’s momentum into electrical energy, since a conductor moving in Earth’s magnetic field will act like a generator. To keep a current flowing in the cable, electron guns will expel electrons into space and prevent the buildup of a static charge. If used long enough, this system will bring down a satellite from low-Earth orbit, and thereby save the roughly 20 percent of rocket fuel that is reserved to de-orbit spent satellites. If solar cells are used to produce a current in the tether, the generator becomes a motor, and the spacecraft’s orbit will be raised. Because of the air resistance that exists in low-Earth orbit, the International Space Station (ISS) needs a boost from time to time. If it were boosted with tethers powered by the station’s solar panels, up to two billion dollars in fuel costs might be saved over ten years. A bolo consists of two masses connected by a tether and set spinning. The end masses are equipped with grapples and thrusters to adjust position. Long tethers will probably be Hoytethers, a loosely woven Kevlar web. Their open structure makes Hoytethers less likely to be severed by meteoroids. If a bolo station (at the center of mass of the bolo) is in low-Earth orbit and therefore has a speed of 7.7 kilometers per second and the rotating tether’s tip speed is 2.4 kilometers per second with respect to the station, the bolo’s rotation direction is such that the tip speed subtracts from the orbital speed for the tip closest to Earth. A spacecraft launched from Earth need only be traveling at 5.3

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kilometers per second when it rendezvouses with, and is seized by, the lower grapple. If the bolo is much more massive than the spacecraft, the spacecraft will be lifted and accelerated by the tether so that the spacecraft is traveling at 10.1 kilometers per second when the tip is farthest from Earth. At the appropriate time, the bolo will release the spacecraft to travel to its next destination, perhaps a second bolo in geosynchronous orbit, which in turn might pass it on to a bolo in lunar orbit, which might set it on the Moon. The great efficiency of such a system is that it minimizes the fuel that must be lifted and accelerated from Earth. However, the bolos will slow down or fall into lower orbits as they give energy to the spacecraft. The bolo in low-Earth orbit could be boosted by using solar panels and a conducting tether. Other bolos might be boosted with solar-powered ion engines. Only steering energy would be required if the amount of mass going from the Moon to low-Earth orbit were the same as that going from low-Earth orbit to the Moon. (The falling mass would provide the energy to lift the rising mass.) A nearly constant flow of traffic would be required to make a bolo system cost-effective. CONCLUSION While many of the technologies described above remain of interest, the exploration of space has proceeded forward on many fronts, and by several different nations as well as private developers. China, India, Brazil, Iran, England, France, Japan, United States, and other nations all have active programs with the goal of establishing colonies on the Moon and on Mars. Commercial space flights are currently available for the ultrawealthy—at US$400,000 per person—to make a brief foray into space. The cost of launch vehicles has been greatly reduced by the SpaceX company, such that all flights to the ISS are now delivered by SpaceX at roughly one-tenth the cost of previous delivery method used by NASA. SpaceX’s great coup was in the development of reus-

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able booster systems that can return and land safely rather than being lost into the depths of the ocean. All of these developments rely on standard chemical rocket technology. Some would say this demonstrates that previous rocket-based projects were extremely inefficient and wasteful of resources. —Charles W. Rogers Further Reading Czysz, Paul A., and Claudio Bruno. Future Spacecraft Propulsion Systems: Enabling Technologies for Space Exploration. Praxis Publishing Limited, 2006. Emrich, William J. Principles of Nuclear Rocket Propulsion. Elsevier Science, 2016. Farokhi, Saeed. Future Propulsion Systems and Energy Sources in Sustainable Aviation. Wiley, 2020. Mauldin, John H. Prospects for Interstellar Travel. Univelt, 1992. Musha, Takaaki, and Yoshinuri Minami. Field Propulsion System for Space Travel: Physics of Non-Conventional Propulsion Methods for Interstellar Travel. Bentham Science Publishers, 2011. NASA. Evaluation of Advanced Propulsion Options for the Next Manned Transportation System. CreateSpace Independent Publishing Platform, 2018. Sabry, Fouad. Plasma Propulsion: Can Space-X Use a Plasma Prop for Starship? One Billion Knowledgeable, 2021. Wright, Jerome L. Space Sailing. Gordon & Beach Science, 1992. See also: Scramjet; Space shuttle; Spacecraft engineering; Spaceflight

Aerobatics and Flight Fields of Study: Physics; Pilot training ABSTRACT Aerobatics refers to any aerial maneuver involving abrupt or extreme bank or pitch angles, unnecessary for normal flight. Aerobatics are an integral part of military flight tactics, air show demonstrations, and sport flying. An aero-

Principles of Aeronautics

batic pilot’s ability to retain spatial orientation and control an airplane in three dimensions provides an extra measure of safety in the event of an accidental upset. KEY CONCEPTS bank: the act of turning an aircraft while in flight, either to the left or to the right centrifugal force: the force felt by an object undergoing rotation about a central axis, directed outward from the center of rotation pitch, roll, yaw: the three natural motions of an aircraft in flight that must be controlled by the pilot to maintain stable flight REGULATIONS Most aerobatic flying is for pleasure, but regional and national contests are held every year, and a world championship contest is held every other year. Although there is no separate aerobatic rating, aerobatics can be safely learned only in an aircraft that is certified for the maneuvers and only under the tutelage of an experienced instructor. Specifically, the US Federal Aviation Regulations (FARs) require approved parachutes when two or more occupants in an airplane intentionally exceed a bank of 60 degrees or a pitch angle of 30 degrees relative to the horizon. The basic aerobatic maneuvers are the slow roll, loop, spin, snap roll, aileron or barrel roll, and the wingover/hammerhead stall. Competition and air show figures combine these basic maneuvers into complex upright and inverted versions. In the absence of a special waiver and to protect passengers and the general populace, intentional aerobatic maneuvers must be performed away from crowded air space, above only sparsely populated areas, and at altitudes greater than 1,500 feet above the surface. Aerobatic aircraft include some gliders and helicopters. Because aerobatics places extra structural and stability demands on an aircraft, only approved maneuvers may be performed in a particular air-

Aerobatics and Flight

craft. For aerobatic certification in the United States, an airplane must be capable of withstanding g-load factors from minus 3 to 6 without permanent deformation and loads of up to 50 percent greater (minus 4.5 to 9) without structural failure. The g-load factor, popularly known as the number of “g’s,” refers to the acceleration of gravity. Sitting still on Earth, one experiences an acceleration of 1 g, or a gravitational force of 1, the normal sensation of gravity. During periods of changing acceleration, such as a banking turn in an airplane, the so-called g-loading will change. Although the g-load factor in upright level flight is 1, it becomes minus 1 in inverted level flight. The best aerobatic aircraft, including those suitable for competition at the highest level, are stressed for load factors of 12 or more g’s. Aerobatics places extra physical demands on the pilot as well: loss of consciousness (positive g-load factors) or burst blood vessels (negative g-load factors) result from sustained high load factors. Military pilots have g-suits that help keep blood in their heads during positive load factors, whereas competition pilots use reclined seats and muscle tensing. A pilot’s tolerance to g-loads increases with practice. SLOW ROLL The slow roll is the most basic roll maneuver and the hardest to learn. It must be mastered before solo aerobatic flight should be considered. In this maneuver, the aircraft is rolled about its longitudinal, or nose-to-tail, axis without altering the direction of flight. Differential aileron deflection provides the torque that produces the roll. The other two controls, the elevator and the rudder, are used to keep the airplane from turning. When the roll is initiated, the opposing rudder must be used, and this reaches a maximum at about one-quarter, or about 90 degrees, through the roll. As the wings lose lift, the elevator must simultaneously be moved toward neutral. For the next quarter-roll, the rudder pressure is

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Principles of Aeronautics

The “Frecce Tricolori,” the aerobatic demonstration team of the Italian Air Force, 2021. Photo by Lawrence of Italia, via Wikimedia Commons.

reduced, and forward elevator is added, as the wings are asked to generate negative lift. For the next 90 degrees of roll, rudder pressure in the direction of the roll is added and the elevator is gradually neutralized. In the last 90 degrees, elevator pressure is increased to the value before the roll was initiated, in level flight. The roll can be stopped at any point by neutralizing the ailerons; a momentary stop every 90 degrees, for example, yields a four-point roll. The slow roll is difficult to learn because elevator and rudder inputs are constantly changing in a manner completely different from those of other maneuvers, because the forces on the pilot are so different and constantly changing, and because even a small

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error can place the aircraft in an inverted dive from which a safe recovery can be difficult. If the roll is initiated from level flight, the pilot senses an apparent weight that varies from normal to zero to upside down to zero to normal, corresponding to g-load factors varying from (at least) 1 to 0 to -1 to 0 to 1. Attempts to teach oneself this maneuver will almost certainly cost a great deal of altitude and exceed design speeds and loads. Beginning pilots often fail to add enough opposite elevator as they near inverted flight, causing the nose to drop and allowing the speed to drop and then build very rapidly. At this point, pilots are disoriented and distracted by hanging from their shoulder harness and will relax

Principles of Aeronautics

the aileron pressure, causing the roll to stop. The natural and almost guaranteed reaction is then to pull back on the stick or wheel, attempting a recovery with a dangerous half-loop. A similar disastrous reaction can be expected from a nonaerobatic pilot when wingtip vortices or atmospheric turbulence flips the plane well past a 90-degree bank. The slow roll has been mastered when the control inputs are instinctive, based on what the pilot wants the nose to do. Rudder pressure on one side always moves the nose in that direction, and forward movement of the stick or wheel always moves the nose away from the pilot. Once this concept is learned, slow rolls in any direction—straight up, straight down, or at an angle to the horizon—can be safely executed. However, the vertical, climbing roll is always a challenge, because it lacks a forward reference point and poses the danger of an inadvertent tail slide. A slow roll is anything but slow in a modern, competition aerobatic airplane, in which roll rates of 720 degrees per second are not uncommon. The roll can be completed so rapidly that there is little time in which to encounter difficulties. Jet fighters can roll very rapidly without requiring rudder input. LOOP A loop is one of the prettiest and most enjoyable aerobatic maneuvers, but skill is required to perform it safely and well. If the pull-up is made too abruptly, the aircraft can suffer either structural damage or a high-speed stall and will not complete the top of the loop. If the pull-up is too gradual, or if there is inadequate speed, the aircraft will run out of speed and fall inverted out of the maneuver. A smooth but noncircular loop requires a g-load factor of 3 to 3.5, whereas a competition-quality circular loop may require a g-load factor of 6. Good aerobatic aircraft are fully symmetrical and can loop from level flight from either erect or inverted flight. A wingtip can be used for spatial reference during

Aerobatics and Flight

the second quarter of the loop, when the horizon will be hidden but, once over the top, the pilot will look overhead for the beautiful sight of the reappearing horizon. Competition-quality “square” loops can generate momentary g-load factors of 10 or more. The first pilots to perform the loop, in 1913, were Petr Nesterov of Russia and Adolphe Pégoud of France. In 1928, Speed Holman of Minnesota broke the world’s upright looping record by performing 1,433 consecutive loops in a five-hour period. SPIN A spin’s downward spiral makes it a crowd pleaser, although it is not a particularly pleasant maneuver for the occupants of the plane. A spin is normally initiated at a speed close to the stall speed with power off, neutral aileron, and full rudder and elevator deflection. After about one turn, the spin should stabilize in a nose-low position, and the airspeed should stabilize at a relatively slow speed, because both wings should be stalled, one more than the other, creating considerable drag. Recovery is usually effected with full opposite rudder to stop the rotation and then at least a relaxation, if not a reverse deflection, of the elevator control. Pulling out of the resulting dive generates a g-load factor of 2 or more. All aerobatic pilots must be very well versed in the spin characteristics of their aircraft, because any failed maneuver often degenerates into a spin. In a true, stable spin, the spin can be continued if altitude remains and the airspeed does not increase. Utility aircraft certificated for spinning may appear to give a good spin entry, but the spin may become a diving spiral, increasing the speed. The same will happen in a good aerobatic airplane if the pilot does not hold full elevator and rudder deflection. Heavy aircraft such as fighter aircraft may show wild gyrations upon spin entry and an oscillating pitch attitude once the spin is established. The World War II P-51D Mustang, for example, would

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oscillate from near-vertical to above the horizon and would lose about 1,000 feet per turn; spins were not to be performed below 12,000 feet. Fully aerobatic aircraft can perform inverted spins as well as upright spins, but the aircraft recovers to inverted, stalled flight when the rotation is stopped, from which recovery to level flight should be made with a slow roll to minimize altitude loss. The rudder may suffer less blanking in inverted spins, allowing recovery to be faster. The inverted spin is much more disorienting than an upright spin and the pilot must concentrate on maintaining full elevator deflection, or the spin will transition to a diving spiral with rapidly increasing speed. If recovery from an upright spin is forced with down elevator and power, some aircraft will flick into an inverted spin. If the aircraft is not certified for spins, or if the center of gravity is too far aft, the spin may be an unrecoverable flat spin with the nose on the horizon, yawing almost entirely rather than exhibiting nearly equal yaw and roll. Modern aerobatic aircraft with fully inverted fuel and oil systems, however, can force an upright or inverted spin to go flat with power and aileron deflection against the spin. These flat spins not only are recoverable but also form an important part of many air show routines. Because it is such an important maneuver, the spin is the only aerobatic maneuver required of pilots seeking to become flight instructors. The requirement for parachutes is waived if an instructor is teaching an instructor-student. Considering that a low-altitude stall that degenerates into even an incipient spin remains a leading cause of fatal accidents, it would seem reasonable for more pilots to become familiar and comfortable with efficient recoveries from incipient spins, entered in the same fashion as accidental spins. Lieutenant Wilfred Parke of England is generally credited with first using what became the classic spin recovery method, in 1912.

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SNAP ROLL A snap roll, also known in England as a flick roll, uses the same control inputs as the spin, but in a snap roll, the controls are applied with power on and at speeds well above the unaccelerated stall speed. The resulting differential lift of the wings produces a rapid roll that can be very difficult to stop at a precise point. Good aerobatic aircraft can execute three or more consecutive snap rolls, both upright and inverted, before the axis of the roll changes excessively and the roll degenerates into a power spin. The load factor varies, as the square of the entry speed is divided by the unaccelerated stall speed, but a considerable twisting moment is also applied to the fuselage. This maneuver, among others, teaches the aerobatic pilot that an aircraft can exceed the critical angle of attack at any airspeed and at any angle relative to the horizon. AILERON ROLL The most comfortable rolling maneuver is the aileron roll, also known as the barrel roll. It is performed through coordinated use of the ailerons and rudder, basically continuing a climbing steep turn to a 90-degree bank, letting the nose fall through the horizon with reducing elevator pressure as the roll continues to inverted flight and then recovering with increasing elevator pressure back to upright flight. The nose will trace out a sort of circle around a point on the horizon. The radius of the circle depends on the roll rate; if the roll is slow, the circle must be large and the top of the circle must be far above the horizon to keep the nose from dropping too low and building up a great deal of speed in the lower half of the maneuver. G-load factors of close to 1 throughout the maneuver are achievable. An expert pilot can perform this kind of roll in almost any airplane; in 1955, test pilot Tex Johnston barrel-rolled the prototype Boeing 707 airliner at a flight demonstration for potential customers.

Principles of Aeronautics

WINGOVER AND HAMMERHEAD STALL The hammerhead stall and the wingover are the most common turnaround maneuvers used by air show performers to maintain their presence in front of the audience. A wingover is a maneuver that changes the flight direction through 180 degrees with negligible net change in altitude. It is performed by simultaneously raising the nose and smoothly banking to a 90-degree bank angle as the flight direction changes by 90 degrees and then smoothly reducing the bank angle to 0 degrees in a descending turn to level flight in the opposite direction. Load factors should be in the range of 0 to 2 for a smoothly executed wingover, because there is no attempt to maintain level flight in the steeply banked turn. In the hammerhead stall, known in England as the stall turn, the aircraft is pitched straight up with power on until it is pointing straight up. Shortly before the craft runs out of airspeed, full rudder is used to rotate the nose to the right or the left, and the rotation is stopped when the aircraft is heading straight down. Recovery may be to either upright or inverted level flight. Load factors need not exceed 2 or 3 if the initial entry and the pullout in recovering are smooth and to upright flight. The “stall” part of the maneuver’s name is a misnomer, because the angle of attack is close to zero during the maneuver, and no stall buffet should be felt. An aircraft with a clockwise propeller rotation from the pilot’s view will rotate best to the left. The greatest danger is waiting too long to use full rudder, allowing the aircraft to slide back on its tail, known as a tailslide, which could damage some of the control surfaces on otherwise aerobatic aircraft. ADVANCED AEROBATIC MANEUVERS The Immelmann turn, named after German World War I fighter pilot Max Immelmann, is a half-loop followed by a half-roll to upright flight. If the speed

Aerobatics and Flight

is low or the loop is stopped too abruptly, a sudden flick into an inverted spin is possible. The Cuban Eight combines three-quarters of a loop, a roll to upright, another three-quarters of a loop, and a roll to upright again. From the ground it appears in the form of a horizontal eight. The rolling turn, a very demanding maneuver to do well, combines a 360-degree turn with a roll, either to the inside or the outside of the turn. The square loop attempts to minimize the radius of the turns at the top and bottom of the loop and generates some of the highest momentary load factors. The Lomcovák is a spectacular, twisting, tumbling maneuver invented by the Czech Ladislav Bezák in 1957. It is usually entered from a climbing, inverted snap roll and is commonly demonstrated at air shows. Another spectacular maneuver is the torque roll, in which the airplane is rolled pointing straight up, and the roll is continued, with the help of engine torque, for a few fuselage lengths in the ensuing tailslide. Powerful aerobatic airplanes can generate enough fuselage lift and horizontal thrust component to maintain level flight in a 90-degree bank, known as knife-edge flight. Russian pilots have demonstrated the cobra maneuver, in which a jet fighter, flying in level flight, is abruptly pitched up through 90 degrees of rotation or more, recovering to level flight with relaxation of the stick. ARESTI SYMBOLS The distinguished Spanish aerobatic pilot Colonel José Luis de Aresti Aguirre developed a shorthand notation for aerobatic maneuvers. First published in 1961, Aresti symbols have become universally used to outline aerobatic routines for both contests and air shows. Each figure in Aresti’s dictionary includes a difficulty, or “K,” factor, by which, in contests, judges’ scores—from 0 to 10—are multiplied. —W. N. Hubin

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Further Reading Carson, Annette. Flight Fantastic: The Illustrated History of Aerobatics. Haynes, 1986. DeLacerda, Fred. Surviving Spins. Iowa State UP, 1989. Kershner, William K. The Basic Aerobatic Manual, With Spin and Upset Recovery Techniques. Iowa State UP, 1987. Luger, Jim. Loop, Roll, and Keep Control—A Step-by-step Aerobatic, Spin and Upset Manual. James Luger, 2020. Marrero, Frank. Lincoln Beachey: The Man Who Owned the Sky. Tripod Press, 2017. Pilkington, David J. Aerobatics Down Under. Revised and updated ed., David J. Pilkington, 2019. Thomas, Bill. Fly for Fun with Bill Thomas. 3rd ed., Yellow Schoolhouse Press LLC, 2022. Williams, Neil. Aerobatics. Crowood Press, 2014. See also: Flight roll and pitch; Flight schools; Flight simulators; Forces of flight; Plane rudders; Training and education of pilots; Weather conditions; Wind shear

Aerodynamics and Flight Fields of Study: Physics; Aeronautical engineering; Fluid dynamics; Mathematics ABSTRACT Aerodynamics refers to the study of airflow over bodies. Knowledge of aerodynamics allows for the prediction of the forces and moments on airplanes. This allows the design of safe and efficient aircraft that can perform a large variety of tasks ranging from small radio-controlled craft to airliners and supersonic military airplanes. KEY CONCEPTS drag: the resistance to motion through a fluid due to friction between the moving object and the fluid medium lift: the upward pressure experienced by wings due to differential pressure between upper and lower surfaces of the wing as it moves through a fluid medium

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pitch: the tendency of the nose of an aircraft to move up or down vertically as it moves through a fluid medium roll: the tendency of the body of an aircraft to rotate about its central axis as it moves through a fluid medium yaw: the tendency of an aircraft to turn horizontally about its center of mass as it moves through a fluid medium HISTORICAL ASPECTS In the late seventeenth century, English physicist Isaac Newton laid the foundations for not only modern mechanics and calculus but also fluid mechanics. Newton’s analysis of fluid flow considered air to be composed of individual particles that struck a body’s surface. This analysis was applied to determine the drag of an object in a moving fluid stream but gave poor results, because it did not account for the effect of the wing or body on the oncoming air. Interestingly, it later proved to be far more valuable in hypersonic flow analysis. Swiss mathematician Daniel Bernoulli and his father, Johann I, both published treatises in the 1740s that greatly clarified the understanding of the behavior of fluid flows. Eighteenth-century Swiss mathematician Leonhard Euler noted the problems with Newton’s model and proposed a more accurate formula for drag in 1755. Subsequent aerodynamic theories developed in the 1800s and early 1900s were based on the works of Newton, Euler, and the Bernoullis. In 1894, British inventor Frederick William Lanchester developed a theory that could predict the aerodynamics of wings. However, Lanchester published this work many years later, in 1907. An acquaintance with Lanchester’s theory might have saved considerable effort for Orville and Wilbur Wright, who first flew a heavier-than-air craft in 1903. Instead, the Wrights gained an understanding of aerodynamics through numerous wind-tunnel experiments conducted in their homebuilt wind tunnel. Subsequent advances

Principles of Aeronautics

in aerodynamics are associated with individuals, including Max Munk, Adolf Busemann, Ludwig Prandtl, and Robert Jones, who developed the principles of aerodynamic analysis. AERODYNAMIC FLIGHT REGIMES Fluids comprise both gases and liquids. A major difference between a fluid and a solid is that a fluid deforms readily. The major distinction between a gas and liquid is that a liquid is difficult to compress. The

Aerodynamics and Flight

atmosphere is a gas composed of approximately 78 percent nitrogen, 20.9 percent oxygen, 0.9 percent argon, 0.042 percent carbon dioxide, and in very small quantities, neon, helium, krypton, hydrogen, xenon, ozone, and radon, based on their volume. The study of the behavior of a body immersed in a moving liquid is called hydrodynamics; in a moving gas, gas dynamics; and in air, aerodynamics. Aerodynamics may be categorized as either lowor high-speed, depending on where the fluid behav-

A NASA wake turbulence study at Wallops Island in 1990. A vortex is created by passage of an aircraft wing, revealed by colored smoke rising from the ground. Vortices are one of the many phenomena associated with the study of aerodynamics. Photo via Wikimedia Commons. [Public domain.]

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ior changes. A common demarcation is subsonic and supersonic flow, where the latter has airspeeds greater than the speed of sound. Transonic flow, where both sub- and supersonic flow may exist, is also usually treated as a distinct regime. Increasing airspeed sees supersonic flow evolving into hypersonic flow at about five times the speed of sound. Difficulty in the analysis of airflow has additionally resulted in airflows being divided into viscous flows and inviscid flows, in which the latter are assumed to have no viscosity and are generally much simpler to analyze. The basic principles underlying aircraft flight are well described assuming inviscid flow. BASICS The flow of air over a body is governed by the so-called continuity equation and the momentum equations. These equations state that mass can be neither created nor destroyed and that the sum of the forces experienced by a body equals its rate of change of momentum, or its quantity of motion. Analysis of these equations applied to various flight problems laid the foundations of aerodynamics. As air flows over an airplane, the plane causes the air to change its velocity, which also leads to changes in the static pressure distribution over the aircraft. The static pressure is the pressure that is felt when moving at the speed of the airstream. The static pressure distribution causes forces and moments, or torques, over the aircraft. The equation that relates velocity and static pressure is referred to as Bernoulli’s principle. SUBSONIC AIRFLOW OVER AIRFOILS The forces and moment that an aircraft experiences are affected by the air density, which in turn is affected by air pressure, temperature, and the amount of moisture in the air, as well as the speed and size of the airplane. As the aircraft flies through the air, it displaces air downward. By pushing the air down, the aircraft’s wings experience a reaction force that

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tends to push the airplane up, creating lift. The lift is defined as being perpendicular to the oncoming airstream. One may imagine the lift of a wing flying along a wave of high pressure to be somewhat like a surfer on a surfboard riding an ocean wave. In cross-section, the wing of an airplane presents as an airfoil profile. The shape of the airfoil profile’s camber line, which is the line equidistant between the upper and lower surface of the airfoil, increases the lift generated at a given angle of attack if the airfoil has positive camber. Positive camber indicates that the leading edge and trailing edges, or the front and back of the airfoil, are curved down. If the airfoil has negative camber, the lift generated at a particular angle of attack is reduced compared to that of a flat airfoil with no camber. Consequently, positive camber or curvature of the camber line has the effect of increasing the lift by a constant amount for a given angle of attack, compared to a flat or symmetrical airfoil. The larger the camber of the airfoil, that is, its curvature, the greater the lift the airfoil will generate. This effect is most pronounced as the location of the maximum camber, or highest point of the airfoil, moves to the trailing edge, or back of the airfoil. The thickness of the airfoil, with reasonable accuracy, does not directly affect the lift the airfoil section generates, but it may affect the nature of the airfoil’s stall. The shape of the airfoil profile and its thickness distribution have a profound effect on the nature of the airfoil’s stall. When an airfoil’s angle of attack is greater than approximately 12 degrees, many airfoils will stall. This condition is due to the air’s viscosity and is caused by a disruption and separation of airflow over the airfoil’s upper surface. Stall causes lift to decrease as the airfoil’s angle of attack is increased. Flow separation also causes a large increase in drag, referred to as pressure drag. For very thin wings, or a flat plate, for example, the stall is quite moderate, and the loss of lift is gradual. For airfoils with a maximum thickness in the 9 percent

Principles of Aeronautics

range, where the maximum thickness of the airfoil divided by the length, or chord, of the airfoil is 0.09, the nature of the stall is quite sharp, and the loss of lift is dramatic. Thicker airfoil profiles, analogous to very thin airfoils, also have weak stalls, with a gradual loss of lift. Numerous methods and devices have been developed to delay the stall of airfoils. These usually comprise a modification to the nose of the airfoil and typically involve pointing the nose down or extending it off the airfoil and rotating it down. These devices are referred to as leading-edge flaps, or slats. Some birds use a similar concept with a feather called the alula, which forms a slat and stops the bird’s wing from stalling. For a given angle of attack below stall, these leading-edge devices generally do not much change the lift of the airfoil. However, they do extend the lift range of the airfoil and can increase it up to 10 degrees beyond the typical stall angle. These types of devices can be seen, and often heard extending or retracting, on airliners extending from the front of the wing at takeoff or landing. On modern aircraft, all components are streamlined, that is, smoothly blended. The importance of streamlining became evident in the 1920s, when it was found that smoothly faired, or joined, bodies, such as aircraft wheels with aerodynamic fairing, had much lower drag than nonfaired bodies. The fairing allowed the air flowing over the wheel to conform smoothly to the surface. Without the fairing, air would separate off the wheel and form large turbulent eddies, or swirling motions, in the wake behind the wheel, greatly increasing drag. The effect of streamlining is to reduce the tendency of the flow to separate off the surface. This separation is caused by the viscosity of the air. Theoretically, at low speed in an inviscid airstream, an airfoil does not suffer any drag. This condition is known as d’Alembert’s paradox, after eighteenth-century French mathematician Jean le

Aerodynamics and Flight

Rond d’Alembert, who calculated this apparent anomaly but was unable to explain it. The reason for the paradox was the exclusion of the effects of the air’s viscosity in d’Alembert’s calculations. Due to viscosity, airfoils experience a component of drag called skin friction drag. The skin friction is caused by the viscosity of the fluid layers near the airfoil surface. On the wing surface, the speed of the air is zero, a condition referred to as the no-slip condition. However, at some small distance above the airfoil surface, the airspeed reaches that which would occur if the flow were inviscid. The region between the surface and this point is referred to as the boundary layer. The nature and behavior of this boundary layer have a significant impact on the skin friction drag and stalling characteristics of the airfoil. The boundary layer can either be laminar, turbulent, or transitional from laminar to turbulent. A laminar boundary layer is composed of air moving in orderly lines. A turbulent boundary layer has air moving close to the airfoil surface in swirling motions. A laminar boundary layer has far lower skin friction drag than the turbulent boundary layer; however, it is also more prone to separate from the airfoil surface. Thus, most airfoils have an initially laminar boundary layer that flows from the front of the airfoil back along the surface. At some point, the boundary layer transitions from laminar to turbulent and is typically turbulent from this point to the trailing edge of the airfoil. Boundary layer transition can be caused by disturbances of insects, ice crystals, high airspeeds, and roughness or imperfections on the airfoil surface. To improve performance at high angles of attack by keeping the boundary layer attached to the airfoil upper surface, an aircraft designer may choose to cause the boundary layer to transition from laminar to turbulent at some point on the airfoil. This may be achieved using small protuberances attached to the airfoil’s surface.

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ULTRALOW-SPEED FLIGHT Efficient flight at very low speeds, such as those of slow-moving birds and insects, presents unique complications. Typical airfoil shapes do not generate much lift at these low airspeeds. The boundary layer at these low speeds is normally always laminar, and so easily separates off the airfoil surface. When this occurs, the lift of the airfoil decreases significantly and its drag increases. Insects and small birds such as hummingbirds use complicated wing motions to create lift at their low airspeeds. These insects and birds develop both so-called steady and unsteady lift, the latter of which is caused by the acceleration of the wing and its carefully performed motion through the air. SUBSONIC FLOW OVER WINGS If the wingspan of the aircraft were infinitely long and the air were assumed to have no viscosity, the wing would theoretically generate a lift force and a moment but no drag. However, aircraft do not have infinite wings, and thus an aircraft in steady cruise experiences lift and drag, as well as a pitching moment, which tends to move the aircraft nose up or down, and possibly either a side force, or yawing moment, which tends to displace the nose from side to side, or a rolling moment, which causes the aircraft to roll about its fuselage such that one wing is higher than the other. The lift of an airfoil is reduced when the airfoil is incorporated into a wing of finite length. The shorter the wingspan is relative to the chord of the wing, the less lift is generated. The largest losses of lift are near the wingtips. On a finite-length wing, air from the lower surface of the wing tries to curl up around the wingtip to the upper surface, causing the formation of two tornado-like structures, known as wingtip vortices, that trail from both wingtips backward. These vortices possess high rotational speeds and pose a significant threat to other aircraft that may fly through them. These vortices require delays between takeoffs and

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landings of aircraft using the same runways at airports, in order that vortices may have time to weaken. The component of drag due to the aircraft having a finite-length wing is called vortex drag. Generally, a wing’s vortex drag is independent of its span. Thus, wings with either a large or a short span will, to a first approximation, develop the same vortex drag. The larger-span wing will, however, generate far more lift, and thus the vortex drag will have a greater effect for short-span wings. To keep the amount of vortex drag low compared to the lift generated, a wing should have a large span. This is the reason why airliners have wings with large spans, and also the reason why gliders have narrow-chord, large-span wings. Aircraft may have many different types of wing shapes that are dictated by the aircraft’s function. A glider flies at low speed but needs to generate a large amount of lift with little drag. As a result, glider wings have large spans but small chords. An airliner needs to fly efficiently but is limited in its wingspan by airport considerations. A large wingspan results in a heavy wing, which is required to support the wing structure. As a result, airliner wings have a large span but not as large as their chord. As aircraft fly faster and approach the speed of sound, the flow over the wing changes. Shock waves may appear on the wings, even though the aircraft is still flying subsonically. An airfoil accelerates the air flowing over its upper surface such that it may become locally supersonic. A shock wave is a very thin flow discontinuity that occurs in supersonic flow and causes the airflow through it to slow down significantly. Shock waves are accompanied by large increases in drag on the airplane and are thus undesirable. A way to delay the onset of shock waves on wings is to sweep the wings back, a commonly seen design on airliners, in which most wings have a sweep of about 20 to 30 degrees. This sweep effec-

Aeroelasticity

Principles of Aeronautics

tively reduces the airspeed that causes the shock waves to form and so allows the plane to fly closer to the speed of sound, normally about 760 miles per hour at sea level. The speed of sound varies with the square root of the air’s temperature. SUPERSONIC AERODYNAMICS When the airspeed is greater than the speed of sound, the airflow is said to be supersonic. Aircraft that are designed to fly supersonically have distinctive design features. At supersonic speed, a new component of drag, called wave drag, appears in addition to the vortex, pressure, and skin friction drag. The wave drag is usually caused by the presence of shock waves on the wing or airfoil. This drag component is sensitive to the thickness of the wing and the lift that the wing is generating and increases with both. To keep wave drag as low as possible, supersonic airplanes may have very thin wings, such as those seen on fighter aircraft, highly swept wings, or a combination of both. The wing on the Concorde is an excellent example of a supersonic wing design. A popular wing planform shape is the delta, or triangular, wing, upon which the Concorde’s wing is based. The design requirements for efficient flight at supersonic speed and subsonic speed are contradictory. At low speeds, a large-span wing is desirable, whereas at high speeds, a highly swept wing is most effective. These requirements have led to the development of the so-called swing wing, seen on aircraft developed in the 1960s and 1970s, such as the European Panavia Tornado and the US B-1 bomber. For low-speed flight, the wings sweep forward, whereas for high-speed flight, the wings sweep rearward. However, this design’s prohibitive cost and structural weight have generally hindered its widespread use. A problem with wings designed for supersonic flight is that, due to their large sweep and small wingspans, they are poor lift generators. That is, they do not develop a large amount of lift for a par-

ticular angle of attack, which can pose serious difficulties when these aircraft either take off and land at very high speeds requiring long runways. One way to alleviate this problem is by designing the highly swept wing to have a sharp nose or leading edge. This design causes the airflow over the wing to form two tornado-like vortices that lie above the wing. These vortices may be clearly seen in photographs of the Concorde taking off or landing on humid days. These vortices greatly increase the lift of the wing, but they also significantly increase drag. —Lance Wayne Traub Further Reading Anderson, J. D. Fundamentals of Aerodynamics. 3rd ed., McGraw-Hill, 2001. Badick, Joseph R., and Brian A. Johnson. Flight Theory and Aerodynamics: A Practical Guide for Operational Safety. Wiley, 2021. Dalca, Cezar. Aerodynamics, Aeronautics and Flight Mechanics. Scitus Academics, 2015. Drela, Mark. Flight Vehicle Aerodynamics. MIT Press, 2014. Ianiro, Andrea, and Stefano Discetti. Experimental Aerodynamics. CRC Press, 2017. Liu, Peiqing. Aerodynamics. Springer Nature, 2022. Phillips, Warren F. Aerodynamics of Flight. John Wiley & Sons, 2014. Wild, Jochen. High-Lift Aerodynamics. CRC Press, 2022. See also: Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Animal flight; Flight roll and pitch; Fluid dynamics; Forces of flight; Plane rudders; Pressure

Aeroelasticity Fields of Study: Physics; Aeronautical engineering; Fluid dynamics; Mathematics ABSTRACT Aeroelasticity is a field of science that studies the interaction between an object that is encountering moving air and

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Aeroelasticity

the forces generated by that air. It specifically looks at what happens when air hits an object that is both solid and elastic, or movable, and the way the air can bend or distort that object. Since it was first defined in the mid-twentieth century, aeroelasticity has become an important factor in designing aircraft, buildings, and a number of other items for maximum function and safety. KEY CONCEPTS buzz: vibration experienced by an aircraft in flight as a result of forces exerted by the air against the aircraft as it moves wake: residual disturbance or turbulent air currents such as vortices caused by the movement of a body through a fluid medium BACKGROUND The word “aeroelasticity” comes from the Greek words aero, meaning “air,” and elastikos, meaning “propulsive.” In the middle of the seventeenth century, the Latinized form, elasticus, became “elastic,” and adopted the meaning of “expanding spontaneously to fill the available space.” In 1947, English engineer Arthur Roderick Collar defined a new term, “aeroelasticity,” as the study of the forces at work when moving air encounters inert and elastic forces. The forces created by moving air are known as “aerodynamics.” Aerodynamics can affect an object that is moving through the air, such as a car, airplane, or bird, or an object that is stationary with air moving around it, such as a building, tree, or flagpole. The force exerted by the air can be significant and can be affected by some characteristics of the object. For instance, a sports car that is designed to be low to the ground with a hood angled to allow air to easily flow over it is going to encounter less resistance from the air than a large eighteen-wheeled truck, and a thin flagpole will withstand the forces of a heavy wind better than a large billboard will. Objects that encounter the forces of air are generally a combination of solid and elastic components.

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For example, the metal of an airplane body is solid and inert, but the body is made of multiple parts that move in relation to one another, no matter how solid they may appear. The parts themselves may be rigid, but the areas where they connect can flex and move, giving the entire object an elastic quality. Air pushing against the parts in a way that brings them together can compact the object, while air forces that push one of the parts away from the other can cause the object to flex and expand. In either case, the object is distorted by the air from the form that it has when there is no air pushing against it. Aeroelasticity is the study of how these three forces interact with one another. It also encompasses the craft of designing items to withstand the distortion that results in elastic objects as the result of the wind. OVERVIEW Aeroelasticity is a factor in the design and use of items that must withstand the effects of moving air. Individuals who study or work with aeroelasticity are concerned with several specific aspects of this effect. These include flutter, divergence, buffeting, dynamic response, load distribution, control effectiveness, and control system reversal. Some of these factors affect objects that are moving through the air, such as a plane, while others are also encountered by objects that are stationary with air moving past, such as a skyscraper. These effects can be minor, causing slight, almost imperceptible, vibrations; or they can be serious, resulting in damage to the object. Of these, the two most significant to the study of aeroelasticity are divergence and flutter. Divergence occurs when the force of the wind is greater than the ability of a structure, such as a plane or building, to overcome that force. This leads to twisting of the structure and can result in structural failure. Divergence can be overcome by adjusting the angle at which the structure encounters the wind (such as angling the wings of a plane) and/or by increasing the stiffness of the structure.

Aeroelasticity

Principles of Aeronautics

Mass balance protruding from an aileron used to suppress flutter. Photo by Badobadop, via Wikimedia Commons.

Flutter is instability in the structure that is caused by its elasticity. The movement of the components of the structure will cause slight vibrations as the structure adjusts to the wind forces. Pilots often refer to this as “buzz.” Other phenomena that are considered in the study of aeroelasticity include buffeting, or the

forces of variations in air movement, such as the “wake,” or air disturbance, left by another aircraft; dynamic response, which are structure distortions that result from wind gusts or sudden movements of an aircraft; load distribution, which refers to how these distortions are spread over the entire surface of a structure; control effectiveness, which addresses

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Aeronautical Engineering

Principles of Aeronautics

how the aerodynamic disturbances affect the handling ability of an aircraft; and control system reversal, which occurs when the aerodynamics outside an aircraft cause the opposite of the expected effect when the controls are used. Any of these factors can cause structural failure, which is a particular danger in aircraft. To avoid potentially disastrous effects from aeroelasticity, engineers use careful design and extensive product testing. This testing can involve wind tunnels, in which models of a plane or parts of a plane can be exposed to forces of wind that will match and exceed those that will be experienced in use. These tests can also simulate the effects of winds at various temperatures, such as the very cold temperatures experienced by large passenger aircraft flying tens of thousands of feet in the air. Aircraft models can also be subjected to vibrations that will simulate what they will experience in the air. This testing is conducted while the plane is safely on the ground. Components of buildings, such as beams or wall structures that will be part of a tall skyscraper, can also be tested in this way. Testing can simulate the flutter that could be encountered in certain environmental conditions. The study of aeroelasticity is helping to create safer buildings and aircraft. These structures can be built to withstand the effects of the maximum aerodynamic forces that can be anticipated during their use. The study of aeroelasticity scenarios can also aid with the development of simulations that will help pilots be prepared to deal with the effects of aeroelasticity they may encounter in actual flight. —Janine Ungvarsky Further Reading “Aeroelasticity: Wing—Flutter and Divergence.” Aerodynamics for Students, s6.aeromech.usyd.edu.au/ aerodynamics/index.php/sample-page/aeroelasticity/. Accessed 18 Dec. 2017.

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Bishop, R. E. D. “Arthur Roderick Collar, 22 February 1908-12 February 1986.” Royal Society,1 Dec. 1987, rsbm.royalsocietypublishing.org/content/roybiogmem/3 3/163. Accessed 18 Dec. 2017. Dimitriadis, G. “Aircraft Design.” University of Liege, www.ltas-cm3.ulg.ac.be/AERO0023-1/ConceptionAeroAe roelasticite.pdf. Accessed 18 Dec. 2017. Lucas, Jim. “What Is Aerodynamics?” Live Science, 20 Sept. 2014, www.livescience.com/47930-what-is-aerodynamics.html. Accessed 18 Dec. 2017. Moraguez, Matthew. “Aeroelasticity.” University of Florida, plaza.ufl.edu/moraguezma/Aeroelasticity.pdf. Accessed 18 Dec. 2017. Myers, Andrew. “Good Vibrations: Stanford Engineers Put a Damper on ‘Aeroelastic Flutter.’” Stanford Report, 24 Mar. 2011, news.stanford.edu/news/2011/march/airplane-aeroelastic -flutter-032411.html. Accessed 18 Dec. 2017. Names, Ben. “5 Things You Should Know about Flutter.” Structural Design and Analysis, structures.aero/blog/5-things-should-know-flutter/. Accessed 18 Dec. 2017. “So Just What Is Aeroelasticity?” Georgia Tech School of Aerospace Engineering, www.msmith.gatech.edu/aeroelasticity. Accessed 18 Dec. 2017. See also: Aerodynamics and flight; Aeronautical engineering; Airplane accident investigation; Airplane safety issues; Forces of flight; Pressure; Temperature; Wind shear; Wind tunnels

Aeronautical Engineering Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Aeronautical engineering is the study, design, and manufacture of aircraft and spacecraft. Aeronautical engineering is responsible for the development of and advancements in aviation and spaceflight.

Principles of Aeronautics

KEY CONCEPTS bomber: an aircraft designed for the delivery of bombs, water, and other ordinance to be dropped on a target crewed flight: air travel requiring the presence of a pilot and other personnel to maintain the aircraft’s flight fighter: an aircraft designed for engaging other aircraft in aerial combat SST: acronym for supersonic transport supersonic: at speeds greater than the speed of sound tanker: an aircraft designed to transport fuel for mid-air refueling ENGINEERING In the first century of crewed flight, which began in December, 1903, the application of the new science of aerodynamics was translated into flying machines by people who understood engineering and problem solving. The industry that grew from this small beginning made amazing strides in the first century of air travel. It is an industry built around visionary engineers and pilots. Aeronautical engineering had its true beginning before Orville and Wilbur Wright but the two brothers were pioneers in the techniques, processes, and system testing that were at the heart of the engineering design and development of aircraft and spacecraft. The conceptualization of an aircraft begins with the identification of something useful to be accomplished by an air machine. The process begins with sketches of an air vehicle to fulfill the performance expectations for the aircraft. In the first two decades of aircraft design and operations, many concepts were proposed, but by the end of World War I, the basics of successful aircraft design were established. Future refinements would come through better tools, materials, and concepts. At the beginning of the second century of crewed flight, the process involves digitally created draw-

Aeronautical Engineering

ings that are sent to machines that make the basic parts, which are then assembled, tested, and prepared for flight test. Twentieth-century aircraft engineering refinements moved at a speed unseen in any previous period of the industrial world. The motivation and excitement of flying higher, faster, and with larger payloads seemed to drive innovation and to demand engineering solutions. By the end of World War II, the aviation industry was fully established as a significant contributor to the economic and military strength of the United States. European aerospace also produced leaders in this field. Companies were built on the talents of engineers and the skills of craftsman. Engineering disciplines expanded, and in the late 1950s, aeronautical engineering became aerospace engineering. In most aircraft manufacturing firms, the engineering department was second in size only to the production groups. Typically, in the middle of the twentieth century, modern aerospace companies spread their products between commercial enterprises and government contracts. The bread-and-butter contracts came from the federal government until the end of the Cold War. Commercial applications of engineering ideas were spun off from aircraft and missiles that had been developed for the military. However, by 1990, the industry was in decline. Following the Gulf War in 1991, the downsizing of the air arms of the military accelerated. The demand for large numbers of new and different military aircraft came under such scrutiny that few of the new programs survived. On the commercial side of the industry, the engineering of new and better transports and aircraft destined for the air carrier markets stopped in favor of building on existing concepts to build bigger aircraft with bigger engines. Airspeed, comfort, and passenger loading ceased to be major requirements and took a back seat to economically viable air transport.

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RESEARCH AND DEVELOPMENT There are three significant eras in the expansion of the aerospace industry. These coincide with technology improvements as well as political changes that affected the industry. The first period started with the Wright brothers’ successful efforts at powered flight and ended with the advent of the jet engine. The next period began when jet engines were being put into all new aircraft designs, and this period saw rapid advances in aircraft performance. The last period began with the introduction of digital computer controls for the aircraft. This development made it possible to design and build incredibly safe and reliable aerospace systems. Out of World War II came large bombers and cargo aircraft. When jet engines were added to these

Principles of Aeronautics

aircraft they held promise for faster and higher, hence more efficient and comfortable, air transportation for the public. The first such jet transport built for the British Overseas Airway Corporation (BOAC, which became British Airways) was the Comet. However, the understanding of structural issues arising from rapid changes in pressure on certain parts of the aircraft, along with manufacturing techniques from the 1940s, resulted in an unsafe aircraft. After two exploded in flight due to structural failure and one burst during ground pressure testing, the world of aeronautical engineering became aware of fatigue failures and the need to design fail-safe structures. At the time, the US Air Force had Boeing designing and building a jet tanker using technology like that applied to its highly successful swept-wing B-47 jet

NASA engineers, seen here in mission control during Apollo 13, worked to ensure the safety of the operation and astronauts onboard. Photo via Wikimedia Commons. [Public domain.]

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bomber program. What came out of that work was the most successful aircraft transport design in history. The Boeing 707 model was the forerunner of all of today’s large jet transports. THE INDUSTRY After World War II, the growth in the aviation industry, both commercial and military, saw a proliferation of new prime contractors who were building and selling aircraft. A prime contractor was defined as the company that was responsible for the concept, design, development, and final introduction of the new aircraft into operational use. In short, a prime contractor was responsible for all aspects of the life cycle of the aircraft. The prime would have subcontractors, perhaps hundreds, with which it did business. At the start of the 1970s, and at the height of the Vietnam War, there were many primes in the aerospace business. The biggest and most successful were Boeing Aircraft, Douglas Aircraft, McDonnell Aircraft, Lockheed Aircraft, Republic Aircraft, General Dynamics, Grumman Aircraft, North American Aviation (North American Rockwell), Northrop Aviation, LTV Aerospace (part of LTV, which used to be Chance Vought), Northrop Aircraft, Bell Airplane and Bell Helicopter, Sikorsky Helicopter, and a handful of general aviation companies, including Cessna, Beech, Piper, and others. At the end of the twentieth century there were only three major aerospace companies left, with all others being absorbed into the remaining companies or having gone out of business. Boeing took over McDonnell Douglas, which used to be McDonnell Aircraft and Douglas Aircraft. Lockheed Martin absorbed General Dynamics Aircraft Division and Martin Marietta. Northrop and Grumman joined, adding pieces of LTV and others. In addition, Raytheon Corporation, which was a small missiles and electronics outfit in the 1960s, took over Beech Aircraft and other subsidiary companies. Cessna and Piper nearly went out of business during the 1970s and 1980s, due to

Aeronautical Engineering

changes in liability laws. Chance Vought became Ling Temco Vought in the mid-1960s and changed its name to LTV Corporation in the 1970s. It was one of the first prime contractors that attempted product diversification, with markets in steel, appliances, missiles, and aircraft; the corporation went bankrupt in 1986. FUTURE DEVELOPMENTS Compared to the days of the Wright Flyer and the Curtiss JN-4, aircraft which were very difficult to control and which carried very small payloads, the F-22 automated advanced fighter and the Boeing 777 automated, large twin-engine transport are engineering marvels. At the beginning of the twenty-first century, there are several different paths that may provide the next major step forward in aeronautical engineering. In June, 1963, President John F. Kennedy, speaking at the commencement of the fifth class to graduate from the US Air Force Academy, announced that the federal government would seek to develop the world’s first supersonic passenger transport (SST). This never happened, for two reasons. The first was the economic issue. Such an aircraft, designed using late 1950s and early 1960s technology, would be very expensive. Airlines could not justify the costs to operate them. The second issue was environmental. Warnings and concerns about the pollution or damage to the upper atmosphere from turbojet engines and problems with sonic booms, which are caused by the shock waves from a supersonic aircraft, led to a premature end of the SST. Europe, in a cooperative move between British and French aircraft firms, did pursue a smaller version of the SST, called the Concorde. It operated successfully starting in January, 1976, although it was under a limitation forbidding it from flying supersonically over the United States. Technology improved during the twenty-two years the Concorde was operating, and by the late 1990s, the National Aeronautics and Space Adminis-

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tration (NASA) attempted to resurrect the SST concept. By then, the problems of jet exhaust and its impact upon the upper atmosphere had been nearly resolved. Ways to reduce the pressure from the sonic booms were being planned. The program ended in 1999 when, for the second time, the economic issues surrounding operational costs of a large SST overrode advances in the aerospace engineering field. The next hope for large transport aircraft lies in engineering a craft that will cruise just under Mach 1. Most large aircraft can cruise efficiently at Mach .75 to .9 (the percent of the speed of sound) but if they could fly efficiently at 95 percent of the speed of sound this would mean a 5 to 20 percent increase in true airspeed (35 to 155 miles per hour). A speed increase of that magnitude would shorten the flight time from New York to Paris by approximately an hour and fifteen minutes. The potential savings in fuel, the increase in the number of aircraft that could fly the same route, and other factors make this an appealing possibility. It is not an easy engineering task, but then, most of the history of aviation has faced such challenging engineering tasks. The ultimate flight would be one that takes the passenger into low-Earth orbit and flies across both continents and oceans. That aircraft will probably come about once the space program has fully established the safety and reliability of such travel. Aeronautical engineering and the companies that have come to the forefront in both engineering and applied sciences for aerospace purposes will be able to achieve these ideas. —R. Smith Reynolds Further Reading Jandusay, D. E. P. 165 Solved Problems in Aeronautical Engineering. Explained. Solved. Final Answer Boxed. Book Publishing House Gate 5, 2020. Lopez, Francisco Gallardo, and Jens Strahmann. Fundamentals of Aerospace Engineering (Beginner’s Guide). CreateSpace Independent Publishing Platform, 2016.

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Mikel, Russell, editor. Aerospace and Aeronautical Engineering. Wilford Press, 2017. Soler, Manuel. Fundamentals of Aerospace Engineering: An Introductory Course to Aeronautical Engineering. CreateSpace Independent Publishing Platform, 2017. Spagner, Natalie. A Researcher’s Guide to Aerospace Engineering. Clanrye International, 2019. Ziegler, Margaret. Aeronautical Engineering. Wilford Press, 2016. See also: Aerospace industry in the United States; Air transportation industry; Airplane manufacturers; Avro Arrow; Glenn H. Curtiss; DC plane family; Federal Aviation Administration (FAA); Homebuilt and experimental aircraft; National Aeronautics and Space Administration (NASA); National Advisory Committee for Aeronautics (NACA); National Transportation Safety Board (NTSB); Howard R. Hughes; Igor Sikorsky; Spacecraft engineering; Wright brothers’ first flight

Aerospace Industry in the United States Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Chemical engineering; Mathematics ABSTRACT The aerospace industry consists of manufacturers directly involved in the production of aircraft, engines, and ancillary products for use in aviation and space travel, as well as the maintenance and operation of those aircraft. The aerospace industry became a critical part of the US economy following World War II. The industry benefited from the postwar emphasis on military and commercial aviation, as well as the development of spaceflight. KEY CONCEPTS Cold War: a period during the 1950s and early 1960s during which the Soviet Union and the United States essentially dared each other to start a nuclear war though neither one would

Principles of Aeronautics

jet aircraft: aircraft that are propelled by thrust from the exhaust of a continuous combustion of fuel piston aircraft: aircraft that are propelled by the action of propellers driven by an internal combustion piston engine THE AEROSPACE INDUSTRY THROUGH 1945 The United States’ adventure in aviation went from its first flight in 1903 to flights to the Moon in 1969, and continues in the present day with efforts by many nations toward the exploration and colonization of the Moon and Mars. Despite this impressive record of accomplishment, aircraft manufacturing proved to be a difficult business. Early companies, notably the Wright Company, founded by Wilbur and Orville Wright, and the Curtiss Aeroplane Company, established by Glenn H. Curtiss, sold a handful of planes to the military but did not find a lasting market for their aircraft. The federal government recognized the importance of aviation by establishing the National Advisory Committee for Aeronautics (NACA) in 1915 but did little to help struggling manufacturers. Although World War I forced the United States to produce greater numbers of aircraft, most American pilots flew French planes during the conflict. Although US airplanes did not make an impact during the war, they did serve to train postwar aviation enthusiasts. The availability of surplus planes left over after the war undercut manufacturers to some degree, but the demand for increased performance gave companies an opportunity to introduce new designs. Despite widespread interest in aviation in the years leading up to World War II, the industry catered primarily to the military. Even companies, such as Boeing and Lockheed, that aggressively targeted the commercial market with private planes and airliners looked to the military for a significant proportion of their business. Other companies, notably Grumman and Douglas, dealt almost exclusively with the military.

Aerospace Industry in the United States

World War II put a stop to commercial aviation plans and forced all manufacturers to focus on military aircraft. The leaders of the US postwar industry clearly emerged during this period. Boeing, North American, Lockheed, Grumman, and Douglas all established themselves as mainstays in aerospace manufacturing. The war also necessitated enormous advances in technology. By the end of the war, jet fighters had taken to the air, heralding the future of the aviation industry. Finally, World War II established aviation as an indispensable component of both military and civilian life in the years to follow. POSTWAR INDUSTRY TRENDS The aerospace industry became increasingly important during the Cold War. The United States relied on technology to offset the numerical advantage of the Soviet Union. Many of the aircraft manufacturers that had done well during World War II remained at the forefront of the industry. These companies concentrated on four areas: military aircraft, missiles, rockets and space exploration vehicles, and commercial aircraft. The advent of space exploration prompted journalists to coin the term “aerospace” in the 1950’s, reflecting the new era of US aviation manufacturers. The aircraft, aerospace, and parts industry had become the largest US employer by 1959, and cities connected to the industry, including Los Angeles, Seattle, and Phoenix, exploded in population. MILITARY AIRCRAFT Aerospace manufacturers worked hard to win lucrative government contracts following World War II. The United States demanded advanced fighters and bombers to meet the Soviet threat. While these contracts provided the backbone of the industry, they also placed extraordinary demands on the manufacturers. The new planes required expensive engines and complicated alloys, both of which added a great deal of expense to the planes. Construction of the air-

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Principles of Aeronautics

NASA’s AIM (Aeronomy of Ice in the Mesosphere) satellite, assembled in clean room. Photo via Wikimedia Commons. [Public domain.]

craft usually necessitated new techniques and equipment. Government designs often included overly complicated ideas that added to the weight of the aircraft. The industry did not help matters by overpaying executives and using unnecessarily expensive components. These problems led the US Congress to require new levels of bureaucracy and paperwork to control costs. Furthermore, Congress could decide at any point to cancel a project, leaving the contractors heavily in debt with no potential market. Despite these problems, aerospace companies could not disregard the billions of dollars that the

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government contracts offered. The enormous sums granted to various manufacturers also allowed much more funding for research and development, accelerating advances in technology. The United States ended World War II somewhat behind Great Britain in jet engine construction, but by the mid-1950s, American manufacturers Pratt &Whitney and General Electric had become the leaders in jet technology. The increasing reliance on computers in the design stage led to continual improvements in microtechnology. Talented individuals such as Clarence “Kelly” Johnson at Lockheed and Ed

Principles of Aeronautics

Heinemann at Douglas created brilliant designs that exceeded government specifications and kept costs down. The biggest problems aerospace manufacturers faced after World War II were not the technical demands of the new aircraft. Given enough time and money, men such as Johnson and Heinemann could overcome those obstacles. Unfortunately, the political demands of the defense issue often took precedence. Companies simply could not afford to spend several years and millions of dollars to develop an aircraft that would not enter service. Consequently, manufacturers went to great lengths to make their projects successful. Lockheed received a considerable amount of bad publicity in the 1970s when investigations revealed that the company had relied heavily on bribery to ensure foreign contracts for its F-104 fighter during the preceding decade. Northrop also suffered for its use of bribery in the Middle East in an effort to find a market for its F-S fighter. Even companies that avoided politics could not disregard the new era in the industry. In the late 1960s, Grumman expanded its facilities to begin manufacturing the Gulfstream II corporate jet. The company, which had always eschewed marketing, placed its new facility in Savannah, Georgia, which was represented by an important member of the House Armed Services Committee and the home state of another influential member of the Senate Armed Services Committee. American defense cutbacks forced manufacturers to consider other markets. In the mid-1970s, General Dynamics designed the single-engine F-16 fighter. The lower cost associated with using only one engine made the plane attractive to European nations with limited budgets. General Dynamics did have to allow European countries to manufacture some of the planes, but the consortium reduced costs for all companies and promoted sales around the world. Difficulties in controlling costs finally forced US competitors to begin working together as

Aerospace Industry in the United States

well. Northrop, with little experience in carrier aircraft, had to turn to McDonnell Douglas for help with a new carrier-based fighter. The result, the F-18 Hornet, became a great success. Not only did the Navy and Marine Corps adopt the plane, but its low cost ensured brisk sales to air forces around the world. The F-18 program convinced manufacturers that collaboration had become necessary to control spiraling costs. The early 1980s saw a resurgence in Cold War tensions. President Jimmy Carter reinstated previously canceled programs such as the MX Peacekeeper intercontinental ballistic missile (ICBM) and the B-1 bomber. The new US military buildup offered greater opportunities for military manufacturers, but these advantages were offset by the fact that the government demanded small numbers of extremely complex aircraft. This trend accelerated after the end of the Cold War, as the United States slashed its defense budget even further. The Air Force could not afford advanced programs such as the F-22 fighter and B-2 bomber, the Navy canceled its search for a new attack plane after well-publicized cost overruns, and crashes of new aircraft eroded public confidence, leaving manufacturers to fight over a shrinking sector. MISSILES As military aircraft contracts forced manufacturers into hard-fought competition, America’s missile program gradually came to represent a larger share of the industry’s production. Between 1956 and 1961, airframe companies increased the percentage of missiles within their military business from 5 to 44 percent. In missile technology, many of the same manufacturers that dominated aircraft production also took a leading role in missile development, but companies such as TRW and Morton Thiokol made significant contributions to the industry. The United States saw missiles as an important part of the nation’s Cold War arsenal. The govern-

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ment took great pains to secure the services of Germany’s leading missile designers at the end of World War II, but the growing Soviet threat made the development of ballistic missiles a high priority. These weapons, like the aircraft and space vehicles of the Cold War, proved much too expensive for individual companies. Missile projects required subcontracting and cooperation between manufacturers. By 1960, US ballistic missile projects included two thousand contractors and forty thousand employees. In the late 1950s, the United States’ first intermediate range ballistic missiles (IRBM) entered service. Thor, produced by Douglas, and Jupiter, produced by Chrysler, went into installations in Britain, Italy, and Turkey. The United States soon succeeded in fielding ICBMs, which could be launched from the United States and attack targets within the Soviet Union. The first two ICBM programs were Atlas and Titan. These programs used separate contractors for each major system in order to facilitate competition and force companies to deliver their products on time. The Air Force did not like the complicated Atlas and Titan missiles and granted a contract to Boeing to manufacture the Minuteman, which entered service in 1962. The Minuteman program did not use separate contractors for each system, but allowed Boeing to subcontract the component manufacturing. Morton Thiokol, Aerojet-General, Hercules Incorporated, North American, Sylvania, Avco, and General Electric all supplied systems for the Minuteman, which were then assembled by Boeing. This approach proved much more effective, and Boeing produced more than one thousand Minutemen, making the missile the foundation of the US ICBM arsenal, even after the MX Peacekeeper missile entered service in the 1980’s. SPACE EXPLORATION American interest in rocket technology before World War II scarcely existed. Robert H. Goddard con-

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ducted pioneering research in the field, but few people gave his theories much notice. During World War II, tactical rockets for battlefield use proved their effectiveness and teams at American universities and corporations began work on the weapons. The success of these weapons combined with the German breakthroughs in ballistic missile technology ensured that rockets would be a key component of national defense. A logical outgrowth of work on ballistic missiles was the idea of space travel. Goddard had theorized about using rockets to reach the Moon, and the conquest of space quickly became an important Cold War achievement. The Soviet Union’s successful launch of Sputnik in October, 1957, revealed that the US space program lagged behind its rival. In response, the United States took several drastic steps to improve the nation’s position in the space race. Schools instituted new curriculums with heavy emphasis on math and science, while the government combined military and civilian rocket research and in 1958 created a new agency, the National Aeronautics and Space Administration (NASA), to replace the NACA. The Soviet lead in the space race allowed it to put the first human in space in 1961, but the United States soon made up the gap. The focused space program administered by NASA stressed corporate cooperation rather than competition. The tremendous cost of developing space vehicles prevented any one company from dominating the field. Instead of using one contractor, NASA used components from a wide variety of manufacturers to create finished products. Companies that failed to meet NASA’s specifications and deadlines risked losing contracts after having spent millions of dollars on research and development. Grumman, General Electric, and North American all revamped their manufacturing and management techniques after aggressive analysis from NASA.

Principles of Aeronautics

The Apollo Moon-landing program illustrated NASA’s approach. No individual company could develop the equipment necessary for such a task. The agency used a variety of contractors to produce a handful of rockets and spacecraft. The Saturn rockets that carried the crews to the moon were a result of components produced by companies including Chrysler, Boeing, and North American. The Saturn V rocket stood 363 feet high and had a diameter of 33 feet, dwarfing any rocket the United States had yet produced. The huge size required companies to invest in new jigs and welding fixtures, new techniques in fabrication, and static test stands that were far larger than any in existence. The research and development and production costs of the Saturn rockets alone totaled $9.3 billion. Grumman, the main contractor for the Lunar Module, also faced tremendous challenges and suffered through numerous delays and cost overruns before delivering the finished product. The companies involved often complained about NASA’s unrealistic expectations, but the two sides generally found mutually agreeable solutions and manufacturers often found ways to streamline their manufacturing processes. Following the conclusion of the Apollo Program in 1972, American interest in space exploration waned. NASA conducted Skylab missions and a joint mission with the Soviet Union in 1975, but these offered little financial security for contractors. When the United States launched the first space shuttle mission in 1981, the space program enjoyed a brief resurgence, but this comeback ended with the explosion of the shuttle Challenger in 1986. NASA resumed crewed flights two years later, but the enthusiastic days of Apollo had gone forever. The increasing costs of space missions forced NASA to increase its participation in joint international missions. Despite these setbacks, contractors found new ways to remain active in space missions. In 1989, private corporations took over the launching of commercial payloads from NASA. McDonnell

Aerospace Industry in the United States

Douglas, Martin Marietta, and General Dynamics all sent satellites into orbit at less than half the cost of a space shuttle mission. COMMERCIAL AIRCRAFT The United States’ affluence and desire for travel following World War II represented an important market for aerospace manufacturers. Companies used the technology developed for the military during the war to produce faster and more comfortable passenger planes. Just as with military aircraft, the more advanced civilian designs proved more costly, and a failed project could leave a manufacturer deeply in debt. Even a successful design could require years to become profitable. Douglas and Lockheed led the immediate postwar commercial programs. The DC family from Douglas and Lockheed’s Constellation provided both intercontinental and transatlantic service and proved very popular. However, these piston-engine planes did not represent the future of the commercial airline industry. Britain’s De Havilland Comet, the world’s first jet airliner, entered service in 1952, proving that just as in the military sector, American companies trailed their British competitors in passenger jet technology. Unfortunately for De Havilland, several mysterious accidents grounded the Comets for two years, giving American manufacturers time to cut into De Havilland’s technological lead. Leaders Douglas and Lockheed did not embrace jet airliners as enthusiastically as did Boeing. The Seattle-based company realized that the company’s development costs for the B-52 bomber, KC-135 tanker, and a civilian airliner would be prohibitive unless Boeing could coordinate efforts on all three aircraft. Boeing used the same basic design for both the KC-135 and what would become known as the 707, the most successful US first-generation jet airliner, which entered service in 1958. This method of combining operations helped manufacturers offset some of the risk involved in de-

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veloping new aircraft. Douglas managed to lengthen its DC-8 jet by 37 feet in the mid-1960s, offering room for seventy more passengers. Boeing found that its 707 design did not allow for the same modifications, giving Douglas a significant advantage in the market. Boeing soon recaptured its position at the forefront of airliner manufacturing by developing the world’s first jumbo jet. Based on Boeing’s failed attempt to win the Air Force’s competition to build an enormous new transport, the Boeing team modified their design into the 747, which rolled off the assembly line in 1968. These methods helped manufacturers to control costs and to insure themselves to some extent against failure. Companies also advertised their planes in travel magazines, hoping to win passenger loyalty. However, creating a new design always entailed financial risk. When Boeing began work on the new 727 in the early 1960s, the company found that it would have to sell three hundred of the planes simply to break even. The 727 became remarkably successful, but the three-hundred-plane total was the equivalent of the entire production runs of commercial airliners twenty years earlier. The enormous sums of money that aerospace companies spent on research and production of military, space, and civilian aircraft eventually came back to haunt the manufacturers. In the late 1960s and early 1970s, companies faced the twin threats of reduced military budgets and a slumping economy. Boeing had to cut its workforce by nearly two-thirds, and Lockheed, staggering under the burden of producing the massive C-S Galaxy transport and new L-1011 airliner, nearly went out of business. Lockheed remained afloat solely because the federal government guaranteed the company’s credit to potential lenders. High costs also forced some companies to merge, including the 1965 merger of McDonnell and Douglas. Merger trends continued through the remainder of the twentieth century, as

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manufacturers found themselves unable to compete in the changing marketplace. This time of transition and economic distress eventually passed, and the commercial sector of the industry emerged with a clear structure. Boeing led US airliner producers and followed up its earlier designs with new airplanes, including the 737, 757, 767, and the next generation of airliners, the 777. McDonnell Douglas and Lockheed maintained secondary positions, while European consortium Airbus entered the US market, providing stiff new competition for Boeing. The US aerospace industry finished the twentieth century as the world’s leader, but changing government and commercial needs forced manufacturers to cut costs in order to remain competitive. —Matthew G. McCoy Further Reading Lawrence, Philip K., and Derek Braddon. Aerospace Strategic Trade: How the U.S. Subsidizes the Large Commercial Aircraft Industry. Taylor & Francis, 2017. National Aeronautics and Space Administration (NASA). U.S. Aerospace and Aviation Industry: A State-by-State Analysis. CreateSpace independent Publishing Platform, 2018. Ohlandt, Chad J. R., Lyle J. Morris, Julia A. Thompson, and Andrew Scobell. Chinese Investment in U.S. Aviation. RAND Corporation, 2017. Shah, Mumtaz. Aerospace Industry in the U.S.: Monopoly to Competitive Market. SSRN, 2017. Spreen, Wesley E. Marketing in the International Aerospace Industry. Taylor & Francis, 2016. ———. The Aerospace Business: Management and Technology. Taylor & Francis, 2019. U.S. Government Accountability Office. U.S. Aerospace Industry. CreateSpace Independent Publishing Platform, 2017. See also: Aeronautical engineering; Air transportation industry; Airplane guidance systems; Airplane maintenance; Airplane manufacturers; Airplane safety issues; Avionics; DC plane family; Federal Aviation Administration (FAA); Flight schools; Homebuilt and experimental aircraft; Na-

Principles of Aeronautics

tional Aeronautics and Space Administration (NASA); National Advisory Committee for Aeronautics (NACA); National Transportation Safety Board (NTSB); Space shuttle; Training and education of pilots

Ailerons, Flaps, and Airplane Wings Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Chemical engineering; Mathematics ABSTRACT Ailerons and flaps are hinged sections on the trailing edges of wings. Both ailerons and flaps can be deflected to change local wing camber and to increase or decrease local lift. Ailerons are used to control the airplane in roll, while flaps allow flight at lower speeds for landing and takeoff. KEY CONCEPTS aileron: a small secondary structure at the outer end of a wing, used to alter the lift of a wing for control the roll and pitch of the aircraft in motion camber: the curvature of the cross-sectional shape of a wing flap: a secondary structure of a wing near the fuselage, used to increase or decrease the lift of the wing for speed control during takeoffs and landings AILERONS Early experimenters with gliders turned their vehicles by shifting their bodies so their weight was to the left or right of their wing’s lifting center. This action made the glider roll or bank to help it turn. Wilbur and Orville Wright improved on this effect by twisting or warping their wood and fabric wing with ropes and pulleys so that one wingtip was at a higher angle of attack than the other and the difference in lift on the two wingtips helped it roll. This design gave their airplane much greater maneuver-

Ailerons, Flaps, and Airplane Wings

ability than early European designs, which tried to turn using only a rudder. This wing-warping control system was the essential element in the Wright patent on the first successful airplane. Glenn H. Curtiss, another American aviation pioneer, patented a different way to control an airplane in roll, using ailerons, originally small, separate wings that were placed between the upper and lower wings of his biplane near the wingtips. The pilot could change the angle between these small wings and the flow to increase the lift on one wing and decrease that of the other. The Wrights claimed that this was a violation of their patent, and the case spent many years in the courts until the US government stepped in to resolve the dispute. Today’s ailerons are built into the trailing edge of wings near the wingtips, and they work by changing the wing’s camber, or curvature, instead of its angle of attack. The ailerons deflect either up or down opposite to each other to increase the lift near one wingtip while lowering lift on the other wingtip. This makes the wing roll, with one wing moving up and the other down. Usually, the aileron deflecting up produces more drag than the one moving down, which helps the airplane turn. In most turns, the aileron movements are coordinated with the movement of the rudder to create a turn which is balanced so that the airplane passengers feel only a downward force and no sideward force. A coordinated turn not only feels better but also is more aerodynamically efficient. If the pilot wants to roll the airplane without turning, the rudder must also be used to oppose the turning motion caused by aileron drag; this is called a cross-control maneuver. A similar cross-control use of rudder and ailerons can make the airplane rotate to the left or right in a sideslip motion without rolling. FLAPS Flaps often resemble ailerons except that they are placed on the wing near the fuselage rather than

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near the wingtips. Flaps normally are only deflected downward since they are used to increase temporarily the wing’s lift on landing and sometimes on takeoff. This maneuver allows flight at lower speeds and landing and takeoff in shorter distances. Early aircraft did not need flaps because they flew at low speeds and could land in much shorter distances than today’s planes; however, as airplanes became more streamlined and could cruise at higher speeds and altitudes, they needed higher speeds for

Photo via iStock/blacklight_trace. [Used under license.]

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takeoff and landing. Designers added flaps to give additional lift and drag and to reduce landing speeds. The famous DC-3 airliner was one of the first commercial planes to use flaps to combine good cruise performance with reasonable landing and takeoff distances. There are many types of flaps, from simple plates that deflect down from the bottom of the wing to very sophisticated combinations of little wings that extend down and behind a wing. The split flap was

Air Flight Communication

Principles of Aeronautics

used on the DC-3 and many World War II airplanes. Fowler flaps are more common today, but many smaller aircraft use simple hinges on the rear part of their wings to deflect a plain flap. The Fowler flap increases the wing camber while increasing the wing area. The space that opens between the deployed Fowler flap and the wing allows an airflow that helps control the pressures over the flap and delay wing stall. Many airliners designed in the mid-to-late twentieth century used complex flap systems that worked like the Fowler flap but had two or more flap elements that opened out below and behind the wing. These flap systems were very carefully designed temporarily to give very high lift at low speeds on sleek, modern wings that were shaped for flight near the speed of sound. They allowed airplanes that cruise at 500 to 600 miles per hour to land at low speeds and come to a stop on relatively short runways. Today’s commercial transport designs do not need these complex flap systems and tend to use simpler Fowler flaps, which are lighter and easier to build and maintain. This shift is partly because of improvements in wing and airfoil design and partly because most major airports now have longer runways. FLAPERONS AND SLATS Occasionally, an airplane design needs extra flap area to get lower landing speeds and the ailerons are also used as flaps. This kind of aileron is called a flaperon, and it requires a more complex hookup to the aircraft controls than a standard aileron and flap system. Some aircraft have flaps on the front of their wings that can also be deflected downward to increase the wing camber. These leading-edge flaps, or slats, help control the flow over the wing at high angles of attack and allow the wing to go to a higher angle of attack before it stalls. —James F. Marchman III

Further Reading Kundu, Ajay Kumar, Mark A. Price, and David Riordan. Conceptual Aircraft Design: An Industrial Approach. Wiley, 2019. National Aeronautics and Space Administration. Aircraft Wing Structural Detail Design (Wing, Aileron, Flaps, and Subsystems). CreateSpace Independent Publishing Platform, 2018. Sabry, Fouad. Adaptive Compliant Wing: No More Flaps, the Aircraft Wing Shape is Now Morphing. One Billion Knowledgeable, 2022. See also: Aerodynamics and flight; Airfoils; DC plane family; Flight roll and pitch; Forces of flight

Air Flight Communication Fields of Study: Aeronautical engineering; Pilot training; Flight control training; Mathematics ABSTRACT Air flight communication is the practice of exchanging safety and operating information between aircraft in flight and ground stations. Communication enables aviation to serve society more completely by expanding the conditions and geographical areas of its operations. KEY CONCEPTS phraseology: the standard use of words and phrases in a particular language to facilitate efficient communication EARLY FLIGHT COMMUNICATION Because they were few, underpowered, and only slightly engaged in commerce, airplanes before 1914 needed no communications between themselves or with ground-based stations. As World War I progressed, airships and specially equipped airplanes carried Morse code radio equipment for military purposes. It was not until the 1930s, however, that civil aviation communications radio became a

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Principles of Aeronautics

Photo via iStock/Jeremy Poland. [Used under license.]

truly useful appliance. Fledgling airlines in the United States began to install radios aboard their airplanes and at their dispatch hubs to monitor each airliner’s progress. This practice brought about the earliest, most rudimentary form of what has become the air traffic control (ATC) system. Early pilots considered radios an unwelcome intrusion in the cockpit, and some pilots refused to use them. Despite these protests, aviation communications provided undeniable benefits to safe and efficient operation, so the system expanded. Following World War II, aviation radios had become widespread in all but the smallest airplanes, as airspace around major cities became congested. By the 1960s, radios were familiar even in small airplanes. By the 1970s, air travel had become sufficiently pervasive that medium-sized

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and smaller cities attracted enough air traffic to make communications important to safety. The number of control towers rose accordingly, and radio communication frequencies soon became congested. Few pilots could realistically consider their airplanes as operating apart from the air traffic system, but standardization of communications procedures and phraseology lagged hardware technology. INTERNATIONAL STANDARDIZATION Standard phraseology is essential for several reasons. Flying is increasingly an international venture, for even those pilots who never venture far from their home airports encounter fliers from other lands. At the end of World War II, industry leaders of various nations recognized aviation’s interna-

Principles of Aeronautics

tional tendency and formed the International Civil Aviation Organization (ICAO). The ICAO established English as the standard aviation language; international aviation communication was and is to be conducted in English. Pilots from non-English-speaking countries must be able to read, write, and speak English sufficiently to use the aviation system, but at the beginning of the twenty-first century, reliably judging that ability in every corner of the industry was still uncertain. The twentieth century’s worst aircraft accident, the Tenerife, Canary Islands, collision of two loaded Boeing 747s, hinged solely on unclear communications. Responding to these deficiencies, the ICAO’s Proficiency Requirements in Common English Study Group (PRICESG/2) completed its second meeting and final report in May, 2001. The ICAO’s goal is to implement an English language proficiency standard for aviation in the twenty-first century. That standard is to address pronunciation, stress and intonation, grammar and syntax, vocabulary, fluency, comprehension, and interaction. The group suggested a list of items to be included in ICAO guidance material. These included the full ICAO scale with a glossary of terminology, elaboration of each level, and examples; an English language competencies chart specifying language performance objectives appropriate to the air traffic controller and pilot work domain; an introduction to English language acquisition and learning theories and methodologies; a manual describing the characteristics and attributes of sound English language training programs; a discussion of the importance of “extended” English, relevant to a controller and pilot’s ability to handle unusual aviation circumstances and emergencies; and approaches to testing English language speaking and listening proficiency. AVIATOR’S ALPHABET At the beginning of the twenty-first century, aviation was largely dependent on radio communications for

Air Flight Communication

both safety and efficiency. Air traffic control has developed from what was basically a trial-and-error experiment in the 1930s to an essential segment of the aviation industry. It works best when all participants understand the system and use it properly. Understanding is the most important commodity in pilot-controller communications. To establish a solid basis for understanding, in the early 1970s the Federal Aviation Administration (FAA) of the United States established a pilot/controller glossary. In that glossary, words and phrases to be used in flight have specific meanings. Aviation communication relies on these standardized meanings. The FAA calls this “phraseology,” and sets forth these words, phrases, and their meanings in the Aeronautical Information Manual (AIM). The AIM divides its treatment of communications into a user-friendly general discussion, placing the pilot/controller glossary handily at the end of the book. The FAA also had to deal with the issue of letters and numbers spoken over aviation radios. Each nation registers its airplanes using letters and numbers or letters alone; these tail numbers establish an airplane’s identity in radio communication. To facilitate this, one segment of the AIM displays a phonetic alphabet wherein individual letters are pronounced as specific and familiar words. The AIM treats numbers just as thoughtfully, rendering easily confused numbers with distinct sounds. For example, in conversational use, the numbers “five” and “nine” can be impossible to distinguish in noisy environments or when accents blur them. Aviation pronounces “five” as “fife” and “nine” as “niner.” Number sets such as “fifteen” and “fifty” are easily misheard even in the quiet of casual office conversation. Aviation addresses this by instructing pilots to, in most cases, speak each number separately. “Fifteen,” therefore, becomes “wun fife” and a correctly speaking pilot or controller says “fifty” as “fife zero.” On the other hand, the AIM instructs pilots and controllers to speak airliner call signs and airways in

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the more conversational format. Airway V12 would be spoken “vik-tah twelve.” Airliner 523 (the assigned flight number, not the tail number) would be spoken “Airliner fife-twenty-tree.” Aviators accepted the phonetic alphabet as they did the radio: Some loved it, some ridiculed it. As aviation brought regions, states, and nations into ever closer contact, the existing hodgepodge of dialects and accents justified the FAA’s wisdom in detailing even phonetic pronunciation. This practice bolsters understanding between pilots and controllers, making the aviation system far safer than it was before standardization had become a goal. PILOT/CONTROLLER GLOSSARY Even pilots native to English-speaking countries may have widely diverging accents, and syntax differs from region to region in many countries. In the United States, after 1972 the FAA established a pilot/controller glossary in the AIM that put forth words and phrases that were largely compatible with those of the ICAO. These words had developed by trial and error since the 1930s, and the FAA found them both efficient and effective. Common words include “Affirmative” to answer a question “yes,” while “negative” answers such a question with “no.” Flight students soon learn that on the radio, monosyllabic words such as “yes” or “no” might not transmit over the radio. Within the United States alone, different regions say “yes” in fashions confusing to the inhabitants of other localities. A commonly misused aviation word, “Roger,” means simply that the hearer has received all of the last transmission. It does not indicate compliance with an instruction, nor understanding of information. When pilots or controllers do not understand a transmission, they should ask the sender to “Say again.” Because radio communications frequencies are usually very busy, the ATC system has words that encapsulate entire sentences into a single word, easily understood by anyone without regard to their first language, accent, or any

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impediment. One example would be “Wilco,” which the AIM defines as meaning, “I have received all of your last transmission, I understand it, and I will comply with it.” Spoken altitudes, radio frequencies, and headings have traits that mesh with the basic rule of pronouncing numbers. Pilots in the United States speak altitudes as thousands and hundreds of feet. In aviation English, the phrase “Two thousand, five hun-

Letter

Word

Pronunciation

A

Alpha

al-fah

B

Bravo

brah-voh

C

Charlie

char-lee or shar-lee

D

Delta

dell-tah

E

Echo

eck-oh

F

Foxtrot

foks-trot

G

Golf

golf

H

Hotel

hoh-tel

I

India

in-dee-ah

J

Juliet

jew-lee-ett

K

Kilo

key-loh

L

Lima

lee-mah

M

Mike

mike

N

November

no-vem-ber

O

Oscar

oss-cah

P

Papa

pah-pah

Q

Quebec

key-beck

R

Romeo

row-me-oh

S

Sierra

see-air-rah

T

Tango

tang-go

U

Uniform

you-nee-form or oo-nee-form

V

Victor

vik-tah

W

Whiskey

wiss-key

X

X ray

ecks-ray

Y

Yankee

yang-key

Z

Zulu

zoo-loo

Principles of Aeronautics

dred” spoken alone only refers to altitude; any other subject would follow the numbers, such as “two thousand, five hundred RPM” if discussing engine or propeller speed, or “two thousand, five hundred miles” when discussing range. The AIM also admonishes US pilots to address radio frequencies by speaking the numbers individually, and to use the word “point” to define tenths and hundreds of a frequency allocation. Internationally, non-US pilots use the three-syllable word “decimal” instead of the single-syllable “point,” which the Americans find clearer and more succinct. A common ground control frequency is spoken as “wun too wun point seven” (121.7). Controllers and pilots use good procedure when they speak aircraft headings (the direction in which the aircraft travels in a straight line) by enunciating each number separately. To head east, therefore, is spoken as “zero niner zero.” This system, properly used, allows the person familiar with it the ability to understand a message because the more it uses specific, meaning-rich words or phrases, the less aviation is encumbered by ambiguous, nonstandard ones. The result is increased safety (saving lives and property) and efficiency (saving money and resources). For pilots and controllers, the pride of professionalism should be a third benefit. BENEFITS OF STANDARDIZED COMMUNICATION Not all pilots agree with the principle of standard phraseology. To teach standard phraseology takes time, and its benefits are not readily apparent with each use. Articles in aviation magazines occasionally have derided established phraseology, some authors belittling aviators who used it or instructors who taught it. Many of these too quickly embraced the AIM’s allowance that, should a pilot’s understanding of phraseology fail, he might simply speak conversational English. Others retorted that every pilot’s public duty is to learn the system and be a fully func-

Air Flight Communication

tioning part of that system, which includes established communications standards. Within the aviation community, as in most others, effective communication remains elusive. Yet while other industries tend to have codes or jargon for internal use, the decades have forged aviation’s communications system into an English-based specialty language. As such, aviation-speak is inefficient for face-to-face conversation but very succinct for time-critical communications in a fluid environment. That fact and its implications are only just beginning to make inroads into the flight training environment. Flight schools still concentrate on teaching aerodynamics, airplane systems, maneuvers, regulations, weather, or myriad other subjects that at the time seem far more immediate than communications. Overall, the aviation industry continues to awaken to communications as a serious public safety issue. —David R. Wilkerson Further Reading Barshi, Immanuel, and Candace Farris. Misunderstandings in ATC Communication. Language, Cognition, and Experimental Methodology. CRC Press, 2016. Becker, Mike. Aviation Communication and Flight Radio. Becker Helicopter Services Pty Ltd., 2022. Conforti, Facundo. Communications for Pilots. Biblioteca Aeronautica, 2022. Estival, Dominique, Candace Farris, and Brett Molesworth. Aviation English: A Lingua Franca for Pilots and Air Traffic Controllers. Taylor & Francis, 2016. Gardner, Bob. Say Again, Please: Guide to Radio Communication. Aircraft Supplies and Academics Inc., 2019. National Aeronautics and Space Administration. Re-Examination of Mixed Media Communication: The Impact of Vocie, Data Link, and Mixed ATC Environments on the Flight Deck. CreateSpace Independent Publishing Platform, 2018. Pacheco, Aline. English for Aviation. Editora da PUCRS, 2022. See also: Air transportation industry; Airplane accident investigation; Airplane safety issues; Federal Aviation Administration (FAA); National Transportation Safety Board (NTSB); Training and education of pilots

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Air Transportation Industry Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Chemical engineering; Mathematics ABSTRACT Business sector that uses aircraft to transport passengers, cargo, and mail. One of the leading business sectors in the American economy, the air transportation industry employed nearly half a million people during the early twenty-first century, not including travel agencies, hotels, and car rental companies. In 2004, the average American flew 2.2 times a year. KEY CONCEPTS deregulation: elimination of many of the government controls over aspects of the air transportation industry, essentially removing the possibility of favoritism flushed rivets: rivets whose heads are made flush with the surface of the aircraft’s fuselage rather than protruding, thus providing a uniformly smooth surface and reducing drag THE BUSINESS OF FLYING One of the leading business sectors in the American economy, the air transportation industry employed nearly half a million people during the early twenty-first century, not including travel agencies, hotels, and car rental companies. In 2004, the average American flew 2.2 times a year. The Wright brothers flew the first powered airplane in 1903, and World War I demonstrated the airplane’s military potential. In 1919, Deutsche Luft-Reederei (later Lufthansa) began flying passengers between Berlin and Weimar, Germany. The air transportation industry began in the United States in 1925, when Juan T. Trippe and others persuaded Congress to privatize the airmail system. The US Post Office initially granted twelve contracts.

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Trippe’s company, Colonial Aviation, won the New York-Boston route, but Trippe later lost control of the company. Airplane manufacturer William Boeing received the contract for Chicago-San Francisco and founded the airline that later became United Airlines. Pitcairn Aviation obtained the New York-Atlanta and Atlanta-Miami contracts and later became Eastern Air Lines. A company called Robertson Aviation flew the St. Louis-Chicago route and employed a then-unknown pilot named Charles A. Lindbergh. In 1927, Trippe’s new company, Pan American World Airways (Pan Am), received the contract to fly the mail from Key West, Florida, to Havana, Cuba. Trippe felt that he could increase profits by transporting a few passengers along with the mail. One of his first customers was the gangster Al Capone. In 1930, the US Post Office awarded the following contracts: New York-California via Chicago to United, New York-California via St. Louis to Trans World Airlines (TWA), New York-California via Dallas to American, and several routes along the east coast to Eastern. Two regional airlines that later became international also received routes: Braniff International Airways got the Chicago-Dallas route and Delta Air Lines got Atlanta-Chicago. The controversial millionaire (later billionaire) Howard Hughes made three important technical innovations during the 1930s. They were retractable landing gear, flushed rivets, and an oxygen feeder system. The first two streamlined airplane designs and increased their speed. The third allowed planes to fly at higher altitudes and increased their speed. Two aircraft, the Douglas DC-3 and the Boeing 315, boosted the air transportation industry during the 1930s. The Douglas DC-3 had two engines, flew at 180 miles per hour, was easier to fly than previous passenger planes, and was more comfortable for passengers. The Boeing 315, also known as the China Clipper, was a four-engine plane with pontoons as big as fishing boats. It landed and took off

Principles of Aeronautics

from water, so it could land anywhere in the ocean in an emergency, carried seventy-four passengers, had a 175-mile-per-hour cruising speed, and offered a range of 3,500 miles without refueling. As the nickname indicates, it was designed to fly from the United States to China, so Pan Am built refueling stations on islands such as Oahu, Wake, and Guam for its Hong Kong-San Francisco and New Zealand-San Francisco routes. It was the largest passenger plane ever regularly flown until the Boeing 747 came along. THE CIVIL AERONAUTICS BOARD PERIOD By 1938, there were 250 passenger flights each day in the United States. However, the system was per-

Air Transportation Industry

ceived as too chaotic by the administration of Franklin D. Roosevelt, which considered airlines to be a kind of utility. In line with the prevailing proregulation ideology, Congress passed the Civil Aeronautics Act of 1938. Not only did it create the Civil Aeronautics Board (CAB) to regulate routes and rates, but it also froze all existing airmail contracts in perpetuity. Prices for flights were determined by the CAB based on the costs provided by the airlines themselves so that the airlines were guaranteed to make a profit. Eventually airlines made a distinction between first class and coach, but even flying coach was so expensive that Pan Am partnered with the Household Finance Corporation to help mid-

Aerial view of Hartsfield–Jackson Atlanta International Airport, the world’s busiest airport by passenger traffic. Photo by formulanone from Huntsville, United States, via Wikimedia Commons.

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dle-class travelers pay for tickets through installments. Hughes bought most of the TWA stock in 1939 and worked with the Lockheed Corporation to develop the L-049 Constellation. It had a pressurized cabin that allowed it to fly at high altitudes, four engines that made it twice as fast as the DC-3, and the same range as the China Clipper. In 1955, Pan Am began flying the first passenger jet, the Boeing 707, and flight times were reduced even further. Pan Am started flying the Boeing 747, the first jumbo jet, in 1970. American Airlines developed the first computerized reservation system, Sabre, during the early 1960s. It enabled American to manage its inventory of planes and seats more efficiently and eventually accumulated reams of data. United built the second system, called Apollo, and other airlines such as Eastern, Delta, and TWA built their own systems as well. In 1976, United offered to place its terminals in the offices of travel agents, although American placed more Sabre terminals in those offices than any other airline. By the mid-1980s, American’s terminals were in 34 percent of the 30,000 travel agencies in the United States, and United’s were in about 25 percent of travel agency offices. In 1969, the CAB allowed the airlines to offer discount fares such as youth and family fares. Two airlines, however, began offering low-price tickets as the norm, not the exception. Pacific Southwest Airlines (PSA) and Southwest Airlines both flew within the borders of just one state, PSA in California, and Southwest in Texas. Consequently, they were not subject to CAB regulations and could set their own prices. The volume on PSA’s route between San Francisco and Los Angeles was so high that the airline could sell a one-way ticket for $10. In 1971, Southwest began flying between Houston, San Antonio, and Dallas Love Field Airport and charged $26 for a one-way ticket, except for the last flight of the day, for which it charged $10.

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Braniff, also based in Texas, cut ticket prices within Texas even further, to $13 for a one-way ticket. Texas International Airlines received permission from CAB to offer “peanuts fares,” which were 50 percent off their regular rates. Continental Airlines countered with “chickenfeed fares,” and American introduced “super saver” fares that required advance purchase and one-week layovers. A service company, founded in 1971, revolutionized the air cargo business. Originally based in Little Rock, Arkansas, Federal Express (better known as FedEx) moved to Memphis in 1973. Its concept was to guarantee next-day delivery of packages via a central hub. The company started with fourteen airplanes connecting twenty-five cities. DEREGULATION Deregulation began as a post-Watergate affair reform. American and Braniff were caught giving illegal campaign contributions to Richard M. Nixon’s 1972 reelection campaign in return for favorable treatment by the CAB. Also, the 1970s were the years of the highest inflation in American history, and pro-free-market economists proposed deregulation as a means of lowering airline ticket prices. Deregulation laws were enacted in 1978. Southwest was the airline best prepared for deregulation because of its low costs. Before deregulation, Southwest had added more cities in Texas to its schedule. However, before it could expand outside the state, it had to deal with one last remnant of regulation. Democrat Jim Wright represented the congressional district that included the Dallas-Fort Worth International Airport and the headquarters of Braniff and American. In 1979, he introduced a bill to prevent any airline from flying from Dallas Love Field Airport to any airport outside Texas. Fortunately for Southwest, it had enough support in Congress to force a compromise. It was allowed to fly from Love Field to airports in the adjacent states of Louisiana, Arkansas, Oklahoma, and New Mexico.

Principles of Aeronautics

(Wright’s law was repealed in 2006.) Southwest’s first interstate flight went from Dallas to New Orleans. Southwest added Chicago’s Midway Airport in 1985 and Baltimore-Washington International Airport in 1993. With flights to cities in California, it became a national airline, not just a regional one. Acquisitions were the first result of deregulation. For instance, Pan Am acquired National Airlines, American purchased AirCal, United obtained Air Wisconsin, PSA was taken over by US Airways, FedEx took over the Flying Tiger Line, and even Southwest bought a small airline called Morris Air, based in Salt Lake City. Texas International Airlines formed a holding company called Texas Air, which acquired Continental, People Express Airlines, and Eastern. It also created a new airline called New York Air. Texas International, People Express, and New York Air were eventually merged into Continental. Eastern continued to operate as a separate company but was forced to sell its computerized reservation system, its gates in Newark, New Jersey, and several wide-body jets to Texas Air at bargain prices. Eastern also had to pay Texas Air a management fee and buy its fuel from an affiliated company. Finally, Eastern’s sales department was transferred to Continental. Texas Air allowed Eastern to file bankruptcy in 1989, and it stopped flying. For the first time since 1938, the airlines had to compete on price, and some never adapted to the new situation. In 1982, Braniff became the first of the old airmail carriers to stop flying. Just before going under, it leased its Latin American routes to Eastern. During liquidation, American bought Braniff’s Dallas to London-Gatwick route. Pan Am survived longer, generating cash by selling its Pacific routes to United in 1985. It kept flying until 1991, when Delta purchased its East Coast and transatlantic routes. United acquired Pan Am’s Latin American routes during liquidation. TWA operated in

Air Transportation Industry

bankruptcy in 1992 and 1995 before it was finally taken over by American in 2001. American created the first loyalty program, using its Sabre system, by assigning different numbers to individual passengers. The airline also used Sabre to develop the concept of yield management, by which programmers could develop algorithms to automatically discount and, even more important, to refrain from discounting fares. This enabled American to increase profits even when involved in price wars. FedEx took advantage of deregulation to expand its fleet of planes and the number of cities it connected. In 1979, the company started using computers to track packages and expanded to Canada in 1981 and Asia in 1984. AFTER SEPTEMBER 11, 2001 Because of the terrorist attacks of September 11, 2001, the entire US air transportation system was shut down for two full days and took months to recover. About 16 percent of flights were eliminated in the process. US Airways took the lead when it cut 24 percent of its flights and laid off roughly the same percentage of employees. United entered Chapter 11 bankruptcy in 2002 and emerged from it in 2006. Both Northwest and Delta filed for bankruptcy, kept flying, and were in the process of merging in early 2009. Of the old airmail carriers, only American has operated without having to merge or file bankruptcy. In 2008, four smaller airlines—Aloha, Skybus, ATA, and Frontier—filed for bankruptcy, and many others cut costs and capacity in the face of rising fuel prices. However, Southwest and other low-cost carriers have increased their market share. In 2007, Southwest became the number one airline in the world in terms of the number of passengers flown. In 2008, as other airlines experienced trouble, Southwest reported a profit in its second quarter, the sixty-ninth profitable quarter in a row. —Thomas R. Feller

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Aircraft Icing

Further Reading Gittell, Jody Hoffer. The Southwest Airlines Way: Using the Power of Relationships to Achieve High Performance. McGraw-Hill, 2003. Hengi, B. I. Airlines Remembered: Over Two Hundred Airlines of the Past. Midland, 2000. ———. Airlines Worldwide. Midland, 2004. Odoni, Amedeo, Cynthis Barnhart, and Peter Belobada, editors. The Global Airline Industry. 2nd edition. Wiley, 2015. Newhouse, John. Boeing Versus Airbus: The Inside Story of the Greatest International Competition in Business. Vintage Books, 2007. Samunderu, Eyden. Air Transport Management: Strategic Management in the Airline Industry. Kogan Page, 2019. Van de Voorde, Eddie, and Rosario Macario, editors. The Air Transportation Industry: Economic Conflict and Competition. Elsevier Science, 2021. Vasigh, Bijan, and Ken Fleming. Introduction to Air Transport Economics from Theory to Applications. Taylor & Francis, 2016. Wensveen, John G. Air Transportation: Management Perspective. Ashgate, 2015. Williams, George. The Airline Industry and the Impact of Deregulation. Taylor & Francis, 2017. See also: Aerospace industry in the United States; DC plane family; Federal Aviation Administration (FAA); History of human flight; Howard R. Hughes; Supersonic jetliners and commercial airfare

Aircraft Icing Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Chemical engineering; Thermodynamics; Fluid dynamics; Mathematics ABSTRACT Icing is the accumulation of frozen moisture on an aircraft. The buildup of ice on an aircraft poses a serious hazard by interfering with the aircraft’s lift and causing additional drag and weight.

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Principles of Aeronautics

KEY CONCEPTS induction icing: formation of ice on internal surfaces affected by induced airflow that results in a lowering of temperature structural icing: ice that forms on exterior surfaces of an aircraft when water droplets contact surfaces that are below the temperature at which water freezes venturi: a device, particularly as a part of a carburetion system, in which the Venturi principle induces a flow of fuel into the carburetor by the action of air flowing through the carburetor body Venturi principle: the movement of a fluid through a channel such as a pipe will induce additional fluid from an outside source to enter the flow DANGERS When an aircraft encounters freezing temperatures and visible moisture, icing, the accumulation of frozen moisture, is possible. Although icing is a serious hazard to the safety of any flight, light aircraft are particularly susceptible to aircraft icing, as such craft have few, if any, anti-icing or deicing systems. Icing can destroy an aircraft’s ability to create lift and engine power when ice builds up on the structure and within the engine-induction system. TYPES OF ICING There are two types of aircraft icing, structural and induction. Structural ice may form on aircraft lifting surfaces, such as the wing and horizontal stabilizer, and on the windshield and protruding devices, such as the propellers, engine air intakes, antennas, struts, and landing gear. Ice adds additional weight to the aircraft. More critical, though, is the additional drag that the ice causes by disrupting the smooth flow of air over the lift-producing surfaces. Moderate to severe accumulations of structural ice can greatly affect the ability of the pilot to control the aircraft in flight. Both wind-tunnel and flight tests have proven that ice accumulations no thicker

Aircraft Icing

Principles of Aeronautics

Aircraft undergoing a de-icing treatment. Photo by Alex Pereslavtsev via Wikimedia Commons.

or rougher than a piece of coarse sandpaper can reduce lift by 30 percent and increase drag by as much as 40 percent. Structural icing may be present as rime, clear, or mixed ice. Rime ice has a rough, milky-white appearance. Rime ice forms when relatively small drops of moisture strike freezing aircraft surfaces and adhere to the surface rapidly. The milky white appearance is caused by the presence of air trapped in the rapidly freezing ice. Deicing systems are generally effective in removing rime ice, because rime ice is less tenacious than other forms of ice. Clear ice forms when large drops of moisture strike aircraft surfaces and freeze at a slower rate. The slower freezing process displaces air from the accreting ice, allowing the formation of a clear, very tenacious coating of ice on the aircraft’s surfaces. Because of the lack of aeration in the ice, clear ice is

difficult to remove and quite heavy. Mixed ice is a combination of rime and clear ice and exhibits characteristics of both. Induction icing can reduce engine performance and may result in complete engine stoppage. Aircraft equipped with carburetors may experience ice buildup in a restricted air passage, called a venturi, that is in the carburetor. An increase in air velocity and a resultant decrease in pressure within the venturi results in a reduction in air temperature. This lowering of air temperature creates the potential for moisture within the air to freeze and create ice accumulations along the sides of the venturi. The ice buildup reduces the flow of air and fuel through the venturi, resulting in decreased engine performance. In severe instances, ice may completely occlude the venturi, resulting in complete loss of engine power. In the case of aircraft equipped with

49

Aircraft Icing

fuel-injection systems, ice can accumulate in air intakes, reducing the flow of air to the engine. Ice occluding engine air intakes can cause reductions in available engine power as well as complete engine failure. FORMATION OF ICE Although all clouds are the manifestation of water in its gaseous state, the moisture content of clouds can vary greatly. Very-cold-winter states such as Montana, North Dakota, and Minnesota often have relatively dry clouds. In contrast, states such as Pennsylvania and New York often produce very wet winter clouds that, when temperatures drop below freezing, have a high potential for icing. Clouds with temperatures at or just below the freezing point, 0º Celsius, are the most likely to result in aircraft icing. Moisture in air that is well below the freezing point is already frozen and therefore will not adhere to aircraft. Wind can move moisture-laden air between regions. Wind moving across large bodies of water, such as oceans or the Great Lakes, will result in greater moisture content within the air. Mountains can cause a lifting phenomenon that may force moisture-laden air upward in the atmosphere where the natural temperature lapse rate cools the surrounding air to the freezing point. Areas of low pressure and fronts are the greatest producers of ice. Although in some instances, isolated air-mass instabilities may also produce sufficient moisture and temperatures capable of producing ice-generating conditions. Freezing rain and drizzle are the most hazardous ice-producing conditions. Freezing rain occurs when temperature inversions exist. Rain falling from clouds in warmer air aloft begins to freeze as it enters freezing air at lower altitudes. Freezing rain and drizzle can produce severe ice accumulations that rapidly overwhelm the ice-shedding capabilities of even the best anti-icing and deicing equipment.

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Principles of Aeronautics

PREDICTING ICE In the United States, the US National Weather Service (NWS) is the government agency responsible for weather forecasting. Utilizing NWS and other weather forecasting sources, the Federal Aviation Administration (FAA) disseminates weather information to the aviation community through a network of Flight Service Stations (FSSs). Flight service station specialists provide comprehensive weather briefings to pilots. These briefings are usually conducted over the telephone but may also be accomplished in person at the FSS. Pilots may also obtain icing and other weather information on the Direct User Access Terminal System (DUATS), utilizing a personal computer and Internet connection. Graphic weather charts available to FSS specialists and DUATS users include predictions of areas of potential icing. In addition, special meteorological notices called AIRMETS and SIGMETS are issued when potentially hazardous icing conditions exist. These notices provide pilots with an additional warning of potential icing. Pilots experiencing icing conditions report these conditions to the nearest FSS. Pilot reports (PIREPS) are usually conveyed directly to FSS specialists via the aircraft radio. PIREPS are an important component of the weather reporting system, because they describe actual conditions and not merely forecasts. —Alan S. Frazier Further Reading Bansmer, Stephan. Aircraft Icing: A Challenging Problem of Fluid Dynamics. Cuvillier Verlag, 2020. Choi, Chang-Hwan, and K. L. Mittal, editors. Ice Adhesion: Mechanism, Measurement, and Mitigation. Wiley, 2020. Gohardani, Omid. Progress in Aircraft Icing and Aircraft Erosion Research. Nova Science Publishers, 2017. National Aeronautics and Space Administration. An Evaluation of an Analytical Simulation of an Airplane with Tailplane Icing by Comparison to Flight Data. CreateSpace Independent Publishing Platform, 2018.

Principles of Aeronautics

Sharifi, David. Developing Superhydrophobic Coatings for Mitigating Aircraft Icing Using Plasma Spray Processes. Concordia University, 2018. See also: Aeronautical engineering; Aerodynamics and flight; Ailerons, flaps, and airplane wings; Airfoils; Airplane maintenance; Airplane safety issues; Aviation and energy consumption

Airfoils Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT An airfoil is a two-dimensional, front-to-back cross-section or slice through a wing from its leading edge to its trailing edge across the wing’s long axis. The shape of a wing’s airfoil section or sections determines the amount of lift, drag, and pitching movement the wing will produce over a range of angles of attack and determines the wing’s stall behavior. KEY CONCEPTS camber: the curvature of the cross-section of a wing lift: the upward force experienced by a wing as determined by its airfoil camber and thickness symmetrical: having equal dimensions on either side of a bisecting plane or axis THE WING IN CROSS-SECTION The shape revealed if a wing were to be sliced from its leading edge to its trailing edge is called the wing’s airfoil section. Although airfoils come in many different shapes, all are designed to accomplish the same goal: forcing the air to move faster over the top of the wing than it does over the bottom. The higher-speed air on the top of the airfoil produces a lower pressure than the flow over the bottom, resulting in lift. The shape of the upper and lower surfaces of the airfoil and the angle that it makes with the oncoming airflow, or angle of attack,

Airfoils

determines the way the flow will accelerate and decelerate around the airfoil and, thus, determines its ability to provide lift. Flow around the airfoil also causes drag, and an airfoil should be designed to get as much lift as possible while at the same time minimizing drag. The shape of the airfoil then determines the balance of lift and drag at various angles of attack. An airplane designer tries to select an airfoil shape that will give the best possible lift-to-drag ratio at some desired optimum flight condition, such as cruise or climb, depending on the type of aircraft. The amount of pitching movement, or tendency for the airfoil to rotate nose up or down, is also a function of the airfoil’s shape and the way lift is produced. Pitch must be evaluated along with the forces of lift and drag. CAMBER AND THICKNESS Early airfoil shapes were thin, essentially cloth stretched over a wood frame, a type of airfoil sometimes seen today in the wings of ultralight or hang glider-type aircraft. Usually, the frame for such an airfoil was curved, or cambered. The camber line, or mean line, of an airfoil is a curved line running halfway between its upper and lower surfaces. If the airfoil is symmetrical, in other words, if its upper surface is exactly the inverse of its lower surface, then the camber line is coincident with its chord line, a straight line from the leading edge to the trailing edge of the airfoil. A symmetrical airfoil is said to have zero camber. The amount of camber possessed by an airfoil is defined by the maximum distance between the chord and camber lines expressed as a percentage of the chord. In other words, an airfoil has 6 percent camber if the maximum distance between its chord and camber lines is 0.06 times its chord length. Experimenters in the late 1800s tried wings built with airfoils with different amounts of camber and different positions of maximum. They found that the location of maximum camber affected both the amount of lift generated at given angles of attack and

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the airfoil’s stall behavior and that too much camber can give high drag. Later researchers learned to create temporary increases in camber by using flaps. Later aircraft used thicker airfoils with both upper and lower surfaces covered first with fabric and then with metal. The thicker airfoils allowed a stronger wing structure as well as a place to store fuel. They also proved able to provide good aerodynamic behavior over a wider range of angle of attack as well as better stall characteristics, but excessive thickness made for increased drag. NACA AIRFOILS In the 1920s, the National Advisory Committee for Aeronautics (NACA) began an exhaustive study of airfoil aerodynamics, examining in detail the effects of variations in camber and thickness distributions on the behavior of wings. This systematic study of variations in the amount and position of maximum

Airfoil nomenclature. Image via Wikimedia Commons. [Public domain.]

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camber and thickness resulted in the wind-tunnel tests of hundreds of airfoil shapes. The NACA also developed a numbering system, or code, to describe the shapes. In the first series of tests, each of the numbers in a four-digit code was used in a prescribed set of equations to draw the airfoil shape. For example, the NACA 2412 airfoil had a maximum camber of 2 percent of its chord with the maximum camber point at 40 percent of the chord from the airfoil leading edge, and the maximum thickness was 12 percent of the chord. Many other series of NACA airfoils were developed and tested. The 6-series airfoils were designed to provide very low drag over a set range of angle of attack by encouraging a low-friction laminar flow over part of the surface. Other series of airfoils were developed for use on propeller blades. The NACA’s successor, the National Aeronautics and Space Administration (NASA), has continued to test and de-

Airplane Accident Investigation

Principles of Aeronautics

velop airfoils including a series of supercritical shapes that give lower drag near the speed of sound, as compared to older designs. MODERN AIRFOIL DESIGN Throughout the twentieth century, airfoil design was essentially a matter of creating a shape based on desired camber and thickness distributions, testing it in wind tunnels and then in flight. Today, airfoils can be selected from hundreds of past designs or custom-developed by specifying a desired distribution of pressure around the surface and using computers to solve for the shape that will give those pressures. Then wind-tunnel tests are done to validate the computer solution. The result is that every airplane can have a wing with a unique distribution of airfoil shapes along its span, all designed for optimum performance. The basic idea is the same as it has always been, to find the combination of camber and thickness that will give the best available mix of lift, drag, and pitching movement for the task at hand. —James F. Marchman III Further Reading Cummings, Russell M., Scott A. Morton, William H. Mason, and David R. McDaniel. Applied Computational Aerodynamics: A Modern Engineering Approach. Cambridge UP, 2015. Pope, Alan. Basic Wing and Airfoil Theory. Dover Publications, 2009. See also: Aerodynamics and flight; Ailerons, flaps, and airplane wings; Fluid dynamics; National Aeronautics and Space Administration (NASA); National Advisory Committee for Aeronautics (NACA); Tail designs; Ultralight aircraft

Airplane Accident Investigation Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Chemical engineering; Mathematics

ABSTRACT Airplane accident investigation is the examination into the causal factors of an aircraft mishap or incident. The investigation of aircraft accidents is important to the aviation industry for many reasons. The study of accident factors helps airlines determine accountability, educate inexperienced pilots, and prevent future accidents. KEY CONCEPTS accident rate: the number of accidents per 100,000 hours of flying time, applies to individual aircraft and aircraft types, pilots, and air carriers aerodynamic flutter: a vibration of wing or fuselage surfaces or of other structural components, caused by disruption of the proper flow of air over those surfaces, referred to as “buzz” by pilots error chain: the sequence of potential causative factors that have combined to result in an accident or crash pushback: the movement of an aircraft from its hangar prior to a flight PERCEPTIONS AND REALITIES Those of the flying public who are not airline pilots, and even some pilots who fly as passengers, are sometimes nervous while doing so. Although airline accidents occur infrequently, those that do occur are typically catastrophic events involving a great loss of life. Media coverage of airline accidents is usually extensive, fueling the uneasy feelings many people have about airline travel. However, the periods after aviation accidents are often the safest times in which to fly. Further, contemporary aviation accident rates are very low, in relation to other types of accidents, such as automobile accidents. Because aviation always involves the risk of an accident, accident investigation is an important element of aviation education. By studying the accidents of other pilots, less experienced aviators can avoid making similar mistakes.

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Airplane Accident Investigation

Principles of Aeronautics

Brazilian Air Force personnel recover the flight data recorder of Gol Transportes Aéreos Flight 1907, which crashed on September 29, 2006. Photo by Alessandro Silva/Força Aérea Brasileira, via Wikimedia Commons.

ACCIDENT PATTERNS A commonly noted pattern in aviation accidents is that there is rarely only a single reason for the accident. Experienced pilots refer to the events leading up to an accident as an “error chain.” Individual links of the chain, when combined, cause an accident to happen. For instance, bad weather alone might not cause an accident, but bad weather combined with darkness and the fact that the pilot became lost might. The error chain is weather, darkness, and becoming lost. The elimination of any one of these factors may prevent the mishap. In other words, if one link of the chain were broken, the accident may very well not happen. The key to accident investigation, then, is to determine the error chain leading up to the event.

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Rectifying the situation regarding an accident under investigation is impossible, because an aircraft accident is a one-time event that has already happened. However, accident investigators can study each link of the chain and prepare documents to report their findings. In turn, other flight crews can study the reports and hopefully avoid the same fate. Many aviation accidents result from human-factor errors. All those who have roles in launching an aircraft, including pilots, mechanics, air traffic controllers, cabin attendants, and baggage handlers, can make mistakes that may cause an airplane accident. ACCIDENT RATES In the study of aviation accident investigation, an important statistic is the number of accidents com-

Principles of Aeronautics

pared to the hours flown. The number of accidents per 100,000 hours determines a figure known as the accident rate. Apart from the mid-1990s, the accident rate has been declining since 1982. The number of accidents has declined, as has the number of fatalities. The Federal Aviation Administration (FAA) and others in the industry attribute better pilot education and technological advances for improvement of aviation safety statistics. CAUSAL FACTORS According to statistics compiled by the industry, approximately 70 percent of air carrier accidents are the results of flight crew error. Maintenance error constitutes another 5 percent, while air traffic control or other airport issues account for about 4 percent of the total. Human error is responsible for a total of almost 80 percent of the commercial airline accidents worldwide. Of the remaining 20 percent, mechanical failures make up 11 percent, weather factors account for about 4.5 percent, with the remainder categorized as miscellaneous or other. Pilots and first officers are responsible for most of the human error accidents. Reasons for the flight crew’s mistakes are many, including loss of situational awareness, flight crew fatigue, and training and operational issues. Accidents can occur during all phases of flight, from pushback to arrival at the destination gate. However, the majority of accidents, almost 56 percent, happen during the approach and landing phase of the flight. The reasons for approach and landing accidents vary. If a flight has been a particularly long one, crew fatigue can play a significantly greater role at the end of a flight than at the start. Being tired or fatigued can impair a crew’s decision-making process. Poor destination weather combined with a tired flight crew could be a recipe for disaster.

Airplane Accident Investigation

POSTACCIDENT SEQUENCE OF EVENTS During an accident investigation, there are many simultaneous issues requiring attention. The first and most important consideration is to assist the injured. Medical personnel are needed immediately to administer to those on-site, and provisions are needed to transport patients to the nearest medical facility as quickly as possible. The rescuers also need to determine where the injured were sitting in the aircraft and where they ended up after the accident. The next task is dealing with the survivors of anyone killed in the accident; if there is even one fatality, the loss touches many people. After the first officials arrive on the scene, their first order of business is to secure the area. Another important task is to observe evidence that is transient in nature. For instance, a popular twin-engine aircraft seemed to be crashing for no reason. It took four such crashes of a similar nature before investigators arrived at the wreckage quickly enough to determine that ice forming on the aircraft’s horizontal stabilizers had caused the accidents. After the previous accidents, the ice had melted before anyone could see or record its presence. After first seeing to the injured, personnel guarding the accident scene have several responsibilities. They must make certain the wreckage is not disturbed, because if someone moves the wreckage, aviation accident investigators will no longer be able to see the parts of the aircraft as they came to rest after the accident. Consequently, investigators will lose many clues that may help them determine a probable cause of the accident. Another essential task is to determine whether hazardous materials were being transported and are present at the scene. If so, personnel must take measures to protect everyone on scene from the dangers of the hazardous materials. It is important for the accident investigators to photograph the scene. Photographs of the wreckage can preserve the visual evidence of the accident for later analysis and should include all aspects of the

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accident: from individual parts of the aircraft to any fatalities where they lie. Marks caused by the craft’s hitting the ground and aerial photos are important to show the path of the aircraft as it crash-landed. In addition to photographs of the crash scene, the investigators should also draw sketches and maps of their observations. By using the photographs, sketches, and maps, they will be able to create a diagram of the last moments of the flight. This diagram of the flight’s last moments is essential to understanding the decisions made by the crew. Guarding the wreckage is an important responsibility for those tasked with the job. Guards should be somewhat familiar with aviation. They must protect the property, the wreckage, and the crash site from being disturbed. They have the difficult duty of making sure that people do not wander through the area. They also collect the names, addresses, and telephone numbers of anyone who may have witnessed the crash. If an accident occurs in a remote location and all aboard sustain fatal injuries, eyewitness statements may be nonexistent. Anyone with knowledge of the accident must be located because witnesses can be very important in helping to determine the cause of an aviation accident. The best witnesses to an aviation accident are not other pilots, or those involved in the industry. In fact, they are not even adults. Children often provide the most accurate and unbiased statements about aviation accidents. Adults often tend to put their own spin on an accident. Pilots who witness accidents may inject far too much opinion into their account of what happened. Children very simply report what they see. NATIONAL TRANSPORTATION SAFETY BOARD Once notification of a major aircraft mishap reaches the authorities, the National Transportation Safety Board (NTSB) launches a “go team.” The team originates from Washington, D.C., where the members

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of the NTSB rotate the duty of being on the team. While the go team is en route, it is the job of the local authorities on scene to organize the agencies to start the rescue or recovery procedures. The accident investigator is indeed a detective. Typically, it is very difficult to determine exactly what caused an aircraft accident. The investigator’s first order of business is to sort through the pieces of wreckage, cataloging the more and less obvious clues leading to the most probable cause of the mishap. In many cases, it is much like looking for the proverbial needle in a haystack. Once the team is on-site, it will survey the wreckage to determine where the aircraft initially struck the ground. Damaged shrubbery and trees may mark the path of the airplane into the crash scene. The investigators will note the general condition of the wreckage while trying to account for all the parts. If parts or components of the aircraft are missing from the accident scene, this may be indicative of an in-flight structural failure. If the empennage, or tail, or the wing of the aircraft came off the airplane at a high altitude, that could well be the cause of the accident. Those parts, once separated from the main aircraft, will fall to the ground sometimes miles away from the main site of the wreckage. The investigators begin with the physical aspects of the accident. They collect parts, examine each one, and map its final placement against the original installation on the aircraft. They also have to be certain that the aircraft is complete; parts of the aircraft missing from the wreckage site suggest an in-flight breakup in which other essential components of the airplane have landed elsewhere. From the physical evidence, the investigators can then determine the aircraft’s approximate angle and speed of impact. They can also determine whether the engines were working at the time of impact. They map and photograph the wreckage to preserve as many of the clues as possible. After this work is complete, they will begin their true detective work.

Principles of Aeronautics

This detective work begins with a review of the pilot’s qualifications and an examination of the pilot’s training and certification documents and medical records. The investigators conduct interviews with the pilot’s friends, peers, and relatives. They review autopsy results. They create a pathological history for a seventy-two-hour time period leading up to the accident and conduct weather data analysis, among other things. The investigators check into the pilot’s physical and psychological makeup and try to determine the pilot’s state of mind at the time of the accident. They question the pilot’s aeronautical decision-making abilities, look into recent history of the pilot’s judgment, and even evaluate the pilot’s training and experience. Investigators also try to determine whether the weather was a factor in the accident, relying upon official weather reports and forecasts. They look for indications of low visibility, turbulence, extreme wind shear, or heavy rains that may been contributors. RECONSTRUCTION Finally, investigators examine the aircraft wreckage to determine whether mechanical malfunctions may have caused the crash. This is one of the more intense segments of the investigation. The aircraft will undergo reconstruction in a secure hangar or other facility. Plans of the aircraft are helpful in determining that all parts of the aircraft have been retrieved from the wreckage site and adjacent areas. The aircraft’s reconstruction helps the examiners find signs of structural failure. The key to determining structural failure involves asking whether engine failure caused the accident or whether it caused the breakup of other parts of the airplane. Investigators try to figure out where such a breakup first occurred. This is the most intriguing part of accident investigation, and it may go well beyond the expertise of the investigators. On many occasions, expert witnesses, such as metallurgists, are necessary to assist in finding the answers.

Airplane Accident Investigation

Every aspect of the aircraft is under scrutiny during the accident examination. Disassembly of each component and system of the aircraft will follow for investigation of any possible failures. Examination of the flight controls may reveal a frayed cable or a broken bearing; a control pushrod may have become bent, allowing aerodynamic flutter to start. That aerodynamic flutter may have caused an actual structural failure of the elevator or rudder, hastening the accident. Investigators also check switch positions at the time of the crash. They are especially interested in the positions of the switches and controls and the relative positions of the associated components. These indicate whether the pilot may have done something improper to cause the accident, such as raising the flaps at the wrong time or unintentionally dumping fuel, resulting in fuel exhaustion. A component failure may be indicated if a switch was properly set and the component discovered is not positioned per the switch selection. Investigators are also intensely interested in the instrument readings at the time of impact. After removal from the crash site, each instrument is sent to an appropriate laboratory for intense postaccident analysis. At the time of impact, each needle on the face of an instrument leaves impact marks that allow the technicians to determine exactly what measurement the instrument was indicating at the time of the crash. With this technique, investigators can determine the speed of the aircraft. They can also make corroborations between engine operations and other instrument indications that may help in explaining the accident. The investigators have a high interest in the navigational instruments and radios, along with everything else in the cockpit. This is particularly true if the accident happened in poor weather. By careful analysis of the frequencies selected, the switch positions of the units, and the readings at impact, examiners can determine if a navigational error factored

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into the cause of the accident. All other systems in the airplane, such as the generators or alternators, the vacuum systems, pneumatics, and hydraulics, are also scrutinized during the investigation. FLIGHT AND COCKPIT RECORDERS Flight recorders and cockpit voice recorders, carried in the tail of an aircraft, are important elements in accident investigation. They provide critical clues in solving the mysteries associated with many of the world’s air disasters and are invaluable in helping to prevent future accidents. Although they are known as black boxes, they are actually painted bright orange to aid in their recovery following an accident. Aircraft flight recorders record many different operating conditions of a flight and provide information that may be difficult or impossible to obtain by any other means. Cockpit voice recorders record the flight crew’s voices, as well as other sounds within the cockpit, including communications with air traffic control, automated radio weather briefings, and conversation between the pilots and ground or cabin crew. Sounds of interest to an investigation board, including engine noise, stall warnings, landing gear extension and retraction, and any clicking or popping noises, are also typically recorded. Based on these sounds, important flight parameters, such as speed, system failures, and the timing of certain events can often be determined. In the event of an accident, an investigation committee creates a written transcript of the cockpit recorder tape. Local standard times associated with the accident sequence are determined for every event on the transcript. This transcript contains all the pertinent portions of the cockpit recording. Due to the highly sensitive nature of the verbal communications inside the cockpit, a high degree of security is provided for the cockpit recorder tape and its transcript. The timing of release and the content of the written transcript are strictly regulated.

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AUTOPSIES The examiners find the pilot’s autopsy results particularly important in determining whether pilot incapacitation caused the accident. The pilot may have experienced a heart attack, stroke, or some other medical factor that caused incapacitation. The pilot may have passed out due to hypoxia or carbon monoxide poisoning. The results of the autopsy enable the investigators to assign probable cause to the pilot or rule out incapacitation. The airplane also undergoes a mechanical autopsy of sorts. After an aviation accident, the maintenance records and logbooks of the accident aircraft are collected. The NTSB investigators examine the records in search of evidence of possible material defects or mechanical malfunction. The inspectors may determine that there were metallurgical or manufacturing defects in the history of the aircraft. They may uncover improper maintenance procedures, such as that a life-limited part has been allowed to exceed its time in service. These are only a few of the possible explanations for the accident. These observations and examinations comprise the bulk of the investigators’ work. The compilation of information on the accident and the examination of evidence may take months, or even years. Many people in the agency are involved in the search for answers about the cause of the accident. As the work is completed, many more people await the results, some patiently, others less so. The meticulous work of the accident investigators takes time, however. Although this challenging work is sometimes tedious and demanding, it must always be thorough. Its reward is the promise of the prevention of future accidents. —Joseph F. Clark III Further Reading Bibel, George, and Robert Hedges. Plane Crash: The Forensics of Aviation Disasters. Johns Hopkins UP, 2018.

Principles of Aeronautics

Dismukes, R. Key, Benjamin A. Berman, and Loukia Loukopoulos. The Limits of Expertise: Rethinking Pilot Error and the Causes of Airline Accidents. CRC Press, 2017. Hales-Dutton, Bruce. Mayday: Air Crash Investigation. Danann Publishing Ltd., 2019. Negroni, Christine. The Crash Detectives: Investigating the World’s Most Mysterious Air Disasters. Penguin Publishing Group, 2016. Pigott, Peter. Brace for Impact: Air Crashes and Aviation Safety. Dundurn, 2016. Wiegmann, Douglas A., and Scott A. Shappell. A Human Error Approach to Aviation Accident Analysis: The Human Factors Analysis and Classification System. CRC Press, 2017. See also: Air flight communication; Aeronautical engineering; Air transportation industry; Aircraft icing; Airplane guidance systems; Airplane maintenance; Airplane safety issues; Autopilot; Avionics; Federal Aviation Administration (FAA); Flight instrumentation; Flight landing procedures; Flight recorder; Flight testing; Gravity and flight; Landing gear; National Transportation Safety Board (NTSB); Stabilizers; Takeoff procedures; Taxiing procedures; Training and education of pilots; Wake turbulence; Weather conditions; Wind shear

Airplane Cockpit Fields of Study: Physics; Pilot training; Mathematics ABSTRACT The cockpit is the area within an aircraft from which the pilot operates the aircraft’s controls. Cockpits provide a central point from which airplane performance can be commanded and monitored. KEY CONCEPTS control yoke: the modern “joystick” with which a pilot controls the operation of an aircraft ergonomics: a design principle that seeks to harmonize mechanical function with the shape and mechanics of the human body in order to minimize physical discomfort and stress

Airplane Cockpit

restraints: seat belts and shoulder harnesses employed to secure the pilot and other aircraft personnel in their respective seats when needed sidestick: a pilot’s control “joystick” mounted to the side rather than directly in front of the pilot EARLY COCKPITS The term “cockpit” originated with the ancient practice of cockfighting. Early pilots had to control unstable airplanes through control levers positioned without regard to one control’s effect on another. Pilots stayed busy; their motions reminiscent of the frenzy in the gaming floor’s cockpit. Although early airplanes accommodated pilots, they had no cockpits by modern definition. The Wright brothers’ Flyer pilot lay prone, having controls in reach but little else. No flight instruments existed until about 1911. In his underpowered, box-kite-like 14-bis, Alberto Santos-Dumont stood erect while becoming Europe’s first airplane pilot. By their first decade, airplanes had evolved cockpits as effective yet inefficient workstations. By World War I, fighter cockpits gave their seated pilots a control stick, a rudder bar, and precious few instruments. Open cockpits were a hallmark of pre-1920 airplanes; rarely were cockpits enclosed. As enclosures became prominent in the 1920s, some pilots disliked them, wanting the wind on their faces to indicate slips or skids. By the 1930s, most airplanes featured enclosed cockpits, although efficient pilot motion stayed a low priority. The layout of cockpits only slowly became logical, with their instruments and installations sometimes cumbersome. Lockheed’s prewar Model 14 Hudson is an example of cockpit inefficiency; its Royal Air Force (RAF) version was a handful for its single pilot. In his 1972 memoir, H. A. Taylor recounted the difficulties of solo flight in the Hudson, beginning with starting the engine. It was a procedure that “was preferably done with three hands, each with more than the usual number of fingers and thumbs,” and involved simul-

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taneously pressing buttons for both the starter motor and booster coil while holding a spring-loaded, three-position switch that selected the engine to be started. Meanwhile, an engine-doping pump and a wobble-pump had to be worked, and as soon as the engine fired, the idle cut-off lever had to be released and the throttle manipulated while the booster button was continually pressed. The layout of these vital mechanisms added to the challenge: “The buttons, switches and doper were on a fore-and-aft electrical panel to the pilot’s right; the wobble-pump handle was at the rear of the throttle pedestal; and the cut-off levers sprouted, among a dozen or more others, from the top of this pedestal.” Not all 1930s manufacturers spurned pilot efficiency. By the early 1930s, Germany’s Junkers Aircraft built its Ju-52/3m, called “Tante Ju” (“Auntie Junkers”) by her adoring crews. Its innovations included dual instruments, a series of mechanical devices to reduce distraction-induced pilot errors, and effective weatherproofing. Logic arranged its flight instruments, and the pilot and copilot could both reach the brake lever. By the climax of World War II, cockpit efficiency had become a manufacturing priority. MODERN COCKPITS Airplane cockpits range from the single-place, where the pilot is the sole occupant and performs all duties, to the multiplace, in which several crew members share duties such as flying, communicating, navigating, and systems monitoring. Cockpit designs demand unique considerations. Accessibility means that the pilot’s station must be easily reached upon entry and easily departed at flight’s end. Restraints must counter turbulence, yet allow quick crew egress in emergencies. Once seated, pilots must be able to reach all the flight and systems controls. The control sticks so favored by early designers provide an unencumbered view of the instrument panel, and fall to hand naturally. Control yokes, or wheels, create an automotive feel

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that comforts new aviators, but blocks pilot vision of parts of the instrument panel. Both amateur airplane builders and conglomerates, such as Europe’s Airbus Industrie, have found value and pilot acceptance of side-sticks, joysticks mounted on the cockpit bulkhead, or side wall, where they can comfortably be reached by the pilot’s hand. These controls can be reliably gripped, even in tense moments or in turbulence, when jolts and jostling fling a pilot’s reaching hand from levers or dials. From the 1920s through the 1950s, training airplanes tended to have tandem cockpits, in which the student and instructor sat on the airplane’s centerline, one behind the other. Advantages included the

Photo via iStock/Maravic. [Used under license.]

Airplane Guidance Systems

Principles of Aeronautics

students’ ability to perform maneuvers in either direction with equal challenge, for their field of vision either way remained identical. Additionally, students tended to develop cockpit skills more quickly because their instructors remained essentially hidden. Disadvantages included the need for duplicate instrumentation and the instructors’ inability to see nuances in student facial expressions. In the 1950s, as most trainer cockpits adopted side-by-side seating, designers strove for cockpit efficiency. Sometimes that goal is still unmet. Ten accidents occurring between 1972 and 1982 prompted development of what is known as cockpit resource management. Accidents underscored the need for physical changes in cockpits. Studies revealed surprising clues to the dangers induced by poor design. By the twentieth century’s close, newly produced airplanes had begun to incorporate cockpit ergonomics. Ergonomics considers the design of the human body, including its ranges of skeletal and muscular motion. Normal operation is the first consideration, but airplanes encounter strong turbulence, operate in daylight and darkness, and can climb in minutes from searing heat at the airport to subzero temperatures at altitude. Designers must consider these factors and more, plus incorporate characteristics to maximize crash survivability. Like the rest of the airplane, the cockpit is a compromise, for which designers cannot rely on tradition. Today’s cockpit designers use recent and exhaustive studies to meet their goals. Despite its claustrophobic faults, the cockpit holds strong allure for millions. Depicting airplanes, artists usually focus on cockpits, for therein sits an airplane’s humanity, and what many see as its ultimate office. —David R. Wilkerson Further Reading Casner, Stephen M. Cockpit Automation for General Aviators and Future Airline Pilots. Aviation Supplies and Academics Inc., 2006.

———. The Pilot’s Guide to the Modern Airline Cockpit. Aviation Supplies and Academics Inc., 2007. Elias, Bart. Cockpit Automation, Flight Systems Complexity, and Aircraft Certification: Background and Issues for Congress. Independently Published, 2019. National Aeronautics and Space Administration (NASA). Learning About Cockpit Automation: From Piston Trainer to Jet Transport. CreateSpace Independent Publishing Platform, 2018. Smith, Patrick. Cockpit Confidential: Everything You Need to Know About Air Travel: Questions, Answers, and Reflections. Sourcebooks, 2018. Taylor, H. A. “Flying the Harassing Hudson.” Air Enthusiast Magazine, Dec. 1972, p. 292. See also: Air flight communication; Airplane guidance systems; Airplane radar; Airplane safety issues; Autopilot; Avionics; Federal Aviation Administration (FAA); Flight instrumentation; Flight landing procedures; Flight recorder; Flight roll and pitch; Flight schools; Flight simulators; Flight testing; Taxiing procedures; Training and education of pilots; Weather conditions; Wind shear

Airplane Guidance Systems Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Electrical engineering; Mathematics ABSTRACT Guidance systems are systems that aid in navigation, that is, in finding and keeping to a route and schedule. Guidance systems enable an aircraft to fly its route safely, even when visibility conditions are less than favorable. KEY CONCEPTS accelerometer: a device that measures the change of stress on an internal component to determine the rate of acceleration or deceleration, and changes in the direction of motion Doppler radar: a type of radar that is able to differentiate the motion towards and the motion away of

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any object being scanned, often used to identify the rotation of storm systems gyrocompass: a device that uses a combination of gyroscopes to maintain a stable indication of “north” inertial navigation system: a system that uses three gyroscopes spinning on mutually orthogonal axes PURPOSE AND FUNCTION The purpose of guidance systems is to aid in navigation. It is a simple point but one that can easily be lost in the overall complexity of some new guidance systems. Navigation has simple, specific objectives. The navigator should select a route and a schedule. There should be a continuous succession of points against which the navigator can check the progress of the voyage. Next, the planned movement is executed; that is, the craft is kept to the route or course set. Guidance systems enable these simple but important tasks to be accomplished accurately. EXTERNAL OBSERVATION GUIDANCE SYSTEMS Guidance systems comprise many parts, including instrument landing systems (ILS), air traffic control (ATC) systems, radar and database systems, and voice communication controls. Satellite landing systems are increasingly important in providing landing guidance. From the earliest days of air flight to the present, there have been consistent improvements in guidance systems. The constant monitoring and correction of position is termed a closed loop. Finding the aircraft’s position is achieved by measuring distance or direction or both. Additionally, guidance systems need to measure altitude. The transmission of sound and light waves, as well as other electromagnetic waves, is used in this process. There are guidance systems to aid in speed measurement, altitude, and every other possible variable for flight. High-speed computers aid in the process,

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warning pilots and navigators when an aspect of the flight requires attention. The Kalman filtering system weights each datum according to its expected quality. It aids in the process of dead reckoning, speed, and direction, as well as continuously updating the craft’s position. It also determines the speed of the plane, its heading, rate of climb or descent, and how each of these must be maintained or adjusted to stick to the flight plan. Air traffic controllers keep a dead reckoning check on each aircraft, using strips that show the height, speed, and timing of each plane. The strips break down the flight plan of each aircraft. Radio navigation uses signals in a true beam system. Narrow beams about 3 feet long are used for landing, even in near-zero visibility. Improved microwave systems allow for even narrower beams and aid instrument landing systems. Laser guidance systems provide pilots with a visual navigation flight path from as far as 35 kilometers from the runway, with the precision of an advanced instrument landing system. Best of all, the installation of laser guidance and cold cathode technologies to replace or enhance conventional landing light systems requires no additional aircraft equipment, and is cheaper to maintain than conventional lighting. For example, the lifetime cost of cold cathode lights is only 20 percent of that of incandescent lights. High-intensity light-emitting diode (LED) lighting bears even lower cost of operation. The combination of enhanced vision technologies with the latest ground proximity warning systems dramatically reduces the number of controlled-flight-into-terrain accidents. INERTIAL GUIDANCE SYSTEMS Inertial guidance is a method of navigation used to guide rockets and airplanes, submarines, and other vehicles. Unlike other methods of navigation, inertial guidance does not rely on observations of land or the stars, on radio or radar signals, or on any

Principles of Aeronautics

Airplane Guidance Systems

Doppler radar. Photo via iStock/Anthro. [Used under license.]

other information from outside the vehicle. Instead, a device called the inertial navigator provides the guidance information. An inertial navigator consists of gyroscopes, which indicate direction, and accelerometers, which measure changes in speed and direction. The principles of inertial guidance have been known since the early 1900s. Gyroscopes have been used as compasses on ships since that time. They can be set so that they point constantly in one direction, such as toward the North Star. Unlike magnetic compasses, these gyrocompasses always indicate true north and are not affected by steel. In 1923, the German engineer Max Schuler described a method for establishing a vertical line that would not tilt when a vehicle changes speed or direction. If the

line tilts, it cannot be used to measure distance. Schuler’s theory is used to build electronic systems that prevent tilting of the vertical line. During World War II, German scientists built an inertial guidance system that guided their V-2 rockets against England. In the late 1940s and early 1950s, Charles S. Draper and other scientists at the Massachusetts Institute of Technology built the first highly accurate inertial guidance systems. Space shuttles and other spacecraft are also equipped with inertial navigators. Inertial guidance systems are required on US commercial overseas flights. The advantages of inertial guidance can be explained by the example of an airplane flight. To reach its destination, an airplane must both fly in the correct direction and cover the correct amount

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of distance. Without inertial guidance, a pilot has to rely on compasses or on signals from radio beacons at known positions on the ground to be sure the airplane is flying in the right direction. With inertial guidance, pilots need only consult the navigation equipment inside the airplane. They can find their way despite poor visibility, faulty communications, and the absence of landmarks. In time of war, enemies cannot jam an inertial guidance navigation system with false or confusing information. The inertial navigator automatically measures changes in a vehicle’s speed and direction, and sends the information to the computer. The computer calculates the effect of all the changes and keeps track of how far and in what direction the vehicle has moved from its starting point. Three gyroscopes inside the inertial navigator spin in different directions on axles. The axles are placed so that they form 90-degree angles with each other, like three edges of a box meeting at a corner. The axles keep their directions if the gyroscopes continue to spin. Each gyroscope is supported by gimbals (movable frames) so that it stays in position as the vehicle rolls, pitches, or turns. Together, the gyroscopes establish an inertial reference system (a stable set of lines). The accelerometers detect changes in the vehicle’s motion in reference to the stable lines defined by the gyroscopes. The inertial navigator measures how far a vehicle has traveled by recording the changes in the position of a vertical line. This line indicates the direction to the center of Earth. Vertical lines from any two points on Earth meet at the center of Earth. The angle between the lines indicates the distance between the points. Each minute (one-sixtieth of a degree) of angle indicates a surface distance of one nautical mile (6,076.1 feet, or 1,852 meters). New York City is 3,006 nautical miles from London. Therefore, a pilot flying from New York City to London knows the airplane has gone far enough when the vertical line of the inertial navigator has

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moved through an angle of 3,006 minutes (50 degrees, 6 minutes). Inertial guidance systems are subject to errors that grow over time. In some systems, a computer periodically combines the system’s outputs with an independent source of position, such as a radio beacon. This procedure helps minimize the size of navigation errors. GYROSCOPES Gyroscopes are essential in the working of inertial guidance systems. The gyroscope functions as a compass when the gyroscope is mounted at the equator of Earth. The spinning axis lies in the east-west plane; the gyroscope continues to point along the east-west line as Earth rotates. Laser gyros provide guidance in the most advanced aircraft systems. These gyros are not inertial devices. Instead, they measure changes in counter-rotating beams of laser light, caused by changes in the aircraft’s direction. The electrically suspended gyro, another advanced system, uses a hollow beryllium sphere suspended in a magnetic cradle. There are also fiber-optic systems in the works to aid in navigation. The gyroscope also aids in the automatic pilot program of a plane through detecting and correcting variations in its selected flight plan, and it supplies corrective signals to the ailerons, elevator, and rudder. There are, in fact, several gyroscopes to detect changes in altitude, barometric pressure, and other factors. These gyroscopes transmit electrical signals to a computer, which combines and amplifies them, and then transmits these corrective signals to servomotors attached to the control surfaces of the aircraft. The pilot is thus able to use an autopilot to make corrections and to combine navigation and radio aids, such as inertial navigation systems, Doppler radar navigation systems, and radio navigation beacons. The autopilot can also couple beams of instrumental landing systems used in airport runways. —Frank A. Salamone

Principles of Aeronautics

Further Reading Binns, Chris. Aircraft Systems: Instruments, Communications Navigation, and Control. Wiley, 2018. Conforti, Facundo. Aircraft Systems. Biblioteca Aeronáutica, 2022. Kabamba, Pierre T., and Anauk R. Girard. Fundamentals of Aerospace Navigation and Guidance. Cambridge UP, 2014. National Aeronautics and Space Administration (NASA). Design and Testing of a Low Noise Flight Guidance Concept. CreateSpace Independent Publishing Platform, 2018. Wyatt, David. Aircraft Flight Instruments and Guidance Systems: Principles, Operations and Maintenance. CRC Press, 2014. See also: Avionics; Flight altitude; Flight instrumentation

Airplane Maintenance Fields of Study: Aeronautical engineering; Mechanical engineering; electrical engineering; Materials science; Advanced composite materials in aeronautical engineering; Advanced composite materials repair ABSTRACT Aircraft maintenance consists of regularly scheduled inspections and periodic adjustment and repair of aircraft. Ongoing maintenance operations enhance the safety of aircraft and the well-being of the flying public. They also ensure that aircraft operators comply with insurance companies’ mandates to reduce risk and liability. KEY CONCEPTS airframe: any aircraft of a specific design downtime: the time in which an aircraft is not able to fly due to mandated inspections and necessary repair or maintenance INSPECTIONS AND REPAIRS Every aircraft registered in the United States must be maintained in accordance with Federal Aviation Regulations (FARs) to ensure continued airworthi-

Airplane Maintenance

ness. This is accomplished through regularly scheduled inspections and periodic maintenance. The largest segment of American aviation is commonly called general aviation and includes thousands of privately owned aircraft as well as those operated for business purposes. General aviation aircraft range from two-seater trainers to fully equipped corporate jets and include experimental, or homebuilt, aircraft. All small aircraft of up to 12,500 pounds gross weight must undergo an annual inspection of the entire airframe structure, the power plant, and propeller, if so equipped, and all accessories and systems. FAR 43 lists the required scope and detail of the annual inspection and includes an approval statement to be written in the aircraft maintenance record, or logbook, which allows that aircraft to be operated for another year. Any necessary repairs must be completed before the inspector signs the approval. In addition to the annual inspection, there is a required inspection for every one hundred hours of operation, if the aircraft is being used for hire. Rental aircraft, flight training, and all passenger-carrying revenue flights fall into this category. The one-hundred-hour inspection is performed to the same scope and detail as the annual inspection. In Appendix A of FAR 43, maintenance is divided into three categories: major, minor, and preventive. Major repairs and alterations pertain to the integrity of the aircraft type design, or, the original configuration chosen by the manufacturer and approved by the Federal Aviation Administration (FAA). An authorized inspector must approve such repairs and alterations before returning the aircraft to service. Minor repairs and alterations may be performed and approved by a certificated airframe or power plant mechanic. A licensed pilot may perform preventive maintenance. All these maintenance actions must be entered in the appropriate aircraft and engine logbooks.

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Large aircraft of more than 12,500 pounds gross weight are usually maintained under a program designed by the aircraft manufacturer and custom-tailored to suit the owner or operator’s needs. Each program is unique and must be reviewed and approved by the FAA. The amount of time an aircraft is not available for use due to inspection and maintenance is known as downtime. To minimize downtime and maximize utility, an operator may place the aircraft in a progressive inspection program. Under the progressive inspection program, the entire aircraft is inspected during a year, but the inspection itself is broken into several smaller segments at specified intervals. A typical progressive inspection program calls for inspection of the wings after one hundred hours of op-

Photo via iStock/ExtremePhotographer. [Used under license.]

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eration, the engine or engines at two hundred hours, the fuselage and tail section at three hundred hours, and the landing gear at four hundred hours, The cycle begins again with the wings at five hundred hours. Each progressive inspection program is designed for a specific operator using a specific aircraft and must be reviewed and approved by the FAA. MAINTENANCE FACILITIES General aviation maintenance is usually performed by a fixed-base operator (FBO) at the local airport. The FBO may be a large, full-service complex offering fuel sales, aircraft rentals, flight instruction, engine overhauls, aircraft refurbishing, and charter flights. Some FBOs are one-person maintenance

Principles of Aeronautics

shops on private airstrips. An FBO may hold a Repair Station Certificate, issued by the FAA, that describes the types of specialized maintenance the FBO is equipped and qualified to perform. Repair stations are regulated under FARs 43 and 145 and may be certified to perform inspections, repairs, and maintenance on instruments, propellers, navigation and communication equipment, and accessory components, as well as on complete aircraft. Many corporations have full-time flight and maintenance personnel to ensure aircraft availability and readiness. This advantage enhances the convenience of executive travel and provides additional support for corporate growth and development. AIRLINE MAINTENANCE Major air carriers are regulated under FAR 121 and usually operate large, transport-category jet aircraft. Their complex and detailed maintenance programs are designed by the aircraft manufacturer. They are tailored to each airline’s operational needs and must be separately approved by the FAA. A typical airline maintenance program may include daily preflight inspections, weekly service checks, and periodic inspections, known as phase checks. Phase checks are usually lettered alphabetically, with each inspection being more detailed and occurring at longer intervals. An A-check may be scheduled every ninety days to check tires, fluid quantities, systems operation, and general aircraft condition. A D-check, however, is a comprehensive inspection and overhaul of the complete airframe, engines, and accessories, along with electronics upgrades, corrosion control and subsequent repainting. This type of inspection and repair usually occurs about every five to six years and may take several months to complete. Commuter airlines operate smaller aircraft to serve the outlying areas away from the major hubs of large cities and large airline activity. Commuter airlines are regulated under FAR 135 to operate and

Airplane Maintenance

maintain less complicated aircraft without compromising safety. A commuter airline maintenance program is generally a progressive inspection and repair schedule designed to interface with that airline’s flight profile. It is usually based on the aircraft manufacturer’s maintenance manuals and must be approved by the FAA. MILITARY MAINTENANCE Military aviation is mission specific. Each type of military aircraft is designed for a particular task. Bombers, fighters, tankers, and trainers play exclusive roles in the overall military aviation effort. Such type division is reflected in military maintenance. Technicians are trained on a specific type of aircraft, on which they may continue to work for several years. In addition, each aircraft is subdivided by system, such as engines, hydraulics, electrical, and fuel. A different team of trained specialists maintains each system. A crew chief, trained and experienced in several systems, is assigned to each aircraft, and serves as the maintenance coordinator for the specialist teams. MAINTENANCE TRAINING The privileges and limitations of aircraft mechanic ratings are listed in FAR 65. Each mechanic must perform maintenance operations in compliance with the regulation. MAINTENANCE RATINGS Airframe mechanics may inspect, repair, and maintain airframe structures, systems, and components according to the applicable manufacturers’ maintenance manuals. They may not repair instruments, navigation equipment, or communication equipment, and they may not approve major repairs and alterations as defined in FAR 43. They may, however, perform one-hundred-hour inspections on airframes and approve them for return to service after repairs.

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Power plant mechanics may inspect, repair, and maintain engines, accessories, and propellers according to applicable manufacturers’ maintenance manuals. They may not approve major repairs and alterations, and they may not perform major repairs on propellers. They may, however, perform 100-hour inspections on power plants and approve them for return to service after repairs. Most aircraft mechanics working in the general aviation maintenance industry hold both the airframe and power plant (A&P) ratings. Both ratings are granted indefinitely and are valid until surrendered, suspended, or revoked. Although not required by regulation in the aircraft manufacturing and major airline industries, the A&P ratings usually bring higher salaries and better job positions. An employee of a FAR-145-certified repair station may qualify for a Repairman Certificate. This certificate is issued after the employee has been sufficiently trained and experienced in the maintenance tasks performed. Unlike Airframe and Power Plant Certificates, the Repairman Certificate is valid for the specified tasks only while the holder is in the employ of that repair station. A Repairman Certificate may also be issued to the primary builder of an experimental, or homebuilt, aircraft. The repairman may then perform condition inspections on that aircraft. A condition inspection is like the annual inspection required on standard-category aircraft. Aircraft mechanics holding both airframe and power plant ratings may also hold an inspection authorization. They may perform annual inspections of the entire aircraft and approve or disapprove it for return to service. They may also inspect any major repairs or alterations to the aircraft and approve it for return to service. Unlike Airframe and Power Plant Certificates, the Inspection Authorization must be renewed every year.

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LICENSE REQUIREMENTS The requirements for the Airframe, Power Plant, and Repairman Certificates and for the Inspection Authorization are given in FAR 65. The regulation includes training and experience requirements, as well as written tests for subject knowledge and practical tests for demonstration of acquired skills. There are two ways to obtain an A&P license: one must either graduate from an approved school or qualify through documented relevant experience. There are approximately 150 certificated aviation maintenance technician schools in the United States. Under FAR 147, these schools must provide a minimum of 1,900 hours of classroom instruction and shop experience. The FAA inspects these schools periodically to ensure regulatory compliance and to assist graduates in the certification process. Aircraft maintenance personnel wishing to obtain A&P licenses may also present documented evidence of their work experience for FAA review and evaluation. The minimum requirements are eighteen months of full-time appropriate maintenance experience for either the airframe or the power plant rating or thirty months for both ratings together. Applicants may obtain their experience while serving in the military in selected job classifications or while being employed by a certified repair station, airline maintenance base, or aircraft modification facility. After reviewing and verifying the applicant’s documents and experience, the FAA issues permission for the applicant to take the written examination for the rating sought. TESTING PROCEDURES The written examination for the A&P license comprises three parts. The general test covers information that could apply to either airframe or power plant maintenance, such as regulations, publications, proper use of tools and equipment, aircraft hardware, and other related subjects. The airframe and power plant tests are subject-specific, as their

Airplane Manufacturers

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names imply. The general test must be taken in conjunction with either of the other tests but is not repeated for the second rating. After successful completion of the written examinations, the applicant schedules an appointment with the local designated mechanic examiner (DME). The DME is an experienced mechanic who has been appointed by the FAA to administer oral and practical examinations. The oral examination consists of a dialog between the examiner and the applicant to ascertain the applicant’s knowledge of aircraft maintenance theory and application. The practical examination is a series of maintenance tasks assigned to the applicant. The DME observes the applicant to evaluate the applicant’s use of technical data, mechanical skill, and proper procedures in performing the assigned tasks. Upon successful completion of the practical examination, the examiner issues a temporary mechanic certificate, that is immediately valid. The permanent certificate is mailed from the FAA registry within a few weeks. The Airframe and Power Plant Certificates are issued for life and continue to be valid unless voluntarily surrendered by the mechanic or suspended or revoked by the FAA. The certificates, however, must be kept current by recent experience. FAR 65 requires that, in order to exercise the privileges of a mechanic certificate, the mechanic must have been actively engaged in aircraft maintenance for six of the preceding twenty-four months. —David E. Fogleman Further Reading Kinnison, Harry A., and Tariq Siddiqui. Aviation Maintenance Management. 2nd ed., McGraw-Hill Education, 2012. Loong, Michael. The Essentials of Airplane Maintenance. Partridge Publishing Singapore, 2015. Patankas, Manaj S., and James C. Taylor. Applied Human Factors in Aviation Maintenance. CRC Press, 2017. Ren, He, Xi Chen, and Yong Chen. Reliability Based Aircraft Maintenance Optimization and Applications. Elsevier Science, 2017.

Yiannakides, Demetris, and Charalampos Sergiou. Human Factors in Aircraft Maintenance. CRC Press, 2019. See also: Advanced composite materials in aeronautical engineering; Advanced composite materials repair; Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Airplane propellers; Avionics; Flight propulsion; Materials science; Propulsion technologies

Airplane Manufacturers Fields of Study: Aeronautical engineering; Mechanical engineering; Electrical engineering; Mathematics; Materials science ABSTRACT Airplane manufacturers are the companies that produce vehicles for travel in air or space, and the components of those vehicles. By the end of the twentieth century, aerospace manufacturers had grown to become one of the most important employers in the industrialized world. KEY CONCEPTS jig: a kind of tool designed for the placement of components that are to be joined together in a precise orientation tooling: the devices used in the formation of individual components and structures BEGINNINGS The Wright brothers made the first powered flight in 1903, but the early years of aviation did not prove very lucrative for aircraft manufacturers. Most designers were sons of wealthy families with the time and money to pursue their interest in flying. Experienced engine designers migrated from the automotive industry, but even relatively successful manufacturers such as the Wright Company and the Curtiss Aeroplane Company had difficulty finding a consistent market for their airplanes. Most companies

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An Airbus A321 on final assembly line 3 in the Airbus plant at Hamburg Finkenwerder Airport. Photo by DearEdward, via Wikimedia Commons.

hoped for military contracts, but armed forces around the world were reluctant to adopt an unfamiliar weapon, and military purchases remained minuscule. WORLD WAR I The demands of modern war soon demonstrated the usefulness of aviation. Aircraft had been used in reconnaissance roles in minor conflicts before the war began, and both sides soon recognized the potential of aviation. At first, aircraft served as spotters for the artillery, but fighters soon developed, followed later in the war by bombers. In Germany, Dutch designer

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Anthony Fokker created a device that allowed a machine gun to fire through a spinning propeller. Fokker’s E-I fighter entered service in 1915 and soon dominated the skies over the western front. Subsequent Fokker models demonstrated continuous improvement, culminating in the highly advanced D-VII, which so frightened the Allies that they demanded the surrender of all D-VII’s as a condition of the armistice. Other German manufacturers followed Fokker’s lead. Albatros and Rumpler produced excellent aircraft, particularly in the reconnaissance sector. Late in the war, Gotha developed a heavy bomber that Germany used to bomb

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targets in Great Britain. Despite Germany’s defeat, the nation’s aviation industry produced more than 44,000 aircraft during the war and utilized twentysix engine and thirty-five airframe manufacturers. To meet the German threat, Britain’s manufacturers expanded their operations. Sopwith proved one of the most effective, and the company’s Pup and Camel models pushed the existing limitations of performance. Other British companies, including Handley-Page and Vickers, competed strongly with Sopwith’s aircraft. Perhaps the most important manufacturing outgrowth of World War I on Britain’s industry was the development of two important engine manufacturers, Napiers and Rolls-Royce. Rolls-Royce would use its wartime experience to become one of the world’s foremost engine designers over the next eighty years. INTERWAR DEVELOPMENTS After World War I, manufacturers turned increasingly to the civilian market. The notoriety of aircraft during the war had done a great deal to increase public interest in aviation. In the United States, the federal government established airmail service and supported research into increased performance. Charles A. Lindbergh’s solo transatlantic flight in 1927 also caused a sensation. Responding to these developments, US manufacturers found an increasing market for their products. Lockheed developed its Vega monoplane, which set several speed and altitude records. Other manufacturers, notably Douglas and Boeing, recognized the developing airliner market. By the end of the 1930s, both companies had developed planes that incorporated such modern features as a comfortable cabin, retractable landing gear, and all-metal construction. US manufacturers even designed airliners for transatlantic service, but World War II interrupted plans to produce these aircraft. Despite the newfound commercial market, many aircraft makers still looked to the military as their

Airplane Manufacturers

primary customer. Despite Douglas’s success in the airliner sector, more than half of the company’s sales went to the military. Military designs took on a greater importance as war seemed more likely in the 1930s. Many theorists, anxious to avoid the stalemate of World War I, saw airpower as the deciding factor in future conflicts. Germany’s rearmament program took place in direct contradiction of the Treaty of Versailles, which expressly forbade Germany to have an air force. Despite some resistance from leaders in the industry, most notably Hugo Junkers, Germany’s new military expansion began in earnest in 1933. Leaders such as Junkers who opposed the idea were swept aside and their companies became integral parts in the development of a new air force. Junkers, Dornier, Messerschmitt, Focke-Wulf, and Heinkel emerged as the leading aircraft manufacturers in Hitler’s Third Reich. Throughout the 1930s, these companies perfected designs such as Messerschmitt’s Bf-109 fighter, Junkers’s Ju-87 dive-bomber, and Heinkel’s He-111 bomber. These planes formed the backbone of Germany’s Luftwaffe at the outset of World War II and represented a serious challenge to the Western Allies. Germany’s aviation industry did not simply rearm the nation, however. Manufacturers provided employment for Germany’s depression-ravaged population and helped reinvigorate the economy. In 1934, Britain determined that it had to maintain parity in aviation with Germany and began its own rearmament program. The new surge in defense spending more than tripled employment in the aviation industry between 1930 and 1936. British companies lagged behind their German and American counterparts in terms of modern production facilities, and the sudden demands created by the decision to rearm revealed serious shortages in machine tools and trained personnel. Some companies, including Rolls-Royce, undertook training programs, but manufacturers often resorted to luring trained workers away from competitors. British companies also had difficulty ad-

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justing their designs and manufacturing techniques to the requirements of mass production, something designers in the United States and Germany had already embraced. Nonetheless, British firms turned out such outstanding designs as the Hawker Hurricane and Supermarine Spitfire fighters and the Avro Lancaster heavy bomber. WORLD WAR II World War II meant enormous changes in the technology of aviation. Wartime demands made a great

Principles of Aeronautics

deal of money available to manufacturers. Companies used these funds to design advanced aircraft and to convert their manufacturing processes to mass-production techniques. Most of the companies that came to dominate world aviation in the latter half of the twentieth century gained notoriety during World War II. In the United States, giants such as Boeing, Douglas, and Lockheed continued work on large bombers and transports, which would give these companies a significant advantage in the postwar

Bell Aircraft Corporation’s main factory in Wheatfield, NY (Buffalo / Niagara Falls) during the 1940s. This unit primarily produced the Bell P-39 Airacobra and P-63 Kingcobra. Photo via Wikimedia Commons. [Public domain.]

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airliner market. Other companies, such as Grumman and North American, concentrated primarily on fighters and established themselves as leaders in the sector. Across the United States, thousands of people moved to take jobs in the aircraft industry, located primarily in Southern California, creating a significant demographic shift. The trend toward mass production in the prewar era became an absolute requirement with the demands of war. US companies streamlined designs and production techniques to allow fast manufacturing with unskilled labor. Britain’s wartime experiments with such ideas as jet propulsion and radar made that country’s manufacturers leaders in those important fields. The success of the Hurricane and the Spitfire in combat proved that Britain could produce aircraft of the highest quality. Unfortunately, the British aviation industry still had difficulty matching its competitors in the area of production. The creative designs of De Havilland helped make that company a fixture in Britain’s aviation industry, but its famed wooden Mosquito fighter-bomber required too much time and skill to produce on the scale demanded by total war. Even more conventional aircraft such as the Spitfire required three times as many man-hours to produce as the German Bf-109. Britain’s aircraft industry emerged from World War II with creative designs and world leaders in engine technology at Rolls-Royce and Bristol Siddley Engines, but the United States’ greater emphasis on mass production would relegate Britain’s manufacturers to a peripheral role in coming years. POSTWAR COMMERCIAL MANUFACTURERS Britain emerged from World War II with a great advantage because of the country’s research into jets. US companies continued to build planes such as Lockheed’s luxurious, piston-powered Constellation, but these models did not represent the future of air travel. The De Havilland Comet became the world’s

Airplane Manufacturers

first jet-powered airliner when it began service to the Middle East in 1952. The Comet’s smooth, fast performance made it a favorite of air travelers and presented a formidable challenge to US manufacturers. Unfortunately for De Havilland, the Comet suffered a series of in-flight explosions due to metal fatigue that grounded the plane for two years while investigators tracked down the problem. During the interim, US manufacturers caught up to De Havilland’s lead. Boeing’s 707 and Douglas’s DC-8 established American supremacy in the airliner sector. In the equally important engine manufacturing sector, US companies Pratt & Whitney and General Electric overtook Rolls-Royce and Bristol Siddley as the world’s foremost manufacturers, adding to the United States’s competitive advantage. France’s Sud Aviation managed to sell a handful of its Caravelle medium-range jets to US air carriers, but could not hope to compete with the highly efficient US companies. By 1970, US manufacturers produced 80 percent of the world’s commercial airliners. European manufacturers realized by the mid-1960s that they could not hope to compete with the powerful American companies and turned to international cooperation in order to maintain the continent’s struggling aerospace industry. Britain and France agreed in 1962 to undertake a supersonic transport program. The resulting Concorde proved to be a commercial disappointment, with only sixteen of the supersonic airliners being produced due to high manufacturing costs. Air France and British Airways began flying the Concorde in the mid-1970’s, but operated the transatlantic flights at a loss. Despite the difficulties, Concorde did give British and French manufacturers increased prestige and income at a time when American airliners had almost eliminated European companies from the sector. In an effort to reestablish a European presence in airliner manufacturing, corporate and government officials in Britain, France, and West Germany established a consortium called Airbus

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Industrie in 1967. After a great deal of political negotiating, Airbus produced the A300B, which entered service with Air France in 1974. Airbus continued to expand its product line to match Boeing’s offerings, and the European conglomerate found customers around the world but fared poorly in the United States. Airbus’s success through the end of the twentieth century assured European manufacturers of a promising future, though the consortium could not match the overwhelming success of Boeing. Boeing’s dominance in the airliner industry was a result of the company’s diverse aircraft designs. The Seattle-based giant produced planes that offered air carriers a few options in terms of range, passenger capacity, and engine configuration. Boeing’s US competitors, Lockheed and McDonnell Douglas, did not provide the same diversity and assured Boeing’s position in the United States market. Airbus provided stiffer competition throughout the rest of the world, but Airbus could not offer anything that matched Boeing’s enormous 747 jumbo jet, which became a fixture on long-distance routes, until the Airbus A380 came into service in 2007. The A380 is the largest airliner in service and the only full-length double-deck passenger aircraft, with a spacious capacity for 509 passengers, greatly exceeding the capacity of what is now Boeing’s largest airliner, the 777. POSTWAR MILITARY MANUFACTURERS The prominence of airpower during World War II grew during the Cold War, requiring nations to spend a significant portion of their defense budgets on aviation. This dependence proved to be very lucrative for manufacturers. In the United States, the demand for varying kinds of fighters, bombers, and transports offered an opportunity for most of the nation’s aircraft companies to find a segment of the market for their products. This new affluence did not come without challenges, the most significant

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being cost. The United States hoped to offset the Soviet Union’s numerical superiority with technology, and the resulting aircraft proved increasingly expensive. Companies sought to combine designs in the hope of saving money. Boeing used the same basic design for both its KC-135 tanker and its 707 airliner. In doing so, the company reduced its expenses by saving time in the design phase and by using many of the same tools, jigs, and other equipment on both aircraft. European companies found the cost of the new high-tech military aircraft prohibitive, and so these manufacturers looked to combine operations with American or other European firms. When several European nations decided to adopt General Dynamic’s single-engine F-16 fighter in the early 1970s, European manufacturers won the right to fill 40 percent of the European orders and 10 percent of American orders. This agreement helped solidify Europe’s aerospace industry while drastically cutting costs by eliminating the need for research and development. In other cases, European nations combined their resources to produce original designs. When North Atlantic Treaty Organization (NATO) countries decided to replace their American-designed F-104 attack fighters in the late 1960s, the various governments decided to create a new European aircraft. The resulting effort, the Tornado, utilized components from Britain, West Germany, and Italy. The Tornado provided European nations with an aircraft that compared favorably with its American counterparts, and though rather expensive, the Tornado program gave European aerospace manufacturers valuable experience. Manufacturers learned the intricacies of managing such an effort in three different countries, each with its own currency, bureaucracy, and interests. The Tornado program also gave European manufacturers much-needed practice in designing high-performance fighters. European nations hoped to repeat the success of the Tor-

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nado with the European Fighter Aircraft, or Eurofighter. Work on the Eurofighter began in the 1980s, but the enormously complex program ran into technical and political difficulties and remained in the developmental phase at the end of the twentieth century. Behind the Iron Curtain, the Soviet Union established its reputation as an aerospace power. The Soviets enjoyed a closed market within their sphere of influence, but they also represented a threat to Western companies by competing in the developing world. The Soviet design bureaus of Antonov, Ilyushin, Mikoyan-Guryevich, Tupolev, and Yakovlev sold military and civilian aircraft to nonaligned nations to strengthen ties between the Soviet Union and the rest of the world. While less capable than Western aircraft, Soviet models were cheaper and generally adequate for most customers. Notably, The Antonov An-225 Mriya is the largest transport airplane ever to fly, with twice the carrying capacity of Boeing’s 747, and served customers worldwide until it was destroyed by Russian forces in Ukraine during the 2022 Russia-Ukraine war. The estimated cost to rebuild this marvelous flying creation is expected to exceed half a billion euros. CORPORATE CHANGES The escalating costs of manufacturing aircraft forced an ongoing series of mergers around the world beginning in the 1960s. In Britain, consolidation throughout the 1960s and 1970s ultimately led to the creation of a single nationalized British company, British Aerospace (BAe) in 1977. The development of BAe followed a government takeover of bankrupt engine-designer Rolls-Royce in 1971, which had already merged with Bristol Siddley Engines in 1966. The Conservative government of Margaret Thatcher privatized both BAe and Rolls-Royce during the 1980s, but the costs of manufacturing modern aircraft had forced British manufacturers to combine under one single parent com-

Airplane Manufacturers

pany. France’s aerospace industry underwent a similar consolidation in 1970 when the government merged the nation’s already nationalized manufacturers into one state-owned consortium, Aerospatiale, to handle France’s commercial aviation production. Military production in France remained the province of Dassault, a privately held but strictly controlled firm. American manufacturers fared better but still went through difficult times. Manufacturing giant Douglas merged with the smaller McDonnell Corporation in 1965, starting a series of mergers that continued throughout the remainder of the century. A downturn in orders in the late 1960s and early 1970s damaged the industry severely. Boeing laid off two-thirds of its workforce, and Lockheed was saved from bankruptcy only when the US Congress guaranteed the company’s credit. Despite these setbacks, US manufacturers maintained their dominant position in the world market. Boeing’s airliners proved enormously popular, while competing models from McDonnell Douglas and Lockheed found niches in the market. Increased military spending during the 1980s also offered greater opportunity for US manufacturers, but the new generation of US military aircraft were extraordinarily expensive, and the government’s spending reductions following the end of the Cold War meant that only a handful of the new planes entered service. Increased competition from Airbus irritated American manufacturers, but the US government did not take direct action against European imports. Throughout the 1980s, US companies pressed for protection, but the government settled for a 1992 agreement in which European governments agreed to limit the direct subsidies they gave to Airbus in return for the US government cutting back on indirect subsidies it offered to its own manufacturers. This agreement did little to limit Airbus’s continued pressure on the US market, but Boeing remained the world’s leading commercial

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aircraft manufacturer through the remainder of the twentieth century. Foreign firms also made headway in the small aircraft market, with models from Brazil, France, and Sweden challenging established US companies such as Cessna and Beech. —Matthew G. McCoy Further Reading Bilstein, Roger. The American Aerospace Industry: From Workshop to Global Enterprise. Twayne, 1996. Dancey, Peter G. British Aircraft Manufacturers Since 1909. Fonthill Media, 2017. Gordon, Yefim, and Dmitriy Komissarov. Antonov’s Heavy Transports from the An-22 to An-225, 1965 to the Present. Schiffer Publishing Ltd., 2020. Hayward, Keith. The World Aerospace Industry: Collaboration and Competition. Duckworth, 1994. Jackson, Robert, and Glen Ashley. Airbus A-380. Pen and Sword, 2021. McGuire, Steven. Airbus Industrie: Conflict and Cooperation in U.S.-E.C. Trade Relations. St. Martin’s Press, 1997. Neubeck, Ken, Leroy E. Douglas, and Long Island Republic Airport Historical Society. Airplane Manufacturing in Farmingdale. Arcadia Publishing Inc., 2016. Newhouse, John. Boeing Versus Airbus. The Inside Story of the Greatest International Competition in Business. Knopf Doubleday Publishing Group, 2008. Pisano, Dominick, and Cathleen Lewis, editors. Air and Space History: An Annotated Bibliography. Garland, 1988. See also: Aerospace industry in the United States; Air transportation industry; Federal Aviation Administration (FAA); German Luftwaffe; Glenn H. Curtiss; Howard R. Hughes; Jet engines; Messerschmitt aircraft; Military aircraft; Otto Lilienthal; Richard Branson; Supersonic jetliners and commercial airfare; Types and structure of airplanes; Wright brothers’ first flight

Airplane Propellers Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics

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ABSTRACT Propellers are rotating airfoils driven by an engine to provide thrust to an aircraft. Propellers were a primary mode of thrust generation for all aircraft up to the development of the gas-turbine engine in the 1940s, and they remain in widespread use, especially on smaller commercial and general aviation aircraft. KEY CONCEPTS airfoil: the cross-sectional profile of an aerodynamic wing angle of attack: the angle at which airflow encounters the leading edge of a wing or propeller relative to its chord line drag: the resistance to motion through a fluid due to friction between the moving object and the fluid medium pitch angle: the angle between a propeller’s chord line and its plane of rotation thrust: the force or pressure exerted on the body of an aircraft in the direction of its motion HISTORY Propellers have long been recognized as an efficient means of generating thrust. They were popularly used in aircraft design even before being used by Orville and Wilbur Wright to power the Wright Flyer in 1903. Leonardo da Vinci sketched propeller designs for helicopters in the 1500’s. They have been commonly used as children’s toys as well. Early propellers were based primarily on designs used for ships and windmills, but experiments soon found that long, thin airfoils provided better thrust than the shorter, thicker hydrofoil designs used in water. NATURE AND USE The function of a propeller is to create thrust to accelerate an aircraft forward. Although a wing creates lift to overcome an aircraft’s weight, a propeller creates thrust to overcome its drag. This thrust keeps an aircraft moving. When the propeller’s thrust is

Principles of Aeronautics

Airplane Propellers

Photo via iStock/Olga Sazonova. [Used under license.]

equal to the aircraft’s drag, the aircraft travels at a constant speed. When thrust is greater than drag, the aircraft accelerates until drag is equal to the thrust. Likewise, when the propeller’s thrust is less than the aircraft’s drag, the vehicle decelerates until the drag and thrust are equal and the aircraft’s velocity becomes constant. Thus, varying the propeller’s thrust will change the aircraft’s velocity. In a helicopter, the propeller is turned upward, so that the thrust is generated vertically to overcome the weight of the aircraft. When a propeller is oriented primarily to overcome weight instead of drag, it is usually called a “rotor.” The engine powering a propeller can be either a conventional piston (reciprocating) engine or a jet (gas turbine) engine. In the latter case, the propeller-and-engine combination is

commonly referred to as a turboprop. Turboprops typically derive 95 percent of their thrust from the propeller, while the remainder comes from the jet-engine exhaust. A propeller may be thought of as a severely twisted wing. In fact, the wings of many aircraft are twisted either to increase or decrease lift on certain portions of the wing by changing the local effective angle of attack. The propeller is twisted for a similar reason. Like an untwisted wing, a propeller could be designed without twist, as some of the first propellers were, but it would create less thrust than would a twisted propeller. A propeller generates thrust in the same way that a wing generates lift. Instead of moving in a straight line, however, the propeller rotates about a hub that

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is turned by the engine shaft. A propeller traces out the shape of a helix as it travels around in flight. For this reason, propellers are often referred to as airscrews and are also analogous to the propeller screws found on a ship. Both the rotating and forward movements of a propeller’s airfoil influence how much thrust is developed. The velocity at each radial location of the propeller will be different, because the total velocity is the vector sum of the propeller radial velocity and the aircraft velocity. Because the propeller is rotating at a certain rotation rate, the propeller velocity at any distance from the axis of rotation is the rotational speed times the radial distance. Thus, the propeller velocity will be almost zero near the hub and a maximum near the tip. This difference in velocity requires that the cross-sections of the propeller’s airfoil be twisted so that the chord line has a large angle of attack near the hub and a small angle of attack near the tip, in contrast to the airfoil of a wing that is nearly flat. The propeller’s chord line increasingly points in the direction of the aircraft motion, as the propeller airfoil sections progress toward the hub. The angle between the chord line of the propeller and the propeller’s plane of rotation is called the “pitch angle.” To determine the local angle of attack of a propeller, one uses the propeller’s pitch angle at each blade section and subtracts the angle of attack of the incoming relative wind. PROPELLER PLACEMENT A propeller can be placed anywhere on an aircraft, either at the nose, tail, wings, or on a pod. In a tractor configuration, the propeller is placed facing forward, usually on the nose, and pulls the aircraft. In a pusher configuration, the propeller is placed facing the rear of the aircraft and pushes the aircraft forward. One design has no real benefit over the other. The tractor configuration is more common, because it allows a better balance of the air-

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craft’s center of gravity about the aerodynamic center of the wing with the engine placed near the nose. Pusher configurations are more common in canard aircraft for the same reason. In a tractor configuration, the slipstream from a propeller is often pushed over the wings, creating a faster flow over that part of the wing. This is sometimes used to generate more lift, but it is not commonly considered in aircraft design. PROPELLER EFFICIENCY The propeller efficiency is a measure of how effectively a propeller transforms the engine power into propulsive power. It is measured by dividing the power output by the power input. The power output is the thrust generated by the propeller multiplied by the aircraft velocity. The power input is the amount of shaft power generated by the engine, measured in horsepower or watts. A propeller that is 100 percent efficient means that all the power from the engine is transferred directly to the air. No propeller can achieve 100 percent efficiency, however, and is hindered by several factors. The propeller, as it rotates, adds energy to the air, and this energy is lost from the aircraft, because it remains with the air long after the aircraft has passed. Indeed, the most efficient propellers are the ones that take a large amount of air and increase the velocity of the air only slightly. Thus, all things being equal, larger-diameter propellers are more efficient than smaller ones. The drag forces that act on the aircraft also act on the propeller. These forces include pressure drag, such as separation of the flow over a propeller, and friction drag, in which viscous effects of the air retard propeller motion. Typical propellers have efficiencies in the 70 to 90 percent range. Fixed-pitch propellers have the lowest efficiency and can drop below 70 percent if they are operating at a velocity for which they were not designed.

Principles of Aeronautics

PROPELLER DESIGNS The Wright brothers and Alexandre-Gustave Eiffel, among others, conducted early experiments on propellers. The Wright brothers were particularly concerned about maximum power output and thrust generation, because their early engines developed very little horsepower. They were able to design propeller blades with an efficiency of up to 70 percent, which was an extraordinary feat for the time. Eiffel, a French engineer and the builder of the Eiffel Tower in Paris, was also an ardent aerodynamicist who performed some of the first detailed wind-tunnel experiments on propellers. He was the first to show that propeller efficiency varied with the propeller’s rotation rate, diameter, and aircraft velocity. This parameter is now called the advance ratio and is used in propeller design, optimization, and selection. FIXED-PITCH PROPELLERS Propellers can be used on aircraft in several different ways. In the fixed-pitch propeller, the propeller blade has a fixed angle of attack. Although the angle varies along the length of the propeller, the blade has a fixed orientation throughout its flight envelope, meaning that the propeller design has been optimized for a single speed. If the aircraft travels at another velocity, the propeller efficiency is reduced. Fixed-pitch propellers were used on all airplanes up to the 1930s, when variable-pitch propellers were introduced. VARIABLE-PITCH PROPELLERS The angle of attack of variable-pitch propellers can be changed by rotating the blade about the hub. This allows pilots to adjust the propellers’ relative angle of attack in flight to account for changes in the aircraft and wind velocity. A complex mechanism in the hub allows the pilot to change the propeller pitch in flight, thereby increasing overall performance. When variable-pitch propellers were introduced in the 1930s, propeller efficiency across the range of flight

Airplane Propellers

conditions was greatly increased. A major drawback, however, was that as the pitch was altered, the torque on the engine was also changed. This would, in turn, change the rotation speed of the engine, resulting in a lower engine-power output. CONSTANT-SPEED PROPELLERS Consequently, the constant-speed propeller was introduced in the 1940s. It is a variant of the variable-pitch propeller in which the propeller pitch is changed automatically to keep the engine speed constant and to maximize total power output. Variable-pitch and constant-speed propellers may be feathered in flight during an engine-out scenario to minimize the propeller drag. To keep the propeller efficiency from dropping, the velocity of the propeller tip must be kept lower than the speed of sound, or Mach 1. If this velocity is exceeded, shock waves form at the tip of the propeller, and the efficiency drops dramatically as the available power is reduced by pressure losses. Shock waves can create other problems, such as severe noise, vibration, and structural damage to the propeller. Because the velocity at the tip is a function of the propeller radius, engine-shaft rotational speed, and aircraft speed, these three factors come into play when determining what size propeller should be used. During the tradeoff analysis of an aircraft design, as the speed of an aircraft increases, the diameter of the propeller decreases. To generate the same thrust for a smaller-diameter propeller given the same engine speed, an aircraft designer may opt to go with a larger number of propellers. The propeller must be balanced, and two blades are the minimum used. However, any number of blades greater than two may be chosen, as long as the blades are evenly spaced to maintain balance. Increasing the number of propeller blades means that to achieve the same thrust, a smaller diameter can be used. This is sometimes done to avoid the sonic tip speeds that may be encountered with long propeller

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blades on fast aircraft. Two-, three-, four-, and five-bladed propellers have been commonly used on aircraft throughout the twentieth century. To overcome the drawback of the sonic tip speed limitation of propellers on some commercial aircraft using turboprops, the use of unducted fan propellers has been proposed. The unducted fan propeller is a many-bladed propeller with short, curved blades that allow craft to overcome the sonic tip concerns that plague high-speed aircraft using traditional propeller designs. —Jamey D. Jacob Further Reading Gudmondsson, Snorri. General Aviation Aircraft Design: Applied Methods and Procedures. Elsevier Science, 2021. Hitchens, Frank. Propeller Aerodynamics: The History, Aerodynamics and Operation of Aircraft Propellers. Andrews UK Limited, 2015. Kinney, Jeremy R. Reinventing the Propeller: Aeronautical Specialty and the Triumph of the Modern Airplane. Cambridge UP, 2017. The Law Library. Air Worthiness Standards—Propellers (US FAA) (FAA) (2018 Edition). CreateSpace Independent Publishing Platform, 2018. National Aeronautics and Space Administration (NASA). An Assessment of Propeller Aircraft Noise Reduction Technology. CreateSpace Independent Publishing Platform, 2018. Steinberger, Victoria. Design of Zephyrus Human Powered Airplane Propellers. Pennsylvania State University, 2018. See also: Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Helicopters; Human-powered flight; Leonardo da Vinci; Propulsion technologies; Shock waves; Sound barrier; Turbojets and turbofans; Turboprops; Wing designs

Airplane Radar Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Chemical engineering; Mathematics

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ABSTRACT Radar is a system that transmits radio waves and receives and analyzes their reflections in order to determine the location and speed of objects, such as aircraft. Radar is essential for air traffic control, aircraft navigation, various weather observations, and many aspects of modern warfare. KEY CONCEPTS Doppler effect: wave impulses of a particular frequency moving toward a detector become compressed and are detected as a higher frequency, while the same wave impulse moving away from a detector becomes elongated and is detected as a lower frequency echo: an electromagnetic, sonic, or physical impulse that has reflected from a surface and returned to its source Hertz (Hz): the unit of frequency; 1 Hz = 1 cycle per second magnetron: an electronic device that generates microwaves wavelength: the distance from one peak of a waveform to the next peak of that waveform NATURE AND USE The word “radar” is an acronym for “radio detection and ranging,” where ranging refers to finding the distance to a target. Radar works in a fashion similar to that supposed by the early Greeks for the operation of the eye. The Greeks imagined that rays shot out from a person’s eye, and that people saw objects as their personal rays struck those objects and somehow returned information. The concept was one of being able to reach out and touch and feel objects from a distance. Radar reaches out by sending out a beam of radio waves oscillating electric and magnetic fields. When a radio wave passes a given point, the electric field strength at that point goes up and down in much the same way that the water level at a point on the ocean

Principles of Aeronautics

goes up and down as a water wave passes. The distance between adjacent crests in a radio wave is the wavelength, and the number of waves that pass a given point during one second is the frequency. The frequency multiplied by the wavelength gives the speed of the waves. The speed of radio waves is very nearly the speed of light, 105 kilometers per second. Light itself is an electromagnetic wave, but it has a much higher frequency than radio waves. At the speed of light, it takes only 2.5 seconds for radio waves to travel to the Moon and back. RADAR COMPONENTS A radar set usually consists of a transmitter, a transmitting antenna, a receiving antenna, a receiver, a computer, and a display. Normally, the same antenna is used both to transmit and to receive. The transmit-

Airplane Radar

ter causes a current to flow back and forth in the antenna, causing radio waves of the same frequency as the current to travel outward from the antenna. When radio waves strike objects, the waves are reflected and absorbed, depending upon the waves’ frequency and the properties of the objects. Metals, for example, are particularly reflective. When waves are reflected, a small fraction of the reflected energy may return to the radar antenna as an echo. The receiver amplifies this echo, and then the computer extracts information from the amplified echo and prepares this information to be displayed. TARGET DIRECTION, RANGE, SPEED, AND SIZE A common type of radar, with a revolving antenna, sends out a short burst of waves and listens for an echo. The direction in which the antenna was

Photo via iStock/HeliRy [Used under license.]

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pointing when it received the echo gives the target’s direction, and the time delay between sending and receiving gives the target’s distance. If the elapsed time is the time between sending the burst and receiving the echo, then the target’s range is one-half the elapsed time multiplied by the speed of radar waves, or about 105 kilometers per second. When the same antenna is used both to transmit and to receive, there must be some way to keep the stronger transmitted signal from completely swamping the weaker return echos. In the pulsed operation just described, this is done by timing. The transmission burst lasts about one microsecond, then the radar listens. The wait time during which the radar set listens for echos before sending out another pulse is keyed to the faintest echo that can be reliably detected. If targets up to 150 kilometers away can be detected, and radar waves can travel this distance and back in one millisecond, the pattern of pulse transmission and listening can be repeated about every millisecond. When a radar wave is reflected from a moving target, the frequency of the wave changes in a fashion described as the Doppler effect. The target’s speed can be determined from this change in frequency. If two targets are at the same distance and have the same radar reflective properties, a brighter echo indicates a larger object. Because radar reflectivity depends upon the shape and composition of the target, a better method to determine size is to send out a series of very short pulses, each lasting only a nanosecond or two. A large target may reflect two or more of these pulses, and the maximum distance between the echos yields the approximate size of the target. ANTENNAS AND OPERATING FREQUENCY A simple wire antenna will send radio waves outward in all directions; however, a carefully spaced group of several antennas can concentrate most of the radio waves into a beam. Such antenna groups

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must be several times the size of the wavelength they broadcast, and they work reasonably well from 3 million to 300 million hertz (cycles per second), or 100-meter to 1-meter wavelengths. The largest radar system in the world is the US Air Force’s over-the-horizon backscatter (OTH-B) air defense radar system, built to detect a Soviet bomber attack from thousands of kilometers away but also used to study ocean currents and waves. Each of the six transmitting antennas are 1.1 kilometers long, and the receiving antennas are 1.5 kilometers long. They operate between 5 and 28 megahertz, from 60- to 1.1-meter wavelengths. These wavelengths bounce off the ionosphere, about 200 kilometers above the ground, and reflect back down to Earth’s surface. The need for finer resolution and more portable radar sets eventually led to the development of radar wavelengths only centimeters long. Such short wavelengths can be formed into a searchlight-like beam by reflecting them from a parabolic metal dish. Because the paths of these wavelengths are not bent by the ionosphere, they must have a straight line of sight to the target. However, they will pass through the ionosphere and can be used to track objects in space. Regardless of the type of antenna used, radar beams spread wider as they travel outward from the antenna. The amount of spreading is smaller for shorter wavelengths and for bigger antennas. That is, the narrowest beams are formed, and the finest details can be seen, with radars using the shortest wavelengths and the largest antennas. The properties of the atmosphere also affect the choice of operating frequency. Atmospheric attenuation is negligible for frequencies up to 1 gigahertz (1 billion hertz). Above 3 gigahertz (1-centimeter wavelength), however, radar absorption by raindrops becomes significant, so weather radars operate at these frequencies. Above 12 gigahertz, clouds begin to absorb the radar waves.

Principles of Aeronautics

MILITARY DEVELOPMENT AND APPLICATIONS The development of radar was such a natural outgrowth of experiments with radio transmission that it was independently invented and developed by several countries during the 1930s. Probably more than any other device, radar dictated the course of World War II. Even before the war, Great Britain had begun installation of chain home (CH) radar stations along its coasts, with radar antennas on towers up to 110 meters high. Germany began massive bomber attacks on Britain in August, 1940. Chain home radars were so effective at giving warning and allowing the badly outnumbered Royal Air Force (RAF) fighters to position themselves for maximum effect, that by November of that year, daytime bomber attacks had stopped. The CH radar system determined the direction and elevation of an approaching aircraft by comparing the intensity of signals received at different antennas in the chain. When night attacks began the following year, CH radars were used to guide friendly fighters toward enemy bombers until the fighters got close enough to pick up the bombers on the short-range (5-kilometer) radar the fighters now carried. This technique was so successful that night attacks were also stopped. Radar was also put to other uses. In order to aid radar operators to distinguish between friendly and enemy aircraft, identification, friend, or foe (IFF) beacons were developed and used by the Allies. These were small radar receiver/transmitters that broadcast a coded radar signal that identified a craft as friendly when they detected a probing radar wave. Another device, a radar altimeter, is simply a small radar set that sends pulses toward the ground and determines the height from the time it takes for the echos to return. The atomic bomb dropped on Hiroshima in 1945 carried four radar altimeters and was fused to explode when any two measured the height as less than 600 meters. Had German submarines been able to cut off the flow of supplies and personnel from the United

Airplane Radar

States and Canada to Great Britain and Europe, the Allied invasion of Europe would have been impossible. At first, the German submarines were very successful in sinking Allied ships, but then the Allies began to hunt the submarines with radar. As submarine losses mounted, the Germans equipped their submarines with radar detectors, and the warning they gained allowed the submarines to be safely hidden underwater by the time attack aircraft arrived. The British then made one of the most important technological advances of the war, the microwave-cavity magnetron, a device for generating high-power radio waves of 10 centimeters or less. Shorter wavelengths meant radar antennas could be smaller, a great advantage in an aircraft, and smaller targets, such as submarine periscopes, could be detected. The German radar detectors could not pick up the short wavelength the Allies were now using, and the tide turned against them. In 1942, the Germans sank 8,245,000 tonnes of Allied shipping while losing 85 submarines. In 1944, they sank only 1,422,000 tonnes, but lost 241 submarines. RADAR TRACKING The familiar weather radar displays distances and directions to radar targets in a maplike image. A moving target such as a storm can be tracked by following its image on the radar screen as its position changes with time. Air traffic controllers use an extension of this method to guide aircraft in the vicinity of busy airports. A sophisticated version of this type of radar is used by the E-3 Sentry, or Airborne Warning and Control System (AWACS) aircraft, a modified Boeing 707 carrying a 9-meter (30-foot) radar dome. When aloft, AWACS can detect low-flying targets more than 375 kilometers away. Special equipment subtracts out the ground clutter that would swamp ordinary radars, thereby allowing AWACS controllers to monitor all the air traffic in the area and to direct friendly aircraft. The AWACS

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assisted in thirty-eight of the forty air-to-air shoot-downs of the 1991 Persian Gulf War. The efforts of civilian air traffic controllers have contributed to making air travel far safer than automobile travel. Airport surveillance radar (ASR) is a medium-range system that detects and tracks aircraft within about 80 kilometers of the radar installation. Controllers use this radar as they direct aircraft landings, takeoffs, and flight patterns. Air route surveillance radar (ARSR) tracks aircraft en route between airports. The ARSR-4 uses a wavelength of about 21 centimeters and has a range of about 400 kilometers. It broadcasts a series of pulses that interrogates the radar beacon or transponder carried by all large aircraft. The transponder broadcasts a reply from which the aircraft’s identity, range, and direction can be determined. An air traffic controller follows the aircraft’s progress and delivers instructions. When the aircraft leaves one controller’s sector, it is progressively handed off to controllers in the sectors through which it flies until reaching the destination airport. Radar sets can be designed to track a target automatically. During the Korean War, the US Army used radar to track mortar shells. A shell follows a parabolic trajectory, and if the radar can follow it for more than one-half of its trajectory, its launch point can be deduced, and artillery fire can be directed against the mortar. The radar dish used could slew, or pivot, quickly in any direction, and a mask partially blocked the center of the radar beam. When the radar locked onto a target, the target was positioned in the center of the beam, where the return echo would be relatively weak because of the mask. If the target drifted from the beam’s center, the echo strengthened, and the radar set used this information to move the antenna and keep the target centered. Although a similar scheme can be used to track aircraft, schemes that maximize the echo are more common. In any case, a relatively narrow beam must be used for tracking.

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Although mechanical systems can neither move quickly enough to track rockets and nearby fast-moving aircraft nor track multiple targets, phased-array radars can. These arrays consist of hundreds or even thousands of small antenna pods mounted in a regular array on a reflecting surface. Each pod is like a four-leaf clover, with each leaf replaced by a pencil-length rod pointing back toward the reflector at an approximate 45-degree angle. The term “phase” refers to position in the wave cycle. When all of the antennas are in phase, they begin broadcasting the beginning of a wave at the same time, and the radar beam is strongest straight ahead. If, instead, neighboring rows of antennas begin to broadcast at progressively later times, the radar beam will be tilted off to one side. When the radar receives a target echo, a computer can calculate where the target should be a fraction of a second later and direct the beam at that point. It takes only millionths of a second to switch the beam between targets so that a phased array can track one hundred or more targets virtually simultaneously. The US Air Force maintains Pave Paws radars at Cape Cod, Massachusetts; Beale, California; and Clear Air Force Station, Alaska. “Pave” is an Air Force program name, and “Paws” is an acronym of phased-array warning system. Each Pave Paws site has twin antennas consisting of 1,792 radiating elements mounted on massive reflecting faces measuring 31 meters across. The primary assignment for these installations is to detect and track intercontinental ballistic missiles or missiles launched from submarines at the United States. The Pave Paws radar beams extend 5,500 kilometers into space and are also used to track satellites. The heart of the US Navy’s Aegis combat system is a 4-megawatt phased-array radar mounted on a special ship that is also equipped with missiles and a Phalanx close-in weapons system (CIWS) for destroying attacking aircraft and missiles.

Principles of Aeronautics

COUNTERMEASURES AND STEALTH TECHNOLOGY The crew of an aircraft carrying a radar detector will know whether the craft is being observed. Once alerted, the crew might eject strips of aluminum foil, called chaff. Clouds of chaff appear as new targets on the radar screen and confuse the radar operator. The US Eighth Air Force dropped more than 4.5 million kilograms of aluminum foil during World War II. Specially equipped aircraft, such as Ferrets, and later, Wild Weasels, determine the location and frequencies of fire-control radars and jam them by broadcasting radar noise. Modern radar countermeasures include recording the fire-control radar signals and then beaming them back at the ground installation, thus making false targets appear at various distances and directions. When the aircraft are close enough, pilots can fire high-speed antiradiation missiles (HARMs) that home in on the fire-control radar. This presents the fire-control radar operator with an impossible choice: In order to shoot down the attacking aircraft, the operator must turn on the fire-control radar. However, if the radar is on for more than a few seconds, a HARM can lock in on its beam. In the initial stage of the Persian Gulf War, F-4G Phantom Wild Weasels flew 2,596 sorties and used this technique to devastate the formidable Iraqi air defenses. Perhaps the best radar countermeasure is to make an aircraft invisible to radar. The radar echos from an aircraft’s rounded fuselage fan out over a broad range of directions, including back toward the originating antenna. Stealth aircraft are made with many flat surfaces that are tilted to deflect the reflected radar beam away from the originating antenna. In order to reduce the radar echo when it is observed from behind, a “W” shape is used for the wing’s trailing edge. Right-angled corners such as those between the tail and fuselage of a normal aircraft are eliminated, because they can return strong radar echos. It is such right angles that make highway

Airplane Safety Issues

signs coated with corner reflector crystals appear to light up when lit by a car’s headlights. Carbon fiber materials and coatings that absorb radar waves are used extensively. The F-117A Nighthawk can get 90 percent closer to ground-based radar than a normal aircraft before it can be detected. During the opening minutes of the Persian Gulf War, eight Nighthawks followed a wave of Tomahawk cruise missiles and arrived at Baghdad undetected by ground radar. Their presence was announced only by bombs falling on their targets. The massive B-2 stealth bomber first saw combat in Yugoslavia during March, 1999. It carries eight times the bomb load of the F-117. —Charles W. Rogers Further Reading Ali, Busejairah Syd. Aircraft Surveillance Systems. Radar Limitations and the Advent of the Automatic Dependent Surveillance Broadcast. Taylor & Francis, 2017. Cushway, Roy T. Air Traffic Control: All’s Not Clear Out the Tower Window. Friesen Press, 2015. Degering, Randall. Radar Contact! The Beginnings of Army Air Forces Radar and Fighter Control. Independently Published, 2019. Lentz, Fleet S. A Backseat View from the Phantom: A Memoir of a Marine Radar Intercept Officer in Vietnam. McFarland Inc. Publishers, 2020. Markin, Evgeny. Principles of Modern Radar Missile Seekers. Artech House, 2022. Westwick, Peter. Stealth: The Secret Contest to Invent Invisible Aircraft. Oxford UP, 2019. See also: Airplane cockpit; Airplane guidance systems; Avionics; Flight instrumentation

Airplane Safety Issues Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Electrical engineering; Mathematics; Airplane accident investigation

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ABSTRACT Safety issues are aspects of the airline industry that affect the number of accidents and incidents, as well as the continuing effort to reduce this number as much as possible. Because millions of people travel every year for both business and pleasure, safe air travel is vital to passengers, businesses, and economies of the world. KEY CONCEPTS error chain: The sequence of potential causative factors that have combined to result in an accident or crash Federal Aviation regulations (FARs): a set of regulations designed for the safe operation of aircraft ground proximity warning system (GPWS): a system designed to provide a warning that the aircraft has become too close to the ground or ground-based obstacle for safe flight microburst: a sudden intense downward flow of air that can cause an aircraft to lose altitude suddenly turbulence: a mass of air moving chaotically that can cause an aircraft to respond in kind, causing passengers to be buffeted about, and potentially damaging the aircraft STATISTICS The aviation industry has a remarkable safety record. The total number of fatalities on board commercial jets in the years from 1959 to 1999 is less than one-half the annual US automobile fatality rate. However, because so many people can be affected by one incident, aviation accidents make headline news. Although the airlines’ safety record is impressive, continuous efforts by the aviation industry, the federal government, and the airlines are aimed at reducing the accident rate to zero. Statistics from the Boeing Company show that the ten-year commercial jet airplane accident rate from 1990 to 1999 was less than one accident per 1 million departures of scheduled air carriers. Even this statistic does not tell the whole story, however, be-

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cause fatal injuries were not present in all of those aircraft accidents. Although accidents are very rare occurrences, reducing the accident rate remains important. If the number of departures doubled from 10 million to 20 million annually and the rate of accidents remained the same, there would be an increase in the number of aircraft accidents. Many organizations, both public and private, are actively involved in research to prevent safety problems before accidents occur. The National Aeronautics and Space Administration (NASA) is very involved in funding basic research into new technologies and cockpit displays to prevent accidents both on the ground and in flight. The NASA Aviation Safety Program is a partnership with the Federal Aviation Administration (FAA), the Department of Defense (DoD), aircraft manufacturers, airlines, and universities. Their collective efforts have contributed significantly to the reduction of the number of aircraft accidents. HUMAN FACTORS Research reveals that more than 70 percent of all airline accidents can be attributed to human error, including that of pilots, air traffic control personnel, airport employees, and others. Government and industry officials have been implicated in some accidents because of delays in implementing certain safety warning devices. However, flight crews are ascribed with most of the errors that result in accidents. Aviation researchers are actively involved in determining how best to relieve this problem. The discipline of human factors in aircraft operations has become focused not only on the causes of accidents but also on the best ways to incorporate lessons learned from them into the aviation system. Rarely does a single event result in an aircraft accident. Research has shown that most accidents can be blamed on a series of uncorrected errors, intervention at any point in which would likely have disrupted the pattern and prevented the accident. Al-

Airplane Safety Issues

Principles of Aeronautics

A NASA air safety project. Photo by NASA, via Wikimedia. [Public domain.]

though aircraft operations attempt to make corrections based on lessons learned, the implementation of such procedures remains a complex issue involving many personalities, agencies, airlines, manufacturers, and governments. HUMAN PERFORMANCE Accidents are rarely caused by a deliberate disregard of procedures. They are more generally caused by a series of uncorrected mistakes or by the development of a situation in which people become overwhelmed or find their capabilities are inadequate for

the situation. Human performance in an accident or serious incident should be measured in terms of what could normally be anticipated and under what circumstances could a reasonable degree of correct performance have been expected from the persons involved. Many aspects of human performance must be considered when evaluating crew behavior. Work experience, working conditions, skill, fatigue, low blood sugar, reduced oxygen, and use of medicines, drugs or alcohol can all affect a person’s capabilities. Environmental conditions, such as noise, vibrations,

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motion, and visual cues may also affect a person’s ability to perform. The least measurable aspect of one’s capability is one’s psychological state. At any given time, one’s emotion, awareness, memory, attention, complacency, boredom, judgment, perceptions, and attitude are all significant contributors to an individual’s psychological capability. The level and quality of interaction with others associated with the flight will affect the tenor of the entire experience. CREW RESOURCE MANAGEMENT Research into an aircraft accident reveals the specifics of the event and most often assigns the blame to the flight crew. Nevertheless, the question of why qualified, demonstratively competent, highly trained, medically fit, well-paid professionals failed to perform the job correctly, resulting in an accident, continues to demand an answer. In 1983, the National Transportation Safety Board (NTSB) established its Human Performance Division to place an emphasis on answering that question. Investigations into crew behavior and organizational cultures reveal that the personalities of the individuals involved have a direct bearing on the flight crew’s general attitude. In the early days of commercial flight, the captain was considered the indisputable boss, and the other crewmembers were required to follow the captain’s orders. Although this hierarchical approach was the norm and expected, especially because most of the airline pilots at the time had been retired from the military, postaccident analysis revealed that if a subordinate crewmember had been more assertive, the accident chain might have been disrupted. A new concept of crew interaction was adopted by United Air Lines in the 1980s and became known as crew resource management (CRM). CRM challenged the paradigm of the captain-as-boss and introduced the concept of teamwork for decision making. It was a revolutionary idea at the time, and airlines hold-

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ing the traditional view of cockpit authority were reticent to embrace this concept. In 1989, United Air Lines Flight 232, whose pilot was able to land a hopelessly crippled DC-10 and saved the lives of half the passengers, forever changed the perception of CRM training from an interesting concept to an indispensable part of crew training. The crew’s remarkable teamwork was identified by the captain as the result of the CRM training that he and his fellow pilots received. The CRM concept is now the accepted norm and required by federal regulations. Airline management uses CRM training as an opportunity to intervene in a broad class of poorly defined problems. Line-oriented flight training (LOFT) is a curriculum of real-time simulator exercises that introduce situations to flight crews that enable them to practice their CRM skills and receive comments on their performance from the instructor. This broad-scale approach to social communication-based behaviors and attitudes is in marked contrast to the previous norm of a top-down captain-copilot relationship. CRM teaches the value of using all members’ experience to solve a problem, even though the captain maintains the legal authority to make final decisions. The success of CRM training has extended beyond cockpit crews. Airlines have discovered that cabin crews can also play a significant role in enhancing flight safety. Flight attendants, when included in preflight briefings by the captain, feel that their role in the safety of the flight is recognized. This inclusion contributes to the healthy tone of the flight and increases the likelihood that cabin crews would intervene in instances where communication between the cabin and cockpit was necessary. TRAINING Training is the single best method of ensuring airline safety. Airlines spend millions of dollars each year to evaluate pilot performance and to teach cor-

Principles of Aeronautics

Airplane Safety Issues

rective actions and procedures based on current research. Training instructs pilots how to perform their tasks. Procedures are designed to dictate the way tasks are implemented by the flight crew, ground crew, and others with direct input to the flight. Training programs, standardization of procedures, quality control, and printed materials such as manuals and checklists are used by all airlines for the safe operation of flight. The prevention and elimination of human error through successful training programs is a vital safety step. CHECKLISTS The purpose of checklists has been to alleviate the burden of pilots from trying to remember all the steps necessary to configure the aircraft for various flight regimes. The use of standardized checklists began about the time of the US Airmail Service and evolved to a complex written list of actions to be performed, a system which has not changed in concept from those early days despite the modern computerized checklists. The checklist is a critical tool for ensuring safe and consistent flight operations. Consistent, accurate use of the checklist is a safeguard to ensure that the aircraft is properly configured, operations are completed sequentially and efficiently, and the aircraft is prepared for flight. The FAA’s Federal Aviation Regulations (FARs) require the checklist to include a starting engines check, a takeoff check, a cruise configuration check, an approach check, an after-landing check, and a shutdown check. The FARs also require a checklist for the emergency operations of fuel, hydraulic, electrical, and mechanical systems and instruments and controls, as well as engine inoperative procedures and any other emergency procedures necessary for a safe flight. Significant research has been conducted in the area of checklist design and usage. The determina-

Air traffic control tower of Mumbai International Airport, India. Photo by Yatrik Sheth, via Wikimedia Commons.

tion of which items should be included, their sequence, redundancy, action and verification, and by whom the checking should be done, is complex. Checklist presentation—on paper, electronically, or mechanically—will vary among airlines and aircraft types. ROLE OF TECHNOLOGY Since the 1950s, continuing improvements in aircraft and engine design have significantly reduced the number of accidents based on these factors. High-bypass engine reliability, aircraft design, warning devices, and automation have all had a significant effect on reducing the airline accident rate.

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Several major improvements in aircraft systems and technology contribute to the safety record of the industry. These include ground proximity warning devices, traffic alert and collision avoidance systems (TCAS), and new cockpit computers and displays that provide updated weather and flight status information directly to the cockpit. GROUND PROXIMITY WARNING SYSTEM The introduction of the ground proximity warning system (GPWS) has significantly reduced the number of accidents involving controlled flight into terrain since its introduction in the 1970s. Controlled flight into terrain occurs when an airworthy aircraft, under the control of the flight crew, is flown unintentionally into terrain, obstacles, or water, usually with no prior awareness by the crew. Because controlled flight into terrain accidents represent the leading cause of aircraft hull losses annually, this safety device is particularly relevant. The GPWS system uses radar altimeter and aircraft configuration information to alert the flight crew of impending terrain. An advanced design, enhanced GPWS (E-GPWS) takes advantage of satellite global positioning system (GPS) technology and cockpit computer technology in third-generation aircraft to combine traditional GPWS with terrain mapping and GPS location information. E-GPWS is expected to reduce or eliminate the number of controlled flights into terrain accidents attributable to the flight crew’s loss of situational awareness. TRAFFIC ALERT AND COLLISION AVOIDANCE SYSTEMS In the decades following World War II, the steady increase in the number of flights by airlines and general aviation aircraft increased the likelihood of midair collisions, especially in the congested airspace over cities. In 1978, a Pacific Southwest Airlines Boeing 727 collided with a single-engine Cessna 172 over a populated area of San Diego, Cal-

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ifornia, resulting in many deaths. In 1986, an Aeromexico DC-9 collided with a single-engine Cherokee over Cerritos, California. The aftermath of this accident and the memory of the 1978 midair accident motivated the FAA and the airlines to develop a technology to augment vision and assist pilots in detecting and avoiding other aircraft. This research led to the development and implementation of traffic alert and collision avoidance systems (TCAS). This system displays other transponder-equipped aircraft within a specified radius. TCAS II, implemented a few years later, gives pilots resolution advisories (RA) either to descend or to climb in order to avoid a collision. Since 1993, TCAS II has been required on all passenger aircraft with more than thirty seats. Commuter aircraft with from ten to thirty seats are required to be equipped with TCAS I. Pilots widely and readily accept TCAS, finding it an indispensable cockpit tool. TCAS enhances pilots’ situational awareness and assists the visual location of aircraft advisories issued by air traffic control. WEATHER Because weather is such an integral part of aviation, improvements in severe weather information, prediction, and depiction have a significant relevance to improving the safety and comfort of flight. Thunderstorms, although easy to detect, have associated hazards, such as lightning, turbulence, heavy precipitation, icing, wind shear, and microbursts, that are more difficult to see and predict. These hazards are most dangerous when the aircraft is low to the ground, as in takeoff and landing. Onboard weather detection systems enable pilots to see the thunderstorm and avoid its associated hazards. TURBULENCE Aircraft encounters with turbulence result in upsets and injuries every year. Turbulence accounted for 103 injuries on board commercial aircraft in the pe-

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Airplane Safety Issues

riod from 1990 to 1999. Although turbulence is not uncommon in flight, the severity of turbulence ranges from uncomfortable to fatal. Types of turbulence include convective turbulence, mountain range turbulence, and clear-air turbulence. Convective turbulence occurs in localized, vertical air movements. The most hazardous types are usually associated with thunderstorms. Mountain range turbulence, as the name implies, occurs when wind blows across rugged hills or mountains, creating updraft on the windward side and strong downdrafts on the lee side. Lenticular clouds that form on the lee side of a mountain range and cumulus-looking rotor clouds that form parallel to the ridge line of a mountain are indicators of strong winds and occasionally severe downdrafts and associated turbulence. Clear-air turbulence is rough, bumpy air that sometimes buffets an airplane in a cloudless sky. It is usually found above altitudes of 15,000 feet and is often located near the jet stream winds. It is associated with a drastic change in wind direction, speed, air temperature, and horizontal or vertical wind shear. Research into the detection and avoidance of clear-air turbulence is important to reduce the injuries and fatalities on board aircraft.

wind directions and velocities to the pilots help them prepare for or avoid encounter with a wind shear.

MICROBURST AND WIND SHEAR Microburst and wind shear are atmospheric phenomena that have been implicated in several major airline accidents. Investigations into these crashes and computer simulations of the events have led to specific training procedures for pilots to escape from these extremely hazardous winds. Low-level wind shear alerting system (LLWSAS) is a system of anemometers implemented in select airports to give air traffic tower controllers information on wind direction and speed at different locations on the airport. If the wind direction and velocity exceed a predetermined parameter, an alarm will sound in the tower. Timely dissemination of the

Further Reading Chabot, B. Elizabeth. Flight Attendants Lost in the Line of Duty: Factual Accounts of Flight Attendant Actions in Life Threatening Incidents. Friesen Press, 2018. Cusick, Stephen K., Antonio I. Cortes, and Clarence C. Rodrigues. Commercial Aviation Safety. 6th ed., McGraw-Hill Education, 2017. Li, Longbiao, editor. Safety and Risk Assessment of Civil Aircraft During Operation. IntechOpen, 2020. National Transportation Safety Board. Emergency Evacuation of Commercial Airplanes Safety Study. CreateSpace Independent Publishing Platform, 2014. Robison, Peter. Flying Blind: The 737 Max Tragedy and the Fall of Boeing. Knopf Doubleday Publishing Group, 2021.

RUNWAY INCURSIONS Crowded skies inevitably lead to crowded airports. Increased congestion at major airports has consequences on the ground as well as in the airspace above. Although it is a rare occurrence, the ground collision of aircraft accounts for the worst aviation disaster in history: that which occurred between two fully loaded Boeing 747 jumbo jets in Tenerife, Canary Islands, in 1977. From 1995 to 2000 there was a 60 percent increase in near-collisions on the ground, according to the NTSB. The FAA places a high priority on the reduction of the number of runway incursions. New methods for pilots to determine their exact location on the airport in low-visibility or night situations are being researched. Improved airport markings, assessing new technologies, strategic plans for foreign air carrier pilot awareness, training, and review of pilot/controller communications phraseology are among the issues being explored to mitigate this safety problem. —Veronica T. Cote

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Soekkha, Hans M., editor. Aviation Safety, Human Factors— System Engineering—Flight Operations—Economics— Strategies—Management. CRC Press, 2020. See also: Air flight communication; Air transportation industry; Airplane accident investigation; Airplane maintenance; Avionics; Federal Aviation Administration (FAA); Flight instrumentation; Flight landing procedures; Flight recorder; Flight simulators; National Transportation Safety Board (NTSB); Takeoff procedures; Taxiing procedures; Wind shear

Animal Flight Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Animal flight refers to sustained and powered airborne travel by birds, insects, or mammals through the use of wings. Animal flight, particularly that of birds, is important to humans, who first learned and dreamt of flight by studying flying animals. The study of animal flight remains a source of information for understanding and design of flying vehicles. KEY CONCEPTS airfoil: the cross-sectional profile of an aerodynamic wing biomechanics: the scientific study of how animals, including man, move as a function of muscles, tendons, and skeleton coordination kinetic energy: the energy an object in motion possesses due to its speed ornithoptic propulsion: flying in the manner of birds, by the flapping of wings potential energy: the energy an object possesses due to its relative position within a frame of reference

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HISTORY Animals have been flying for millions of years. The first flying animals were insects, which appeared approximately 350 million years ago. From that time, flight evolved separately among three other kinds of animals. There are four types of animals capable of flight: insects, birds, bats, and pterosaurs, the last of which are extinct. Each of these groups developed the ability to fly independently, and, in many cases, different species in each group separately evolved the ability to fly. Additionally, some mammals and birds developed the ability to glide but not to fly. Unlike aircraft, which gain lift with wings that are either fixed or rotating, animals almost universally accomplish flight by flapping their wings. The flapping motion provides not only lift but also thrust and is referred to as ornithoptic propulsion. Both animals and manufactured aircraft using this method of achieving flight are commonly called ornithopters. BASIS OF ANIMAL FLIGHT The same aerodynamic laws that apply to man-made aircraft also apply to animals, and animal flight is divided into three categories, based on how it is attained. Gliding animals do not fly but trade potential energy (height) for kinetic energy (speed) to remain aloft. Gliding is only useful for small distances. Flying animals use their wings to generate both lift and thrust to remain in the air. Soaring animals, a cross between gliders and fliers, usually use wing movement only for takeoffs and landings, generally relying on subtle changes in wing geometry, thermal air currents (thermals), and prevailing winds to gain altitude. Many large birds soar rather than fly. For an animal to remain in level and steady flight, the lift that it generates with its wings must be equal to its weight, whereas the thrust it creates must be equal to its aerodynamic drag. All flying animals generate both lift and thrust by the same method: flapping their wings.

Principles of Aeronautics

In flapping, or ornithoptic, flight, the wing must produce lift and thrust at the same time. However, lift and thrust does not have to be produced constantly. During a single wing beat, lift and thrust vary. As long as the average lift and drag over the period of the wing motion are equal to the drag and weight, respectively, this will keep the animal in level and steady flight over time. AERODYNAMICS The primary difference in the aerodynamics of aircraft and animal flight is the slower speed and smaller size of flying animals, compared to that of manufactured aircraft. This difference is characterized by a parameter called the Reynolds number, which measures the effect of aerodynamic inertial forces compared to aerodynamic viscous, or frictional, forces. The lower the Reynolds number is,

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the more important the effects of fluid viscosity, or friction, become. The Reynolds value of most aircraft, whether general aviation craft, commercial airliners, or fighters, is in the millions. For birds and insects, however, the Reynolds value is usually 100,000 or less and is sometimes even less than 1,000 for very small insects. For flying objects with a Reynolds value greater than 100,000, thick, curved, or cambered, airfoils work best, whereas those with Reynolds values of less than 100,000 tend to work best with thinner, flatter airfoils. This difference is demonstrated by examining the value of the lift-to-drag ratio as a function of Reynolds number for a number of given airfoils. Most fat airfoils have a higher lift-to-drag ratio at high Reynolds numbers, whereas thin airfoils have a higher lift-to-drag ratio at low Reynolds numbers. This fact was originally discovered during World

Photo via iStock/pchoui. [Used under license.]

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War I by the Germans, who determined that fat wings worked better on their faster biplane fighters. Likewise, as an animal’s speed and size increases, the shape of its wings changes to reflect the increase in Reynolds number. Thus, large or fast birds, such as pigeons and falcons, have wing cross sections that look surprisingly similar to those of modern aircraft wings. Another important aspect of speed or Reynolds number is how the roughness of a wing affects flight efficiency. The faster an object flies, the smoother the wing needs to be for maximum lift and minimum drag. At low Reynolds numbers, however, the lift dramatically drops for smooth wings, whereas it does not for rough wings. Thus, smooth manufactured wings do not operate as efficiently as rough wings, whether the animal wings are roughened by feathers, scales, or fur. Rough animal wings are most efficient for low speeds. The motion of the feathers and fur allows animals to sense when their wings are about to stall. Whereas avian biomechanics are complex, insect biomechanics are relatively simple and easy to analyze. This simplicity lends itself well to duplication using modern mechanical technology. In the simplest of insect wings, wing motion is controlled by contraction of interior muscles. The motion in this case is indirect; other insect systems have a direct relationship between muscle movement and wing motion. Without examining complex muscle mechanics, however, one can quickly determine the limit to a flying animal’s size by examining the weight in relation to the size. Scaling determines whether ornithoptic propulsion is efficient for a given weight and length scale. The length scale is a measure of an animal’s size, in either length or wingspan. A flier’s weight is proportional to the length scale cubed, whereas the wing area is proportional to length scale squared. This is known as the cube-square law. Thus, one can deduce that the wing loading (weight divided by wing area)

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is proportional to the length scale. As size increases, the wing loading must also increase. Eventually, the wing loading will be too great for the bones and muscles of an animal to withstand, and any animal above this size will be unable to fly. The required power or energy input for a given weight can be determined from commonly known aerodynamic relations and can be shown to increase as the 7/2 power of the length scale. Thus, if the wingspan of an animal doubles, the power required to fly must increase by more than ten times. Based on muscle-mass arguments stating that the amount of energy available is related to the amount of muscle mass, the power available to flying animals can be shown to quadruple as the wingspan doubles. Thus, as the size of a flying animal increases, required power will soon overtake available power, not only limiting the animal’s maximum possible weight but also decreasing the animal’s ability to take off, climb, and hover. Hence, larger flying animals tend to use soaring as the primary flight mode instead of powered flapping. The ratio of unsteady lift, derived from flapping, compared to steady lift, derived from forward motion, shows that flapping frequency can be directly related to the size and weight of a flying animal. Using the flapping frequency as an approximate measure of this ratio and comparing it with the flier’s length scale, it is shown that the frequency is inversely proportional to this length scale, which can also be related to the Reynolds number. Thus, as the Reynolds number increases, or as the speed or size of a flier increases, the frequency at which the wings flap decreases. Eventually, the flapping frequency will decrease to the point where the wings will be stationary, indicating that there is a limit to the efficiency of flapping as a flight mechanism, as size increases. On the contrary, as weight decreases, there is a limit below which flapping is a very efficient flight mode. This principle has direct applications to the development of manufactured microaerial vehi-

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cles: Instead of shrinking down conventional aircraft designs to a smaller scale, it may be more practical to design miniature aircraft that use flapping wings instead of fixed wings for lift generation and engine-propeller combinations for thrust generation. The efficiency of flapping flight for small birds may also be one reason why wings evolved over propellers for thrust generation. As the size of a flying animal decreases, the generation of unsteady lift becomes more important to its flight. This is especially true for insects that derive much of their lift from unsteady effects alone. BIRD FLIGHT Birds are by far the best-known animal fliers. There are more than 9,000 species of birds, of which only a handful, such as the penguin, kiwi, ostrich, and emu, are flightless. Birds are characterized as warm-blooded, egg-laying vertebrates with feathered wings and strong hollow bones, many of which are fused together to increase strength and decrease weight. They have powerful muscles that allow for flight and require large amounts of food for energy. Birds evolved from dinosaurs approximately 150 million years ago. Most birds appear to have evolved flight from ground-up gliding, used both to catch prey and to evade predators. Wings may also have developed as an aid to increase leaping distances and as a display to attract mates. Two scenarios for the evolution of flight include the ground-up scenario, in which running and leaping animals evolved wings, and the tree-down scenario, in which tree-dwelling creatures evolved wings to move from tree to tree. In either case, the ability to survive and gain access to unoccupied niches appear to be the greatest reasons for the development of bird flight. Although it appears that modern birds evolved from dinosaurs, birds are not related to the now-extinct pterosaurs, or flying archosaurian reptiles.

Animal Flight

Pterosaurs were lizards and appeared to be proficient fliers with wing structures, similar to those of bats, that had an outstretched membrane over a thin upper limb. They had large heads that may have assisted their flight stability. Their wingspans ranged from a few inches to almost 40 feet. The pteranodon had a wingspan of up to 25 feet but weighed only 25 pounds. Due to their large sizes, most pterosaurs were probably soaring animals that relied on thermals to fly at high altitudes. The oldest known bird is the archaeopteryx, named for the Greek “ancient wing,” which lived around 150 million years ago. It had a wingspan of approximately 18 inches and weighed about 1 pound. With its feathers and beak, it had similarities to modern birds, and with its teeth and clawed wings, it had similarities to dinosaurs. There is a wide variety of flying birds, including the small hovering hummingbird, the swift falcon, and the lumbering condor. Each adopted a mode of flight suited to its evolutionary niche. There are several differences between flying and flightless birds that illustrate requirements for successful bird flight. Flightless birds tend to have shorter, symmetrical wings, whereas flying birds have long, cambered wings that produce substantially more lift. To keep their weight low, flying birds tend to have fewer feathers than their grounded counterparts. Flying birds also have longer tails, or keels, that aid in flight stability. Birds occupy almost every low-speed flight niche known. They are adept fliers, using their wing muscles and feathers to control the distribution of lift over the wings. This allows them to easily adjust to changes in ambient flight conditions, such as gusts or downdrafts. Their whole bodies are designed for flight. They have strong, hollow bones that minimize weight and withstand impacts. They have unique single-path pulmonary systems that constantly feed fresh oxygen to the lungs to maximize energy. They use their heads, tails, and feet to help control flight.

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Variations across the bird species detail how well-designed birds are for their particular niches. The hummingbird, for example, is well known for its ability to hover in one place in flight, beating its wings at an amazing 60 or more beats per second while feeding on the nectar of flowers, and for the ability to fly backwards. Although other birds, such as kestrels, terns, and gulls, can also hover, only hummingbirds can fly sideways and backward in hovering flight. INSECTS Insects are both the oldest and generally smallest of flying animals. The first winged insects appeared some 350 million years ago and were the first creatures to fly on Earth. There are at least one million species of insects, many of which fly. They range in size from barely visible to almost 1 foot in wingspan. Insects are invertebrate arthropods with a hard exoskeleton and a three-part body consisting of head, thorax, and abdomen, three pairs of jointed legs, and two antennae. The legs and wings are attached to the thorax. Most winged insects have two sets of wings, fore and aft. Most flap their wings in synch, whereas a few, such as the dragonfly, flap their fore and aft wings asynchronously. In the former case, synchronous wing movement appears to be limited to approximately 200 beats per second, because the wing motion is related directly to the nerve inputs to the muscles. For asynchronously flapping winged insects, beat frequencies of more than 1,000 beats per second have been recorded, because the myogenic flight muscles used in asynchronous wing motion can contract more than once per nerve impulse. Most insects cannot fly, according the laws of conventional aerodynamics. Under these assumptions, lift is determined by the steady flow of air over a wing just as in aircraft flight. The wing areas of most insects are too small to obtain the required lift at their measured flight speeds, however. Much of

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their lift is instead derived from unsteady lift as described above. For insects, the clap-and-fling effect is used to generate the required lift. In this method, the wings are beaten together (clap) and rapidly pulled apart (fling). The air rushing in to fill the void develops a fast-moving vortex over the top of the wing that generates a large amount of unsteady lift. Insects must beat their wings rapidly and repeatedly to generate lift. Bees flap their wings more than 100 times per second. The common housefly beats its wings more than 20,000 times per minute, or about 300 times per second. A midge of the genus Forcipomyia has a measured wing-beat frequency of more than 1,000 beats per second. The wings of most insects are less flexible than those of birds or bats. Most insects change direction and speed primarily by altering the motion and frequency of their wing beats. Pitch, yaw, and roll control involve changes in wing-beat amplitude on one wing with respect to the other, lateral wing twisting, or leg and abdominal movement. Some insects can twist their wings like those of a bird to control motion, such that a large area is projected on the downstroke and a small area is projected on the upstroke. These traits give great maneuverability to most insect species. The number of flying insect species is enormous. Typical insect flight speeds range from 24 kilometers per hour for bees to 1.6 kilometer per hour for mosquitoes, and even less for smaller insects. The fastest flying insect may be the tabanid, with a flight speed estimated at 144 kilometers per hour; it has been observed to execute Immelmann maneuvers while in flight. The Australian dragonfly can reach 55 kilometers per hour over short distances, outrunning most horses. Some dragonflies have wingspans of up to 29.5 centimeters, and some butterflies have wingspans of up to 25.4 centimeters. Dragonflies have two sets of high-aspect ratio wings, and butterflies have two pairs of large low-aspect ratio wings covered with colorful,

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iridescent scales in overlapping rows. Lepidoptera, as butterflies and moths are known, are the only insects that have scaly wings. Flight speeds vary among butterfly species. The poisonous varieties fly more slowly than nonpoisonous varieties, because they do not have to fly as quickly to evade predators. The fastest butterflies can fly at about 50 kilometers per hour or faster, whereas slow-flying butterflies fly about 8 kilometers per hour. MAMMALS Only one mammal is truly capable of powered flight: the bat, of the order Chiroptera, a word that means “hand-wing.” All other so-called flying mammalian species do not actually fly but rather glide. Other mammals that fly by gliding include the flying squirrel and the flying lemur, neither of which actually fly and the latter of which is not actually a lemur. Bats, however, like birds, do attain true powered flight. Bats are vertebrates with fur that bear and nurse live young. Nocturnal animals, they are found in all regions of the world except for the North and South Poles. There are more than 900 different species of bats, ranging in wingspan from the 12.5 cm bumblebee bat to the 2-meter flying fox. Some bats migrate, whereas others hibernate. Because the fossil record is limited, the origin and evolution of bats remain unknown. It is believed bats appeared around fifty million years ago. Bats are related to the colugo, or flying lemur, but their common link is a mystery. They probably evolved from arboreal ancestors related to primates that used gliding and climbing as separate means of locomotion. The fact that the earliest bats had tails supports this assertion. Bats are divided into two suborders based upon their method of navigation. Those of the suborder Microchiroptera use a type of sonar called echolocation to navigate and search for prey. They send off high-pitched sounds beyond the range of most human hearing. These sounds echo off sur-

Animal Flight

roundings and other animals, and bats use the echoes to determine the size and distance of the object. Microchiropteras include the vampire bat, the only mammal to feed exclusively on blood. Bats of the suborder Megachiroptera, such as the fruit bat, use their sense of smell to find food. Both types of bats have poor eyesight. Bats are agile fliers. The bat wing is a membrane stretched across elongated fingers of the hand which support the distal, thrust-producing portion of the wing. Bats can change the effective airfoil cross section of the wing by moving their fingers. The fingers are extremely flexible, much like those of humans, and allow a bat to create almost any desired airfoil shape. The uropatagium, a membrane stretched between the hind limbs, helps stabilize the bat during flight and is often used to capture prey. Because gliding animals incorporate their hind limbs into their wings, this membrane is believed to have evolved from gliding. Gliding mammals include the flying squirrel and flying lemur. Flying squirrels have a fold of skin extending from the wrist of the front leg to the ankle of the hind leg that forms a winglike gliding surface when the limbs are extended. The tail serves as a control device during glides to steer and stabilize flight. Colugos, or flying lemurs, arboreal climbers and gliders with lateral skin membranes and large, webbed, clawed feet, are found in certain regions of the Pacific Rim. They resemble large flying squirrels. Like bats, they have a short tail, which is used for stability and is connected to the hind limbs by skin folds. —Jamey D. Jacob Further Reading Alexander, R. McNeill. Principles of Animal Locomotion. Princeton UP, 2006. Glaeser, Georg, Hannes F. Paulus, and Werner Nachtigall. The Evolution of Flight. Springer International Publishing, 2017.

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Norberg, Ulla M. Vertebrate Flight Mechanics, Physiology, Morphology, Ecology, and Evolution. Springer, 2012. Pettigrew, James Bell. Animal Locomotion: Or, Walking, Swimming, and Flying, with a Dissertation on Aeronautics. 1874. Reprint. Good Press, 2019. Reay, D. A. The History of Man-Powered Flight. Elsevier Science, 2014. See also: Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Glider planes

Neil Armstrong Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Pilot training ABSTRACT Neil Armstrong was born on August 5, 1930, in Wapakoneta, Ohio, and died August 25, 2012, in Cincinnati, Ohio. As commander of the Apollo 11 lunar landing mission in 1969, he became the first human to walk on the moon. In addition to his outstanding and pioneering contributions to the National Aeronautics and Space Administration’s (NASA) crewed spaceflight program, Armstrong has served with distinction as a professor of aerospace engineering, chairman and director of several corporations, and member of presidential commissions.

from the flight deck of the USS Essex in Korea in 1950. He won three Air Medals for his combat duty. At the end of the war, Armstrong returned to Purdue and received his baccalaureate degree in 1955.

EARLY LIFE AND EDUCATION Born to Stephen and Viola Louise Armstrong in Wapakoneta, Ohio, in 1930, Neil Armstrong was an avid enthusiast of flying from an early age. He received his student pilot’s license at age sixteen, before receiving a driver’s license. In 1947, he entered the aeronautical engineering program at Purdue University with a scholarship from the US Navy. Two years later, he was called to active duty and earned his pilot’s wings at the Naval Air Station in Pensacola, Florida. As the youngest pilot in his squadron, he flew seventy-eight combat missions

PROFESSIONAL ACTIVITIES AT NASA After graduating from Purdue, Armstrong joined the National Aeronautics and Space Administration’s (NASA’s) Lewis Flight Propulsion Laboratory in Cleveland, Ohio. Later, he transferred to NASA’s High-Speed Flight Station at Edwards Air Force Base, California. There, as an aeronautical research pilot, he flew X-15 airplanes to altitudes over 61 kilometers, at speeds up to 6,438 kilometers per hour. As a test pilot, Armstrong also flew the X-1 rocket airplane, the F-100, F-101, F-102, F-104, F-5D, B-47, and other aircraft. Armstrong’s experience

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Neil Armstrong, 1969. Photo via Wikimedia Commons. [Public domain.]

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with the X-15 led to his selection as a pilot of the X-20 Dyna-Soar, an experimental craft that could leave the atmosphere, orbit Earth, reenter the atmosphere, and land like a conventional aircraft. However, the X-20 project was canceled in 1962, and Armstrong then decided to become an astronaut. In September, 1962, Armstrong was one of the first two civilians selected for astronaut training. In his first flight assignment, he served as a backup command pilot for the Gemini GT-5 mission. On March 16, 1966, Armstrong served as the command pilot for Gemini 8, and, along with pilot David R. Scott, successfully docked two vehicles in space for the first time. The flight was terminated ahead of its three-day schedule due to a malfunctioning thruster. Demonstrating exceptional piloting skill, the crew overcame the problem and brought the craft to a safe landing. Subsequently, Armstrong served as backup command pilot and backup commander for the Gemini 11 and the Apollo 8 missions, respectively. Armstrong’s most significant role as an astronaut occurred during his command of the manned lunar landing mission of Apollo 11 from July 16 to July 21, 1969. The crew for this historic flight consisted of spacecraft commander Armstrong, Lunar Module pilot Edwin “Buzz” Aldrin, and Command Module pilot Michael Collins. On July 20, 1969, their efforts accomplished what many consider the single greatest achievement of all time. For the first time in human history, a human set foot on a celestial body beyond Earth. After landing on the lunar surface at about 4:18 p.m. eastern daylight time, Armstrong radioed back to mission control the now-famous words, “Houston, Tranquility Base here. The Eagle has landed.” Six hours later, Armstrong stepped off the Lunar Module onto the surface of the Moon. Taking his first steps on the Moon, he uttered the immortal words, “That’s one small step for man, one giant leap for mankind.” Shortly thereafter, he was joined by Aldrin. The two astronauts spent twenty-one hours on the lunar surface, collecting

Neil Armstrong

just over 20 kilograms of lunar rocks. Their liftoff from the surface of the Moon was partially captured on a television camera they left behind, and they successfully docked with Michael Collins, who had continued to orbit the Moon alone in the Command Module Columbia. POST-NASA ACTIVITIES Following his historic walk on the Moon, Armstrong received a master of science degree in aeronautical engineering from the University of Southern California. In the fall of 1971, he accepted a position as professor of aerospace engineering at the University of Cincinnati, an interdisciplinary post he held until 1980. Thereafter, he served as the chairman of the board of Cardwell International Corporation in Lebanon, Ohio, until 1982, when he became the chairman of the board of Computing Technologies for Aviation (CTA) Incorporated of Charlottesville, Virginia. In 1984, along with the test pilot Charles E. “Chuck” Yeager, Armstrong joined the National Commission on Space (NCOS), a presidential panel created to develop goals for the space program in the twenty-first century. However, the explosion of the space shuttle Challenger on January 28, 1986, placed the commission’s report on hold. Following the Challenger disaster, Armstrong was named vice chairman of the Presidential Commission on the Space Shuttle Challenger Accident. Over the years, Armstrong, an intensely private and unassuming man, has avoided as much as possible making public appearances. On the occasion of the thirtieth anniversary of the first lunar landing on July 20, 1999, he gave a lighthearted speech before the National Press Club in Washington, D.C., on behalf of the National Academy of Engineering. He described spaceflight as one of the greatest engineering achievements and observed that while “science is about what is, engineering is about what can be.” —Monish R. Chatterjee

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Further Reading Barbree, Jay. Neil Armstrong: A Life of Flight. St. Martin’s Publishing Group, 2014. Hansen, James R. First Man: The Life of Neil A. Armstrong. Simon & Schuster, 2012. ———. A Reluctant Icon: Letters to Neil Armstrong. Purdue UP, 2020. Kuhlmann, Torben. Armstrong: The Adventurous Journey of a Mouse on the Moon. NorthSouth Books, 2016. Okelly, Kevin. Neil Armstrong: The Lives and Careers of the First Man on the Moon (The Life and Legacy of the First Astronaut to Walk on the Moon). Phil Dawson, 2022. See also: Glenn H. Curtiss; Amelia Earhart; Yuri Gagarin; John Glenn; High-speed flight; Johnson Space Center; Charles A. Lindbergh; Military aircraft; Billy Mitchell; National Aeronautics and Space Administration (NASA); Wiley Post; Eddie Rickenbacker; Alan Shepard; Supersonic aircraft; Valentina Tereshkova; Konstantin Tsiolkovsky; Manfred von Richthofen; Wright brothers’ first flight; Chuck Yeager

Atmospheric Circulation Fields of Study: Physics; Aeronautical engineering; Meteorology; Climatology; Fluid dynamics ABSTRACT Atmospheric circulation is the large-scale movement of air that distributes heat from tropical to polar latitudes across the surface of Earth. The global wind patterns are guided by three distinct convection cells that transport heat by circulating air at various latitudes and that extend from Earth’s surface to the upper boundary of the troposphere. The troposphere is the lowest layer of the atmosphere and extends from Earth’s surface upward to approximately 15 kilometers above the surface. KEY CONCEPTS convection cell: a closed-loop system of fluid movement in which warmer, less dense fluid rises and cooler, denser fluid descends, with both upper

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and lower levels exhibiting lateral displacement to replace the fluids that have ascended and descended pressure gradient: the continuous change of pressure over the distance between two regions having different pressures tropopause: the lowest portion of the atmosphere, about 15 kilometers in thickness, extending upward from Earth’s surface Atmospheric circulation is the large-scale movement of air that distributes heat from tropical to polar latitudes across the surface of Earth. The global wind patterns are guided by three distinct convection cells—known as the Hadley cell, Ferrel cell, and Polar cell—that transport heat by circulating air at various latitudes and that extend from Earth’s surface to the upper boundary of the troposphere. The troposphere is the lowest layer of the atmosphere and extends from Earth’s surface upward to approximately 15 kilometers (9.3 miles) above the surface, where it is separated from the stratosphere by an area of temperature inversion known as the tropopause. Because the troposphere is where nearly all of the world’s weather conditions originate, it is important for scientists to understand atmospheric circulation patterns in order to more accurately predict climate conditions that can affect everything from crop production to transportation safety. BACKGROUND English meteorologist George Hadley (1685-1768) was interested in finding out why sailors encountered westerly winds at the midlatitudes and easterly winds, known as the trade winds, closer to the equator. In 1735, Hadley described atmospheric circulation as a massive version of a huge sea breeze in which warm air rises over the equator and sinks over the poles and is moved directionally along latitudinal lines as a result of the rotation of Earth. His is regarded as the first attempt to describe how

Principles of Aeronautics

weather patterns combine and interact to produce a general circulation of the atmosphere. In recognition, the largest of the three convection cells was named after Hadley. The Hadley cells lie nearest to the equator, stretching north and south from the equatorial line to approximately 30 degrees latitude. Within the Hadley cells, warm air rises from along the equator and flows toward the poles within the troposphere before cooling and descending in the subtropics. Near the surface, trade winds blow toward the equator in a westward direction and often develop into thunderstorms as they rise near the equator, in what is called the Inter-Tropical Convergence Zone. The rising warm air from the equator circulates toward higher latitudes and then sinks at approximately 30 degrees latitude, creating high-pressure regions over the world’s subtropical oceans and deserts. The Ferrel cell, named in honor of nineteenth-century American meteorologist William Ferrel (1817-1891), represents the midlatitude segment of Earth’s atmospheric circulation, ranging between 30 and 60 degrees north and south latitude. Air circulation in the Ferrel cell is opposite the flow in the Hadley cell. In the Ferrel cell, air near the surface flows toward the poles in an eastward direction, and air at higher altitudes flows toward the equator in a westward direction. The prevailing winds in this cell, known as the westerlies because they originate in the west and flow eastward, are more susceptible to passing weather systems—particularly subtropical highs— than the prevailing winds in the Hadley and Polar cells and can change direction abruptly. The Polar cells lie at the farthest distance from Earth’s equator, extending from 60 degrees latitude to the North and South Poles. These are the smallest and weakest of the atmospheric circulation cells. Air in the Polar cell rises at lower latitudes and moves toward the poles through the troposphere. When this circulating air reaches the pole, it has cooled signifi-

Atmospheric Circulation

cantly and descends, traveling along the surface back toward the equator in a westward direction. The prevailing winds in this cell are known as the polar easterlies. OVERVIEW Atmospheric circulation occurs when pressurized air moves around the globe in convection cells, with warmer, less dense air rising from the surface and cooler, denser air descending from the troposphere. Air circulation is also driven by movement from dense, high-pressure areas to low-pressure areas. The vertical and horizontal air movements come together to influence climate and weather conditions in the various parts of the world. For example, land is heated more quickly than water during the daytime hours due to the differences in the specific heat capacity of land and water. Therefore, the air above the land becomes warmer and rises (vertical movement), adding to the atmospheric pressure. Horizontal air flow then moves the pressurized air into lower-pressure areas over the sea, creating less air mass over the land. The cycle perpetuates when the pressurized air over the sea makes its way to the lower-pressure atmosphere near the land, where it heats up again and continues the rotation. The flow is reversed in the evening—land loses heat more quickly than water—creating an opposite current of circulating air. Air moves through the atmosphere under the influence of pressure gradients that propel it from high-pressure areas to low-pressure areas. Horizontal winds that travel long distances appear to follow curved trajectories because of the eastward rotation of Earth. The specific arc is a result of air’s speed of movement and its latitude. For example, a mass of air that is flowing from the equator toward the pole appears to be deflected because the air is moving faster to the east at the equator than at its destination at the pole. This is because a stationary object at the equator completes a path of approximately

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40,000 kilometers in one day because of Earth’s rotation, while an object located at 60 degrees latitude travels only half that distance in the same time. This force is known as the Coriolis effect in honor of the French physicist Gustave-Gaspard Coriolis, who described the phenomenon in 1835. Knowing how air pressure—as well as Earth’s natural forces—relates to air movement is essential for making predictions and preparations about regional climates, ocean currents, storm systems, and wind behavior that can impact the safety, well-being, and livelihoods of people all over the world.

Autogyros Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Also known as gyroplanes, gyrocopters, autogyros, and autogiros, an aircraft that, during most of its flight, derives a substantial part of its lift force from a free-spinning rotor system not provided with any form of direct power drive.

—Shari Parsons Further Reading Barry, Roger G., and Richard J. Chorley. Atmosphere, Weather, and Climate. Routledge, 2009. Henderson-Sellers, Ann, and Kendal McGuffie. “Atmospheric Composition Change: Climate-Chemistry Interactions.” The Future of the World’s Climate: A Modeling Perspective. Elsevier, 2012, pp. 309-66. Leroux, Marcel. Dynamic Analysis of Weather and Climate: Atmospheric Circulation, Perturbations, Climatic Evolution. Springer, 2014. Oliver, John. “Atmospheric Circulation, Global.” The Encyclopedia of World Climatology. Springer, 2005, pp. 126-33. Philander, S. George. “Investigating Atmospheric Circulation.” Our Affair with El Niño: How We Transformed an Enchanting Peruvian Current into a Global Climate Hazard. Princeton UP, 2005, pp. 177-88. Satoh, Masaki. Atmospheric Circulation Dynamics and General Circulation Models. Springer, 2014. Seager, Sara. “Atmospheric Circulation.” Exoplanet Atmospheres: Physical Processes. Princeton UP, 2010, pp. 211-28. Vallis, Geoffrey K. Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-Scale Circulation. Cambridge UP, 2006. See also: Aerodynamics and flight; Aircraft icing; Airplane safety issues; Aviation and energy consumption; Conservation of energy; Fluid dynamics; Greenhouse gases; Pressure; Shock waves; Temperature; Training and education of pilots; Viscosity; Wake turbulence; Weather conditions; Wind shear; Wind tunnels

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KEY CONCEPTS autorotating: rotating without being driven by a motor blade pitch: the angle between a propeller’s chord line and its plane of rotation, also called pitch angle jump-takeoff: a takeoff in which the blades of an autogyro are spun at a high speed then pitched to provide immediate lift, instantly “jumping” the aircraft off the ground lift: the upward pressure experienced by wings due to differential pressure between upper and lower surfaces of the wing as it moves through a fluid medium as determined by its airfoil camber and thickness SIGNIFICANCE Historically, the gyroplane is significant in that its invention preceded that of the helicopter and was largely responsible for the helicopter’s success. The need for hinges at the root of helicopter blades was first successfully accomplished in the gyroplane. Many homebuilt sport gyroplanes exist in the United States and throughout the world. The gyroplane’s success as a homebuilt aircraft is largely due to its simplicity as compared to that of the helicopter.

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Principles of Aeronautics

TERMINOLOGY As recognized by the Federal Aviation Administration (FAA), “gyroplane” is the correct generic term for a type of aircraft that, during most of its flight, derives a substantial part of its lift force from a free-spinning rotor system not provided with any form of direct power drive. The term “gyrocopter” is actually a proprietary name originally used by Bensen Aircraft Corporation to designate its B8-M Gyrocopter. The B8-M was the predecessor of most amateur-built sport gyroplanes. “Autogyro” is an older term, often used for this type of aircraft, but it, too, is actually a proprietary name used by the Autogyro Company of America, which built some of the first gyroplanes. “Gyro” is a nickname applied to all these types of aircraft.

FEATURES The gyroplane is any type of aircraft that relies primarily on an unpowered, freewheeling (or autorotating) rotor as the main source of lifting force and has a separate propeller and engine combination providing forward thrust. Modern gyroplanes look much like helicopters with conventional-appearing tail surfaces. An example of a gyroplane is the Air and Space 18-A. The gyroplane was invented by Juan de la Cierva, an early twentieth-century civil engineer born in Spain. Cierva’s first successful flight was made near Madrid, Spain, on January 9, 1923. The first gyroplane to be certified by the FAA in the United States was the Pitcairn PCA-2 gyroplane, which, at that time, was called an autogyro. Early

A Magni two-seat autogyro. Photo by Mike Burdett from Cromer, UK, via Wikimedia Commons.

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gyroplanes looked much like double-wing aircraft with the top wing removed and replaced with a rotor. They had a conventional engine and propeller in front for forward thrust, a small lower wing for auxiliary lift and control, and conventional-looking tail surfaces, with a large rotor on top of the fuselage for primary lift. The performance features of gyroplanes are a combination of those of helicopters and fixedwing aircraft. The rotor is usually in autorotation, turned by the wind, much like the blades of a windmill that has been turned edgewise to the wind. There must be airflow through the rotor to keep it turning, and thus the gyroplane requires separate forward propulsion to keep it moving through the air. For this reason, gyroplanes cannot hover or take off and climb vertically like a helicopter. Although they can make fully controlled vertical descents, the speed is somewhat high, and landings are not performed in this manner. A gyroplane can fly very slowly and has a very short landing roll. Most gyroplanes temporarily use the engine to spin up the rotor before the takeoff run, thus allowing a very short takeoff roll. Engine power is removed from the rotor just before takeoff. A few gyroplanes also have a jump-takeoff capability. In a jump takeoff, the rotor is oversped with the rotor blades in a low pitch setting while the gyroplane is sitting on the ground. Power is then removed from the rotor and transferred to the forward propulsion system. The blade pitch is then rapidly increased to the normal cruise flight setting, and the gyroplane literally jumps off the ground, perhaps 10 to 15 feet into the air before transitioning to forward flight. Amateur-built sport gyroplanes are often single-seat, open-cockpit aircraft that look much like flying lawn chairs, with a rotor on top and engine, propeller, and tail surfaces in the rear. An example of this type of gyroplane is the Brock KB-2.

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ROTOR SYSTEMS A gyroplane’s rotor “disk” is the tip path plane swept out by the individual rotor blades as they spin. The rotor disk is approximately perpendicular to the rotor shaft. A fundamental difference between the rotor disks of helicopters and gyroplanes is that a gyroplane rotor disk is tilted slightly rearward when viewed from the side. This angle provides both upward lift and rearward drag that must be overcome by the forward propulsion system. The individual rotor blades of a gyroplane are set at a low-pitch angle, which allows them to operate in autorotation. In a helicopter, the rotor disk is tilted slightly forward, providing upward lift as well as a component of the forward thrust for propulsion. The helicopter’s engine spins the rotor and must provide the torque necessary to turn the rotor. Because the rotor blades of a helicopter have a higher pitch angle than those of a gyroplane, they require a power input in order to rotate. A number of different rotor systems are in use in gyroplanes. All gyroplanes must have hinges on the blades where they attach to the rotor hub. In forward flight, the blade moving into the wind (advancing blade) would create more lift than the blade moving away from the wind (retreating blade). Without hinges, this would cause a dissymmetry of lift that would tend to roll the gyroplane over. This was the source of many problems in early attempts at rotary wing flight. Hinging the individual rotor blades allow them to flap up and down slightly as they move into the wind and away from the wind. This equalizes the lift and allows controlled flight. Smaller gyroplanes usually have a rotor system that consists of two blades rigidly attached together. The two blades are hinged to the rotor shaft at their center, much like the pivot on a seesaw, and allow the blades to flap as a unit to equalize the lift. In larger gyroplanes, with three or more rotor blades, each blade is individually hinged to the rotor hub, so the blades can flap up and down slightly as they

Principles of Aeronautics

rotate. Flapping stops designed into the rotor hub prevent the flapping motion from becoming excessive and keep the blades from drooping excessively when the gyroplane is on the ground and the rotor is not turning. PROPULSION The forward propulsion in gyroplanes is usually provided by a conventional reciprocating engine turning a pusher propeller located behind the rotor mast and ahead of the tail surfaces. The pusher propeller arrangement has three advantages over an arrangement with the engine and propeller mounted in front, known as a tractor arrangement. First, the pusher propeller system allows for a more balanced gyroplane design, with cabin and crew weight in front of the rotor mast and the engine weight behind the rotor mast. Second, it provides better forward visibility for the pilot. Third, in the pusher propeller arrangement, the propeller slipstream hitting the tail surfaces provides better directional control and stability. Most early gyroplanes of the 1930s vintage had propellers pulling from the front. Some gyroplanes have rotary Wankel-type engines, and one, the Groen Brothers Hawk 4 Gyroplane has a gas-turbine engine driving a three-bladed propeller. TAIL SURFACES The tail surfaces, or empennage, on a gyroplane are used more for stability than for control purposes. As do fixed-wing aircraft, gyroplanes display a wide variety of tail surface designs. Conventional tail designs, as well as V-tail, H-tail, and triple-tail designs can be found. Vertical stabilizers usually have rudders on them that can be deflected to cause the nose of the gyroplane to yaw to the left or right. Large rudder surface areas are usually used to take advantage of the propeller slipstream to provide yaw control at low forward speeds. Unlike in an airplane, the horizontal tail surfaces of a gyroplane are not usually movable, but rather are fixed surfaces pro-

Autogyros

vided for stability. Because a gyroplane can fly very slowly, relatively large tail surfaces are usually necessary for stability at low speeds. For this reason, it is not uncommon to see double or even triple rudders on a gyroplane, used to increase the total surface area without having a single, excessively large tail. Occasionally a large single vertical fin is used if it is centrally placed in the propeller slipstream. CONTROL SYSTEMS The main flight controls of a gyroplane consist of a joystick, rudder pedals, and a throttle. Variations of these do occur. The throttle controls the engine power output and thus the forward thrust of the propeller, much as in a conventional fixed-wing aircraft. This arrangement is different from that of a helicopter, in which the throttle controls the engine power input to the main rotor, usually operating at a constant rate of revolutions per minute. A gyroplane’s joystick, also called a cyclic stick, controls the tilt of the rotor disk either by tilting the rotor shaft or by individually changing the pitch of the blades as they cyclically rotate (hence, the term cyclic pitch). Tilting the stick to the left effectively causes the rotor disk to tilt to the left, causing a sideward component of rotor thrust that makes the gyroplane turn and bank to the left. Tilting the stick to the right does just the opposite. Pulling back on the cyclic stick tilts the rotor disk more rearward, causing an increase in rotor thrust due to the increased angle of attack to the airflow. This makes the gyroplane climb, assuming that sufficient thrust is produced by the propeller. Pushing forward on the cyclic stick tilts the rotor disk more forward, causing a decrease in rotor thrust and making the gyroplane descend. In essence, the cyclic stick controls the mechanical operation of the main rotor much the same as it would in a helicopter, but because the rotor is unpowered, it causes the gyroplane to respond much like an airplane to similar control stick inputs. Some gyroplanes have

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an overhead stick that requires movement in directions just the opposite of a joystick to control the rotor. Rudder pedals operate the rudder as they would in an airplane, causing a yawing motion from right to left. In a helicopter, the rudder pedals are used to control yawing of the helicopter by changing the tail rotor blade pitch. The gyroplane does not use a collective pitch lever in the same way a helicopter does. Instead, the collective pitch of the gyroplane’s rotor blades is factory preset at an optimum angle for normal flight operation. Gyroplanes that have a rotor prespin or jump-takeoff capabilities will usually have a two-position collective pitch control. One position, with the blades in flat pitch, is used for rotor spinup while on the ground. The other position, for normal flight, is engaged just before starting the takeoff roll or making a jump takeoff. In a helicopter, a collective pitch lever is provided to manually change the pitch of all the rotor blades simultaneously, thus changing the rotor thrust as needed. TYPICAL GYROPLANES The following gyroplanes designed for production have been developed in the United States or Canada: Kellet, Pitcairn, Umbaugh (later designated the Air and Space 18-A), the Canadian Avian, McCulloch J-2, and the Groen Brothers Aviation Hawk 4 Gyroplane. Amateur-built sport gyroplanes can be licensed with the FAA in the experimental category if the aircraft is at least 51 percent amateur-built. A number of companies, including Air Command International, Joe Souza Gyroplanes, Barnett Rotorcraft, Ken Brock Manufacturing, Rotor Flight Dynamics, Rotor Hawk Industries, and Rotary Air Force have developed sport gyroplane kits, which can be assembled in various combinations to suit the homebuilder’s ability. The number of companies in the amateur-built field has proliferated so much that

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one must use care to select a well-proven and time-tested design. —Eugene E. Niemi Jr. Further Reading Harris, Tim. Vertical Takeoff Aircraft from Drones to Jump Jets. Hungry Tomato, 2018. Johnson, Wayne. Rotorcraft Aeromechanics. Cambridge UP, 2013. Maslov, Mikhail. Soviet Autogyros 1929-1942. Helion, 2015. US Flight Standards Service. Rotorcraft Flying Handbook. Washington, DC: US Department of Transportation, Federal Aviation Administration, Flight Standards Service, 2000. Venkatasan, C. Fundamentals of Helicopter Dynamics. CRC Press, 2014. See also: Aerodynamics and flight; Aeronautical engineering; Airplane propellers; Boomerangs; Flight propulsion; Flight roll and pitch; Forces of flight; Helicopters; Homebuilt and experimental aircraft; Plane rudders; Propulsion technologies; Rotorcraft; Igor Sikorsky; Tail designs; Training and education of pilots; Wing designs

Autopilot Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Avionics ABSTRACT The autopilot, a device used to control an aircraft in flight automatically, is also known as an automatic flight control system or integrated flight control system. Autopilots are equipped on large commercial, military, and many small aircraft. By reducing pilot workload, autopilots greatly increase flight safety. KEY CONCEPTS actuator: a device driven by an electric motor or by hydraulic or pneumatic pressure to move something to which it is attached, such as a control surface on an aircraft

Principles of Aeronautics

pitch: the tendency of the nose of an aircraft to move up or down vertically as it moves through a fluid medium roll: the tendency of the body of an aircraft to rotate about its central axis as it moves through a fluid medium servo: a device that converts electrical energy or pneumatic or hydraulic pressure into mechanical motion through an actuator yaw: the tendency of an aircraft to turn horizontally about its center of mass as it moves through a fluid medium NATURE AND USE Many aircraft are equipped with autopilots that will fly an aircraft automatically while the pilot accomplishes other tasks. These systems vary greatly in sophistication, from simple wing levelers to completely integrated flight control systems. The simplest autopilot is a single-axis system. Most single-axis autopilots are designed to control the motion of the aircraft around the aircraft’s longitudinal axis, passing from the front of the aircraft to the rear. When movement around the longitudinal axis becomes unstable, then the aircraft will roll, or tip, from side to side. In its simplest form, the single-axis autopilot may be referred to by pilots as a wing leveler. Upon activation, a wing leveler will stabilize the aircraft by leveling the wings. By adding features such as turn, heading, and navigational control, pilots can use a single-axis system throughout most of the flight. Another common type of single-axis system is known as the yaw damper. This autopilot maintains control of the aircraft around the vertical axis, running through the aircraft from top to bottom. When movement around the vertical axis becomes unstable, the aircraft is considered to be slipping or skidding sideways. This motion is known as yaw. Yaw dampers are designed to prevent slipping and skidding.

Autopilot

A form of autopilot commonly used on medium-sized aircraft is the dual-axis system. A dual-axis autopilot will maintain control of the aircraft around both the lateral and the longitudinal axes. The lateral axis of an aircraft is an imaginary line passing from wingtip to wingtip. Movement around the lateral axis causes the front of the airplane to move up or down. For example, a dual-axis autopilot will be able to keep both the wings and the nose of the aircraft level. Pilots may use the dual-axis system to hold a particular direction, follow commands from a navigation system, maintain an altitude, and climb or descend at a specified rate. The three-axis autopilot is a combination of a dual-axis system and a yaw damper. Airliners and large business aircraft are normally equipped with a three-axis autopilot. Three-axis systems are connected with navigation and flight-management systems. In addition, they may include features such as throttle control and ground steering. INTEGRATION Many autopilots can connect to, or be integrated with, a navigation system. In a single-axis autopilot, this may merely be a connection to the directional gyro. In a complex three-axis system, all of the navigation devices may be connected to the autopilot. In this case, the autopilot could be considered an integrated flight control system. Most integrated flight control systems include a special attitude indicator known as a flight director indicator. In addition to the symbolic airplane and horizon reference line found in most attitude indicators, a flight director indicator includes a special set of needles called flight director, or command, bars. The flight director bars will move up, down, right, and left to indicate where the autopilot intends to fly. Often, these bars are operated by a special computer running in parallel with the autopilot computer. In case of an autopilot failure, the flight director computer will still be able to manip-

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The modern flight control unit of an Airbus A340. Photo by Kiko Alario Salom, via Wikimedia Commons.

ulate the flight director bars. Pilots can manually fly a precise flight path by keeping the bars centered. By allowing the flight director computer to make the complex calculations involved in flying a precise flight path, pilots are still able to reduce their workload. HOW THE SYSTEM WORKS In order to control the aircraft, an autopilot must be able to sense attitude. To do this, autopilots rely on gyroscopic instruments, or accelerometer-based sensors. Often, the attitude gyro is used to transmit information regarding pitch and roll attitude to the autopilot computer. A turn and bank indicator or a turn and slip indicator can be used to supply yaw information. The autopilot computer will compare the

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actual flight attitude of the aircraft with the desired flight attitude and, if necessary, move the appropriate control surface. The device that operates the control surfaces of the aircraft is called a servo. A servo converts electrical energy into mechanical energy. Servos may be electric, hydraulic, or pneumatic. Electric and hydraulic servos are quite common. Electric servos are widely used on aircraft with mechanical or fly-by-wire controls, and hydraulic servos are widely used on aircraft with hydraulic controls. Electric servos contain a small, electric motor. In this type of system, the computer sends a voltage to the servo, causing the motor to rotate. The motor is connected to the aircraft controls, and as the motor turns, the controls are moved.

Aviation and Energy Consumption

Principles of Aeronautics

Hydraulic servos contain a small, electrically controlled, hydraulic actuator. In this type of system, the computer sends a voltage to the actuator. Valves within the actuator channel hydraulic fluid in and out of small cylinders containing pistons. The pistons are connected to the control surface, and, as they move, the surface moves. Pneumatic servos contain electrically operated valves. These valves channel air into bellows that are connected to the aircraft controls. The inflation and deflation of the bellows causes the controls to move. —Thomas Inman Further Reading Alshoubaki, Ahmad. Autopilot Design and Commercial Autopilot Evaluation Using Flybarless Helicopter. American University of Sharjah, 2014. Helfrick, Albert. Principles of Avionics. Avionics Communications, 2000. Nagabhushana, S. Aircraft Instrumentation and Systems. I.K. International Publishing House, 2013. National Aeronautics and Space Administration (NASA). Integrated Autopilot/Autothrottle Based on a Total Energy Control Concept: Design and Evaluation of Additional Autopilot Modes. CreateSpace Independent Publishing Platform, 2018. See also: Aeronautical engineering; Ailerons, flaps, and airplane wings; Airplane cockpit; Airplane guidance systems; Avionics; Stabilizers; Training and education of pilots

Aviation and Energy Consumption Fields of Study: Aeronautical engineering; Mechanical engineering; Economics; Mathematics; Business management ABSTRACT Aviation fuel consumption in the United States reached a total of 30 billion gallons of jet fuel in 2015, according to

the US Department of Transportation. Volatility in fuel prices, scarcity, and increasing environmental concerns are moving the sector toward the use of alternative fuels. KEY CONCEPTS flashpoint: the lowest temperature at which combustible vapors at the surface of a liquid will ignite greenhouse gas emissions: emissions of carbon dioxide, water vapor, hydrocarbons, and soot from combustion, especially at higher altitudes in the troposphere kerosene: a distilled blend of 10 different hydrocarbons with 10 to 16 carbon atoms per molecule WANTING TO FLY Flying has been a constant goal for humankind; evidence of the human ambition to fly can be found in the imagery, mythology, arts, literature, and science of all civilizations. For centuries, different types of inventions—ornithopters, gliders, kites, and other flying devices—were envisioned; most of them were human-powered machines emulating movements and principles from nature. Even sacred scriptures speak of flying chariots in the Bible and vimana in the millenia-old Hindu Vedas. Indeed, there are many who firmly believe the sarcophagus of Pakál in the ancient city of Palenque, in Central America, depicts Pakál at the controls of a rocket ship. Pioneering attempts in modern aviation considered thermal energy for lifting purposes, including the Montgolfier brothers’ hot-air balloon in 1783 and Count von Zeppelin’s airship, which performed its first flight in 1900. Aviators experimented with steam engines in the last quarter of the nineteenth century, but with limited success. More significant developments occurred early in the twentieth century, when Wilbur and Orville Wright designed and operated the world’s first heavier-than-air powered airplane at Kitty Hawk, North Carolina, in 1903. In the summer of 1919, the first nonstop transatlantic flight took place, piloted by the British war pilots

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John Alcock and Arthur W. Brown. These first flights used shaft-based engines fueled by gasoline. Military aviation served as a testing ground for both engines and fuel, and during this time the propeller-driven piston engine, operating on aviation gasoline (Avgas), was perfected. Avgas, with an octane rating of 100, replaced the lower-octane fuel of standard gasoline, which also had a dangerous flashpoint (lower than 1 degree Celsius). However, the invention of the jet engine, in the 1940s, is regarded as the key milestone in aviation history. It enabled faster flights and greatly enhanced aircraft capacity. In the 1950s, the British Overseas Airways Corporation (BOAC) led the jet age of commercial aviation, with civil transport of freight and passengers. Energy consumed by com-

Photo via iStock/Lya_Cattel. [Used under license.]

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mercial aviation since then has mostly been a kerosene-based fuel, and the use of Avgas 1001L became residual. Jet A1 is now the most commonly used civil jet fuel, followed by Jet A, which is sold only in the United States, and Jet B. Military aviation can use other fuels, such as JP4, JP5, and JP8. Aviation fuels are characterized by a high energy density and are required to meet stringent and internationally accepted specifications. In the early years, the vertiginous expansion of aviation prompted international concerns over the need for regulation of this new activity. In 1944, the Convention on International Civil Aviation was signed in Chicago, leading to the creation of the International Civil Aviation Organization (ICAO), an agency of the United Nations and the natural forum

Principles of Aeronautics

for the international community to set rules and to facilitate understanding and cooperation for a homogeneous and consistent legal framework for aviation. ICAO developed a body of regulations, recommendations, and standards to promote security, safety, and air traffic. Later, environmental issues became part of the organization’s agenda, including goals related to fuel efficiency and alternative fuels. Aviation energy demand has steeply increased in the last century, reflecting the globalization process, the democratization of air transport, and the difficulty in substituting other means of transportation for air travel. The Intergovernmental Panel on Climate Change in its special report Aviation and the Global Atmosphere (released in 1999), showed how, in future growth scenarios, improvements in fuel efficiency would be undermined by high levels of sustained energy demand in the sector, with an annual average increase of 3 percent between 1990 and 2015. Since 1970, aircraft fuel efficiency has increased by 70 percent, running at an annual average rate of 4.5 percent until it dropped to 1.2 percent at the beginning of the twenty-first century. In addition to this drop-off in efficiency improvements, growth in demand means that absolute levels of energy consumption in the global aviation industry are on the increase. Air travel demand, measured by revenue per passenger kilometer, increased 6.3 percent per year between 1972 and 2007. The ICAO’s environmental report for 2007 states that almost 4 trillion passenger-kilometers were registered in 2006 and that international passenger traffic is due to increase an annual rate of 5.3 percent for the period 2005-2025. With emerging economies joining the Western model of lifestyle and the boost of trade activities between Asia, the United States, and Europe, energy demand from the aviation sector is due to continue, whereas in other sectors it is expected to decline. In China, demand for aviation has grown an average rate of 14.8 percent per year since the 1990s.

Aviation and Energy Consumption

The world’s leading aircraft manufacturers, the American company Boeing and the European corporation Airbus (founded in 1967 to promote economic and technological progress in European aviation), increased their sales in 2010 even in the context of a world economic crisis. Both companies have been fiercely competitive for their share in the market since the 1990s. Airbus delivered its first plane in 1974 and reached 10,000 orders in January 2011, slightly overtaking its competitor. Sales projections of growth in 2011 are driven mainly by the emerging economies and the boom in airlines that specialize in low-cost transportation. As a pioneering sector for technology, aircraft design and engines are constantly improving to perform more efficiently, and operational measures are revised to reduce fuel consumption and environmental impact. Fuel efficiency is achieved mainly by improvements in jet engines, although it is argued that the last piston engines had a better efficiency index than the first jet engines; other variables, such as airplane design, new materials, load factors, operational measures, airspace control, and airport measures, can also help to reduce fuel consumption. New aircraft, such as the Airbus A380, currently the world’s largest passenger plane, use around three liters of fuel per 100 passenger kilometers. Airlines that operate more short-haul flights tend to record lower fuel efficiencies (measured in liters of fuel per passenger kilometer), because aircraft consume higher amounts of fuel during takeoff and landing. However, aviation is considered to be a mature technology, where opportunities for achieving significant new efficiencies are limited. A technological breakthrough in the short term is unlikely to happen, but some leading aeronautic institutions are looking at a forty-year framework for delivering aircraft designs that could save significant amounts of fuel compared with traditional ones. The price of jet fuel is linked to the price of crude oil, and for the first few years of the twenty-first cen-

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tury it fluctuated around $40-$60 (US) per barrel. However, it soon began to climb. In 2008, when global oil prices reached an all-time high, US-based United Airlines claimed that it spent $173,000 to fuel a Boeing 747 for a single flight from Chicago to Hong Kong, the equivalent of $500 per seat. In 2010 a steeper increase took place, and for the next four years the price per barrel hovered between $120 and $140. In early 2015, however, fuel prices plunged, and returned to the $40-$60 range through the end of 2015 and the start of 2016. During the 1970s, the rise in fuel prices stimulated the industry to develop alternative fuels, and the 2008 fuel price spike forced some airlines into bankruptcy. However, it is important to note that the international airline industry is traditionally exempt from paying tax on jet fuel or, indeed, any environmental tax designed to internalize the external cost, such as taxes for carbon emissions. The combustion of aviation fuel affects the environment through noise pollution, deterioration of local air quality, and a release of emissions into the atmosphere that contributes to climate change and ozone depletion. Globally, the aviation sector is responsible for 2 percent of anthropogenic carbon emissions, while its share in global warming in terms of radiative forcing is far higher, accounting for 3.5 percent in 1992 and predicted to be 5 percent by 2050. This significant rate results from the fact that most fuel emissions from aircraft occur in the troposphere, between 8 and 12 kilometers above sea level, where their impact on the environment is more intensive than those of land-based emissions. The main emissions from aircraft include carbon dioxide (CO2), nitrogen oxides, water vapor, sulfur, and soot particles that also cause the formation of condensation trails (contrails) and the enhancement of cirrus clouds. Although CO2 is the most abundant greenhouse gas emitted by aircraft engines, the Intergovernmental Panel on Climate Change (IPCC) estimates that non-CO2 emissions, excluding the

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formation of cirrus clouds (over which there remain scientific uncertainties), are responsible for at least doubling the climate change impact of aviation. In the European Union (EU), CO2 emissions from aviation in the EU25 increased 73 percent between 1990 and 2004. During the same period, according to Eurostat, energy consumed by air transport within the EU rose by 67 percent, and despite fuel efficiency and technological improvements, the projected growth for the sector would undermine the EU’s effort to curb emissions and fulfill its Kyoto Protocol commitments. If business is conducted as usual, the aviation sector’s contribution to emissions in the EU will become more significant and visible as planned reductions are achieved in other sectors. This means that tackling aviation greenhouse gas emissions is now a priority for the EU, and in 2008 the EU passed legislation to include the sector in the EU Emissions Trading Scheme in 2012 for all flights departing from or arriving in any EU member state. However, this policy is highly controversial, and its validity has been questioned with regard to the fairness of imposing charges on aircraft and the possibility that it could contravene international law. The use of alternative energy to replace or supplement conventional jet fuel can help overcome the major environmental challenges faced by the sector, while limiting the dependence on already scarce and expensive petroleum-based fuels. The future of aviation energy is envisioned to follow the development of low-carbon fuels or even emissions-free hydrogen technology, and the scientific community, industry, and governments are already moving forward to develop alternative fuels. These fuels need to have a high energy intensity per unit of weight and volume, and their feasibility depends on their ability to be drop-in fuels, which do not require modifications in current engines and/or which can be blended with traditional jet fuels. The use of synthetic jet fuel derived from Fischer-Tropsch conversion from coal, natural gas, or biomass to liquid hydrocarbons is

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available typically in a 50 percent blend. This synthetic kerosene can also have a mineral origin but has some environmental and supply advantages when compared with traditional jet fuel. It is hoped that biofuels for aviation will be a viable option in the medium term. These include fermented jet fuel from sugars; hydro-treated renewable fuel produced from algae, jatropha, and camelina crops; and high energy raw materials from second-generation biofuels, which have superior sustainability and life-cycle benefits compared to other biofuels. In recent years, biofuel test flights have taken place. Continental Airlines, for example, used a 50 percent blend of jatropha and algae fuels mixed with kerosene in a Boeing 737 flight in January 2009. An increasing number of partnerships are taking place to develop biofuels, such as one signed in 2010 between Boeing, Air China, PetroChina, and research institutions in both the United States and China. In the long run, sustainable energy for aviation might include other sources, although scientific uncertainties remain high. Such is the case with hydrogen fuel technologies, which present weight and volume incompatibilities with air transport. Other renewable energy options for aviation are being tried and tested, including solar-powered aircraft. For example, Solar Impulse, a fixed-wing solar-powered aircraft, set a world record of more than twenty-six hours of continuous flight under solar power alone, including flight during more than nine hours of darkness. —Beatriz Martínez Romera Further Reading Benito, Arturo, and Gustavo Alonso. Energy Efficiency in Air Transportation. Elsevier Science, 2018. Brown, Nelson, and National Aeronautics and Space Administration. Peak Seeking Control for Reduced Fuel Consumption with Preliminary Flight Test Results. BiblioLife, 2013.

Egenhofer, C. The EU Should Not Shy Away from Setting CO2-Related Targets for Transport. Policy Brief 229. Centre for European Policy Studies, 2011. European Expert Group on Future Transport Fuels. Future Transport Fuels. N.p.: European Commission, 2011. “Jet Fuel Price Development.” IATA, 29 Apr. 2016. Accessed 13 May 2016. Khandelwal, Bhupendra, editor. Aviation Fuels. Academic Press, 2018. Lee, D. S., et al. “Aviation and Global Climate Change in the 21st Century.” Atmospheric Environment, vol. 43, nos. 22-23, 2009, pp. 3520-37. McCollum, David, et al. Greenhouse Gas Emissions from Aviation and Marine Transportation: Mitigation Potential and Policies. Pew Center on Global Climate Change, 2009. Mouawad, Jad, and Diane Cardwell. “Farm Waste and Animal Fats Will Help Power a United Jet.” New York Times, 30 June 2015. Accessed 13 May 2016. National Research Council. Improving the Efficiency of Engines for Large Nonfighter Aircraft. National Academies Press, 2007. Seymour, K., M. Held, G. Georges, and K. Boulouchos. “Fuel Estimation in Air Transportation: Modeling Global Fuel Consumption for Commercial Aviation.” Transportation Research Part D: Transport and Environment, vol. 88, Nov. 2020, p. 102528. Wihbey, John. “Fly or Drive? Parsing the Evolving Climate Math.” Yale Climate Connections. Yale University, 2 Sept. 2015. Accessed 13 May 2016. Wit, R. C. N., et al. Giving Wings to Emission Trading: Inclusion of Aviation Under the European Emission Trading System. CE Delft, 2005. See also: Aerodynamics and flight; Aeronautical engineering; Air transportation industry; Contrails; Dirigibles; Federal Aviation Administration (FAA); Flight propulsion; High-altitude flight; High-speed flight; Hot-air balloons; Jet engines; Montgolfier brothers; Propulsion technologies; Supersonic aircraft; Turbojets and turbofans; Turboprops; Wright brothers’ first flight

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Avionics Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Avionics is a combination of the words “aviation” and “electronics.” Many aircraft cannot fly without avionics. Avionic equipment includes a variety of systems designed to assist pilots, aviation maintenance technicians, and passengers. KEY CONCEPTS autopilot: an electronic computerized system that can carry out the control functions of an aircraft in flight transistor: a silicon-based microelectronic component that can be used to control voltages and currents in printed circuits HISTORY From the time avionics were invented in 1903 until approximately 1930, pilots rarely used them, navigating instead by known landmarks on the ground. In the 1930s, however, engineers began installing communications and navigation equipment in airplanes. The first system designed for airplane navigation was the direction finder (DF), also known as a homing beacon. In the late 1930s, the government began installing the first range stations, which allowed pilots to follow a specific course. Before World War II (1939-1945), electronic equipment was large, heavy, and often required an extra person to operate; therefore, only large aircraft used avionics. During World War II, both Allied and Axis forces developed radio detection and ranging, or radar. In addition, the Allies developed the identification, friend, or foe (IFF) system. The IFF system became the air traffic control (ATC) transponder. Throughout the 1940s, engineers made many improvements

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in the size and reliability of avionics. During the late 1940s and early 1950s, the very high frequency (VHF) omnidirectional range beacon was developed, which was a great improvement to the original range stations. In the 1960s, radios became lighter and smaller, mostly due to the application of the transistor to avionic equipment. The first avionics to use transistors were hybrids, or radios containing both vacuum tubes and transistors. In the 1970s, manufacturers introduced the first reliable solid-state avionics, using semiconductor devices rather than electron tubes. Simultaneously, avionics using digital systems were introduced. These developments allowed for even smaller, lighter, and easier to use systems. Consequently, small personal aircraft of the 1970s were able to have more complex avionics than could the large airliners of the 1950s. The introduction of the microprocessor and database technology in the 1980s created a revolution in the avionics industry. For the first time, pilots could use long-range navigation systems, such as loran-C and Omega, for aircraft navigation. This new technology also allowed for increasingly smaller, lighter, and even easier to use avionics. The 1990s brought the introduction of satellite navigation, known as the global positioning system (GPS). By the end of the decade, the US government decommissioned the Omega navigation system, which GPS had made obsolete. In the early twenty-first century, improvements in microprocessors allowed many more improvements in avionics systems. Three-dimensional (3D) moving map displays and low-cost electronic flight instrumentation are a few of the improvements to come about in the first decade of the third millennium. NAVIGATION Avionics assist the pilot to navigate the aircraft in several ways. Many different navigation systems help

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Radar and other avionics in the nose of a Cessna Citation I/SP. Photo by Dtom, via Wikimedia Commons.

pilots find their way across the globe and locate runways. The automatic direction finder (ADF) indicates the direction of special radio navigation stations and AM broadcast stations. This system receives radio signals in the low- and medium-frequency bands. An indicator in the instrument panel simply points toward the source of the radio signals. The VHF omnidirectional range beacon system provides the pilot with directional information relative to a course. This system receives radio signals in the VHF range from a station on the ground. The system is made up of a radio receiver connected to a device that converts the radio signal to visual infor-

mation. The pilot chooses a bearing to fly, and a special indicator in the panel shows whether the airplane is to the left or right of a course, also known as a radial, that passes through the navigation station. Loran-C provides pilots with long-range area navigation. The name “loran-C” is an abbreviation of “long-range navigation,” with the “C” representing the fact that the current system is the third generation of loran. Originally, loran-C worked as a maritime navigation system; however, with microprocessor and database technology, it became available to pilots. Loran-C does not require the pilot to use a navigation station as a reference point, as do the VHF omnidirectional range beacon and the auto-

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matic direction finder. Instead, the pilot simply chooses an origin and destination within the loran-C coverage area, and the loran-C guides the pilot directly from the origin to the destination. The system consists of a low-frequency receiver, computer, database, and an indicator. The receiver listens for pulses from a set of transmitting stations, and the computer measures the time delay between pulses to determine position. The global positioning system (GPS) provides pilots with a worldwide area navigation system. Although GPS is similar in design to the loran-C, it is much more accurate. Twenty-four GPS satellites orbit Earth and provide pilots with 3D navigation signals. Often, the GPS system will work with a moving map display to show exactly where the airplane is. The system consists of an ultrahigh frequency (UHF) receiver, computer, database, and indicator. The re-

F-105 Thunderchief with avionics laid out. Photo via Wikimedia Commons. [Public domain.]

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ceiver listens for pulses from the satellites, and the computer measures the time delay between pulses to determine position. With wide- and local-area augmentation systems, GPS can be used as the sole means of navigation. The instrument landing system (ILS) gives pilots guidance toward runways and consists of three major components. The first, the aircraft’s localizer transmitter, is integrated with the VHF omnirange. When the pilot selects a special ILS channel, the VHF omnirange system switches to localizer mode. Now, instead of having several courses to choose from, the pilot has only one, which will lead to the end of the runway. The course directing indicator (CDI) will indicate whether the course is to the pilot’s left or right. The second ILS component, the glide slope, provides pilots with vertical guidance to the end of the runway. The glide slope consists of a UHF receiver and circuitry that converts navigation signal information to visual information. When the pilot selects an ILS channel with the VHF omnirange system, the glide slope automatically becomes active and provides information on the CDI to indicate whether the pilot is above or below the proper glide path. The final ILS component, the marker beacon, then turns on a light in the cockpit as the aircraft passes over certain checkpoints during the approach to the airport. A special receiver in the airplane is tuned to 75 megahertz and will listen for special signals from marker transmitters placed along the localizer course. Distance measuring equipment (DME) uses radar principles to measure the distance between the aircraft and special navigation stations on the ground. The DME displays distance, speed, and time to or from the navigation station. The aircraft system consists of a transmitter and a receiver. The UHF transmitter sends pairs of pulses to a ground station, which the ground station then sends back to the aircraft. The DME will measure the time elapsed from

Principles of Aeronautics

when the pulses were sent to when they return and will calculate distance, speed, and time. COMMUNICATION There are many communications systems on board aircraft. In small airplanes and helicopters, the system will consist of a VHF transceiver for the pilot to communicate with air traffic controllers. Similar to a citizen’s band radio, this more powerful system can have up to 2,280 channels. Many aircraft also have an intercom with which to communicate with other crewmembers and passengers. In addition to the VHF transceiver and intercom, some aircraft may have high-frequency transceivers or satellite transceivers to allow long distance communication on transcontinental flights. Although similar in purpose, the design of these two systems is quite different. The high-frequency (HF) transceiver transmits and receives frequencies between 3 and 30 megahertz. Radio frequencies within this range have the ability to stay in the earth’s atmosphere and travel around the world. The satellite system uses UHFs and an antenna that swivels to remain pointed at a communications satellite in orbit above the earth. The signal travels from the airplane to the satellite and is then relayed to any place on Earth. Another communications system is the aircraft communications and reporting system (ACARS), a private, low-speed, digital communications system used by the airlines to communicate between the aircraft and the operations center. Aircraft may also include passenger address systems that allow the pilots to speak to passengers and a radio telephone system that allows passengers to call friends, relatives, and business associates. SURVEILLANCE Air traffic controllers use two systems to track the movements of aircraft: the primary surveillance radar and the secondary surveillance radar. The pri-

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mary surveillance radar uses a powerful transmitter and a large rotating antenna to send strong bursts of microwave energy into the air. The microwave energy reflects off the aircraft, returns to the large antenna, and shows up as a dot on the air traffic controller’s radar display. However, not all aircraft reflect microwaves well, and such aircraft may not show up on the radar display. For this reason, all private and commercial aircraft are required to have special equipment on board that acts as part of the secondary surveillance radar system. Secondary surveillance radar sends a pulse code to a special radio in the aircraft called an ATC transponder. The ATC transponder replies with its own pulse code, which may contain a variety of information, such as altitude, speed range, and assigned codes, that will show on the air traffic controller’s radar screen. Aircraft can also perform surveillance on each other. Airliners and large business aircraft use a system called transponder-based collision avoidance system (TCAS). A TCAS-equipped aircraft sends a pulse code to which other aircraft with ATC transponders reply. A special instrument in the first aircraft displays the location of the second, indicates collision threats, and recommends a flight direction to avoid collision. Many aircraft are equipped with weather-surveillance systems. These come in two varieties, active and passive. The active system uses radar. Mounted in the nose of the aircraft, the antenna points forward and sweeps back and forth. The radar transmits energy in front of the aircraft, and water droplets reflect this energy back to the radar antenna. Rain will display on a screen in the instrument panel of the aircraft. The passive weather-surveillance system uses a special loop antenna to detect the electrical activity associated with thunderstorms and air turbulence. The activity is shown on an indicator in the instrument panel of the aircraft. Both systems help pilots

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avoid dangerous weather and, in some cases, can be combined into a single, comprehensive weatheravoidance system. AUTOPILOTS Many aircraft are equipped with autopilots, which fly an aircraft automatically while the pilot accomplishes other tasks or simply rests briefly. The simplest autopilot is a single-axis system. The single-axis autopilot controls the airplane on only one of the axes of flight. For example, a wing leveler will keep the wings level, but the pilot will be responsible for keeping the nose level, and keeping the tail in line. The dual-axis autopilot controls two axes of flight, keeping both the wings and the nose of the aircraft level, for example. The three-axis autopilot maintains control of the aircraft in all axes or directions. Often, two- or three-axis systems are interconnected with navigation and flight-management systems, and may include features such as throttle control and ground steering. In these cases, the autopilot is considered an integrated flight-control system. PASSENGER ENTERTAINMENT AND CONVENIENCE There are many systems designed for passenger entertainment and convenience. Many aircraft have special telephones that passengers may use to make telephone calls. In addition, multichannel sound systems deliver several styles of music from which passengers may choose. In larger airliners, video systems allow passengers to watch movies or play video games. In some aircraft, passengers can keep track of the flight’s progress by viewing a moving map display. In addition, business jets may have a local area network, printers, and modems to allow passengers to work while in flight. —Thomas Inman

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Further Reading Avtin, Igor Victorovich, Vladimir Ivanovich Baburov, Boris Victorovich Ponomarenko, and Yuri Gregorievich Shatrakov. Principles of Integrated Airborne Avionics. Springer Singapore, 2021. Collinson, R. P. G. Introduction to Avionics Systems. Springer US, 2013. Kenney, Scott. Avionics-Fundamentals of Aircraft Electronics. Avotek Information Resources, 2013. Spitzer, Cary R. Avionics Elements, Software and Functions. CRC Press, 2018. Spitzer, Cary R., editor. Avionics Development and Implementation. CRC Press, 2018. Wang, Guoqing, and Wenhao Zhao. The Principles of Integrated Technology in Avionics Systems. Elsevier Science, 2020. See also: Airplane cockpit; Airplane guidance systems; Airplane radar; Autopilot; Flight instrumentation; Flight recorder; Flight simulators; Training and education of pilots

Avro Arrow Fields of Study: Aeronautical engineering; Mechanical engineering ABSTRACT The Avro Arrow was a Canadian-designed jet interceptor, test-flown on March 25, 1958. The Avro Arrow had the potential to become the greatest jet fighter-interceptor of its day, far superior to any fighter aircraft that existed anywhere in the world at that time and even in the present day, but the government of Prime Minister John G. Diefenbaker canceled the project. It is a bitter memory among Canadians to this day. KEY CONCEPTS ceiling: the maximum altitude at which an aircraft is designed to fly fire-control system: an electronic system that controls the functioning of an aircraft’s armaments

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The first Avro Arrow, RL-201, is officially rolled out on 4 October 1957. Photo via Wikimedia Commons. [Public domain.]

THE CALL In April 1953, the Royal Canadian Air Force (RCAF) established requirements for a new and highly sophisticated jet interceptor to replace its aging, subsonic CF-100 fighters. Such an aircraft would have to fly at twice the speed of sound, have an operational ceiling of 58,000 feet, and employ state-of-the-art missiles and fire-control systems to facilitate its role as a bomber destroyer. An estimated six hundred machines were considered necessary to protect North America from attack by Soviet aircraft flying over the North Pole.

The RCAF established rigid specifications for the project, and by 1957, the firm of Avro Canada had finalized design of the CF-105, unofficially known as the Arrow. The Arrow was a large delta-winged fighter, powered by twin turbojet engines. The first Arrow was test-flown on March 25, 1958, and it stunned the aviation world with its many sterling qualities. In fact, this aircraft placed Canada at the forefront of military aircraft design and was considered a source of national pride. However, the plane is claimed to have also experienced problems and was viewed by the Conservative government of

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Prime Minister John G. Diefenbaker as prohibitively expensive. On February 20, 1959, Diefenbaker summarily canceled the entire project and ordered the five prototypes and all related materials destroyed. IMPACT Beyond depriving Canada of one of the world’s greatest aircraft, the Arrow’s demise solidified perceptions that Diefenbaker was soft on defense matters and contributed to his eventual defeat at the polls. Moreover, despite the Diefenbaker administration’s attempts to distance Canada from the foreign policies of the United States, the cancellation of the Avro Arrow meant that Canada was left to rely on American-built BOMARC (Boeing Michigan Aeronautical Research Center) surface-to-air missiles, a path which ultimately required Canada to rely on its southern neighbor. Interest in the Avro Arrow has never died among its many admirers, and there have been many searches, some successful, in efforts to locate and retrieve models of the aircraft that had been tested over Lake Ontario. In addition, there are substantiated reports that not everything to do with the Arrow project was actually destroyed. There are claims by some that copies of the actual blueprints of the aircraft were secreted away by a small number of engineers who worked on the project. There are also claims that at least two of the engines that were destined for the latest version of the Arrow airframe were taken away and put into closely guarded storage by other Arrow engineers. While none of these items have definitively come to light, suggesting that the claims are not true, it is reasonable to think that the claims are indicative

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of a strong desire among Arrow proponents to see the project renewed and the Arrow resurrected from the graveyard of aeronautic history as the pride of Canadian aviation. Given the capabilities that the Arrow displayed in its all-too-brief lifetime, one can only wonder what that aircraft would be like today if it were to be rebuilt with modern engines, avionics, materials, and computers. One interesting footnote of the Arrow story is that after the project had been canceled, many of the engineers who had worked on the project made their way to the United States and were instrumental in the work of the fledgling Administration known today as the National Aeronautics and Space Administration (NASA). The spirit of the Arrow, if not the actual aircraft, continues to fly. —John C. Fredriksen Further Reading Campagna, Palmiro. Requiem for a Giant: A. V. Roe and the Avro Arrow. Dundurn Group, 2003. Campagna, Palmiro. The Avro Arrow: For the Record. Dundurn Press, 2019. ———. Storms of Controversy. The Secret Avro Arrow Files Revealed. 4th ed., Dundurn Press, 2010. Gainor, Chris. Who Killed the Avro Arrow? Folklore Publishing, 2007. Pearce, Nigel. The Avro CF-105 Arrow. United p.c. Verlag, 2013. Peden, Murray. Fall of an Arrow. Stoddert, 2001. Smye, Fred. Canadian Aviation and the Avro Arrow. CreateSpace Independent Publishing Platform, 2014. See also: Aerodynamics and flight; Aeronautical engineering; High-speed flight; Homebuilt and experimental aircraft

B Daniel Bernoulli Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics; Acoustics ABSTRACT Daniel Bernoulli was born February 8, 1700, in Groningen, Netherlands. He died March 17, 1782, in Basel, Switzerland. Eighteenth-century Swiss mathematician Daniel Bernoulli is best known for his work in the field of fluid dynamics, particularly the Bernoulli equation. His book Hydrodynamica gave the field its original name. Bernoulli also worked in the fields of physics and acoustics.

At the age of thirteen, Bernoulli was sent to the University of Basel to study philosophy and logic. He also excelled in mathematics and studied calculus under his older brother Nicolaus, their father having made significant discoveries in that discipline. At the age of sixteen, Bernoulli earned a Master’s degree from Basel. Despite his skill and interest in mathematics, however, his father forbade him to pursue a career in the field. Johann Bernoulli tried at first to force his son to become a merchant, but Bernoulli refused. Johann then told his son to study to become a doctor. Bernoulli obliged, traveling to

KEY CONCEPTS fluid dynamics: the science of the motion and properties of fluids Fourier analysis: application of mathematical principles that simplify a complex waveform as a single expression in sines and cosines hydrodynamics: the science of the motion and properties of water, the forerunner of the broader science that is fluid dynamics EARLY LIFE Daniel Bernoulli was born into a family of prominent Swiss mathematicians in Groningen, Netherlands, on February 8, 1700. His father, Johann Bernoulli, was a professor of mathematics at the University of Groningen, and his uncle Jakob Bernoulli was the chair of mathematics at the University of Basel, Switzerland. In 1705, when Bernoulli was five years old, his father took over his uncle’s position and the family moved back to Switzerland.

Portrait of Daniel Bernoulli, c. 1720-1725. Image by Bammesk, via Wikimedia Commons.

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Italy to study medicine. He finished his studies in 1721. Unable to secure a teaching position, he continued to study medicine as well as mathematics. While studying mathematics in Italy, Bernoulli wrote a treatise on probability and fluid motion. Published in Venice in 1724, Exercitationes quaedam mathematicae (Certain mathematical exercises) brought Bernoulli immediate recognition. He was offered a position as a professor of mathematics at the Saint Petersburg Academy of Sciences in Russia, where his brother Nicolaus also accepted an offer to teach mathematics. Before moving to Saint Petersburg in 1725, Bernoulli won first prize from the French Académie Royale des Sciences (Royal Academy of Sciences, now the French Academy of Sciences) for his essay on the best shape of hourglass to use on ships. It was the first of ten prizes he would win from the Academy. LIFE’S WORK Bernoulli created the basis for his advances in mathematics, probability, and physics while teaching mathematics in Saint Petersburg. In addition to establishing him academically, his Exercitationes contained the origins of his exploration into fluid dynamics and probability. In 1726, he outlined the parallelogram of forces; the next year, he began regularly corresponding and collaborating with his friend Leonhard Euler, one of his father’s pupils. While trying to learn more about the flow of blood with Euler, Bernoulli developed a way of measuring blood pressure. This involved sticking a tube in an artery and measuring the height at which the blood filled the tube. The method became so popular that it was used throughout Europe for approximately the next 170 years. Bernoulli’s method was later borrowed to measure airspeed. Although Bernoulli found success in Russia, he was not happy there, and he left after eight years. Upon his return to Switzerland in 1733, he took a position at the University of Basel teaching botany,

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despite his lack of fondness for the subject. He continued working in other fields as well, such as mechanics and mathematics; in 1737, for example, he delivered a lecture on calculating the work done by the heart. The next year, Bernoulli published his seminal Hydrodynamica (Hydrodynamics, 1738), establishing the field of hydrodynamics. This far-reaching work contained his famous fluid flow equation, called the Bernoulli equation, from which the Bernoulli principle was derived. The principle relates flow, speed, pressure, and potential energy. Hydrodynamica laid the foundation for all later work in hydrodynamics and aerodynamics, referred to collectively as fluid dynamics. Bernoulli devised a number of experiments to demonstrate his theories. He also examined gas pressure, positing that it was composed of fast and randomly moving particles. His analysis confirmed Robert Boyle’s 1660 gas law, which states that pressure multiplied by volume remains constant when the temperature does not change. This perspective paved the way for later studies, such as heat transfer. Bernoulli also published a paper in 1738 that detailed the best shape for a ship’s anchor. The paper won a prize from the Royal Academy. That same year, he published Specimen theoriae novae de mensura sortis (“Exposition of a New Theory on the Measurement of Risk”), in which he investigated the Saint Petersburg paradox as a base for risk analysis and utility investigation. The Saint Petersburg paradox is a probability theory based on the Saint Petersburg gambling game, in which a player flips a coin until the head side appears. The winnings are two guilders (or two dollars) if the head appears on the first toss, four if on the second, eight if on the third, and so on ad infinitum. The probability of winning decreases by half for each flip of the coin: a 50 percent chance the first time, 25 percent the second time, and so on. The paradox is how much a player would be willing to pay to play the game. Bernoulli pro-

Principles of Aeronautics

posed that the solution was not to calculate the expected winnings but instead to calculate by a utility function, determining how useful the winnings would be in comparison to the player’s wealth. In 1743, Bernoulli became a professor of physiology at the University of Basel. He used this opportunity to research subjects such as muscular contraction and the optic nerve. Seven years later, in 1750, he obtained the chair of experimental and speculative philosophy, now called theoretical physics. He was a very popular lecturer and continued to apply mathematics to physical phenomena. Bernoulli was also elected a fellow of the Royal Society of London in 1750. Around this time, Euler and Bernoulli collaborated on the study of beam bending—that is, the sagging of a structure due to stress—and created a system later known as the Euler-Bernoulli beam equation. Their equation became the mathematical base for structural engineering projects such as the Eiffel Tower. Bernoulli also analyzed kinetic energy, which at the time was called vis viva, or living force. He posited that vis viva was conserved across the entire universe, anticipating the law of energy conservation, though he lacked the tools to prove it empirically. As part of a scholarly dispute with Euler, Bernoulli investigated sound. He found that physical objects tend to vibrate at certain proper or natural frequencies. He named the lowest frequency the fundamental frequency and called the higher frequencies overtones. He also discovered that increases in frequency cause an increase in the number of nodes, or points, with no vibration. Bernoulli then built a mathematical framework around his findings, which were confirmed by Jean-Baptiste-Joseph Fourier’s work on harmonics in the early 1800s. Much of Bernoulli’s later work involved the application of probability to disparate fields, including birth rate and inoculation. In one study in 1766, he used smallpox morbidity and mortality rates to illustrate the effectiveness of inoculation. In 1776,

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Bernoulli retired from teaching. He died in Basel, Switzerland, on March 17, 1782. IMPACT Bernoulli’s contributions to mathematics influenced numerous later developments, leading to improvements ranging from better sound quality in MP3s to stealthier submarines. He is considered the father of fluid dynamics, and his work on fluid flows is an integral part of the science used in the design of travel vessels, including airplanes, cars, and ships. Bernoulli’s fluid flow equation led to advances resulting in the modern practice of building ships based on model design, a process pioneered by naval architects such as William H. Froude (1810-79) and David Taylor (1864-1940), who then used fluid dynamics to predict the behavior of the full-size ship. Before these architects developed the idea of building ships in miniature first, the ships had to be built full scale before they could be tested. Applied to aeronautics, Bernoulli’s principle of fluid dynamics was essential in the development of the first airfoils. An airfoil is the part of a travel vessel, particularly an airplane’s wing or propeller, that is designed to give the vessel speed in relation to the surrounding air pressure. The faster an airplane travels, the more lift it can achieve. Because Bernoulli’s approach worked, his equations were expanded upon, and they became the basis for a set of equations governing pressure. Initially used for low speeds, Bernoulli’s equations were extended to all velocity ranges, including modern hypersonic flight. In addition to airplanes and submarines, Bernoulli’s equations became important to the automobile industry, enabling the production of faster and more fuel-efficient cars. Additionally, his application of probability to physics provided better definitions to temperature and other such fundamental ideas, allowing for more accurate descriptions and further work in the various fields of physics. For example, the field of thermodynamics, which studies

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the flow of heat as a group of excited particles, uses Bernoulli’s conjectures. —Gina Hagler Further Reading Baigrie, Brian S. The Renaissance and the Scientific Revolution: Biographical Portraits. Scribner’s, 2001. Bernoulli, Daniel, and Johann Bernoulli. Hydrodynamics and Hydraulics. Trans. Thomas Carmody and Helmut Kobus. Dover, 2005. Chakrabarti, Subrata K. The Theory and Practice of Hydrodynamics and Vibration. World Scientific, 2002. Hanlon, Robert T. Block by Block: The Historical and Theoretical Foundations of Thermodynamics. Oxford UP, 2020. Tent, M. B. W. Leonard Euler and the Bernoullis: Mathematicians from Basel. CRC Press, 2009. Thakur, Rajesh. Daniel Bernoulli. Prabhat Prakashan, 2021. See also: Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Airplane propellers; Forces of flight; Tail designs; Viscosity; Wind shear; Wing designs

Biplanes Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT A biplane is an airplane with two levels of wings, roughly one above the other. Most early aircraft utilized the biplane configuration, and biplanes remain popular for sport flying and aerobatics. KEY CONCEPTS decalage: the difference between the angles of the upper and lower wings of a biplanes interplane struts: structural components, typically of wood, between the upper and lower wings of a biplane or triplane to provide structural strength and stability

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negative stagger: the condition in which the lower wing of a biplane is mounted farther toward the nose than is the upper wing positive stagger: the condition in which the upper wing of a biplane is mounted farther toward the nose than is the lower wing wing chord or chord line: a straight line from the leading edge to the trailing edge of an airfoil REASONS FOR THE BIPLANE CONFIGURATION From the early, pioneering flights of Orville and Wilbur Wright in 1903 through the 1920s and 1930s, biplanes represented the most practical aircraft configuration for both structural and maneuverability reasons. By the 1940s, they remained a practical choice only for training aircraft. Since then, biplanes have retained a certain popularity as sport and aerobatic and air show aircraft. Until sufficiently light and powerful aircraft engines were developed, a large wing area was required to keep an aircraft aloft, and the biplane structure provided the most strength with the least weight. It was initially thought that thin wing sections were necessary for efficient generation of lift. In a biplane configuration, interplane struts and wire bracing provide a bridge-like strength and rigidity to the wing. Biplanes can use lesser wingspans, and both wings can use ailerons, resulting in the added advantage of maneuverability. Thus, for the first few decades of flight, biplanes were the configuration of choice for training aircraft, sport aircraft, military fighters, military bomber aircraft, and transport aircraft. FAMOUS BIPLANES America’s best-known aircraft during World War I was the Curtiss JN “Jenny” trainer. It used a four-bay wing with eight interplane struts and many bracing wires, but it could fly two people with only a 90-horsepower OX-5 engine. After the war, Jennys

Biplanes

Principles of Aeronautics

First World War Sopwith Camel biplane. Photo by Airwolfhound from Hertfordshire, UK, via Wikimedia Commons.

were declared surplus and became the barnstormer’s choice of airplane. The most famous World War I fighters were biplanes. In England, the De Havilland Tiger Moth was the trainer of choice between the world wars. In the 1930s, the Curtiss P-6E Hawk fighter biplane delighted the eye. In World War II, the best-known US trainers were the Piper Cub monoplane and the Boeing-built Stearman PT-17 biplane. The Stearman had a reputation for indestructibility in the air and remains a popular sport biplane. When the Stearman was declared to be surplus after the war, it became the favorite of crop dusters, who took advantage of its great strength and load-carrying ability. It also survives as a popular sporting aircraft. The biplane flowered in the interwar period. Travelair, which began producing biplanes in 1925,

bettered the Jenny in control, comfort, speed, and safety. The Travelair D-4D is arguably the best-looking open-cockpit biplane ever built. During the 1920s and 1930s, the Waco Aircraft Company of Troy, Ohio, was by far the largest airplane manufacturer in the United States, building thousands of open-cockpit and cabin biplanes. The first Waco biplanes were built in 1922, but the Waco 9 appeared at the same time as the first Travelair and was highly regarded. The Waco Taperwing, using a tapered wing planform on both wings, remains popular. DISADVANTAGES OF THE BIPLANE CONFIGURATION One disadvantage of the biplane is related to the extra drag of its wires and supporting struts and the interference drag between its two wings, which result

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in reduced cruising and top speeds for a given engine power. Another disadvantage is a poor lift-to-drag ratio that results in poor glide angles. By the 1920s, the most efficient aircraft were monoplane designs, such as Charles A. Lindbergh’s Spirit of St. Louis. A monoplane is simpler in design and less costly to build. When aircraft designers learned how to make strong, internally braced aircraft entirely of aluminum, and when powerful and relatively light engines became available, the monoplane replaced the biplane as the configuration of choice for all high-speed aircraft. The primary lifting surface of a wing is its upper surface, so the lower wing suffers the most from this; the gap between the wings is therefore usually made at least as large as the wing chord. If the wings are set at different angles (decalage), the relative loading of the wings and stall characteristics can be adjusted. Often the upper wing is mounted ahead of the lower wing, an arrangement known as positive stagger. This is particularly true for open-cockpit biplanes in which the front cockpit is under the wing and the rear cockpit, for stability reasons, is not placed too far back on the fuselage. However, the famous Beechcraft Staggerwing has a closed cabin and uses negative stagger. A biplane that has a smaller lower wing is known as a sesquiplane. Most biplanes use the lighter tailwheel configuration for their landing gear, but the higher center of gravity and the poor view for the pilot upon landing mean that the directional instability of the tailwheel configuration requires significantly more pilot alertness and skill. Usually, the lower wing has a dihedral angle to provide lateral stability and to keep the tips farther from the ground, whereas the upper wing is straight, to simplify its construction. SPORT, AEROBATIC, AND AIR-SHOW BIPLANES The biplane configuration has long been preferred for aerobatics because of its inherently good roll rate

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and because the extra drag of brace wires and struts prevents a rapid buildup of speed in the diving aspect of maneuvers. The 1920s and 1930s Great Lakes Trainer biplane was considered the best aerobatic aircraft of all US-manufactured aircraft until the arrival of the Pitts Special. When the Great Lakes Trainer was first flown, it was found that its center of gravity was too far aft. This problem was corrected most simply by giving the upper wing rearward sweep. This correction had the side benefit of making the airplane a better snap-roll performer. Biplanes remain favored aircraft for many air-show pilots, because of their extra visibility to spectators and because of the additional possibilities for wing-walkers. Only in the last decade of the twentieth century did monoplanes begin to dominate aerobatic competition at the highest levels. The appeal of the open-cockpit biplane, a sort of motorcycle of the air, will live on indefinitely, as pilots feel the sheer joy of flying between two wings in warm summer air. —W. N. Hubin Further Reading Anderson, Dale, Ian Graham, and Brian Williams. Flight and Motion: The History and Science of Flying. Taylor & Francis, 2015. Johnson, Wray R. Biplanes at War: US Marine Corps Aviation in the Small Wars Era 1915-1934. UP of Kentucky, 2019. Ministry of Munition, Aircraft Production. Report on the L.V.G. Two-Seater Biplanes, September 1918: Reports on German Aircraft 16. Naval & Military Press, 2014. Smith, Peter C. Combat Biplanes of World War II. Pen & Sword Books, 2015. Travel Air Manufacturing Company. The Story of Travel Air, Makers of Biplanes and Monoplanes. Periscope Film LLC, 2012. See also: Aerobatics and flight; Aerodynamics and flight; Aeronautical engineering; Airplane manufacturers; Forces of flight; Heavier-than-air craft; Military aircraft; Billy Mitchell; Monoplanes; Eddie Rickenbacker; Training and education of pilots; Triplanes; Types and structure of airplanes; Ultralight aircraft

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Blimps Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT Also known as airships, nonrigid or pressure-airships, dirigibles, or balloon-dirigibles, blimps are lighter-than-air, pressurized airships, comprising an elliptical, gas-filled bag, a means of propulsion, a means to control buoyancy and flight, and one or more gondolas to hold crew, passengers, the power unit, and cargo. The nonrigid airship was the first form of controlled human flight, and the blimp was the last airship to be used in wartime. KEY CONCEPTS dirigible: a lighter-than-air craft whose direction of motion can be controlled gondola: the crew, freight, and passenger cabin carried below an airship DEVELOPMENT The early days of aviation witnessed a competition between two very different vehicles: the heavier-than-air airplane and the lighter-than-air airship. Although the airship initially prevailed, it would, by the 1930s, be largely replaced by the airplane. Airships, however, continue to perform functions that are beyond the capabilities of airplanes. Airships evolved from the free, or hot-air, balloon, first launched in 1783 near Lyons, France, by Jacques-Étienne and Joseph-Michel Montgolfier. This balloon would soon be modified. Henry Cavendish, a British chemist, found that hydrogen gas was at least seven times lighter than air, and by 1785, French army engineer Jean Baptiste Marie-Meusnier designed a bag of an ellipsoidal shape. French inventor Henri Giffard took these notions, added mechanical propulsion and steering, and flew a dirigible-balloon, named from the Latin dirigere,

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“to steer,” on September 24, 1852. This 143-footlong airship, driven by a screw propeller rotated by a 3-horsepower steam engine, traveled at the speed of 10 miles per hour. It was the first successful flight of an airship. Thirty-one years later, the Tissandier brothers, Albert and Gaston, built an electrically powered, 37,000-cubic-foot airship. On August 9, 1884, Charles Renard and Arthur Krebs piloted the 66,000-cubic-foot, electrically driven La France for 5 miles, returning safely to the point of departure. A Brazilian aeronaut, Alberto Santos-Dumont, who “mused on the exploration of the aerial ocean,” launched a series of fourteen airships in France before 1905. His airship Number 6 made headlines when it successfully circled the Eiffel Tower. The eccentric Santos-Dumont popularized airships by parking them over the rooftops of his Parisian hosts and descending to join them for dinner. DESIGN An airship has five crucial components: an elliptical bag filled with either hydrogen or helium and covered with a strong, light “envelope” (an outer skin initially made of cotton and rubber; today synthetic fabrics are used); a means of propulsion, using propellers and engines powered by fuels ranging from steam and electricity to gasoline; control of buoyancy attained by releasing ballast for ascent, gas for descent; flight control, with the pilot using vertically hinged rudders for steering, horizontally hinged elevators for lift; and one or more gondolas for crew, passengers, the power unit, and cargo. There are three classes of airship. One is the nonrigid, or pressurized, airship. Without a metal frame, the bag collapses when the gas is released. During World War I, this type of airship was the most common in the Royal Navy and gave rise to the slang term “blimp,” which took its initial “b” from “British Class B Airship,” and “limp” from its nonrigid nature.

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Another type of airship is the semirigid, in which, to maintain the form, gas pressure acts in conjunction with the longitudinal keel. A third type is the rigid airship, or zeppelin, named for the German count, Ferdinand von Zeppelin, who perfected it. With a skeleton, it retains its shape when deflated. USE The Germans stressed the rigid, the British the nonrigid type. During World War I, the Germans had some sixty-seven zeppelins flying a variety of missions. The British Navy favored blimps, deploying over two hundred of them for submarine and mine detection, aerial observation, coastal patrols, scouting, and escorting troop and merchant vessel convoys. Following World War I, rigids were preferred. That popularity ended dramatically when Germany’s pride, the Hindenburg, perished in fire at Lakehurst, New Jersey, in 1937. The United States, fortunately, had not abandoned blimps. By 1930, the Goodyear Tire and Rubber Company had a fleet

Photo via iStock/luismmolina. [Used under license.]

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of twelve blimps, used primarily for advertising. The only nation to make effective use of blimps in World War II, the United States had a fleet of 150 of them, serving in fifteen airship squadrons on three continents, patrolling 7,680,000 square kilometers. The first nonrigid airship crossing of the Atlantic occurred from May 29 and June 1, 1944, when a US Navy blimp squadron made the 5,032-kilometer flight from South Weymouth, Massachusetts, to Port Lyautey, French Morocco. Blimps proved effective in detecting German submarine wolfpacks. Not a single blimp-escorted convoy lost a ship. Only one blimp was downed by enemy fire. During the Cold War, blimps were of value not only for coastal patrols, but also as an early-warning device against piloted bomber flights. In 1958, the US Navy commissioned a series of four ZPG-3W airships, each 146 meters in length, 30.8 meters in diameter, with a capacity of 42,475.27 cubic meters. These were the largest blimps ever. When, after 1962, the piloted bomber gave way to the intercontinental ballistic missile, the value of blimps declined.

Principles of Aeronautics

By the 1990s, however, there was a renewed interest in blimps. From a commercial standpoint, they could carry passengers and cargo cheaply and efficiently. Television networks used them for aerial views of sporting events. Advertising (as with the well-known Fuji and Goodyear blimps) was profitable. The recreational use of airships was appealing. Synthetic fibers, computer-aided design, and enhanced engineering led to such “super blimps” as the Sentinel 5000 launched in 1997. It had a three-story pressurized gondola. Virtually impervious to weather (icing, snow, sleet, rain, fog, hail) and radar, it traveled in excess of 60 miles per hour. Blimps, because of their range, fuel efficiency, low cost of development and maintenance, capacity for in-flight refueling, and lack of negative environmental impact, proved attractive to both military and civilian agencies for a variety of surveillance work. The blimps promise to have a long and useful future. Further Reading Dick, Harold G., and D. H. Robinson. The Golden Age of the Great Passenger Airships. Reprint. Smithsonian Institution Press, 1992. Krisha, K. R. Aerial Robotics in Agriculture. Parafoils, Blimps, Aerostats, and Kites. Apple Academic Press, 2021. Rechs, Robert J. A Practical Guide to Building Small Gas Blimps. CreateSpace Independent Publishing Platform, 1998. See also: Aeronautical engineering; Air transportation industry; Dirigibles; First manned balloon flight; Flight balloons; Gravity and flight; Hindenburg; Hot-air balloons; Lighter-than-air craft; Montgolfier brothers; Wind shear

Boomerangs Fields of Study: Physics; Aerodynamics; Mathematics ABSTRACT A boomerang is a curved, multiwinged projectile which, when properly thrown, returns nearly to the point from

Boomerangs

which it was thrown. Originally an offensive weapon and hunting tool by Australian Aborigines, boomerangs are used now primarily as a toy and in sport. The boomerang’s unique wing configuration generates flight characteristics that return the projectile back to the point from which it was originally thrown. KEY CONCEPTS aerodynamic forces: lift, drag, gravity and angular momentum for rotating objects in flight discus: a circular weighted disk of wood with a metal rim, constructed so that the radial cross-section resembles an airfoil, making the entire shape rather like a circular wing when spinning through the air killer-stick: essentially a heavy hardwood stick shaped to an airfoil cross-section that could be thrown horizontally with deadly effect sufficient to decapitate an adult kangaroo EVOLUTION OF THE BOOMERANG The boomerang originated in Australia and was used as a hunting tool by the Aborigines. Although the boomerang is often thought of as a weapon, it has primarily served as a recreational and sport toy. The killer-stick, believed to be the predecessor of the boomerang, was used for hunting and fighting. The killer-stick has a similar shape and shares many of the boomerang’s properties with one important difference: the killer-stick does not return to the thrower. The stick was smoothed, sanded, and shaped to provide an airfoil cross-section like a wing and could be thrown fast, far, and with great accuracy. Like many other sports projectiles, such as the discus, the killer-stick was thrown with rotational spin stabilizing its flight path. The boomerang, which is smaller, lighter, and has a more pronounced separation of wings than the killer-stick, was not used to kill game, but to trap birds. An Aboriginal hunter would imitate a hawk’s call and throw the boomerang over a flying bird

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The evolution of the boomerang. Image by Augustus Henry Lane-Fox Pitt-Rivers, Sir John Linton Myres,and Henry Balfour, via Wikimedia. [Public domain.]

flock. To evade the hawk, the flock would swoop down into the hunter’s waiting nets. SHAPE AND CONSTRUCTION The boomerang is composed of two connected wings. The point of connection is called the elbow. There is a front, or leading, wing and a rear, or trailing, wing. The elbow separates the two wings at an angle generally ranging between 105 and 110 degrees. Each of the boomerang’s wings has a traditional airfoil cross-sectional shape with a leading and trailing edge. As with any other flight vehicle, the leading edge strikes the air first and the air flows

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over the top and bottom of the wing, past the trailing edge. Unlike a bird’s or aircraft’s wings however, the two boomerang wings are not mirror images of one another. One is slightly larger than the other so they generate slightly different amounts of lift in flight. Thus, there are right-handed and left-handed boomerangs. When thrown vertically into the wind, the upper wing’s leading edge is located on the inner concave portion of the boomerang. The lower wing’s leading edge is on the outer convex portion. Air first strikes the upper leading edge. As the boomerang rotates, this allows the lower wing’s leading edge to meet and strike the air.

Principles of Aeronautics

AERODYNAMIC FORCES AND STABILITY Common to all sports projectiles, the aerodynamic forces acting on the boomerang are lift, drag, and gravity, or the boomerang’s own weight. The spin imparted to the boomerang stabilizes the flight path. When a boomerang is thrown correctly, these forces cause the boomerang to circle around and return. As the boomerang flies through the air, each wing produces lift. Although the shape of the wing generates lift, the lifting force is not enough to sustain the boomerang’s flight. A boomerang is thrown with spin similar to that of a discus. Without spin, a boomerang will wobble and fall to the ground; the boomerang’s flight is not stable. Airplanes and birds have tail configurations that provide stability, while the rotational spin of a boomerang stabilizes its flight and produces a curved flight path. Stabilizing effects of spinning also are observed in a toy top and a bicycle wheel. The turning force produced is a result of the unequal airspeeds over the spinning wings. The wings of a stationary, spinning boomerang produce the same amount of lift. When launched with a forward velocity, the forward-moving wing experiences more lift than the retreating wing. The net result is a force which turns the boomerang. As with anything flying through the air, a boomerang is subject to drag and its own weight. The drag slows the boomerang down, limiting the flight time. However, given enough spin and initial velocity, the boomerang might circle above the thrower’s head a few times before landing. BOOMERANG THROWING TECHNIQUE A boomerang is launched almost vertically, based on the speed of the wind. The boomerang incurs a continuous turn throughout the duration of its flight, which causes the boomerang to lay down as it turns. Thus, the boomerang returns to the thrower in a

Boomerangs

horizontal hover. If a boomerang were thrown horizontally, it would climb until the wings stalled and simply fall to the ground. The boomerang is launched at an angle to the wind. The thrower faces the wind and turns approximately 50 degrees to the right or left, depending on whether the person is right-handed or left-handed. Thrown at the proper angle, the boomerang will return. MODERN DESIGNS Simple and sleek in design, the boomerang’s unique motion utilizes complex aerodynamics and physics. Based on these same scientific principles, some modern boomerangs have advanced technical or artistic designs. Several wings may be joined at a centralized hub. Modern boomerangs may be constructed to resemble letters of the alphabet or birds, for example. Some boomerangs are constructed so that the wings’ tips are slower, making the boomerang easier to catch. All boomerangs use the same basic aerodynamic and physical principles to return to the thrower at the end of their flight. —Jani Macari Pallis Further Reading Bryden, Paul. The First Boomerang. ETT Imprint, 2018. Lorenz, Ralph D. Spinning Flight: Dynamics of Frisbees, Boomerangs, Samaras, and Skipping Stones. Springer New York, 2007. Mason, Bernard S. Boomerangs: How to Make and Throw Them. Dover Publications, 2012. Ruhe, Benjamin, and Eric Darnell. Boomerangs: How to Throw, Catch, and Make Them. Workman, 1985. Walker, Pearl. “Boomerangs! How to Make Them and Also How They Fly.” Scientific American, vol. 240, 1979, pp. 130-35. See also: Aerodynamics and flight; Airfoils; Conservation of energy; Flying wing; Forces of flight; Gravity and flight; Wing designs

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Richard Branson Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Richard Branson was born on July 18, 1950, in Shamley Green, Surrey, England. He is a British entrepreneur and in 2021, Branson was listed in Forbes magazine as the 589th-richest person in the world, with an estimated net worth of $4.9 billion. A self-made man, Branson became a very successful entrepreneur through a wide variety of business ventures, including music sales, music production, air and rail transportation, banking, and soft drinks.

Principles of Aeronautics

many notable figures of the time, including John Lennon and Yoko Ono; convinced numerous large corporations to advertise in the periodical; and also managed to get arrested. The arrest was a result of his advertising venereal disease counseling and condom availability in Student. The police alleged that Branson violated two statutes, the 1889 Indecent Advertisements Act and the 1917 Venereal Disease Act. He was acquitted on the charge related to the Venereal Disease Act but was later convicted on the Indecent Advertisements Act charge and sentenced to pay a token fine equivalent to $14. Branson’s next significant business venture was the opening of a record store in London in 1969. He expanded to mail-order record sales the follow-

EARLY LIFE Richard Branson was born on July 18, 1950, in Shamley Green, Surrey, England. His father, Edward James Branson, was an attorney; his mother, Eve, was a former airline flight attendant. Branson’s parents were loving and supportive, and his mother was fond of setting challenging goals for him. Branson was educated in boarding schools, initially Scaitcliffe Preparatory in Windsor Great Park and subsequently Stowe in Buckinghamshire. He did not finish secondary school or go on to attend college. Branson has dyslexia, a learning difference associated with above average intelligence that results in difficulty with reading and spelling, and he performed poorly in academics. However, he was popular in school because of his ability to converse effectively and connect with others. FIRST VENTURES Branson’s first significant business venture occurred in 1967, when he and a childhood friend established Student, a periodical focused on the interests of secondary school and college students. Student was notable because it seemed to bring out the best in Branson’s can-do nature. He secured interviews with

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Richard Branson. Photo by Chatham House, via Wikimedia Commons.

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ing year and eventually opened additional stores, ultimately creating the huge Virgin megastore chain of music shops. This business venture also resulted in Branson’s arrest after British customs officials charged him with selling nontaxed records, which he claimed to export. Branson settled out of court by paying the required customs duties and penalties. MATURE WEALTH Throughout the 1970s, Branson expanded his line of Virgin record shops, opening stores throughout the United Kingdom and ultimately throughout the world. The stores boasted extensive inventory and an informal atmosphere that appealed to young people. This formula proved successful and was the start of the Virgin empire. In 1971, Branson bought a country manor in Chipton-on-Cherwell near Oxford, England, with the intention of converting the home into a recording studio. He felt there was a need for a more relaxed environment in which music artists could record. He also saw the potential for making a profit. Later that year Mike and Sally Oldfield arrived at the manor to make a recording. Mike Oldfield’s choice of the manor as a recording location was fortunate for Branson because Oldfield went on to record the multiplatinum Tubular Bells album at the manor. Tubular Bells earned a great deal of money for the new Virgin Records label that Branson launched in 1972. Over the years, income from sales of Tubular Bells would pull the Virgin Group, Ltd., back from the brink of bankruptcy and provide vital capital for the establishment of new business ventures. Throughout the 1970s, Virgin Records struggled to stay solvent. Mike Oldfield was a consistent earner, but other artists did not prove as successful, and in 1980 the company lost $13.5 million. This financial picture changed drastically in 1982, when Virgin Records signed Boy George and Culture Club to the label. That year, the company earned a profit

Richard Branson

of $3 million, and in 1983, profits climbed to $16.5 million. The little Virgin label that other music publishers had derided was now a major force in the record industry. Branson established Virgin Atlantic Airlines in 1984 with a single leased Boeing 747 jumbo jet. Virgin Atlantic initially had only one route, from New York’s John F. Kennedy International Airport to London’s Gatwick Airport. The maiden flight from Gatwick to Kennedy started with a bang, literally, when one of the jet’s four large engines ingested birds on takeoff and had to be shut down. Not to be deterred, Branson continued to move forward, ultimately building Virgin Atlantic into a profitable and popular airline. Along the way, he had to endure a lengthy legal battle with British Airways. Branson accused British Airways of libel, and the Virgin Records label had to be sold in order to raise capital to keep the airline viable. The lawsuit against British Airways concluded with Branson and Virgin Atlantic settling for more than $900,000 in damages. In 1986, Branson made the decision to publicly trade stock in the Virgin Group. A huge level of interest accompanied this initial public stock offering. Unfortunately, the stock did not appreciate as hoped. In addition, Branson did not like the additional oversight and direction that the operation of a public company entailed. Consequently, in late 1987 all issued stock was bought back at the original purchase price and Virgin once again became a private company. In 1991, Virgin Records signed Janet Jackson for a record $25 million single-album deal. The signing of an artist for a single record instead of several was unheard of and the music industry perceived this deal as extremely risky. However, Virgin Records continued to prosper and began looking to purchase another label, Thorn/EMI, in order to gain control of this firm’s lucrative record rights inventory. Ironically, when Virgin Atlantic Airlines fell on hard economic times, it was Thorn/EMI that purchased the

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Virgin recording label for an astounding $1 billion. This huge infusion of cash allowed Virgin Atlantic Airlines to survive and gave Branson the financial wherewithal to continue expanding his empire. The 1990s and early 2000s found Virgin expanding into a very diverse collection of business ventures, including Virgin Cola, a soft drink company; Virgin Money, a banking and investment company; Virgin America, an American-based airline; Virgin Mobile, a telecommunications company; Virgin Trains, a British rail provider; and Virgin Galactic, a space tourism company. Virgin Cola did not pan out, and Virgin America was sold to Alaska Air in 2016 amid increasing US airline consolidation. Although all of the ventures are significant in some way, Virgin Trains and Virgin Galactic are probably the most intriguing. Virgin Trains is Branson’s attempt to revitalize and modernize the British rail system. By 2009, Virgin was managing a significant portion of Britain’s rail transportation, with the west of England served by a modern high-speed train system designed and implemented by the company. Virgin Galactic is Branson’s attempt to beat big government at its own game, namely, spaceflight. In 2004, Branson’s partners in Virgin Galactic, Paul Allen, cofounder of Microsoft Corporation, and aircraft designer Burt Rutan, won the Ansari X prize for conducting the first civilian launch of a reusable space vehicle twice within a two-week period. Allen and Rutan were awarded $10 million for their efforts. Rutan also designed a spacecraft in which Virgin Galactic planned to take clients on brief spaceflights. On December 7, 2009, SpaceShipTwo was rolled out in the Mojave Desert—the first public appearance of a commercial passenger spacecraft. Tragically, after more than fifty flights, the craft broke up on October 31, 2014, as a result of human error; a replacement SpaceShipTwo was unveiled on February 19, 2016. After a 2018 test flight saw the craft first reach suborbital space and a 2019 mishap regarding a seal led to alterations to one of its stabi-

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lizers, it successfully reached suborbital space for the first time with a full crew after detaching from a carrier craft in July 2021. Branson himself was aboard for the flight, which touched back down less than twenty minutes later, meaning that he had just beat his main space tourism competitor, Jeff Bezos, into suborbital space. Branson has publicly stated that his goal is to continue to expand his enterprises whenever the Virgin Group has money. This formula has proven highly effective for the Virgin empire, which in 2021 employed more than sixty thousand people in a highly diverse collection of companies. Branson sees himself as a serial entrepreneur, as opposed to a corporate executive. He regularly licenses the Virgin brand name to subsidiary owners and, even in the companies in which Virgin does have an ownership stake, prefers to delegate operations management. LEGACY Richard Branson has made significant impacts on each of the industries into which he has ventured. By some estimates, Branson has started five hundred companies and about half have survived. Yet from music to airlines and on to spaceflight, Branson has raised the bar of quality and competitiveness. Consumers have benefited greatly from Branson’s Virgin Group by receiving higher quality at lower prices. His adventurous and exciting life are both fascinating and inspirational. Truly a self-made man, Branson triumphed over dyslexia to become one of the richest people in the world. Like many ultrarich individuals, Branson has turned his efforts toward altruistic causes. He has used his money to build medical clinics in Africa and, in 2007, pledged $25 million to fund the Virgin Earth Challenge, an award designed to stimulate innovation in the sequestration of carbon dioxide, a potent greenhouse gas in Earth’s atmosphere. Though eleven organizations were named as final-

Richard Branson

Principles of Aeronautics

ists for the prize, some investment funds were raised, and others were inspired to launch similar challenges, by 2019 the Virgin Earth Challenge had been controversially discontinued without the prize having been awarded. Branson was also instrumental in bringing together a group of worldly intellectuals known as the Elders, including South African leaders Nelson Mandela and Desmond Tutu and Kofi Annan, the former secretary general of the United Nations. The Elders seek to objectively solve difficult global conflicts. —Alan S. Frazier Further Reading Bower, Tom. Branson. HarperCollins UK, 2009. Branson, Richard. Business Stripped Bare: Adventures of a Global Entrepreneur. Virgin Books, 2008. Branson, Richard. Losing My Virginity: How I’ve Survived, Had Fun, and Made a Fortune Doing Business My Way. Crown, 2011. ———. Losing My Virginity: Richard Branson, the New Autobiography. Ebury Publishing, 2022.

———. Screw It, Let’s Do It: Fourteen Lessons on Making It to the Top While Having Fun and Staying Green. Virgin Books, 2008. Dearlove, Des. Business the Richard Branson Way: Ten Secrets of the World’s Greatest Brand Builder. Capstone, 2007. Feloni, Richard. “Why Richard Branson Is So Successful.” Business Insider, 11 Feb. 2015. Accessed 24 June 2016. Gordon, Sarah. “Virgin Group: Brand It Like Branson.” FT.com. Financial Times, 5 Nov. 2014. Accessed 24 June 2016. MacGregor, J. Richard Branson, The Force Behind Virgin: Insight and Analysis into the Life and Successes of Sir Richard Branson. CAC Publishing LLC, 2018. Masunaga, Samantha. “Richard Branson and Virgin Galactic Crew Go to the Edge of Space and Back.” Los Angeles Times, 11 July 2021, www.latimes.com/business/ story/2021-07-11/virgin-galactic-richard-branson-spaceflight. Accessed 4 Aug. 2021. Wachman, Richard. “Virgin Brands: What Does Richard Branson Really Own?” Observer. Guardian News and Media, 7 Jan. 2012. Accessed 24 June 2016. See also: Air transportation industry; Neil Armstrong; Glenn H. Curtiss; Steve Fossett; Howard R. Hughes; Billy Mitchell; Wiley Post; Eddie Rickenbacker; Wright brothers’ first flight; Chuck Yeager

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C Conservation of Energy Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics; Classical mechanics; Electromagnetism; Nuclear physics ABSTRACT The motion of an object can be described by considering the various forms of energy the object has. In the absence of dissipative forces, the energy of a system can be neither created nor destroyed. This principle can be used to find the varying amount of kinetic energy of an object, which directly relates to the speed of that object. KEY CONCEPTS dissipative forces: forces that function to reduce the amount of energy an object possesses kinetic energy: energy due to any kind of motion, be it rotation, vibration, or translation potential energy: energy that is stored in objects and has the potential to become other forms of energy, such as kinetic energy total mechanical energy: the sum of all the kinetic and potential energies of an object in a closed system CONSERVATION OF ENERGY AND MASS To study the motion, or kinematics, of an object, one can apply Isaac Newton’s (1642-1727) laws of motion and obtain results that match what happens in the real world. However, there is one small issue with this approach: it gets a lot more complicated when dealing with variable accelerations. When a car stops at a red light, it is not accelerating at that moment. When the light turns green, the driver applies a variable amount of pressure to the gas pedal. That variable

amount of pressure produces a variable acceleration of the car. In these cases, applying Newton’s laws of motion produce different results depending on the acceleration value used. Calculating energy is a simple approach to solving these kinds of problems. This is due to the fact that the total amount of energy an object has never changes. Physicists call this principle the conservation of energy. Energy is conserved by all objects in the absence of friction or any other dissipative force. If a person rubs a finger on a table for a long period of time, his or her finger will get hotter and hotter. If the person hits the table with an open palm, his or her palm will also be warmer than before. This is because some of that person’s energy was turned into heat, which is a form of energy. Heat then moves away from the person’s hand in the form of infrared radiation and goes back into the environment. While it may seem like the energy was not conserved, it is still part of a larger overall system and did not disappear completely. The law of conservation of energy states that the total energy in the universe is always the same, and therefore energy can be neither created nor destroyed. In other words, it is not possible to add energy to the universe or take some energy out of the universe. This law can be applied to any system in which one can assume no dissipating forces exist. In 1905, Albert Einstein (1879-1955) recognized in his theory of relativity that mass is itself a form of energy. Thus, the law of conservation of energy also addresses the conservation of mass, in that the total amount of mass and energy in the universe is constant. Inside the Sun, mass is constantly being turned into energy. This is the energy that warms Earth. In

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a way, fossil fuels are a form of energy from the sun, if one considers the law of conservation of mass and energy. Eons ago, the sun converted mass into energy in the form of light and heat. When that light and heat reached Earth, plants, bacteria, and algae turned the sun’s energy into mass in the form of food, which provided them with energy to live and grow. That energy was converted back into mass in the form of the newly grown plants, bacteria, and algae. Animals then ate those plants, bacteria, and algae as food, which they converted to energy. Over time, the remains of flora and fauna fossilized, becoming coal and oil. Humans use coal and oil as fuel to provide energy. Conservation of energy is all around, and it affects everyone in more ways than one might think. DIFFERENT FORMS OF ENERGY In order to understand conservation of energy, one must understand the different forms and properties of energy. Everything that is in motion has a form of energy called kinetic energy. In fact, the temperature of a room is defined as the average kinetic energy of the particles in the room. The air molecules in a room are in a constant state of motion. Not only are they moving around, they are also vibrating. If on average they are moving faster, then they have more kinetic energy, which makes the whole room warmer. If on average they move slower, then less kinetic energy leads to lower temperatures. The kinetic energy (K) of an object, in joules (J), is mathematically defined by the object’s mass (m) and its velocity (v), as in the following equation: EK = 0.5mv2 But that is not the only form of energy objects can have. Objects tend to fall to Earth’s surface due to the planet’s gravitational pull. Another simple way to say this is that the object has extra amounts of energy when it is above the surface of Earth. This energy is known as the gravitational potential energy

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(Ug), and it is determined by the object’s height above the surface (h), its mass (m), and the strength of Earth’s gravitational field (g). When a pencil is held, it has potential energy. As the pencil falls to the ground, it starts to lose some of that potential energy, which becomes kinetic energy. The pencil’s velocity increases as it continues to fall. When it hits the ground, all of its potential energy has been turned into kinetic energy, and it lands at its highest possible speed. At that point, dissipative forces cause the energy to be lost to the surrounding environment. Mathematically speaking, the gravitational potential energy, in joules, is expressed as Ug = mgh where g equals 9.8 meters per second per second, or meters per second squared (m/s2), near the surface of Earth. There are many other forms of stored or potential energy. There is stored energy in chemicals that are about to react in a chemical reaction. Some of this energy is released in the reaction as heat. Another form of potential energy is found by the stretching and compression of a spring. If there is a mass (m) attached to a spring that has been fixed to a wall, and someone pulls on the mass without letting go, the mass now has potential energy. If the mass is released, it will begin to oscillate, gaining kinetic energy. At one point during this oscillation, the mass will compress the spring to the maximum possible amount and stop moving for a fraction of a second. When this happens, all the kinetic energy gained has been transformed back into potential energy. Then the spring will push back on the mass, allowing the stored energy to be transformed into kinetic energy. The entire process repeats itself for as long as the mass is allowed to oscillate. The potential energy stored in a spring is a function of the distance the spring is stretched or compressed (x) and the properties of the particular spring used, summarized as its unique spring constant (k). The spring constant is a measurement of how rigid the spring is

Principles of Aeronautics

and how it reacts to being stretched or compressed. In the International System of Units, it is measured in newtons per meter (N/m). Mathematically, the potential energy of a spring in joules, known as the elastic potential energy (Ue), is found using the following equation: Ue = -x Elastic potential energy only exists if a spring is part of the system in question. An object can have multiple forms of energy at once. A pendulum that is oscillating is moving, therefore it has kinetic energy, and is at a distance from the surface, so it has gravitational potential energy. When the pendulum is at its highest point, all its energy is in the form of gravitational potential energy. When it is at its lowest point, it has zero potential energy and the highest amount of kinetic energy it can have. In between, it has different amounts of potential and kinetic energies. As described above, energy in a closed system is conserved. That means that all of the energy in the pendulum is always the same. Physicists have defined the total mechanical energy (E) as the sum of all the kinetic and potential energies of an object in a closed system: E = K + Ug + Ue When physicists say that the energy is conserved, they mean that mechanical energy is conserved. This means that there is no change in the total mechanical energy, or that the initial mechanical energy (Ei) equals the final mechanical energy (Ef):

Conservation of Energy

ENERGY PRODUCTION By using energy to solve problems about motion, one can arrive at the same result without having to deal with variable forces and accelerations. This has wide implications and applications for the real world. When hydroelectric power plants produce energy, they do so by converting potential and kinetic energy into electrical energy. As water falls down from the top of a lake behind a dam through pipes called penstocks, it loses potential energy and gains kinetic energy. This allows the water to move faster and faster as it falls. It then hits the blades of a turbine and transfers its energy into the turbine, causing it to spin. The turbine turns a generator, which produces electrical energy. Conservation of energy can also be seen in the production of energy by other means. Much of the electricity produced in the United States comes from the burning of coal. When coal is used to heat water to produce electricity, the power plant cannot produce more energy than is stored in the coal as chemical potential energy. The same can be said about gasoline-powered cars. The kinetic energy obtained by the explosive reaction of gasoline in an engine cannot be greater than the chemical energy stored in that gasoline before the reaction. This means that no one will ever be able to attain infinite speeds by means of propulsion, as an ever-increasing need for kinetic energy comes from an ever-increasing amount of stored potential energy, and there is a limited, and not infinite, amount of energy in the universe. Nothing in the universe that has mass can propel itself at or faster than the speed of light.

Ei = E f Substituting the definitions of the different forms of energy into the mechanical energy conservation equation, the equation becomes Ki + Ug,i + Ue,i = Kf + Ug,f + Ue,f

CONSERVATION OF ENERGY IN AERONAUTICS Flight is subject to several dissipative forces. An aircraft flying through the air is subject to air resistance and must expend energy to counteract the direct force of resistance from the air that it must displace as it moves. The aircraft is also subject to drag,

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which is the friction of the air moving across the surfaces of the aircraft. While the aircraft’s wings generate lift because of the air passing over the airfoil, they also produce the greatest amount of drag felt by the aircraft. The aircraft must expend energy to counteract the effects of drag. Gravity is another dissipative force affecting an aircraft in flight, and represents the greatest expenditure of energy by an aircraft in flight, simply because this energy is used to maintain the flight of the aircraft against the force of gravity and the other dissipative forces. Failure of the aircraft to maintain the expenditure of energy to overcome these effects generally has a catastrophic outcome. —Angel G. Fuentes Further Reading “Circus Physics: Conservation of Energy.” Circus. PBS, 2010. Accessed 21 Apr. 2015. “Conservation of Energy: Physics.” Encyclopedia Britannica, 23 Jan. 2014. Accessed 21 Apr. 2015. “Conservation of Energy.” Khan Academy, 2015. Accessed 21 Apr. 2015. Giambattista, Alan, and Betty McCarthy Richardson. Physics. 2nd ed., McGraw, 2010. Moskowitz, Clara. “Fact or Fiction? Energy Can Neither Be Created nor Destroyed.” Scientific American, 5 Aug. 2014. Accessed 21 Apr. 2015. Young, Hugh D., and Francis Weston Sears. Sears & Zemansky’s College Physics. 9th ed., Addison, 2012. See also: Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Airplane accident investigation; Airplane propellers; Autogyros; Aviation and energy consumption; Flight propulsion; Fluid dynamics; Forces of flight; Gravity and flight; Propulsion technologies; Rockets; Wake turbulence; Wind shear

Contrails Fields of Study: Physics; Fluid dynamics; Mathematics

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ABSTRACT Condensation trails, or contrails, are long, narrow cirrus clouds composed of ice crystals that form behind aircraft or rocket engines flying in the upper atmosphere. Water vapor in the hot exhaust rapidly condenses and freezes when it mixes with cold, humid atmospheric air, forming a trail of ice crystals. Contrails typically form where the air temperature is 4°C to -60°C and relative humidity exceeds 100 percent, with low wind turbulence. KEY CONCEPTS condensation: the change of the physical state of water from gas to liquid vortex: a column of fluid spinning rapidly about a central locus or axis wing-tip vortex: a turbulent swirling motion of the air behind the wing-tip of an aircraft in flight DEFINITION Condensation trails, or contrails, are long, narrow cirrus clouds composed of ice crystals that form behind aircraft or rocket engines flying in the upper atmosphere. When fuels containing hydrogen, such as hydrocarbons, burn in air, the engine exhaust contains water vapor. The vapor in the hot exhaust rapidly condenses and freezes when it mixes with cold, humid atmospheric air, forming a trail of ice crystals. Contrails typically form where the air temperature is 4°C to -60°C and relative humidity exceeds 100 percent, with low wind turbulence. Favorable conditions typically occur above 8,530 meters. Contrails have been reported since 1915, but they became much more common as jet aircraft traffic increased. Geese flying in cold air have been seen to leave small contrails as they exhale moist air. The condensation engendered by contrails starts to form on microscopic dust particles. Contrails become visible approximately 0.1 second after leaving an engine, as the ice particles grow large enough to scatter sufficient light for them to be seen. Although contrails start as one exhaust behind each engine,

Principles of Aeronautics

Contrails

Photo via iStock/alexandrumagurean. [Used under license.]

they often merge into one wing-tip vortex from each wing tip. The lower pressure and temperature in the core of each tip vortex also helps accelerate condensation. Contrails move down in reaction to the aircraft’s lift and the higher density of ice. During the daytime ice crystals absorb sunlight. The warming air around them can convect the contrails several hundred meters up. When the air is dry, the ice particles sublimate to vapor quickly, resulting in a short contrail. In humid air, contrails can persist for several hours or thousands of kilometers, given the speed of aircraft, and they grow into cirrus clouds as thick as 500 meters and several kilometers wide. Ice-particle size in such clouds is on the order of 200 micrometers, and their density is on the order of 1 to 50 particles per cubic centimeter of air. Many trails are over two hours old, and they continue to accumulate moisture from the air during that time. Indeed, little of such a cloud

comes from the original jet exhaust. A contrail cloud may contain one thousand to ten thousand times the water released by the aircraft engine itself. SIGNIFICANCE FOR CLIMATE CHANGE The cirrus cover due to contrails has been estimated to cover as much as 0.1 percent of Earth’s surface. The most famous experiment on contrails was conducted by the National Aeronautics and Space Administration (NASA) in the days following the terrorist attacks of September 11, 2001. US civil air traffic was grounded for three days. During those three days, the difference between daytime high and nighttime low temperatures over the continental United States increased by roughly 1 degree Celsius when compared to the thirty-year average. At the same time, the trails left by six military aircraft persisted and eventually covered over 19,700 square kilometers.

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The increase in cirrus cloud cover due to contrails has been studied as an anthropogenic factor in global warming. Ice crystals absorb, scatter, and reflect radiant heat. Some studies indicate that contrail cloud cover inhibits outward radiation from Earth’s surface and lower atmosphere more than it reflects incoming solar radiation, contributing to the greenhouse effect. Some argue that long-wave infrared radiation is absorbed more by ice crystals than by air or water vapor, so that cirrus clouds have a net warming effect. Others studies suggest that night flights, which constitute only 20 to 25 percent of air traffic, may contribute to 60 percent of contrails’ greenhouse effect. Published data project that a fivefold increase in air traffic would cause a net global warming effect due to contrails of 0.05°C. Some argue that any detectable anthropogenic change is a cause for concern. Others point to a more severe localized effect due to the heavy traffic over industrialized nations in the temperate zone, where the air is moist and cold for a greater part of the year compared to equatorial regions. Changing from hydrocarbon to hydrogen fuel will not reduce water vapor, but may reduce dust particles in the exhaust. A different aspect is the deposition of carbon dioxide and heat in the upper atmosphere. aircraft emissions are believed to contribute 2 to 3 percent of all anthropogenic global warming. Contrails are highly amplified reminders of that problem. —Narayanan M. Komerath Further Reading Agarwal, Akshat. Quantifying and Reducing the Uncertainties in Global Contrail Radiative Forcing. MIT, 2021. Atlas, David, Zhien Wang, and David P. Duda. “Contrails to Cirrus: Morphology, Microphysics, and Radiative Properties.” Journal of the American Meteorological Society, Jan. 2006, pp. 5-19. Daley, Ben. Air Transport and the Environment. Taylor & Francis, 2016.

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Freeland. Elana. Chemtrails, HAARP, and the Full Spectrum Dominance of Planet Earth. Feral House, 2014. Grewe, Volker, et al. “Reduction of the Air Traffic’s Contribution to Climate Change: A REACT4C Case Study.” Atmospheric Environment, vol. 94, 2014, pp. 616-25. Accessed 19 Mar. 2015. Hoyle, C. R., B. P. Luo, and T. Peter. “The Origin of High Ice Crystal Number Densities in Cirrus Clouds.” Journal of the Atmospheric Sciences, vol. 62, no. 7, July 2005, pp. 2568-79. Pandarhinath, Navale. Aviation Meteorology. BS Publications, 2014. See also: Atmospheric circulation; Aviation and energy consumption; Flight altitude; Flight propulsion; Greenhouse gases; High-altitude flight

Glenn H. Curtiss Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Glenn Curtiss was the most prolific aeronautical inventor and manufacturer of airplanes and airplane engines in the United States well into the 1920s. He was born on May 21, 1878, in Hammondsport, New York, and died on July 23, 1930, in Hammondsport, New York. Curtiss developed ailerons for flight control, designed the first American amphibious airplanes, and built the first airplane to cross the Atlantic Ocean. He was also the first US licensed pilot and the first to make a public flight. KEY CONCEPTS aileron: a small secondary structure at the outer end of a wing, used to alter the lift of a wing for control of the roll and pitch of an aircraft in motion roll: the tendency of the body of an aircraft to rotate about its central axis as it moves through a fluid medium

Principles of Aeronautics

BACKGROUND Born in 1878, Glenn Hammond Curtiss, a champion bicycle racer, developed gasoline engines to power his bicycles, and he set international speed records on bicycles powered by engines with up to eight cylinders. It was Curtiss’s engine expertise that led him to join Alexander Graham Bell’s Aerial Experiment Association (AEA) in 1907 as its director of experiments. In the AEA, Curtiss quickly became instrumental in the design of a series of successful airplanes. The Curtiss White Wing became the first American airplane to take off on wheels instead of skids and the first to use ailerons for roll control in turns. His June Bug made the first public flight that was filmed and witnessed by the press in 1908. This flight would win Curtiss the Scientific American prize. The French press proclaimed Curtiss the “Champion Aviator of the World” after he set new speed records in winning the Gordon Bennett trophy in France in 1909. Curtiss was sued by Wilbur and Orville Wright, who claimed that his use of ailerons violated their patents for controlling the roll of an airplane. Although many experts believed that ailerons were different from the Wright’s use of wing-warping, the courts were harder to convince. Repeated lawsuits kept Curtiss tied up in the courts for years, until the government intervened in the national interest, as the country entered World War I. Curtiss further angered the Wrights when, at the request of the Smithsonian Institution, he agreed to prove that Samuel Pierpont Langley’s aerodrome, an uncrewed flying machine driven by a gasoline-fueled, steam-powered engine which had crashed in the Potomac River twice in attempting the first airplane flight, was actually capable of flight. Curtiss made significant modifications to Langley’s design, and when it flew, the Smithsonian proclaimed the aerodrome to be the first heavier-than-air craft capable of flight. The Wrights never forgave Curtiss for trying to usurp their rightful claim as the first to fly.

Glenn H. Curtiss

Glenn H. Curtiss. Photo via Wikimedia Commons. [Public domain.]

Curtiss established flying schools throughout the country and contracted to train US Navy and Army aviators as he continued to develop newer airplanes. His Curtiss JN-4, or Jenny, a trainer aircraft, was the best American-designed plane to come out of World War I, and surplus Jennys became the airplane of choice for hundreds of aspiring pilots after that war. A prolific designer of land-and-water-based airplanes for the Navy, Curtiss designed the NC-4, the first airplane to fly across the Atlantic, in 1919, and a series of racing planes that set world speed records in the early 1920s. His OX series of airplane engines were dominant in the US market. Curtiss died in Hammondsport, New York, on July 23, 1930, of a pulmonary embolism suffered after a bout with acute appendicitis. —James F. Marchman III

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Further Reading Goldstone, Laurence. Birdmen: The Wright Brothers, Glenn Curtiss, and the Battle to Control the Skies. Random House Publishing Group, 2015. Hatch, Alden. Glenn Curtiss, Pioneer of Aviation. Lyons Press, 2007. House, Kirk W. Curtiss-Wright. Arcadia, 2005.

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Tremaine, Jenna. Soaring Through Glenn Curtiss’s Growth Mindset: Dream Big, Work Hard, Fly High. Halo Publishing International, 2022. Trimble, William F. Hero of the Air: Glenn Curtiss and the Birth of Naval Aviation. Naval Institute Press, 2013. See also: Aeronautical engineering; Ailerons, flaps, and airplane wings; Wright brothers’ first flight

D Leonardo da Vinci Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Leonardo da Vinci was an Italian engineer and artist. The archetypal Renaissance man, da Vinci pursued interests in art, architecture, mathematics, astronomy, anatomy, biology, botany, philosophy, science, and engineering. In thousands of pages of detailed notes and drawings, which included hundreds of inventions, Leonardo examined the physical world around him, leading the world away from the superstitions of the Middle Ages and toward the modern era of science and reason.

his life, Leonardo dedicated many in-depth studies to the flight of birds, such as his 1505 Codex on the Flight of Birds. Leonardo’s designs for a flying machine progressed over time. His earliest designs consisted of machines in which the human pilot, laying prone in a wooden frame, placed his feet in stirrups and moved his feet together, causing the downstroke of

KEY CONCEPTS anemometer: a device that interacts with moving air to show the direction and speed of the wind downstroke: the downward motion in the imitation of the flapping of wings inclinometer: a device that indicates the angle at which an aircraft is ascending or descending relative to level flight THE DREAM OF FLIGHT In his notebooks, Leonardo wrote “I have always felt it is my destiny to build a machine that would allow man to fly.” To fly like a bird was a lifelong dream for Leonardo da Vinci. Leonardo said that he had a childhood memory of a bird flying down to his cradle and brushing its feathers against his face, and one of Leonardo’s earliest drawings was an aerial bird’s-eye view of the Arno River Valley. Throughout

This portrait attributed to Francesco Melzi, c.?1515–1518, is the only certain contemporary depiction of Leonardo. Image via Wikimedia Commons. [Public domain.]

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A design for a flying machine (c.?1488), first presented in the Codex on the Flight of Birds. Image via Wikimedia Commons. [Public domain.]

the wings; the pilot’s hands directed the upstroke by means of a lever. Over time, Leonardo modified the movement of the pilot’s legs, having the legs slide up and down, assisted by the hands, in order to make the wings beat. A still further advance was the addition of a head harness that manipulated a rudder to control direction. Leonardo drew numerous sketches of wing types, from a wing based on that of a bat to adjustable tilt wings, beating wings, and articulated wings, each in an effort to refine his flying machine. The most ambitious of his designs included a large enclosed cabin, capable of holding two pilots who operated the flapping bat wings with a complex system of screws and cranks. As inspired as Leonardo’s designs were for his early flying machines, two factors would have prevented his designs from ever taking off. First, the materials that were available at the time were just too heavy to be manipulated by even multiple human pilots. In addition, Leonardo’s understanding of the flight of birds was fundamentally flawed. Leonardo believed that birds flew when the wing was moved down and back, as if the bird were swimming through the air like a swimmer through water. In ac-

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tuality, the lift required for flight is created when birds move their wing feathers up and forward. As time progressed, Leonardo replaced the flapping-wing designs with designs for fixed-wing glider crafts. At first, Leonardo did not provide a means for the pilot to control the machine through body movements. Eventually, however, he enabled the pilot to balance the craft by moving the body’s extremities, effectively inventing a predecessor to the modern controlled glider. Leonardo wrote a journal entry in which he stated that a great bird will take flight from Mount Cerceri, a mountain near Leonardo’s residence at the time. Legends recount that one of Leonardo’s assistants piloted the craft, crashed, and broke his leg. Whether one of Leonardo’s great birds ever actually took flight will probably never be known. What is known is that in an era of holdovers of medieval traditionalism and superstition, Leonardo’s dream of flight opened up a new vision of possibilities for the future. EARLY LIFE Leonardo da Vinci was born on April 15, 1452, in Vinci, a small town located in the hills above the lower Arno River Valley in the vicinity of Florence. Because he was the illegitimate son of a wealthy notary, Ser Piero, and a local peasant girl, Caterina, few records exist to document Leonardo’s early years. In an era in which surnames were not yet in common use, Leonardo’s birth name was Leonardo di ser Piero da Vinci, which meant “Leonardo, son of [Mes]ser Piero from Vinci,” later shortened to Leonardo da Vinci, or Leonardo from Vinci. Leonardo apparently spent his first five years in Anchiano, a small village just outside Vinci, in the care of his mother, who may have been the first to introduce the young Leonardo to the beauty of the Tuscan countryside and to lay the groundwork for his later insatiable desire to understand the wonders of the natural world.

Principles of Aeronautics

When Leonardo was age five, Ser Piero brought the boy to live with him and his family in Vinci, where Leonardo was groomed to follow in his father’s occupation as a notary. Before long, however, Leonardo’s artistic talents were recognized as too impressive to be ignored, and Ser Piero took his son to Florence, the hub of fifteenth-century cultural and political activity. In 1466, Ser Piero secured for Leonardo the position of apprentice in the workshop of Verrocchio, one of the foremost artists of the day. There, the young Leonardo met not only important civic leaders and patrons but also other young apprentices such as Perugino, Ghirlandaio, and Botticelli, who, along with Leonardo, would later become some of the greatest artists of their era. In this invigorating atmosphere, Leonardo learned skills that would serve him as an artist and as an inventor and engineer as well, skills such as the vital role of keen observation, the importance of rational investigation, and the need for careful documentation. In 1472, when he was twenty, Leonardo was admitted into the Guild of St. Luke, the organization that oversaw both artists and doctors of medicine, a fortuitous combination for a young man with Leonardo’s extraordinary proclivities. LIFE’S WORK Upon becoming a master, Leonardo established his own workshop, where, in 1473, he created his earliest extant drawing, a view of his beloved Arno River Valley, drawn from a bird’s-eye perspective, which reflected his lifelong interest in birds and flight. In 1482, on the advice of Lorenzo de’ Medici, Leonardo traveled to Milan to visit the court of Ludovico Sforza, known as Ludovico il Moro (Ludovico the Moor, due to his dark complexion). In a letter to Ludovico, Leonardo offered his services as a military engineer, listing the many warfare devices and military benefits he could proffer, including portable bridges, mortars, mines, guns, can-

Leonardo da Vinci

nons, covered chariots, and catapults. At the end of the letter, almost as an afterthought, Leonardo mentioned that he could also serve Ludovico as an architect and painter. In an era of intense rivalry, both in cultural status and military prowess, Leonardo’s claims appealed to Ludovico, who wisely hired the prodigious young man. Leonardo remained in the Ludovico court from 1482 to 1499, during which time Leonardo worked intensely on mechanisms for both defensive and offensive military purposes. Whether it was out of his own innate interests or because of the necessity of defending his homeland, Leonardo was always clearly involved in developing machines for warfare. Among his drawings, Leonardo included sketches for both a single sling and a double sling to hurl stones, as well as a giant crossbow. Leonardo also drew designs for a scything wagon, which, when pulled by horses, moves a system of gears that spin huge scythes protruding from the side of the wagon, capable of cutting down anything in their path. Leonardo missed no opportunity to enlarge and embellish upon cur-

An aerial screw (c.1489), suggestive of a helicopter, from the Codex Atlanticus. Image via Wikimedia Commons. [Public domain.]

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rent firearms. He multiplied the efficiency of the traditional single-barrel artillery by designing a three-barreled cannon, an eight-barrel machine gun, and even a thirty-three-barrel machine gun. Each of these guns was designed to be mounted on easily movable gun carriages that were adjustable both horizontally and vertically in relation to the desired target. In order to lift heavier artillery, Leonardo devised a winch in which the lifting and lowering was done with a worm screw and a spiral wheel. To improve the accuracy of his artillery, Leonardo explored the use of aerodynamic projectiles to control the trajectory of the ammunition. Concerned with the challenge of rapid ignition of the multiple artillery barrels he was designing, Leonardo drew plans for automated igniting devices and automatic strikers. The covered chariot that Leonardo had promised Ludovico Sforza in his letter of 1482 was, in essence, the forerunner of the modern tank. Leonardo proposed the construction of a round, metal-covered enclosure, which, when operated by eight men protected inside, could be turned and moved in any direction by men on the lower level, while men on the upper level of the machine fired through narrow openings in the top. Leonardo was also interested in innovations at sea. He drew plans for fast boats that could ram and hold the enemy’s craft while men protected by shields attacked the enemy with artillery. Leonardo also drew designs for paddle-propulsion boats, with the energy being provided by men in the hull, whose power was increased by a system of cogwheels and gears that multiplied the revolutions of the paddlers. He designed a ship with a double hull that would minimize the water intake if the ship’s outer hull were damaged. In order to inflict as much damage as possible on an enemy vessel, Leonardo designed ship bottom breakers to facilitate the sinking of enemy ships. Leonardo was also concerned with spanning bodies of water. He designed fast construction bridges to enable armies to

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quickly build bridges with a system of wood logs and ropes. Leonardo also designed movable bridges, such as completely revolving bridges that could be turned in case of enemy advances; parabolic bridges, which, secured to one bank, could be turned on vertical hinges; and an impressive design for a single-span bridge with a double support that would permit the bridge to be of sufficient height to permit even vessels with tall masts to sail underneath. If a bridge was not available, Leonardo designed floats so that a man could simply walk across a body of water. Leonardo even drew plans for a diving suit that would allow a man to dive underwater and to breathe through a system of respiration pipes that reached the surface of the water. The political and military events of the late fifteenth and early sixteenth centuries drove Leonardo from place to place. In 1499, Louis XII of France invaded Milan and defeated the ruling Sforza family. Leonardo was forced to flee to Venice, where he used his talents to invent devices to defend that city from attacks by sea. The next year found Leonardo back in Florence, where he rejoined his guild and resumed painting. During the ensuing years, Leonardo served as military engineer for Cesare Borgia, traveling between Italian cities devising defensive mechanisms and designing a functioning canal up to the Porto Cesenatico. Only after the Sforzas regained control of Milan in 1506 did Leonardo return once again to that city. In his later years, Leonardo lived and worked in Florence, Rome, and Bologna, making the acquaintance of his younger fellow artists, Michelangelo and Raphael. Leonardo spent his last years in France as the court engineer, architect, and painter to King Francis I. Throughout these tumultuous years, Leonardo recorded his observations, insights, and inventive devices in drawings and notations that filled thousands of notebook pages. In addition to his copious studies on military matters, Leonardo also turned his attention to civil

Principles of Aeronautics

matters, with special attention to devising labor-saving devices and improving on existing work-related machines. Leonardo drew intricate plans for machines for making ropes, lifting columns, and threading screws. One of his particular interests was the fabrication of textiles, a major industry in Italy at the time. Leonardo drew designs for several spinning and weaving machines and included plans for an automated bobbin winder. He also devised teasing machines that processed cloth by running the cloth over a series of rollers. He designed a winged spindle that would stretch, twist, and wind thread simultaneously. Although most of the textile manufacture in Italy at the time was done with wool, silk was also a coveted material. Leonardo designed a silk-doubling machine that increased silk production output. Leonardo was always fascinated by the possibility of locomotion, whether on land or in air. It is his experiments with flight for which Leonardo’s inventive talents are perhaps best remembered. Leonardo seemed obsessed with enabling humans to fly like the birds. He drew many sketches of wings, both the natural wings of birds and human-made wings that were worked by various mechanisms. To assist the act of flight, Leonardo devised an inclinometer to control the horizontal positioning of the craft and anemometers to show the wind direction. Leonardo’s dreams of flight ranged from designs for flying machines, hang gliders, and parachutes to a complex rendering for an air screw, anticipating the future helicopter. IMPACT Leonardo was one of the foremost visionaries in history. In addition to creating such artistic masterpieces as The Last Supper and the Mona Lisa, Leonardo is credited with being one of the most prolific inventors of all time. His inventions were based on careful observation and copious documentation. Leonardo was unique for his time in that he ex-

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plained not only the purpose of machines but also how machines actually worked. Leonardo was the first to conceive of machines as both a whole and a sum of its parts, realizing that the parts, such as flywheels, cogs, and gears, could be modified, improved upon, and utilized in innovative new ways. With this realization, Leonardo was able to combine individual parts into hundreds of new machines. In his designs, Leonardo anticipated many modern inventions. He drew diagrams of a hydrometer that anticipated the modern science of hydraulics. His plans for devices for the canalization of rivers are still in use today. His designs for diving suits, parachutes, flying machines, and helicopters inspired generations of later inventors. In 1903, the Wright brothers achieved Leonardo’s dream of heavier-than-air human flight. A few years later, the modern helicopter was developed. His plan for a single-span bridge, conceived in 1502 for the sultan of Istanbul but never realized because it was thought to be impossible, was brought to reality in 2001, when a smaller bridge based on Leonardo’s design was constructed in Norway. On May 17, 2006, the Turkish government commenced construction of Leonardo’s bridge over the Golden Horn at the mouth of the Bosporus, just as Leonardo had originally planned 504 years earlier. Recognized now as some of the most farsighted inventions ever devised, Leonardo’s achievements are displayed in museums around the world, chief among them the Leonardo da Vinci National Museum of Science and Technology in Milan, Italy, and the Leonardo da Vinci Museum at the Château du Clos Lucé in Amboise, France. His notebooks are the highlights of major collections in the Louvre, the National Library of Spain, the Ambrosian Library of Milan, and the British Library. Only one of Leonardo’s notebooks is in a private collection: The Codex Leicester is owned by Bill Gates, an acclaimed inventor of the late twentieth century. Five

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centuries later, inventors can still draw inspiration from Leonardo da Vinci, the most prodigious of Renaissance men. —Sonia Sorrell Further Reading Bark, Jasper. Journal of Inventions: Leonardo Da Vinci. Silver Dolphin Books, 2009. Da Vinci, Leonardo. The Notebooks-The Original Classic Edition. Lightning Source Inc., 2012. Isaacson, Walter. Leonardo Da Vinci. Simon & Schuster, 2017. Kemp, Martin. Living with Leonardo: Fifty Years of Sanity and Insanity in the Art World and Beyond. Thames & Hudson, 2018. Laurenza, Domenico, Mario Tadei, and Edoardo Zanon. Leonardo’s Machines: Da Vinci’s Inventions Revealed. David & Charles, 2006. Lewis, Ben. The Last Leonardo: The Secret Lives of the World’s Most Expensive Painting. Random House Publishing Group, 2019. McCurdy, Edward. The Mind of Leonardo Da Vinci. Dover Publications, 2011. Nicholl, Charles. Leonardo da Vinci: Flights of the Mind. Viking Penguin, 2004. Richter, Irma A., editor. Leonardo da Vinci Notebooks. Oxford UP, 2008. Suh, H. Anna, editor. Leonardo’s Notebooks. Black Dog Leventhal, 2005. See also: Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Forces of flight; Heavier-than-air craft; History of human flight; Human-powered flight; Parachutes; Rockets; Wing designs; Wright Flyer

DC Plane Family Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Business administration and economics ABSTRACT The DC series of passenger aircraft was the most widely used passenger airplane series between the 1930s and the

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1970s. From 1933 through modern times, the DC series of planes made air travel possible for most Americans by introducing such innovations as sleeper cabins, nonstop coast-to-coast flights, pressurized cabins, and ever-expanding fuselages to allow for more passengers per plane and to allow airlines to fly profitably. KEY CONCEPTS cargo plane: aircraft designed and put into service solely for the transport of material goods; cargo plane designs generally are modified passenger planes with all passenger seating removed nonstop: a flight that does not require landing part-way to its final destination in order to refuel or change flight personnel passenger-hours: the number of hours of flying time multiplied by the number of passengers passenger plane: aircraft designed and put into service solely for the transport of commercial travelers; passenger plane designs are often modified to remove passenger-carrying capability to make room for material goods THE BEGINNING OF THE DC SERIES The greatest contributor to the expansion of domestic and international air travel was the family of planes known as the Douglas Commercials or DC series. Built by the Douglas Aircraft Company, the DCs became the dominant brand of commercial passenger plane starting in the 1930s, and later served the needs of the American military beginning in World War II. The first DC model, the DC-1, was built in 1933. Capable of carrying twelve passengers, the two-propeller plane could travel coast-to-coast in a little over eleven hours. The DC-1 took passenger comfort into account in comparison to its main rivals. To combat the noise from the propeller-driven plane, the company used carpeted floors, sound-absorbing fabric, and rubber supports for the seats. The only DC-1 built was purchased by Trans World Airlines (TWA), which saw the plane as the one that

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Principles of Aeronautics

The Douglas DC-4, 1940s. Photo via wikimedia Commons. [Public domain.]

would allow it to compete with the more established air carriers. Within a year of the DC-1 rolling off the assembly line, the Douglas company built the DC-2, also for use in passenger flight by TWA. Known as the Sky Chief, the DC-2 could carry fourteen passengers, and in terms of physical size it had 2 feet more space in the fuselage and nearly 6 feet more in the wingspan. While it had a limited range of 1,000 miles, the DC-2 proved to be a workhorse, with 134 produced between 1934 and 1937. The third of the line was appropriately known as the DC-3 and was first flown as a passenger plane in 1935. This was the best known and the most popular of the DC series and is frequently called the greatest cargo plane ever built. American Airlines was the first to use the craft, after seeing its competitors tie up the other aircraft manufacturers with large or-

ders of other passenger plane models. The airline sought a plane that would allow passengers to rest during the lengthy flight from New York to Los Angeles. The DC-3 had fourteen seats that folded into sleeping berths for passengers. The plane could carry fourteen passengers on an extended coast-to-coast flight or use all of the seats to fit twenty-eight passengers per flight for a shorter trip. The DC-3s larger capacity, its sleeping berths, and its nearly 1,500-mile range provided a boon for the passenger airline business and more importantly for the Douglas Aircraft Company. By the 1940s, approximately 90 percent of all passenger planes flying in the United States were either DC-2s or DC-3s. Some 455 DC-3s were built for commercial use, but the start of World War II saw a surge in the need for military transports that the DC-3 also filled. Over

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10,000 DC-3s were produced for the military to carry both men and matériel to the European and Asian war zones. Even after the war and the end of production in the 1940s, the DC-3 continued to influence the passenger and freight airline markets and it continued to be flown in both capacities at the turn of the century. FROM DC-4 TO DC-8 The highly popular and profitable DC-3 was followed by a less successful version, the DC-4. Nearly twice the size of its predecessor, the DC-4 could carry up to forty-two passengers, but its size made maintenance and flight expensive, relegating the DC-4 to use almost exclusively as a military transport. In this role, the DC-4 was known as the C-54 Skymaster. The DC-4s were used mainly to fly supply missions across the North Atlantic. The four-engine plane proved to be reliable in this task and was used as a cargo carrier for civilian purposes at the end of the war. In 1939, the DC-5 made its first flight. However, only five DC-5 aircraft, with seven more as R-3D military transports, were ultimately built. The next in the series, the DC-6, was best known as the first regular aircraft to make around-theworld flights. Flying for the first time in 1946, the DC-6 was used by American, United, and Pan American airlines. Featuring the first pressurized cabin in the DC series, the DC-6 was able to fly at 20,000 feet while keeping passengers comfortable within the fuselage. The new DC-6 was a considerable improvement over its predecessors, carrying 102 passengers and traveling at a speed of 493 kilometers per hour, a full 144 kilometers per hour faster than the DC-4. The DC-6 became the workhorse for the airlines in their extended international flights. In 1951, the DC-6B, with modifications of the original DC-6, became first official presidential airplane. Known as the Independence, it was first used by President Harry S. Truman to allow him to travel quickly across the

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country or around the world. The DC-6B was also adapted for use as a cargo carrier in the Korean War. Over 700 of them were built for military and civilian use, and by end of the century, scores continued to be used. The DC-7 proved to be the last propeller-driven plane in the DC series. It represented the greatest increase in range among the models, with each plane able to fly 8,216 kilometers. By increasing the distance it could fly, the DC-7 became the first passenger plane to fly nonstop from New York to Los Angeles. Because the DC-7 did not have to stop for refueling, the flying time of the trip was reduced. This reduced flying time increased profits and lowered the ticket price for the flight, while the shorter flying time made a cross-country trip less burdensome for most people. The DC-7 was also known as the Seven Seas because its long range allowed for flights around the world. The DC-7 was introduced in 1953 and it could carry 110 passengers, a small improvement over the DC-6. There were 338 of the planes built and a few continued to operate a half-century later. The DC-8, introduced in 1959, was the first jetpowered plane of the DC series. The four jet engines allowed the plane to reach speeds exceeding 960 kilometers per hour. The DC-8 became the first commercial jet to break the sound barrier. Along with its speed, the DC-8 had an expanded fuselage that doubled the passenger load to 260. While the plane had a slightly shorter range—7,200 kilometers—than its predecessor, its passenger capacity and freight-hauling abilities made it one of the largest commercial planes at that time. Over 550 of the planes were built, with more than 350 continuing to fly through the 1990s. Three different models of the DC-8s were built: the DC-8-61, the DC-8-62, and the DC-8-63. THE MODERN DCS The DC-9 has the distinction of having the largest number of commercial airplanes produced of any of

Principles of Aeronautics

the DC series. Some 976 planes were built, of five different types, each one extending the fuselage and allowing for more passengers. The DC-9-10 was the smallest version, carrying only ninety passengers and used primarily for shorter range flights. The DC-9-20 also had a smaller fuselage, carrying fewer than 100 passengers while utilizing larger engines to create greater thrust and carry larger payloads. The DC-9-30 added 15 feet to the fuselage and carried 115 passengers. The plane was specifically designed for rapid takeoff, allowing it to be used on smaller air fields. This made the DC-9-30 the most frequently used of all the aircraft. The DC-9-40 added another 6 feet to the fuselage and expanded passenger cargo to 125. The DC-9-50 was the largest plane in the family, with 8 more feet of fuselage beyond the DC-9 40, a passenger capacity of 139, and more space for cargo. Each of the DC-9s was introduced in the 1960s and many continued to fly both passengers and cargo at the turn of the century. The DC-10 was the last of the series to be produced. While many of the features of the series would be found in its successor, the MD, the merger of Douglas Aircraft with McDonnell Aircraft led to the end of the name DC. The first model DC-10 flew in August, 1971. The DC-10-30 and the DC-10-40 were both extended-flight airplanes, with ranges of 9,440 and 9,280 kilometers, respectively. Three other types of DC-10s were used, mainly for carrying freight. The DC-10 Convertible was able to carry passengers or freight, though it was mainly a cargo carrier. The DC-10-15 resembled the original DC-10 but had a longer range of approximately 9,600 kilometers. The last of the DC-10s was the 30F. It was used exclusively as a freight carrier and became one of the standard planes for package delivery companies. The 30F was renamed the KC-10 cargo plane for the US Air Force. When DC-10 production was halted in 1989, approximately 380 planes were flying commercially, while 60 more were being used as cargo carriers for the Air Force. Yet even with this commercial success, the

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DC-10 had a mixed safety record. A 1974 crash near Paris killed 346 people and was blamed on a cargo door blowing open in flight. Similar problems were discovered in other DC-10s. In a six-month period in 1979, some 500 people died in three DC-10 crashes. This was attributed to structural fatigue, with one crash caused by a pylon collapsing in flight. In July, 1989, in Sioux City, Iowa, the most spectacular crash occurred, when a DC-10s hydraulic system failed. Over 100 people died, although more than twice that many survived. These safety problems gave the DC-10 a bad reputation but it continues to fly in many airline fleets. The DC series ended with the DC-10. In 1967, the Douglas Company, suffering from severe financial losses caused by problems in the production of DC-8s and DC-9s, merged with the McDonnell Corporation to form McDonnell Douglas. The next series of DC planes were renamed the MD series, and when McDonnell Douglas merged with Boeing in 1997, the planes took on the 700-family name associated with that company. THE LEGACY OF THE DCS The DC series of planes may have been the most important of all families of passenger carriers. With their start in the 1930s, the DC series helped make air travel affordable for the individual and profitable for many airlines. The DC planes also established such innovations as nonstop flights across the United States, larger fuselages to carry ten times the passengers of the original DC models, and a dependability that sees many DCs flying local routes to smaller airports and others longer routes across countries or continents. While the DC line ended with the DC-10 and the original company that developed the model was merged into oblivion, the plane series continues to strike the imaginations of both those who study passenger airlines and those who fly them. —Douglas Clouatre

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Further Reading Borgmann, Wolfgang. McDonnell Douglas DC-10/MD11: A Legends of Flight Illustrated History. Schiffer Publishing Limited, 2021. Francillon, Rene. Douglas Propliners: Skyleaders, DC-1 to DC-7. Haynes Publishing UK, 2012. Godson, John. The Rise and Fall of the DC-10. D. McKay Company, 1975. Holden, Henry M. The Douglas DC-3. Aero, 2007. Peeler, Jodie. Douglas DC-9/MD-80: Later Variants Included. Squadron/Signal Publications, 2007. Singfield, Tom. Classic Airliners. Midland, 2000. Waddington, Terry. McDonnell Douglas DC-10. Motorbooks International, 2000. See also: Aeronautical engineering; Aerospace industry in the United States: Air transportation industry; Airplane accident investigation; Airplane manufacturers; Avro Arrow; Richard Branson; Glenn H. Curtiss; Federal Aviation Administration (FAA); Howard R. Hughes; Burt Rutan; Sound barrier; Supersonic jetliners and commercial airfare; Supersonic jets invented; Turbojets and turbofans; Turboprops

Differential Equations Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics; Advanced mathematics ABSTRACT Many laws of physics are best expressed by prescribing relationships between a function describing the phenomenon and its rate of change. Once these laws are supplemented by other conditions, such as the value of the functions at a specified time, it becomes possible to find out what actually happens. KEY CONCEPTS differential equation: a relationship between the derivatives of one or more functions and the functions themselves

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general solution of a differential equation: a formula involving arbitrary constants such that, by assigning values to the constants, one gets all the solutions of the differential equation order of the differential equation: the order of the derivative of highest order appearing in the differential equation parameters: variables that do not enter in the differential equation but that correspond to magnitudes which can be set from the exterior solution of a differential equation: a function or group of functions whose derivatives are related to the functions in the way prescribed by the differential equation OVERVIEW From the point of view of physics, perhaps the most important contribution of Sir Isaac Newton was the realization that many laws of nature are expressed by relations between functions and their derivatives. For example, in mechanics, if a body is moving under the gravitational influence of others, the position determines the force acting on it and, by Newton’s law of motion, dividing this force by the mass of the body, one can determine the acceleration. Since the acceleration is the second derivative of the position with respect to time, one finds that there should be a relationship between the position of a body and its second derivative with respect to time. Conversely, every motion described by functions satisfying the differential equation is possible for the system if one puts it in the appropriate initial state. Similar examples can be found in many physical sciences. For example, the rate of cooling of an object such as an airplane or any of its parts—the rate of change of its temperature with respect to time—is very often proportional to the difference of temperatures between the body and its surrounding media. Such relationships between functions and their derivatives are called “differential equations.” Given

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a differential equation, one may attempt to find all the possible functions that satisfy the equation. This is called “finding the general solution.” If one succeeds in doing that, by reading out the formulas one may discover all possible motions that the system may experience. This may be useful if one wants to find some particular type of motion with desirable properties. For example, if one has an explicit general solution for the pendulum, one may inquire about whether there is a motion in which the pendulum rotates twice and stops. Unfortunately, this procedure of finding explicit solutions is very difficult and, for some systems, even impossible. Many times, one must settle for simpler problems. For example, one may prescribe the initial position and velocity of a spaceship and ask where it will be one year from now. This is called the “initial value problem.” One important advantage of the differential equations method compared with other methods of encapsulating the laws of nature is that it allows its user to obtain approximate solutions systematically. One can, for example, use numerical methods, or one can systematically find out what the corrections are, starting from a simpler model. This is usually called a “perturbation analysis.” Perhaps the biggest triumph of the method of perturbation analysis is in solar system studies. If the planets had very small masses, the solar system could be understood using explicit formulas and one could derive Kepler’s laws. In the time of Newton, it was known that these laws did not fit the observations exactly. Newton and others, notably Pierre-Simon de Laplace, found that most of these observations could be accounted for by the fact that the planets are massive. Even if they could not find exact formulas for the motions of the planets, they could find formulas that, even if approximate, were of comparable or better accuracy than the experimental observations of the time. As the techniques to obtain data were refined, it became necessary to

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work harder at improving the accuracy of the approximate formulas. It was believed for a long time that this procedure of deriving more and more approximate solutions could be carried out to any degree of approximation. Nevertheless, in the last decades of the nineteenth century, Henri Poincaré started a systematic study of perturbation methods and discovered that some of them have intrinsic limitations. He also proposed a new way of studying differential equations, now called “the geometric approach.” The basic idea is that many of the questions one asks about differential equations are really geometric questions. For example, when one asks about an orbit going from Earth to the Moon and back, one is really asking about a line passing through regions in space. Even if one were to succeed in finding explicit formulas for the motions, it would be necessary to analyze the formulas to verify whether such orbits are possible. It then becomes natural to devise methods of reasoning that work with geometric objects using geometric arguments without making use of the crutch of deriving explicit formulas, whose geometric interpretation has to be worked out afterward. One result along this line—derived before Poincaré—is the continuity with respect to initial conditions. For many differential equations, it is possible to show that any initial condition determines uniquely where one will be one unit of time later. Moreover, if one makes a small error in the initial condition, the corresponding error in the position one unit of time later is also small. This propagated error can be made as small as one wishes by ensuring that the error in the initial conditions is small. For many differential equations whose defining functions admit derivatives of high order, one can get considerably sharper results. The results derived from the geometric program have had an enormous influence not only in applications but also in mathematics. Many new disciplines (such as topology) were

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created to serve as tools for the study of differential equations and then took on a life of their own. One important development has been the availability of digital computers. It is possible to write algorithms that produce very approximate solutions of the ordinary differential equations. In fact, one of the first problems that was tackled by computers was the production of “artillery tables”—which solve differential equations that model the motion of a shell in the atmosphere subject to gravitation and friction. The influence of computers has been very profound. For concrete applications in which the goal is to compute an actual orbit whose initial conditions are known, they are now the tool of choice. Perhaps more important, through judicious simulation of key cases, it is possible to develop intuitions that help solve the problem and lead to the understanding of new phenomena. Graphical representations that are easy to grasp have been developed, and the intuitions obtained through these graphical simulations are particularly helpful when used together with the mathematical results coming from the geometric approach. There are already several commercially available programs for the visual exploration of differential equations. APPLICATIONS Differential equations arose from the needs of classical mechanics, but they are fundamental tools for almost all branches of physical sciences and, more tentatively, for biology and economics. In application to mechanics, the success has been spectacular. Almost all features of the solar system have been accounted for (notable facts that still lack a convincing explanation are the rings of Saturn and the fact that the Moon always faces Earth and that Mercury rotates exactly three times around its axis every two revolutions around the Sun). It is also possible in a routine way to compute the motion of artificial satellites and of the Moon in such a way that the effects of all planets are included so as to get a

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precision of about a meter for the position of the Moon over a century. More important, it is possible to find orbits that are immune to disturbances or that use them to get several effects. Differential equations gave rise to many mechanical inventions that dominated science and technology until the beginning of the twentieth century. When mechanical devices were replaced by electric and electronic ones, differential equations remained the method of choice. By using very simple rules, it is easy to derive differential equations that model circuits. The difference of voltage across a capacitor is proportional to the charge that it stores; the derivative with respect to time of this charge is the current flowing to the capacitor. The difference of voltage across a resistor is proportional to the current flowing through it (Ohm’s law). For an electronic device such as a diode, the voltage as a function of the current is a complicated nonlinear function. The voltage attributable to self-induction is proportional to the rate of change of the current. For some more complicated devices, such as transistors or vacuum tubes, the current flowing across a pair of legs is a function of the voltage applied to them as well as the voltage of a third leg. Such equations are at the basis of all electronic applications. For example, it is possible to understand how to build circuits that will keep oscillating with a fixed frequency. Such circuits are found in many useful devices, such as radio transmitters and receptors, television sets, and computers. Another important application of differential equations is in the kinetics of chemical reactions. The rate of change of the concentration of chemicals in a reactor tank is proportional to the number of reactions that take place. Those, in turn, are proportional to the number of collisions of appropriate molecules, which in turn depends in a simple way on the concentrations. By using differential equations, it is possible to predict conditions in which the end product will be a

Principles of Aeronautics

useful material rather than a useless waste. It is also possible to predict regimes which stay safely away from dangerous behaviors such as explosions. Besides the direct applications of differential equations to physics, many problems in mathematics—frequently arising from physics—can be reduced to differential equations. A particularly important one is the use of the method of separation of variables in partial differential equations. Other problems that frequently lead to differential equations are variational problems in which one tries to determine functions which are optional in a certain sense. CONTEXT Differential equations appeared with calculus in the hands of Newton as a tool for mechanics, and quickly developed into the method of choice for modeling physical systems. A drastic revolution took place at the end of the nineteenth century, when several mathematicians realized that it was advantageous to think of differential equations in geometric terms. This geometric program remained dormant for a long time. In the West, it was used mainly by mechanical engineers and those who studied celestial mechanics, whereas in the East it was developed mainly by electrical engineers. In the 1960s, it again caught the attention of mathematicians who, in the meantime, had developed many disciplines—such as topology—that had made precise many intuitions. Important leaders in this revival were the mathematicians Stephen Smale, Jack K. Hale, and J. Moser in the West and A. N. Kolmogorov, V. I. Arnold, and Y. Sinai in the East. One important tool that made progress quicker and easier was the increasing availability of computers. The graphic displays made it easy to obtain a geometric intuition of phenomena and to test conjectures. Computers were also the only means of obtaining quantitative predictions that were useful in applied problems.

Differential Equations

From the late 1970s, the fact that differential equations could solve physical problems that had been unsolved for a long time was increasingly recognized by physicists. A notable turning point was the realization by Mitchell Feigenbaun that renormalization group methods could produce quantitative results for chaotic systems. From that time on, the field has experienced an explosive growth, and one can now find scientists in many disciplines (mathematics, physics, engineering, and even biology) making important contributions by using differential equations. Besides the specialized journals devoted specifically to differential equations, one can now find important results in many journals devoted to chemistry, physics, or biology. There are two types of functions that together describe the physical Universe; continuous functions and discontinuous functions. A continuous function is one whose value persists without interruption throughout its range. A discontinuous function, on the other hand, may have a nonzero starting point and be continuous only for small segments of its range, looking like the teeth of a saw when graphed. Both are fundamental to many aspects of aerospace, from the workings of avionics to consideration of the flow of air over wings and other parts of an aircraft. All digital electronics operate using discontinuous functions, simply because the information is encoded in a digital signal as a series of abrupt changes of the wave function between “high” and “low” signals. Analog electronics, however, function using continuous waveforms as the variation of voltage, current, and other electrical values. Calculation of fluid dynamics in regard to the design of an aircraft is essential to the design process, and consumes many hours of computation time to determine optimal design features such as the smooth, continuous shape of an aircraft’s fuselage. —Rafael de la Llave

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Further Reading Goodwine, Bill. Engineering Differential Equations: Theory and Applications. Springer New York, 2010. Leonides, C.T. Control and Dynamic Systems V.38: Advances in Aeronautical Systems. Advances in Theory and Applications. Elsevier Science, 2012. Miele, Angelo, and Attilio Salvetti, editors. Applied Mathematics in Aerospace Science and Engineering. Springer US, 2013. Wilson, Edwin Bidwell. Aeronautics: A Class Text. CreateSpace Independent Publishing Platform, 2017. See also: Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Airplane guidance systems; Airplane propellers; Airplane radar; Avionics; Conservation of energy; Flight instrumentation; Rocket propulsion; Rockets

Dirigibles Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT Dirigibles are aircraft that float because they are filled with lighter-than-air gas, and have power to direct their course of flight. Dirigibles were the leading edge of aviation from the 1850s until they were supplanted by airplanes and helicopters. Dirigible aviation developed many techniques that were later adopted for airplanes. In the twenty-first century, dirigibles may serve a number of niche functions, such as telecommunications repeaters, high-altitude science platforms, and heavy cargo transporters. KEY CONCEPTS buoyancy: the extent to which an object will float in a fluid medium on the basis of their relative densities density: the mass of an object in relation to its volume; the density of water = 1 gram per cubic centimeter; density of air = approximately 0.00129 grams per cubic centimeter (at 0ºC and 760 mm)

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fluid: a form of matter that has the ability to flow and adapts its shape to its container NATURE AND USE OF DIRIGIBLES Dirigibles, like balloons, are often referred to as lighter-than-air (LTA) craft, in contrast with airplanes and helicopters, which are heavier-than-air (HTA) craft. The term “dirigible” is a shortened form of “dirigible balloon,” from the French word meaning directable, or steerable, balloon. Buoyancy is the key to dirigible flight. The ancient mathematician Archimedes stated that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. For dirigibles, two LTA gases, hydrogen and helium, are combined to provide the buoyancy that lifts the dirigible and any payload. Typically, hydrogen lifts 30 kilograms per 31.15 cubic meters. Helium lifts 14 percent less (24 rather than 30 kilograms) per 31.15 cubic meters. Helium has a major safety advantage over hydrogen, in that it does not burn, whereas hydrogen can ignite explosively. Unfortunately, helium did not become available until the 1920s, and even then, the US government, which controlled most of the world’s supply, was slow to allow exports. Consequently, dirigibles manufactured outside the United States flew using highly flammable hydrogen, which caused many catastrophic fires. A third LTA gas, hot air, has only one-third the amount of lift of hydrogen, meaning the propulsion unit must have proportionately more thrust. Another dirigible concern is that the density and pressure of the surrounding air decreases with altitude. Hence, there is less lift available per unit volume, so the craft must be larger to carry a given payload to higher altitudes. Consequently, dirigibles with heavy payloads tend to be limited to low altitudes of a few thousand feet. For higher altitudes, designers can compensate for decreased lift per unit volume by using lighter payloads, such as remotely controlled instruments instead of people.

Principles of Aeronautics

TYPES Dirigibles are divided into three categories: nonrigid, semirigid, and rigid. Nonrigids are essentially large, streamlined cylindrical balloons, nicknamed “blimps,” supposedly for the sound made by a finger thumping into the side of the envelope, or gas bag. Nonrigids, such as the Goodyear blimps, get their shape from the gas within a single gas bag or envelope. The engines and a car or a gondola hang below the gas bag. The design of nonrigid dirigibles simplifies cost and minimizes structural weight, which in turn reduces the net lifting capability. However, nonrigids are limited in size, because an unsupported gas bag may bend unpredictably under heavy loads or strong winds. In a worst-case scenario, a partially deflated gas bag may flop over the gondola or propellers. Conversely, the gas bag cannot be filled too tightly, lest it burst. The one gas bag is a single point of failure that could cause a crash, although the large size of dirigibles means that operations could continue for some hours, even with significant leaks. Another method to compensate for pressure loss in the gas bag is the use of an inner ballonet, which can be inflated with outside air. Semirigid dirigibles have a keel on the bottom to support a larger gas bag, and the keel can hold the gondola and engines, at the cost of additional weight. The risks associated with a single gas bag also apply to semirigid dirigibles. The most famous semirigid was the airship Norge, which made the first transpolar flight from Spitsbergen Island to Alaska. Rigid dirigibles have a framework to support an outer skin and individual gas bags. Although the term “zeppelin” is sometimes used to describe any rigid dirigible, the name legally applies only to the type of craft manufactured by the Luftschiffbau Zeppelin company of Germany. In rigid dirigibles, an individual gas bag can fail without damaging the aerodynamic integrity of the craft, and there are usually sufficient reserves among

Dirigibles

Von Zeppelin’s first airship. Photo by Peter Scherer, Library of Congress Prints & Photographs division, via Wikimedia. [Public domain.]

the other cells to maintain buoyancy. Those advantages cost additional weight. However, greater weight can be compensated for by greater size. There is theoretically no limit to a rigid’s size. The German passenger airship Hindenburg had an LTA gas capacity of nearly 200,000 cubic meters, and designs of twice that size have been proposed. HISTORY Beginning in the 1790s, balloons made true humankind’s dream of the possibility to drift like clouds. Like clouds, however, balloons drifted wherever the wind blew. Thus, inventors realized their craft must be directable (dirigible) as well as lighter than air. The key to this directional ability was generating sufficient power while remaining light enough to fly. Repeated attempts in the first half of the nineteenth century showed that human power was insufficient against even slight winds. A number of inventors flew models powered by springs or clockworks during that time, but none of the models’ mechanisms could be sufficiently scaled up to power a craft carrying a person. Henri Giffard of France had the first partial success on September 24, 1852, with a dirigible powered by a steam engine. The engine, advanced for its time, produced 3 horsepower and weighed as much as two large men. Giffard’s aerial steamer, as

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it was known, launched from the Paris Hippodrome and hissed sedately to a landing 27 kilometers away. In a later flight, Giffard circled around Paris. However, because his craft’s top speed was only 9.6 kilometers per hour, it was not steerable against even a breeze. Fortunately for dirigible designers, the development of metallurgy and power plants advanced in the second half of the nineteenth century. In 1886, an electrolytic process was invented for producing aluminum inexpensively enough so that it could be used to replace the heavier steel in dirigible support structures. In 1876, German engineer Nikolaus August Otto began marketing a four-stroke, internal combustion engine yielding more power per unit

Ferdinand von Zeppelin. Photo by Nicola Perscheid, via Wikimedia Commons.

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weight than the external-combustion steam engines. In 1885, another German engineer, Gottlieb Daimler, patented significant improvements to the internal combustion engine and offered it for use in dirigibles. On November 12, 1897, an airship built by Austrian David Schwartz sported a 10-horsepower Daimler motor. Before it was ready to launch, a gust of wind pulled the craft loose from its moorings and toward nearby buildings. The pilot panicked and valved out so much gas that he crashed on the field. Despite its misfortune, this ship represented the first rigid dirigible, with a solid structure and a thin aluminum skin around the gas bag. By this time, both airships and balloons had developed a bad reputation. Fortunately, public relations assistance and superb flying skill arrived from Brazil in the form of wealthy experimenter Alberto Santos-Dumont, who took a single-cylinder engine from each of two tricycle automobiles to make a single, 66-pound, two-cylinder engine delivering 3.5 horsepower, roughly five times the power-to-weight ratio that had been available to Giffard. Santos-Dumont launched this 82-foot nonrigid craft with 1,812 cubic meters of gas volume, along with himself and a basket. On September 20, 1898, Santos-Dumont began flying around Paris in his airship, usually flying low enough to greet people on the streets. As both his flying skills and his dirigibles became progressively more advanced over the next several months, he aroused tremendous public interest, especially because he commuted around Paris in his compact dirigibles, mooring his craft above the spots to which he traveled. Santos-Dumont engendered so much interest in flying that a prize was offered to the aviator who could fly an eleven-kilometer course to the Eiffel Tower and back within thirty minutes. After several heroic attempts, Santos-Dumont won the prize. He became a global celebrity and inspired many others built nonrigid airships.

Principles of Aeronautics

In Germany, Count Ferdinand von Zeppelin built a large rigid dirigible, Luftschiff Zeppelin Number 1, or LZ-1, which was 128 meters long and 12.8 meters in diameter with a gas volume of 11,327 cubic meters, sixty times greater than that of Santos-Dumont’s model number 1. LZ-1, which first flew in July, 1900, had seventeen separate gas cells held together by an aluminum framework and covered with fabric. However, the two 15-horsepower engines gave LZ-1 a top speed of only 25.75 kilometers per hour, still insufficient to fly against moderate winds. Zeppelin raised more money to build LZ-2 and LZ-3, both of which had two 65-horsepower engines. LZ-2 was destroyed at its mooring by winds, but successful flights of LZ-3 led the German government to offer payment for a still-larger craft, if it could stay aloft for twenty-four hours. On August 4, 1908, the LZ-4 began a majestic tour from its home base on the Swiss border, heading north along the Rhine River. People along the way cheered the giant airship. LZ-4 flew for eleven hours, as far as Mainz, Germany, and had begun its return when one engine failed. Rather than press on in the dark with only one engine, Zeppelin set LZ-4 down near the town of Echterdingen. That night a storm pulled the craft loose and destroyed it. Yet Zeppelin’s story continued, as envelopes of cash began arriving from all over Germany. The so-called Miracle of Echterdingen supplied his company with more money than the German government had offered. The count continued to build, and by 1910, Zeppelin dirigibles had begun carrying sightseeing passengers and mail. By 1914, a number of Zeppelin dirigibles were in regular service. That year, World War I began. At first, the dirigibles dominated the skies, and the competing airplanes posed no threat to them. In 1915, German rigid dirigibles conducted the first long-range bombing attacks against targets in Great Britain, with little effective resistance from airplanes.

Dirigibles

However, faster and larger airplanes were soon able to catch the dirigibles, which proved to be large, slow targets. A single incendiary round of fire passing through a hydrogen gas cell could transform an airship into a fireball. In order to escape the airplanes, the Germans piloted their dirigibles to higher altitudes, where at 6,100 meters, water froze in the crew’s canteens. The airplanes, however, were improved enough to reach the dirigibles. By the end of the war, large airplanes had replaced rigids for long-distance bombing. The only dirigibles successful throughout the war were 200 nonrigids the British used to guard convoys against submarines. The long flights made by rigid dirigibles during the war suggested that dirigibles might be used for intercontinental passenger service, or even as flying warships. Continued research was conducted by four countries: France, Great Britain, the United States, and Germany. France had a number of smaller nonrigids, as well as one large rigid taken from Germany as part of its war reparations. The airship was renamed the Dixmude and flew for several years, making a record-breaking flight over the North African desert. After the airship exploded in flight during a storm in 1923, France abandoned large dirigibles. During World War I, the British had built an R (for rigid) series of dirigibles, which the British continued to develop after the war by reverse engineering from a captured German dirigible. On July 2, 1919, the R-34 left England, and, four days later, it had completed the first east-to-west aerial crossing of the Atlantic Ocean. In 1924, the British government started two competing programs to build dirigible airliners. The R-100, built with private funding, was known as the capitalist ship, and it flew well on a demonstration flight to Canada and back. The R-101, built by the government, was known as the socialist ship and was heavy with safety features. To increase lift, the builders cut the ship in half and inserted an additional gas bag. They also loosened

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wire netting around the gas cells, so they could be expanded. Unfortunately, this adjustment allowed the cells to rub against the framework, causing many small leaks. Because officials wanted to use the airship for a prescheduled demonstration flight to India, the major changes were not flight-tested. The R-101 launched from England on October 4, 1930, and early the following morning, it crashed into a hillside and exploded 64 kilometers northwest of Paris; forty-eight of the fifty-four people aboard died. As a result of this accident, Great Britain abandoned passenger airships and even scrapped the successful R-100. In the 1920s and 1930s, the US government operated four rigids as military ships intended for long-range reconnaissance. Two of the airships, the USS Akron and the USS Macon, actually carried their own fighter planes for defense. Because the United States held most of the world’s helium supply and used helium for its LTA gas, none of these craft exploded. However, three were lost in storms, and the United States abandoned the giant rigids after the last, the Macon, broke up in a storm and went into the sea off Point Sur, California, on February 12, 1935. The Luftschiffbau Zeppelin company of Germany, with its experience in building more than 100 rigids and its thorough design details, had the best safety record of any dirigible manufacturing company. For several years after World War I, Germany was forbidden by the Treaty of Versailles from possessing dirigibles larger than 28,317 cubic meters. However, in 1922, the U.S. Navy placed an order for a dirigible, which was named the Los Angeles. Zeppelin’s brilliant manager, Hugo Eckener, flew the craft to the United States. After the size limit on German dirigibles was lifted in 1925, Eckener organized construction of the Graf Zeppelin. Beginning in 1928, the Graf Zeppelin circled the world, flew regularly to Brazil and North America, made an Arctic expedition, and traversed 1,609,344 kilometers before being retired.

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The last and greatest rigid was the Hindenburg, launched in 1936. The Hindenburg was 245 meters long and 41.15 meters in diameter. Its 198,218 cubic meters of gas allowed it to carry fifty passengers and sixty crew in absolute luxury at a speed of 135 kilometers per hour for a range of 17,703 kilometers. The Hindenburg and the older Graf Zeppelin represented great profits for Luftschiffbau Zeppelin and good propaganda for Germany’s Nazi regime. Then disaster struck. Although Luftschiffbau Zeppelin was negotiating with the US government for helium, it still employed hydrogen in its airships. As the Hindenburg was docking at Lakehurst, New Jersey, on May 6, 1937, several crew members noticed a small fire in one gas cell. Within one minute, the craft had exploded into a ball of fire and lay on the ground, a smoldering wreckage. Although many theories proposed causes such as lightening, leaking gas, and anti-Nazi sabotage, filmed footage of the event convinced the public that large dirigibles were unsafe. After the Hindenburg disaster, only nonrigids remained, and they played a major role in the antisubmarine warfare of World War II. However, they were retired in the 1950s, after it became clear that helicopters provided the same hovering capability with greater dash capability and easier storage. In the last third of the twentieth century, the few working nonrigid dirigibles were limited to use as advertising billboards and as vehicles for television cameras providing overhead views of sporting events. ECONOMICS AND PROSPECTS Although dirigibles at the beginning of the twentyfirst century enjoyed a small resurgence in several niche markets, they will probably never recover their primacy in aviation for five major reasons. The first reason is the massive investment cost of building and developing dirigibles. Several factors make dirigibles more efficient as their size increases. However, the increase in size increases the cost of

Jimmy Doolittle

Principles of Aeronautics

design and building. Large size also reduces the number of units made, so dirigibles have less chance for lower costs and improved designs than do HTA craft, which are typically made by the hundreds or thousands. Second, hangar costs are high. Dirigibles are kept inflated because their helium lifting gas is expensive and would require too much time and effort to pump back into tanks. However, inflated dirigibles can easily be swept off their parking areas by winds. Consequently, dirigibles must be housed in their own special hangars instead of being parked on runways as airplanes are. Third, dirigibles are vulnerable to bad weather, which limits their performance. The giant buoyant structures can be seized by gusts of wind on takeoffs and landings and are more vulnerable than airplanes to icing. Zeppelin passenger flights were not scheduled in winter. Dirigibles are so large that winds may pull them in different directions while they are in flight, destroying them. The USS Shenandoah, Akron, and Macon were all destroyed in this way. Moreover, unless they are specially designed for high altitude, dirigibles cannot readily climb above storms as jet-propelled airplanes can. Fourth, because dirigibles’ great size causes more drag per unit mass of cargo, dirigibles are significantly slower than their HTA competition. They can at best obtain one-half the speed of propeller-driven planes and one-fifth that of jets. Thus, a jet with one-fifth of the cargo capacity of a dirigible can deliver the same cumulative mass of cargo. For the passenger market, shorter flight times are crucial. Still, dirigibles have potential for certain markets because they can run quietly and smoothly, linger for long periods, carry heavy and awkwardly large payloads, and land without runways. These advantages have been increased by lighter and more fireproof materials. The number of advertising dirigibles increased steadily beginning in the 1980s. At the start of the twenty-first century, the present-day

Luftschifftechnik Zeppelin company marketed sightseeing semirigids one-third the size of the Hindenburg. A German-American company called CargoLifter designed a cargo-carrying rigid larger than the Hindenburg. Meanwhile, an entirely new concept was being developed: the use of dirigibles in the lower stratosphere as high-altitude platforms. Such platforms could serve many functions of communications satellites and astronomical satellites at a fraction of the cost of spacecraft. —Roger V. Carlson Further Reading Charles River Editors. Famous Dirigibles: The History and Legacy of Lighter Than Air Vehicles from the Renaissance to Today. Independently Published, 2019. ———. Modern Balloons and Airships: The History and Legacy of Dirigibles During the 20th Century. Independently Published, 2019. Hiam, C. Michael. Dirigible Dreams: The Age of the Airship. ForeEdge, 2014. Talbot, Frederick Arthur Ambrose. Aeroplanes and Dirigibles of War. Outlook Verlag, 2022. Tsiolkovsky, Konstantin E. Dirigibles. UP of the Pacific, 2004. See also: Aerodynamics and flight; Aeronautical engineering; Atmospheric circulation; Blimps; Flight balloons; Gravity and flight; Hindenburg; Hot-air balloons; Lighter-than-air craft; Konstantin Tsiolkovsky

Jimmy Doolittle Fields of Study: Aeronautical engineering; Mechanical engineering; World War II history ABSTRACT James “Jimmy Doolittle” was born on December 14, 1896, in Alameda, California and died on September 27, 1993, in Pebble Beach, California. He was a pilot and pioneer of military aviation and instrument flying. As a member of

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the US Army Air Service in 1922, Doolittle made the first transcontinental flight in less than twenty-four hours. He was most noted for leading the first air raid over Japan during World War II. EARLY LIFE James Harold “Jimmy” Doolittle was born in California but spent much of his youth in Alaska. He left the University of California in 1917 to enlist in the US Army Reserve and was assigned to the Signal Corps. During World War I (1914-18), he served as an aviator and flight instructor. Commissioned as a first lieutenant in 1920, he spent much of the following decade in the development of military aviation. During this period, Doolittle combined his interest in aviation with the sport of flying. He took part in numerous races, winning a number of trophies. In September, 1922, he carried out the first transcontinental flight from Florida to California, a distance of more than 2,100 miles, in fewer than twenty-four hours. The purpose of the flight was to support the growing role of the US Army Air Service in the nation’s defenses. At the same time, the flight brought Doolittle to national prominence. In 1930, Doolittle resigned from the Army to work for the Shell Petroleum Company. He continued to race, setting a world speed record in 1932. In 1940, Doolittle rejoined the Army Air Corps with a rank of major. On April 18, 1942, as a lieutenant colonel, he led a force of sixteen B-25 bombers that had been highly modified to reduce their weight and carry extra fuel from the USS Hornet, hitting targets in Japan more than 1,300 kilometers across the Pacific. Although the targeted cities, Tokyo, Yokohama, Kobe, and Nagoya, received negligible damage, the raid shattered the impenetrable image of the Japanese islands. Most of the planes and their seventy-five fliers crash-landed in China, having carried only enough fuel for a one-way trip. Doolittle was awarded the Congressional Medal of Honor for his action.

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Jimmy Doolittle. Photo via Wikimedia Commons. [Public domain.]

During the war, Doolittle rose to the rank of lieutenant general, commanding the Twelfth Air Force in North Africa and the Fifteenth Air Force elsewhere in the region. In 1944, Doolittle assumed command of the Eighth Air Force, directing bombing of Germany until the end of the war. From 1948 to 1958, Doolittle served on both the National Advisory Committee for Aeronautics and the President’s Science Advisory Committee. He became director of Space Technology Laboratories following his retirement from the Air Force in 1959. Doolittle’s autobiography, I Could Never Be So Lucky Again, covered his extensive career in the air service, with emphasis on the Tokyo raid. —Richard Adler

Principles of Aeronautics

Further Reading Daso, Dik A. Doolittle, Aerospace Visionary. Potomac Books Inc., 2014. Doolittle, James, and Carroll V. Glines. I Could Never Be So Lucky Again: An Autobiography. Random House Publishing Group, 2001. Glines, Carroll. The Doolittle Raid. Schiffer, 1999. Hoppes, Jonna Doolittle. Calculated Risk: The Extraordinary Life of Jimmy Doolittle-Aviation Pioneer and World War II Hero. Santa Monica Press, 2022.

Jimmy Doolittle

Mann, Carl. Lightning in the Sky: The Story of Jimmy Doolittle. Verdun Press, 2016. Scott, James M. Target Tokyo: Jimmy Doolittle and the Raid That Avenged Pearl Harbor. W. W. Norton, 2016. See also: German Luftwaffe; John Glenn; Howard R. Hughes; Charles A. Lindbergh; Billy Mitchell; Eddie Rickenbacker; Igor Sikorsky; Manfred von Richthofen; Wright brothers’ first flight; Chuck Yeager

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E Amelia Earhart Fields of Study: Aeronautical engineering; Mechanical engineering; Aviation ABSTRACT Amelia Earhart was an American aviator, born July 24, 1897, in Atchison, Kansas. It is believed she died in early July, 1937, although there are unconfirmed reports that she was seen alive for some time after that. By being the first woman to fly across the Atlantic and by establishing numerous other flying records, Earhart helped to promote commercial aviation and advance the cause of women in aviation.

learned to ride horseback. When her father accepted a job in Des Moines, Iowa, in 1905, Earhart and her sister remained for a year in Atchison, where she later recalled, “There were regular games and school and mud-ball fights, picnics, and exploring raids up and down the bluffs of the Missouri River.” After joining her father in Des Moines, Earhart attended school and began reading the books that fur-

KEY CONCEPTS autogyro: forerunner of the helicopter, an aircraft with short wings and an overhead rotor for propulsion magnetometric: by measurements acquired using a magnetometer solo: a first flight by a trainee pilot without the accompaniment of an instructor pilot trimotor: an airplane with three motors, typically each driving a propeller EARLY LIFE Amelia Earhart, the daughter of Amy Otis and Edwin Stanton Earhart, was born in the home of her maternal grandparents in Atchison, Kansas. Her grandfather was Alfred G. Otis, a pioneer Atchison settler who became a prominent lawyer, banker, and federal district court judge. Her father worked for a railroad as an attorney and claims agent. Earhart’s early childhood was spent in Kansas City, Kansas, where she and her younger sister

Amelia Earhart. Photo via Wikimedia Commons. [Public domain.]

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ther encouraged her spirit of adventure. Sir Walter Scott, Charles Dickens, George Eliot, and William Makepeace Thackeray were her favorite authors, and she and her sister made up imaginary journeys while they played in an abandoned carriage. When her father went to work for the Great Northern railroad, the Earharts moved to St. Paul, Minnesota, but Edwin’s alcoholism grew worse, and her mother took her daughters to Chicago, where Earhart graduated from Hyde Park High School in June 1916. She attended the Ogontz School in Rydal, Pennsylvania, then went to Toronto, Canada, where her sister was in school. In Toronto, she saw wounded veterans of World War I and became a Red Cross volunteer. She worked at Spadina Military Hospital, where she came to know and admire the young fliers of the Royal Flying Corps. In 1918, she was ill with pneumonia and went to live with her sister in Northampton, Massachusetts. While her sister was enrolled at Smith College, Earhart took a course in automobile repair. In 1919, she enrolled at Columbia University to study medicine but left after a year to join her parents in Los Angeles. The aviation industry was just beginning to develop in Southern California, and Earhart was attracted to the air shows and flying demonstrations at local airports. She took her first airplane ride from the Glendale airport and soon convinced her parents to help her take flying lessons with a pioneer woman pilot, Neta Snook. In June, 1921, Earhart made her first solo flight in a Kinner Airster. One year later, she had saved two thousand dollars to buy a three-cylinder Kinner Canary, a plane in which she set a woman’s altitude record of fourteen thousand feet. Her career as a pilot was launched. LIFE’S WORK Even in 1922, however, flying was expensive, and paid employment for women in aviation was limited. When her parents were divorced, Earhart sold her plane and returned to Massachusetts, where she

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taught English to immigrants and became a social worker at Denison House, a Boston settlement. She was able to combine her interests in social work and aviation, on one occasion flying over Boston and dropping leaflets announcing a Denison House street fair and on another judging a model airplane contest for the National Playground Association. In 1928, she was selected by the publisher George P. Putnam to fly with pilot Wilmer Stutz and mechanic Lou Gordon in a Fokker trimotor across the Atlantic. The plane, named Friendship, had been purchased from the explorer Richard Byrd by Amy Phipps Guest, an American flying enthusiast who had married and settled in England. When Guest was unable to make the flight herself, she asked Putnam to find a young woman (he found Earhart) to represent her in the promotion of women in aviation. On June 3, Friendship left Boston for Halifax, Nova Scotia, and Trepassy, Newfoundland. Delayed by bad weather for several days, the plane left Trepassy on June 17 and landed the following day at Burry Port, Wales. Earhart was given a hero’s welcome on her return to New York. Because her flight came only a little more than a year after the solo flight by Charles A. Lindbergh, and because of her tall, slender build and short, blond hair, she was nicknamed Lady Lindy, but she preferred to be called “AE.” Within a few months Putnam rushed her account of the flight, Twenty Hours Forty Minutes (1928), into print. The book is part autobiography, part journal of the flight, and part advocacy of flying in general. It is the third part that is most interesting because of her observations on the future of flying and on the role of women in aviation. After stating that the remarkable thing about flying is that it is not remarkable, Earhart goes on to discuss the need for more attractive airports, a review of safety regulations, and better weather reporting. Women will have a role to play in all these areas, she asserts, because they have already had a

Principles of Aeronautics

major impact on the automobile industry. The airplane will be used for leisure and recreation, and the growing purchasing power of American women will help to shape the airline industry. Earhart concludes her book with a characteristically honest assessment of the ways in which her life has been changed by her sudden fame. For the remainder of her life, Earhart campaigned tirelessly for the cause of women in flying. She participated in many cross-country air races, flew an autogyro (a forerunner of the helicopter), and was one of the founders of an organization of licensed women pilots, the Ninety-nine Club. In 1932, she was elected a member of the Society of Women Geographers. She also wrote a column on aviation for Cosmopolitan magazine. Her advice was sought by many airlines and airplane manufacturers, and she became a model for young women throughout the country. In 1931, she married Putnam, who had been managing her career. Her second book, The Fun of It, was published in 1932. In it Earhart adds details about her childhood and further explains her attraction to flying, especially to unusual aerial maneuvers known as “stunting”: I had fun trying to do [stunts]...so much so, in fact, I have sometimes thought that transport companies would do well to have a ‘recreation airplane’ for their pilots who don’t have a chance to play in the big transports or while on duty. If a little stunt ship were available, the men could go up 5000 feet, and ‘turn it inside out’ “turn it inside out” to relieve the monotony of hours of straight flying. Her assurance that flying was safe and fun and her example as the first woman to fly the Atlantic alone increased her popularity with the public. Earhart’s solo flight from Harbor Grace, Newfoundland, to Culmore, Ireland, May 21-22, 1932, won for her the Distinguished Flying Cross from the US Congress of

Amelia Earhart

the United States, an award from the French Legion of Honor, and a medal from the National Geographic Society. In 1935, she became the first person to fly alone from Hawaii to California and the first to fly nonstop from Mexico City to Newark, New Jersey. The trustees of Purdue University purchased a twin-engine Lockheed Electra for her, and she began to plan a round-the-world flight. After several false starts and minor accidents, Earhart and her navigator, Fred Noonan, took off from Miami, Florida, on June 1, 1937. A month of flying brought them across the Atlantic, Africa, and southern Asia to Lae, New Guinea. She and Noonan took off July 2, intending to land and refuel on tiny Howland Island in the middle of the Pacific Ocean. Several hours later, the Coast Guard cutter Itasca, anchored off Howland Island, heard a radio message from Earhart that she was lost and running low on fuel. Neither the plane nor its pilot and navigator were ever found. Because the Japanese claimed many of the islands in the mid-Pacific, rumors grew that Earhart and Noonan had crashed on a Japanese-held island and been captured and killed. After World War II, attempts were made to find the wreckage and confirm the rumors, but no convincing evidence had come to light. However, in late 2014, the International Group for Historic Aircraft Recovery (TIGHAR) claimed that a small piece of aluminum plane wreckage discovered on a nearby island in 1991 could very well have belonged to Earhart’s Electra. The group spotted a similar metal patch in the photograph of Earhart and her plane captured before taking off from Miami. They believe that the piece served as a modification in the field. To further support this connection, the group compared the patch’s size, shape, and rivet holes with an Electra being restored in Kansas, deeming it a match. This discovery would bolster the theory that the pilot and her navigator were forced to land on Nikumaroro after running out of fuel rather than crashing in the

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Pacific. While TIGHAR wants to travel back to Nikumaroro to see if an earlier anomoly found close to the island on sonar could be more of the wreckage, critics and skeptics have argued that there are still too many variables involved to make this possible link definitive. In recent years, however, researchers using sophisticated magnetometric methods found what appeared to be one of the wheel assemblies from her lost aircraft. SIGNIFICANCE Earhart was one of the most appealing heroes in an age of American hero worship. Like Lindbergh and Byrd, Earhart pioneered air travel by establishing flying records and opening new routes. Like Babe Didrikson Zaharias the athlete and Louise Arner Boyd the Arctic explorer, Earhart showed that women had a place in fields that were generally restricted to men. Although she was criticized during her life for using her fame for profit—at various times she promoted Lucky Strike cigarettes, luggage, and sports clothes—Earhart remained essentially a private person. Because her parents believed that girls should have the same opportunities as boys, she was able to learn to fly. Because she believed that she should help others by sharing her experiences, she maintained a hectic schedule of flying and lecturing. Once she had been given the opportunity to be the first woman to fly across the Atlantic, Earhart dedicated herself to flying. She was able to combine pleasure with business, and she worked hard at both. Success brought her into contact with other notable women, from First Lady Eleanor Roosevelt to film star Mary Pickford. Earhart was also a celebrity, and her untimely death at the age of thirty-nine enshrined her in the hearts of her generation. Earhart was a product of the social changes in the United States between the world wars. In many ways she epitomized her generation’s desire to break with the past and to create a better world. She captured

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something of that spirit in one of her poems, which begins, Courage is the price that life exacts for granting peace. The soul that knows it not, knows no release From little things; Knows not the livid loneliness of fear Nor mountain heights, where bitter joy can hear The sound of wings. —Bernard Mergen Further Reading Backus, Jean L. Letters from Amelia: 1901-1937. Beacon, 1982. Burke, John. Amelia Earhart. Flying Solo. Voyageur Press, 2017. Campbell, Mike. Amelia Earhart: The Truth at Last. 2nd ed., Sunbury Press Inc., 2016. Earhart, Amelia. The Fun of It: Random Records of My Own Flying and of Women in Aviation. 1932. Reprint. Gale Research, 1975. ———. Last Flight: The World’s Foremost Woman Aviator Recounts, in Her Own Words, Her Last, Fateful Flight. Crown, 2009. ———. Twenty Hours Forty Minutes. 1928. Reprint. Arno, 1980. Fleming, Candace. Amelia Lost: The Life and Disappearance of Amelia Earhart. Yearling Reprint, 2019. Gillespie, Ric. Finding Amelia: The True Story of the Earhart Disappearance. Naval Institute Press, 2006. Harris, Mike. Amelia Earhart’s Final Flight: On Amelia Earhart’s Final Flight She Landed on Mili Atoll and Was Captured by the Japanese. CreateSpace Independent Publishing Platform, 2017. King, Thomas F., Randall S. Jacobson, Karen Ramey Burns, and Kenton Spading. Amelia Earhart’s Shoes: Is the Mystery Solved? University Press of America Inc., 2004. McCoy, Terrence. “The Metal Fragment That Could Solve the Mystery of Amelia Earhart’s Disappearance.” Washington Post, 30 Oct. 2014. Accessed 13 Jan. 2015. Slowson, Larry. What Happened to Amelia Earhart? Larry Slowson, 2019. Van Pelt, Lori. Amelia Earhart: The Sky’s No Limit. Forge, 2005.

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See also: Glenn H. Curtiss; Jimmy Doolittle; Yuri Gagarin; Howard R. Hughes; Otto Lilienthal; Charles A. Lindbergh; Billy Mitchell; Wiley Post; Eddie

Amelia Earhart

Rickenbacker; John Alan Shepard; Valentina Tereshkova; Manfred von Richthofen; Wright brothers’ first flight; Chuck Yeager

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F Federal Aviation Administration (FAA) Fields of Study: Physics; Mathematics ABSTRACT The Federal Aviation Administration (FAA) is the US government organization primarily responsible for overseeing aviation safety, air traffic control and navigation, federal funding for airport and airway facilities, and civil aviation security. It is responsible for issues of aviation safety regulation, inspection, examination, certification, and issuance of licenses. The FAA oversees pilots, aircraft, airports, airlines, air traffic control and navigation, aircraft and parts manufacturing, repair, civil aviation security, and even commercial space transportation—blasting private satellites into space.

traffic control and navigation, aircraft and parts manufacturing, repair, civil aviation security, and even commercial space transportation—blasting private satellites into space. FUNCTIONS AND STRUCTURE The FAA has three main areas of responsibility: air traffic control and navigation; civil aviation safety regulation, certification of airlines and aircraft, and licensing of pilots, mechanics, and other aviation personnel; and civil (as opposed to criminal) aviation security regulation and enforcement to safeguard airports, airplanes, and personnel and passengers from terrorism and other criminal threats to aviation. To accomplish these functions, the FAA has its headquarters in Washington, D.C., nine regional offices, and hundreds of other field offices in the

KEY CONCEPTS air traffic control system: monitored radar stations on which aircraft signals appear accompanied by unique identifying information, course prediction, and altitude regulation and deregulation: establishment and removal of federal rules governing the operation of the aviation industry THE FAA The Federal Aviation Administration, the FAA, was formed on October 15, 1966, with the creation of the Department of Transportation. It oversees all aspects of air transportation and is responsible for issues of aviation safety regulation, inspection, examination, certification, and issuance of licenses. The FAA oversees pilots, aircraft, airports, airlines, air

Logo of the Federal Aviation Administration. Image via Wikimedia Commons. [Public domain.]

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United States and worldwide. The FAA has two major research centers in Oklahoma and New Jersey. The FAA employs about 45,600 employees. The majority of these employees are air traffic controllers. The job of regulating, inspecting, and licensing airlines, aircraft, pilots, and mechanics is performed by a smaller regulation and certification workforce. The remaining personnel work in the area of civil aviation security, in administration, in research, or even in the overseeing the safety of commercial space launches to put satellites into orbit for telecommunications companies or other businesses. The FAA is headed by an administrator who serves a five-year term under the US secretary of transportation. A deputy administrator and several associate and assistant administrators oversee the different areas of FAA responsibility. Although the size of the FAA workforce may seem extraordinary, it is appropriate to the role of aviation in the United States. FAA air traffic controllers handled approximately 42,700 flights per day, for a total of 15,631,000 flights in 2016. As of that year, there were 19,601 airports in the United States, 5,116 of which were public-use airports. As of the end of 2017, there were 609,306 active certified pilots in the United States: whether they were students in small propeller planes or airline captains commanding jetliners carrying hundreds of passengers, more than one-half million people held licenses permitting them to fly. The FAA expends about $16 billion yearly in the performance of its functions. Much of that total is paid by the traveling public through excise taxes added to the price of airline tickets. These taxes, totaling almost $12 billion annually, go into a national aviation trust fund to pay for improvements to airports and airways. ORIGINS Soon after Jean-François Pilâtre de Rozier and the marquis François d’Arlandes completed the first

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untethered balloon flight on November 21, 1783, the first effort at aviation regulation was made. In April, 1784, a French police ordinance required permits for balloon flights over Paris. Early laws were hardly conducive to aviation. Roman law proclaimed that whoever owned land also owned the sky above that land. Early property law provided that if one owned the land on the surface, then one owned it to the center of the earth and to the heights of the sky. This legal concept did not prove especially troubling until air travel became possible. At the time of Wilbur and Orville Wright’s success on December 17, 1903, the prevailing legal concepts made the dominion of the skies somewhat like that of the ocean: Both were considered to belong to all people but not to any one person. By the end of the World War I, that theory had been replaced by the realization that a nation’s skies were the key not only to its defense but also to its prosperity. Thus, each nation’s skies became protected airspace. Treaties were drafted to keep nations’ aircraft from entering other nations’ airspace and to regulate the economics and safety of international aviation. REGULATORY FRAMEWORK In 1919, the world powers met in Paris to devise a plan for the implementation of an international regulatory framework to carry out civil aviation in a peaceful, safe, and efficient manner. The sovereignty of each nation’s airspace was recognized, and the group proposed minimum standards for certification and safety regulation as well as general rules for air traffic control. Each nation would be required to adopt regulations to certify its airlines, aircraft, and pilots and to oversee the safety of its operations. Although the United States sent representatives to attend the Paris conference, it did not adopt the convention’s agreements. The United States would become a signatory to later air commerce and navigation treaties, and

Principles of Aeronautics

eventually the international oversight of aviation would be governed by the International Civil Aviation Organization, a part of the United Nations. To this day, the international regulatory plan depends on each nation having aviation safety laws and a government agency to enforce them. Although the FAA would eventually fulfill that role for the United States, it was still decades in the making. THE US AIR MAIL SERVICE The US Post Office was the beneficiary of the first US aviation regulation. In 1920, after numerous airmail accidents, the head of the US Air Mail Service set about to improve the situation. Pilots were required to complete 500 hours of flight training, pass an examination, and undergo a physical to establish medical fitness. Orville Wright assisted the effort to qualify and license the nation’s pilots, personally signing some of the earliest US pilot’s licenses. Air mail was privatized with the Air Mail Act of 1925, known as the Kelly Act. The routes were put up for bid, and wealthy American industrialists, such as Henry Ford, William Rockefeller, Cornelius Vanderbilt, and Marshall Field, garnered the first contracts. On May 20, 1926, the Air Commerce Act was passed at the urging of the aviation industry, after the aviation industrialists realized aviation could not reach its significant commercial potential without the federal government providing safety regulation. It is unsurprising, then, that the job of aviation safety was given to the US Department of Commerce. The secretary of commerce was charged with promoting air commerce, enforcing air traffic rules, licensing pilots and planes, certificating aircraft, establishing airways, maintaining aids to air navigation, and generally working to improve aviation’s dismal safety record. With that, the seeds of the future FAA were planted, and there was much to be done. There were only 6,000 passengers willing to brave the airlines in 1926.

Federal Aviation Administration (FAA)

FORERUNNERS OF THE FAA By 1933, the nation’s system of 28,800 kilometers of airways with 1,500 beacon towers and 263 landing fields was finished. Aerial navigation was very much a ground-based enterprise, with a cross-country system of ground beacons, small towers with a flashing rotating light and two course lights. In addition, ninety radio navigation stations had been built to provide aural and visual guidance to pilots. In 1930, the first radio-equipped air traffic control tower was built in Cleveland, Ohio, with twenty more to follow by 1935. In 1935, the cities of Chicago and Newark set up air traffic control systems to control their flights. The Bureau of Air Commerce was formed in 1934 within the Department of Commerce, and, two years later, it took over the responsibility of air traffic control. By the 1930s, the airlines, in the throes of destructive price-cutting competition spurred by mail-contract bidding, were themselves clamoring for federal regulation. The airlines wanted to upgrade their fleets with the new, sleek, metal marvels of the aviation world: the Douglas DC-3 aircraft. To afford these airplanes, the airlines needed to be spared from cutthroat price wars. The airlines’ solution was federal economic regulation. By having the federal government regulate not only air traffic control, safety, and certification, but also airline profits, they would be protected from huge losses caused by destructive competition, and they could afford to buy the marvelous new DC-3s. Thus, in 1938, the Civil Aeronautics Act created the Civil Aeronautics Authority to regulate safety and economics. In 1940, the authority was split into the Civil Aeronautics Board (CAB), which had the powers of safety regulation, accident investigation, and economic regulation and also established airline fares and routes, and the Civil Aeronautics Administration, which was responsible for air traffic control, pilot and aircraft certification, safety enforcement, and airway development. The airline plan worked.

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Americans loved the DC-3, which remains the most successful transport plane ever. Almost 11,000 were built in the United States and at least as many were manufactured overseas. By 1941, there were 3 million US airline passengers, who looked to the US government to protect their safety. During World War II, both military and civil aviation changed dramatically. Newly developed radar technology was applied to air traffic control. In 1944 alone, the United States produced 96,318 airplanes. Aviation was credited by many historians with winning the war. During the Cold War, the federal government provided money for airports and instrument landing systems. Equipment, such as pressurized airplanes, airborne weather radar, and autopilots, were dramatically improved, and passenger comforts were increased. By 1956, US airline passengers outnumbered rail passengers, a trend that was never reversed. However, during this time, several tragedies shook the confidence of the flying public. The world’s first commercial jet airliner, the British De Havilland Comet commenced passenger jet service in May, 1952, but its success was short-lived. Of the nine Comets in commercial passenger service, three seemingly came apart in midflight. In 1956, a United Air Lines flight collided in midair above the Grand Canyon with a Trans World Airlines (TWA) flight, killing 128. After it was discovered that an air traffic controller had seen the planes’ collision course on his radar and had failed to warn the pilots, the reputation of the CAA was tarnished. After two more US midair collisions, between US Air Force planes and civilian airliners, it was recognized that changes were required. FEDERAL AVIATION AGENCY In 1958, the new, independent Federal Aviation Agency was created and took over the CAA’s functions. It also took from the CAB the job of promul-

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gating safety regulation and coordinating military and civilian air traffic control. The CAB retained the responsibilities of economic regulation and accident investigation, but not for long. A midair collision on December 18, 1960, between a United Air Lines and a TWA flight in the skies above New York killed 135 people, including 8 on the ground, and intensified the public demand for improvements. The 1960s brought the radar-based air traffic control (ATC) system, with its banks of green screens that enabled controllers to monitor the nation’s airports and airways. In 1966, the Department of Transportation (DOT) was formed to coordinate the regulation of all modes of transportation within one department. The Federal Aviation Agency, now operating under the DOT, became the Federal Aviation Administration, the FAA. The accident investigation function was removed from the CAB’s jurisdiction, and an independent accident investigation organization, called the National Transportation Safety Board (NTSB), was established. Although the FAA may assist in aircraft accident investigation, the NTSB remains primarily responsible. REACTING TO TRAGEDIES Unfortunately, in the coming decades, air tragedies continued to direct the course of the FAA. Aircraft hijackings in the 1960s caused the FAA to institute security regulations and requirements, followed years later with more-stringent requirements following more deadly and catastrophic aircraft crimes, such as the terrorist bombing of Pan American Flight 103 in 1988. Plastic explosives were hidden in a personal tape recorder, which was loaded in Frankfurt, Germany, into the baggage compartment of the doomed plane. There was no passenger on board the flight to accompany the baggage. Major domestic security changes were ordered after a tragic episode in 1987, in which a fired Pacific Southwest Airlines (PSA) employee boarded the airliner with his old employee badge and, after takeoff,

Principles of Aeronautics

shot his former boss, a passenger. The killer then shot the aircraft’s pilots, and the plane plunged to the ground, killing all on board. The most shocking tragedy occurred on September 11, 2001, when teams of terrorists hijacked four commercial jets. Two 767s out of Boston’s Logan Airport, American Airlines Flight 11 and United Air Lines Flight 175, were intentionally crashed into the Twin Towers of the World Trade Center in New York City, causing both buildings to collapse. The third plane, American Flight 77, a 757 out of Washington’s Dulles Airport, was crashed into the Pentagon in Washington, D.C. The fourth plane, United Flight 93 out of Newark, New Jersey, crashed in a field near Pittsburgh, Pennsylvania, when passengers stormed the cockpit of the 757. In total, more than 5,500 people, from the planes and on the ground, were killed. The hijackers smuggled box cutters on board the aircraft, gained access to the cockpits, either killed or incapacitated the flight crews, switched off the transponders, and took over the controls. Two of the terrorists, Islamic fundamentalists associated with Osama bin Laden’s al-Qaeda network, were on a Federal Bureau of Investigation (FBI) watchlist but were allowed to purchase tickets. Eventually, the NTSB would discover the trend that the most frequent among many causes of accidents was the failure of the FAA to act to avert catastrophe. AIRLINE DEREGULATION In 1978, the FAA faced another problem, when the airline economic deregulation unwittingly dealt airline safety an insidious blow. Bowing to intense political pressure, the federal government hastily freed airlines from almost all economic regulation. The debate over the wisdom of deregulation has continued ever since. The CAB was abolished, and with the elimination of economic regulation, a substantial part of airline regulation disappeared. The govern-

Federal Aviation Administration (FAA)

ment no longer regulated the routes that airlines could fly or how frequently they could fly them. The airlines could set fares and invent combinations of arbitrary fare restrictions. The airlines could price their tickets below the cost of buying, flying, and maintaining the planes. As the US airline fleet aged, cash-strapped carriers delayed maintenance, cut corners on safety, and, in some cases, even falsified maintenance records. The FAA, however, did not change the way it policed the airlines. In the years following deregulation, dozens of upstart carriers entered the airline business. Many of these companies operated with meager financing, old planes, little experience, and low-paid employees. They planned to meet vital functions, such as maintenance and safety, by contracting with the lowest bidders. Such airlines came to be called virtual airlines, and almost all of them went bankrupt or otherwise ceased to exist within a few years. One such carrier, ValuJet, caused the biggest FAA crisis in history, but also caused the FAA to increase by 267 percent its remedial action. On May 11, 1996, a ValuJet flight crashed into the Florida Everglades, killing all 110 on board. The American public’s faith in the FAA was shaken when both the FAA administrator and the DOT secretary of transportation stood at the crash site and, before any investigation, on national television pronounced the airline safe. The airline was not safe, however, and days later, ValuJet was grounded for safety violations. A document produced from within the FAA showed that FAA inspectors had recommended grounding ValuJet months before the crash. Congressional and Senate hearings probed the problems within the FAA, and the FAA admitted to Congress that with the advent of virtual airlines, its ability to inspect and oversee the airlines had been significantly hampered. The dual mission of the FAA, both to promote aviation and to regulate

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safety, inherited decades before from its predecessors within the Commerce Department, was an obvious inherent conflict. The FAA was tasked by Congress to set about improving its own safety record, as well as that of the US airlines. AIR TRAFFIC CONTROL AND CROWDED SKIES By the 1980s, the nationwide air traffic control system of banks of blinking green screens and paper strips tracking thousands of planes across the skies had become antiquated. Congressional hearings examined the problem of demand for air travel exceeding the ability of the old air traffic control system to handle the traffic. The FAA’s initial efforts to replace the system had failed by the mid-1990s. The first replacement program was woefully behind schedule and well over budget. The FAA, ordered by Congress to start over, began phasing in over many years parts of the new air traffic control replacement system. The overall completion date was targeted for 2015, with most of the system projected to be finished by 2008. The completed system allows for completely computerized and automated air traffic control aided by global positioning system (GPS) satellites. The aircraft itself will be able to communicate with the air traffic control system. Should something happen to the pilots, a verbal or electronic command from the aircraft’s home base or air traffic control can tell it to return to its home airport or to another designated airport. Under the new system, pilots are finally able to choose paths legally and safely across the sky, without bonfires or beacons, without cumbersome air routes dictated by green blinking radar screens and strips of paper, and without needless tragedy that imperils pilots, passengers, and even pedestrians on the ground below. —Mary Fackler Schiavo

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Further Reading “Air Traffic by the Numbers.” Federal Aviation Administration, 2017, www.faa.gov/air_traffic/by_the_numbers/. Accessed 7 Aug. 2018. FAA 2015 Performance and Accountability Report. US Department of Transportation, Federal Aviation Administration, 2016. FAA. Aviation Weather: FAA Advisory Circular (AC) 00-6B. Aviation Supplies & Academics Inc., 2016. FAA. Instrument Flying Handbook (Federal Aviation Administration) FAA-H-8083-15B. Skyhorse Publishing, 2017. FAA. Instrument Procedures Handbook (Federal Aviation Administration) FAA-H-8083-16A. Skyhorse Publishing, 2017. FAA. Pilot’s Handbook of Aeronautical Knowledge (Federal Aviation Administration) FAA-H-8083-25B. Skyhorse Publishing, 2017. Federal Aviation Administration, www.faa.gov/about/plans_reports/media/2015-FAA-PAR. pdf. Accessed 7 Aug. 2018. See also: Aerospace industry in the United States; Air transportation industry; Airplane maintenance; Flight schools; National Aeronautics and Space Administration (NASA); National Transportation Safety Board (NTSB)

First Airplane Flight across the English Channel Fields of Study: Aeronautical engineering; Mechanical engineering ABSTRACT On July 25, 1909, Louis Blériot accomplished the first international airplane flight when he flew from Calais, France, to Dover, England, across the English Channel. He was aided in this venture by English newspaper publisher Alfred Harmsworth (Viscount Northcliffe, 1865-1922), and his competitor French-English big-game hunter, boat racer, race car driver, and aviator Hubert Latham (1883-1912).

Principles of Aeronautics

SUMMARY OF EVENT In 1908, Wilbur Wright presented demonstrations of powered flight in France that caused an uproar throughout Europe. Wright’s ability to control his airplane astonished European pilots and other observers. One of these was the aeronautical correspondent for the Daily Mail, an English newspaper owned by Alfred Harmsworth, a man with a deep interest in aviation. To spur the interest of others in his country, Harmsworth offered a prize of five hundred pounds for the first flight across the English Channel, in either direction, in a heavier-than-air device unsupported by any lifting agent; the flight was to be completed between sunrise and sunset. This was not the first time Harmsworth had attempted to interest the people and government of England in aviation. In 1906, he had offered a prize

First Airplane Flight across the English Channel

for the first flight from London to Manchester; ironically, it was won, in 1910, by a Frenchman, Louis Paulham. No one stepped forward in response to the prize offered by the Daily Mail, so late in 1908 Harmsworth increased the sum to one thousand pounds, an amount that succeeded in bringing out contestants. The first to announce his intentions was Hubert Latham, a debonair young flyer of French and English descent who immediately won the hearts of the public. Even after other contestants had declared themselves, Latham remained the favorite. Like many early aviators, he was wealthy; his money allowed him to pursue a variety of interests, from big-game hunting to racing boats and automobiles to aviation. He had, in fact, already flown the Channel, from England to France,

Louis Blériot in his aircraft just before takeoff for his cross-channel flight, July 25, 1909. Photo via Wikimedia Commons. [Public domain.]

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by balloon; he had decided to become an airplane pilot after witnessing some of Wright’s demonstrations. Latham chose to fly an Antoinette, a beautiful but rather unstable monoplane designed and built by Leon Levasseur. Latham and the designer set up an airfield in France at Sangatte, not far from Calais, in the summer of 1909. The weather over the English Channel, known for its unpredictability, was worse than normal: High winds, rain, fog, and mist conditions precluded any attempt until mid-July. On July 19, at 6:42 a.m., Latham finally took off for his first attempt to cross the Channel, escorted by the destroyer Harpoon. Seven minutes into the flight, his engine failed, and he landed in the Channel, unhurt. The crew of the Harpoon rescued him and attempted to bring his airplane on board, but unfortunately in the process the machine was damaged beyond repair. Once on shore, Latham ordered another plane to be sent right away. It arrived in a short time and was quickly assembled, and he was ready for another try. In the meantime, however, word of Latham’s attempt had reached the ears of his most serious competitor, Louis Blériot, who traveled to Calais immediately to challenge Latham. Blériot, a Frenchman, was heavyset and dour-looking, with a large, red mustache. His less-than-dashing appearance, together with his no-nonsense attitude, made him less popular with the public than Latham. He had amassed a fortune through the business of designing, manufacturing, and selling acetylene headlights for automobiles, and, having become infatuated with flying, he spent most of his fortune on aviation. From 1901 to 1909, Blériot had designed and built a number of airplanes, evolving from devices with flapping wings to biplanes and, eventually, monoplanes, culminating in his Blériot No. XI. It has been estimated that Blériot had invested approximately $150,000 in building airplanes by the time he came up with his

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No. XI, the plane that would bring him fame and more fortune. Arriving in Calais, Blériot chose as his airfield site a farm near Les Baraques, a small village not far from Sangatte. His plane—driven by a 25-horsepower Anazini engine, crude in design, and not known for its reliability—was carted to the farm. Both aircraft were now standing ready for the flight, but the two pilots could only fret while the weather over the English Channel kept them grounded. On July 24, it appeared that there might be a change in the unfriendly weather; accordingly, that night M. Charles Fontaine, a reporter for the Paris newspaper Le Matin, took the night ship to Dover, England. His job was to find a safe landing place for Blériot and to signal him by waving the French flag. At 2:30 a.m. on July 25, Blériot was awakened and given the news that the weather seemed to be improving. He dressed and ate a quick breakfast, then went to the field to prepare for departure while his wife went to Calais to alert the escort ship Escopette to Blériot’s forthcoming departure. At 4:10 a.m. he made a short test flight, and by 4:35 he was ready to leave, waiting only to make sure that his takeoff would be after sunrise. According to witnesses, just before he took off he asked, “Au fait, ou est-ce exactement, Douvres?” (By the way, where exactly is Dover?). After a short time airborne, Blériot passed over his escort ship; now he was alone, flying through patches of mist. Suddenly, fog engulfed him, obscuring everything. With no instruments on board his airplane to guide him, not even sure he was heading in the right direction, Blériot released the controls, letting the airplane fly itself. He flew this way for ten minutes, during which his engine began to overheat, intermittently losing and then recovering power, causing him to lose altitude. The aircraft’s descent took it through a small rain shower, which seemed to resolve his problem: The engine cooled down and regained full power.

Principles of Aeronautics

A few minutes later, the mist and fog began to thin, and Blériot could see the coast of England. He realized that the winds had blown him off course: He was near St. Margaret’s Bay, east of Dover. He turned and headed for the Dover lighthouse, visible through the haze, noting that the winds had increased in velocity and turbulence. Flying over the English fleet, which was anchored in Dover Harbor, he proceeded along the cliffs looking for Fontaine. Spying the reporter standing in a depression above the cliffs, Blériot made a half circle and headed toward him. Once over the cliffs, the airplane experienced severe turbulence, which spun it around. Blériot responded by cutting the engine, and the plane descended rapidly from approximately eighteen meters, making a “pancake” landing that smashed the landing gear and broke the propeller. Thirty-seven minutes after taking off from Calais, Blériot had survived his fifty-first crash, won the coveted Daily Mail prize, and secured his place in aviation history. SIGNIFICANCE Although the Wright brothers had proved that sustained, controlled flight was possible, most people in the early 1900s viewed the airplane as a frail, unreliable, dangerous device, a rich person’s toy with no practical use. Blériot’s flight across the English Channel was not particularly noteworthy for its length in either time or distance, but as the first airplane flight to traverse national boundaries and to cross a large body of water, it awakened awareness in both governments and the general public that this new invention could be something more than a passing fad—it could, in fact, have practical uses. The worldwide fame that Blériot achieved through his successful flight, together with his investment of the prize money in his recently acquired aviation company, established him as the leading European airplane designer and manufacturer of the time and for many years thereafter. Al-

First Airplane Flight across the English Channel

though debates over the relative superiority of biplane versus monoplane would continue for a number of years, Blériot’s monoplane was the most widely accepted design of that era and would become the prototype for most twentieth-century airplanes. France was quick to capitalize on the excitement and fervor surrounding Blériot’s achievement. Within a month after his English Channel crossing, the champagne industry, in cooperation with the municipality of Reims, had organized the world’s first international air meet, bringing together most of the leading aviators of the day to compete for prize money. The Reims air meet was soon followed by other tournaments and cross-country air races, events that fed the technological development of the airplane much as early automobile races fed the development of the automobile. Within two years, air races had become international in scope. With contestants flying from one country to another, it soon became obvious that national borders, as drawn on maps, had lost much of their effectiveness as obstacles to the movement of people and goods between countries. No longer was it necessary to stop at the border and gain permission to enter or pass through a country. While the general public was still caught up in the excitement and joy of Blériot’s successful flight across the English Channel, many leaders in England and continental Europe were quick to realize the political and military significance of the accomplishment. For hundreds of years, England’s security had been guaranteed by two things: the English Channel and the Royal Navy. Blériot’s flight had negated both of these as the country’s protectors. Harmsworth was the first to predict that in the future, the airplane would play a dominant role in England’s survival. Although his efforts to spur the development of aviation in England were supported by other farsighted leaders and reporters, as well as the general public, they were for a number of years

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thwarted by the Royal Navy’s domination of the English military establishment. Observers on the Continent, meanwhile, pointed out that airplanes could fly in both directions, hence it was not England alone that had lost a measure of security. As a result, while England vacillated, France took the lead in nurturing the development of aviation, followed closely by Germany. By the time World War I broke out, the military establishments of both countries had fledgling aviation branches, and the outstanding fighter planes produced by Blériot’s company, the Société Pour l’Aviation et ses Dérivés (better known by its acronym, SPAD), played an important part in the Allies’ eventual victory. It was not until World War II, however, that Harmsworth’s prediction concerning the crucial role of the airplane in England’s defense against attack would come true—in 1940, when the Royal Air Force defeated the German Luftwaffe in what later became known as the Battle of Britain. As the first international airplane flight and the first flight over a large expanse of water, Blériot’s 1909 English Channel crossing demonstrated the potential of the airplane for transporting people and goods. It may, therefore, be considered the forerunner of both military and peaceful commercial flight between countries. —P. John Carter Further Reading Abbott, Malcolm, and Jill Barnforth. The Early Development of the Aviation Industry. Taylor & Francis, 2019. Dick, Ron, and Dan Patterson. The Early Years: Vol. 1 in Aviation Century. Boston Mills Press, 2003. Gwynn-Jones, Terry. “1909-Lord Northcliffe’s Channel Challenge: The Dawn of Air Racing.” The Air Racers: Aviation’s Golden Era, 1909-1936. Pelham Books, 1984. Hales-Dutton, Bruce. Cross-Channel Aviation Pioneers: Blanchard and Bleriot, Vikings and Viscounts. Pen & Sword Books, 2021. Prendergast, Curtis. “The Great Show at Reims.” The First Aviators. Time-Life Books, 1980.

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Taylor, Michael J. H. Aviators: A Photographic History of Flight. Collins, 2005. See also: Neil Armstrong; Amelia Earhart; First manned balloon flight; John Glenn; Robert H. Goddard; Charles A. Lindbergh; Billy Mitchell; Montgolfier brothers; Eddie Rickenbacker; Alan Shepard; Igor Sikorsky; Valentina Tereshkova; Wright brothers’ first flight; Chuck Yeager

First Cross-Channel Balloon Flight Fields of Study: Aeronautical engineering ABSTRACT On January 7, 1785, Jean Blanchard and John Jeffries successfully crossed the English Channel in a balloon from Dover, England, to Calais, France, demonstrating that travel by air was practical and opening the door to military and scientific observations using balloons. SUMMARY OF EVENT Jean-Pierre-François Blanchard (1753-1809), a French pioneer in aviation and ballooning, and John Jeffries (1745-1819), a Boston medical doctor, successfully crossed the English Channel in a balloon, demonstrating that travel by air was practical and opening the door to military and scientific observations using balloons. The balloon, the simplest of all flying machines, consists of a fabric envelope that is filled with a gas that is lighter than air. If the entire balloon, including a suspended basket used to carry instruments or passengers, has an overall density less than the surrounding air, then the balloon and its payload rise to a height where their density equals the density of the air. The first balloon that carried humans into the sky used hot air to provide the lifting force. In September of 1783, brothers Joseph-Michel Montgolfier

Principles of Aeronautics

(1740-1810) and Jacques-Étienne Montgolfier (1745-99) demonstrated a hot-air balloon for the king and queen of France, but it carried aloft only animals—a sheep, a duck, and a rooster. On November 21, 1783, Jean-François Pilâtre de Rozier (1756-85) and Marquis d’Arlandes (1742-1809) flew above Paris for twenty-five minutes, becoming the first human “aeronauts” in a balloon designed by the Montgolfier brothers. Also in 1783, Jacques-Alexandre-Cesar Charles (1746-1823), a French chemist who studied the properties of gases, experimented with using hydrogen instead of hot air in balloons. Hydrogen has two advantages over hot air. First, hydrogen is the lightest gas, so it provides more lifting force than an equal volume of hot air; a smaller balloon filled with hydrogen would be able to lift the same payload, that is, the weight of the basket and its contents, as would a larger balloon filled with hot air. Second, hydrogen does not cool in the same manner as hot air, so it retains its lifting capacity, allowing for a much longer flight. On December 1, 1783, Charles, accompanied by Nicolas Robert, took off from the gardens of the Tuileries in Paris before a crowd of 400,000 people and flew 43 kilometers in a hydrogen balloon. Hydrogen, however, also has one serious disadvantage: It can burn or explode when combined with oxygen. A worldwide interest in ballooning developed quite rapidly after these two successful flights. Showmen began to stage balloon ascents and charged fees to the crowds of spectators. A flight across the English Channel, the body of water that separates England from continental Europe (in this case, France), was considered to be the flight-distance challenge for early balloonists. If the English Channel could be crossed, they believed, it would be proven that balloons were practical for long-distance flight. One person who developed an interest in ballooning was Jean-Pierre-François Blanchard. Blanchard

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began inventing mechanical devices as a boy, including a rat trap, a hydraulic pump, and a “velocipede,” which was a predecessor of the bicycle that was driven simply by walking while sitting on it. Blanchard also constructed his own balloon, which he flew for the first time on March 2, 1784, from Champ de Mars Park in Paris. His balloon measured 8.5 meters in diameter, had a parachute in case it burst, and was filled with hydrogen rather than hot air. Since ballooning was developing rapidly in France, Blanchard decided to move to England, where he would have fewer competitors for fame. Blanchard’s first flight demonstrated that he had perfected the basic components of his balloon, so he began a series of experiments to improve the design. Ballooning, however, was expensive, so Blanchard needed financial support for his efforts. He publicized his experiments and found a group of wealthy sponsors. An American medical doctor, John Jeffries of Boston, provided £700. On November 30, 1784, Blanchard and Jeffries made their first flight together, taking off from Rhedarium Garden, London, and landing in Kent, England. On January 7, 1785, Blanchard and Jeffries became the first to cross the English Channel by air. Jeffries paid Blanchard an additional £100 for the flight across the Channel, but Blanchard did not want Jeffries to share in the glory. He said that Jeffries’s weight might keep the balloon from completing the crossing, so, before Blanchard agreed to take him, Jeffries had to promise he would jump overboard into the Channel if the balloon could not stay aloft. Before their flight began, a slight breeze started to blow toward France from the cliffs of Dover, a city located on the Channel. The flight got off to a good start at about 1:00 p.m., but after only about 13 kilometers, the balloon began to descend. Blanchard and Jeffries jettisoned the ballast (the extra weight carried for the balloon’s ascent) but,

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still, they continued to settle toward the water. According to Jeffries’s account, the two men had been arguing about what they should throw overboard when the basket bounced on the water. Neither could swim, so they finally threw ropes, anchors, seats, and scientific instruments overboard. With the balloon skimming just above the water, Blanchard and Jeffries tossed their clothes, except their underwear, into the sea. As they crossed the coast, the natural updraft that occurs as warm air rises from ground heated by the Sun caused the balloon to climb. Because they had thrown their landing ropes and anchors into the Channel, Jeffries grabbed some treetops to slow the balloon, but the balloon continued to rise. When they got over a field, Blanchard released some of the hydrogen and the balloon sank to a landing. A group of men, who had watched the landing, rode up on horseback. The adventurers were given clothes and taken to the nearby town of Calais, where they were greeted by cheering crowds. French king Louis XVI awarded Blanchard about $12,000 as well as a lifetime pension. On June 15, 1785, Pierre Romain and Jean-François Pilâtre de Rozier, who had flown on the Montgolfier balloon, also attempted a crossing of the Channel, ascending from Boulogne, France, in a hydrogen-filled balloon. About thirty minutes into the flight, however, at an altitude of about three thousand feet, Pilâtre de Rozier’s balloon exploded, killing both men. In 1792, Blanchard traveled by ship to Philadelphia, bringing with him several balloons and the apparatus for generating hydrogen gas. On January 9, 1793, Blanchard made the first balloon flight in North America, from a prison yard in Philadelphia to Gloucester County, New Jersey. US president George Washington attended the ascent, and Blanchard carried a letter from Washington in the balloon, possibly the world’s first “airmail” delivery.

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SIGNIFICANCE Balloons did not emerge as practical methods of transportation because their flight paths are subject to the changing directions of the wind. However, the success of the earliest balloons led to the development of blimps and dirigibles, which use motor-driven propellers to direct the flight path. The military quickly recognized the utility of balloons for observation. In 1793, the French government began using balloons for reconnaissance. Hot-air balloons were used during the American Civil War. Flying over the battlefield, military observers directed Union Army gunners to fire on Confederate positions without the gunners being able to see the enemy position. Hydrogen-filled observation balloons were widely used during World War I to detect troop movements and direct artillery fire. During World War II, gas-filled barrage balloons were used to intercept low-flying aircraft in the Battle of Britain. The Japanese launched thousands of balloon bombs toward the United States and Canada. The military’s use of balloons has continued into the twenty-first century, as surveillance balloons, equipped with high-tech optics, have observed enemy movement from many miles away; surveillance balloons were used during the American invasion of Iraq in 2003. Long-distance balloon flights continue to challenge adventurers. It was not until 1978, though, nearly two hundred years after the first successful piloted balloon flight across the English Channel, that the Double Eagle II, carrying three passengers, was able to cross the Atlantic Ocean, the first balloon to do so with humans aboard. The first crossing of the Pacific Ocean was accomplished in 1981, when the Double Eagle V flew from Japan to California. —George J. Flynn Further Reading Blanchard, Jean-Pierre. First Air Voyage in America. Applewood Books, 2002.

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Hales-Dutton, Bruce. Cross-Channel Aviation Pioneers: Blanchard and Bleriot, Vikings and Viscounts. Pen & Sword Books, 2021. See also: Neil Armstrong; Amelia Earhart; First airplane flight across the English Channel; First manned balloon flight; Flight balloons; John Glenn; Hindenburg; Hot-air balloons; Lighter-than-air craft; Charles A. Lindbergh; Montgolfier brothers; Eddie Rickenbacker; Alan Shepard; Valentina Tereshkova; Chuck Yeager

First Flights of Note Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT The first occurrence of many things that are taken for granted in the modern age often took years of research and development before they could take place, yet they are now little more than footnotes in history despite their importance to modern society. Three such events are the first solo around-the-world flight, the first jet aircraft flight, and the first successful helicopter flight. Had any of these firsts failed. air transportation in the present day would not be what it is now. SUCCESSFUL FIRSTS The first occurrence of many things that are taken for granted in the modern age often took years of research and development before they could take place, yet they are now little more than footnotes in history despite their importance to modern society. Three such events are the first solo around-the-world flight, the first jet aircraft flight, and the first successful helicopter flight. Had any of these firsts failed air transportation in the present day would not be what it is now. Consider the first world-spanning solo flight of Wiley Post. This flight, it required many refueling stops along the way, demonstrated that a single air-

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plane and its pilot possess the capability of long-range flight. Had this flight failed, development of long-range aircraft would have been delayed to an unknowable degree. While trans-Atlantic flight was already well known at the time just prior to the start of World War II, trans-Pacific flight was something of an unknown, with the operational range of military aircraft being too limited to reach Japan from Hawaii. Jimmy Doolittle’s raid on Japan required his squadron of B-17 bombers to take off from an aircraft carrier in mid-ocean in order to reach Japan with no fuel remaining for a return flight. Post’s successful long-range flight made the idea of this bold step possible. Today, we travel by air in jet-powered aircraft and think nothing of it. Successful jet-powered airplanes were invented at the start of World War II as the latest addition to the arsenal of Nazi Germany’s Luftwaffe. The German Messerschmitt fighter aircraft were as good as the twin-tailed Allies’ P-38 Mosquito, which German pilots referred to as the “two-tailed devil.” and the P-51 Mustang, both of which flew through the air at more than 800 kilometers per hour. To be better meant going faster, and that was the goal of the development of the Heinkel-178. Fortunately, despite their speed, the bugs had not been worked out completely before the Nazi Reich was defeated. But their success revealed the promise of what jet-powered aircraft could become. So today we have jet-powered transport aircraft that can carry 300 passengers and more, and until it was destroyed in the 2022 Russo-Ukraine War, the largest airplane ever to have flown was the Antonov AN-225 Mryha. The helicopter is today a mainstay of modern society for everything from excursion flights to remote locations, to vital search-and-rescue missions and medical evacuation. Would these exist in their present state if Igor Sikorsky’s first flight, however short that it was, had failed? Would developers have concluded that the helicopter was an unworkable idea

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He 178 replica at Rostock-Laage Airport, Germany. Photo via Wikimedia Commons. [Public domain.]

and ceased to develop the aircraft and its possibilities for an indeterminate length of time, perhaps relegating the idea to the dustbin of history? How would our world be different if these three things had not happened and succeeded? FIRST SOLO FLIGHT AROUND THE WORLD The United States established many milestones in the field of aviation during the twentieth century, beginning with the Wright brothers’ first heavier-than-air flight on December 17, 1903, and including the first Moon landing on July 20, 1969. Another significant pioneering flight was made by

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the American Wiley Post, who became the first person to fly solo around the world on July 22, 1933. Post was born on November 22, 1898, near Grand Saline, Texas. He had an adventurous youth, even serving prison time for stealing an automobile, and worked a variety of jobs. Post had been fascinated with aircraft ever since he was a child, so when he received a cash settlement as compensation for an eye injury he suffered while working in an oil field, he used the money to buy his first plane. He became an experienced aviator and flew around the world in 1931 with another man, his navigator, Harold Charles Gatty.

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In 1933 Post decided to attempt to become the first person to fly around the world alone—a dangerous proposition at that time. Not only did he face the challenge of being both pilot and navigator, but he also had to worry about possessing the sheer physical stamina required for such a flight. He departed on July 15, 1933, leaving New York City aboard a Lockheed Vega aircraft named the Winnie Mae and traveled eastward. Post returned on July 22 of that year after having made a number of stops at airports along the way for fuel and various repairs. He died a few years later on August 15, 1935, in a

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plane crash in Alaska, which also killed his passenger, the comedian Will Rogers. FIRST JET AIRCRAFT FLIGHT The first true jet aircraft made its maiden flight on August 27, 1939, when German scientist and engineer Hans Joachim Pabst von Ohain’s axial-flow turbojet was used to power the new Heinkel-178 for the Heinkel Company, a major German aircraft manufacturer. Others had experimented with jet engines and jet aircraft designs before Ohain, and a Frenchman named Henri Marie Coanda even built and

Wiley Post’s plane, Winnie Mae, at the Edmonton Municipal Airport, Alberta, during his solo flight around the world in 1933. Photo via Wikimedia Commons. [Public domain.]

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VS-300. Photo via Wikimedia Commons. [Public domain.]

flew a form of jet-powered biplane in 1910, but Ohain’s was the first true jet aircraft flight. At the time, Germany was mobilizing for war under the Nazi regime of Adolf Hitler, and World War II broke out shortly thereafter. Although traditional propeller-driven fighter planes and bombers dominated the war in both the German military and the air forces of the other combatants, by the end of the war both sides had developed fast and powerful jet aircraft, thanks to the work of Ohain and other pioneers. After the war, all of the major air forces converted to jet aircraft, and this conversion spread to the civilian aircraft and airline industry as well beginning on

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May 2, 1952. It was on this date that the British Overseas Airways Company (BOAC) inaugurated the first jet passenger service, employing a Havilland DH 106 Comet on the route from London, England, to Johannesburg, South Africa. As for Ohain personally, he moved to the United States following World War II and served on various research and development projects for the American military. He retired in 1979 and died on March 13, 1998, in Melbourne, Florida, at the age of 86. FIRST SUCCESSFUL HELICOPTER FLIGHT Although there had been a variety of experiments with vertical ascent aircraft since the early twentieth

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century, the first truly successful hovering helicopter test took place on September 14, 1939, when Ukrainian-born Igor Sikorsky took his VS-300 aircraft aloft at his facility in Stanford, Connecticut. Although the test flight lasted for only a few seconds and the vehicle rose above the ground only a few inches, it was a success. Sikorsky had incorporated a tail rotor in addition to the main overhead rotor, which enabled the vehicle not only to rise above the ground but also to avoid spinning out of control due to torque. Speculative conceptions about devices roughly analogous to modern helicopters have existed since the time of Leonardo da Vinci. These conjectures did not take positive form until the twentieth century, however, when it became possible to build propeller-driven aircraft sufficiently powerful to rise into the air. The advantage of helicopters over airplanes is that they can rise vertically, enabling them to take off and land in restricted terrains, and can stay aloft in a stationary position to provide a range of operational possibilities. Designs based on Sikorsky’s helicopters are now common in the military and civilian air fleets of countries throughout the world. Sikorsky, born on May 25, 1889, in Ukraine (then part of the Russian empire), died on October 26, 1972, in Easton, Connecticut. His highly successful helicopter company eventually merged into the American conglomerate known as United Technologies. Further Reading Hirschel, Ernst-Heinrich, Horst Prem, and Gero Madelung. Aeronautical Research in Germany, from Lilienthal Until Today. Springer Berlin, 2012. Johnson, Wayne. Rotorcraft Aeromechanics. Cambridge UP, 2013. Leishman, Gordon J. Principles of Helicopter Aerodynamics with CD Extra. Cambridge UP, 2006. Myhra, David. Heinkel He-178-Redeaux. RCW Technology & Ebook Publishing, 2013. Sterling, Bryan B., and Frances N. Sterling. Forgotten Eagle: Wiley Post, America’s Heroic Aviation Pioneer. Carroll & Graf, 2001.

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Stinson, Patrick M. Around the World Flights: A History. McFarland Inc. Publishers, 2011. Wagner, Wolfgang. The History of German Aviation: The First Jet Aircraft. Schiffer Publishing, 1998. Warsitz, Lutz, and Geoffrey Brooks. The First Jet Pilot: The Story of German Test Pilot Erich Warsitz. Pen & Sword Books, 2009. Wittreich, Paul. Forgotten First Flights. Xlibris US, 2009. Wyckoff, Edwin Brit. Helicopter Man: Igor Sikorsky and His Amazing Invention. Enslow, 2010. See also: Aerodynamics and flight; Aeronautical engineering; Neil Armstrong; Glenn H. Curtiss; Leonardo da Vinci; Amelia Earhart; First airplane flight across the English Channel; First cross-channel balloon flight; First manned balloon flight; German Luftwaffe; John Glenn; Helicopters; Howard R. Hughes; Jet engines; Charles A. Lindbergh; Billy Mitchell; Eddie Rickenbacker; Burt Rutan; Alan Shepard; Igor Sikorsky; Valentina Tereshkova; Wright brothers’ first flight; Chuck Yeager

First Manned Balloon Flight Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT The first manned balloon flight took place on November 21, 1783, at Château de la Muette, near Paris, France. The balloon was designed and constructed by French inventors Joseph-Michel Montgolfier (1740-1810), and his brother Jacques-Étienne Montgolfier (1745-99). The actual flight was overseen by the Montgolfier brothers, but was piloted by the Marquis d’Arlandes (François Laurent d’Arlandes, 1742-1809) and Jean-François Pilâtre de Rozier (1756-85). SUMMARY OF EVENT On November 21, 1783, the people of Paris cheered as they watched the slow passage of the hot-air balloon created by Joseph-Michel Montgolfier and Jacques-Étienne Montgolfier. With

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the technology of hot-air ballooning nearly perfected, the Montgolfiers had been testing their invention since the preceding June, first with empty tethered flights and then with farm animals aboard. On November 21, they attempted the first piloted free flight, carrying Jean-François Pilâtre de Rozier and the Marquis d’Arlandes. Departing from the garden of the Château de la Muette in the northwest of Paris, the balloon sailed through the air for twenty-six minutes before landing safely. Joseph-Michel Mongolfier was born in Vidalonles-Annonay in Ardèche in 1740. He was the twelfth child of a family of sixteen children, of which only five survived childhood. He was five years older than his brother Jacques-Étienne. Their father, Pierre Montgolfier, owned a very successful paper factory in Dauphiné, a region near the Alps. Jacques-Étienne was a serious and disciplined student who excelled in mathematics and studied architecture with the famous French architect Jacques-Germain Soufflot. Joseph-Michel, on the other hand, was an indifferent student. Sent to a Jesuit college in Toumon to study for the priesthood, he showed little interest in theology or Latin, soon leaving his studies and migrating to Paris. There, he met and was fascinated by many of the great scientists and mechanics of his day, including Benjamin Franklin, the naturalist Louis Jean-Marie Daubenton, Jean le Rond d’Alembert, and Jacques Vaucanson, who was engaged in creating automatons. Jacques-Étienne was given charge of the family business in 1772, where he industriously began attempting to perfect the papermaking process. Joseph-Michel likewise became the manager of a paper factory in Voiron, Dauphiné, but lacked his brother’s commitment to the profession and his business sense. Though he was a born inventor, gifted in mechanics and the sciences, he was also an absent-minded romantic and dreamer who had been known to walk home from a tavern, forgetting that he had come gone there on his horse.

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A model of the Montgolfier brothers’ balloon at the London Science Museum. Photo via Wikimedia Commons. [Public domain.]

Though their personalities were so different, the brothers got on well together, forming an alliance between dreamer and diligent mechanic that made them an ideal team. Having read a physics treatise on the laws of gases, they began experimenting with lighter-than-air flight in 1782, designing and building small silk or paper balloons they filled with hot air. Modest successes prompted them to continue their work. On June 5, 1783, the brothers’ latest hot-air balloon rose to about 2,000 meters, landing in a vineyard 2.5 kilometers from Annonay. The Montgolfiers were not, however, the only ones experimenting with flight. Jacques-Alexandre Charles and Marie-Noel Robert developed a hydrogen balloon that, on August 27, 1783, rose from the

Principles of Aeronautics

Champ de Mars, an open grassy area in the center of Paris, and came to rest in Gonesse, 25 kilometers away. Though Charles and Robert’s balloon was attacked and destroyed by peasants, frightened by its sudden and mysterious descent from the heavens, it had beaten the Montgolfiers’ record. Fostering the competition that led to scientific advances and engineering breakthroughs, the French Academy of Sciences encouraged the Montgolfiers to continue their experiments. A presentation of their work before King Louis XVI and Marie-Antoinette at Versailles was arranged on September 19, 1783. As the king watched, the brothers placed a sheep, a rooster, and a duck in the basket suspended from their balloon. When the craft sailed into the air, landing eight minutes later and 3 kilometers away in the woods of Vaucresson, the king was delighted. Soon thereafter, he ennobled Joseph-Michel and Jacques-Étienne, who were henceforth called de Montgolfier. A month later, on October 15, the brothers were approached by Jean-François Pilâtre de Rozier as they prepared for a flight in a park in Paris. Pilâtre de Rozier, intrigued by the preparations that he had observed, offered to take the place of the domestic animals, becoming the first person to rise 20 meters in a tethered balloon. On October 20, he reached 60 meters. Discovering that sustained flight was almost impossible because of the difficulty of maintaining the fire that provided the hot air, Pilâtre de Rozier suggested that a second passenger would be useful. A few hours later, Giroud de Villette accompanied Pilâtre de Rozier on a flight that reached 80 meters and lasted for ten minutes. Though the problems of sustaining flight had been solved, if not the considerable danger involved in maintaining an open fire so near to a canopy composed of silk and paper, still, free flight had not been achieved. The Montgolfiers announced that the first free flight of a piloted balloon would take place on November 21, 1783. As the population of Paris awaited

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the spectacle, at 1:54 p.m. Pilâtre de Rozier, accompanied by an infantry officer, the Marquis d’Arlandes, rose from the field surrounding the Château de la Muette at the northwest boundary of Paris. A strong wind blowing from the northwest pushed the balloon over the roofs of Paris, as both aeronauts worked diligently to feed their fire with straw. When they reached the Seine River, the air in contact with the water was colder, and the balloon dropped steadily toward the ground. Feeding more straw into the furnace brought the balloon back up to 1,000 meters, however. The flames from the larger fire ignited the balloon envelope in scattered areas, threatening disaster until Pilâtre de Rozier extinguished the flames with a wet sponge. After flying near Notre Dame Cathedral and the windmills of Montmartre, the balloon landed safely at 2:20 p.m. on the Butte-des-Cailles, near the present-day Place d’Italie. For twenty-six minutes, and for the first time in history, two people had traveled freely in the air. Soon after the Montgolfiers’ success, on December 1, 1783, Charles and Robert flew for fifty-six minutes in their hydrogen balloon and reached an altitude of 3,500 meters carrying meteorological instruments—a thermometer and a barometer. Joseph-Michel de Montgolfier finally flew in his own balloon on January 19, 1784. The dangers that the early balloonists faced became clear when, on June 15, 1785, Pilâtre de Rozier decided to cross the English Channel with Pierre Ange Romain. Soon after taking off, their balloon caught fire and both aeronauts were killed. SIGNIFICANCE The key to the success of the Montgolfiers resided in the complementarity of their characters. The empirical method and imagination of Joseph-Michel was tempered by the order, method, and conservatism of Jacques-Étienne. Their discoveries were integral to the explosion of scientific theory and application

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resulting from the Enlightenment’s faith in the power of rational thought. Like Benjamin Franklin in the United States, they were a convincing demonstration of the ability of intelligent people to make an understanding of the nature of the physical world useful. The hot-air balloons that they pioneered provided the technological basis for most lighter-than- air flight in the nineteenth century. Used for scientific observation, hot-air balloons also were adapted to military use, primarily for observation and communication. They were successfully employed during the US Civil War, in the 1870-71 Siege of Paris during the Franco-Prussian War, and during World War I. Charles and Roberts’s hydrogen balloon was the predecessor of the dirigibles and blimps of the early twentieth century. Though hot-air ballooning has become to a great extent merely the hobby of a community of dedicated enthusiasts, it has led to more efficient and dependable technologies that are still employed whenever modern meteorological balloons are launched to study atmospheric pressure, humidity, and the ozone layer. —Denyse Lemaire and David Kasserman Further Reading Christopher, John. Riding the Jetstream: The Story of Ballooning, from Montgolfier to Breitling. John Murray, 2001. Gillipsie, Charles Coulston. The Montgolfier Brothers and the Invention of Aviation, 1783-1784: With a Word on the Importance of Ballooning for the Science of Heat and the Art of Building Railroads. Princeton UP, 1983. Holmes, Richard. The Age of Wonder: How the Romantic Generation Discovered the Beauty and Terror of Science. Knopf Doubleday Publishing Group, 2009. Morton, Tyler. From Kites to Cold War: The Evolution of Airborne Reconnaissance. Naval Institute Press, 2019. See also: Blimps; Leonardo da Vinci; Dirigibles; First airplane flight across the English Channel; First cross-channel balloon flight; Flight balloons; Hindenburg; History of human flight; Lighter-than-air craft; Montgolfier brothers

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Flight Altitude Fields of Study: Physics; Aeronautical engineering; Mathematics ABSTRACT Altitude is a measured or calibrated height above the ground or above sea level. Pilots use indicated altitude to maintain height separation from other aircraft and from ground obstructions. However, the actual, or true, altitude is usually not the same as the indicated altitude. KEY CONCEPTS altimeter: a device that indicates the aircraft’s altitude ideal gas law: the relationship of pressure (P), temperature (T), volume (V), mass in moles (n), and the universal gas constant, R, as PV = nRT Kollsman window: a secondary window at the 3 o’clock position on the face of an altimeter dial, showing a small dial that the pilot can calibrate the altimeter with the current local pressure mean sea level (MSL): true altitude, as the average height above standard sea level where atmospheric pressure is measured in order to calibrate altitude INDICATED ALTITUDE The standard aircraft altimeter is an aneroid (without liquid) barometer that measures the ambient or static air pressure outside the airplane. It is calibrated through the use of a Standard Atmosphere model so that it presents this pressure to the pilot as an altitude. Because the air pressure on the ground varies a great deal with the movement of air masses across the country, an offset can be introduced by the pilot to make the indicated altitude equal to the actual altitude of an airport before takeoff and while approaching to land. The offset, if any, is indicated by the reading in a window, known as the Kollsman window, on the face of the altimeter.

Principles of Aeronautics

THE STANDARD ATMOSPHERE The Standard Atmosphere model is based on an arbitrarily chosen, midlatitude, average value for the pressure, temperature, humidity, and density of the air at sea level. It assumes a sea-level pressure of 76.00 centimeters of mercury, a sea-level temperature of 15 degrees Celsius, 0 percent humidity, and an air density calculated from the ideal gas law. It further assumes that the temperature decreases linearly with an increase in altitude in the troposphere, at the rate of 1.9811 degrees Celsius for every 304.8 meters for the first 10,972.8 meters, and then is constant in the stratosphere. These assumptions, along with the gravitational and thermodynamic laws, yield the Standard Atmosphere, uniquely defining the standard air pressure, density, and temperature at every altitude. It should be noted that pressures expressed as a height of mercury are not using true pressure units but are reflecting a common way to measure pressures. An accurate mercury thermometer can be made by bending a 6-foot-long glass tube into the shape of an upright U, filling it half-full of mercury, and attaching a vacuum pump to one end. Atmospheric pressure at the other end then pushes the mercury down, and the difference in mercury heights is a direct measure of the atmospheric pressure. Pressure expressed in inches of mercury can be converted to pressure expressed in units of pounds per square inch (psi) by multiplying by 70.73. PRESSURE ALTITUDE Because the altimeter is calibrated in feet of altitude, or in kilometers in Europe and elsewhere, but is really measuring only atmospheric pressure, the actual air pressure can be obtained by consulting tables of the Standard Atmosphere. This is important because the performance of engines and airplanes depends directly on the air density, and the only way to determine air density is to calculate it from the measured air pressure and temperature, using the ideal

Flight Altitude

gas law. Pilots adjust their altimeters so that 29.92 appears in the Kollsman window—that is, with no offset from the Standard Atmosphere—and the indicated altitude is then called the pressure altitude. Whenever a pilot is flying above an indicated altitude of 18,000 feet, the altimeter must be set to 29.92 (no offset) to simplify vertical separation of aircraft. All aircraft flying in busy airspace are also required to use transponders that report both position and pressure altitude to air traffic control. DENSITY ALTITUDE When the pressure altitude is combined with the outside air temperature through the ideal gas law, the density of the air can be calculated. It is most convenient to express this air density in terms of the altitude in the Standard Atmosphere, which is defined to have this density. This calculation is called the density altitude. The performance of airplanes, in terms of the available engine thrust or power, takeoff distance, climb rate, cruise speeds, and landing distances, depends directly on density altitude, or air density, and is specified as such in aircraft flight manuals. It is very helpful for a pilot to realize intuitively that low pressures (especially due to high elevations) and high temperatures result in very low air density (high-density altitudes) and that in high-density altitudes, aircraft performance will be greatly reduced from sea-level values. Density altitude is easily calculated from the pressure altitude and the temperature using either an E6-B circular slide rule or an electronic calculator. The current density altitude is also broadcast at many high-altitude airports. TRUE ALTITUDE When approaching land, it is important to enter the appropriate offset, or altimeter setting, into the altimeter, so that it will both give guidance regarding obstacle clearance on the approach and read field elevation after landing. However, usually the varia-

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tion of pressure with altitude above the airport will not follow the Standard Atmosphere model, and the indicated altitudes of obstructions will still not exactly equal their true altitudes. Air pressure varies more rapidly with altitude if the air is colder than that assumed by the Standard Atmosphere. Therefore, if a pilot flies into air that is colder than standard, or if the altimeter has not been adjusted en route while flying toward a region of lower pressures, the airplane’s true altitude is lower than the indicated altitude, and safety may be compromised, especially in mountainous terrain. Unstable weather conditions and high winds around mountain ranges can also produce locally lower air pressures that result in erroneously high indicated altitudes. OTHER ALTITUDE-MEASURING INSTRUMENTS The radar altimeter measures an aircraft’s height above the ground by measuring the time it takes a radio wave to return, using the known speed of light. Transport and other complex aircraft find the radar altimeter to be a valuable aid in avoiding obstructions during the landing approach. Altitude can also be derived by geometry from three or more satellites in the global positioning system (GPS) used for navigation. This may become the preferred altimeter for high-altitude, oceanic flight.

Lafayette, Porter. Flight Times: Cruising Altitude. Lulu.com, 2018. See also: Airplane cockpit; Airplane guidance systems; Airplane radar; Autopilot; Flight instrumentation; High-altitude flight; Wiley Post

Flight Balloons Fields of Study: Physics; Aeronautical engineering ABSTRACT Balloons are large fabric sacks holding a lighter-than-air (LTA) gas so that the containers and any attached payload are buoyed up and float in the sky. The gas may be hot air, hydrogen, or helium. KEY CONCEPTS aerial reconnaissance: viewing ground-level activities from a vantage point above the ground born aloft by balloon or other type of aircraft density: the weight of a material per unit volume, typically grams per cubic centimeter or kilograms per cubic meter gondola: the passenger and cargo carrier suspended or affixed below a balloon or other type of floating airship inflammable air: the early chemists’ name for hydrogen before it was identified chemically

—W. N. Hubin Further Reading Blair, Jason. An Aviator’s Field Guide to Middle-Altitude Flying: Practical Skills and Tips for Flying Between 10,000 and 25,000 Feet MSL. Aviation Supplies & Academics Inc., 2018. Campbell, R. D., and Michael Bagshaw. Human Performance and Limitations in Aviation. 3rd ed., Wiley, 2008. Jenkins, Dennis R. Dressing for Altitude: U.S. Aviation Pressure Suits, Wiley Post to Space Shuttle. National Aeronautics and Space Administration (NASA), 2012.

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THE SIGNIFICANCE OF BALLOONS One of humanity’s oldest dreams has been to float in the sky. Balloon flights first transformed that dream into adventure, and they continue to do so. For more than a century after the first balloon flight, balloons were the cutting edge of science and aviation technology, and they remain the best craft for most scientific missions operating in altitudes of roughly 16 to 50 kilometers, which are too high for airplanes and too low for orbiting vehicles.

Principles of Aeronautics

NATURE AND USE The term “balloon” may refer to the gas bag, or envelope, or to the balloon and any additional objects attached to it, which are usually hung below. Objects are attached to smaller balloons by a single line, but larger balloons require netting to spread the load over the entire gas bag. A large cargo below is called a gondola or basket, often a large wicker basket. Buoyancy is the key to balloon flight. The ancient mathematician Archimedes stated that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. For balloons, a lighter-than-air (LTA) gas provides buoyancy to lift the balloon containing it as well as any payload. LTA gases include gases with densities lower than that of air and heated air that has expanded and is thus lighter than the surrounding air. The two low-density gases used for balloons are hydrogen and helium, which require the balloon to be sealed so that they do not mix with the heavier air. Air is usually heated for buoyancy by burning propane or kerosene. The heated air rises into the balloon through an open base, and air that has cooled drains from the balloon through that same orifice. Warmth is constantly drained away at the surface of the balloon so hot-air balloons require frequent firings of their burners. Consequently, they tend to have shorter ranges than balloons with low-density gas. More importantly, hot air has less lifting capacity than hydrogen or helium. Typically, hydrogen has a net lift of 30 kilograms per 310 cubic meters, but hot air provides only 7.5 to 9 kilograms of lift. Thus, hot-air balloons must be three times larger to lift the same payload. However, hot-air balloons are less expensive to operate, because they do not have to accommodate the complexities of hydrogen and helium. Although helium lifts 14 percent less than hydrogen (about 25 rather than 30 kilograms) per 310 cubic meters, it has the major safety advantage of be-

Flight Balloons

ing nonflammable, whereas hydrogen can ignite explosively. As a balloon increases its altitude, the density and pressure of the surrounding air decreases, meaning there is less lift available per unit volume, so the balloon must be larger to carry a given payload to higher altitudes. A partially compensating factor is that the buoyant gas also grows less dense as the pressure decrease allows it to expand, but the trend is toward miniscule lift per unit volume, as most of the atmosphere is left below. Balloon builders can compensate with lighter payloads, such as remotely controlled instruments, but at some point, the weight of balloon fabric alone matches the lift from

In 1999, Bertrand Piccard and Brian Jones achieved the first nonstop balloon circumnavigation in Breitling Orbiter 3. Photo by BetaCommandBot, via Wikimedia Commons.

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the gas volume, and even the largest balloons can go no higher. Balloonists have two other ways to vary the buoyancy of their craft. They can descend by decreasing buoyancy or land by valving out some of the lifting gas. They can increase buoyancy by dropping ballast, which is water, sand, or other material carried along for that purpose. In extreme conditions, balloonists have dropped all articles in the gondola and even the gondola itself. HISTORY For centuries, the Chinese made toy hot-air balloons of a design that could and might have been scaled up to carry passengers. There are accounts from twelfth century BCE China of people in balloons, but the records are too old and incomplete to be confirmed. Likewise, drawings on pottery associated with the Nazca Lines, constructed more than two thousand years ago in southern Peru, suggest that these massive earthen line drawings were made with overhead direction from hot-air balloons. Confirmable accounts begin in the eighteenth century. In 1782 and 1783, Joseph-Michel and Jacques-Étienne Montgolfier, two French brothers, flew hot-air balloons larger than toys, with animals as their first passengers. Ironically, in their first balloons, the Montgolfiers had wanted to use hydrogen, which British chemist Henry Cavendish had discovered in 1776, noting in his experiment reports that this “inflammable air” was lighter than ordinary air. The French Academy in Paris was working toward a rubberized or varnished fabric to contain the troublesome gas that seeped through ordinary fabrics and escaped. When Joseph-Michel Montgolfier experienced the same problem, he noted that scraps of paper in a fireplace rose up the chimney. Paper, which the Montgolfier family manufactured, could contain smoke, so the Montgolfiers made hot-air balloons and successfully flew three animal passengers: a rooster, a sheep, and

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a duck. King Louis XVI’s permission was required for people to fly because it was not known whether leaving the ground might be harmful to people. Jean-François Pilâtre de Rozier, a young doctor who wanted to take the risk of human flight, recruited the Marquis François d’Arlandes to serve as copilot and, more importantly, to secure the king’s permission. On November 21, 1783, having obtained permission, the two men flew over Paris for twenty-five minutes while desperately stoking their lifting fire and sponging out fires in their rigging caused by sparks. Below them, nearly the entire populace of Paris watched.

Gay-Lussac and Biot ascend in a hydrogen balloon, 1804. Illustration from the late 19th century. Image via Wikimedia Commons. [Public domain.]

Principles of Aeronautics

Only a few days later, on December 1, 1783, Jacques-Alexander-César Charles, of the French Academy, flew a hydrogen balloon. The preparation required the production of large quantities of hydrogen gas and the careful varnishing of cloth to render it relatively airtight. The flight illustrated the advantages of hydrogen balloons over hot-air balloons. Because hydrogen is more buoyant than hot air, the hydrogen balloon could be a third the size of a comparable hot-air balloon. Charles flew for two and one-half hours, dropped off his passenger at sunset, and then rose high enough to be the first person to see the sun set twice in one day. Shortly thereafter, balloonists began attempting not only to fly but also to reach destinations. Jean-Pierre Blanchard, another Frenchman, and John Jeffries, an American, decided to be the first aeronauts to fly across the English Channel to France, which they did on January 7, 1785. However, they were somewhat humbled upon arrival, because they had jettisoned most of their clothes, along with the gondola and the articles within it, in order to avoid falling into the Channel. A rivalry ensued with the French wanting to have a flight from France back to England. Pilâtre de Rozier, who had piloted the first hot-air balloon, had another balloon made that advanced balloon technology. This hybrid de Rozier balloon had a hydrogen balloon that rode over a hot-air balloon. The pilot could vary the balloon’s buoyancy, and thus its altitude, by adjusting the fire under the hot-air balloon instead of valving out hydrogen gas or dropping ballast, neither of which could be replenished. On June 15, 1785, Pilâtre de Rozier and his copilot floated in this balloon toward England. Unfortunately, Pilâtre de Rozier fell prey to two problems of early balloonists. He had no reliable weather reports, and when the wind changed, he floated back toward France. Worse, his hydrogen lifting gas was very flammable and the varnished cloth only slightly less so. As the audience who had

Flight Balloons

joyously witnessed the launch watched, the balloon caught fire, lost buoyancy, and plummeted the two aeronauts to their deaths. After Pilâtre de Rozier’s death, hot-air balloons fell out of favor until experiencing a renaissance in the 1960s. Similar problems plagued ambitious balloon flights for the next century. Attempts at crossing the Atlantic Ocean lost credibility as balloonists waited vainly for suitable weather. Balloon flights crossed the Alps in Europe, and John Wise crossed a third of North America, but the final destination of long-distance flights was always a surprise, and disaster was always just a spark away. Despite its shortcomings, the balloon went to war in 1793 when revolutionary France was attacked by a number of neighbors. At the Battle of Fleurus in 1794, the fledgling French balloon corps fielded a single reconnaissance balloon tethered on a line several hundred feet above the ground. Observers in the balloon, who could see several miles past the line of battle, provided tactical reports via notes dropped from the gondola. The most important observation was that the attacking Austrian army had pitched an empty tent city in an effort to overawe the French commander into retreating. Because of that vital bit of intelligence, the French did not retreat but rather fought on with their exotic new technology looming over and unnerving their opponents, until they eventually won the battle. Aerial reconnaissance was reinvented in the American Civil War (1861-65), in which several groups operated observation balloons. The most successful was inventor Thaddeus Sobieski Coulincourt Lowe, who organized an aeronautic corps of balloon observers for the Union Army. Although balloon technology had not advanced tremendously, communications technology had. Lowe’s observers transmitted their reports either by signal flags or by telegraph wire running down to the ground.

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The potential of Lowe’s reports is shown by accounts of one 1862 engagement, in which Lowe directed Union cannon fire at an area invisible to Union guns because it was behind a hill. When the Confederate horsemen rode away from the shell impacts, Lowe had the guns redirect their fire. After the war, Confederate accounts revealed that one of the horsemen was showered by so much dirt from a near-miss that his colleagues feared he had been hit. That horseman was Jefferson Davis, the Confederate president. Nearby, also in danger, was Robert E. Lee, Davis’s commander of all Confederate armies. Despite Lowe’s successes, a change in the Union Army high command caused Lowe to fall from favor and come under stringent control by an unsympathetic regular officer. Lowe ultimately resigned from his post, and his entire corps withered away. Military ballooning was next reinvented by the French during the Franco-Prussian War of 1870-71. The Prussians smashed the regular French army and surrounded the French capital city of Paris. The plucky Parisians responded by raising a militia to hold off attacks and launching balloons to carry observers and send messages out of the city, rallying the countryside. Fifty-four of sixty-two balloons got through, carrying one hundred people and two and one-half million pieces of mail. Although France eventually accepted harsh peace terms, the utility of war balloons was established. By the time World War I began in 1914, observation balloons were in use by both sides. By the end of the war, they were being replaced by heavier-than-air airplanes. Surprisingly, small balloons did evolve into an important military and civilian use during World War II. The development of small radio transmitters combined with remotely operating weather instruments made possible balloon-borne radiosondes that reported temperature, pressure, and relative humidity. Angle data from antennas tracking the radiosondes yielded the more important factors of wind speed and direction at dif-

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ferent heights. The use of weather balloons has continued into the twenty-first century to help predict weather, to plot sky conditions for aircraft, and to fire artillery more accurately. Finally, in the 1990s, tethered balloons returned to service as aerostats, providing platforms at altitudes as high as 10,000 to 15,000 feet for radar stations and communications repeater stations. SCIENTIFIC APPLICATIONS Some have said that balloons are a pacifist technology. They are big, slow, and cannot be piloted accurately, particularly when the wind changes. Yet, balloons do a number of things well. They move gently and can carry large payloads that would not fit in an airplane fuselage. Most importantly, they can reach high enough altitudes to perform many of the research tasks generally performed by spacecraft. However, balloons can accomplish this research more cheaply and quickly than can spacecraft, and without the vibration and acceleration forces of a rocket launch into space. In the twentieth century, a number of supporting technologies radically improved, allowing the balloon to become much more practical for research applications. Most importantly, helium became widely available as a nonflammable lifting gas. Synthetic fibers, such as nylon, polyethylene, and Kevlar, supplied fire-resistant materials with the lightness of silk and strengths approaching steel. Vulcanized rubber allowed light, cheap, disposable balloons. Virtually all measuring instruments shrank in size. Finally, worldwide weather databases and communications links made it more possible to guide balloons on long voyages. The quest for altitude began as both an adventure and a science. The first balloonists had no idea whether the atmosphere continued indefinitely or became lethal a short distance above the ground. They soon discovered that pressure and temperature decreased with increasing altitude. Those who

Principles of Aeronautics

attempted altitude records discovered temperatures tens of degrees below the freezing point of water. However, the greatest risk was hypoxia, or oxygen deprivation, which causes weakness, shakiness, mental confusion, and eventual death. To deal with these problems, balloonists developed oxygen-supply systems and learned to use equipment that would not freeze in the bitter cold. However, even the breathing of pure oxygen was found to be insufficient above altitudes of 15,000 meters. Swiss balloonist Auguste Piccard surmounted this problem with a pressurized cabin that essentially represented the first space capsule. On May 27, 1931, Piccard and an assistant launched from southern Germany and reached 15,786.5 meters, making them the first fliers ever to reach the stratosphere. More importantly, they discovered that cosmic rays increased with altitude, proving that these rays came from somewhere in space and not from radioactivity within the earth. From 1933 to 1935, the governments of the United States and the Soviet Union foreshadowed the space race that would begin nearly a quarter-century later. Balloon flights carried personnel and instruments to steadily greater heights and developed many technologies that were later used in the space race. For example, on May 4, 1961, the American Stratolab High V balloon reached a world-record-breaking altitude of 34,625 meters, with an open gondola so that the two pilots could test space suits in near-space conditions for the Mercury orbital-flight program. In retrospect, the best high-altitude science data began to be collected in the 1960s, after improved robotic instrumentation allowed shedding the weight of the balloonists and their life-support gear. Over the closing decades of the twentieth century, astronomic balloon-borne instruments conducted sky surveys in a number of frequency bands that cannot penetrate the lower atmosphere and provided valuable weather data from the lower stratosphere.

Flight Balloons

By the late twentieth century, the National Aeronautics and Space Administration (NASA) began using superpressure balloons for relatively small payloads of several tens of pounds. Balloons called zero-pressure balloons expand when warmed by the sun and contract at night when cooled. When warmed at high altitude, they must vent excess helium to prevent bursting. This gas loss limits mission duration to only several days. Superpressure balloons, in contrast, keep the same maximum shape when the balloon is warmed. Because no gas is lost, such balloons can operate for weeks or months, and some of these balloons have circled the globe one or more times. By the early twenty-first century, NASA had begun flying large superpressure balloons in a

Auguste Piccard. Photo by Bundesarchiv, via Wikimedia Commons.

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program called the Ultra Long Duration Balloon (ULDB). These large balloons could carry several tons of instrument payload for weeks at a time. Less well-documented are flights of small superpressure balloons by US intelligence agencies since the 1950s.

and Pacific Oceans. In March, 1999, another Piccard, Auguste’s grandson, Bertrand, and Brian Jones spent twenty days flying 30,000 miles to make a complete circumnavigation of the globe.

RECREATIONAL APPLICATIONS Although ballooning is no longer the world’s primary means of aviation, a balloon ride remains a beautiful and awe-inspiring experience. Balloonists enjoy panoramic views that float by below them and sounds that float up from the ground. However, a recreational balloon ride was a rare experience until the so-called renaissance of hot-air ballooning, which was started by American balloonist Edward Yost. While Yost was developing high-altitude balloons for the US government in the 1950s, it occurred to him that polyethylene-coated nylon would be a lighter, less flammable material than that used for the Montgolfiers’ balloons. He used an acetylene welding torch as a less labor-intensive source of hot air than that used by the Montgolfiers. After some development, such as replacing the welding torch with a propane burner, Yost made the first “modern” hot-air balloon launch from Bruning, Nebraska, on October 10, 1960. Beginning in the 1960s, the new hot-air balloons radically reduced the cost and complexity of supplying buoyant gas. Thus, were born ballooning clubs, competitions, and tour services. Hot-air balloons have been flown, primarily for advertising, in whimsical shapes, including those of spark plugs, lightbulbs, human faces, and even a mansion. A combination of Yost’s hot-air technology, lightweight insulating material lining the gas bag, and helium made the de Rozier balloon practical for more ambitious, long-distance flights. Varying the amount of heat in the inner balloon provides altitude control for hunting favorable winds. That capability, along with worldwide weather reports, made it possible to make balloon flights across the Atlantic

Further Reading Burton, Anthony. Balloons and Airships: A Tale of Lighter-Than-Air Aviation. Pen & Sword Books, 2019. Charles River Editors. Modern Balloons and Airships: The History and Legacy of Dirigibles During the 20th Century. Independently Published, 2019. Lynn, Michael R. The Sublime Invention: Ballooning in Europe, 1783-1820. Taylor & Francis, 2015. Mallard, Graham, and Stephen Glaister. Transport Economics: Theory, Application and Policy. Macmillan Education UK, 2020. von Ehrenfried, Manfred. Stratospheric Balloons: Science and Commerce at the Edge of Space. Springer International Publishing, 2021. Yajima, Nobuyuki, Naoki Izetsu, Takeshi Imamura, and Toyoo Abe. Scientific Ballooning: Technology and Applications of Exploration Balloons Floating in the Stratosphere and the Atmospheres of Other Planets. Springer New York, 2016.

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—Roger V. Carlson

See also: Aeronautical engineering; Blimps; Dirigibles; Hindenburg; Lighter-than-air craft

Flight Control Systems Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Mathematics ABSTRACT Flight control systems are simple and combined electric, mechanical, and hydraulic systems that help to maintain the flight of an aircraft while flying. Flight control systems allow pilots to adjust the speed, attitude, and direction of an aircraft.

Principles of Aeronautics

KEY CONCEPTS actuator: a device driven by an electric motor or by hydraulic or pneumatic pressure to move something to which it is attached, such as a control surface on an aircraft flap: a secondary structure of a wing near the fuselage, used to increase or decrease the lift of the wing for speed control during takeoffs and landings hydraulic: a system that transfers power through pressure exerted using an oil or other noncompressible fluid lift: the upward pressure experienced by wings due to differential pressure between upper and lower surfaces of the wing as it moves through a fluid medium as determined by its airfoil camber and thickness positive control: maintaining the operation of an aircraft by the actions and commands of the pilot rather than flying free without human control EARLY HISTORY The early experimenters and inventors who preceded Orville and Wilbur Wright, who made their first flight in 1903, did not fully appreciate the necessity for positive control of the machine. Prior to 1903, the prevailing ideas about aircraft control were that the airplane must have some kind of inherent stability and that the pilot’s only function was to make small directional changes. Much of inventors’ efforts prior to the beginning of the nineteenth century were directed at obtaining a lightweight engine. The Wright brothers realized that a proper engine was a necessary ingredient in mechanical flight. However, they appreciated the importance of control and the fact that the pilot must be an active participant in the control of the airplane. By 1909, a control system had evolved consisting of ailerons, a rudder, and an elevator, which, in its essentials, remains in use today.

Flight Control Systems

TYPES OF CONTROLS Modern aerodynamic flight control systems, as opposed to engine controls, are essentially the same for all airplanes. Flight controls can be separated into two categories: primary and secondary controls. The primary controls change the angles that the airplane makes relative to the ground. The secondary controls are flaps that control the lift of the airplane, especially at low speeds, and tabs that reduce or eliminate the forces the pilot must exert on the controls in the cockpit. All controls, whether primary or secondary, have three important subdivisions. The first are external movable surfaces on the airplane, such as the rudder, aileron, and elevator. The second are the cockpit controls, which are moved by the pilot to change the direction of the airplane. The third are the links between the cockpit controls and the external surfaces of the airplane. These connections might be cables, electrical-conducting wires, electrical motors and computers, hydraulic lines, and hydroid motors. PRIMARY CONTROLS There are three categories of primary controls. Category A refers to the three hinged panels that are rotated about their hinge line to change the angular attitude of the airplane. Category B controls are those which the pilot moves to change the direction of the aircraft and, to a limited extent, the speed of the aircraft, particularly the descent rate. These controls consist of the stick or wheel, which is moved to pitch and roll the airplane, and the rudder, which is moved to yaw the airplane. These controls have not changed significantly since 1915, during the second decade of mechanical flight. Category C controls vary the most widely between different types of airplanes. These types of controls have also evolved most radically over the history of mechanical flight. A small, low-cost training plane connects the pilot’s control to the aerodynamic con-

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trols with cables or push rods; hydraulic lines and associated motors perform the same function in high-speed commercial airliners. Electrical conducting wire or even fiber optic lines might be used to carry the control signal from the cockpit to an electrical motor at the surfaces in other commercial airplanes or high-performance military airplanes. The airplane responds to the movement of the primary category A cockpit controls in a number of ways. The elevator is deflected to change the pitch angle of the airplane: When the trailing edge of the elevator is moved upward, a down force is generated on the horizontal stabilizer. The result is that the nose of the airplane pitches upward. The airplane will pitch in the opposite direction if the trailing edge of the elevator is moved downward. When the rudder is moved to the left side of the airplane, from the point of view of the pilot, a side force to the right is applied to the vertical stabilizer. This force swings, or yaws, the nose of the airplane to the left. Reversing the direction of the rudder movement will reverse the yaw direction. Finally, movement of the ailerons causes the airplane to roll. The ailerons move differentially; when one aileron moves upward, the other moves downward. On the wing with the downward aileron, there is a slight increase in the lift. On the wing with the upward aileron, there is a slight decrease in lift. The unbalance in lift between the two wings causes the airplane to roll. Cockpit controls are connected to the airplane’s external controls. Moving the stick back brings up the elevator trailing edge, placing a down force on the horizontal stabilizer. The horizontal stabilizer and tail goes down while the nose goes up. Reversing the direction of the stick movement reverses the motion of the pitch of the airplane. The rudder is moved by pushing on the rudder pedals located on the floor of the airplane. Pressing the left pedal causes the trailing edge of the elevator to move to the left, resulting in the application of a

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side force to the right on the vertical stabilizer. The tail of the airplane moves to the right, and the nose moves to the left. Moving the nose left or right is called yawing the airplane left or right. Finally, the ailerons are moved by either sideways motion of the stick or rotation of the control wheel. To roll the airplane to the right, for example, the stick is moved to the right, lowering the left aileron and raising the right aileron. SECONDARY CONTROLS The secondary aerodynamic controls are the tabs and the flaps, both of which can be operated by the pilot from the cockpit. The tab is a small elevator hinged to the trailing edge of the elevator. To hold the nose up for a prolonged period of time, the pilot must continually apply a backward force on the stick to keep the elevator in the up position. By moving the tab downward, in this case, a small force through leverage balances the much larger force on the elevator, with the result that there is little or no stick force required from the pilot to keep the elevator trailing edge upward. Tabs are found also on the rudder and aileron. On an airplane with two wing-mounted engines, a rudder tab is nearly essential in helping the pilot set and hold the extreme rudder deflection required for single-engine flight. Flaps deflect in unison, unlike ailerons, which move differentially. Flaps help maintain lift, especially during low-speed flight. The pilot can control the deflection angle of the flap. Flaps are deflected at maximum deflection for landing and at a small angle for takeoff. There are three basic flap designs. The split flap, the simplest but least effective, consists of a small plate that comes down from the lower surface of the wing. Because this flap does not change the contour of the wing, it primarily produces drag. The plain flap changes the shape of the wing and therefore produces lift as well as drag. The slotted flap is derived from the plain flap with special attention given

Flight Instrumentation

Principles of Aeronautics

to the junction of the flap and the wing. The design of this junction is crucial to the flap’s effectiveness. The Fowler flap is the most effective and the most mechanically complicated flap. When deflected, it changes not only the shape but also the area of the wing. There are two types of control used on airplanes: primary and secondary. Primary controls are the elevator, rudder and ailerons, and the primary cockpit controls are the stick and rudder, located in the cockpit. The secondary controls are tabs and flaps. The flaps allow the airplane to fly at lower speeds than would otherwise be possible. The tab allows the pilot to remove any forces required to hold control deflections. Tabs are usually located on the elevator and can also be found on the rudder and ailerons. —Frank J. Regan Further Reading Kundu, Ajoy Kumar, Mark A. Price, and David Riordan. Conceptual Aircraft Design: An Industrial Approach. Wiley, 2019. National Aeronautics and Space Administration (NASA). Aircraft Wing Structural Detail Design (Wing, Aileron, Flaps, and Subsystems). CreateSpace Independent Publishing Platform, 2018. Sabry, Fouad. Adaptive Compliant Wing: No More Flaps, the Aircraft Wing Shape is Now Morphing. One Billion Knowledgeable, 2022. See also: Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Airplane guidance systems; Avionics; Flight roll and pitch; Forces of flight; Pressure; Plane rudders; Stabilizers; Training and education of pilots; Wing designs; Wright Flyer

Flight Instrumentation Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics; Mathematics

ABSTRACT Flight instrumentation consists of the gauges and instruments used by the pilot to monitor the condition of an aircraft and the condition of its flight. Instrumentation contributes to flight safety and the usability of aircraft. Without instruments, aircraft would be able to fly only for short periods on sunny days, dependent upon the pilot’s ability to fly by sight and “the seat of his pants.” KEY CONCEPTS altimeter: a device that indicates the aircraft’s altitude inclinometer: a device that indicates the angle at which an aircraft is ascending or descending relative to level flight Kollsman window: a secondary window at the 3 o’clock position on the face of an altimeter dial, showing a small dial, which the pilot can calibrate the altimeter with the current local pressure pitot tube: a tube that is open to the front of an aircraft to transmit the static air pressure outside to various instruments inside the aircraft HISTORY The earliest aircraft had no instruments at all. Pilots controlled the airplane and the engine using their senses of sight, hearing, and touch. As airplanes grew more complex, pilots needed more instruments to control the planes and monitor the engines. In addition, pilots required instruments to help them navigate and to maintain control of the aircraft in fog or clouds. The first instruments installed in aircraft monitored the crafts’ engines and fuel. By World War I (1914-18), aircraft had compasses, inclinometers, and simple altimeters to help pilots navigate and maintain control. In 1928, Paul Kollsman invented the first sensitive altimeter. A year later, on September 24, 1929, Army lieutenant James H. “Jimmy” Doolittle, using Kollsman’s altimeter and other instruments, demonstrated that an aircraft could be successfully controlled by reference to instruments

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Six flight instruments in an aircraft cockpit, sometimes refered to as “basic-T” or “basic flight instruments.” Photo by Contributions/Meggar, via Wikimedia Commons.8

alone. With a safety pilot in the forward cockpit, Doolittle climbed into the rear cockpit and covered it so he could not see out. Then he took off, flew a 15-mile triangular course, and landed. For the first time, an aircraft had been flown by reference to instruments alone. Although engineers improved instrument accuracy and reliability, the basic design of flight instruments remained the same from the 1930s to the 1960s. During the 1960s and 1970s, as the transistor and, later, the integrated circuit came into general usage, instruments began to change dramatically.

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The instrument could be mounted away from the cockpit, and the information could be displayed on a simple indicator. In the 1980s, as microprocessors came into general usage, the indicators could be replaced with cathode ray tubes and then liquid crystal displays. MAGNETIC COMPASS The most basic instrument used for navigation is the magnetic compass. The magnetic compass uses two small magnets attached to a floating compass card inside a container filled with kerosene. These mag-

Principles of Aeronautics

nets point toward Earth’s magnetic north pole. The compass card has letters and numbers printed on it that allow the pilot to determine the direction of flight. The movement of the airplane during flight causes the magnetic compass to move freely and reorient itself to north accordingly. This free movement also permits oscillation of the compass needle, which limits the pilot’s ability to determine flight direction with precision. PITOT-STATIC FLIGHT INSTRUMENTS The static system is designed to measure the ambient air pressure surrounding the aircraft. A static port consisting of small holes drilled through the side of the aircraft is connected to tubing that leads to the pressure-sensing instruments. The pitot tube is usually a cylindrical device with a hole at one end, installed so that the end with the hole faces forward. The other end is connected to a hose that leads to airspeed sensing instruments. With this arrangement, as the aircraft moves forward, it will create a positive air pressure within the pitot tube. Used together, the pitot tube, the static port, and the hoses associated with each are known as the pitot-static system. Three flight instruments are based on measuring air pressure and are connected to the pitot-static system. These are known as pitot-static instruments and, in general, need no external power source. AIRSPEED INDICATOR The airspeed indicator is connected through hoses to both the pitot tube and the static port. The basic function of the airspeed indicator is to compare air pressure caused by aircraft movement to ambient air pressure. Within the instrument, a small set of bellows connects to the hose leading to the pitot tube. The bellows are also mechanically connected through gears and springs to a needle on the face of the airspeed indicator. The case of the instrument is connected to the hose leading to the static port. As

Flight Instrumentation

the aircraft moves through the air, the pressure in the pitot tube inflates the bellows. As the bellows expand, the needle will move to indicate airspeed. Airspeed indicators can be calibrated in nautical miles per hour (knots), miles or kilometers per hour, or both. ALTIMETER The altimeter is connected through hoses to the static port. The basic function of this instrument is to measure barometric air pressure. If a tube filled with mercury is inverted and is placed into a mercury reservoir, the mercury will drain from the tube into the reservoir, leaving a certain amount in the tube that balances the atmospheric pressure on the mercury reservoir. Measuring the length of tubing filled with mercury will give an indication of the atmospheric pressure. At sea level, the length of tube filled will be approximately 760 millimeters of mercury. At 6,100 meters, the length would only be 349.25 millimeters of mercury. This pressure is commonly known as barometric pressure. Inside the altimeter is a sealed pressure capsule connected to needles on a dial calibrated for altitude. As the aircraft climbs skyward, the capsule expands, causing the needles to indicate an altitude above sea level. The altimeter is only accurate when the pilot sets the altimeter for the local barometric pressure by adjusting a secondary setting visible in the Kollsman window (named after Paul Kollsman) on the face of the altimeter. For example, before takeoff, the pilot gets a weather report that indicates the local barometric pressure and then enters the pressure into the altimeter. Once entered, the altimeter will read the field elevation, or altitude above sea level. VERTICAL SPEED INDICATOR The vertical speed indicator is connected through hoses to the static port. The function of this instrument is to measure the rate of altitude change. In-

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side the vertical speed indicator is a pressure capsule with a calibrated leak. This capsule is connected to a needle on the face of the instrument. As the aircraft increases or decreases in altitude, the capsule will expand or contract, and air will either leak in or out of the capsule, causing the needle to indicate the rate of either climb or descent. GYROSCOPIC FLIGHT INSTRUMENTS The gyroscopic instruments work on the principle that a spinning wheel will rigidly maintain its orientation in space due to torque. The gyroscopic instruments are constructed around a spinning wheel called a gyroscope. Once spinning, the gyroscope will maintain its spatial orientation; therefore, it is mounted on special devices called gimbals. The gimbals allow the aircraft to move freely around the rigid gyroscope. Gyroscopic instruments are different from pitot-static instruments in that they require a power source. These instruments may be either air or electrically powered. DIRECTIONAL GYRO Unlike the magnetic compass, the directional gyro remains stable in spite of aircraft movement. The gimbal of the directional gyro connects to a circular compass card. A number or letter under a lubber line at the top of the instrument indicates the direction in which the aircraft is pointed. ATTITUDE GYRO This instrument is also known as the artificial horizon, or attitude indicator. Two gimbals within the attitude gyro are connected to a horizon-reference arm. Using two gimbals allows the aircraft to move freely in all directions around the rigid gyroscope. The reference arm rotates right and left and moves up and down. When the gyroscope is spinning and rigid in space, the horizon reference bar will remain level. As the airplane climbs, descends, or banks, the pilot can compare the position of the reference bar

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to an airplane symbol and bank index on the face of the instrument. In this manner, the pilot can then determine the attitude of the aircraft relative to the horizon. RATE GYROS There are two different types of rate gyros, the turn and slip indicator, and the turn and bank indicator. Both types feature an inclinometer on the face of the instrument that indicates whether or not the aircraft is sliding sideways. A sideways slide is known as either a slip or a skid. The turn and slip indicator uses a gyroscope in a horizontally mounted gimbal connected to a needle in the face of the instrument. As the aircraft turns, the gimbal rotates and forces the needle to the left or right, depending on the direction of the turn. The faster the turn, the greater the deflection of the needle. In a turn and bank indicator, also known as a turn coordinator, the gimbal is mounted at an angle and connected to a symbolic airplane on the instrument face. This instrument senses both bank rate and turn rate. ENGINE INSTRUMENTS All aircraft are equipped with a tachometer. A tachometer measures the rotation speed of the engine in revolutions per minute or in percent of maximum. In piston-powered aircraft, the tachometer may also include an hour meter to measure the time that the engine has been running. In helicopters, the tachometer will have two needles, one to measure engine speed and the other to measure rotor speed. Jet-powered aircraft have two tachometers labeled N1 and N2. The N1 tachometer measures the speed of the low-pressure compressor, and the N2 tachometer measures the speed of the high-pressure compressor. Oil temperature and pressure gauges are also found on all aircraft. Many piston-powered aircraft are cooled by a combination of air and oil. By moni-

Principles of Aeronautics

toring the oil temperature, the pilot can determine if the engine is operating within its proper temperature range. All engines are lubricated with oil, and the oil pressure gauge alerts the pilot to any changes in oil pressure that would indicate an engine problem. Many aircraft are equipped with an exhaust-gas temperature gauge. This instrument measures the temperature of the exhaust gases, and pilots use this instrument to monitor the efficiency of the engine. Piston-powered aircraft may be equipped with a manifold pressure gauge. This instrument is similar in construction to, yet less sensitive than, an altimeter. The manifold pressure gauge measures the air pressure within the intake manifold. For aircraft equipped with constant-speed propellers, this instrument is the only reliable way to measure the power output of the engine. Jet engines will have instruments that measure pressure at both the low- and high-pressure compressors. These pressure gauges allow the pilot to monitor the performance of the engine. SYSTEMS INSTRUMENTS Pilots monitor the condition of the electrical system in the aircraft by using an ammeter. Ammeters measure electrical current flow in amperes. Some aircraft also have a voltmeter. The voltmeter measures electrical potential in volts. By monitoring these instruments, the pilot can determine whether the battery is charging or discharging and whether the generator is working properly. All aircraft are equipped with fuel quantity indicators, the equivalent of a fuel gauge in an automobile. Since many aircraft have more than one fuel tank, there may be more than one fuel gauge. In some cases, a single gauge can be used with a selector switch so that the pilot must measure fuel quantity one tank at a time. Some aircraft are equipped with fuel flow gauges. These instruments monitor the rate at which the en-

Flight Instrumentation

gine or engines are using fuel. Pilots use these gauges to monitor the condition of the engines and to plan when fuel stops will be necessary. ELECTRONIC FLIGHT INSTRUMENTATION SYSTEM Electronic flight instrumentation systems (EFIS) can be used in place of every instrument except the magnetic compass. By nature, instruments with no moving parts are more reliable than their mechanical counterparts. Various techniques are used to replace the mechanical components of an instrument. For example, by using accelerometers coupled to microprocessors, engineers can duplicate the operation of the gyroscope. In addition, a laser beam shining through a ring of optic fiber can duplicate the operation of a mechanical accelerometer. By using technology similar to computers and televisions, information can be displayed on cathode ray tubes or liquid crystal displays. In most EFIS designs, all of the flight instrument information is exhibited on one or two displays, while navigation, engine, and other information will be shown on other displays. EFIS-equipped aircraft may a have special engine monitoring system called the engine indicating and crew alerting system (EICAS). With EICAS, engine data are not all displayed continuously. During normal operation, only a minimum amount of information is displayed. If a malfunction occurs, important information will appear automatically on the electronic display. —Thomas Inman Further Reading Department of the Army. Instrument Flight for Army Aviators (Tc 3-04.5). CreateSpace Independent Publishing Platform, 2017. Federal Aviation Administration (FAA). Instrument Flying Handbook: ASA FAA-H-8083-15B. Revised ed., Aviation Supplies & Academics Inc., 2017.

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Kershner, William K. The Instrument Flight Manual: The Instrument Rating and Beyond. Aviation Supplies & Academics Inc., 2019. Nagabhushana, S. Aircraft Instrumentation and Systems. I. K. International Publishing House Pvt Ltd., 2013. Walmsley, Stephen. Flight Instruments for the Private Pilot. Amazon Digital Services LLC-Kdp, 2022. Wyatt, David. Aircraft Flight Instruments and Guidance Systems: Principles, Operations and Maintenance. CRC Press, 2014. See also: Airplane cockpit; Airplane guidance systems; Airplane radar; Autopilot; Avionics; Flight altitude; Flight recorder; Flight simulators

Flight Landing Procedures Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Pilot training ABSTRACT Landing procedures for aircraft are steps which, when followed, achieve the safe return of an aircraft from the sky to the surface. Landings, an essential and one of the most dangerous parts of a flight, allow little room for error, because speed, ground proximity, winds, and momentum must all be balanced for reasons of safety and economy. KEY CONCEPTS center of gravity: the point within an aircraft, or any other body, about which the entire mass of that body is equally distributed crosswind: wind blowing across the direction of an aircraft’s direction of motion slip: a maneuver in which an aircraft is made to turn sideways slightly while maintaining the same direction of motion, thus presenting a greater surface area in the wind direction and acting to slow the aircraft somewhat tailwheel: a wheel assembly located under the tail section of an aircraft, thus preventing the tail of the aircraft from dragging on the ground

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wheel sideload: force exerted perpendicularly to a wheel parallel to its axis of rotation rather than in the direction in which the wheels rotate BACKGROUND A common aviation joke attests that although takeoffs are optional, landings are mandatory. Although landings may seem effortless to nonpilots, landing procedures comprise a large portion of any student pilot’s flight training. As student pilots become more comfortable and proficient at landings, however, they may treat them more lightly. The first aviators, who had no teachers, had to learn how to fly through trial and error. Some early fliers could commit their attention only to getting airborne and allowed landings to take care of themselves, often with tragic consequences. Landing procedures have an obvious purpose: to return the aircraft and its passengers safely to the surface. The first generation of pilots seemed happy to walk away after just about any landing. Today, although flying is statistically the safest mode of transportation, it is nonetheless true that the take-off and landing are the stages of flight that have the highest accident rate. Orville and Wilbur Wright equipped their first Flyer with skids instead of wheels, expecting the sands at Kitty Hawk, North Carolina, to intervene and soften the blow of the first landings and the area’s average 16-mile-per-hour winds to allow the Flyer to touch down as slowly as possible. Fortunately, the brothers had gained previous landing experience with gliders designed similarly to their Flyer. However, the powered Flyer differed from the Wrights’ earlier kites and gliders not only in its engine and propellers but also in its substantial pair of skids, which traversed the machine’s length. Later versions of the Flyer repositioned the pilot from a prone to a seated position and strengthened the landing skids and their supports. Wheels remained absent from Wright airplanes until 1910,

Principles of Aeronautics

when the US Army’s purchase demanded specific modifications. Pilots and designers learned quickly that landing was to be as new a science as was flight itself. At first, there were as many designs and combinations of skids and wheels as there were airplanes. For example, although the Wright brothers did not add wheels to the Flyer design until 1910, in 1909, Louis Blériot used two main wheels on a single axle under the engine and equipped his airplane with a non-steerable tailwheel. In the same year, the Antoinette airplane was built, with two main wheels behind the engine, a spoon-shaped skid poking ahead of the main wheels, and another skid beneath the tail. These skids absorbed the shock of landings performed by inexperienced pilots.

Flight Landing Procedures

LANDING FIELDS Early aviators used landing fields that were, as their name implies, open fields, in which pilots could point their airplanes directly into the prevailing winds. This orientation ensured that each landing could be made directly into the wind, for early airplanes’ controls were usually too weak or unbalanced to permit reliable crosswind landings. Crosswind landing capabilities are essential in modern airplanes, because runways long ago replaced landing fields. Whenever winds blow cleanly down the runway’s length, crosswinds pose no challenge. The greater the wind’s angle to a runway’s centerline, the more skill a pilot must demonstrate to make a safe landing. Much of the reason for this difficulty is because airplanes in flight move about the concentration of mass that pi-

Photo via iStock/bfk92. [Used under license.]

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lots call the center of gravity. If an airplane could be held off the ground by a cable attached at its center of gravity, it would remain balanced, with both wings and nose level. As runways became more prevalent after the 1930s, pilots had to develop techniques to prevent any crosswinds from pushing on their airplanes’ vertical stabilizers. As an airplane slows after landing, side winds hit the vertical stabilizer, much as they fill the sail of a boat. More force concentrates on the tail, as the wind pushes against the entire airplane, and the tail moves downwind, as the nose swings in the opposite direction. As the airplane slows, the crosswind’s force can become great enough that the rudder can no longer overcome it, causing the pilot to lose control and forcing the airplane off the runway. Crosswind landing techniques emerged to counter this threat. In some cases, it is even necessary for the pilot to bring the aircraft in sideways to the runway and turn into the runway just as the wheels touch down. LANDING TECHNIQUES The earliest and most basic landing technique involves the pilot crabbing the airplane into the wind until just a moment before touchdown. At the split second before the tires contact the runway, the pilot straightens the nose relative to the runway using the rudder. This technique causes the airplane’s wheels to touch the surface with little sideload but requires that the airplane be stopped quickly. Because so many early airplanes were tailwheel types, quick stops were not always possible. Many airplanes ran off the runway, or ground-looped. However, the technique found wide acceptance on broad grass runways. From the 1950s on, more costly, and, therefore, more narrow, paved runways became the norm. Landing accidents increased, not because pavement was a more difficult landing medium to master, but because crosswind techniques on pavement required a crisper, more certain control technique.

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In the days before airplanes had landing flaps, pilots could lose altitude quickly and safely by slipping, a technique wherein the pilot lowers one wing and keeps the airplane from turning by using the opposite rudder. The same technique, refined by a pilot’s delicate touch, worked well to land an airplane in a strong crosswind. By lowering the wing into the wind, a pilot could use the airplane’s lift to maintain position on a runway’s centerline. Touching down on the upwind wheel allowed pilots to maintain directional control by using the rudder. An airplane’s fuselage, no longer streamlined into the wind, provided welcome aerodynamic drag to slow the airplane quickly, so the moment between the flight controls losing effectiveness and the airplane slowing to the point that most crosswinds would not push the tail became minimum. The point at which a wind becomes too strong to allow a proficient pilot to land safely is called the maximum crosswind component of the airplane’s performance envelope. At the end of the twentieth century, the US Federal Aviation Administration (FAA) recognized only the slip-to-landing crosswind technique. Straightening the nose at the last moment required too unreliable a sense of timing and was simply less safe than the slip-to-landing technique. The particular technique that a pilot uses to land an aircraft depends on several things, including the airplane’s landing gear, the length of the runway, and the runway’s surface. The three basic landing techniques are the normal landing, the soft-field landing, and the short-field landing. NORMAL LANDINGS Pilots elect to make normal landings when the available runway length allows plenty of room, there are no obstructions to approach, and the runway surface is smooth, hard, and dry. Practiced normal landings appear effortless to observers but require much skill and judgment on the part of the pilot. Student pilots

Principles of Aeronautics

normally begin their flight training with normal landings, the simplest of landing techniques. In normal landings, pilots must align their airplanes with the runway centerline and, maintaining an appropriate airspeed, plan a stable approach path to the runway. Airspeed control is critical during all types of landings, because the goal is always to touch down with as little downward motion as possible in order to prevent damage to the landing gear. The second critical part of landing is airspeed control. If the airspeed is too slow on approach, the pilot may lose control of the airplane. If the airspeed is too fast, the pilot may not be able to touch down at the appropriate point, using up too much runway and damaging any airplanes at the end of the runway. SOFT-FIELD LANDINGS Soft-field landings require a high degree of pilot awareness, because the pilot essentially handles the controls as if to keep the airplane flying until the wings simply stop producing enough lift for flight. This procedure must be timed so that all of the aircraft’s wheels touch the runway surface at the same moment. After the wheels touch, the pilot must apply just enough power to reduce the nosewheel’s pressure on the runway by applying back pressure on the stick, or yoke, by pulling the stick forward with very light hand pressure. The pilot continues to apply back pressure until the airplane slows so much that the weight of the nose finally rests fully on the rolling nosewheel. Because there are so many types of runway surfaces, pilots must use extreme care and near-faultless judgment to analyze and properly land on a soft field. Pilots must avoid portions of the runway that might damage their airplanes. Obstacles such as broken concrete, badly eroded asphalt, snow packed to iceberg hardness, or windborne debris can contaminate a landing field. Special caution is also essential on grass or dirt runways after a rain. Muddy

Flight Landing Procedures

surfaces can stop an airplane so suddenly as to flip the airplane over. SHORT-FIELD LANDINGS Pilots use the short-field landing technique when a runway is shorter than normal. Short-field landings demand skill and practice, because they require pilots to touch down on or near a specific point at the lowest safe airspeed. After all wheels have contacted the surface, pilots must apply heavy braking to stop the airplane in the shortest distance. Successful short-field landings require a pilot’s heavy reliance on the pilot’s skill and judgment. Student pilots practice short-field landings throughout most of their training, and their flight instructors emphasize them with increasing frequency as students approach their practical test. Airspeed control, pitch attitude control, and power control blend together through the pilot’s hand in a ballet of momentum management that ends in a thrilling dissipation of energy. US Navy pilots are, in effect, making short-field landings when they land on aircraft carriers. They rely on shipboard signal officers, who manually signal essential corrections, as they concentrate on lighted approach-slope aids. A properly flown approach to the short field of an aircraft carrier results in the airplane’s tailhook grabbing a landing cable, which slows the airplane violently but certainly on a pitching, rolling runway. A civilian pilot has only the airplane’s brakes and flap retraction to stop the airplane on the runway after the pilot’s visual judgment places the airplane on its touchdown point. Regardless of the type of landing a pilot selects, consistency is the key to success. Pilots attain and maintain consistency by keeping in practice. US regulations have long required pilots to have landed at least three times within the ninety days preceding a flight carrying passengers. Landings have fulfilled aviators, met schedules, thrilled passengers, and even saddened those expe-

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riencing a flight’s end. Aviation has inspired poets in most of its aspects, but landings have received rare poetic treatment. In 1956, F. Pratt Green recounted his emotion at the moment of landing, and of exiting the airplane to meet loved ones at the fence in his five-part poem “Return to Earth.” Odd, then, that to alight on a runway was to die another death. Required to declare our love, we found nothing to say to those who at barriers waited to embrace us. Our return to earth, we felt, was to be mourned, not fêted.

driving propellers on small airplanes to conceptual models that may use magnetic fields to propel spacecraft in the future. KEY CONCEPTS geosynchronous orbit: an orbit in which an object remains in the same location relative to a location on Earth’s surface grain: a shaped charge of solid rocket fuel designed to undergo oxidation combustion at a predetermined rate to provide a desired amount of thrust specific impulse: thrust developed per second per weight of propellant consumed under standard gravity

—David R. Wilkerson Further Reading Federal Aviation Administration (FAA). Airplane Flying Handbook (FAA-H-8083-3A). Skyhorse Publishing, 2011. Young, Trevor M. Performance of the Jet Transport Airplane: Analysis Methods, Flight Operations, and Regulations. Wiley, 2019. See also: Aerodynamics and flight; Ailerons, flaps, and airplane wings; Flight instrumentation; Flight simulators; Forces of flight; Landing gear; Plane rudders; Takeoff procedures; Taxiing procedures; Training and education of pilots; Weather conditions; Wind shear

Flight Propulsion Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics; Mathematics ABSTRACT Propulsion is the process of forcing an object to move. The word is also used to refer to the entire system of engines for achieving propulsion in the context of flight vehicles. Aircraft propulsion systems include engines, nozzles, and propellers. Methods of propulsion range from piston engines

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PROPULSION Propulsion is the force that allows aircraft to fly. Aircraft propulsion systems include engines, nozzles, and propellers. Methods of propulsion range from piston engines driving propellers on small airplanes to conceptual models that may use magnetic fields, laser beams, or antimatter to propel spacecraft in the future. TYPES OF PROPULSION SYSTEMS While the machinery is complex, the principles of operation are common to most propulsion systems. According to Newton’s second law of motion, the net force exerted on an object is equal to the rate of change of its momentum. According to Newton’s third law of motion, every action (of a force) produces an equal and opposite reaction. For flight in the atmosphere, air is used as the working fluid whose momentum is changed by the propulsion system. The reaction to the resulting force acts on the propulsion system and drives the aircraft forward. Since momentum is the product of mass and velocity, designers can choose to produce a given increase of momentum by either accelerating a large mass of fluid per second through a small change in velocity, or accelerating a smaller mass of fluid

Principles of Aeronautics

through a large increase in velocity. For flight at low speeds, it is more efficient to do the former. For example, helicopters and propeller-driven airplanes use large rotating blades to capture a large amount of air and accelerate it through a relatively small change in velocity. For flight at high speeds, turbojet and ramjet engines, which usually have small intake areas, add heat to the captured air. This heat is then converted to the work done in accelerating the air through a large velocity change, leaving hot jets of air behind. In effect, a force is exerted on the air by the engine to accelerate it backward from the aircraft. The reaction to this force acts on the engine and hence drives the aircraft forward. The same principle applies to rocket propulsion, in the atmosphere or in outer space. Rockets generate gas at high pressure by burning chemicals, and this gas escapes at high speed through a nozzle. The reaction to the force used in doing so accelerates the rocket. The key idea is that the engine and the propellant gases are pushing against each other: no other medium is needed to be pushed. In the early days of rocket flight, several experts, including editorials in the New York Times, sneered at rocket pioneer Robert H. Goddard for his insistence that rockets could thus work in the vacuum of space, but today such flight is taken for granted. PISTON ENGINES AND PROPELLERS Early aircraft propulsion systems used piston engines to drive propellers. The revolving blades of the propeller are like rotary wings, producing a force and accelerating the air encountered within the large area swept by the blades. Propellers were termed pusher or puller props, depending on whether they were mounted behind or ahead of the wings. Propellers are highly efficient as propulsion for slow-flying aircraft. Today many short-range aircraft and general aviation aircraft are powered by turboprop engines, where the engine uses the gas turbine principle, but the power generated is used to

Flight Propulsion

drive a propeller. For flight at more than about half the speed of sound (Mach 0.5), the speed at the tips of the blades exceeds the speed of sound, and shocks waves form, generating unacceptable levels of noise and drag. SOLAR-ELECTRIC PROPULSION Renewed interest in propeller-driven aircraft comes from the idea of continuously flying airplanes in the upper atmosphere using solar power to drive a motor and propeller. The National Aeronautics and Space Administration (NASA) Solar Pathfinder demonstrated ascent to over 80,000 feet using wings covered with solar panels. The energy absorbed from the Sun during the daytime can drive the vehicle to such high altitudes that it can glide all night without coming down too low. Thus automatic, continuously flying aircraft can be propelled using solar power. ROCKET ENGINES The earliest evidence of rocket usage is from China, where black-powder rockets stabilized with bamboo poles, perhaps with multiple stages, were used in the twelfth century. The South Indian king Tippu Sultan of Mysore used iron-cased rocket-powered projectiles with 2,400-meter range from 1780 to 1799 in order to protect his nation from British invaders. Using rockets captured from India, Britain’s William Congreve developed solid rockets with a 3,000-yard range, used against Napoleon’s forces in Bologne in 1806, and in the War of 1812 against the United States. Russia’s Konstantin Tsiolkovsky (1857-1935) developed the idea of multistage rockets to escape Earth’s gravity in a 1903 paper titled “Isslyedovanye mirovykh prostranstv ryeaktivnymi priborami” (“Exploration of Space with Reactive Devices,” 1957) discussing the use of liquid oxygen and liquid hydrogen. American Robert H. Goddard (1882-1945) registered a patent in 1914 for the design of a rocket combustion-chamber nozzle and propellant feed system. He published “A Method of Reaching

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Extreme Altitudes” in 1919 through the Smithsonian Institute, and conducted experiments with liquid-oxygen and gasoline propellants between 1920 and 1940. In Germany, Hermann Oberth published Die Rackete zu den Planetenraumen (1923; The Rocket into Interplanetary Space) and Wege zur Raumschiffart (1929; The Road to Space Travel). During World War II, air-launched rocket-powered unguided missiles were used, followed by Russian use of rockets in artillery barrages, and the German V-1 and V-2 ballistic missiles, which were launched into Britain. After the war, with German rocket engineers inducted into American and Soviet research organizations, the missile race accelerated. On October 4, 1957, the Soviet Union’s Sputnik became the first artificial satellite of Earth, and by 1969, Apollo 11 had taken two men to walk on the Moon and return to Earth. SOLID, LIQUID, CRYOGENIC, AND HYBRID ROCKETS The simplest rocket engine has a propellant grain of fuel and oxidizer in solid form, ignited at one end. As the solid melts and vaporizes due to the heat, the chemical reaction starts, releasing much more heat. The hot gases reach high pressure in the combustion chamber and exhaust through a nozzle, reaching high velocities. Rocket designers shape the propellant grain (the shape of the interior core of the solid propellant) in various ways to tailor the rate at which the solid material is consumed, thus predetermining how the thrust will vary with time. In general, the thrust of a solid rocket cannot be controlled once it starts, aside from releasing the pressure and thus stopping the combustion: most modern solid propellants do not burn unless the pressure is several atmospheres. Liquid propellants are stored in one or more tanks, and pumped into the combustion chamber, where the pressure is usually much higher than in the storage tanks. While liquid rockets are more controllable, the pumps often pose failure risks;

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however, the lack of control of the solid rocket is also a disadvantage. Hybrid rockets use a bi-propellant, where the liquid propellant is metered to flow over a solid propellant grain. The performance of a propulsion system is characterized by its specific impulse (Isp), which is the thrust developed per second, per unit weight of the propellant consumed, at the standard value of Earth’s gravitational acceleration, and expressed in units of seconds. The specific impulse of solid-fueled rockets is limited to about 270 seconds. Liquid-fueled rockets using storable fuels are limited to about 250 seconds. Rockets with cryogenic fuels such as liquid oxygen and liquid hydrogen reach 390 to 450 seconds. Proponents of nuclear thermal propulsion hope to achieve an Isp of 825-925 seconds. Electrothermal propulsion, where the propellant gas is heated by an electric arc, promises 800 to 1,200 seconds; electromagnetic acceleration, 5,000 seconds; and ion propulsion, 10,000 seconds. High Isp does not tell the whole story, since the higher Isp systems usually required heavy machinery, and produce very small amounts of thrust. The specific impulse of engines in space is proportional to the exhaust velocity of the propellant gas. For a given addition of momentum per unit mass, hydrogen, having the lowest molecular weight, provides the highest specific impulse. An efficient type of rocket engine is the solar-hydrogen engine used in orbit transfer vehicles shuttling between low-Earth orbit and geosynchronous Earth orbits. Here solar energy is focused by a collector to heat hydrogen, which then flows out at high speed through a nozzle. NUCLEAR PROPULSION A heat source is crucial to propulsion, and one which generates the most heat with the least expenditure of fuel weight would produce the highest specific impulse. Nuclear reactions satisfy this criterion, but the weight of the shielding needed for the reactor, and the consequences of a crash, have limited

Principles of Aeronautics

their use in flight propulsion. The slow neutron reactors used in ships and submarines proved to be too heavy for use in aircraft, while other designs, which could heat air to high temperatures quickly, operated at temperatures too high for available materials and posed extreme radiation hazards. In the 1950s, an Aircraft Nuclear Propulsion (ANP) project led to several advanced designs for nuclear-powered intercontinental bombers, but none appear to have been flight-tested. Project Pluto, a secret project conducted in Nevada, developed a nuclear-powered ramjet supersonic cruise missile. Small nuclear reactors have been used in deep-space probes such as the Galileo mission, and it is expected that missions to other planets, such as an exploration of Jupiter’s atmosphere, will require nuclear propulsion to provide the required specific impulse. Proposed nuclear thermal rockets will heat propellant gas (hydrogen) through the coolant channels of a solid-fuel reactor core at about 3,000 degrees Kelvin, and expand hydrogen through a nozzle. ION PROPULSION Ionized gases are accelerated to high exhaust velocity using electromagnetic fields in engines used to produce low thrust, available for station-keeping orbit corrections over long durations on spacecraft. The Boeing 702 Xenon Ion Thruster claims an Isp of 3,800 seconds and thrust of 165 million newtons (by comparison, the Saturn V at liftoff produced over 33 million newtons). The weight of the system required to produce the electromagnetic field has restricted the usage of ion propulsion to low-thrust applications, perhaps until superconducting electromagnets become available for use in such systems. AIR-BREATHING JET PROPULSION For flight in the atmosphere, the effective specific impulse can be increased greatly by using oxygen in the air as oxidizer, and air as the working fluid: air does not have to be added to the fuel cost or vehicle

Flight Propulsion

weight. There are three principles of jet propulsion: heat addition to the working fluid is most efficient if the heat is added at the highest pressure possible; the conversion of heat to work is most efficient if the temperature difference is largest; and the thrust is most efficient in driving the aircraft if the exhaust velocity is close to (but greater than) the flight speed. In the gas turbine cycle, the working fluid is first compressed, then heat is added at constant pressure, and finally work is extracted from the hot, high-pressure fluid as it expands and flows out. Thus, gas turbine engines incorporate a compressor to increase pressure, a combustion chamber to add the heat through a combustion reaction between the fuel and air, a turbine to extract work and run the compressor, and a nozzle to expand the flow out. For large engines used by commercial aircraft, the optimal value of pressure ratio (between the highest pressure after compression and the outside) is as high as 50. At supersonic speeds, the deceleration of the air at the front of the engine itself raises the pressure substantially; the optimum pressure ratio may be only 7. As Mach number increases beyond 2.5, the need for a mechanical compressor vanishes, and ramjet engines can operate. Here the incoming air is decelerated, so that its pressure increases to such large values that mechanical compressors and the turbines to operate them are not needed. All other gas turbine engines require compressors to increase the pressure of the incoming air, and turbines which drive the compressor and extract work required to run other components including propellers, rotors, and fans. These turbomachines change pressure through several stages. Each stage has a rotor where work is done on the fluid to change its momentum, and a stator, or counter-rotating rotor, to recover the momentum change and convert it into a pressure change. Turbomachine stages may be centrifugal or axial. In centrifugal stages, air comes in near the axis and is flung out

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pressurized at the periphery. In axial stages, the flow is predominantly parallel to the axis, with rows of blades successively increasing momentum by swirling the flow and recovering the pressure by reducing the swirl. TURBOFANS, TURBOJETS, AND PROPFANS The first jet engines were turbojets, where all of the airflow went through the same compressor and combustion chamber. The first jet engine was patented in 1930 by Sir Frank Whittle (1907-96). The PowerJets Model W.1 engine was first tested in April, 1937, and according to Sir Whittle, “made a noise like an air raid siren,” sending onlookers running for cover. It weighed 700 pounds and produced 860 pounds of thrust, using a double-sided centrifugal compressor. The first British aircraft to use the engine was the Gloster Meteor, a night fighter that first flew in March, 1943, eventually reaching 420 miles per hour. The first jet-powered flight, however, was on a Heinkel aircraft powered by Hans von Ohain’s (1911-98) axial-compressor turbojet engine in Germany. The first jet fighter took off on July 18, 1942, a Messerschmitt Me-262 fighter piloted by Fritz Wendel of the German Luftwaffe, using a Junkers Jumo 004 turbojet engine producing 2,200 pounds of thrust. Earlier attempts had been made using BMW003 turbojet engines, which used a seven-stage axial-flow compressor and an annular combustion chamber with sixteen burners. Today, centrifugal compressors are used in the turbopumps of rocket engines, while axial compressors are dominant in most aircraft applications. Helicopter turboshaft engines use both centrifugal and axial stages. The thrust-to-weight ratio of modern jet engines has improved to well over 4:1. Turboprop engines use a small turbine to extract enough work from the hot combustor gases to run the compressor, and a large power turbine to extract most of the work from the air to run a propeller. The propeller is connected through a gearbox to reduce

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the speed of revolution; this adds considerable weight to turboprop engines. The Soviet Bear long-range bomber used turboprop engines with a pair of counter-rotating propellers on each engine. The design trade-off between high thermal efficiency (requiring high pressure and temperature) and high propulsive efficiency (requiring a small increase of air velocity from the flight speed) is addressed using bypass or turbofan engines, where a part of the captured air goes through a fan and a nozzle, bypassing the main compressor, combustor, and turbine. The bypass ratio is the ratio of the air bypassing the hot core of the engine to the air which goes through the core and has fuel burned in it. Fighter aircraft turbofan engines use a bypass ratio of approximately 1, while modern commercial aircraft engines, such as the GE90 used on the Boeing 777 and Airbus 340 airliners, use bypass ratios up to 12. In the 1980s, propfans or unducted fans were explored to bridge the gap between the propeller and the ducted turbofan engine. Using modern computational aerodynamics technology, large fan blades of complex shape were designed to operate with supersonic tip speeds and large pressure rise across each stage. Some designs had counter-rotating rows of fan blades. To increase the capture area, the blades were left without the outer cowling used by turbofan engines. These engines promised large improvements in fuel efficiency for short-haul aircraft, but encountered severe problems of development cost and noise levels high enough to damage the aircraft structure through sonic fatigue. For air-breathing flight at supersonic speeds, a supersonic inlet must slow down the supersonic flow with minimal losses due to shock waves, so that the fan, compressor, and combustion chamber can operate at subsonic speeds. Inlets vary in complexity from the normal-shock inlet of the early MiG and Sabre fighters, through the movable spike inlets of the MiG-21 or the SR-71, to the multiple-ramp inlets of the F-15 or Concorde. Hypersonic aircraft use the

Flight Recorder

Principles of Aeronautics

compression across the shock produced by the aircraft fuselage to decelerate, so that engine-airframe integration is vital to such designs. Instead of varying geometry, supersonic flows can also be decelerated and compressed using heat addition (thermal compression). At the other end, nozzles vary from simple convergent nozzles of subsonic aircraft, to the converging-diverging nozzles of fighters with afterburners, to the rectangular nozzles of modern fighters where the thrust can be vectored for maneuvering or vertical takeoff. High-speed aircraft concepts (NASA’s X-33, Lockheed’s VentureStar, and the Japanese ATREX turboramjet) use the Aerospike or Plug Flow nozzles to enable external variation of the nozzle expansion. Several other types of propulsion devices are being studied by researchers. In the Mini-Magnetospheric Plasma Propulsion (M2P2) concept developed by Robert Winglee at the University of Washington, jets of heated gas plasma, fired from a spacecraft, interact with the magnetic field generated by the spacecraft to produce a mini-magnetosphere around the craft. The interaction of this magnetosphere with the plasma wind from the Sun (the solar wind) produces forces in a fashion somewhat similar to the interaction of an airfoil shape with flowing air generating lift. This force can be tailored to drive the spacecraft around the solar system at very high speeds. Unlike solar sails, which work better to drive a spacecraft in the inner solar system, M2P2 is seen as an option for travel to the outer planets. Scientists have long speculated that photons could exert pressure on a spacecraft and drive it to speeds approaching the speed of light. Practical systems for focusing high-power lasers onto spacecraft are not yet in use in space. Experiments by Leik Myrabo of Rensselaer Polytechnic Institute and the US Air Force had succeeded, by the year 2000, in lifting small objects to a height of a few dozen meters using ground-based lasers. In extended forms of this concept, the focused laser beam creates an “aerospike”

of heated gas ahead of the vehicle, which helps reduce drag as the vehicle is driven up through the atmosphere by a shock created by expanding air beneath the vehicle. FUSION AND ANTIMATTER PROPULSION Scientists hope that in the distant future, power generation by nuclear fusion or matter-antimatter interaction will allow the development of propulsion systems with immense thrust levels and very high specific impulse. For now, such systems remain in the realm of science fiction. —Narayanan M. Komerath Further Reading Anvekar, Mayur R. Aircraft Propulsion. Prentice Hall India Pvt Ltd., 2018. El-Sayed, Ahmed F. Aircraft Propulsion and Gas Turbine Engines. CRC Press, 2017. ———. Fundamentals of Aircraft and Rocket Propulsion. Springer London, 2016. Farokhi, Saeed. Aircraft Propulsion. Wiley, 2014. ———. Aircraft Propulsion: Cleaner, Leaner, and Greener. Wiley, 2021. Haran, Kiruba, Nateria Madavan, and Tim C. O’Connell. Electrified Aircraft Propulsion: Powering the Future of Air Transportation. Cambridge UP, 2022. See also: Advanced propulsion; Aeronautical engineering; Airplane propellers; Daniel Bernoulli; First flights of note; Forces of flight; Robert H. Goddard; Greenhouse gases; Hypersonic aircraft; Jet engines; Pressure; Propulsion technologies; Rocket propulsion; Rockets; Scramjet; Shock waves; Spacecraft engineering; Spaceflight; Supersonic aircraft; Konstantin Tsiolkovsky; Turbojets and turbofans; Turboprops

Flight Recorder Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology

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ABSTRACT The flight recorder, also known as the black box (though it is bright orange in color), is an instrument that records the performance and condition of an aircraft in flight. The data retrieved from a flight data recorder can be used to generate a computer-animated video reconstruction of the flight of an aircraft, making possible the investigation and analysis of aircraft accidents or other unusual occurrences. KEY CONCEPTS stall: ascending at an angle that causes the wings of an aircraft to lose their ability to provide lift vertical acceleration: the rate at which an aircraft gains or loses altitude FLIGHT DATA RECORDERS An aircraft flight recorder records many different operating conditions of a flight and provides infor-

Principles of Aeronautics

mation that may be difficult or impossible to obtain by any other means. By regulation in most countries in the world, newly manufactured aircraft must monitor at least twenty-eight important parameters. These include time, altitude, airspeed, heading, vertical acceleration, and aircraft pitch. Some recorders can record the status of more than three hundred additional in-flight characteristics that can aid in an accident investigation. Some of these include flap position, autopilot mode, and even smoke alarms. To ensure that a large amount of information is recorded, a flight recorder is able to record for at least twenty-five hours. Computer programs have been written to take flight recorder data and reconstruct animated videos of aircraft flight. The animation allows the investigation team to view the last moments of a flight prior to an accident. In the event of an accident, investiga-

A modern flight data recorder; the underwater locator beacon is the small cylinder on the far right. (Translation of warning message in French: “Flight recorder do not open”.) The warning appears in English on the other side. Photo via Wikimedia Commons. [Public domain.]

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tors can visualize the instrument readings, power settings, airplane’s attitude, and other important characteristics of a given flight. COCKPIT VOICE RECORDERS A cockpit voice recorder records the flight crew’s voices, as well as other sounds within the cockpit. Communications with air traffic control, automated radio weather briefings, and conversation between the pilots and ground or cabin crew are recorded. Sounds of interest to an investigation board, including engine noise, stall warnings, landing gear extension and retraction, and any clicking or popping noises, are typically recorded. Based on these sounds, important flight parameters, such as speed, system failures, and the timing of certain events can often be determined. In the event of an accident, an investigation committee creates a written transcript of the cockpit recorder tape. Local standard times associated with the accident sequence are determined for every event on the transcript. This transcript contains all the pertinent portions of the cockpit recording. Due to the highly sensitive nature of the verbal communications inside the cockpit, a high degree of security is provided for the cockpit recorder tape and its transcript. The timing of release and the content of the written transcript are strictly regulated. HISTORY The idea of a device to record both the voices and the instrument readings in the cockpit of an aircraft was originally conceived by Dr. David Warren at the Aeronautical Research Laboratory in Melbourne, Australia, in the 1950s. A demonstration unit was constructed in 1957. Although Australian aviation authorities did not initially approve the device, it was taken to Great Britain and the United States for further development. On June 10, 1960, a Trans-Australian Airlines Fokker F-27 crashed while landing at an airport in

Flight Recorder

Queensland, Australia, killing all twenty-nine people on board. The subsequent board of inquiry was unable to arrive at any definite conclusions as to the factors underlying the accident. The board recommended that all airliners be fitted with flight recorders. In 1961, Australia became one of the first countries to make flight recorders mandatory in aircraft. Any craft with a takeoff weight greater than 12,568 pounds must carry both a cockpit voice recorder and a flight data recorder. Flight recorders and cockpit voice recorders, also known as black boxes, are actually painted bright orange to aid in their recovery following an accident. They have provided critical clues in solving the mysteries associated with many of the world’s air disasters and have also been invaluable in helping to prevent future accidents. SPECIFICATIONS Flight recorders and cockpit voice recorders are housed in titanium boxes that are lined with many layers of insulating material. This design protects the recorders against impacts that produce accelerations up to 3,400 times the acceleration of gravity, against fires of up to 1,093 degrees Celsius, and against pressures at water depths of up to 6,100 meters. The recording devices are protected against contact with seawater and inadvertent erasure of recorded information. These specifications preserve the devices in the most serious accidents and in extreme climatic conditions. In addition, the boxes are fitted with battery-powered ultrasonic beacons that aid with underwater recovery. The beacons can transmit pulses from water depths of up to 4,300 meters for at least thirty days over a range of 3.2 kilometers. However, factors such as water temperature and depth can have a negative effect on the life of the battery of the beacon, giving investigators less time to track down the black box under water. In a more high-profile case, it took almost two years for crews to find the black box from Air France Flight 447 that had crashed in the Atlantic

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Ocean in 2009; statisticians were brought in to calculate the most likely resting place of the wreckage on the ocean floor. Because they are more reliable and require minimal maintenance, computer memory chips have replaced most magnetic tapes as the recording media. Flight recorders are connected to a flight data acquisition unit that processes, digitizes, and formats the data for recording on the memory chips. Both the flight recorder and the cockpit voice recorder are carried in the tail of an aircraft. Flight data recorders reveal what happened in an accident, whereas cockpit data recorders reveal why it happened. Even before a crash occurs, it is possible to monitor the safety of flights by using a quick access recorder. This device records even more parameters than a typical flight recorder and samples the data at higher rates for a longer duration of time. The data are stored on an optical disk and can be studied to identify problems before they become fatal. A ground-based computer analyzes the data and determines what is going wrong, rather than what went wrong. Particularly after plane accidents and crashes in which the black box has not been recovered, leaving investigators largely in the dark about what actually caused the crash, experts have pushed for possible improvements to further ensure that the black box can be found in the event of an incident. Following the disappearance and assumed crash of Malaysia Airlines Flight 370 in March 2014 and recovery crews were unable to find the wreckage and black box, Congressman David Price proposed a bill once more calling for all black boxes to be made ejectable and able to float on the water’s surface. Other suggestions have included video recording capabilities for the cockpit and having the recorders transmit data in real time to ground crews who could help use the information to possibly avert disasters. —Alvin K. Benson

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Further Reading Adair, Bill. The Mystery of Flight 427: Inside a Crash Investigation. Smithsonian Institute Press, 2013. Barreveld, Dirk. Air Crash Investigations—Chaos in the Cockpit: The Crash of Northwest Airlines Flight 255. Lulu.com, 2014. Federal Aviation Administration (FAA). FAR/AIM 2022: Up-to-Date FAA Regs/Aeronautics. Skyhorse Publishing, 2022. Jeffries, Stuart. “Secrets of the Black Box: How Does MH370’s Flight Recorder Work?” The Guardian, 31 Mar. 2014, www.theguardian.com/world/2014/mar/31/ airplane-black-box-flight-recorders-investigators. Accessed 25 Jan. 2017. National Archives and Records Service. Code of Federal Regulations; Aeronautics and Space: Part 10. Office of the Federal Register, National Archives and Records Service, General Services, 2013. Siegel, Greg. Forensic Media: Reconstructing Accidents in Accelerated Modernity. Duke UP, 2014. See also: Air transportation industry; Aircraft accident investigation; Airflight communication; Airplane cockpit; Airplane safety issues; Autopilot; Avionics; Federal Aviation Administration (FAA); Flight instrumentation; National Transportation Safety Board (NTSB); Training and education of pilots

Flight Roll and Pitch Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Pilot training ABSTRACT Roll is the angular motion of an airplane about its centerline, a line of equal distance between the wings through the fuselage. Pitch is the angular motion of an airplane about a line from wingtip to wingtip perpendicular to its centerline. KEY CONCEPTS center of gravity: the point within an aircraft, or any other body, about which the entire mass of that body is equally distributed

Flight Roll and Pitch

Principles of Aeronautics

centerline: the longitudinal axis of an aircraft in the direction it is designed to fly pennage: the vertical structure comprising the tail of an aircraft rudder: a vertical aileron on the pennage of an aircraft SIGNIFICANCE OF ROLL AND PITCH Roll is important because pilots can use this motion to differentiate the lift on the wings and change the path of the airplane; by pitching the airplane, the pilot can change the magnitude of the lift on the airplane. For example, in setting up an airplane for a landing, the pilot must continually redirect the lift force to keep the airplane aligned with the runway. In landing, changes to the pitch angle can make small changes to the lift force and therefore alter the descent rate of the airplane. AILERONS AND ELEVATORS The primary airplane controls that generate roll are the ailerons, which are located on each side of the wing and are identified as the left and right ailerons. Both ailerons are of the same size and are located at the same distance from the centerline of the airplane. These controls are essentially the same in both expensive, high-performance general aviation jets and low-performance training airplanes. In both, each aileron is attached to its corresponding wing by a hinge. The ailerons deflect upward and downward about the hinge line. When one aileron deflects upward, the other aileron deflects downward. The pilot deflects the ailerons by moving the control wheel. If the control wheel is rotated counterclockwise, the left aileron moves upward and the right aileron moves downward. This movement is in contrast to that of the elevator; both right and left elevators move together. The elevator is the primary control for changing the pitch angle or the angle that the centerline makes with the horizontal. The pilot deflects the elevator

by moving the control wheel or stick backward and forward. Rearward movement of the wheel raises the elevator and therefore the nose of the airplane. RUDDERS Even though the ailerons are the primary roll control, the rudder is often moved with the ailerons in making turns. The rudder moves the nose of the airplane in the direction of the lower wing. However, movement of the rudder can also affect the roll of the airplane. In a turn (to the left, for example), the aileron on the left wing is raised and the aileron on the right wing is lowered. In this aileron position, the rudder is moved to the left. This rudder movement pushes the tail to the right and therefore the nose to the left, essentially making the airplane pivot about its center of gravity. A force at the tail to the right will push the nose of the airplane to the left. Since this force due to the rudder is to the right and above the centerline of the airplane, the result is an initial rolling action opposite to that resulting from the aileron movement. Therefore, even though the rudder is required to move left to help turn the airplane to the left, there is a secondary effect as soon as the rudder is applied, which detracts from the rolling motion of the ailerons. However, because the rudder forces the nose to the left, in spite of its contrary rolling effect, the nose is moved to the left

Image by Auawise, via Wikimedia.

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of the direction of the oncoming air. This deflection produces a cross-flow coming from the right to the left for the left turn. Airplane wings are designed with a slight upward cant. This bend is called the wing dihedral, and the angle that the wing makes with the horizontal is called the dihedral angle. As a result of both dihedral angle and cross-flow, the right wing has a slight increase in upward flow, and the left wing has a slight decrease in upward flow. The result is that the lift on the right wing is increased and the lift on the left wing is decreased. The result is a roll angle, left wing down and right wing up, that is in the same direction as that caused by the deflection of the ailerons. The rudder initially causes the airplane to roll in a direction opposite to that of the ailerons; however, the yawing motion of the rudder causes a cross-flow to develop, and that flow, along with the built-in dihedral angle, causes the airplane to roll in the proper direction for the turn: left wing down for turn to the left, right wing down for turn to the right. An airplane can, however, have an excessive amount of dihedral, the result of which would be that wind gusts from the left or right would cause a rolling motion that would increase with the dihedral angle. The airplane would have an unpleasant rocking motion in response to even small gusts. As has been pointed out, the rudder can cause the airplane to roll, but the ailerons can cause the airplane to yaw. The aileron can cause the airplane to yaw because there is always a drag associated with lift. If lift is increased, then the drag is increased. When the right aileron is deflected downward, the lift increases on the right wing. At the same time, the left aileron is moved upward, decreasing the lift on the left wing. Thus, there is an increase in drag on the right wing and a decrease in drag on the left wing, which will cause the nose of the airplane to swing to the right. Because the ailerons are moved to turn to the left, the yaw that results from aileron deflection is called adverse yaw. The main purpose

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of the rudder is to counteract this adverse yaw. When the airplane rolls to the left, the nose-right adverse yaw of the ailerons is countered by moving the rudder left. The primary roll control on an airplane is managed by the ailerons, one on each wing. The pilot controls airplane roll by rotating the control wheel in the direction of the desired roll. Aileron deflection produces an adverse yaw, which is countered by the rudder. Pitch is controlled by movement of the elevator, moved in turn by the pilot by a backward, or nose-up, and forward, or nose-down, movement of the control wheel. —Frank J. Regan Further Reading Durham, Wayne. Aircraft Flight Dynamics and Control. Wiley, 2013. Falangas, Eric T. Performance Evaluation and Design of Flight Vehicle Control Systems. Wiley, 2015. Federal Aviation Administration (FAA). Airplane Flying Handbook (FAA-H-8083-3A). Skyhorse Publishing, 2011. Padfield, Gareth D. Helicopter Flight Dynamics, Including a Treatment of Tiltrotor Aircraft. Wiley, 2018. Sinha, Nandan K., and N. Ananthkrishnan. Advanced Flight Dynamics with Elements of Flight Control. CRC Press, 2017. Vepa, Ranjan. Flight Dynamics, Simulation, and Control. Taylor & Francis, 2014. See also: Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Forces of flight; Plane rudders; Pressure; Rotorcraft; Tail designs

Flight Schools Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Flight schools are institutions that provide the education and training necessary for individuals to learn to pilot an

Principles of Aeronautics

aircraft. Flight schools educate pilots and prepare them for certification. Flight schools teach pilots who intend to fly for their personal enjoyment, who intend to pilot commercial aircraft, and who intend to pilot military and other service aircraft. KEY CONCEPTS instrument rating: a certification for the ability to take off, fly and land an aircraft without visible cues using only instrument readings multiengine rating: a certification allowing a pilot to fly aircraft with two or more engines HISTORY In the early days of aviation, there were no government regulations to control the certification of pilots. Learning to fly was largely a matter of experimentation, observation of others who knew how to fly, and trial and error. As the field of aviation evolved, the need for more formal methods of training pilots became apparent. Flight schools first began to appear in the late 1920s. Parks College was the first flight school to be awarded a Transport and Limited Commercial Ground and Flying School Certificate, granted in 1929 by the US government. During the Great Depression years of the 1930s, the few flight schools in existence were fortunate if they were able to stay in business, and significant growth in flight training did not occur until the early 1940s. The outbreak of World War II generated a need for a number of pilots, each of whom needed to be trained to a certain standard in a relatively short amount of time. In 1939, the US Congress appropriated $4 million to create the Civilian Pilot Training Program. The flight training done under this program was conducted at more than 400 colleges nationwide. After World War II, there continued to be a strong interest in aviation, particularly by the returning veterans. The GI Bill (1944) provided funding for veterans to obtain flight training, and thousands of students took advantage of this program.

Flight Schools

This source of income provided a foundation for flight schools to continue to grow and prosper. Pilot training today is regulated in the United States by the Federal Aviation Administration (FAA) and in Canada by Transport Canada, agencies of the respective federal governments. The FAA issues Federal Aviation Regulations (FARs), which are the rules that govern aviation in the United States. These rules include the certification of pilots and aircraft and the governance of flight operations. Flight schools train prospective pilots to meet the certification requirements specified by the FARs for various levels of pilot certificates and ratings. For the most part, Transport Canada has adopted these FARs as applicable to Canadian aviation. TYPES OF SCHOOLS There are a number of different types of institutions that provide flight training and education in the United States. These include fixed-base operators (FBOs), collegiate aviation programs, proprietary professional aviation academies, and military programs. The type of flight school best suited to a particular student depends on that student’s goals and intentions in aviation. FBOs are businesses that operate at airports. They often provide a variety of services to the aviation community, including aircraft rental, maintenance, refueling, and the sale of aviation equipment, in addition to pilot training. Pilot training at this type of facility is typically tailored to an individual’s schedule and personal goals. This type of flight school is usually attended by students who are interested in flying for pleasure or for personal business transportation. Collegiate aviation programs, available at both two-year and four-year institutions, are designed for those students who wish to pursue a career as a pilot. Both types of institutions typically provide flight training through at least the Commercial Pilot Certificate, and usually the Certified Flight Instructor

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A Canadian aeroplane flight instructor (left) and her student, next to a Cessna 172 with which they have just completed a lesson. Photo via Wikimedia Commons. [Public domain.]

Certificate. Graduates of two-year programs receive an associate of science degree, whereas graduates of four-year programs receive a bachelor of science degree. In addition to completing the required ground and flight training for a Commercial Pilot Certificate, students at these institutions complete course work in a variety of areas important to understanding aviation. These may include maintenance, weather, aerodynamics, and aviation management courses. There are more than one hundred colleges and universities, large and small, that offer flight training as part of the curriculum for a degree. The Council on Aviation Accreditation is the accrediting body for collegiate aviation programs, and most reputable college programs have received accreditation by this organization.

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Proprietary professional aviation academies are also designed for those students who wish to enter the aviation profession as a pilot. These schools typically provide training through at least the Commercial Pilot Certificate, and often through the Certified Flight Instructor Certificate. Enrollment in this type of school is most often a full-time endeavor. Since the late 1980s, the educational requirement for career advancement to a position as a pilot for a major airline has been a four-year college degree, so a number of proprietary aviation academies are also associated with a collegiate institution. Military flight schools are utilized to train those personnel accepted into a branch of the US Armed Forces for a pilot position. These programs provide high-quality initial training in basic piloting skills,

Principles of Aeronautics

followed by training in the specific type of aircraft and operation to which the person is to be assigned. The training period for both the initial course and the advanced course is typically one year each. The US Army, Air Force, Navy, Marine Corps, and Coast Guard each have personnel assigned to pilot positions. The training for these personnel is conducted at various military bases throughout the country. Whatever the type of institution, a flight school will provide a fleet of aircraft in which to conduct training, and a staff of certified flight instructors (CFIs) to provide flight training. Both the size of the aircraft fleet and the size of the CFI staff may vary from one to more than one hundred. TYPES OF TRAINING OFFERED At any of the civilian institutions described above, flight training may be conducted under either FAR Part 61 or FAR Part 141. Part 141 specifically describes minimum requirements regarding training facilities, personnel, course syllabi, and student performance rates for FAA-approved flight schools. Programs conducted under Part 141 are subject to continuing oversight and approval by the FAA. Collegiate and proprietary aviation academies are typically certified under FAR Part 141, although a number of FBOs also have Part 141 certification. FAR Part 61 specifically governs the certification of aircraft and pilots, and flight training can also be conducted under this part. Often, training for students who are interested in flying for their personal benefit or enjoyment is conducted under Part 61 at a local airport FBO, whereas training for students who desire a career as a pilot is conducted at a Part 141 school. Part 141 schools tend to be more structured and formalized, and Part 61 schools tend to be tailored more toward the individual requirements of the person receiving training. For example, a businessperson who wants to obtain a pilot certificate for transportation purposes may desire to participate in flight training only twice a week and at a different

Flight Schools

time each week. This type of schedule is often best accommodated at a local airport FBO under Part 61. A person interested in a career as a pilot would most likely desire to pursue this goal in a full-time capacity, and many Part 141 schools can accommodate this arrangement. TYPES OF CERTIFICATES There are a number of types of pilot certificates issued by the FAA. These include the Recreational Pilot Certificate, the Private Pilot Certificate, the Commercial Pilot Certificate, the Certified Flight Instructor Certificate and the Airline Transport Pilot (ATP) Certificate. Training to obtain any one of these certificates involves a specified minimum of both flight and ground training, often called ground school. Both the Recreational and Private Pilot Certificates are designed for individuals who wish to fly for their own personal enjoyment. The Recreational Pilot Certificate has a number of limitations, such as the requirement that recreational pilots remain within 50 nautical miles of the departure airport, carrying no more than one passenger, and that a recreational pilot not fly an aircraft with more than four seats. The Private Pilot Certificate allows more freedom, with no limit on passengers or distance from the departure airport. The Commercial Pilot Certificate is required in order for a pilot to be paid for flying an aircraft. The CFI Certificate is required to be able to instruct others in flight training, and the ATP Certificate is required to be a captain (or pilot in command) of an aircraft operated by a commercial air carrier. In addition to these certificates, an important rating that can be added to the Private and Commercial Certificates is the instrument rating. The ATP Certificate essentially includes an instrument rating as part of its privileges and limitations. The instrument rating allows pilots to fly in bad weather, called instrument meteorological conditions, which include such things as clouds or low visibility. Before obtaining an

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instrument rating, pilots are restricted to visual meteorological conditions, which means they must maintain a minimum visibility and distance from clouds. If a person intends to use aviation as a dependable and regular means of personal transportation, obtaining an instrument rating is essential. If a pilot is to fly for hire, an instrument rating is likewise required. One additional rating that must be obtained before flying an airplane that has more than one engine is a multiengine rating. Since most flight students first learn to fly in a single-engine aircraft, this rating is usually added to an existing Private or Commercial Single-Engine Certificate. CFI and ATP Certificates also specify whether the pilot has single-engine privileges, multiengine privileges, or both. GROUND TRAINING Training conducted at flight schools, while often termed flight training, in reality consists of both ground training and training in an actual aircraft. Ground training may be conducted in a formal classroom setting, with a number of students receiving instruction from a teacher, or it may be conducted informally by a student’s flight instructor before or after a flight. Typically, Part 61 flight schools tend to use more informal methods, whereas larger Part 141 flight schools and college programs tend to use traditional classroom settings for ground school. Again, the best method depends on the interests and background of the flight student. Ground school covers a variety of topics, including applicable FARs, aircraft systems and performance, aerodynamics, weather, flight planning, and navigation. Often, flight schools own one or more flight-training devices in addition to their fleet of aircraft. These flight-training devices are more simplified versions of what are commonly known as flight simulators. Most often, they have a cockpit mock-up and a rudimentary visual display. However,

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there is no movement of the device in response to aircraft control movements. These devices are used most heavily during training for the Instrument Rating. Students working on this rating receive training in these flight-training devices in addition to conventional ground school and flight training in an aircraft. OBTAINING A PILOT CERTIFICATE To obtain any level of pilot certificate or rating, an applicant must do a number of things. First, the ground and flight instruction specified by the FARs must be obtained from and certified by a CFI. A knowledge test, administered in a computer-based testing format, must be taken and passed with a minimum score of 70 percent. An appropriate medical certificate must be obtained from an aviation medical examiner for the level of certificate desired. For example, for a Private Pilot Certificate, a third-class medical certificate is required. For a Commercial Pilot Certificate, a second-class medical certificate is required, and for an Airline Transport Pilot Certificate, a first-class medical certificate is required. Finally, a practical test is conducted by a pilot examiner. This test consists of both an oral exam and a flight exam. During the oral exam, the examiner will cover items such as aerodynamics, weather, aircraft systems, aircraft performance, and flight planning. During the flight, a series of maneuvers will be evaluated to determine whether the applicant meets the minimum standards specified for the certificate for which he or she is applying. If the check ride is satisfactory, the student will be issued the certificate for which he or she applied. SELECTING A FLIGHT SCHOOL A flight school is best selected by considering the needs of an individual. Such items as the location of the school and the schedule of lessons are key issues, as are the types of training typically conducted and the structure of the school, for instance, whether it is

Principles of Aeronautics

geared toward those interested in aviation as a profession or toward those interested in learning to fly for fun. Other things to consider are the size and availability of the training aircraft fleet and the availability of instructional staff. The school’s safety record, how long the school has been in operation, and its reputation are also important. In addition, maintenance of the training fleet should be examined. Many schools offer an introductory flight lesson, during which a CFI will allow a prospective student to manipulate the controls of the airplane in flight. This provides an opportunity for the prospective student to examine the flight environment firsthand, as well as a chance to experience a representative training aircraft and instructor. One aspect of the decision regarding a flight school selection involves whether to select a FAR Part 141-approved flight school or a FAR Part 61 flight school. To obtain a Private Pilot Certificate, thirty-five hours of flight training are required under Part 141, whereas forty hours of flight training are required under Part 61. However, the national average of flight hours to obtain a Private Pilot Certificate ranges from sixty-five to seventy hours, so it would be an error to base a decision to use a Part 141 school instead of a Part 61 facility solely on the flight-time requirement for a Private Pilot Certificate. If a student is interested in pursuing a Commercial Pilot Certificate, there is a flight-time benefit in utilizing a Part 141 flight school. The flight time required for a Commercial Pilot Certificate is 250 hours under Part 61 and 190 hours under Part 141. COST OF FLIGHT TRAINING The cost of flight training varies widely depending on the area of the United States in which a student resides and the type of flight school attended. Many flight schools offer package deals for flight instruction, but it is important to understand what items are included in the package. During flight training

Flight Schools

in an aircraft, both an airplane rental fee and a flight instructor’s hourly fee are charged. Aircraft used for instruction usually have a digital recording clock, called a Hobbs meter, which records the amount of flight time for a given flight by subtracting the Hobbs meter reading at the beginning of the flight from the Hobbs meter reading at the end of a flight. Preflight and postflight briefing time, which is conducted by a student’s CFI and which is necessary for effective flight training, is also billed. The most common type of package offer includes the cost of these items up to a certain number of hours, with excess hours becoming the student’s responsibility if they are required. Other packages may guarantee obtainment of a certificate, with no maximum number of hours specified, although there are often many other stipulations in this kind of package. The minimum time required by the FARs to obtain a Private Pilot Certificate under FAR Part 61 is forty hours: twenty hours with an instructor, called dual instruction, and twenty hours of solo flight time. For flight-school package offer-comparison purposes, however, an average student usually requires from sixty-five to seventy hours to obtain a Private Pilot Certificate, with forty to forty-five flight hours of dual instruction and twenty-five hours of solo flight time. The total cost of a university education, including obtaining Commercial Pilot and Certified Flight Instructor Certificates as well as a four-year degree at a private university, can equal more than $100,000. However, much of this cost would also be incurred in the course of obtaining a bachelor’s degree from a private university in a field other than aviation. The cost is typically less at state-supported universities and less still at junior colleges or community colleges. Often, two-year program graduates can continue their studies at a four-year university to complete a bachelor of science degree. The cost to attend a proprietary professional academy, usually resulting in the obtaining Com-

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mercial Pilot and Certified Flight Instructor Certificates, can range from approximately $50,000 to $85,000. This type of program is often selected by individuals who have already obtained a four-year college degree and who are interested in changing careers. The sole focus on flight training allows such individuals to accelerate their training so they can begin to pursue their new career path. In addition, there are students who choose to enroll in this type of program right after high school, and then serve as certified flight instructors while earning their college degrees. OTHER TYPES OF FLIGHT SCHOOLS The preponderance of flight schools in existence in the United States and Canada are for airplane pilots. However, in addition to training for pilot certificates for airplanes, there are also flight schools that conduct specialized training in other types of aircraft or operations. For example, helicopter pilots, glider pilots, pilots involved in agricultural operations, and seaplane pilots are required to receive appropriate ground and flight training for the type of operation and aircraft they pilot. Some large flight schools conduct these types of training in addition to more traditional airplane pilot training, whereas other schools choose to specialize in a niche market. —Wendy S. Beckman Further Reading Garajedaghi, Reza. Want to Be a Commercial Pilot? Your Complete Guide to Finding the Right Flight School and Fulfilling Your Dreams. Flight Schools Consulting, 2019. Meyer, Alan. Weekend Pilots: Technology, Masculinity, and Private Aviation in Postwar America. Johns Hopkins UP, 2015. The Pilot’s Manual Editorial Board. The Pilot’s Manual: Ground School: All the Aeronautical Knowledge Required to Pass the FAA Exams and Operate as a Private and Commercial Pilot. Aviation Supplies & Academica Inc., 2020. Pisano, Dominick A. To Fill the Skies with Pilots: The Civilian Pilot Training Program, 1939-1946. Smithsonian Institute Press, 2001.

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Turpeinen, Vesa. Learn to Fly and Become a Pilot! The Ultimate Guide to Determining Your Capabilities of Becoming a Professional Pilot and Getting Started with Flight Training. Vesa Turpeinen, 2019. See also: Aerodynamics and flight; Aeronautical engineering; Airflight communication; Airplane cockpit; Airplane maintenance; Airplane safety issues; Avionics; Federal Aviation Administration (FAA); Flight instrumentation; Flight landing procedures; Flight roll and pitch; Flight simulators; Flight testing; Forces of flight; Glider planes; Landing gear; Plane rudders; Takeoff procedures; Taxiing procedures; Training and education of pilots; Wake turbulence; Weather conditions; Wind shear

Flight Simulators Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics; Pilot training ABSTRACT Flight simulators are computer-controlled electronic-hydraulic devices that are used to enable a person to experience flight situations and/or movements without actually flying in an aircraft or spacecraft. Flight simulators can save time and money in the training of pilots. They are also used in flight testing to investigate the stability, control characteristics, and behavior of aircraft and spacecraft, allowing detailed and realistic simulations of all aspects of flight with no risk to either vehicle or pilot. KEY CONCEPTS simulation: the recreation of a real or imagined event in an artificial environment rather than in a real environment FLIGHT SIMULATIONS Flight simulation involves the use of a ground-based device to enable a pilot, student pilot, or an aerospace engineer to experience or evaluate the behav-

Principles of Aeronautics

ior of an aircraft or spacecraft in flight. The inside of the simulator looks like the cockpit of an airplane or spacecraft. Computers are used to present simulated exterior views that would be seen through actual cockpit windows, as well as to control hydraulic systems attached to flight controls. The hydraulic systems generate movements of the simulator cockpit that mimic the response of the actual aircraft being simulated, according to the pilot’s operation of the flight controls. In the course of the first century of human aviation, flight simulators have evolved from crude devices consisting of little more than a chair and a set of imitation controls mounted on a wood platform that can be pitched and rolled by training person-

Flight Simulators

nel, to multimillion-dollar computer-controlled aircraft or spacecraft cockpits that can duplicate every conceivable motion and reaction of the real vehicle. Every child who has placed a chair in a large cardboard box and used anything from a broomstick to a baseball bat to pretend to control a make-believe airplane has experienced flight simulation at a very basic level. Flight simulators allow people to “pretend” to fly. TEACHING TOOLS An important use of flight simulators is to help teach pilots how to fly an airplane with only their instruments to tell them the position, attitude, and direction of flight of their airplane. An important aspect

F/A-18 Hornet flight simulator aboard the USS Independence aircraft carrier. Photo via Wikimedia Commons. [Public domain.]

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of such training is teaching pilots that they cannot rely on the body’s natural senses of sight and balance to fly under “instrument flight conditions”; they learn to fly using the information provided by the flight instruments alone. A simple desktop computer screen and a set of airplane-like controls can be used with any of many flight simulator computer codes to accomplish this task. Older pilots will recall training in simulators that were made to resemble little airplanes with small wings and tails and that were mounted on mechanically or hydraulically powered platforms designed to move the small cockpit like an airplane as the pilot “flew” the trainer, using an array of instruments identical to those on a real instrument panel. Pilots of craft—from fighters to general aviation craft to space shuttles—train in sophisticated flight simulators in which the pilot can see realistic in-flight images of sky, terrain, and airports and learn to fly the vehicle using both its instruments and the simulated view from the cockpit. The simulator can, with the flick of a switch or the turning of a knob, subject pilots to the conditions they would face with the loss of an engine, severe turbulence and weather, loss of part of the control system, or almost any other emergency imaginable. There is continued debate about whether training is more or less effective when the simulator moves to replicate the body forces which pilots might experience in training maneuvers. Both moving and nonmoving simulators are used in teaching pilots how to react to almost any situation that may be encountered, ranging from an ordinary flight to a severe emergency. RESEARCH AND DEVELOPMENT TOOLS These same simulators are used to study ways to improve the control systems of airplanes and spacecraft. Engineers can write “control law” routines that will alter the way the vehicle behaves in flight, simulating everything from a shift in payload weight, to

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the loss of a rudder in combat, to a complete redesign of the airplane wing or tail. Simulators are used to investigate such changes and events without risk of loss of life or vehicle in a flight test. If there is any question of control system failure or problems in an aircraft accident, simulators are used to determine the effect of that loss on the performance and handling of the plane and to compare the test results to the facts known about the accident. Using these control laws, every newly designed aircraft is “flown” for hundreds of hours in the simulator before a test airplane ever leaves the ground; it has become commonplace for the test pilot to report after the first flight that the plane flew just like it did in the simulator. A very interesting application of this can be seen in the motion picture Falling from the Sky: Flight 174 (1995), which recounts the incident that came to be known as “the Gimli Glider” (Air Canada Flight 143). In the actual event, the pilot of the aircraft, a brand-new Boeing 767, was faced with the unthinkable loss of both engines en route to the Winnipeg airport and having to pilot the aircraft as though it were a gigantic glider. The pilot, Captain Robert Pearson, had a great deal of experience as a glider pilot as a young man and he was successful in this, landing the craft safely at an abandoned airfield in Gimli, Manitoba, rather than crashing catastrophically in Winnipeg. This scenario was subsequently programmed into the 767 flight simulators, but none of the pilots who then attempted the scenario in the simulators were able to repeat the operation successfully. Some of the world’s most sophisticated flight simulators are used in the design and development of military aircraft such as fighters. Several government facilities have twin simulators in which two fighter pilots can fly simulated dog-fights against each other with the “enemy” simulator programmed to handle like real enemy aircraft. The simulators are coupled in such a way that the two pilots can see the opponent aircraft projected onto huge screens surround-

Flight Testing

Principles of Aeronautics

ing their multimillion-dollar full-motion flight simulators. This type of simulation allows the military to determine the best maneuvers for use in aerial combat and to design or redesign their aircraft and control systems to give them the edge in a fight. Dozens of very sophisticated flight simulator programs and games now on the market allow anyone with a home computer to experience flight simulation. Many of these programs provide excellent simulations of actual airplane motion and control effectiveness, rivaling that of real flight training simulators. Some of the best such programs have been developed using the control laws of real aircraft, both modern or historic, and can give users an outstanding feel for the thrill of flight in their airplane of choice. —James F. Marchman III Further Reading Allerton, David. Principles of Flight Simulation. Wiley, 2009. ———. Flight Simulation Software: Design, Development and Testing. Wiley, 2023. Hamel, Peter G. In-Flight Simulators and Fly-by-Wire/Light Demonstrators: A Historical Account of International Aeronautical Research. Springer International Publishing, 2017. Jentsch, Florian, and Michael Curtis. Simulation in Aviation Training. CRC Press, 2017. Lee, Alfred T. Flight Simulation: Virtual Environments in Aviation. CRC Press, 2017. Yurkov, Nikolay Kondratyevich, Nin Ivanova Romancheva, Dmitry Alexandrovich Zatuchny, and Evgeny Yuryevich Goncharoc. Designing Aircraft Simulators. Springer Nature Singapore, 2022. See also: Airplane accident investigation; Airplane cockpit; Airplane safety issues; Autopilot; Avionics; Flight instrumentation; Flight landing procedures; Flight recorder; Flight roll and pitch; Flight schools; Flight testing; Forces of flight; Glider planes; Homebuilt and experimental aircraft; National Transportation Safety Board (NTSB); Spacecraft engineering; Spaceflight; Takeoff procedures; Taxiing procedures; Training and education of pilots

Flight Testing Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Flight testing is the process by which an experimental aircraft is proved to be safe, functional, and economically viable to produce. Testing allows airplane engineers and designers to ensure that their design visions are practical and safe before an aircraft is put into commercial production. The procedure involves both real and computer-simulated tests of all elements of the craft, both individually and systematically. KEY CONCEPTS airframe: the essential structure of an aircraft design without the additional items required for its intended use hypersonic: at speeds greater than 3.5 times the speed of sound (Mach 3.5) supersonic: at speeds greater than the speed of sound (Mach 1 to Mach 3.5) TESTING MODE Testing of aircraft is often thought of as only encompassing test flights, more often than not conjuring a picture of a brave pilot climbing into the latest supersonic or hypersonic craft and driving it to the edge of space or beyond. This image has some validity, but aircraft testing also includes less glamorous processes involving computer simulations, laboratory tests, and other types of trials intended to assess how the aircraft will perform when it finally runs a test flight. However, such flights are of the utmost importance to any aircraft research program, which is the basis for the future of aviation. Military flight tests are usually run at locations such as Edwards Air Force Base, California; the

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China Lake Naval Weapons Test Center, California; or the Patuxent River Naval Air Warfare Center, Maryland. Commercial tests are performed at sites such as Yuma, Arizona, and Moses Lake, Washington. These test flights are the extension of laboratory tests and computer simulations. Advances in technology must be tested in order to be put to use, and yesterday’s cutting edge soon becomes run-of-the mill. For instance, after World War II, Douglas Aircraft tested its bright red experimental Skystreak model D-558, which reached speeds of Mach .80 to Mach .90. In 2001, approximately 1 billion airline passengers sat comfortably on airliners traveling at roughly the same speed. TESTING OVERSIGHT There is much more to flight testing than trying out new concepts. Commercial and military aircraft are delivered daily to their ultimate users by Boeing, Lockheed, Airbus, and other manufacturers. When a B-777, an A380, or an F-22 comes off the production line, it is first turned over to production test personnel, whose function is to test the components of the new aircraft to ensure that it is ready for the ultimate customer. The customer, American Airlines or the US Air Force, for example, then conducts a production acceptance flight to determine whether the aircraft meets specifications and is ready for delivery and payment. The military has its own personnel that perform these acceptance tests, and the Civil Aeronautics Authority (CAA)—the forerunner of the Federal Aviation Administration (FAA)—did likewise. During the 1950s and 1960s, FAA inspectors were on the scene, personally checking the output of the manufacturer’s engineers and technicians. Beginning in the 1970s, however, the FAA became increasingly dependent upon manufacturers to perform both standard testing and FAA inspections themselves, as “designated representatives of the FAA.” With the increasing use of computer simula-

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tion in place of hard testing, the FAA has neither the personnel nor the training to oversee the manufacturer. The Boeing B-777 is a prime example in which the manufacturer performed all the testing oversight and approval on the extensively computer-designed aircraft. There is also an ever-widening gap between the requirements of commercial and military aircraft performance envelopes. The modern subsonic commercial airline transport is designed to cruise at about Mach .85 at altitudes of 8,500 to 12,000 meters. The passengers are afforded living-room comfort, complete with pressurization holding the cabin at a comfortable maximum altitude of 2,400 meters, in-flight entertainment, meals on longer flight legs, cool drinks of their own choice, and an atmosphere of relaxation and safety. Although some modern commercial transport aircraft have found their way into military service as aerial refueling/cargo transports, medical evacuation airplanes, and VIP carriers, the requirements of the modern military have diverged into very specialized equipment. The military is interested in high speed, enhanced maneuverability, rugged survivability, deadly weapon delivery systems, and the ability to escape enemy radar detection. Although the armed forces are interested in economy of operations, cost is a secondary consideration compared to military performance. As a result, advances in engine performance have historically taken place in military development; in fact, the core of every engine model currently used on commercial aircraft had its beginning as a military engine. The testing requirements for new engines can run into billions of dollars, and if the military requirements diverge too far from commercial interests, there will be a significant demand for the establishment of a government-sponsored commercial jet development center. The FAA, which is part of the Department of Transportation, is responsible for licensing all general aviation, business aviation, and commercial air-

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Principles of Aeronautics

A LTV XC-142 experimental V/STOL aircraft. Photo via Wikimedia Commons. [Public domain.]

line aircraft, a task that consumes a significant part of its $11 billion yearly budget. Aside from the few homebuilt experimental aircraft that fly in fairly quiet traffic areas, the FAA’s mission is to certify that any new airplane will safely carry passengers and is airworthy. A new model is taken to the edge of its performance envelope by testing it under the most unusual and exacting conditions, conditions that might not occur in real life during many hundreds of thousands of flight hours. MATERIALS TESTING Flight testing, which is the ultimate appraisal of the finished vehicle, differs from the many preliminary tests, computer simulations, and other explorations

that contribute to the success of the complete airplane. Laboratory testing of the airframe starts off with material science, and engineers must make decisions considering questions of the material’s strength, cost, weight, and ease of manufacture, maintenance, and repair. The first aircraft frames, constructed prior to World War I, were composed of fine-grain spruce or similar wood covered with fabric stitched over the skeleton. Steel was used for the engine mounts, some fuselage structures, guy wires and braces, landing gear or skids, and various attachment fittings. Aluminum was still in short supply, very expensive, and had some serious fabrication drawbacks. Aircraft through World War I and up to the mid-1920s continued to utilize these mate-

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rials, and they allowed new aircraft models to be constructed very rapidly. Fabric skins slowly became replaced by plywood, and aluminum crept into use in castings and other high-strength-to-weight parts. The wide use of aluminum in the 1930s allowed the rapid expansion of the airline industry with the Douglas DC family of aircraft. World War II combat requirements saw the development of new high-strength aluminum, then called 75S, and this material was immediately applied to the commercial airline business after the war in aircraft such as the Douglas DC-6 and the Lockheed Constellation. By 1950, titanium was becoming available, but the cost of $25 per pound was about the same cost as sterling silver. The continued search for new and better materials opened the door to nonmetallic materials such as fiberglass, carbon fiber/epoxy, and other synthetics such as Nylon, Dacron, Kevlar, Mylar, Spectra, and Technora in advanced composite materials. Aircraft material science both creates designs that demand the invention of new fabrics and utilizes new technology to create new designs. Once it is possible and desirable to start the fabrication of parts, these components are tested either through computer simulation or on actual test fixtures. Testing is done to determine ultimate strength, fatigue resistance, crack propagation, notch sensitivity, and wear resistance. Once the individual parts are assembled into minor or major subassemblies they are again run through tests or simulations in a structural test laboratory to prove their suitability for incorporation into the finished airframe. Computer-controlled hydraulic jacks and fixtures, or sometimes just sand bags such as the Wrights used, torture the structures until they turn to scrap material. In other laboratories, the major systems of the finished aircraft are also “wrung out.” As an example, the DC-8, Douglas Aircraft’s first jetliner, developed in the late 1950s, had the complete pneumatic, pressurization, and air-conditioning systems laid out in a

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ground test laboratory. Giant compressors and heaters simulated the “bleed air” from the aircraft’s four jet engines, which was ducted through the simulated wings and fuselage to test the anti- and deicing system, and to run the cabin pressurization superchargers and the refrigeration compressors. The pressurized and air-conditioned air was then fed to a huge steel tank simulating the cabin volume. This pneumatic test program proved that a water separator was needed on the DC-8 air-conditioning system. On an early production DC-8 that lacked such a separator, a jet of cold water came out of the air-conditioning system, hitting Donald W. Douglas Sr., the chairman, on the head. This same early jetliner had an “iron horse” fixture to test the control system layout for the flaps, ailerons, rudders, and elevators; an engine test stand to develop noise suppressors and thrust reverses; a fuselage panel test rig to test explosive decompression resistance of the DC-8’s rip-stopper fuselage skin; a landing gear drop fixture to test impact resistance; a hydraulic laboratory to test the Skydrol-based system; a heavy, concrete bomb shelter and noise generator to design cabin noise insulation; a power generation laboratory to check out the aircraft’s alternators; a complete layout of the radio rack and flight deck instrumentation; a microwave test facility to test the aircraft antennas; a full-scale mockup of the interior so that interior designers could simulate the look and feel of the passenger cabin (this facility was also used to examine the toilet and galley layout); and finally, a toilet test rig. Many of these test programs are now be simulated with computers in three-dimensional models, which prevent the interesting arrangements found on the DC-8, where hydraulic, pneumatic, and electrical lines were planned to occupy the same space at the same time. The use of fiber optics and fly-by-wire (completely computerized flight control) will not only save weight, but also free space that in the past was taken up with heavy cables and giant wire harnesses.

Principles of Aeronautics

FRAME TESTING Once the major systems and components are tested, there is the final “proving-up” of the aircraft. Sometimes a sacrificial aircraft structure is constructed, a complete structural aircraft without electronics, hydraulics, pneumatics, and other such items. This aircraft is placed in a giant fixture surrounded by a steel framework and computer-controlled hydraulic jacks that bend and twist the airframe to simulate flight loads up to what is called limit load. Limit load is the load that the designers expect the airframe never to exceed in even the most violent maneuvers. Once the airframe has passed the limit load test, it is repaired, if necessary, and then undergoes a fatigue life test program. The airframe is cycled. One airliner cycle includes pushback from the gate at maximum takeoff weight; taxi along a bumpy taxiway to takeoff position; the takeoff run itself; the pressure cycle, in which the fuselage is pressurized to 8 pounds per square inch; flight loads due to turbulence and normal flying; the depressurization of the fuselage on descent; the landing load, sometimes known as an organized “crash” on the runway; and the taxi back to the gate. Military aircraft have similar cycles, especially in the Navy where the plane is catapulted from the deck and arrested by a wire upon landing. Experience tells the designers how many cycles per day, week, or year the aircraft can expect to undergo. Fatigue testing simulates these cycles by taking the airframe through the equivalent of twenty-five to thirty years on a commercial jetliner. Military aircraft life is usually shorter since the planes do not fly every day except in combat. However, both military and civilian jets are being used over longer periods, far beyond their original design specifications. As a result, service life extension programs (SLEPs) have been initiated on numerous aircraft models. These aircraft are twisted, bent, and tortured in the test rigs until something breaks. The broken parts are either replaced or repaired and the torture continues,

Flight Testing

until airframes designed for twenty thousand to thirty thousand hours are SLEPed to sixty thousand to eighty thousand hours. Finally, when the engineers have tortured the airframe to its most extended life, it is taken to destruction by loading up the hydraulic jacks until catastrophic failure of a major component, such as the wing spars, occurs. The designers now know how much safety they have in their calculations—110 percent, 120 percent, and so on. The percent over 100 is how much load it took over maximum design load to create a catastrophic failure, and this is the margin of safety. FLIGHT TESTING The finished aircraft is now on the flight ramp ready for its first flight. The major components have been tested, the systems have been tested, and the airframe has been tested, but there are a few things yet to be done. During the construction of the first example of a new craft, extensive instrumentation is routed throughout the aircraft to measure parameters from every conceivable point of view. Stress and strain on the structure, pressures, temperatures, voltages, and frequencies are to be measured and recorded both on the craft and remotely. Safety equipment and systems have been installed, for as the old saying goes, “I told the Wrights, and I’m telling you, it’ll never get off the ground.” Preliminary tests are run before the plane takes off for the first time: low-speed taxi tests, braking tests, and high-speed taxi tests, in which the nose wheel is lifted off the ground. Finally, the first flight is undertaken, often with much media fanfare. Once the first flight is accomplished, testers slowly expand the envelope by flying ever higher and faster, thereby testing all the systems of the aircraft. Back on the ground, or rather very near it, rejected takeoff tests are performed, in which testers accelerate the aircraft to flight speed, chop the power, and slam on the brakes to see if it can stop in the required field length. Burning tires usually accompany

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this test, since on the rejected takeoff test the engine thrust reversers cannot be used. Another exciting ground test on commercial aircraft is the evacuation test. The number of passengers that will be allowed on a commercial aircraft is determined by the FAA requirement that all passengers must be evacuated within ninety seconds with 50 percent of the doors or escape windows blocked. Furthermore, the test passengers, usually manufacturer’s employees and relatives, must be a cross-section of the traveling public, so there must be children as well as senior citizens in the mix. They are seated in the aircraft’s seats with seat belts fastened and an alarm is sounded. They have not been told which exits will not work, and all must go down the typical airline exit slide. The only unrealistic aspect of this test is that all test passengers know the test is coming, all know their jobs are dependent upon good test results, and they have not been frightened out of their wits by an actual emergency on the aircraft and know that the threat of real fire is not present. The final elements to prepare for commercial flight include training the airline crews, both in the cockpit and in the cabin, readying the ground handling equipment and ground crew, positioning spares, training the maintenance workers, and actually flying the routes the aircraft will fly when full of passengers. —James S. Douglas Further Reading Corda, Stephen. Introduction to Aerospace Engineering with a Flight Test Perspective. Wiley, 2017. Gao, Yakui, Gang An, and Chaoyou Zhi. Test Techniques for Flight Control Systems of Large Transport Aircraft. Elsevier Science, 2021. Gregory, James W., and Tianshu Liu. Introduction to Flight Testing. Wiley, 2021. Larsson, Roger. Flight Test System Identification. Linköping U Electronic P, 2019. Perkins, Courtland D. Stability and Control: Flight Testing. Elsevier Science, 2014.

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Schrader, Steffen Haakon. Flight Testing: Analysis of the Spin Dynamics of a Single-Engine Low-Wing Aeroplane. Springer Berlin Heidelberg, 2022. See also: Aeronautical engineering; Air transportation industry; Airplane accident investigation; Airplane manufacturers; Airplane safety issues; Federal Aviation Administration (FAA); Flight landing procedures; Flight propulsion; Flight roll and pitch; Flight simulators; Takeoff procedures; Taxiing procedures

Fluid Dynamics Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Fluid dynamics is an interdisciplinary field concerned with the behavior of gases, air, water, and other liquids in motion. An understanding of fluid dynamic principles is essential to the work done in aerodynamics. It informs the design of air and spacecraft. An understanding of fluid dynamic principles is also essential to the field of hydromechanics and the design of oceangoing vessels. Any system with air, gases, or water in motion incorporates the principles of fluid dynamics. KEY CONCEPTS fluid: the type of matter that does not have a three-dimensional shape of its own but conforms to the shape of the surrounding matter that is containing it; gases and liquids incompressible: a material that cannot be reduced in volume by the application of pressure and maintains its characteristic density or mass-to-volume ratio streamline: the continuous flow of matter such as air or water in its direction of motion relative to a surface

Principles of Aeronautics

DEFINITION AND BASIC PRINCIPLES Fluid dynamics is the study of fluids in motion. Air, gases, and water are all considered to be fluids. When the fluid is air, this branch of science is called aerodynamics. When the fluid is water, it is called hydrodynamics. The basic principles of fluid dynamics state that fluids are a state of matter in which a substance cannot maintain an independent shape. A fluid will take the shape of its container, forming an observable surface at the highest level of the fluid when it does not completely fill the container. Fluids flow in a continuum, with no breaks or gaps in the flow. They are said to flow in a streamline, with a series of particles following one another in an orderly fashion in parallel with other streamlines. Real fluids have some amount of internal friction, known as viscosity. Viscosity is the temperature-dependent phenomenon that causes some fluids to flow more readily than others. It is the reason that molasses flows more slowly than water at room temperature and barely at all at cold temperatures. Fluids are said to be compressible or incompressible. Liquids are generally incompressible fluids because their density does not change when pressure is applied. Incompressible fluids are subject to the law of continuity, which states that fluid flows in a pipe are constant. This theory explains why the rate of flow increases when the area of the pipe is reduced and vice versa. The viscosity of a fluid is an important consideration when calculating the total resistance on an object. The point where the fluid flows at the surface of an object is called the boundary layer. The fluid “sticks” to the object, not moving at all at the point of contact. The streamlines further from the surface are moving, but each is impeded by the streamline between it and the wall until the effect of the streamline closest to the wall is no longer a factor. The boundary layer is not obvious to the casual observer,

Fluid Dynamics

but it is an important consideration in any calculations of fluid dynamics. Most fluids are Newtonian fluids. Newtonian fluids have a stress-strain relationship that is linear. This means that a fluid will flow around an object in its path and “come together” on the other side without a delay in time. Non-Newtonian fluids do not have a linear stress-strain relationship. When they encounter shear stress, their recovery varies with the type of non-Newtonian fluid. A main consideration in fluid dynamics is the amount of resistance encountered by an object moving through a fluid. Resistance, also known as drag, is made up of several components, all of which have one thing in common: they occur at the point where the object meets the fluid. The area can be quite large, as in the wetted surface of a ship, the portion of a ship that is below the waterline. For an airplane, the equivalent is the body of the plane as it moves through the air. The goal for those who work in the field of fluid dynamics is to understand the effects of fluid flows and minimize their effect on the object in question. BACKGROUND AND HISTORY Swiss mathematician Daniel Bernoulli introduced the term “hydrodynamics” with the publication of his book Hydrodynamica in 1738. The name referred to water in motion and gave the field of fluid dynamics its first name, but it was not the first time water in action had been noted and studied. Leonardo da Vinci made observations of water flows in a river and was the one who realized that water is an incompressible flow and that for an incompressible flow, V = constant. This law of continuity states that fluid flow in a pipe is constant. In the late 1600s, French physicist Edme Mariotte and Dutch mathematician Christiaan Huygens contributed the velocity-squared law to the science of fluid dynamics. They did not work together, but they both reached the conclusion that resistance is pro-

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portional not to velocity itself but to the square of the velocity. Sir Isaac Newton put forth his three laws of motion in the 1700s. These laws play a fundamental part in many branches of science, including fluid dynamics. In addition to the term hydrodynamics, Bernoulli’s contribution to fluid dynamics was the realization that pressure decreases as velocity increases. This understanding is essential to the understanding of lift; increasing the airflow on top of an airfoil decreases the pressure there relative to the underside of the airfoil, hence lift is generated. Leonhard Euler, the father of fluid dynamics, is considered by many to be the preeminent mathematician of the eighteenth century. He is the one who derived what is known as the Bernoulli equation from the work of Daniel Bernoulli. Euler also developed equations for inviscid flows. These equations were based on his own work and are still used for compressible and incompressible fluids. The Navier-Stokes equations result from the work of French engineer Claude-Louis Navier and British physicist George Gabriel Stokes in the mid-nineteenth century. They did not work together, but their equations apply to incompressible flows. The Navier-Stokes equations are still used. At the end of the nineteenth century, Scottish engineer William John Macquorn Rankine changed the understanding of the way fluids flow with his streamline theory, which states that water flows in a steady current of parallel flows unless disrupted. This theory caused a fundamental shift in the field of ship design because it changed the popular understanding of resistance in oceangoing vessels. Laminar flow is measured by use of the Reynolds number, developed by British engineer and physicist Osborne Reynolds in 1883. When the number is low, viscous forces dominate. When the number is high, turbulent flows are dominant. American naval architect David Watson Taylor designed and operated the first experimental model

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basin in the United States at the start of the twentieth century. His seminal work, The Speed and Power of Ships (1910), is still read in the twenty-first century. Taylor played a role in the use of bulbous bows on US Navy vessels. He also championed the use of airplanes that would be launched from naval craft underway in the ocean. The principles of fluid dynamics took to the air in the eighteenth century with the work done by aviators such as the Montgolfier brothers and their hot-air balloons and French physicist LouisSébastien Lenormand’s parachute. It was not until 1799, when English inventor Sir George Cayley designed the first airplane with an understanding of the roles of lift, drag, and propulsion, that aerodynamics came under scrutiny. Cayley’s work was soon followed by the work of American engineer Octave Chanute. In 1875, he designed several biplane gliders, and with the publication of his book Progress in Flying Machines (1894), he became internationally recognized as an aeronautics expert. The Wright brothers are rightfully called the first aeronautical engineers because of the testing they did in their wind tunnel. By using balances to test a variety of airfoil shapes, they were able to correctly predict the lift and drag of different wing shapes. This work enabled them to fly successfully at Kitty Hawk, North Carolina, on December 17, 1903. German physicist Ludwig Prandtl identified the boundary layer in 1904. His work led him to be known as the father of modern aerodynamics. Russian scientist Konstantin Tsiolkovsky and American physicist Robert Goddard followed, and Goddard’s first successful liquid propellant rocket launch in 1926 earned him the title of the father of modern rocketry. All of the principles that applied to hydrodynamics—the study of water in motion—applied to aerodynamics: the study of air in motion. Together these principles constitute the field of fluid dynamics.

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HOW IT WORKS When an object moves through a fluid such as gas or water, it encounters resistance. How much resistance depends upon the amount of internal friction in the fluid (the viscosity) as well as the shape of the object. A torpedo, with its streamlined shape, will encounter less resistance than a two-by-four that is neither sanded nor varnished. A ship with a square bow will encounter more resistance than one with a bulbous bow and V shape. All of this is important because with greater resistance comes the need for greater power to cover a given distance. Since power requires a fuel source and a way to carry that fuel, a vessel that can travel with a lighter fuel load will be more efficient. Whether the design under consideration is for a tractor trailer, an automobile, an ocean liner, an airplane, a rocket, or a space shuttle, these basic considerations are of paramount importance in their design. APPLICATIONS AND PRODUCTS Fluid dynamics plays a part in the design of everything from automobiles to rockets. Fluid dynamic principles are also used in medical research by bioengineers who want to know how a pacemaker will perform or what effect an implant or shunt will have on blood flow. Fire flows are also being studied to aid in the science of wildfire management. Previously the models focused on heat transfer, but in the twenty-first century studies are looking at fire systems and their fluid dynamic properties. Sophisticated models are used to predict fluid flows before model testing is done. This lowers the cost of new designs and allows the people involved to gain a thorough understanding of the trade-off between size and power, given a certain design and level of resistance. RELEVANCE IN CAREERS Fluid dynamics plays a part in a host of careers. Naval architects use fluid dynamic principles to design

Fluid Dynamics

vessels. Aeronautical engineers use the principles to design aircraft. Astronautical engineers use fluid dynamic principles to design spacecraft. Weapons are constructed with an understanding of fluids in motion. Automotive engineers must understand fluid dynamics to design fuel-efficient cars. Architects must take the motion of air into their design of skyscrapers and other large buildings. Bioengineers use fluid dynamic principles to their advantage in the design of components that will interact with blood flow in the human body. Land-management professionals can use their understanding of fluid flows to develop plans for protecting the areas under their care from catastrophic loss due to fires. Civil engineers take the principles of fluid dynamics into consideration when designing bridges. Fluid dynamics also plays a role in sports, from pitchers who want to improve their curveballs to quarterbacks who are determined to increase the accuracy of their passes. Students should take substantial course work in more than one of the primary fields of study related to fluid dynamics (physics, mathematics, computer science, and engineering), because the fields that depend upon knowledge of fluid dynamic principles draw from multiple disciplines. In addition, anyone desiring to work in fluid dynamics should possess skills that go beyond the academic, including an aptitude for mechanical details and the ability to envision a problem in more than one dimension. A collaborative mind-set is also an asset, as fluid dynamic applications tend to be created by teams. SOCIAL CONTEXT AND FUTURE The science of fluid dynamics touches upon a number of career fields that range from sports to bioengineering. Anything that moves through liquids such as air, water, or gases is subject to the principles of fluid dynamics. The more thorough the understanding, the more efficient vessel and other designs will be. This will result in the use of fewer resources in the form of power for inefficient designs and help

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create more efficient aircraft and launch vehicles as well as medical breakthroughs. —Gina Hagler Further Reading Anderson, John D., Jr. A History of Aerodynamics and Its Impact on Flying Machines. 1997. Cambridge UP, 2001. Çengel, Yunus A., and John M. Cimbala. Fluid Mechanics: Fundamentals and Applications. 4th ed., McGraw-Hill, 2017. Darrigol, Olivier. Worlds of Flow: A History of Hydrodynamics from the Bernoullis to Prandtl. Oxford UP, 2005. Eckert, Michael. The Dawn of Fluid Dynamics: A Discipline Between Science and Technology. Wiley-VCH, 2006. Ferreiro, Larrie D. Ships and Science: The Birth of Naval Architecture in the Scientific Revolution, 1600-1800. MIT Press, 2007. Johnson, Richard W., editor. The Handbook of Fluid Dynamics. 2nd ed., CRC Press, 2016. Magoules, Frederic, editor. Computational Fluid Dynamics. Chapman and Hall/CRC, 2018. Moon, Young J. Introduction to Fluid Dynamics: Understanding Fundamental Physics. Wiley, 2022. Rientord, Michel. Fluid Dynamics: An Introduction. Springer International Publishing, 2014. Vallis, Geoffrey K. Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-Scale Circulation. Cambridge UP, 2017. Visconti, Guido, and Paolo Ruggieri. Fluid Dynamics: Fundamentals and Applications. Springer, 2020. Wendt, John F. Computational Fluid Dynamics: An Introduction. Springer, 2009. See also: Aerodynamics and flight; Aeronautical engineering; Airfoils; Airplane propellers; Daniel Bernoulli; Biplanes; Blimps; Boomerangs; Conservation of energy; Leonardo da Vinci; Differential equations; Dirigibles; Flight balloons; Forces of flight; Glider planes; Robert H. Goddard; Paper airplanes; Konstantin Tsiolkovsky; Viscosity; Wake turbulence; Wind shear

Flying Wing Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics

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ABSTRACT Also known as an all-wing or nurflügel, the flying wing is the American term given to airplanes that are predominantly the lifting component, the wing. The all-wing design was among the earliest aerodynamic ideas for reducing drag and costs through high efficiency. Its simplicity has challenged generations of designers, teaching them many aerodynamic fine points. KEY CONCEPTS drag: the resistance to motion through a fluid due to friction between the moving object and the fluid medium fuselage: the longitudinal central body of an airplane glider: an aircraft that has no engine or other power source to provide forward motion, relying instead on the pilot’s skill in maneuvering the aircraft in existing air currents to remain aloft EARLY DEVELOPMENT Before the invention of the airplane, English physicist Sir George Cayley, who in 1853 built the first manned glider, suggested that flying machines would be most efficient if they were only a wing. After the airplane became a reality in the early twentieth century, most successful airplane designs were linear. Their noses sported vertical or horizontal stabilizers, or perhaps an engine, and worked backward toward the cockpit, wings, rudimentary fuselage, and vertical or horizontal stabilizers, or both. Airplanes are engineered to suit mathematical logic and economic reality. Pilots seek aerodynamic poise, while passengers seek comfort and amenities. Operators measure an aircraft’s reliability, and accountants measure its economy. Most people also judge airplanes for their inspiring beauty. One design, the “flying wing,” exhibited grace, economy, and performance. Inspired by Cayley’s belief in eliminating the drag and weight of fuselages and tails, the flying wing has long been aviation’s Holy Grail. Because all-wing airplanes need fewer parts

Principles of Aeronautics

and construction steps than do conventional designs, they are more energy-efficient both to build and to operate. Still, flying wings are rare. During and after World War I, the development of aircraft engines quickly overpowered the aerodynamic drag produced by early airplanes. This development spelled doom for the flying wing design. Because airplane designs were still new, people had little preconception of how airplanes should appear. By the 1940s, airplanes had proved viable, and society’s view of airplanes included a fuselage and tail. The economic boom and low energy costs had made conventional design inefficiencies tolerable. The earliest flying wings, sporting vertical stabilizers, were not purely wing-only designs. In 1907, British airplane designer John William Dunne showed that conventional tails were unnecessary. His balanced aerodynamics have infused tailless and all-wing airplanes ever since. By 1930, Germany’s Walter and Reimar Horten first flew their model all-wing airplane, called a nurflügel in Germany. In 1933, the Hortens flew a manned all-wing glider model called the HO-1. Knowledge gained from the HO-1 inspired the HO-5, a twin-engine machine potentially leading to an all-wing fighter. German general Ernst Udet, long appreciating the Hortens’ nurflügel ideas, succumbed to political blame for other project failures and in November, 1941, took his own life. Germany’s all-wing idea had lost a patron, and the proposed all-wing fighter languished until 1945, when advancing American soldiers discovered one nearly completed twin-jet HO-9 fighter. Other Horten all-wings flew as developmental projects. However, the most ambitious project, the HO-18 long-range heavy bomber, remained only an idea. NORTHROP’S ALL-WING DESIGNS In the United States, John Knudsen Northrop dreamed of flying wings. His single-seat 1933 de-

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sign, the Model 1, had thin, tubular twin booms supporting a conventional horizontal stabilizer with twin rudders. Northrop began testing a true nurflügel in July, 1940, when the N-1M first flew. It was a flying laboratory, designed to change configurations between flights. Changes in wingtip droop, sweepback (the taper of the wing’s leading edge), and wing dihedral (angle of the wingtips) could provide a bank of information. The Americans considered building a medium bomber based on the N-1M. Encouraged, Northrop continued testing predominantly all-wing airplanes, including four N-9M engineering test airplanes. Although one N-9M crashed, the remaining crafts saw duty as trainers, giving pilots firsthand experience with the flight characteristics of all-wing airplanes. Following World War II, Northrop built the prototype XB-35 all-wing bomber, a six-engine, propeller-driven pusher design that was a true wing-only machine. Jet engine technology was making propellers obsolete for combat airplanes, so Northrop soon rebuilt the XB-35 into the YB-49, an eight-engine all-jet bomber. Although the YB-49 project did suffer airplane loss, the design was sound, and officials who were influential in procurement favored the US Air Force’s adoption of the YB-49. However, the project was mired in political intrigue, dooming America’s flying wing. So sour were the feelings of those involved that Air Force secretary Stuart Symington ordered not only the flying wing’s cancellation in 1950 but also the destruction of all XB airframes. None remained even for museum display. THE STEALTH BOMBER Four decades later, on November 22, 1988, the Air Force unveiled the B-2 stealth bomber, designed by the Northrop Corporation, to the American public. Several years earlier, Northrop company officials had secretly revealed the airplane to the eighty-five-year-old Northrop, who died the follow-

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ing year, knowing that his dream would finally fly. In 1996, the McDonnell Douglas Corporation revealed its idea of a BWB-1 (blended-wing body) airliner to seat 800 passengers. Efficient design would permit the three-engine jet to operate at about two-thirds the cost of a conventional airplane of the same capacity. In 2001, Northrop Grumman used company funds to construct an all-wing uncrewed combat air vehicle (UCAV) for aircraft carrier use. With leading edges swept back 55 degrees and trailing edges sweeping forward 30 degrees, the inherently stealthy design was a logical step in airplane development toward autonomous, or “smart” aircraft. To meet the military’s need for extended endurance, the Northrop Grumman Corporation began to look at extensions on each wingtip, transforming the UCAV’s aggressive arrowhead shape into the more elegant, traditional all-wing design. —David R. Wilkerson Further Reading Anderson, Fred. Northrop: An Aeronautical History. Wipf & Stock Publishers, 2016. Cole, Lance. Secret Wings of WW II: Nazi Technology and the Allied Arms Race. Pen & Sword Aviation, 2015. Davies, Peter E. Northrop Flying Wings. Bloomsbury Publishing, 2019. Myhra, David. The Horten Brothers and Their All-Wing Aircraft. Schiffer, 1998 Simons, Graham M. Northrop Flying Wings. Pen & Sword Books, 2013. United States Air Force (USAF) and USAF Staff. Northrop YB-49 Flying Wing Pilot’s Manual. Periscope Film LLC, 2010. Wall, R., and D. A. Fulghum. “New Demonstrator Spurs Navy UCAV Development.” Aviation Week and Space Technology, vol. 154, 19 Feb. 2001. See also: Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Flight testing; Fluid dynamics; Military aircraft; Stealth bomber; Types and structure of airplanes; Unidentified aerial phenomena (UAP); Wing designs

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Forces of Flight Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics ABSTRACT There are four so-called forces—gravity, drag, lift, and thrust—that act upon an airplane in straight-and-level unaccelerated flight. Weight and drag are forces of nature inherent of any object lifted from the ground and moved through the air. The forces of lift and thrust are artificially caused to overcome the forces of weight and drag and enable an airplane to fly. KEY CONCEPTS mass: the amount of matter that an object comprises as an absolute intrinsic quantity Reynolds number: defined as the product of the density of the fluid, the flow speed and an object’s length, divided by the dynamic viscosity of the fluid; the Reynolds number represents the ratio of inertial forces to viscous forces for an object moving through a fluid unaccelerated: having a constant velocity in one direction only weight: the perception of the amount of matter an object comprises within a gravitational field, as the product of the mass and the acceleration due to gravity (or gravitational force) TRYING TO FLY Humans’ first attempts to fly, inspired by birds, were limited until humans realized they could not actually fly like birds. Birds, with their very light weight, comparatively great strength, and complex biological design, can use their wings to create both lift and thrust to overcome the natural forces of weight and drag, and to maintain control. Humans, in contrast, had to invent a different approach to have any success in aviation. The functions of lift and thrust had

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to be separated. For that, wings and engines were introduced. While wings produce lift, engines work to produce thrust by propellers or jet exhaust. Following the first flights made by Orville and Wilbur Wright in December, 1903, the pace of aeronautical development accelerated, and the progress made in overcoming the natural forces in the aviation industry in following decades was dramatic. The understanding of natural forces is thus as important for an airplane’s aerodynamics as the creation of artificial forces to counterbalance these natural forces. The engine and propeller combination is designed to produce thrust to overcome drag. The wing is designed to produce lift to overcome weight, or gravity. In unaccelerated, straight-and-level flight, which is coordinated flight at a constant altitude and heading, lift equals weight and thrust equals drag. Nevertheless, lift and weight will not equal thrust and drag. In everyday vocabulary, the upward forces balance the downward forces, and forward forces balance the rearward forces. This statement is true whether or not the contributions due to weight,

drag, lift, and thrust are calculated separately. Any inequality between lift and weight will result in the airplane entering a climb or descent. Any inequality between thrust and drag while maintaining straight-and-level flight will result in acceleration or retardation until the two forces become balanced. However, there are a couple of paradoxes surrounding this information. The first paradox is that in a low-speed, high-power climb, the amount of lift is less than the amount of weight. In this situation, thrust is supporting part of the weight. The second paradox is that in a low-power, high-speed descent, the amount of lift is again less than the amount of weight. In this situation, the drag is supporting part of the weight. In light aircraft, the amount of lift ordinarily is approximately ten times the amount of drag. The motion of an aircraft through the air depends on the size of these four forces. The weight of an airplane is determined by the size and material used in the airplane’s construction and on the payload and fuel that the airplane carries. The lift and

Main forces acting on a heavier-than-air aircraft. Image by NASA, via Wikimedia. [Public domain.]

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drag are aerodynamical forces that depend on the shape and the size of the aircraft, air conditions, and the flight speed and direction relative to the air velocity. The thrust is determined by the size and type of the propulsion system used in the airplane and on the throttle setting selected during the flight. The relative wind velocity acting on the airplane contributes a certain amount of force, called total aerodynamic force. This force can be resolved into two components perpendicular to each other along the directions of lift and drag. Lift is the component of aerodynamic force directly perpendicular to the relative wind velocity. Drag is the component of aerodynamic force acting parallel to the relative motion of the wind. Weight is the force directed always downward toward Earth’s center of gravity. It is equal to the mass of the airplane multiplied by the acceleration due to the gravity, or the strength of the gravitational field. Thrust is the force produced by the engine and is usually more or less parallel to the long axis of the airplane. WEIGHT Weight, or gravity, is the force that always acts downward, toward Earth’s center of gravity. It is the total sum of the masses of all its components and contents multiplied by the strength of the gravitational field, commonly referred to as the number of g’s. The weight may be considered to act as a single force, representing all its components and contents, through a single point called the center of gravity. Weight is the most reliable force, which always acts in the same direction and gradually decreases as airplane fuel is used. The center of gravity shifts as the weight is redistributed. Although the terms “mass” and “weight” are often confused with each other, it is important to distinguish between them. Mass is a property of a body itself and measures a body’s quantity of matter. Weight, in contrast, is a force representing the force of gravity acting on a body. It is also loosely called gravity. In practicality,

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weight and mass are the same thing within the same gravitational field, but they differ accordingly in different gravitational settings. In space, for example, an object in orbit has the same mass as on Earth, but it may have no discernible weight. To illustrate the difference, one could describe an object that is taken to the Moon, where the force of gravity is weaker, about one-sixth that on Earth. On the Moon, the object will weigh only about one-sixth as much as it did on Earth. The mass of the object, however, will be the same on the Moon or anywhere else. In other words, it will continue to have the same amount of matter. DRAG When an object moves relative to a fluid, either a gas or a liquid, the fluid exerts a frictional force on the object. This force which is referred to as a drag force, is due to the viscosity, or stickiness, of the fluid and also, at high speeds, to the turbulence behind and around the object. To characterize the motion of an object at different speeds relative to the fluid and to understand the associated drag, it is useful to understand a designation called the Reynolds number. The Reynolds number depends on the properties, such as length and velocity, of the fluid and the object relative to the fluid. In the case of an airplane, which flies through air, the Reynolds number for air is smaller than that for water because of the lower density of the air. For example, an object of 1-millimeter long moving with a speed of 1 millimeter per second through water has the same Reynolds number as an object 2-millimeters long moving at a rate of 7 millimeters per second in the air. The drag manifests itself differently for different Reynolds numbers associated to it. When the Reynolds number is less than 1, as in the case of fairly small objects, such as raindrops, the viscous force is directly proportional to the speed of the object. For large Reynolds numbers, usually above a value between about 1 and 10, there will be

Principles of Aeronautics

turbulence behind the body, known as wake, and hence, the drag force will be larger and it increases as the square of the velocity instead of its linear dependence on the velocity. When the Reynolds number approaches a value of around 1,000,000, the drag force increases abruptly. For above this value, turbulence exists in the layer of fluid lying next to the body all along its sides. For streamlined objects, however, there will be less turbulence and, hence, less drag. The flow is said to be streamlined of laminar flow if the flow is smooth, such that neighboring layers of the fluid slide by each other smoothly. There are several types of drag, subdivided and classified according to their action on an airplane. Pressure drag is the force pushing a horizontally moving object against the front vertical surface of the object. Friction drag is produced on a horizontally moving object by applying a force along the surface of the object. Friction drag is proportional to the viscosity of the fluid. Fortunately, air has rather low viscosity, so in most situations the amount of friction drag is small compared to that of pressure drag. In contrast, pressure drag does not depend very strongly on viscosity. Instead, it depends on the density of the air. Both friction drag and pressure drag create a force proportional to the area involved and the square of the airspeed. Part of the pressure drag that a wing produces depends on the amount of lift it is producing. This part of the drag is called induced drag. The rest of the drag is called parasite drag. The part of the parasite drag that is not due to friction is called form drag, because it is extremely sensitive to the detailed form and shape of the airplane. A streamlined object can have ten times less form drag than a nonstreamlined object of comparable frontal area. The peak pressure in front of the two shapes will be the same. However, the streamlined shape causes the air to accelerate, so the region of highest pressure is smaller, and more importantly, the streamlined shape cultivates high pressure behind

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the object that pushes it forward, thus canceling most of the pressure drag. This situation is called pressure recovery. An object moving through the air has a high-pressure region in front, but a properly streamlined object will have a high-pressure region in back as well. However, streamlining is never perfect; there is always at least some net pressure drag. Induced drag also contributes to pressure drag whenever lift is being produced, even for perfectly streamlined objects in the absence of separation. The flow pattern near a nonstreamlined object is not symmetric fore and aft because the streamlines separate from the object as they go around the sharp corners of the plate. Except in the cases of very small objects or very low speeds, pressure drag is larger than friction drag, even for well-streamlined objects. The pressure drag of a nonstreamlined object is much larger still. For this reason, even the smallest parts of high-performance aircraft, such as fuel-cap handles, are precisely aligned with the airflow. An inevitable exception involves the air that has to flow through the engine compartment to cool the engine. A lot of the air has to flow through narrow channels. The resulting friction drag, called cooling drag, amounts to 30 percent of the total drag in some airplanes. Unlike pressure drag, friction drag cannot possibly be canceled. It can, however, be minimized. The way to minimize friction drag is to minimize the total area, called wetted area, that has high-speed air flowing along it. The way to reduce form drag is to minimize separation by streamlining all parts. It is often convenient to express the drag force as a dimensionless quantity by the coefficient of drag. In that case, the drag force is proportional to the coefficient of drag, the density of the air, the square of the true airspeed, and the relevant area, which is typically taken to be the wing area excluding the surface area of the fuselage. In the mushing regime, most of the drag is induced drag. As the airplane goes more slowly, in-

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duced drag increases dramatically, and parasite drag becomes almost negligible. At high airspeeds, parasite drag is dominant, and induced drag becomes almost negligible. In a high-speed regime that includes normal cruise, the power required increases rapidly with increasing airspeed. Parasite drag is the dominant contribution to the coefficient of drag, and it is more or less independent of airspeed. Induced drag decreases as the airspeed increases, but this is a relatively minor contribution in this regime. Ways of reducing induced drag include wing tapering, wingtip modification, and employing washout and a high aspect ratio. The aspect ratio is defined as the ratio between the span and the mean chord. The mean chord, in turn, is the ratio between the wing area and the wingspan. LIFT Airplane wings and other airfoils are designed to deflect the air so that, although streamline flow is largely maintained, the streamlines are crowded together above the wing. Just as the flow lines are crowded together in a pipe constriction where the velocity is high, so the crowded streamlines above the wing indicate that the airspeed is greater than below the wing. Hence, according to Bernoulli’s principle which states that velocity increases as pressure decreases, the air pressure above the wing is less than that below the wing, and there is a net upward force, which is called dynamic lift, or lift. In fact, Bernoulli’s principle is only one aspect of the lift on a wing. Wings are usually tilted slightly upward so that air striking the bottom surface is deflected downward. The change in momentum, a product of mass and velocity, of the rebounding air molecules results in an additional upward force on the wing. As the air passes over the wing, it is bent down. The bending of the air is the action; the reaction is the lift on the wing. To generate sufficient lift, a wing must divert air down. Anyone who has stuck a hand out the window of a moving vehicle

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and twisted that hand up or down has felt this effect. To increase the lift, either or both the diverted air and downward velocity must be incremented. The downward velocity behind the wing is called downwash. The vertical downward airspeed varies as the angle of attack. The angle of attack is the angle of the chord line. The direction of the relative airflow on the wing, along the chord line, or chord length, is the distance from the loading edge of the wing to the trailing edge. As the wing moves along while the air is diverted at the rear end of the wing, it is pulled up at the leading edge, also giving rise to upwash. This upwash contributes negatively to the lift. Turbulence also plays an important role in contributing to the lift. Like drag, lift can also be expressed in a dimensionless quantity in terms of the coefficient of lift. In that way, the lift force is proportional to the coefficient of lift and the density of the air, the square of true airspeed and relevant area. The coefficient of lift is a ratio that basically measures how effectively the wing turns the available dynamic pressure into a useful average suction over the wing. The dynamic pressure is the product of the air density and the square of the velocity. This is the difference between total pressure and static pressure. The total pressure is the pressure in air that has been brought to rest from the free stream, and the static pressure is the ambient pressure at the same level as the aircraft. In actual flight, pilots are not free to make any amount of lift they want. The lift is nearly always equal to the weight multiplied by the load factor; the coefficient of lift depends directly on the load factor, and inversely, on the square of the airspeed. Because of the airspeed squared, the airplane must fly at a very high coefficient of lift in order to support its weight at low airspeeds. As there is a center of gravity, there is also a center of pressure, which is a point through which the resultant lift acts. The center of pressure changes with change of wing shape. A number, called the

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Principles of Aeronautics

lift-drag ratio, is considered best when it produces the most efficient speed for maximum range with minimum drag. THRUST A force pushing an airplane, or any object, forward is called thrust. The thrust is produced by the engines of the airplane or by the flapping of a bird’s wings. The engines push fast-moving air out behind the plane, by either propeller or jet. The fast-moving air causes the plane to move forward, countering drag. Since the Wright brothers first flew in 1903, aeronautical engineers have created a multitude of airplane types, every one of which has dealt with the same four forces of weight, drag, lift, and thrust. All people have to deal with the challenges of stability with respect to these forces. Flying faster than the speed of sound has its own special demands, but the underlying forces of weight, drag, lift, and thrust remain the same. In some sense, it is easier to fly in space, which is devoid of air, than it is to fly in air. However, spaceflight has its own special challenges. In space, one must deal with only two forces, weight and thrust. Thrust provides the force to lift a rocket into space. Once in orbit, a spacecraft no longer needs propulsion. Short bursts from smaller rockets are used to maneuver the spacecraft. To change its orientation, a spacecraft applies torque, a twisting force, by firing small rockets called thrusters or by spinning internal reaction wheels. —M. A. K. Lodhi Further Reading Dole, Charles E., James E. Lewis, Joseph R. Badick, and Brian A. Johnson. Flight Theory and Aerodynamics: A Practical Guide for Operational Safety. Wiley, 2016. Etkin, Bernard. Dynamics of Atmospheric Flight. Dover Publications, 2005. Rathakrishnan, Ethirajan. Introduction to Aerospace Engineering: Basic Principles of Flight. Wiley, 2021. Volkov, Konstantin. Flight Physics: Models, Techniques and Technologies. IntechOpen, 2018.

See also: Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Airplane propellers; Daniel Bernoulli; Boomerangs; Flight roll and pitch; Fluid dynamics; Gravity and flight; Jet engines; Pressure; Stabilizers; Viscosity; Wake turbulence; Wind shear; Wing designs

Steve Fossett Fields of Study: Aeronautical engineering; Mechanical engineering; Fluid mechanics; Mathematics ABSTRACT Steve Fossett was born on April 22, 1944, in Garden Grove, California, and died September 3, 2007. He was a prominent balloonist, aviator, and yachtsman who has set many world records for flight. From his twenties, Fossett engaged in daredevil adventures, sailing marathons, flying jet planes, climbing mountains, and racing automobiles. His most publicized and spectacular feats have been in ballooning. STEVE FOSSETT’S LIFE James Stephen “Steve” Fossett was a millionaire stockbroker who engaged in adventurous hobbies. As a youngster, he started rock climbing. As a college student at Stanford University, he climbed mountains and swam the Hellespont in Turkey—a classical test of strength and endurance that he swam both ways. After earning his master of business administration (MBA) degree at Washington University in St. Louis, Missouri, Fossett moved to Chicago, where he made his fortune. However, he continued to engage in dangerous adventurous sports, continuing to try, despite many failures, until he succeeded. “I always thrive under pressure,” he claimed. In the late 1990s, he concentrated on sailing and ballooning, sports in which he believed he could set world records. In January, 1997, he attempted to be the first

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person to circumnavigate the world in a hot-air balloon. He competed against three teams, the British Virgin Group of Richard Branson, an Australian team, and a Swiss team. Compared to the millions that his competitors were spending, Fossett’s barebones Solo Spirit balloon was relatively inexpensive, at $300,000. Fossett’s 1997 flight failed when he had to land in northern India after six days, but he set a record for endurance and distance. His balloon traveled at heights of 5.5 to over 10.5 kilometers. With a broken heater, temperatures in the balloon were -9.5 degrees Celsius. Fossett failed in a second attempt, in August, 1998, when he had to land in Russia. Later that year, he began his third attempt in North Africa, but he was forced to land in the Pacific after China refused permission to fly over their airspace. A rival team of Bertrand Piccard and Brian Jones, however, accomplished the feat shortly afterward. Fossett then sought to set sailing records in his Play Station Maxi Catamaran. In February, 2000, along with copilots Darrin Adkins and Alex Tai, Fossett set the around-the-world record for medium-weight airplanes in his Citation X two-engine business jet. The trip took 41 hours, 13 minutes, and 11 seconds, about 6 hours less than the previous record. His average speed was 559 miles per hour. The same year, he set the US coast-to-coast records for private planes in both directions in his Citation. His August, 1998, balloon flight set the record for the longest solo aircraft flight and the second longest balloon flight. He also holds the record for several other distance and speed flights in his private plane. In August of 2001, Fossett made a fifth attempt at a transglobal balloon flight, but bad weather forced him to land in southern Brazil, just one day after he had reached the halfway point of his trip. Despite the curtailed effort, however, Fossett still managed to set a new record for the longest solo balloon flight, with a trip lasting 12 days and 13 hours. With

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Steve Fossett. Photo by John Mathew Smith, via Wikimedia Commons.

Fossett’s failed attempt, the solo transglobal balloon record remained unclaimed. —Frederick B. Chary Further Reading Austin, Elizabeth. Treading on Thin Air. Pegasus Books, 2016. Conniff, Richard. “Racing with the Wind.” National Geographic, vol. 192, no. 3, Sept. 1997. Fossett, Steve, and Will Hasley. Chasing the Wind: The Autobiography of Steve Fossett. Virgin, 2009. Gannon, Robert. “The Great Balloon Race.” Popular Science, vol. 248, no. 5, May 1996. Hogan, David. “Up, Up, and Away.” Current Science, vol. 83, no. 6, 14 Nov. 1997. Zimmerman, Tim. The Race: The First Nonstop, Round-the-World, No-Holds-Barred Sailing Competition. Houghton Mifflin Harcourt, 2004. See also: Aerodynamics and flight; Blimps; Richard Branson; First flights of note; Flight balloons; Hot-air balloons; Lighter-than-air craft; Montgolfier brothers; Jules Verne

G Yuri Gagarin Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology ABSTRACT Yuri Gagarin was a Russian cosmonaut whose 108-minute Earth orbital flight on April 12, 1961, represented mankind’s first space travel. He was born on March 9, 1934, in Klushino, near Gzhatsk, Smolensk Oblast, Soviet Union, and died on March 27, 1968, near Moscow, Soviet Union. As pilot of the Soviet orbital mission Vostok 1, Gagarin ushered in the space age by proving that a human being could endure the rigors of liftoff, reentry, and weightlessness and still perform the manual operations essential to space flight. YURI, THE SMILING COSMONAUT After a primarily vocational education, Russian cosmonaut Yuri Alekseyevich Gagarin entered pilot training at the First Chkalovsky Orenburg Military School for Pilots. In the autumn of 1957, he graduated with high honors from Orenburg and joined the Soviet Air Force as a junior lieutenant. From late 1957 until the spring of 1960, he served as a military fighter pilot in the Arctic. In 1960, he was selected as a member of the first group of Soviet cosmonauts. On the morning of April 12, 1961, Gagarin literally flew into history on board the spaceship Vostok 1, which launched at 9:07 a.m. Moscow time. The flight was automated for fear that the weightlessness of space might disable the pilot. A key was available in a sealed envelope in case it became necessary to

take control in an emergency. In a preflight speech, Gagarin commented that he had always waited for this moment and that he was glad to “meet nature face to face, in an unprecedented encounter.” The rocket accelerated to a peak of 5 g’s, indicating that Gagarin felt five times his normal weight. Fourteen minutes after liftoff, Gagarin reported that the capsule had achieved Earth orbit. He then tested his food and water samples and reported no side effects to the weightlessness. During the 108-minute flight, he made one elliptical Earth orbit, the apogee of which was about 324 kilometers above sea level. The orbital speed was approximately 27,359 kilometers

Yuri Gagarin. Photo via Wikimedia Commons. [Public domain.]

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per hour. The payload included life-support equipment as well as communications equipment that relayed information on Gagarin’s condition. As planned, at about 6 kilometers, Gagarin ejected and descended under his own parachute and landed southwest of the Saratov region, near Smelovka, Saratskaya. Following his historic flight, Gagarin received many honors in recognition of his Vostok mission. He was named a hero of the Soviet Union and was awarded the Order of Lenin and the K. E. Tsiolkovsky Gold Medal of the USSR Academy of Sciences. Later, a crater on the far side of the Moon was named after him. On March 27, 1968, Gagarin was killed in an accident while test piloting a MiG-15 aircraft near Moscow. The event caused a great deal of shock and spawned numerous conspiracy theories and rumors within the Soviet Union, whereas Western powers alleged that Gagarin had been drunk at the time. Two years after his death, he was posthumously inducted to the International Aerospace Hall of Fame. —Monish R. Chatterjee Further Reading Bizony, Piers, and Jamie Doran. Starman: The Truth Behind the Legend of Yuri Gagarin. Bloomsbury Publishing, 2011. Brennan, Gerald. Public Loneliness: Yuri Gagarin’s Circumpolar Flight. Tortoise Books, 2014. Hall, Rex D., David J. Shayler, Shayler David, and Bert Vis. Russia’s Cosmonauts: Inside the Yuri Gagarin Training Center. Springer New York, 2005. Jenks, Andrew L. The Cosmonaut Who Couldn’t Stop Smiling: The Life and Legend of Yuri Gagarin. Cornell UP, 2019. Walker, Stephen. Beyond: The Astonishing Story of the First Human to Leave Our Planet and Journey into Space. HarperCollins, 2021. See also: John Glenn; Robert H. Goddard; Rocket propulsion; Rockets; Russian space program; Alan Shepard; Space shuttle; Spacecraft engineering; Spaceflight; Valentina Tereshkova; Chuck Yeager

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German Luftwaffe Fields of Study: Aeronautical engineering; Mechanical engineering; World War II history ABSTRACT The Luftwaffe was Nazi Germany’s air force from the early 1930s to the end of World War II. The Luftwaffe used fast-moving offensives called blitzkrieg to destroy enemy aircraft. Although it was effective in the early years of World War II, it became less so as the war progressed, losing its air superiority due to aircraft obsolescence, economic inferiority, poor organization, and poor leadership. KEY CONCEPTS blitzkrieg: a World War II German war strategy of rapid air attacks on selected targets; the word means “lightning war” LUFTWAFFE HISTORY After World War I, in which Germany had been roundly defeated by the war’s length as much as by the economic superiority of its opponents, General Hans von Seeckt, the commander of the German Army, realized that fast, mobile offensives would be necessary to avoid prolonged future wars that Germany could not win. He therefore devised the military strategy of the blitzkrieg, or lightning war, fast-moving surprise attacks. The Luftwaffe, designed around the blitzkrieg concept, was initially very effective during World War II in gaining air superiority via short, independent operations all over the European continent. As the European air war drew on, however, the Luftwaffe was engaged in a contest that it could not win. From the beginning, the Luftwaffe’s leaders saw the necessity of air superiority. Its chiefs of staff noted that air support of ground forces at the start of a war did not mitigate the damage inflicted by functional enemy air forces. From the start of any

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campaign, the Luftwaffe’s primary efforts were focused on the destruction of all enemy aircraft. Its bombers crushed enemy bombers on the ground, disrupting potential sorties, and its fighters hunted down any enemy aircraft able to become airborne. The Luftwaffe carried out its activities via autonomous air fleets, known as the Luftflotten. Each Luftflotte comprised both aircraft and support units. Technologically, the aircraft were suited for offensive counterair missions (OCAMs) that destroyed enemy aircraft on the ground. German Stuka bombers had the range and payload to reach and damage the air bases that held the most enemy aircraft. Twin-engine fighters escorted bombers, warding off enemy fighters until the OCAM was completed. Single-engine Messerschmitt fighters fought enemy aircraft. Luftwaffe aircraft enabled short-offensive campaigns but had little use in other types of air warfare, such as attacks on training bases in the rear and other distant sources of enemy air power. German bombers had low ranges, meager payloads, and very little defensive armament. Later failure of newer escort fighters and the short ranges of all existing escorts exacerbated the problem, restricting German air power to use in the battlefield. Germany successfully applied its OCAM doctrine during the first two years of the war. These attacks destroyed numerous aircraft and caused the remainder to operate inefficiently. However, these victories cost the Luftwaffe huge aircraft losses. For example, in the two-month battle for France, 36 percent of all German aircraft were either damaged or lost. Such losses were initially acceptable, because they were suffered in the defeat of several Allied air forces. However, over the course of prolonged warfare, this high loss rate was damaging to the German cause, especially after German offensive campaigns failed against both England and the Soviet Union. In 1940, the Luftwaffe was unable to defeat the Royal Air Force (RAF) in the Battle of Britain. After unsuccessful battles over the English Channel, a

German Luftwaffe

three-week German campaign against RAF bases made some progress but was changed to unsuccessful day attacks and then to nocturnal terror attacks. The German inability to win was due to RAF defense strategy and absence of a ground war to distract the RAF. Similarly, the Germans had initial success in their air war against Russia by employing the OCAM doctrine, but the German ground forces ultimately proved unsuccessful. The unanticipated Siberian air power, the untenable Russian winter weather, and the vastness of the eastern front all led to the campaign’s failure and the Luftwaffe’s weakening. As General von Seeckt had noted, opponents pushed to defense are broken by destruction of aircraft. German failures in Britain and Russia forced the Luftwaffe into defensive counterair battle (DCAB), as the need to win air battles over its homeland exhausted Germany’s hope of air superiority. The prolonged defensive air war forced the Luftwaffe to adopt a defensive strategy in its organization, equipment, and deployment. Overwhelmed by Allied aircraft production, German strategy consisted largely of annihilating Allied bombers. This strategy was impractical, given the Allied air superiority. Luftwaffe generals, however, clung to false hope that if they could decimate enough Allied bombers, they could cause the cessation of Allied air offensives. By 1944, two whole Luftflotten were used in this way. As the Allied threat grew, the German expansion and refinement of its air defense included the development of radar and automated fighter control systems, an increase in armor and armament, the use of aerial bombs and cannon on board fighter planes. German aircraft manufacturers shifted from the production of bombers to production of fighters, giving the Germans success throughout 1943. Although Allied bomber raids were not stopped entirely, they evolved into less effective, nighttime operations with huge bomber losses. However, these German victories were only temporary, because at

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the same time, hundreds of German planes and pilots were lost in the battles. The air war became a lost cause. Although the Germans made other technological advances, including the use of jets and surface-to-air missiles, these came too late to be useful. Huge numbers of Allied air forces drove the Luftwaffe from the sky. Amid failing defenses, the German air force stubbornly held to its offensive practice. Its forces kept declining, however, and the last major OCAM achievements occurred as follows. In June, 1944, a night raid on Poltava, a Ukrainian city on the eastern front, destroyed many US bombers caught on the ground. Operation Bodenplatte, the last major German fighter operation, was waged in Belgium, Holland, and France, in January, 1945. It used the entire German fighter force to raid Allied airfields. There, single-engine fighters and green German pilots carried out a mission in which 30 percent losses occurred. Both operations were destructive, but barely altered the numbers of Allied aircraft available in Europe. LEADERSHIP Possibly, the Luftwaffe had always been in an untenable position, because Nazi Germany was a dictatorship ruled by its inflexible chancellor, Adolf Hitler. The German General Staff was faced with this problem as well as with the politicking, drug addiction, and incompetency of high-ranking government officials. An example is that of Hermann Göring. A former World War I fighter ace and the head of the Luftwaffe, he was unable to function adequately or consistently due to his drug abuse and his inability to counter Hitler’s preference for the production of bombers instead of fighters. These weaknesses in the Luftwaffe’s command led to the decrease and eventual collapse of the force’s fighting ability. A contrasting example is that of the able Adolf Galland, a fighter ace of the Condor Legion who

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also participated in the German invasions of Poland and France and the Battle of Britain. In 1941, he became the commander of the Luftwaffe’s fighter arm and by 1943, had been promoted to the rank of major general. In 1943 and 1944, he ably commanded Germany’s already-failing fighter squadrons against Allied bombers. Despite Galland’s resourceful leadership of a crumbling air operation, Hitler and Göring blamed him for weakened Luftwaffe air defenses in 1944 and, soon thereafter, relieved him of his command. Many critics blame the Luftwaffe’s failure on its leaders, based on three flaws. First, is the fact that the Luftwaffe high command used training units in battles such as Poltava. This decision was seen as damaging, because continued training operations were essential to winning the war. Second, is the perception that the Luftwaffe was too slow in recognizing the war’s attritional nature and implementing the defense measures needed for any chance of eventual success. Göring has been accused of overconfidence in the offensive air-war strategy and failure to see the need to prepare for failure. The third major Luftwaffe shortcoming was in its inferior equipment and inability to modernize or build heavy bombers that could compete with those of the Allies. AIRCRAFT Much of the basis for eventual Luftwaffe failure may lie in Hitler’s long-standing preference for bombers over fighters. Under Hitler’s dictatorship, it was difficult for military leaders to work around such a prejudice. This theory may partly explain the relative obsolescence of the Luftwaffe aircraft toward the end of the war. Although efforts were made to produce new aircraft, promising new designs apparently did not work out well. For example, the Junkers Ju-88, a twin-engine bomber planned as the successor to the Ju-87 Stuka, was not airworthy. In its production,

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designers increased the Ju-88’s weight to enable dive-bombing, thereby reducing its effective range and speed and rendering it ineffective against the increasingly faster aircraft being produced by the Allies. Furthermore, the Heinkel He-117 aircraft, an attempt to produce long-range bombers, was a disaster. Its planned weight was doubled in order to make it suitable for dive-bombing. More devastating was the aircraft’s tendency to fall apart during dives and to explode in flight. Moreover, the Messerschmitt Me-210 fighter, planned to replace the Luftwaffe’s fighter workhorse, the Me-109, was a dismal failure. This aircraft and others were canceled early in production, and the Germans were never able to build a successful four-engine bomber. These failures occurred at times when Germany could not afford to squander its slim resources. Consequently, the Luftwaffe relied on a few tried-and-true aircraft, such as the Junkers Stuka bombers and the Messerschmitt Me-109 fighters. These aircraft were, by the early 1940s, relatively obsolete. The Stuka dive-bomber, a low-winged, single-engine aircraft, was a very successful weapon during the first half of the war. Stukas employed dive-bombing techniques developed by the US Navy, dropping bombs while diving and then moving into getaway flight mode. Special brakes slowed Stuka dives and gave pilots time to aim bombs. The bombers were armed with four 8-millimeter machine guns, two of which were operated by a rear-gunner. Late in the war, the rear-mounted guns were replaced with a heavier gun. The Stuka carried 1,100- or 550-pound bombs and had two 110-pound bombs under each wing. Although the plane was periodically modified throughout the war, its maximum speed remained 210 miles per hour. Eventually, it proved no match for faster Allied fighters. The Me-109 fighter was used to great effect in World War II. Powered by a fuel-injected, Daimler-Benz engine, this low-winged, single-seater,

monoplane had a top speed of 350 miles per hour and a ceiling of around 40,000 feet. It held two 20-millimeter cannons and two machine guns. Me-109s were the pride and joy of the Luftwaffe, faster and much more maneuverable than most Allied fighters. However, the Me-109’s range was limited by a small fuel capacity, and by 1944, Allied fighters had outstripped it in every way. —Sanford S. Singer Further Reading Baumbach, General Werner. The Life and Death of the Luftwaffe. Tannenberg Publishing, 2016. Heath, Tim. In Furious Skies: Flying with Hitler’s Luftwaffe in the Second World War. Pen and Sword, 2022. McNab, Chris. German Luftwaffe in WW II. Amber Books, 2009. Pavelec, S. Mike. The Luftwaffe: Facts, Figures and Data for the German Air Force, 1933-45. Amber Books Ltd., 2018. Thomas, Geoffrey J., and Barry Ketley. Luftwaffe KG 200: The German Air Force’s Most Secret Unit of World War II. Stackpole Books, 2015. See also: Messerschmitt aircraft; Military aircraft

John Glenn Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT John Glenn was born on July 18, 1921, in Cambridge, Ohio, and died December 8, 2016, in Columbus, Ohio. He was the first US astronaut to orbit Earth (1962) and became the world’s oldest astronaut (1998). Glenn is a symbol of the evolution of the American space program. His first space mission restored American pride in the space race with the Soviet Union, while his last space mission demonstrated that the elderly can make important contributions to society.

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EARLY LIFE Reared in New Concord, Ohio, John Herschel Glenn Jr., developed a great love and respect for his parents, who taught him that he had unlimited possibilities, and that, with hard work, he could achieve whatever goals he set for himself. His mother, an elementary school teacher, taught Glenn to love reading and learning. When Glenn was eight years old, he accompanied his father, a plumbing contractor, on a job to Cambridge, Ohio. During this trip, Glenn’s father arranged for his son’s first flight in an airplane, after which the young Glenn was hooked on flying. Model airplanes became his favorite hobby, and he dreamed of someday becoming a pilot. In high school, Glenn participated in football, basketball, and tennis; played the trumpet in orchestra; and served as a school newspaper reporter and student

Glenn in his Mercury spacesuit in 1962. Photo via Wikimedia Commons. [Public domain.]

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body officer. After graduating from high school in 1939, he enrolled in Muskingum College to study chemical engineering. He also entered a civilian pilot training program and earned his flying license in 1941. Upon the US entry into World War II (1939-45), Glenn decided it was his patriotic duty to enlist for naval aviation training. After graduation, Glenn received a commission in the US Marine Corps Reserve. By March, 1943, he had earned his wings and was promoted to a Marine second lieutenant. He married his childhood sweetheart, Annie Castor, on April 6, 1943. WAR EXPERIENCE Assigned to Marine Fighter Squadron 155, Glenn spent a year flying F-4U Corsair fighters on a variety of bombing and reconnaissance missions against Japanese garrisons in the Marshall Islands. He flew fifty-nine combat missions and was hit by enemy fire five different times. After returning to the United States, his principal duties were as a flight instructor. He was promoted to the rank of captain in July, 1945. In December, 1946, he was assigned as a member of Marine Fighter Squadron 218 to patrol North China in support of General George C. Marshall’s World War II peace terms. From June, 1948, until December, 1950, he served as an instructor in advanced flight training in Corpus Christi, Texas. During the Korean War (1950-53), Glenn flew jets in ground-support missions for the Marines and in air-to-air combat as an exchange pilot in the new Air Force F-86 Sabre jets, completing a total of ninety missions between February and September, 1953. Glenn had many close calls that often caused him to return to base with a seemingly unflyable aircraft. In the last nine days of fighting in Korea, Glenn downed three Soviet-built MiG-15s in fierce combat along the Yalu River. For his military service during World War II and the Korean War, Glenn received four Distinguished Flying Crosses and eighteen Air Medals. He rose

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steadily through the ranks, becoming a captain in 1945, a major in 1952, and a colonel in 1959. In 1954, he was assigned to the Navy’s test pilot school in Patuxent River, Maryland. Upon graduation, he served as a project officer on a number of aircraft. On July 16, 1957, he set a record for the first coast-to-coast, nonstop, supersonic flight in an F-8U Crusader jet fighter, flying from Los Angeles to New York in three hours and twenty-three minutes. For this event, Glenn received his fifth Distinguished Flying Cross. SPACE FLIGHT AND POLITICS Spurred by the success of the Russian satellite Sputnik, the United States established Project Mercury in 1958. Glenn was named as one of the seven Mercury astronauts in April, 1959. Motivated by his deep religious faith, hard work ethic, and tenacious devotion to duty, he helped win the widespread public support that the space program needed. Glenn was selected to serve as backup pilot for the suborbital flights of Alan Shepard and Virgil “Gus” Grissom in 1961. He was then chosen as the first American to orbit Earth, orbiting three times in the Friendship 7 orbiter on February 20, 1962. The mission restored American pride in the space race with the Soviet Union. After convalescing from a severe inner-ear injury caused by a fall in February, 1964, Glenn retired from the Marines in January, 1965. He was elected to four consecutive terms as a US senator from Ohio, beginning in 1974. He made an unsuccessful bid for the Democratic presidential nomination in 1984. On the thirty-fifth anniversary of his historic flight (February 20, 1997), Glenn announced that he would retire from the Senate at the end of his fourth term in 1998. THE OLDEST ASTRONAUT While Glenn sought additional funding for the National Aeronautics and Space Administration

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(NASA) in 1995, he reviewed some documents on the physical changes that happen to astronauts in orbit. He was amazed at the similarities between the effects of zero gravity on the body and the natural aging process on Earth. Consequently, he began petitioning NASA for the opportunity to go back into space and study the effects of weightlessness on older Americans. After much perseverance, on January 15, 1998, he was granted his wish of going back into space. After a thirty-seven-year hiatus from space flight, Glenn spent months of training, experimenting, and baseline medical tests to become the oldest person to travel into space. As a member of the nine-day space shuttle Discovery mission from October 29 to November 7, 1998, the seventy-seven-year-old Glenn conducted numerous experiments that focused on osteoporosis and the immune system’s adjustments to the aging process. Glenn’s contributions demonstrated that the elderly can and do still make important contributions to society. Glenn stands out as a symbol of courage, honor, and lifelong devotion and service to his family and his country. —Alvin K. Benson Further Reading Burgess, Colin. Friendship 7: The Epic Orbital Flight of John H. Glenn, Jr. Springer International Publishing, 2015. Chaikin, Andrew. John Glenn: America’s Astronaut. Smithsonian Institution Press, 2014. George, Alice L. The Last American Hero: The Remarkable Life of John Glenn. Chicago Review Press, 2020. Green, Robert. John Glenn. Facts on File Inc., 2009. Pierce, Philip N. John H. Glenn, Astronaut. Papamoa Press, 2018. Shesol, Jeff. Mercury Rising: John Glenn, John Kennedy, and the New Battleground of the Cold War. W. W. Norton, 2021. See also: Neil Armstrong; Amelia Earhart; First flights of note; Yuri Gagarin; Charles A. Lindbergh; Montgolfier brothers; Wiley Post; Russian space program; Alan Shepard; Space shuttle; Spaceflight; Valentina Tereshkova; Chuck Yeager

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Glider Planes Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Pilot training ABSTRACT A glider plane is any one of a number of types of winged, heavier-than-air craft, having no motive power other than gravity and existing air currents. Sailplanes are a specific type of glider than can ascend as well as descend. Gliders were the predecessors to motorized flight. Information gained from experiments with gliders made motorized flight possible. KEY CONCEPTS biplane glider: a glider bearing two pairs of wings ornithopter: aircraft that were designed to imitate birds by using the principle of flapping wings as the means of propulsion ornithoptic propulsion: flying in the manner of birds, by the flapping of wings sink rate: the ratio of a glider’s rate of descent to the distance covered during a flight triplane glider: a glider bearing three pairs of wings HOW GLIDERS FLY A glider is launched from a raised elevation and is capable only of forward movement through the air while at the same time losing altitude. The relation between forward momentum and loss of altitude is a glider’s sink rate, and gliding is the motion of the craft’s controlled descent. The history of glider development is essentially the process of experimentation to minimize a glider’s sink rate, while giving the glider pilot increasing control over the movement or flight of the glider while airborne. Eventually, after centuries of experimentation, aviation technology developed to the point where gliders could be constructed and flown in ways that permitted the glider pilot to slow and even reverse the rate of descent. The process of flying a glider using the energy from

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thermal air currents to regain altitude lost by the downward force of gravity is called soaring, a practice used by many species of high-flying birds such as condors and eagles. The type of glider capable of being flown in such a way is termed a sailplane. It is basically a high-performance glider designed specifically for soaring. Post-World-War-II gliders are more correctly called sailplanes to distinguish them from earlier gliders, regardless of size and precise configuration, that were not capable of regaining lost altitude in a controlled manner after they had been launched. EARLIEST HISTORY The process of experimentation with gliders that led to modern sailplanes took place over the course of centuries. As long as humans have watched birds in flight, humans have wanted to imitate them. Many of the earliest attempts at human flight are thinly documented or are mythological. One of the earliest stories of human flight is the account of Daedalus and his son Icarus. As related in Metamorphoses (ca. 8 CE; Eng. trans. 1567), a collection of tales by the Roman writer Ovid, Daedalus was imprisoned by the Cretan king Minos. While watching seagulls soaring in flight, Daedalus got the idea to fashion wings from discarded seagull feathers held together with candle wax. Using these birdlike wings, Daedalus and Icarus escaped. Daedalus wisely kept to a course midway between earth and heaven, but Icarus flew too close to the sun. The wax holding his wings together melted and he plummeted to his death. It seems likely that Daedalus and Icarus were actually using their wings as gliders, and Daedalus avoided direct sunlight in his descent, but Icarus did not so the heat from the direct sunlight doomed his wings. This cautionary tale of Daedalus and Icarus set the stage for much later thinking about human flight. Most people were of the opinion that humans had no business trying to fly, but there was a small group of adventurers and inventors who disregarded this opinion.

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Glider Planes

Smallest glider in the world, the BrO-18 “Boru?” (Ladybird), constructed in Lithuania in 1975. Photo by Ke an, via Wikimedia Commons.

MEDIEVAL ATTEMPTS AT FLIGHT There are numerous undocumented passages in medieval historical sources stating that humans achieved flight aboard or attached to gigantic kites, perhaps similar to present-day hang gliders. The Italian mathematician Giovanni Danti is reported to have tried to fly over Lake Trasimeno in Italy in the late 1500s. John Damian, another Italian, reportedly constructed a pair of wings and jumped off the wall of a castle belonging to King James IV of Scotland. He plummeted to Earth, breaking his leg. Leonardo da Vinci, a fifteenth-century Italian artist, scientist, and inventor, seriously examined the possibility of human flight. Using comparative zoology and architectural and mathematical studies, da Vinci concluded that humans were too heavy to be kept aloft by feathered wings modeled on the wings of birds. Da Vinci thought that batlike wings in which the skin is stretched over a lightweight skeleton was more likely to sustain the weight of a human in flight. Da

Vinci also designed rudimentary parachutes and a type of ornithopter or bird-imitating flapping machine that is considered an early prototype to the modern helicopter. Although da Vinci’s flying inventions are theoretically possible, it was almost three hundred years before they were actually built, tested, modified, and put into practice. NINETEENTH CENTURY The Englishman Sir George Cayley systematically examined the problems associated with human flight. In 1809, he published the results of his experiments with small, uncrewed glider models, each of them with V-shaped wings and a tail stabilizer. Using his horses to supply the forward momentum, Cayley performed a brief, barely controlled glider flight in 1853. William Henson tried to develop Cayley’s experiments further by adding a steam-powered motor to the aircraft. Such an engine made the aircraft far too heavy to get off the

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ground, but Henson improved Cayley’s glider designs, eventually designing a fixed, single-wing glider with a bird-tail-shaped tail, a rudder, and landing gear. Henson’s friend John Stringfellow built a small model glider with a small steam engine that could fly under specific circumstances, but he did not build a model big enough to carry the weight of a human. Francis Herbert Wenham, another Englishman, also studied birds to investigate possibilities for human flight. Wenham concluded that a slightly arched wing set at an angle, rather than a flat wing surface, could lift more weight. He also thought that a connected series of shorter, arched wings rather than one set of long, flat wings might sustain a person in flight, if only a means could be found to lift the craft off the ground initially. Frenchmen Jean-Marie Le Bris and Felix Du Temple both built uncrewed, motorless gliders. Le Bris fashioned his glider in the shape of an albatross and Du Temple constructed the first propeller-driven aircraft to lift off from the ground under its own power. Neither craft could stay aloft for more than a few seconds nor could their flight path be controlled. In the late nineteenth century, the German Otto Lilienthal built numerous single-winged gliders, each with a fixed tail for stability. The pilot stood in the center of the glider with the glider frame attached around his waist. By making over two thousand flights off a small hill, Lilienthal learned how to move his weight to steer the glider. His longest flight was approximately 61 meters. On August 9, 1896, Lilienthal attached a small motor to his glider and launched himself off the hill. The wind shifted and he crashed, suffering fatal injuries. Percy Pilcher, a Scotsman who had known Lilienthal, modified his own triplane glider based on Lilienthal’s experiments. Pilcher conducted numerous glider flights, one as long as 750 feet, before being killed in a glider accident in September, 1899. A naturalized American, Octave Chanute, was also in-

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fluenced by Lilienthal’s experiments. He designed numerous gliders and tested them on the beach at Lake Michigan near Chicago, Illinois. He had two-, three-, and five-winged models with rear stabilizers, each controlled in flight by shifts of the pilot’s weight. Chanute kept careful records of his experiments with equilibrium while aloft, information he shared with the Wright brothers. EARLY TWENTIETH CENTURY Wilbur Wright and his brother Orville Wright grew up primarily in Dayton, Ohio. They initially made their living repairing bicycles while pursuing aeronautical experiments as a hobby. Beginning with a series of kites, the Wright brothers developed a system of wing-warping that greatly increased the pilot’s ability to control the flight of an aircraft. The Wright brothers spent part of each year from 1900 to 1905 at Kitty Hawk, North Carolina, testing gliders they had designed in Dayton. The 1900 glider weighed 23.6 kilograms and had 15.5 meters by 1.5 meters wings. It was not substantial enough to lift a pilot in a controlled flight. In 1901, the glider was much bigger, having 6.7- by 2-meter wings. The longest piloted flight, by Wilbur, was 121 meters. During the winter of 1901, the Wright brothers reworked information from Chanute and Lilienthal in order to solve problems with both lift and control. Using this new information, the 1902 biplane glider weighed 52.6 kilograms and had a 9.7-meter wingspan. It incorporated various design changes to provide more lift, including a forward monoplane elevator, as well as a fixed rudder linked to the wing-warping or shaping system that allowed the pilot to control the glider’s flight. The longest flight of the 1902 testing session was 189.5 meters, lasting 26 seconds. The original patent issued to the Wright brothers covered the modifications included in the 1902 glider design. Returning to design experiments, the Wrights constructed a glider that could carry a 12-horsepower engine and have two propel-

Principles of Aeronautics

lers. On December 17, 1903, Wilbur Wright flew 260 meters, staying aloft for 59 seconds, the first documented pilot-controlled motorized flight in history. The Wrights continued to refine their aircraft designs in 1904 and 1905, gradually increasing both the length of and control over motorized flights. In 1911, Orville returned briefly to gliders, setting a glider flight record of 9 minutes, 45 seconds. WORLD WAR II Once motorized flight had been demonstrated, gliders seemed rather primitive. All the major powers in World War I used motorized airplanes, not gliders. After the end of World War I, however, attention returned to gliders. The Treaty of Versailles ending the war prohibited the Germans from building new planes with engines. The treaty did not mention the building of motorless gliders. Thus, throughout the 1920s and early 1930s, thousands of young German men learned to fly as glider pilots. They formed the core of the Nazi Luftwaffe in World War II. In 1930, the three Schweizer brothers—Bill, Paul, and Ernest—began to build gliders for sale to enthusiasts in the United States. In 1932, the Soaring Society of America was founded to regulate the small but growing hobby in America. Soaring Magazine, still in publication, debuted in 1937. The Germans were the first to recognize the potential military applications of gliders. The first military glider capable of carrying troops and equipment was the DFS-230. On May 11, 1940, ten DFS-230s carrying seventy-eight glider troops attacked and captured Eben Emael in Belgium, due in large measure to the element of surprise. Other countries quickly took notice. The United States produced thousands of small TG-2 and TG-3 gliders, as well as jumbo gliders such as the Laister-Kauffman CG-10A Trojan. The British also built large numbers of various types of gliders to use in aerial observation, as well as in troop and equipment transport.

Glider Planes

The idea of parachute troops or airborne infantry was in its infancy in early World War II. Rather than trying to coordinate hundreds of individual soldiers in parachute drops, the conventional wisdom of the time thought it made more sense to airlift troops in platoons in gliders. Unfortunately, glider pilots and troops suffered very high casualty rates, losing in excess of 50 percent of the gliders and the personnel they carried. The Germans tried an unsuccessful glider assault on Crete. Many gliders were blown off course, some crashed, some landed intact but far from the designated landing zone. On July 9, 1943, the Allies tried a joint American-British glider assault on Sicily. Of the 144 troop gliders involved in the assault, 69 landed in the ocean rather than on land, 10 were apparently shot down, only 12 were able to land intact, and only 4 of those landed within the designated landing zone. The Allies also tried glider assaults in Burma, with similar disastrous results. POST-WORLD WAR II After World War II, many military pilots turned to gliding as a recreational pursuit. Inexpensive military surplus gliders were readily available. By the mid-1950s, there was a large enough recreational market to spur further refinements in glider design. Invented in 1928, the variometer, a piece of equipment that allowed the pilot to measure even small differences in altitude, became standard on every glider, which became technically sailplanes, able to both ascend and descend. National and international championships are held annually for different design classifications of sailplanes, with various contests for speed, altitude, duration of flight, distance covered and accuracy in landing at a designated spot. All rules and standards concerning sailplane construction and classification, as well as sailplane pilot training requirements in the United States, are regulated by the Federal Aviation Administration (FAA). —Victoria Erhart

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Further Reading Federal Aviation Administration (FAA). Glider Flying Handbook. Aviation Supplies & Academics Inc., 2004. Marriott, John. Aerotowing Gliders: A Guide to Towing Gliders, with an Emphasis on Safety. Author House, 2011. Mrazek, James E. Airborne Combat: The Glider War/Fighting Gliders of World War II. Stackpole Books, 2011. Pajno, Vittorio. Light Airplane and Glider Static and Dynamic Stability: Basic Theory and Calculation Examples. IBN, 2015. See also: Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Glenn H. Curtiss; Leonardo da Vinci; Flight propulsion; Flight roll and pitch; Forces of flight; Gravity and flight; Paper airplanes; Propulsion technologies; Stabilizers; Training and education of pilots; Weather conditions; Wind shear; Wright brothers’ first flight; Wright Flyer

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until fifteen years later, when Mrs. Goddard’s health dictated their return to Worcester. Various bronchial ailments plagued their only son, who, because of frequent absences from school, did not graduate from high school until his twenty-second year. Like many boys of his time, Goddard devoured such prototypes of science fiction as Jules Verne’s From the Earth to the Moon (1865) and H. G. Wells’s The War of the Worlds (1898). Goddard dated the discovery of his vocation, however, from an experience that, like the story often told of George Washington, involved a cherry tree but a story whose authenticity is not in doubt. On October 19, 1899, shortly after his seventeenth birthday, he climbed a cherry tree on the family property and, while in its branches, imagined a spaceship that might travel to Mars. He later claimed that when he descended from the tree, he was “a different boy,” and for the remainder of

Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Robert H. Goddard was born October 5, 1882, in Worcester, Massachusetts, and died August 10, 1945, in Baltimore, Maryland. He devised the first successful liquid-fueled rocket and was a tireless explorer of the theoretical and practical problems of rocketry decades before the subject gained substantial support in the United States. Goddard stands as the great American pioneer of space travel. EARLY LIFE Robert H. Goddard was born in the central Massachusetts industrial city of Worcester. Nahum Goddard, then a bookkeeper for a manufacturer of machine knives, and his wife, the former Fannie Louise Hoyt, moved to Roxbury, Massachusetts, when Robert was only an infant but continued to spend considerable time at the family homestead

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Robert H. Goddard. Photo via Wikimedia Commons. [Public domain.]

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his life he would solemnly celebrate the date as “Anniversary Day.” Whenever possible, he visited the tree on October 19, as long as it stood. Single-minded in his dedication to the idea of space flight, he entered the local engineering college, Worcester Polytechnic Institute, in 1904. Although he pondered space travel in his spare time, the nature of his collegiate work suggested to his physics professor the likelihood of a career in radio engineering. On graduation in 1908, he continued his study of physics at Clark University, also in Worcester; Clark had been founded as a graduate school and emphasized the natural and social sciences. Goddard taught physics briefly at Worcester Polytechnic Institute, but, on receipt of his PhD in physics in 1911, he accepted a research fellowship at Princeton University, realizing that his aptitude for research exceeded that for teaching. In March of 1913, he learned that he had contracted his mother’s illness, tuberculosis, and physicians gave him little chance to survive. He spent a year at home recuperating, and by 1914, was well enough to conduct a series of experiments with tiny rockets propelled by a smokeless powder of his own devising. The struggle with disease, however, had exacted its toll, leaving him nearly bald in his early thirties. He remained thin and frail throughout his life, and he developed a stoop while relatively young. The young scientist was of average height, his two most prominent facial characteristics being a trim brown mustache and expressive brown eyes under dark brows. Rejecting offers from Princeton and Columbia, which he feared might not leave him sufficient time for research, he accepted a position as instructor in physics at Clark, an association that would last the remainder of his life. LIFE’S WORK The two world wars bounded, and greatly influenced, Goddard’s working life. Of the 214 patents

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issued to him, the first two came in the summer of 1914, as World War I was beginning. One, for a cartridge-feeding mechanism, turned out to be impractical in rocketry; the second, for a liquid rocket fuel, presaged his greatest accomplishment, still more than a decade of hard work away from fruition. Whereas science fiction had fired his imagination early, Goddard invariably approached his investigations in a matter-of-fact way and never seems to have wasted time on romantic but scientifically dubious schemes for space travel. At this time, weaponry, not space flight, occupied the American military, and Goddard, aware that the Germans had pursued applications of the Wright brothers’ great invention more quickly than had Americans, and anxious that they not take the lead in rocket development, wrote to the US Navy about his experiments. Although he provoked some interest, President Woodrow Wilson’s declaration of American neutrality discouraged research in military rockets. Furthermore, despite his success at sending his tiny powder rockets nearly five hundred feet into the air over Worcester by 1915, Goddard had not yet developed a suitable liquid fuel. When his university salary proved inadequate to support his research, he obtained a five-thousand-dollar grant from the Smithsonian Institution, and by the fall of 1918, he had devised a rocket that was capable of being fired from a trench and of delivering a payload three-quarters of a mile away. In November, he demonstrated his rockets at the Army’s proving grounds at Aberdeen, Maryland. Impressed that these rockets outperformed existing trench mortar, the Army agreed to appropriate money for production. A few days later, however, Germany surrendered. It would require another global conflict to revive high-level interest in Goddard’s rockets. Returning to his first love, the goal of space travel, Goddard, under the auspices of the Smithsonian, published in 1919 a treatise explaining how rockets might ascend to the moon. This work, “A

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Method of Reaching Extreme Altitudes,” brought him unwelcome publicity as an eccentric professor, and references to him in the popular press as “moon man” stung this serious investigator. He was fortunate in his academic affiliation, however, for Clark granted him leaves of absence when necessary, and his work proceeded steadily. Between 1920 and 1923, he conducted experiments at the Navy ordnance facility at Indian Head, Maryland. Back in Worcester in 1923, he was appointed director of laboratories at Clark in addition to his professorship. Goddard took time out, in 1924, to marry Esther Kisk, who proved a devoted helpmate; after his death his widow would spend years editing his voluminous papers. On March 16, 1926, on a farm in nearby Auburn, Goddard achieved his greatest success. The ten-foot rocket he sent up that day traveled for only two and one-half seconds and flew only 184 feet. His sponsor at the Smithsonian, Charles G. Abbot, was not impressed, for Goddard had talked in terms of hundreds of miles, and this rocket had attained a maximum altitude of forty-one feet. In retrospect, however, this short flight looms as momentous as that of the Wright brothers’ airplane twenty-three years earlier at Kitty Hawk, North Carolina. This first successful flight of a liquid-propellant rocket established the feasibility of the spectacular space ventures that Goddard did not live to see. His experiments having outgrown New England pastures, Goddard received the assistance of Colonel Charles A. Lindbergh, whose 1927 transoceanic flight had earned for him international fame. In 1929, Lindbergh talked philanthropist Harry Guggenheim into granting Goddard fifty thousand dollars for research on a larger scale. This largess enabled Goddard to spend much of the following decade at Roswell, New Mexico, improving his rockets. Clark issued another monograph on his work to date in 1936, but not until after his death would his major publication, Rocket Development: Liquid-Fuel

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Rocket Research (1948), appear. Unlike Hermann Oberth, his younger German contemporary, who independently achieved results comparable to Goddard’s, the Clark professor shunned publicity and avoided joint projects with other scientists. As a consequence, he spent the latter part of his career laboring in obscurity while Oberth’s work led directly to the V-1 and V-2 rockets of World War II. During the war, Goddard worked for the Navy in Maryland but found the required teamwork uncongenial. As the war dragged on, declining health and the knowledge that German rocketry was outdistancing that of the United States seized Goddard, and the American government’s decision to concentrate on atomic research left him in a military backwater. In May of 1945, a few weeks after Germany’s surrender, a physician detected a growth in Goddard’s throat. Despite two operations at the University of Maryland Hospital in Baltimore, Goddard continued to fail. On August 10, 1945, America’s rocket pioneer died; his body was returned to Worcester and buried on August 14, the day of Japan’s surrender. Assessing Goddard’s achievement later that year, Science magazine credited him with investigating virtually every principle vital to the theory and practice of jet propulsion and rocket guidance. Nevertheless, most Americans did not know him until the early 1960s, when the successes of the American space program provoked interest in its historical background, and publications describing Goddard’s life and work began to appear. When his story was told, it was often with an emphasis on the solitariness and obscurity of his endeavors. His biographers have tended to depict him as a lonely hero, obliged to endure at first scorn and later neglect. Historians of rocketry and space travel, however, have pointed out the extent to which he imposed his plight on himself. Retiring by nature, Goddard appears to have been driven further inward by the facetious tone of early journalistic accounts of his research. He conducted his experi-

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ments in a secretive and possessive manner, sharing his discoveries with only a few trusted assistants. Although a true scientist, Goddard evinced an inventor’s interest in protecting his work by patent much more often than a scientist’s desire to share his discoveries with fellow scientists in scholarly monographs. Pursued by both professional and amateur societies in his field, he generally remained aloof. As a result, it is likely that he fell behind other researchers in the 1930s and 1940s. One of Goddard’s biographers has noted that his brown eyes could radiate warmth and friendliness at times and turn cold and austere at others. Many anecdotes testify to his congeniality when at ease and among friends, but he seems to have been a man who coveted and cherished his professional isolation. Whatever the explanation of his proprietary attitude toward his work, he was a true pioneer in rocketry. He made original discoveries in many aspects of this subject, producing innovative igniters and carburetors, pumps and turbines, gyroscopic stabilizers and landing controls, jet-driven propellers and variable-thrust engines. His ceaseless dedication and the thoroughness of his research and testing complemented his sheer brilliance. Robert H. Goddard’s legacy, so little recognized at the time of his death, is now manifest in the space age.

Tyson, Neil deGrasse. “Fueling Up.” Natural History, vol. 114, no. 5, June 2005, pp. 18-25. West, Doug. Dr. Robert H. Goddard, A Brief Biography: Father of American Rocketry and the Space Age. CreateSpace Independent Publishing Platform, 2017. See also: Advanced propulsion; Neil Armstrong; Yuri Gagarin; John Glenn; Jet Propulsion Laboratory (JPL); Johnson Space Center; Charles A. Lindbergh; National Aeronautics and Space Administration (NASA); Propulsion technologies; Rocket propulsion; Rockets; Russian space program; Alan Shepard; Space shuttle; Spacecraft engineering; Valentina Tereshkova; Konstantin Tsiolkovsky; Unidentified aerial phenomena (UAP); Jules Verne

Gravity and Flight Fields of Study: Physics; Aeronautical engineering; Astronautics; Pilot training; Mathematics ABSTRACT Gravity is an intrinsic property of matter and is interpreted as the force that all objects in the universe exert on all other objects as a result of their mass. The origin of weight and the cause of the downward acceleration (“falling”) of unsupported objects, gravity must be overcome by lift in order to sustain aerial flight, and must be properly exploited during spaceflight to successfully achieve orbit.

—Robert P. Ellis Further Reading Clary, David A. Rocket Man: Robert H. Goddard and the Birth of the Space Age. Hyperion, 2003. Goddard, Robert. Rockets: Two Classic Papers. Dover Publications, 2012. Heister, Stephen D., William E. Anderson, Timothée L. Pourpoint, and Joseph Cassady. Rocket Propulsion. Cambridge UP, 2019. Morgan, George D. Rocket Age: The Race to the Moon and What It Took to Get There. Prometheus, 2020. Spilsbury, Louise. Robert Goddard and the Rocket. Rosen Publishing Group Inc., 2015.

KEY CONCEPTS drag: the resistance to motion through a fluid due to friction between the moving object and the fluid medium free fall: unobstructed and uncontrolled motion from an altitude toward Earth’s center of gravity lift: the upward pressure experienced by wings due to differential pressure between upper and lower surfaces of the wing as it moves through a fluid medium as determined by its airfoil camber and thickness

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GRAVITATIONAL FORCE Physicists identify four fundamental forces that account for all known physical phenomena: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. Gravity is the weakest of the four, despite its overwhelming influence in everyday life, and it has a cosmic role in controlling the structure and evolution of the universe. Gravitation is dominant on a cosmic scale because it is long range, extending to infinity. The strong and weak nuclear forces, while much stronger than gravity, are of very short range and confined to the interior of the atomic nucleus. Electromagnetism also extends to infinity, but electric charges, which are the source of electromagnetic forces, come in both positive and

Photo via iStock/Georgethefourth. [Used under license.]

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negative forms that by and large cancel each other out, leaving only a relatively tiny net effect. Gravity, by contrast, is always attractive, therefore always additive and reinforcing. With a sufficient amount of mass, gravity can be made arbitrarily large. It is only because of the tremendous mass of Earth that gravity becomes the dominant force in everyday life. With respect to the normal flight of aircraft, the gravitational attraction of Earth must be overcome if an aircraft is to maintain its flight. This is the job of the wings and their airfoil. As the aircraft flies through the fluid medium of air, the air must separate before it to flow over the top and bottom surfaces of the wings. The cross-sectional shape of the wing—its airfoil—is such that the velocity of the air

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streaming over the upper surface of the wing is greater than that of the air flowing across the underside of the wing. In agreement with Bernoulli’s principle, the pressure underneath the wing is increased relative to its upper surface, and the air pressure atop the wing is decreased relative to the underside of the wing. This pressure differential is defined as the “lift” that counteracts Earth’s gravitational attraction. Sir Isaac Newton in 1684 recognized that the gravitational force between two widely separated bodies must be proportional to the mass of each and weaken as the square of the distance between them. The gravitational force of Earth on an object is called its weight, and Newton’s law states that one object twice the mass of another will weigh twice as much. It is on this basis that the mass of an object can be measured using devices such as balances and scales that actually determine weight. The law also specifies that weight will diminish with distance, so that objects at high altitudes will weigh less than they do at sea level. This loss of weight is real and easily measured with modern instruments. It must be stressed, however, that there is no corresponding loss of mass. Because gravity extends to infinity, weight never vanishes completely and there is no such thing as true weightlessness. What is typically thought of as “weight” is actually the counterforce of the ground that supports objects and prevents them from falling due to the gravitational force. The “weightlessness” experienced by astronauts in orbit is actually free fall. In the 1580s, Simon Stevin experimentally discovered that all objects fall in a gravitational field at exactly the same rate, a result whose importance was first recognized and widely disseminated by Galileo Galilei in 1638. Astronauts in orbit are continuously falling toward Earth, but the spacecraft enclosing them is falling in exactly the same direction at exactly the same rate. As there is no relative motion, the astronauts float in the cabin as though gravity has gone away.

Gravity and Flight

ORBITAL MOTION Orbital motion is a combination of free fall with a large velocity at right angles to the direction of fall. Absent the gravitational force, a satellite would move away from Earth in a straight line that would eventually carry it off toward infinity. The gravitational force pulls the satellite from this straight-line motion onto a path that curves around Earth and closes onto itself with each cycle around the planet. This is an orbit. Newton’s law of gravitation explains Johannes Kepler’s three laws of orbital motion: satellites travel in ellipses with the gravitational source (the primary) at one focus of the ellipse; a line joining the primary to the satellite sweeps out equal areas in equal times as the satellite moves around the orbit; and the cube of the average distance from the primary to the satellite is proportional to the square of the orbital period. Moving objects possess energy of motion called kinetic energy, equal to one-half of their mass multiplied by the square of their velocity. Satellites in orbit are continuously speeding up as they fall toward the primary, thereby gaining kinetic energy, and slowing down as they coast away from it, and so are losing kinetic energy. Since the total amount of energy in a system, kinetic plus potential, can never increase or decrease, the gain or loss of kinetic energy must be balanced by a gain or loss from another source called gravitational potential energy. The gravitational potential energy of two objects mutually attracted by a gravitational force is proportional to the product of their masses divided by the distance between them. An object in free fall decelerates as it coasts upward against the force of gravity, eventually coming to a stop when all of its kinetic energy has been converted to potential energy. It then starts to fall downward, converting potential energy back to kinetic energy and accelerating as it does so. Because the gravitational force weakens with distance, the

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amount of additional kinetic energy needed to reach ever-greater heights is limited. Objects moving fast enough to have kinetic energies that exceed this limit will never stop rising and will coast away from Earth forever. The velocity associated with this energy is referred to as the local escape velocity. Escape velocity at Earth’s surface is approximately 11 kilometers per second. The total energy of a satellite determines the size of its orbit, its orbital speed, and its orbital period through Kepler’s second and third laws. Satellites in low-Earth orbit travel at slightly less than 7.7 kilometers per second. The atmosphere at that altitude is extremely thin but still capable of exerting significant drag on objects traveling at such high velocities. Drag is a dissipative force which converts kinetic energy to heat and ordinarily slows objects down, but satellites under the influence of drag drop closer to the earth, converting potential energy to kinetic energy as they do so, and surprisingly end up traveling faster. When the total energy is no longer sufficient to maintain orbit, the satellite reenters the atmosphere. In order for the satellite to reach the ground at rest, all of its orbital kinetic and potential energy must be converted into heat. Temperatures become so great that the air around the reentering satellite becomes hot enough to glow. In uncontrolled reentry, too much of the heat builds up within the satellite and the satellite vaporizes, a fate common to small meteors. Crewed spacecraft control reentry and survive by discharging the heat overboard.

the Moon’s orbit increases in size a small but measurable amount. The orbital period of the Moon increases as a result, and the month gets slightly longer. Correspondingly, the drag of the tides on the ocean floor slows down Earth’s rotation, increasing the length of the day. Although Earth and the Moon appear to be made of hard, rigid rock, each is flexible enough to bend in response to their mutual gravitational attraction. This allows tides to rise in the rock itself. Rock tides on Earth contribute to the braking effect of the ocean tides, but are very small in comparison. Earth’s gravity also raises rock tides on the Moon, which have, over billions of years, slowed the Moon’s rotation down to the point that the length of the lunar day exactly equals the orbital period: one month. As a consequence, the Moon always keeps one face toward Earth, and humankind is only privileged to see the other side of the Moon through photographs taken from lunar orbit. This curious circumstance is called tidal locking and it is not at all rare. A majority of the natural moons in the solar system are tidally locked to their parent planet. Tidal locking is the inevitable result of the gravitational interaction of one flexible body orbiting another. When deliberately used by satellite designers to keep one end of an oblong satellite pointed toward Earth, it is referred to as gravity-gradient stabilization. (Space shuttle pilots put the shuttle into gravity-gradient stabilization during sleep periods so that noisy thruster firings to maintain attitude could be avoided.)

GRAVITATIONAL EFFECTS The gravitational pull of the Moon is felt daily in the rising of the tides. Additionally, as Earth rotates underneath the tides, it pulls the bulge of water from west to east, working against the attraction of the Moon. This produces a small tug on the Moon in the direction of its orbital motion and slightly increases the Moon’s total energy. As a consequence,

GENERAL RELATIVITY Although gravity was the first force to be mathematically described by physicists, it remains the least understood. Stevin’s and Galileo’s observation that all objects fall at exactly the same rate in response to the gravitational force inspired Albert Einstein in 1915 to go beyond Newton’s law of gravitation to propose the theory of general relativity. Based on

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Einstein’s theory of special relativity, which unites space and time into a four-dimensional universe, general relativity describes gravity as the result of localized space-time curvature in this four-dimensional universe. General relativity predicts that clocks at high altitudes will run faster than identical clocks at low altitudes. This prediction has been verified and this phenomenon had to be included in the design of the global positioning system (GPS) in order to achieve required accuracy and precision. Very accurate and stable atomic clocks flown on GPS satellites consistently run faster than identical clocks on the ground. General relativity also explains the cosmological expansion of the universe and the bizarre properties of black holes. The expansion of the universe was discovered by Edwin Hubble in 1925 through measurement of the frequency shifts of light emitted by distant galaxies. Almost all proved to be moving radially away from the Milky Way, with a speed of recession proportional to distance away: A galaxy twice as far away as another recedes from Earth twice as fast. This shocking phenomenon proved to be a direct and natural expectation of the general theory of relativity. Apparently, the universe originated billions of years ago in a “big bang” that flung matter outward in all directions. Over the course of billions of years, gravity pulled the matter into clumps out of which galaxies, stars, and planets formed. Because the galaxies attract each other gravitationally, the expansion should slow as time goes by. If the universe does not contain enough matter to make the local escape velocity of the galaxies everywhere greater than the current recession velocity, then the expansion will go on forever. If the universe does contain enough matter, then the expansion will eventually slow to a halt and the universe will contract back into a single mass, possibly to explode again and expand into a brand new and different universe.

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Black holes are objects whose surface gravity is so strong that in regions inside what is called the escape horizon, the local escape velocity is greater than the speed of light. As a basic tenet of special relativity is that nothing can travel faster than light, nothing that ever falls through the escape horizon can ever get out again. A second consequence is that anything that falls through the escape horizon continues to fall all the way to the center of the black hole where it and all other infalling matter are crushed to zero volume and infinite density. It appears that the laws of physics themselves cease to hold under these conditions. GRAVITY AND UNIFIED FIELD THEORY Certain aspects of general relativity have not been reconciled with quantum theory, the branch of physics that explains the behavior of objects at atomic and subatomic levels. Physicists have succeeded in uniting the theory of electromagnetism and the theory of the weak nuclear force into one theory of electroweak interactions. They are confident that eventually the theory of electroweak interactions and the theory of the strong nuclear force will be united into a grand unified theory. The ultimate quest of theoretical physics is a single theory uniting this eventual grand unified theory with general relativity, capable of explaining all four fundamental forces, and by extension, everything in the universe. —Billy R. Smith Jr. Further Reading Chown, Marcus. The Ascent of Gravity: The Quest to Understand the Force that Explains Everything. Orion Publishing Group Ltd., 2018. Clifton, Timothy. Gravity: A Very Short Introduction. Oxford UP, 2017. Federal Aviation Administration (FAA). Aircraft Weight and Balance Handbook FAA-H-8o83-1b. Aviation Supplies and Academics Inc., 2016. Newman, David. High G Flight: Physiological Effects and Countermeasures. CRC Press, 2016.

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Pletser, Vladimir. Gravity, Weight and Their Absence. Springer Singapore, 2018. Teitel, Ang Shura. Breaking the Chains of Gravity: The Story of Spaceflight Before NASA. Bloomsbury Publishing, 2015. See also: Aerodynamics and flight; Airfoils; Daniel Bernoulli; Flight propulsion; Fluid dynamics; Rocket propulsion; Space shuttle; Spacecraft engineering; Spaceflight

Greenhouse Gases Fields of Study: Atmospheric science; Physiology; Mathematics; Environmental studies ABSTRACT Carbon dioxide, methane, nitrogen oxides, sulfur hexafluoride, chlorofluorocarbons, and water vapor are the most important greenhouse gases. Over the past two centuries they have been emitted in increasing amounts as a result of human activities, including air transportation. KEY CONCEPTS anthropogenic: caused by or existing as a result of human activity carbon sink: any process or mechanism by which carbon dioxide is removed from the atmosphere and sequestered global warming potential: a measure of the extent to which a particular atmospheric gas is able to increase the average temperature of the atmosphere greenhouse gases (GHGs): atmospheric gases that are able to absorb infrared radiation emanating from Earth’s surface, then re-emit a large percentage of that heat energy back into the atmosphere and towards the surface while the remainder goes out into space CARBON FOOTPRINTS One method to measure human impact on climate is expressed in what is called carbon footprints. Instead of recording emissions associated with produc-

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tion, carbon footprints focus on emissions associated with consumption (of natural resources). A country’s carbon footprint summarizes emissions from imported goods that are produced elsewhere, as well as emissions associated with international transport and shipping. Such aspects are not accounted for in standard national inventories. According to the Carbon Dioxide Information Analysis Center and the United Nations Development Programme, in 2004 the average resident of the United States had a per capita carbon footprint of 20.6 tonnes of CO2 equivalent, some five to seven times the global average. Over the past century, ongoing measurements demonstrate that the temperature on Earth has increased by about 0.75 degree Celsius. As generally agreed among scientists, this increase tied to greater atmospheric concentration of key greenhouse gases (GHGs)—including carbon dioxide, methane, and nitrous oxide, as well as sulfur hexafluoride and chlorofluorocarbons (CFCs)—is due to human activities, and is considered to be the direct cause of the observed climate changes. Levels of several important GHGs have risen by about 25 percent since large-scale industrialization began around 200 years ago. Since the early 1990s, about three-quarters of anthropogenic (human-induced) emissions have come from the burning of fossil fuels. Concentrations of carbon dioxide in the atmosphere are naturally regulated by numerous processes, which together are defined as the carbon cycle. Human impact on the global climate since the Industrial Revolution has been difficult to interpret, given our incomplete understanding of how some factors operate and interact with changes in surface temperature. Also, while emissions of GHGs such as carbon dioxide and methane have had a net warming effect, emissions of sulfate aerosols from volcanism have had a net cooling effect. The movement of carbon between the atmosphere, land, and oceans is dominated by natural processes such as plant photosynthesis and weather.

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Greenhouse Gases

Photo via iStock/:Petmal. [Used under license.}

Although these processes can absorb some of the net 6.2 billion tonnes of anthropogenic carbon dioxide emissions produced each year—that is, 7.2 billion tonnes less approximately 1 billion tonnes reabsorbed by what are termed carbon sinks (areas such as forests that absorb carbon dioxide), as measured in carbon equivalent terms—an estimated 4.1 billion tonnes are added to the atmosphere annually. This positive imbalance between GHG emissions and absorption results in the continuous net increase in atmospheric concentrations of GHGs. Earth, understood as a physical system, has an energy budget that comprises incoming and outgoing energy. Thermal solar radiation is absorbed by the surface of Earth, causing it to warm. Part of the absorbed energy is then reradiated back to the atmo-

sphere as long-wave infrared radiation. Some of this reradiated energy escapes into space, and some is absorbed by atmospheric GHGs. Then, the GHGs re-radiate the thermal waves in all directions. Part of this re-radiation goes back toward Earth’s surface, transferred to the lower atmosphere again, resulting in higher temperatures. This mechanism differs from that of an actual greenhouse, in that the latter isolates warm air inside the structure so that heat is not lost by convection. Human activities since the Industrial Revolution—especially fossil fuel combustion, land-use change, increasingly intensive agriculture, and an expanding global human population—are primarily responsible for the recent steady increases in atmospheric concentrations of various GHGs, causing the atmosphere to warm.

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The global warming potential (GWP) of the known GHGs is a factor given in relation to that of carbon dioxide. Methane, for example, has a GWP of 25. That is, every kilogram of methane in the atmosphere has, on a timescale of 100 years, the equivalent global warming potential of 25 kilograms of carbon dioxide. Methane is much more effective than carbon dioxide at absorbing infrared radiation, but its lifetime in the atmosphere is shorter, about twelve years, compared to anywhere from 30 to 1,000 years for a molecule of carbon dioxide. The Intergovernmental Panel on Climate Change (IPCC) is an organization sponsored by the United Nations and made up of 2,500 scientists from around the world. Its 2007 report on global warming projected that global warming would have severe impacts on human health, natural ecosystems, agriculture, and coastal communities. However, an opposite view, taking longer periods into account, states that the global warming of the last century is part of the planets natural cycle driven by emissions by tectonic activities, wetlands, oceanic sources and sinks, and other factors. Currently, only one thing is certain: Human-produced GHGs have been emitted at extremely high rates. Fossil fuels including petroleum, coal, and natural gas, made up of hydrogen and carbon, release carbon dioxide and other GHGs upon combustion. One scientific forecast projects that during our children’s lifetimes, global warming will raise the average temperature of the planet by 1 to 3.5 degrees Celsius. In contrast, Earth is only about 3 to 6 degrees Celsius warmer today than it was 10,000 years ago, during the last ice age. Human-driven global warming is thus occurring far more quickly than warming at any other time in at least the last 10,000 years. Higher temperatures, changes in precipitation patterns, and acidification of the oceans may lead to reduction of the land and ocean carbon sinks, thereby unleashing a feedback-induced acceleration in the concentration of GHGs in the atmosphere.

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Another phenomenon to be considered is the so-called Little Ice Age period, which occurred between 1550 and 1850. Conditions around the world were cooler than usual; many bodies of water froze over. The average global temperature since then has risen by 0.5 degree Celsius, which some consider an argument against global warming. CARBON DIOXIDE Carbon dioxide (CO2) is a colorless, odorless, nonflammable gas. It is one of the many trace gases that naturally occur in the atmosphere, currently making up 0.0422 percent of the gaseous composition. It is considered to be the most important GHG because of its predominance in emissions yielded from combustion of fossil fuels. CO2 is produced naturally: as an exhalation product of many organisms, during the decay of organisms, in the weathering of carbon-containing rock strata, and as volcanic emissions. Forests and oceans are natural carbon sinks as the CO2 is recycled by photosynthesis. Besides the day-and-night change in plant respiration, plants absorb CO2 during their growth. Terrestrial ecosystems emit approximately 119 billion tonnes of carbon each year via the process of respiration and absorb approximately 120 billion tonnes of carbon each year via photosynthesis—a net sink of 1 billion tonnes of carbon. However, with the decline of forests, especially alarming in the tropics, less CO2 can be absorbed. Approximately 88 billion tonnes of carbon are emitted annually by the oceans, and about 90 billion tonnes of it are absorbed, a net sink of about 2 billion tonnes of carbon. Over the last 150 years, these sinks have absorbed about 40 percent of the CO2 emissions released by human activities. For as long as 600,000 years before 1750, the generally agreed date of the beginning of the Industrial Revolution, atmospheric levels of carbon dioxide were about 280 parts per million (ppm). Today, measurements show concentrations of CO2 in the at-

Principles of Aeronautics

mosphere of about 422 ppm, a 50.1 percent increment, chiefly due to industrial development. Besides the combustion of fossil fuels, yielding about 65 percent of the CO2 emissions, deforestation and land-use change have caused the remaining 35 percent of emissions. This is an estimated number that takes tree logging, charcoal production, slash-and-burn agricultural practices, pulpwood and fuelwood production, and forest degradation into account. Another important contributor is the clearing of new farmland and rangeland. Natural ecosystems can store 20 to 100 times more CO 2 per unit area than agricultural systems. Current land use activities in Africa, Asia, and South America contribute the greatest CO2 emissions due to deforestation. The fossil fuels, such as coal, oil, and natural gas, were created chiefly by the decay of plants from millions of years ago. Today they are used to generate electricity and produce heating energy to power factories, as well as provide many comforts of civilization. With energy stored in their hydrocarbon molecules, upon burning the energy is released—as is CO2. The World Energy Council reported that global CO2 emissions from burning fossil fuels rose 12 percent between 1990 and 1995. The United States was responsible for 25 percent of all emissions worldwide, the leading share at that time, with China on the cusp of overtaking that rank, which it did in 2006. Carbon footprint estimates and analyses can be useful in revealing the consequences of consumption activities to individuals, businesses, and societies. Still, it is emissions that can be most directly measured and around which national policy effects can be structured. The Organisation for Economic Co-operation and Development (OECD) in 2012 projected that world GHG emissions would rise by 37 percent above year 2005 levels by 2030, and by 52 percent above 2005 levels by the year 2050—if no new policies were adopted. The OECD projected that a CO2 atmospheric concentration of 450 ppm—well above historical maximums—could be

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achieved as a GHG stabilization target if countries reduced year 2050 GHG emissions by 39 percent below year 2000 levels. METHANE Methane (CH4) is a colorless, odorless, flammable gas. Its colloquial name, swamp gas, indicates that it is formed when plants, organic matter, decay under anaerobic (oxygen-free) conditions. Since 1750, methane emissions have doubled, and they could double again by 2050. Each year, 600 million tons of methane are emitted into the air by livestock (especially cattle), coal mining, drilling for oil and natural gas, crop production in rice paddy fields, and direct emissions from organic matter breaking down in landfills. The changes in agriculture and land use in response to the worlds growing population have caused additional methane emissions that are currently under research in the effort to find ways to reduce them. Frozen methane clathrate deposits found on the sea floor and in deep permafrost are additional natural sources of potential GHGs and have the potential for future exploitation and energy use, as well as the potential to boost rates of global warming. The potentially uncontrolled release of methane into the atmosphere could occur as permafrost areas warm; also, releases related to exploitation of these reservoirs could occur, and from slippage of the continental shelf once the supporting reservoirs are reduced. Recent scientific research has reported significant amounts of methane released into the atmosphere already from methane clathrate deposits found in the Arctic, due to global warming. NITROUS OXIDE Nitrous oxide (N2O) is a colorless gas with a slightly sweet odor. Its colloquial name is laughing gas; it is used as an anesthetic and has other uses as well. Occurring naturally, it is emitted from oceans and by bacteria in soils; industrially, it is mostly an exhaust

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product. The current 13 million tonnes of nitrous oxide gas emitted annually seems to be low compared to CO2 emissions, but N2O emissions have increased more than 15 percent since 1750, and they endure in the atmosphere for 100 years. This long atmospheric lifetime leads to a GWP of 298. The reduction of these emissions is a big challenge because today nitrous oxide is the most hazardous ozone-depleting substance released by human activities, since hydrochlorofluorocarbons (HCFCs) have been largely controlled. Most of the N2O added to the atmosphere each year comes from deforestation and the conversion of forest, savannah, and grassland ecosystems into agricultural fields and rangeland; and from the use of nitrate and ammonium fertilizer. In the past fifteen years, the use of nitrogen-based fertilizers has doubled, but plants absorb only 30 percent of the added nitrogen. In extreme cases, artificial fertilization can lead to the death of forests, eutrophication of aquatic biomes, and species extinctions. N 2O is also released into the atmosphere when fossil fuels and biomass are burned. In the future, it is estimated that N2O emissions will increase due to more agriculture activity related to the production of supposedly green biofuels. FLUOROCARBONS AND HALONS Fluorocarbons are a group of synthetic organic compounds that contain fluorine and carbon. Many of these compounds, including CFCs, have properties that are favorable for many technical applications. They are relatively nontoxic, nonflammable, odorless, and colorless, and they can be easily converted from gas to liquid or liquid to gas. In the 1970s, studies showed that when CFCs were emitted into the atmosphere, they destroyed the ozone layer in the stratosphere. These compounds are highly resistant in the atmosphere, which makes them thousands of times more potent as GHGs than carbon dioxide. Currently, all halogenated hydrocar-

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bons together add 0.337 watt per square meter to the greenhouse effect. Chlorodifluoromethane (HCFC-22), is, after carbon dioxide and methane, the third most important anthropogenic greenhouse gas. The Montreal Protocol, an international agreement adopted in 1989, which phases out ozone-depleting substances, requires the end of HCFC-22 production by 2020 in developed countries and by 2030 in developing countries. CFCs and HCFCs are being replaced by less-damaging hydrofluorocarbons (HFCs) for such widespread uses as the coolant in air conditioners and refrigerators. Halons are organic compounds containing bromine, derived from methane or ethane. They were used as extinguishing agents. Like CFCs, they are ozone-depleting substances, and their destructive potential is 10 times greater than that of CFCs. Since 1994, their production has been banned globally, with an extended deadline for developing countries. The Montreal Protocol described a special regulation for methyl bromide, enacting a global successive phaseout of its production by 2005. FUTURE EFFECTS AND STRATEGIES The long-term environmental problems resulting from the increased atmospheric concentration of GHGs are intertwined, and pose a huge ecological challenge. One GHG, greenhouse water vapor, is increasing in the atmosphere. Water vapor has an immense greenhouse effect due to its great capacity to absorb thermal energy and lock it within the atmosphere. That causes further warming, leading to still more water vapor rising from ocean surfaces; this positive feedback loop leads to a more intense greenhouse effect. Consequences are likely to include more and stronger tropical cyclones, as well as changing ocean current patterns, the potential for more tidal waves or tsunamis, and faster erosion along the coastlines. Besides such looping effects, there are one-way developments that escalate with each step and di-

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rectly cause secondary effects. Melting glaciers and polar ice caps, more severe floods and droughts, and rising sea levels (on average between 4 and 10 inches since 1990) are considered in this manner. Rises in sea level, for example, can increase the salinity of wetlands and upstream freshwater, and endanger coastal lands and communities. Global warming is also regarded as introducing health concerns, such as the spread of tropical diseases into temperate zones. The Kyoto Protocol, adopted in 1997 as part of the United Nations Framework Convention on Climate Change, was the first international treaty to establish carbon emissions reduction programs. However, it did little to enforce the parties meeting their emissions targets. The United States, until recently the leading emitter of GHGs, has not ratified the Kyoto Protocol, while China, now the leading emitter, retains its status as a developing country and is therefore not required to reach the strict targets imposed on developed nations. One core strategy for reducing greenhouse gas emissions is to enhance energy-saving technologies while generating more energy from renewable sources, thereby reducing the demand for fossil fuels. By the year 2050, renewable sources could provide 40 percent of the energy needed in the world. Use of renewable energy can help to slow global warming even while reducing air pollution. The World Business Council for Sustainable Development Greenhouse Gas Protocol, a joint initiative of the World Resources Institute and the World Business Council for Sustainable Development, has become the most commonly used international accounting framework for government and business leaders to understand, quantify, and manage greenhouse gas emissions.

Further Reading Agarwal, Avinash Kumar, Narasinha Shurpali, and V. K. Srivastava, editors. Greenhouse Gas Emissions: Challenges, Technologies and Solutions. Springer Nature Singapore, 2018. “Emissions of Potent Greenhouse Gas Increase Despite Reduction Efforts.” US Department of Commerce, 2010, www.noaanews.noaa.gov/stories2010/20100127_ greenhousegas.html. de Mestral, Armand L.C., Md. Tanveer Ahmad, and P. Paul Fitzgerald, editors. Sustainable Development, International Aviation, and Treaty Implementation. Cambridge UP, 2018. Hansen, James. Storms of My Grandchildren: The Truth About the Coming Climate Catastrophe and Our Last Chance to Save Humanity. Bloomsbury Press, 2009. “The Kyoto Protocol on Climate Change.” US Department of State, www.state.gov/www/global/oes/fs_kyoto_ climate_980115.html. Lankford, Ronald D. Greenhouse Gases. Greenhaven Press, 2009. Llamos, Bernardo, and Juan Pour. Greenhouse Gases. IntechOpen, 2016. McCarthy, James E., Cianni Marino, Nico Costa, Larry Parker, and Gina McCarthy. EPA Regulation of Greenhouse Gases: Considerations and Options. Nova Science Publishers, 2011. Piera, Alejandro José. Greenhouse Gas Emissions from International Aviation: Legal and Policy Challenges. Eleven International Publishing, 2015. Reddy, D. R., and Emily S. Nelson, editors. Green Aviation: Reduction of Environmental Impact Through Aircraft Technology and Alternative Fuels. CRC Press, 2018. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Avery, M. Tignor, and H. L. Miller, editors. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. Cambridge UP, 2007. See also: Air transportation industry; Atmospheric circulation; Aviation and energy consumption; Contrails; Rocket propulsion

—Manja Leyk

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H Heavier-than-air Craft Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT A heavier-than-air (HTA) vehicle is one driven through the air by a self-carried power source, supported by air pressure against the wings or rotors, and controlled in flight path and destination by the pilot. HTA craft, such as airplanes and helicopters, are faster, more controllable, and safer than lighter-than-air (LTA) craft, and thus have become the dominant instrument in aerial transportation and warfare. KEY CONCEPTS chassis: the basic frame of an airplane (or other type of vehicle) monocoque: an aircraft in which the chassis is integrated with the body ornithopter: aircraft that were designed to imitate birds by using the principle of flapping wings as the means of propulsion ornithoptic propulsion: flying in the manner of birds, by the flapping of wings EARLY EXPERIMENTS In religions, mythologies, legends, and imaginings, human levitation and flight are old and familiar concepts. Birds, bats, and insects were visible proof that flying through the air with wings was possible in nature, and for centuries humans imitated birds by attempting to fly with flapping, birdlike wings carried by human arms. These “ornithopters,” frequently launched from hillsides, towers, or barns,

formed a long and frequently farcical or fatal tradition in humankind’s attempt to fly. Some early Greek physicists appreciated that a jet of compressed air could be a motive force, but saw no practical way to achieve this. During the Renaissance, Leonardo da Vinci sketched out a few ideas regarding helicopters and propellers, but with no suggestion for a power source. Still, the evidence that moving air could exert a tangible and usable force on sails, kites, and windmills was plain enough. In the eighteenth century, some “whirling arm” experimenters, such as John Smeaton, began to quantify the lift and drag forces exerted by moving air upon flat surfaces. In France, Launoy and Bienvenu devised a model helicopter in 1784. Late eighteenth-century technology developed steam as a power source, but by 1783 Jacques-Étienne Montgolfier’s balloons had captured public interest and also had given the French the premier place in aeronautics development. PRACTICAL APPLICATIONS Significant heavier-than-air (HTA) research was done in early nineteenth-0century England by Sir George Cayley, an inventor, scholar, and publicist whom many authors describe as “the father of modern aviation.” Cayley’s studies and experiments confirmed that a curved-wing, or cambered, surface supplied more lift than did a flat one, that reduced pressure on the upper surface and increased pressure on the lower surface exerted considerable lift, and that air pressure on an adjustable plane surface in an airstream varied in extent and location. He drew attention to the problem of stability, and also built model helicopters and gliders. One glider was

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capable of supporting his coachman in a short airborne hop. The pioneer pilot’s verdict was “Please, Sir George, I wish to give notice. I was hired to drive, not to fly.” Cayley’s extensive publications were not widely known in his lifetime, but they had later influence. Cayley’s English followers, such as William Samuel Henson and John Stringfellow, attempted an aerial steam carriage, but of greater importance was the first wind tunnel, built in 1871 by Francis Herbert Wenham and John Browning. The French dominated aeronautical study and experimentation in the nineteenth century, but their more elaborate machines were less successful than a device of great simplicity. In 1871, Alphonse Pénaud

Principles of Aeronautics

employed twisted rubber as a power source for a model aircraft. His “planophore” was a stick fuselage holding curved and angled monoplane wings with their extremities tipped up, a vertical rudder, and a pusher propeller at the rear, powered by a twisted rubber band directly under the fuselage. In an apparently simple toy, Pénaud incorporated the essentials of airplane structure and flight, including lift, inherent stability, and elementary vertical and horizontal control. The major challenge remained to find a better power source. One key to the progress of aviation was the development of the internal combustion engine by Nikolaus August Otto, Gottlieb Daimler, and Carl Benz.

Aerobatic glider with tip smoke, pictured on July 2, 2005, in Lappeenranta, Finland. Photo by Dave S. from Witney, England, via Wikimedia Commons.

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GLIDERS AND POWERED CRAFT In the 1890s, hang gliding was greatly developed and popularized by the exploits of Germany’s Otto Lilienthal. The author of Des Vogelflug als Grundlage der Fliegekunst (1889; Bird Flight as the Basis of Aviation, 1911), Lilienthal believed that gliders copying bird wings would lead to successful powered flight. From 1891 to 1896, he built five monoplane gliders and two biplane gliders and made two thousand flights with them, measuring lift and drag. These glides of up to 229 meters in distance drew spectators, reporters, and photographers. The “German bird-man” was a hero to the air-minded, especially in the United States, and remained an inspiration even after his August 9, 1896, fatal crash. Other European aviation pioneers were concentrating on powered flying machines. Alexander Feodorovich Mozhaiski attempted a steam-powered hop in 1884. In the 1890s, Victor Tatin and Charles Rivet built a steam-powered model plane, which in one test flew about 140 meters. Clément Ader claimed to have flown about 50 meters in 1890 in his steam-powered Eole and to have surpassed this distance on October 14, 1897, with a flight of 300 meters in his government-financed Avion III. Whether this was a continuous flight or the total length of a series of hops in unclear, but the French army observers were less impressed than Ader was, and the project was dropped. In the 1890s considerable press attention was given to the construction and testing of a £30,000 steam airplane by Sir Hiram Maxim. It had a lifting area of 371.6 square meters, two 180-horsepower steam engines, twin propellers of 5.4 meters, a 549-meter launching track, and a total weight of 3,629 kilograms. On July 31, 1894, with a steam pressure of 145 kilograms per square inch (6.45-square centimeters), this monster barely left the ground, colliding with the guard rails. Maxim’s craft had ample power, but lacked all the other requirements for flight. This experiment was not pur-

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sued further and made no advance in aviation technology, but it did keep attempts to fly in the public mind. At the turn of the century, European aviation interests were turning to semirigid powered airships of increasing size, culminating in Germany’s Graf zeppelins. Hang gliding was continued, however, by Percy Pilcher, a Lilienthal disciple and English engineer. Pilcher was briefly joined by Lawrence Hargrave of Australia for testing some of the latter’s box-kite designs. Pilcher’s career was ended by a fatal crash in 1899. The next major experiment in HTA flight was made in America. AMERICAN EXPERIMENTS The gliding school of aviation in America was continued, encouraged, and publicized by Octave Chanute, a French-born American civil engineer. He improved glider design, using the ideas of Lilienthal, Pilcher, Hargrave, and others. Collecting information on past and current aviation experiments in the United States, France, and England, he developed the Chanute biplane glider using the Pratt truss used in bridge building. Augustus Moore Herring acted as Chanute’s assistant and pilot for several hundred glides launched from the Indiana dunes in 1896 and flew up to 107 meters. In 1900, Chanute was contacted by the Wright brothers and gave them information and encouragement, while he was also in communication with the telephone inventor Alexander Graham Bell and the Smithsonian secretary Samuel Pierpont Langley regarding their own aviation projects. Thanks largely to Chanute, meetings and publications began to connect American aeronautical researchers into an informal group of scientific minds. Langley, secretary of the Smithsonian Institution and respected in academic circles as America’s leading expert in aeronautic science, succeeded in the 1890s in constructing steam-powered model airplanes. In 1898, during the Spanish-American War,

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he succeeded in gaining a grant from the US Army for building a human-lifting, power-driven, controllable airplane. The result was the Pénaud-type aerodrome, with tandem wings, a tailpiece rudder, and twin-pusher propellers driven by a water-cooled gasoline engine of radial design, with five cylinders providing 52 horsepower. On October 7, 1903, at Widewater on the Potomac River, witnessed by officials and the press, the 385.5-kilogram craft, with Charles Manly as pilot, was propelled from the roof of a houseboat, and in the Washington Post‘s description, “simply slid into the water like a handful of mortar.” The New York Times decided that a practical flying machine “might be evolved . . . in from one to ten million years.” After a repetition of this failure on December 8, one congressman described Langley’s aerodrome as a “mud duck which will not fly fifty feet.” The US Army quickly canceled Langley’s project, and he died in 1906 a disappointed man. However, the Smithsonian Institution until 1948 prominently displayed the great aerodrome as “the first aircraft in history capable of flight with a pilot and several hundred pounds of useful load.” THE FIRST SUCCESSFUL FLIGHT Wilbur Wright and his brother Orville were bachelors, living with their father, Milton Wright, a bishop in the United Brethren Church, and their sister Kate in Dayton, Ohio. The brothers operated a shop for building, selling, maintaining, and repairing the popular safety bicycles of the 1890s. Their joint interest in aviation may have been sparked by a childhood gift of a toy helicopter. It was certainly inspired by Otto Lilienthal, whose personal role in practical gliding they admired, and whose inductive, step-by-step approach to airplane design they followed. The Wrights were competent enough in algebra, solid geometry, trigonometry, and physics to understand the aeronautical problems involved in aviation, and as practical mechanics they were able

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to do most of the production themselves, saving expense and minimizing errors. They attacked the task in stages, concentrating first on the problem of wing lift, then on mastering flight control, and finally on adequate propulsion. In May, 1899, Wilbur Wright wrote to the Smithsonian Institution requesting titles of books and articles on flying, and a current list was sent to him. The following August, the Wrights built their first aircraft, a biplane box kite 1.5-meters wide, with a fixed-tail plane, in order to test wing twisting, later called wing warping, as a method of controlling side roll. In May, 1900, Wilbur wrote to Chanute to exchange ideas on gliding, and the Wrights’ later gliders somewhat resembled Chanute types. In September, 1900, the off-season in the bicycle trade, the Wrights took a camping vacation at Kitty Hawk, a sparsely inhabited stretch of sand dunes and mosquitoes on the Outer Banks of North Carolina. Here they flew their Glider I, mostly as a kite. The following year, a larger model, Glider II, failed to achieve the lift and drag results reported by earlier experimenters. The Wrights decided to check existing aeronautic tables with their homemade wind tunnel. These tests indicated that the Smeaton coefficient and the Lilienthal and Chanute tables from which they had been working were significantly inaccurate. Developing their own (confidential) tables, the Wrights built their successful 1902 Glider III. This craft included the mechanical linkage of wing warping to rear rudder control, which formed the chief basis of their 1902 patent application, granted in 1906. By a process of research, experiment, and checking for flaws, the Wrights developed an air frame which solved the problems of lift and flight control. The Wrights were then ready to attempt powered flight in 1903. Much of their 1903 season, however, was consumed by problems and delays. Not finding a gasoline engine meeting their lightweight, high-power needs, they designed their own four-cylinder, wa-

Principles of Aeronautics

ter-cooled, in-line engine, weighing about 68 kilograms, producing 12 horsepower, and linked by bicycle chains to a pair of pusher propellers. Marine propellers being entirely unsuitable, the Wrights used their wind tunnel to design propellers as “moving wings” traveling in a forward spiral. Altogether, testing the new machine, Flyer I, at Kitty Hawk was delayed until December 17, 1903. That day’s consecutive flights were Orville’s initial hop of 36.5 meters Wilbur’s of 53.3 meters, Orville’s flight of 61 meters, and Wilbur’s flight of 260 meters into a wind of 20 to 27 miles per hour for 59 seconds. These straight-line distances at a low level were not revolutionary, but to take off and be airborne under power for nearly a minute was new in the annals of aviation. There were photographs and five witnesses, but the press generated only a few garbled reports. In 1904, the Wrights practiced on a new Flyer II with a slightly larger engine, flying at Huffman Prairie near Dayton. These low-altitude flights culminated in successful circles and, on November 9, a flight of five minutes. The 1905 Flyer III had a wing area of 46.7 square meters, a 12.34-meter span, and wing camber of 1 in 20. Its wings were horizontally flat, with a built-up elevator and rudder, and with an engine of about 20 horsepower. Another series of Huffman Prairie flights included one of 38.6 kilometers in 38 minutes. The local audience and photographs increased, and as one foreign visitor put it, “Dayton knows the Wrights fly, but America isn’t sure.” The 1905 Flyer III represented the completion of the Wrights’ project to build a human-carrying, powered flying machine capable of controlled flight. The Wrights offered the plane to the US Army, then the British, French, and Germans. Their asking price of $250,000 or more was too steep for the war departments, who shrewdly suspected that the Wrights were reluctant to demonstrate their machine for fear of easy copying. Octave Chanute’s 1903 Paris lecture on the Wrights’ gliding experi-

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ments, Wilbur Wright’s US lectures, and visits to the Wrights by European observers gradually spread the conviction that powered aviation was indeed at hand. IMPROVING THE Wright Flyer Meanwhile, powered glider hops, particularly of box-kite construction types, increased in Europe. Some models were advertised for sale as “Wright-type flyers.” The popular Brazilian sportsman Alberto Santos-Dumont was hailed for his 1906 flight at Bagatelle, France, as the “the first to fly.” The Wrights brought a flyer plane to Europe in 1907, but left it in storage, deciding that in 1908 Orville would compete for a US Army contract, while Wilbur would demonstrate the model which they left in France. In 1908, Orville won the US Army contract to considerable public acclaim, while in France, Wilbur had a Cinderella experience. Ridiculed for weeks for his lengthy delays in assembling and repairing the stored plane, Wilbur’s August 8 demonstration flight at Le Mans, with circles, figure eights, and graceful landings under complete control, came as a revelation to Europeans who had not gotten beyond short, straight-line hops. Aviators, press, and public hailed Wilbur Wright as a hero and companies were quickly formed in France, Britain, and Germany to build Wright biplanes under license. The 1908 Wright Flyer clearly outclassed its European counterparts in construction, performance, and ability to control. However, at the Rheims air exhibition of 1909, there were several French types which had improved on the Wright Flyer. Henri and Maurice Farman offered stable biplanes, and Louis Blériot showed the monoplane type with which he would cross the English Channel to become the French hero of the year. Gabriel Voisin promised quick delivery and reliable construction. Léon Levavasseur’s Antoinettes were becoming popular. Glenn H. Curtiss upheld the United States’ reputa-

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tion by winning the Gordon Bennet Speed Trophy. All these represented some form of advancement over the 1908 Wright machine. Several nations also established airplane sections in their armies in 1909. The year 1910 saw a great increase in the number of airplane manufacturers, but a more modest growth in airplane sales. Clearly, even the largest firms would not survive without large government orders for military purposes, so patriotic public agitation was organized to that end. This brought about a major change in production types. Pre-1914 war departments wanted planes that excelled in range, stability, load, and altitude, solid and simple in design, built for careless handling with easy maintenance and repair under wartime conditions. From 1911 on, Europe’s war departments were deciding which plane types and which manufacturing firms would survive, and trying to find a remedy for the French predominance in the light engine market. By 1913, airplanes had wheeled landing gear, more efficient tractor propellers were replacing pusher types, and cantilevered wings were the key to larger monoplanes. Monocoque fuselage construction made possible the airliners of the future, and ailerons were beginning to replace wing warping, which would clearly not be practical with the heavy wings of a large plane. Also, Igor Sikorsky had already built a four-engine plane and would later build a practical helicopter. None of these improvements on the Wright Flyer matched the difficulty or importance of the problems of flight which the Wright brothers had solved, but they marked modern aviation as a field of constant and rapid change. —K. Fred Gillum Further Reading Christienne, Charles, and Pierre Lissarague. A History of French Military Aviation. Smithsonian Institution Press, 1986. Crouch, Tom D. A Dream of Wings: Americans and the Airplane, 1875-1905. W. W. Norton, 1981.

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Gibbs-Smith, Charles Harvard. Aviation: An Historical Survey from Its Origins to the End of World War II. Her Majesty’s Stationery Office, 1970. Goldstone, Lawrence. Birdmen: The Wright Brothers, Glenn Curtiss, and the Battle to Control the Skies. Random House Publishing Group, 2015. Harwood, Craig S., and Gary B. Fogel. Quest for Flight: John J. Montgomery and the Dawn of Aviation in the West. U of Oklahoma P, 2012. Holcombe, Colin. The Story of Flight. Colin Holcombe, 2020. Tobin, James. To Conquer the Air: The Wright Brothers and the Great Race for Flight. Free Press, 2012. See also: Aerodynamics and flight; Aeronautical engineering; Airfoils; Airplane manufacturers; Airplane propellers; Flight propulsion; Glider planes; History of human flight; Monoplanes; Wright brothers’ first flight; Wright Flyer

Helicopters Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Helicopters, often referred to as choppers, helos, whirlybirds, or copters, are any rotary-wing aircraft having powered, fixed rotors that provide lift and propulsion for the aircraft. The helicopter was the first operational vertical takeoff and landing (VTOL) aircraft and remains the most prevalent. KEY CONCEPTS Coanda effect: the tendency of a jet of air to adhere to a curved surface and to entrain adjacent air thus creating a region of low pressure fixed-wing aircraft: aircraft having the traditional structure of a fuselage and nonmoving wings on either side of the fuselage rotary-wing aircraft: aircraft in which the function of wings and propellers has been replaced by the operation of spinning blades on a rotor

Principles of Aeronautics

swashplate: a mechanical device on the rotor hub of a helicopter that translates control movements from the pilot’s control stick into various pitch adjustments of the rotor blades HELICOPTER CONFIGURATIONS The helicopter is the principal type of vertical takeoff and landing (VTOL) aircraft in service throughout the world. The name “helicopter” was coined by Viscomte Gustave de Ponton d’Amecourt, circa 1863. Helicopters can be distinguished from other rotary-wing aircraft by the fact that their rotors are fixed in position on the aircraft fuselage and simultaneously provide lift and propulsion. The vast majority of modern helicopters have either one or two rotors that provide lift and propulsive force. Although helicopters can take off and land vertically, their maximum forward speed is much lower than that of fixed-wing aircraft. This limitation is due to the fact that the rotor or rotors must provide both propulsion and lift. Under high-speed flight conditions, the vibratory forces on the rotor blades become very large, thereby limiting the top speed of the helicopter. In order to increase the top speed, some helicopters, known as compound helicopters, have been equipped with auxiliary means of propulsion, such as propellers or jet engines. Helicopters are built in a variety of configurations, including the single-rotor, the tandem, the coaxial, and the side-by-side helicopters. The single-rotor helicopter is the most common configuration currently in use. It can be identified by the single main rotor that provides thrust and propulsion, as well as pitch and roll control. A smaller tail rotor usually provides antitorque directional yaw control. However, other devices may be used instead of a tail rotor. Another common configuration is the tandem helicopter. The tandem helicopter has two large rotors, one at the forward end of the helicopter and the other at the aft end. The two rotors rotate in op-

Helicopters

posite directions, thus eliminating the need for an antitorque device, such as a tail rotor. This configuration is particularly well-suited for the transport of heavy cargo, because the two rotors can accommodate large changes in the aircraft center of gravity due to the cargo load. Less common configurations include side-by-side and coaxial helicopters. Like the tandem helicopters, side-by-side helicopters have two main rotors, but one is located on the right side of the aircraft, and the other is located on the left side. The rotors rotate in opposite directions, again eliminating the need for a tail rotor. A variant of the side-by-side helicopter is the synchropter, on which the two rotors are placed close together, so that the rotors intermesh. The synchropter has the advantage of being able to take off and land in more confined areas than either a side-by-side or tandem helicopter, because the close proximity of rotor masts reduces the area required for clearance around the rotors. The coaxial helicopter has two counterrotating rotors that share a common mast. Because the rotors rotate in opposite directions, no tail rotor is needed for this configuration either. Coaxial helicopters also have the advantage of being able to land in more confined areas than any other configuration, because the swept area of the rotors is the smallest of all configurations. HISTORY Although the development of an operational helicopter is a relatively recent accomplishment, many of the concepts necessary for designing a practical helicopter have been known for a very long time. In fact, one could argue that a maple seed falling from a tree is nature’s model for the helicopter. The Chinese top, which predates the Roman Empire, is perhaps humankind’s first step toward modern helicopters. In addition, Leonardo da Vinci considered the possibility of vertical flight, and made sketches of his concept for such a vehicle.

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Helicopter. Photo by Ronnie Robertson, via Wikimedia Commons.

The development of a practical helicopter was made possible by overcoming three major technological barriers. The first barrier, and the easiest to overcome, was the design of a rotor system with rotor blades and a rotor hub that were strong but lightweight, with adequate aerodynamic efficiency. The second was to engineer a power plant with a sufficiently high ratio of power to weight, required in order to lift the aircraft off the ground. This barrier was overcome with the invention of the internal-combustion engine. The third technology barrier was to devise a method for controlling the helicopter in flight. The principles leading to controlled helicopter flight were developed gradually by helicopter pioneers.

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Early helicopter pioneers tried a variety of power plants in their helicopter designs. During the latter half of the eighteenth century, Mikhail Vasilyevich Lomonosov in Russia, Launoy and Bienvenu in France, and Sir George Cayley in England provided power to their helicopters by using different spring mechanisms. While spring-driven power plants have a good ratio of power to weight, they cannot provide sufficient sustained power for long flights. In the nineteenth century, steam-powered helicopters were designed by Horatio Frederick Phillips in England, d’Amecourt and Alphonse Pénaud in France, Enrico Forlanini in Italy, and Thomas Edison in the United States. In contrast to spring power, steam power could provide sufficient sus-

Principles of Aeronautics

tained power, but its ratio of power to weight was very low. Like that of the airplane, the concept of the helicopter did not become truly feasible until the invention of the internal combustion engine. Developments leading to a practical helicopter began to be achieved not long after Orville and Wilbur Wright flew their first airplane, but the availability of an adequate power plant brought problems of control to the fore. Paul Cornu and Charles Renard in France, Emile Berliner and Henry Berliner in the United States, and Igor Sikorsky and Boris Yuriev in Russia made significant contributions prior to 1920. Renard introduced the flapping hinge, which improved rotor control; and Yuriev introduced the antitorque tail rotor for yaw control. In 1907, Cornu made the first piloted, free-flight, vertical takeoff, but the aircraft had to be stabilized manually by a ground crew. In the 1920s and early 1930s, George de Bothezat in the United States, Etienne Oemichen and Louis-Charles Breguet in France, Raoul Pescara in Spain, the Berliners in the United States, Louis Brennan in England, A. G. von Baumhauer in Holland, and Corradino D’Ascanio in Italy, M. B. Bleeker in the United States, and Yuriev in Russia all built prototype helicopters. Unfortunately, all of these designs either had controllability problems or were too complex to be practical. However, important contributions toward improved control were made by Bothezat, in differential collective pitch control; Pescara, in cyclic pitch control; von Baumhauer, in the area of the swashplate; and d’Ascanio, in servotab cyclic pitch control. In 1936, German aircraft designer Heinrich Focke introduced the first practical helicopter, the Focke-Achgelis Fa-61, a side-by-side design in which all of the stability problems had been solved. In 1938, Hanna Reitsch flew the Fa-61 inside the Deutschland-Halle in Berlin, demonstrating its flying precision. In 1939, in the United States, Igor Sikorsky introduced the VS-300, a single-rotor heli-

Helicopters

copter, which may have been the world’s first useful helicopter. Germany continued its development of the helicopter during World War II, and Anton Flettner’s synchropter design, the FL-282 Kolibri, became the first production helicopter. At about the same time, other individuals, including Arthur Young, Frank Piasecki, and Stanley Hiller in the United States, and Nikolai Kamov, Mikhail Mil, and Ivan Bratukhin in the Soviet Union were developing their own independent helicopter designs. MODERN HELICOPTERS The basics of helicopter design have not changed greatly since the early days of helicopters in the 1940s. However, technological improvements have been incorporated that make the modern helicopter safer, easier, and more efficient to fly. One of the most significant advances in helicopter performance resulted from the introduction of the gas-turbine engine. The maximum power-to-weight ratio achievable with piston engines by the end of World War II was approximately 1 horsepower per pound. However, by the 1960, turbine engines had achieved power-to-weight ratios of 3 horsepower per pound, and by 2000 they had achieved weight ratios of up to 6 horsepower per pound. Helicopter rotor systems have also undergone significant changes. In the early years, rotor blades were made exclusively of wood, one of the principal materials used for aircraft construction. In 1944, Hiller introduced metal rotor blades on the XH-44, but it was not until 1952 that metal blades were delivered on a production aircraft, the Sikorsky S-52. The use of composite materials for rotor blade construction began in the early 1960s, and, by the 1970s, the Messerschmitt-Bölkow-Blohm company in Germany had built all-composite blades for the BO-105 helicopter. Virtually every modern helicopter is now equipped with composite blades. The rotor hub has also undergone changes in the way that

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the blades are attached. Many helicopter rotors are fully articulated. That is, each blade has physical hinges, which allow the blade to flap out of the plane of rotation and lag in the plane of rotation. A bearing also allows the blade to pitch. The concepts of a hingeless rotor that eliminates the flap and lag hinges and a bearingless rotor, which is basically a hingeless rotor without a pitch bearing, have found their way into the designs of many modern helicopters. Technological improvements, such as vibration control devices in the rotor system and the fuselage, have improved the comfort level for passengers, as well as the performance of the flight crew due to reduced fatigue. Crash-worthy structural design, seats, and fuel systems have improved the safety of helicopters in emergency situations. Hydraulic control systems have replaced the mechanical control systems of early helicopters, and modern helicopters are often equipped with electronic flight control and stability augmentation systems to reduce pilot workload. Digital fly-by-wire and fly-by-light control systems, as well as glass cockpits, have been introduced in advanced production helicopters. The late twentieth and early twenty-first centuries also saw an explosion in the development of unmanned aerial vehicles (UAVs), or drones, many of which used helicopter designs. Pioneered by militaries and operated with sophisticated remote-control systems, improved technology and miniaturization as well as declining costs soon led to consumer versions. Production models range from fairly large craft capable of various operations to handheld toys meant simply to fly a few feet in the air. FLIGHT CONTROL One of the first problems of helicopter flight control that must be solved is the question of how to keep the fuselage from rotating opposite the rotor. In order to spin the rotor, torque is applied by the engine to the rotor driveshaft. Therefore, the rotor has an

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angular momentum, which must be counteracted in some manner. If the angular momentum of the rotor is not equalized, the fuselage will begin to rotate in the opposite direction to the rotor rotation. Single-rotor helicopters equalize the angular momentum with countertorque devices, such as a tail rotor or a NOTAR (no tail rotor) system. The tail rotor is a smaller rotor mounted vertically at the end of a tail boom that generates a lateral thrust. The NOTAR system also generates lateral thrust but does so using the slipstream of the rotor and air ejected from a slot in the tail boom to produce the Coanda effect. Helicopters with more than one rotor, such as the tandem, side-by-side, and coaxial types, equalize the angular momentum by employing equally sized rotors rotating in opposite directions. In order to fly a helicopter, the pilot must be able to control the translation of the aircraft in the vertical, lateral (side-to-side), and longitudinal (forward-and-back) directions, as well as rotation in roll, pitch, and yaw. The pilot’s controls include a collective lever beside the pilot seat, a cyclic stick between the pilot’s knees, and foot pedals. It is interesting to note that in helicopters, the pilot sits in the right seat and the copilot sits in the left. In fixed-wing aircraft, the pilot sits in the left seat, and the copilot sits in the right. This seating arrangement is an artifact from one of Igor Sikorsky’s early helicopters, which had such “backward” seating. To explain helicopter control, consider a single-rotor helicopter. The main rotor of a single-rotor helicopter produces a thrust, which acts in a direction roughly normal to the rotor disk. Therefore, in order to control the helicopter, the pilot must be able to control the magnitude and direction of this thrust. The magnitude of the thrust is controlled by the collective lever, which equally increases or decreases the pitch angle of all rotor blades, thereby increasing or decreasing the thrust. In order to control the direction of the thrust, the pilot must be

Principles of Aeronautics

able to control the orientation of the rotor disk. One way to change the orientation of the rotor disk is to physically tilt the rotor hub. For very small helicopters, hub tilt is a practical control method. However, for larger helicopters, the rotor acts like a large gyroscope, which makes tilting the hub extremely difficult. The alternative is to increase the thrust on one half of the disk, while simultaneously decreasing the thrust on the other half. This cyclic change in thrust causes the rotor disk to tilt and does so with much less effort than hub tilt. In all but a few modern helicopters, the pilot’s cyclic stick, acting through a swashplate, is used to change the cyclic pitch of the rotor blades. The swashplate consists of two parts: a nonrotating plate and a rotating plate. The nonrotating plate, which is connected to the pilot collective and cyclic pitch controls, slides up and down for collective-pitch changes and tilts for cyclic-pitch changes. The rotating plate sits on top of the nonrotating plate and spins with the rotor. Pitch links, attached to the rotating plate and the rotor blades, mechanically change the pitch angle of the blades. Yaw control is obtained through the foot pedals, which are connected to the collective pitch controls for the tail rotor. MISSIONS Because helicopters are able to take off and land vertically and hover in midair, they are ideal vehicles for a wide variety of missions. They do not require large runways or prepared landing areas, so they can take off and land in forest clearings, on the tops of buildings, and on ships at sea. As a result, they can be used in civil and military applications for which fixed-wing aircraft are unsuitable. The transportation of passengers is one of the primary missions of helicopters. The largest civilian user of helicopter transportation is the petroleum industry. Helicopters regularly transport petroleum workers to and from offshore oil platforms, because

Helicopters

they are much faster and more cost effective than boats. Many large corporations use helicopters to ferry their executives between sites. Commercial helicopter operators in scenic locations, such as the Grand Canyon and Hawaii, regularly carry passengers on sight-seeing tours, although increasingly stringent noise regulations have somewhat curtailed their business. Commercial helicopter airlines have not been economically viable, despite the obvious advantages of ferrying passengers between airports and between airports and inner-city heliports. The US military services, particularly the Army and the Marines, make extensive use of helicopters for troop transport. Naval helicopters are often used for ship-to-shore and ship-to-ship transportation of personnel. In all services, helicopters are used for the insertion and extraction of special-operations forces at remote sites. Cargo transportation is another important helicopter function. In the logging industry, helicopters may be used to transport logs from remote areas either directly to a mill or to rivers in which the logs are floated to a mill. Construction projects often use helicopters to transport heavy equipment, such as heating, ventilation, and air conditioning units, to the tops of tall buildings. The Statue of Freedom, atop the US Capitol Building, was removed by helicopter in 1993 for restoration and was later replaced in the same manner. Helicopters adapted as firefighting aircraft, such as with large buckets slung beneath them, are used to transport water from nearby lakes or flame-retardant chemicals to the site of a forest fire. On the military side, pilots often use helicopters to transport supplies and even small- and medium-sized vehicles from rear areas to troops in the field. Navies frequently use helicopters to transport supplies from shore to ships at sea and between ships at sea, due to the ability to land on ships smaller than aircraft carriers.

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Many police departments, particularly in large cities, use helicopters for airborne patrol and surveillance. Because they operate at altitude, helicopters have a wider field of view than ground patrols. In cases of pursuit, it can be much easier for a helicopter to keep a fleeing suspect in view and safer for the ground units and the general public. In addition, when on patrol, a helicopter can often reach the crime scene more rapidly than can a ground unit. In a similar application, radio and television stations use helicopters for acquiring traffic reports and news gathering. News helicopters can often reach the scene of a news event more rapidly than can ground vehicles. Another mission for which helicopters are particularly well-suited is search and rescue, including medical evacuation (medevac) using craft outfitted as air ambulances. Although this is primarily a military mission, police departments and the US National Park Service (NPS) also use helicopters to find and rescue hikers, campers, and others who find themselves in dangerous situations. The US Coast Guard is very active in search and rescue, patrolling the waters off the coast of the United States. A typical Coast Guard rescue mission would be to extract passengers from foundering sailing vessels. Combat search-and-rescue missions are flown primarily by the air forces and navies to locate and return aircrews of aircraft downed in combat. During the Vietnam War, the Jolly Green Giant (CH/HH-3E) helicopters were a welcome sight for many pilots who had been shot down while flying over North Vietnam. Combat close air support and antiarmor are purely military missions that may be carried out by helicopter. Close air support involves using helicopters to support friendly ground troops by directing fire on enemy troops in the near vicinity. Helicopters used in antiarmor missions are equipped with ordnance that is capable of disabling or destroying tanks and other armored vehicles. The US Marines long made use of the versatile Bell AH-1

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SuperCobra, while the US Army typically operated the similarly versatile Boeing AH-64 Apache for these missions. Virtually all mission types traditionally carried out by manned helicopters have the potential to also be carried out by UAVs, or drones. While not all UAVs are helicopters, many do use rotor designs, and therefore have similar VTOL and hovering capabilities to ordinary helicopters. Drones have particular value in situation that may be dangerous for a human pilot, such as combat or firefighting, or in which small size is important, such as surveillance. They have already been extensively employed by the US military for drone warfare, though this has often drawn controversy due to the possibility of targeting malfunctions and other ethical issues. Public use of helicopter drones has also increased dramatically in the twenty-first century, making formerly difficult and expensive tasks like aerial photography relatively easy, cheap, and unobtrusive. Scientists have embraced such technology for low-cost remote sensing missions. However, civilian use has also drawn controversy over the potential to breach privacy, as well as regarding license or permit issues and safety risks. —Donald L. Kunz Further Reading Bichlmeier, Magnus. Certifiable L1 Adaptive Control for Helicopters. Cuvillier Verlag, 2016. Croucher, Phil. The Helicopter Pilot’s Handbook. CreateSpace Independent Publishing Platform, 2016. “History of Helicopters.” American Helicopter Museum & Education Center, 2016, americanhelicopter.museum/exhibits-and-resources/hist ory-helicopters. Accessed 6 Dec. 2016. Krasner, Helen. The Helicopter Pilot’s Companion: A Manual for helicopter Enthusiasts. Crowood Press UK, 2008. Padfield, Gareth D. Helicopter Flight Dyanamics Including a Treatment of Tiltrotor Aircraft. Wiley, 2018. Ren, Beibei, Shuzhi Sam Ge, Chang Chen, and Cheng-Heng Fua. Modeling, Control and Coordination of Helicopter Systems. Springer New York, 2011.

Principles of Aeronautics

Venkatesan, C. Fundamentals of Helicopter Dynamics. CRC Press, 2015. Wagtendonk, Walter J. Principles of Helicopter Flight. Aviation Supplies & Academics Inc., 2015. See also: Aerodynamics and flight; Aeronautical engineering; Airfoils; Autogyros; Avionics; Leonardo da Vinci; First flights of note; Gravity and flight; Heavier-than-air craft; Propulsion technologies; Rotorcraft; Igor Sikorsky

High-altitude Flight Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT High-altitude flight occurs at altitudes higher than most flights but lower than orbital flight—roughly between 15 kilometers and 30 kilometers. High-altitude flight has often been the frontier of aviation technology, meteorology, astronomy, and aerial reconnaissance. Many tasks can still be done more economically, or can only be done, in high-altitude flight. KEY CONCEPTS control surfaces: the surfaces of wings, ailerons, flaps and tail rudders used to control the flight characteristics of an aircraft hyperbaric atmosphere: a high-pressure atmosphere hypobaric atmosphere: a low-pressure atmosphere payload: the weight of people and cargo an aircraft can carry HIGH-ALTITUDE CHARACTERISTICS The meaning of the term “high altitude” has changed over the years. Balloonists struggled to reach altitudes between 6 and 9 kilometers, yet by the last third of the twentieth century these altitudes were routine for commercial and military jet trans-

High-altitude Flight

ports. The only constant is that the frontier always lies at the current definition of high altitude. Decreasing pressure is the most important feature of high altitude. Most of Earth’s atmosphere is in the troposphere, roughly the first 12 kilometers from the surface, and 99 percent of the atmosphere is below 39 kilometers. This has many implications. For high-speed jet aircraft, lesser air density allows greater speed, reduces heating problems, and allows greater engine efficiency until the available oxygen is too dilute to support combustion. For rockets, which require no external oxidizers, there is no limit except available fuel and oxidizer. For slower aircraft utilizing maximum lift for minimum energy, progressively less air density requires progressively wider wingspans, bigger control surfaces, cleaner aerodynamics, or more power to lift the same payload. For lighter-than-air (LTA) craft, such as balloons and dirigibles, decreasing air density with increasing altitude means there is less lift available per unit volume, so LTA craft must be larger to carry a given payload to higher altitudes. For living creatures, such as human crewmembers, a low-pressure (hypobaric) environment can be deadly. For instance, at about 5.5 kilometers the total air pressure is halved from that at sea level, and the amount of oxygen available to the body is similarly halved. The result is hypoxia (low oxygen) with progressively more severe symptoms as pressure declines: euphoria, headache, nausea, irritability, confusion, unconsciousness, and death. Aircraft crews can compensate for low pressure by breathing a greater percentage of oxygen. However, above 15 kilometers even pure oxygen does not have sufficient pressure to sustain life, so crews must have either pressurized cabins or pressure suits. Flying above much of the atmosphere means that much of the radiation usually stopped by the atmosphere will impact the craft. The lack of atmosphere allows clearer astronomical observations at light

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wavelengths stopped by the atmosphere, such as infrared. However, increased radiation in ultraviolet and shorter wavelength bands can attack a number of plastics that might be used in aircraft structures, and flight crews are subject to higher doses of ionizing radiation than people on the ground. Cold is another feature of high altitude. A rough formula is that in temperate zones every 300 meters of altitude is equivalent to traveling 120 kilometers farther from the equator. Temperatures drop steadily with altitude in the troposphere, stay the same or even rise slightly in the lower stratosphere, and then become somewhat irrelevant as declining air density begins to approach vacuum. Cold is not a serious problem for supersonic craft, for which avoidance of overheating is the prime concern. However, it can be life-threatening for slower craft. Lastly, every 1.6 kilometers of altitude yields roughly 50 kilometers of line-of-sight to the horizon. This has great importance for airborne radars and communications platforms. A radar plane flying at 12 kilometers has a range 418 kilometers, compared to 840 kilometers for a craft at 24 kilometers. In communications, an aircraft holding position or flying in tight circles to stay nearly in the same place can replace satellite communications service at lower cost and allow for ground stations that use much less power. They can also be put in place or upgraded more quickly than can satellite launches. There are three types of high-altitude craft: highly efficient propeller and jet craft, supersonic jets and rockets, and LTA craft, such as balloons and dirigibles. THE U-2 AND ITS COMPETITORS The Lockheed U-2, sometimes called the Dragon Lady, is the most famous formerly secret high-altitude aircraft. In the early 1950s, during the most intense part of the Cold War, the US intelligence community wanted a spy aircraft that could fly higher

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than any interceptors in the Soviet Union. Kelly Johnson at Lockheed proposed radically reconfiguring an F-104 Starfighter as a glider body with a 25-meter wingspan and a jet engine so it could fly at 21 kilometers. Johnson and his “Skunk Works” flew the first craft in August, 1955. On July 5, 1956, a U-2 flew over Moscow, the Soviet capital. Although the Soviets protested, they could do nothing about the overflights, and the United States denied its existence. However, the U-2 flew slowly, turned slowly, had not been designed for stealth by minimizing its radar and infrared signature, and Soviet antiaircraft missiles improved. On May 1, 1960, the Soviets downed a U-2 1,600 kilometers inside their border, and captured the pilot, precipitating a major diplomatic incident. Another U-2 was shot down during the Cuban Missile Crisis in 1962, and the U-2s were pulled back from well-defended areas. However, they continued to be used into the twenty-first century as high-flying signal-intelligence craft, obtaining data without crossing into hostile territory, and as conventional reconnaissance craft once air superiority was achieved, as in the Gulf War of 1991. This long life required a series of upgrades. The most important was the U-2R, beginning in 1967, which was a larger, stealthier aircraft that accommodated a two-person crew, a fourteen-hour maximum mission operations time, and a ferrying range of 13,000 kilometers. The civilian U-2 is the ER-2, which has done mapping and atmospheric sensing for several decades. The most recent U-2 competition has come from two planes from the company Scaled Composites: the Raptor and the Proteus. Both use lightweight composite materials and advanced aerodynamics pioneered by Burt Rutan, designer of the nonstop world-circling Voyager. The remotely controlled Raptor is a propeller-driven slower competitor, but it is stealthier than the U-2, and it can linger over an

Principles of Aeronautics

area for forty-eight hours. It demonstrated a 13,000-kilometer flight range in 2001. The Proteus is a direct, cheaper competitor to the U-2, with jet propulsion, a 1,000-kilogram payload, a fourteen-hour operations length, and an operational altitude of nearly 21.4 kilometers. As with its shape-changing namesake in Greek mythology, the Proteus can be configured for several other missions. Most important, it is a telecommunications repeater station, and for this mission, the Proteus demonstrated stable flight at 16.8 kilometers in late 2000. A new altitude record of 29.4 kilometers was set on August 13, 2001, by the Helios, a robotic flying wing designed by Paul MacCready of AeroVironment. (MacCready had also designed the human-powered Gossamer Condor.) Although its payload is only 100 kilograms, the Helios is direct competition for the Scaled Composites’ Proteus repeater stations. Helios has solar cells for daylight power and for electrolyzing water into hydrogen and oxygen for nighttime fuel-cell power. Consequently, Helios can fly for six months at a time. LIGHTER-THAN-AIR CRAFT AT HIGH ALTITUDES Balloons were the first craft capable of reaching high altitude. On December 1, 1783, Jacques-Alexander-César Charles made the first flight in a hydrogen balloon and also made the first high-altitude flight, limited only by the uncomfortable cold he encountered. For the next 120 years, balloons were the only means of observing the atmosphere. Swiss balloonist Auguste Piccard demonstrated the first pressurized cabin on May 27, 1931, when he and an assistant reached 15.8 kilometers, making them the first fliers ever to reach the stratosphere. More importantly, they discovered that cosmic rays increased with altitude, proving that they came from somewhere in space rather than the other suggested source, radioactivity within the earth.

High-altitude Flight

American and Soviet flights from the 1930s through the 1960s carried personnel and instruments to steadily greater heights and developed many technologies later used in the space race. In fact, on May 4, 1961, the American Stratolab V reached an altitude of 34.7 kilometers with an open gondola, testing space suits in near-space conditions for the Mercury orbital-flight program. After the 1960s, improved robotic instrumentation allowed LTA craft to shed the weight of the balloonists and their life support gear. By the late twentieth century, the National Aeronautics and Space Administration (NASA) began using super-pressure balloons for relatively small payloads in balloons weighing several tens of pounds. These balloons are different from zero-pressure balloons that expand when warmed by the sun and contract at night. When warmed at high altitude, zero-pressure balloons must vent excess helium to prevent bursting. This gas loss limits mission duration to only several days. With stronger materials, super-pressure balloons keep the same maximum shape even when warmed. Because no gas is lost such balloons can operate for months, and some of these balloons have circled the globe several times. By the early twenty-first century, NASA began flying large super-pressure balloons in a program called the Ultra Long Duration Balloon (ULDB), with balloons carrying several tons of instrument payload. These balloons compete with spacecraft for carrying astronomic payloads because they are cheaper, turnaround time is shorter, and awkward payloads can be accommodated that might not fit in a rocket or aircraft fuselage. Dirigibles have greater difficulty reaching high altitudes because the volume of buoyant gas needed to lift the payload as well as a body structure, engines, and control surfaces can become truly immense compared to the weight of payload being carried. Yet, dirigibles can fly slowly enough into the wind to remain stationary over one spot for weeks, ideal for

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communications repeating stations. Thus, by the early twenty-first century, Sky Station International was building a dirigible to compete against those of AeroVironment and Scaled Composites. SUPERSONIC HIGH-ALTITUDE CRAFT The most important supersonic high-altitude craft was the North American X-15 rocket-propelled research airplane, used from 1959 through 1968 to test materials and aerodynamics at speeds as great as 6.7 times the speed of sound (Mach 6.7, or 7,275 kilometers per hour) and altitudes as high as 107.9 kilometers. Lessons learned from these tests were later applied to the space shuttle and many supersonic airplanes. High-altitude supersonic flight development reached a peak in the early 1960s and then languished until the beginning of the twenty-first century. As noted, supersonic craft operate best in high-altitude regimes because thinner air causes less heat through friction and allows greater efficiency. Higher altitudes had also been a general direction of military flight since World War I (1914-18). These two trends led to the North American XB-70, planned as a heavy bomber flying at Mach 3 and a flight ceiling of 21.3 kilometers. The XB-70 flew in the early 1960s. By 1964, the Soviet Union responded with the Mach-2.8 Mikoyan-Gurevich MiG-25 interceptor. After the 1960s, other developments intervened. First, intercontinental ballistic missiles (ICBMs) were widely deployed. ICBMs could deliver bombs much faster than could aircraft. Furthermore, they did not require the expensive high-temperature alloys and vast amounts of fuel in operational training. Second, increasingly effective surface-to-air missiles caused large military aircraft to switch from high-altitude flight to flying low while ducking around missile sites. The XB-70 was never produced in volume, and the F-25 became largely a high-speed reconnaissance craft. The Lockheed SR-71 Blackbird was the

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best high-speed reconnaissance craft, with a maximum speed of more than Mach 3 (3,540 kilometers per hour and a maximum altitude of 27.5 kilometers). The XB-70 also demonstrated that no matter how high supersonic transports flew, sonic booms were a major irritant to people on the ground. The booms and the cost of heavy fuel use both limited the market for commercial supersonic transports, such as the Concorde and the similar Tupolev Tu-144. However, research has continued to develop better supersonic craft as the first stages for launch into orbit because oxygen carried by rockets weighs sixteen times as much as hydrogen fuel whereas jets get their oxygen from the atmosphere. Also, there have been reports of secret military craft with speeds of Mach 5 through Mach 10 and flight ceilings of 45 kilometers. —Roger V. Carlson Further Reading Davis, Jeffrey, Jan Stepanak, Jennifer Fogarty, and Rebecca Blue. Fundamentals of Aerospace Medicine. Wolters Kluwer Health, 2021. Hagland, Mark. “Helios: A State-of-the-Art Solar Plane.” Solar Today, vol. 5, no. 3, May/June 2001, pp. 32-35. Jenkins, Dennis R. Dressing for Altitude: U.S. Aviation Pressure Suits, Wiley Post to Space Shuttle. National Aeronautics and Space Administration (NASA), 2012. Martin, Steven C. Aerospace Physiology: Aeromedical and Human Performance Factors for Pilots. Gatekeeper Press, 2021. Newman, David G., and David Newman. Flying Fast Jets: Human Factors and Performance Limitations. Ashgate Publishing Ltd., 2014. Sóbester, Andras. Stratospheric Flight: Aeronautics at the Limit. Springer Verlag New York Inc., 2011. See also: Aerodynamics and fight; Aeronautical engineering; Neil Armstrong; Aviation and energy consumption; Contrails; Dirigibles; Lighter-than-air craft; Ernst Mach; Pressure; Propulsion technologies; Ramjets; Rocket Propulsion; Rockets; Scramjet; Space shuttle; Spacecraft engineering

Principles of Aeronautics

High-speed Flight Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT High speed refers to flight airspeeds greater than the average, especially speeds close to the maximum attainable speed for the era. The utility of civilian or military aircraft is always enhanced by increases in practical flight airspeeds. KEY CONCEPTS drag: the resistance to motion through a fluid due to friction between the moving object and the fluid medium radial engine: an engine in which the pistons and cylinders are arranged radially about a common crankshaft struts and wires: the supporting and strengthening connections between the wings of a biplane V-12 engine: an engine with twelve cylinders and pistons arranged in two parallel banks of six in a V configuration relative to each other and attached to a common crankshaft water/methanol injection: a fuel delivery system that injects a solution of methanol and water directly into the cylinders of an engine as a means of increasing power output as the methanol/water mixture flashes into steam and combustible methanol vapor PROGRESS IN AVIATION Much of the historic progress in aviation has revolved around solving the aerodynamic, structural, power, and heat problems associated with ever increasing speeds. Races and speed records, with their promise of prize money or prestige, have often stimulated individuals and governments to advance the art and science of high-speed flight.

High-speed Flight

As the first humans to make a controlled, powered flight in 1903, Orville and Wilbur Wright held the first unofficial speed record, at 48 kilometers per hour. Official speed records, though, are those that have been authenticated by the rules of the Fédération Aéronautique Internationale (FAI), formed in Paris in 1906, and these speed records begin with Alberto Santos-Dumont’s 42 kilometers per hour in 1906, increasing to 133.6 kilometers per hour by 1911, 191.5 kilometers per hour in 1913, 309 kilometers per hour in 1920, 754.8 kilometers per hour on the eve of World War II in 1939, 975.3 kilometers per hour with the first jets in 1945, 2,455.9 kilometers per hour with second-generation fighter aircraft in 1959, and 3,531 kilometers per hour by the SR-71A in 1976. EARLY HIGH-SPEED FLIGHT Initially, flight speeds were limited primarily by the lack of lightweight power plants, not surprising in view of the fact that development of the gasoline engine was still in its infancy. Thus, the first airplanes required a very large wing area and the most efficient structure was the bridge-based biplane, but its large size and attendant struts and wires generated a great deal of drag. By 1909, Glenn H. Curtiss was able to take his draggy biplane to first place in the first Gordon Bennett closed-circuit race at Rheims, France, with a speed of 75.6 kilometers per hour, mostly due to his development of a 50-horsepower V-8 engine that bettered the Wright brothers’ original 12-horsepower engine. Invention of the relatively lightweight, reliable, air-cooled rotary engine, in which the crankshaft is bolted to the aircraft and the cylinders and propeller rotate, led to the next great increment in high speed, culminating in a winning 200.4 miles per hour by a special Deperdussin monoplane racer in the last prewar Gordon Bennett race in 1913. World War I led to improvements in structures and power plants but negligible increases in speed because of

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the emphasis on climb rate and maneuverability, which favored biplanes. After World War I, the Schneider Trophy race for seaplanes inspired great advances in engines. The liquid-cooled engine assumed prominence because of its low frontal area. By 1927, Reginald Mitchell’s 900-horsepower Supermarine S.5 had established an absolute speed record of 453.8 kilometers per hour; it was the last time a biplane would win the race. The Supermarine S.6 retired the Schneider Trophy with an uncontested win at a speed of 547.2 kilometers per hour in 1930, with 2,300 horsepower available from its supercharged, liquid-cooled Rolls-Royce V-12. These racers, however, were impractical aircraft because they used low-drag skin radiators to dissipate the tremendous heat from their liquid-cooled engines. The winning Rolls-Royce V-12 engine in the S.6 had been inspired by the Curtiss V-12 engine that had powered the Curtiss CR-2 biplane to first place in the Pulitzer race of 1921 (and later to an absolute speed record of 318.7 kilometers per hour) and by the CR-3 that won the Schneider Trophy in 1923 at 286.5 kilometers per hour. In the United States, however, by the mid-1920s, water-cooled engines were taking second place to newly developed, reliable, air-cooled radial engines that were much lighter and much more suited to commercial applications, but presented a great deal more frontal area and accompanying drag. Charles A. Lindbergh’s historic New York-to-Paris flight in 1927 was made possible by a 220-horsepower Wright Whirlwind radial engine, for example. The monoplane route to higher speeds, now that brute power had accomplished about all it could do, was shown by the features of the unheralded, trouble-prone Dayton-Wright RB-1 racer of 1921: a small unbraced (cantilevered) wing that used flaps to yield acceptable takeoff and landing speeds, a closed cockpit for less drag, and a retractable landing gear. With a low-drag NACA cowling for its radial engine

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(developed by the National Advisory Committee for Aeronautics or NACA, the predecessor of NASA), the 450-horsepower TravelAir Mystery Ship won the 1929 National Air Races in Cleveland, besting all the military biplanes. James H. “Jimmy” Doolittle set a land plane record of 473.1 kilometers per hour in 1932 in the small-winged, bottle-shaped GeeBee R-1, using an 800-horsepower Pratt & Whitney radial engine. By 1939, however, Lockheed had flown the aerodynamically clean prototype of its P-38 twin-engined Lightning to a top speed of 664.7 kilometers per hour, and civilian aircraft were forever out of the race for the highest speeds. In World War II, piston-engined fighters reached speeds of around 805 kilometers per hour, but only at altitudes above about 6,100 meters, where their supercharged or turbocharged engines could take advantage of the less dense air. The liquid-cooled P-51H Mustang reached 783.8 kilometers per hour at 7.620 meters using water/methanol injection and high-octane fuel. An experimental version of the Republic P-47 Thunderbolt, using an 18-cylinder, air-cooled radial engine, reached a speed of 811.1 kilometers per hour at 10,500 meters. The Goodyear F-2G, developed from the Chance Vought F-4U Corsair, could reach speeds close to 804.7 kilometers per hour. Nonetheless, two facts threatened to forever prevent higher speeds: propeller and wing compressibility (Mach) effects. THE SOUND BARRIER Propellers were becoming less and less efficient as their tips approached the speed of sound; the air would break away from the tips and form ear-splitting shock waves. Worse, planes and pilots were being lost when the airflow over the wing approached the speed of sound. Because air is speeded up over the top surface of a wing to produce a lower air pressure there and thereby generate lift, the speed of sound is reached at that point before (sometimes well before) aircraft speed reaches sonic speed

Principles of Aeronautics

(Mach 1). Shortly after a local airspeed of Mach 1 is exceeded, a shock wave is formed where the air suddenly has to be slowed back to subsonic speeds; because the pressure waves that inform the air that it must change its speed or direction cannot propagate into the region ahead of this point, the shock wave represents an extremely narrow region perpendicular to the wing surface where the pressure and density and temperature of the air greatly increase. Shock wave formation not only greatly increases the power requirement, it also causes the airflow to break away from the wing at that point, producing effects very similar to the low-speed stall created when the wing is at a high angle relative to the oncoming air. At less than 70 percent of the speed of sound (Mach 0.675), a speed easily reached in a dive, the P-38 became uncontrollably nose-heavy as the wing lost lift and the horizontal tail surface lost its downward force; dive flaps were added to the sides of nacelles to save future pilots. Of US fighters, the P-51 Mustang suffered the least from compressibility, thanks to its laminar-flow wing with the thickest point well back from the leading edge, but its pilots

Transonic flow patterns on an airfoil showing the formation of shock waves at different Mach numbers (M) in high-speed flight. Image via Wikimedia Commons. [Public domain.]

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were warned that in high-speed dives, uncontrollable violent porpoising preceded a loss of altitude of 10,000 feet or more, at which point a recovery might be possible, because the Mach number decreased as the air temperature increased. The British Spitfire, with its very thin wing section, was eventually dived successfully to Mach 0.9, but it still was not at all clear that controlled supersonic flight would ever be possible. The propulsion problem was solved by the invention of the turbojet engine by British and German engineers, an engine which obtains thrust by taking in air and using it to burn fuel, with the byproducts exiting to the rear at a much higher speed. The thrust generated is equal to the rate of change in momentum (the product of mass and speed) generated by the engine. The jet engine is most efficient in the less dense air at altitudes above 20,000 feet. Rocket engines produce even more short-term thrust than jet engines, for their weight, and were used for the earliest transonic and supersonic flights. (Transonic flight is flight for which there is mixed subsonic and supersonic flow, approximately Mach 0.8 to Mach 1.2.) By 1944, the German rocket-powered Messerschmitt Me-263B had reached a speed of 1,131.4 kilometers per hour and the much more practical, jet-powered Messerschmitt Me-262 had reached 1,004.2 kilometers per hour. Higher speeds required better aerodynamics, including a recognition of the advantages of the swept-back wing and solutions for its disadvantages. Adolf Busemann, in 1935, first published the finding that a swept wing permits a wing to be effectively thinner because the chord (width) of the wing is greater than for a similar unswept wing. It spreads the lift and the cross-section of the wing over a greater percentage of the fuselage, reducing the suddenness of the drag rise and the pitch-down tendency. However, the spanwise flow on a swept wing also tends to cause the wingtips to stall first at low speeds or when maneuvering, making the ailerons

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ineffective and producing a violent pitch-up tendency. The swept wing also suffers from a Dutch roll (coupled yaw and roll) tendency, which can be serious enough to destroy the aircraft. The stall problem can be treated by using high-lift devices (slats) on the leading edge of the outer wing panels and by chordwise plates (fences). The Dutch roll tendency can be treated by a gyroscopic-based yaw damper, as well as aerodynamically. In May, 1948, the swept-wing, jet-powered North American F-86A Sabre jet achieved an official world speed record of 1,078.3 kilometers per hour. SUPERSONIC FLIGHT The sound barrier, however, was still to be breached. The United States chose the bullet-shaped, rocket-propelled Bell X-1 to make the attempt. (Bullets were known to reach supersonic speeds in flight, but they had to spiral for stability and they did not try to use lift to stay in the air.) With only 2.5 minutes’ worth of rocket fuel, the only available route was to use an air launch from a modified B-29 bomber and glide to a landing afterward. This was possible only because the United States had California’s clear skies and vast Muroc Dry Lake (the present Edwards Air Force Base) for landing. On October 14, 1947, test pilot Charles E. “Chuck” Yeager reached Mach 1.06 at 13,106.4 meters and the first human-generated sonic boom was heard. He glided back at 402 kilometers per hour, approached at 354 kilometers per hour, and landed at 305.8 kilometers per hour. The Russian MiG-19 was the first fighter capable of supersonic speeds in level flight, followed shortly by the Republic XP-91 and North American’s F-100 Super Sabre. The secret was obtaining a short burst of extra thrust by dumping raw fuel directly into the exhaust (after the turbine), a practice called afterburning in the United States and reheat in Great Britain. However, the Super Sabre also had to be cured of a new disease: inertia coupling. With a

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long, heavy fuselage supported by short, light wings, an aircraft has roll inertia (resistance to changes in roll around the nose-to-tail axis) that is much less than its pitch and yaw inertia, and rolling motion can induce a pitching moment that sends an aircraft into a disastrous tumble. Additional tail and wing area solved the problem for the Super Sabre. The FAI set new rules for speed records at high altitudes and Great Britain quickly claimed an absolute speed record in March, 1956, when its Fairey Delta 2 (FD.2) flew at 1,821.8 kilometers per hour. The official speed record as of 2001 was held by Lockheed’s SR-71 Blackbird (a reconnaissance aircraft capable of over Mach 3 at 30,480 meters altitude); in 1976 it averaged officially 3531 kilometers per hour, but is known to have gone considerably faster unofficially (the SR-71 Blackbird’s actual speed and altitude capabilities are kept guarded to this day). Transport aircraft have followed in the path blazed by research and fighter aircraft. The Douglas DC-2 almost won the London to Melbourne race in 1933 against specialized racing aircraft. The de Havilland DH-106 Comet was the first successful jet transport, going into service in 1952 until 1997. The Comet was succeeded by Boeing’s 707, which became the workhorse of the air transportation industry until it was succeeded by more advanced designs in the Boeing line. The French/British Concorde began Mach 2 airline service in 1976 and flew for 22 years before being discontinued; it used a highly swept delta planeform with sharp leading edges generating vortices that greatly enhance lift at low speeds (vortex lift). HYPERSONIC FLIGHT Hypersonic flight (greater than Mach 3.5) is the new frontier and in this flight realm, heat is the primary foe. Even on the Concorde, skin temperatures of 127 degrees Celsius are reached and the fuselage lengthens by 23 to 25 centimeters in flight. On the

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titanium SR-71, temperatures reach 316 degrees Celsius. The North American X-15 showed that a rocket-powered research aircraft could reach hypersonic speeds; by 1967 the X-15 had flown to 107,899 meters and Mach 6.7, but it had also been seared by 1,649-degree-Celsius temperatures. Ablative (sacrificial) coatings were used; they melt away at high temperatures while absorbing and dissipating the heat. After the Soviet Union sent Yuri Gagarin into Earth orbit in 1961, beginning the race to the Moon, spaceflight, rather than higher-speed atmospheric flight, became the next US challenge. It was followed by space stations and the space shuttle, the latter of which must attain just the right speed for its orbital height and then, with braking rockets, reenter the atmosphere at about 42,672 meters and Mach 6.7. The X-43, making its first flight in 2001, is an uninhabited hypersonic research aircraft. It remains to be seen whether it will make hypersonic flight a regular occurrence. —W. N. Hubin Further Reading Chittum, Samme. Last Days of the Concorde: The Crash of Flight 4590 and the End of Supersonic Passenger Travel. Smithsonian Institute Press, 2018. Hans-Reichel, Michael. Subsonic Versus Supersonic Business Jets-Full Concept Comparison Considering Technical, Environmental and Economic Aspects. Diplom.de, 2012. National Aeronautics and Space Administration Staff. The Sr-71 Test Bed Aircraft: A Facility for High-Speed Flight Research. Independently Published, 2018. Sabry, Fouad. Active Aeroelastic Wing: Improve Aircraft Maneuverability at the Transonic and Supersonic Speeds. One Billion Knowledgeable, 2022. Torenbeek, Egbert. Essentials of Supersonic Commercial Aircraft Conceptual Design. Wiley, 2020. See also: Advanced propulsion; Aerodynamics and flight; Aeronautical engineering; Air transportation industry; Airplane manufacturers; DC plane family; First flights of

note; Fluid dynamics; Forces of flight; Yuri Gagarin; High-altitude flight; Hypersonic aircraft; Jet engines; Military aircraft; Propulsion technologies; Ramjets; Rocket propulsion; Russian space program; Scramjet; Shock waves; Sound barrier; Space shuttle; Spacecraft engineering; Supersonic aircraft; Supersonic jetliners and commercial airfare; Supersonic jets invented; Temperature; Konstantin Tsiolkovsky; Turbojets and turbofans; Wing designs; X-Planes (X-1 to X-45)

Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Combustion science ABSTRACT The famous zeppelin Hindenburg, officially Luftschiff Zeppelin 129 (LZ-129), flew from March 4, 1936, to May 6, 1937. It was the largest rigid lighter-than-air passenger transport vehicle ever constructed. Dirigibles were becoming an important means of overseas transportation when the Hindenburg exploded while docking at Lakehurst, New Jersey, eliminating any hope that this means of transoceanic travel might become widespread. KEY CONCEPTS buoyancy: the extent to which an object will float in a fluid medium on the basis of their relative densities payload: the weight of people and cargo an aircraft can carry zeppelin: a self-propelled airship having a rigid body framework and using hydrogen gas as its lifting agent GERMANY AND THE DEVELOPMENT OF DIRIGIBLES Lighter-than-air flight began in Europe as early as 1783, when a cloth balloon filled with hot air carried three animals aloft in France. By 1898, European aviation pioneer Alberto Santos-Dumont had fash-

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ioned a cylindrical balloon that flew over Paris and the surrounding countryside powered by a motorcycle engine and steered by a rudder. This vehicle, however, carried only one person. By 1900, Count Ferdinand von Zeppelin had built a huge oblong aircraft with a cloth-covered steel frame inside of which were large bags of hydrogen that lifted the vehicle into the air. Several such aircraft, built by Zeppelin’s company, Luftschiffbau Zeppelin, were built and were used to carry passengers on sightseeing trips across Germany. In 1909, the world’s first passenger airline, Deutsche Luftschiffahrts Aktien-Gesellschaft (Delag), was established. Its lighter-than-air fleet, consisting of the Schwaben, the Victoria-Luise, and the Sachsen, carried 37,250 passengers 1,600 flights. During their 3,200 hours aloft, the airships covered more than 160,000 accident-free kilometers. During World War I (1914-18), the German army used dirigibles for reconnaissance and for bombing missions over London. During the following decade, many civilian uses were found for dirigibles. Arctic explorer Roald Amundsen bought a dirigible, in which he flew over the North Pole, traveling the 5,118 kilometers from Spitsbergen, Norway, to Teller, Alaska, in about 71 hours. By 1929, Germany had built the Graf Zeppelin, which carried twenty passengers on the first nonstop flight around the world. This feat marked the beginning of regular overseas passenger travel in lighter-than-air craft. At this time, it took at least five days to cross the Atlantic Ocean by steamship, and crossings were often rough during storms. In contrast, a dirigible, averaging 129 kilometers per hour, could make the transatlantic crossing in two and one-half days, floating like a cloud above storms and turbulent seas alike. When Adolf Hitler came to power in Germany during the mid-1930s, political storm clouds gathered over Europe. Hitler accomplished a tactical victory by luring the 1936 Olympic Games to Berlin.

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He conceived the idea of building the largest dirigible in the world, to be flown over the Olympic stadium during the games. The project, referred to simply as LZ-129, was completed quickly. A 245-meter rigid frame of steel was covered with a superstrong, hand-stitched cotton fabric, and the ship’s interior amenities were refined to the point that it unquestionably offered the most luxurious means of crossing the Atlantic. On March 4, 1936, German aeronaut Hugo Eckener took LZ-129 on its maiden flight. As Eckener hovered over Munich, the city’s mayor radioed to ask him the name of the ship. He unhesitatingly responded with the name Hindenburg, after German field marshal and former president of Germany’s Weimar Republic Paul von Hindenburg, who had died two years earlier. Joseph Goebbels, Germany’s minister of propaganda, reprimanded Eckener severely for presuming to name the ship without authorization, telling him that the Reich had been planning to call it the Adolf Hitler. A change could not be made gracefully after Eckener’s public statement, so the ship continued to be called the Hindenburg. THE HINDENBURG’S AMENITIES The luxurious Hindenburg was three and one-half times the length of a Boeing 747 and about the same length as the steamship Titanic. It was outfitted with extremely lightweight furniture, including a 180-kilogram aluminum piano. It had a lounge, a writing room, a smoking room, and a dining room, whose tables were set with exquisite floral arrangements, silver, and china. Lavish meals prepared by superb chefs issued forth from its kitchens. Banks of windows along the bottom portion of the aircraft provided dramatic vantage points from which to view the scene below. The Hindenburg’s staterooms were small but efficient, with bunk beds and foldout tables and sinks. Originally the ship could accommodate fifty passen-

Principles of Aeronautics

gers, but with the success of the 1936 season, during which every stateroom was usually filled, the ship was modified to serve seventy-five passengers. The new staterooms, unlike the old ones, had windows that offered spectacular views. The May 3, 1937, flight of the Hindenburg carried thirty-six passengers and a crew of sixty-one. Those who were traveling alone had staterooms to themselves. Above the passenger-crew areas of the ship were cavernous spaces that could carry up to 100 tonnes of cargo. These spaces contained bags of hydrogen, required to lift the craft, and water, used for ballast. Because hydrogen is a highly explosive substance, every precaution was taken to prevent the hydrogen on board from being ignited accidentally. Crew and passengers wore slippers with felt soles. Matches and cigarette lighters were confiscated and later returned to debarking passengers. The smoking room contained only one lighter, secured by a chain, and the room was tightly sealed so that no sparks could escape. Helium, another gas, which is not explosive, would have lifted the craft as well as hydrogen. Hydrogen, however, took up less space, permitting the Hindenburg to carry a larger payload. In addition, the United States, a major supplier of helium, was reluctant to sell this substance to Germany as it moved increasingly toward fascism and helium is an important component of nuclear research. THE HINDENBURG’S FATEFUL LANDING On Monday, May 3, 1937, the Hindenburg drifted from its moorings in Frankfurt at 7:30 p.m. to begin its first flight of the season from Germany to the United States. Although it had made ten trips to New York in 1936, in winter, when the North Atlantic was stormy, the Hindenburg flew the Frankfurt-to-Rio de Janiero route instead, resuming its North Atlantic flights when the weather improved. Eighteen flights to the United States were scheduled for 1937.

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The Hindenburg’s May 3 flight was to have taken about thirty-six hours, with touchdown at Lakehurst, New Jersey, outside New York City, scheduled for the morning of Thursday, May 6. Headwinds across the Atlantic delayed the ship’s arrival. By the time it flew over New York City, it was nearly twelve hours late. The weather was bad, so even though the aircraft flew over Lakehurst, it did not land immediately. Rather, it flew down the coast toward Atlantic City, New Jersey, before circling back for its landing at Lakehurst, where it was awaited by a ground crew and people who had come to meet arriving family and friends. As the ship inched toward its metal mooring at about 7:30 p.m., it dumped some of its water ballast to slow its descent, dousing some of the ground crew below. Those on the ground looked up at the gleaming ship with rapt expressions. Suddenly a thunderous noise shook the area, and the observers’ expressions turned from joy to horror, as the ship trembled violently with reverberating explosions. As the hydrogen quickly ignited, fireballs engulfed the ship. Chaos ruled as people on board, many with their clothing and hair on fire, jumped from the craft to the ground 33 meters below. Others, such as cabin boy Werner Franz, who, two weeks short of his fifteenth birthday, was the Hindenburg’s youngest crew member, were trapped. As fire rolled toward Franz from two directions, his situation seemed hopeless. Suddenly a ballast tank ruptured, immersing him in 2 tonnes of water. Soaking wet, Franz jumped from the inferno onto the ground, emerging with only minor injuries. He arrived home in Germany on May 22, his birthday. In all, twenty-two of the Hindenburg’s crew of sixty-one died in the disaster, including Captain Ernst Lehmann who, although badly burned, had returned to the inferno in an attempt to rescue trapped passengers and crew. Twenty-three of the

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Photograph of the Hindenburg descending in flames, May 1937. Photo via Wikimedia Commons. [Public domain.]

thirty-six passengers survived, although a number of them were severely injured. POSSIBLE CAUSES OF THE DISASTER Following the Hindenburg’s destruction, speculation about its causes was widespread. Certainly, the hydrogen used to lift the craft, once ignited, exploded to create a fire of great intensity. However, what caused the hydrogen to ignite remained a mystery. Some experts believed that as the aircraft had flown through the electrical storms that had raged

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along its course, static electricity had collected on its exterior, so that when it contacted its metal mooring, sparks flew and ignited the hydrogen, small quantities of which could already have been leaking. The US Department of Commerce established a commission to probe into the cause of the explosion, but no firm conclusion was forthcoming from that commission. Among the possible causes mentioned were a ball of lightning, demon protons, static electricity, and St. Elmo’s fire, a discharge of atmospheric electricity that commonly collects on aircraft

Principles of Aeronautics

flying in thunderstorms. However, eyewitnesses verified that the fire started inside the ship; if any of these possibilities been invalid, the fire would have begun on the outside. Given the strained relations between the United States and Hitler’s Germany, the US government wanted to prevent the disaster from escalating into an international incident. The official finding of the committee identified the disaster’s cause as St. Elmo’s fire, although in all of aviation history, no similar incident had ever been recorded. In Germany, General Hermann Göring ordered the German commission investigating the explosion to “discover nothing.” He officially declared the event an act of God, foreclosing further investigation. CONSPIRACY THEORIES AND OTHER THEORIES Accidents such as the explosion of the Hindenburg often spawn conspiracy theories, which are sometimes given serious consideration. One cannot forget that Adolf Hitler, in promoting the development of the Hindenburg, sought to bring favorable attention both to Germany and to his despotic regime, in only its second year when the airship was conceived. Germany had already planned to build other transoceanic dirigibles, and those opposed to Hitler did not want Germany to fulfill this dream. The destruction of the largest dirigible in the world would thwart plans for expanding Germany’s lighter-than-air passenger service and would be a great personal blow to the country’s dictator. Further, one must remember that threats had been made against the Hindenburg. Not long before its ill-fated trip, a bomb had been found in the dining salon of the Graf Zeppelin and was removed before it exploded. Some conspiracy theorists noted that one of the passengers on board the Hindenburg had been Joseph Späh, a German who had fled the country as

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Hitler was coming to power, and who was an outspoken opponent of the Nazi government. Späh had been observed in restricted areas and explained his presence there by saying that he had wanted to visit his dog, who was being carried in the ship’s hold. Those doubting the conspiracy theories pointed out that if a passenger or crew member had sought to destroy the Hindenburg by planting a bomb somewhere aboard, that person would have had to die in the explosion. The counterargument to this objection is that a time bomb might have been hidden somewhere in the craft’s vast superstructure with the intention of destroying the ship after it had landed. Because the Hindenburg arrived twelve hours behind schedule, such a bomb might have detonated just as the dirigible was landing. A more technically interesting theory, also based on eyewitness accounts, was that the fire originated on the outside of the aircraft, caused by a fluke chemical reaction between the silvery powdered aluminum coating on the outward side of the fabric covering and the red iron oxide coating on the inward side of the fabric. Iron oxide and aluminum are the two components of the highly exothermic “thermite reaction.” When in contact with each other the reaction requires a significant input of energy for its initiation, such as can be obtained from an electrical spark. This theory states that the large amount of static charge built up on the Hindenburg initiated just such a reaction in the fabric skin of the aircraft, and eyewitness accounts of the dark orange flame and billowing black smoke of the fire do not correspond with the combustion characteristics of burning hydrogen. Several years later, a remnant of the fabric used to construct the Hindenburg was discovered in the manufacturer’s facility in Germany, and was subjected to testing to determine the viability of this theory. Indeed, when subjected to an electrical spark of the power that would have existed aboard the Hindenburg the fab-

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ric did in fact erupt into flame as had been described. This of course could not prove the cause of the Hindenburg’s demise, but it does endow the theory with some validity. —R. Baird Shuman Further Reading Burgan, Michael. The Hindenburg in Flames: How a Photograph Marked the End of the Airship. Capstone Press, 2016. McCarthy, Michael. The Hidden Hindenburg: The Untold Story of the Tragedy, the Nazi Secrets, and the Quest to Rule the Skies. Rowman & Littlefield, 2022. Otfinoski, Steven. The Hindenburg Explosion: Core Events of a Disaster in the Air. Capstone Press, 2014. Regis, Edward. Monsters: The Hindenburg Disaster and the Birth of Pathological Technology. Basic Books, 2015. Warbel, Barbara. The Zeppelin Airship LZ-129 Hindenburg. Sutton, 2013. See also: Aeronautical engineering; Blimps; Dirigibles; Flight balloons; Gravity and flight; Hot-air balloons; Lighter-than-air craft; Materials science

History of Human Flight Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT The history of human flight comprises a series of developments that have allowed people to travel through the air in manufactured aircraft. Since the earliest recorded history, people have dreamed of flying, often ascribing the power to mythical gods. The invention of means for human flight has changed the world as has no other invention before or since by conquering the problem of distance. KEY CONCEPTS aerostat: a lighter-than-air vehicle that can remain in one position in the air

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glider: an aircraft that has no engine or other power source to provide forward motion, relying instead on the pilot’s skill in maneuvering the aircraft in existing air currents to remain aloft ornithoptic propulsion: flying in the manner of birds, by the flapping of wings pectoral muscles: a set of muscles on the upper torso of animals that are responsible for the lateral motion of arms, wings, and legs EARLY SCIENTIFIC THEORIES Seventeenth-century English physicist Sir Isaac Newton postulated that heavier-than-air flight was impossible (apparently there were no birds in Newton’s time...). Newtonian physics concluded that the resistance encountered by a wing would require an even heavier engine, which in turn would require an even larger wing, which would require an even heavier engine, and so on, in a circuitous conundrum. The history of the science of human flight arguably began in 1680, when Italian physicist Giovanni Alfonso Borelli definitively proved that humans could not fly under their own power, because their pectoral muscles were simply too weak to support flight, regardless of the wing structures one might employ. That evidence should have ended tower jumping and flapping wing contraptions, but it did not. Some persisted, even into the twentieth century, in their preoccupation with the impossible. Others searched for less fatal alternatives, turning from flapping to floating. HOT-AIR AND HYDROGEN BALLOONS An artist captured the supposed earliest recorded hot-air balloon demonstration, which was a small model, sent briefly aloft in 1709. Scientifically studying and documenting their efforts with public demonstrations, French brothers Joseph-Michel and Jacques-Étienne Montgolfier achieved the first true hot-air balloon ascents. On June 4, 1783, near Lyon, France, they demonstrated for the public their

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uncrewed aerostat, a huge linen bag lined with paper, 100 feet in circumference, which rose 6,000 feet aloft when a straw-fed fire heated the air inside the bag. The Montgolfiers were not alone, however, in their quest for the sky. A series of scientific discoveries followed, and in 1766, hydrogen was discovered to be one-fourteenth the density of air. French physicist Jacques-Alexander-César Charles created his own version of the aerostat, crafting a rubber-coated silk balloon filled with hydrogen, which he publicly demonstrated on August 24, 1783, before a large crowd that included the American diplomat and inventor Benjamin Franklin. During this period, the field of aviation experienced many pioneering firsts. In a demonstration before French king Louis XVI and his wife Marie-Antoinette at Versailles in September, 1783, the Montgolfiers sent aloft aviation’s first living voyagers: a rooster, a sheep, and a duck. The first crewed balloon flight came on October 15, 1783, with volunteer Jean-François Pilâtre de Rozier, a young doctor, on board. The original plan had been to send aloft two criminals from prison, in case they did not come down alive. However, Pilâtre de Rozier insisted on taking the place of the prisoners. On November 21, 1783, he and the marquis François d’Arlandes flew untethered across Paris. On December 1, 1783, the first crewed hydrogen balloon ascended with Charles and a passenger. After a lengthy two-hour, 43.5-kilometer flight, Charles set down, left off his passenger, and made a second flight, at sunset. With one less person aboard, the balloon rose to 9,000 feet, and Charles became the first person to see the sun set twice in one day. With these first successes also came tragedy. The first to fly was also the first to die. Pilâtre de Rozier, in attempting to cross the English Channel on June 15, 1785, combined the hot-air-and-hydrogen balloon technology, putting a fire under a hydrogen

History of Human Flight

balloon. The flight lasted only four minutes before exploding and killing its pilot. Military use for the balloon was not far behind, starting a familiar sequence which would be repeated throughout history. In June, 1793, the French Republican government put observers in tethered balloons to report on enemy movements. In April, 1794, the French formed the first balloon corps, used in several European campaigns but disbanded by Napoleon in 1802. POWERED FLIGHT The matter of balloon steering remained unresolved until the twentieth century, when the elongated steerable dirigible was developed. At about the same time, the first powered airplane was invented. These near-simultaneous achievements were not serendipitous. Both enterprises depended on the same thing to succeed: a suitable engine to power the aircraft. Although early engines were problematic, they held great promise. In 1876, German engineer Nikolaus August Otto designed and built the world’s first practical internal combustion engine to use liquid petrol as fuel. In 1885, simultaneously and independently, German engineers Gottleib Daimler and Carl Benz built the first lightweight, high-speed petrol engines. GLIDERS AND WINGED FLIGHT Although Orville and Wilbur Wright’s secret to success in achieving the first powered flight was the engine they designed and built themselves, the two brothers researched, designed, tested, and built the plane, propellers, and control surfaces and structures that ultimately helped them to succeed. They systematically and scientifically studied the work of early glider pioneers, most notably that of the British Sir George Cayley, known as the father of aeronautics. In 1804, Cayley had built the first known heavier-than-air flying model. Over the next fifty

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years, he built three full-scale gliders capable of flight with a passenger on board. In 1871, the wind tunnel was invented to enable the aerodynamic study of scale models. The Wrights perfected their own wind tunnel and used it systematically to study and perfect their wing and propeller designs. Their scientific approach enabled them to succeed where others had failed. Another glider great, the German aviation pioneer Otto Lilienthal, developed and flew controllable gliders, working from 1891 until his death in 1896 while attempting a powered glide. He tested his gliders by leaping from an enormous hill he had built near Berlin. The Wrights, too, began by experimenting with gliders. Still others would play a role in the Wright’s story by disseminating information. Octave Chanute was a French-American engineer credited with building the first bridge across the Missouri River and the stockyards in Chicago and Kansas City. Bored with engineering for trains and livestock, he turned his attention to aeronautics. In 1894 he published Progress in Flying Machines, which collected and summarized the information humans had learned to date about heavier-than-air flying machines. On May 13, 1900, Wilbur Wright wrote to Chanute to introduce himself and inquire about available information that would assist the Wrights’ experimentation. The Wrights had learned of Chanute after having written the Smithsonian Institution seeking information on flying. They learned that Samuel Pierpont Langley, the director of the Smithsonian, was experimenting with heavier-than-air flight. Langley had received the first federal government contracts to build a powered human-carrying plane, but both his attempts, with planes launched from atop a houseboat in the Potomac River, were failures. He gave up in disappointment, most of his calculations proven embarrassingly incorrect when, one week later, he was beaten by the Wrights in the race to fly.

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THE WRIGHT BROTHERS’ ACHIEVEMENT It is almost impossible to overstate the significance of the Wrights’ accomplishment. Through careful study and diligent testing, they unlocked the secret to controlled flight. By wing warping, or twisting, they could cause the plane to turn. This mechanism was the forerunner of the aileron on modern planes. The Wrights systematically studied aspect ratios, comparing the wing’s length to its width, and devising tables to decide on the most suitable wing sizes and shapes. They researched propellers in their wind tunnels, understanding that the propeller was a rotary wing with forward lift. They defeated torque by using two propellers rotating in opposite directions, connected to sprockets by a bicycle chain. When they could find no satisfactory engine, they designed and built their own. At their testing grounds at Kitty Hawk, North Carolina, on December 17, 1903, at 10:35 a.m., the Wright brothers made their first flight, with Orville at the controls, and Wilbur running alongside him. Orville had positioned a camera before the flight, and John T. Daniels, a volunteer helper who had run up from the Kill Devil Hill Life Saving Station, snapped the picture of the first powered, sustained flight by humankind. The Wrights made four flights that day. The longest, with Wilbur at the controls, lasted 59 seconds and covered 260 meters. In many ways the most important part of the history of flight was over on the day it began. The scant one hundred and twenty years in the history of human flight show astonishing discovery and achievement, but in reality, much remains exactly as it was researched and recorded in the Wrights’ early records. In the next two years, the Wrights built more airplanes and made more discoveries about piloting them: how to control, turn, and avoid stalls. In January, 1906, the Wrights offered to sell their plane to the US War Department, which declined. In 1907, they took their plane to Europe, seeking in vain to find a buyer overseas. The Wrights left their air-

Principles of Aeronautics

plane stored in a shed in Europe and returned, dejected, to Dayton. In 1908, however, the British Army acquired the Wrights’ aeroplane number 1 and the US Army agreed to pay the Wrights $25,000 for one of their airplanes. A French group agreed to pay $100,000. Wilbur went to France, got the plane out of the shed and upgraded it with some recent advancements. To fulfill the US government contract, Orville went to Fort Myer to test-fly the airplanes. Tragedy struck on the last day of the test flights, and a twenty-six-year-old lieutenant, Thomas Selfridge, became the first person to lose his life in an airplane accident. Thereafter, the aviation firsts came in rapid succession. In 1909, French aviator Louis Blériot flew across the English Channel in a self-built monoplane. The public’s fascination with aviation was fueled by air meets that occurred on both sides of the Atlantic Ocean. By 1910, there were nighttime takeoffs, makeshift lighted beacons and runways, stunt flying, and the first takeoff from a ship deck. The next few years brought ship-deck landings, the first US transcontinental flights, the crossing of the Mediterranean and the establishment of the British Royal Flying Corps. With heavy initial losses and little early success, the US Signal Corps was also established. AIRPLANES AS WARPLANES On June 28, 1914, Europe was thrown into World War I. At the start of the war, there were 1,200 German planes, and 1,000 French and British planes. On April 6, 1917, the United States entered World War I, and a US military aviation section was established. Although 1,000 men enlisted, fewer than 250 planes were amassed. Fighter planes and their pilots would earn fame and affection during World War I. Eddie Rickenbacker of Columbus, Ohio, who later helped found Eastern Air Lines, was the top US ace, and

History of Human Flight

also flew for France with the US volunteer group, the Lafayette Escadrille. General William “Billy” Mitchell returned home in 1919 from his European service advocating an independent US air power. He predicted the attack on Pearl Harbor decades before its occurrence and forced the US government to examine what had happened to money appropriated by the US Congress for World War I fighter planes that were never delivered. Justice Department investigations and Congressional hearings were held, but eventually Congress tired of the issue and agreed not to pursue the matter further. AVIATION IN THE INTERWAR YEARS After the war was over, airplane travel offered a way around the war’s destruction and devastation, and, in combination with ground transportation, allowed access to far-flung colonial outposts. European governments directly subsidized the development of commercial air carriers, such as KLM, Lufthansa, British Airways, and Air France. The US government, in contrast, opposed direct subsidies for the development of commercial air carriers, but instead used US airmail contracts to establish and subsidize the aviation industry. In 1925, air mail was privatized, and wealthy American industrialists snapped up the first contracts. As aircraft improvements were made and more reliable engines were developed, navigational aids helped to make flying a more certain venture. In 1926, government regulation and certification of pilots, aircraft, air traffic rules, maps, weather reports, and accident investigation aided the private air carriers by improving the safety, efficiency, economics, and reputation of flying. American fliers and planes soon established their worldwide dominance in the field of aviation. The first around-the-world flight was accomplished by Douglas Aircraft World Cruisers, manufactured in Santa Monica, California. Four Cruisers departed Seattle, Washington, on April 6, 1924, and two

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Cruisers returned to the same spot 44,342 kilometers and 175 days later, on September 28. Seven years later, a similar feat was accomplished in just eight days, when American aviator Wiley Post circumnavigated the globe. In May 1927, Charles A. Lindbergh, flying from New York to Paris, became the first pilot to make a solo transatlantic crossing. He was immediately lauded as a hero and remains among the most famous fliers in history. Lindbergh would go on to devote his life to the promotion of both civil and military aviation. In 1932, Amelia Earhart crossed the Atlantic solo, and fame followed her flight. She was lost in 1937, attempting to circumnavigate the globe via the 43,452-kilometer equator route. The record-setting flights of the 1920s and 1930s bear witness to the fact that during this period, planes were undergoing dramatic technical improvements. No airplane before or since has captivated the world, or its aircraft sales, as did the Douglas DC-3. Still the most successful transport plane ever, with 10,926 manufactured in the United States, and perhaps as many as 5,000 more manufactured overseas by other countries, the twin-engine DC-3, also known as the Gooney Bird, Dakota, or Skytrain, along with the DC-4, also known as the Skymaster, with double the DC-3s engines, passenger capacity, and range, revolutionized air transportation. ALTERNATE METHODS OF FLIGHT Other methods of human flight rose, or fell, in the 1920s and 1930s. In 1926, American physicist Robert H. Goddard demonstrated the liquid fuel rocket. In 1936, the first truly successful helicopter, the Focke-Wulf, was developed in Germany. In 1937, British engineer Sir Frank Whittle established the world’s first turbojet engine development program. Zeppelin airships, two and one-half times the length of a football field, were first launched in 1909 and began commercial service in 1911. Zeppelins pro-

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vided the world’s first passenger airline service. On May 6, 1937, the world’s largest airship, the hydrogen-filled Hindenburg, exploded while approaching moorings in New Jersey, after completing a trip from Germany. Although lives had been lost in zeppelin travel until the Hindenburg disaster, the loss of the airship, captured on film and rebroadcast worldwide, ended the era of the airship, a scant thirty years into its history. Today, the sight of a helium-filled blimp overhead is a rare occurrence usually confined to the airspace over sporting events. WORLD WAR II As the 1930s drew to a close, war again loomed in Europe and Asia. In 1932, Japan employed aircraft carriers in conflicts with China. In 1935, Italy used air forces to invade Ethiopia, and in 1936, sent air forces to aid General Francisco Franco in the Spanish Civil War, as did Germany, which had sent troops to Italy for secret pilot training. By 1938, Germany had reached wartime levels of airplane production. The Soviet Union sent planes to Spain to help defend against Franco’s forces. In 1939, the Soviet Union aided China against Japan, as did the United States, with its American Volunteer Group, also known as the Flying Tigers. On September 1, 1939, chancellor Adolf Hitler ordered the invasion of Poland. The Luftwaffe, the German air force, struck the Polish airfields, taking out most of Poland’s planes. In 1940, Denmark, the Netherlands, Belgium, and France fell to the Germans. Beginning on September 7, 1940, Hitler’s Luftwaffe commenced nightly bombing of London. Aided in part by radar warning stations, London and Britain held, and Hitler turned his attention elsewhere when he failed to achieve control of the air in the Battle of Britain. In 1941, Hitler invaded the Soviet Union. Although the Soviets suffered significant casualties and the loss of 8,000 planes, the German planes could not reach the Soviet aircraft factories, and the Soviets kept building.

Principles of Aeronautics

In the United States, airplane production had been dramatically increasing throughout the late 1930s. In 1938, airplane production was increased to 10,000 units per year. On December 7, 1941, the Japanese attacked Pearl Harbor, Hawaii, much as Billy Mitchell had long before predicted. In rapid succession, the Philippines, Hong Kong, Singapore, Malaya, the Dutch East Indies, Borneo, and Burma fell to the Japanese. In 1942, American bomber groups were deployed over Tokyo, Europe, and Africa. In 1943, the Eighth Air Force began attacks on Germany, and the Fifteenth Air Force began attacks on Italy. US aircraft manufacturers had produced 96,318 planes in one year by 1944. On May 7, 1945, Germany surrendered, even though it had developed formidable new weapons in the V-1 pilotless bomber jet and the V-2 missile, with a 2,000-pound warhead, a range of 354 kilometers, and a speed of up to 5,800 kilometers per hour. Meanwhile, the air war raged on in the Pacific. The United States continued its strategic bombing of Japan with B-29 aircraft. The threat posed by Japanese kamikaze suicide bombers increased, as it surfaced that Japan intended to use the majority of its remaining planes as kamikazes. The awful and unnecessary, and entirely political, decision was made to use the atomic bomb on Japan. On August 6, 1945, the first bomb was dropped on the city of Hiroshima, and a second was dropped on Nagasaki on August 9. Within six days, Japan had surrendered. BERLIN AIRLIFT After World War II, Germany was literally divided in two. East Germany was walled off by the Soviet Union and placed under a Communist government. West Germany was free and protected by US forces. Landlocked within East Germany was the city of Berlin, half of which was not under Communist control and was protected by the US and its allies. In 1948 and 1949, the United States airlifted food and

History of Human Flight

supplies into Berlin, using Douglas C-47 and C-54 aircraft, in what was at the time the largest humanitarian effort in history. AN INDEPENDENT US AIR FORCE Following World War II, the United States altered its development and use of military aircraft. In 1947, the Army Air Corps became the newest branch of the armed forces, and the US Air Force and the Central Intelligence Agency (CIA) were created. These two acts were not independent. The CIA, the Air Force, and private aerospace contractors worked together to develop new aircraft and intelligence technologies. One such facility, Lockheed Corporation’s secret development division was dubbed the “Skunk Works,” and its projects were so classified that they did not appear even in federal budgets. In 1947, a military plane broke the sound barrier. THE COLD WAR Although World War II had ended, the Cold War took its place. A handful of the world’s most powerful nations possessed the capability to make and deploy nuclear weapons. The United States and the Soviet Union, former World War II allies, both developed intercontinental ballistic missiles (ICBMs) capable of attacking the other nation when launched from home soil. The deterrent effect of these weapons was called mutual assured destruction (MAD). The destructive capabilities and the distrust between the two superpowers spurred the space race, as well as military actions in other parts of the world. THE KOREAN WAR On June 25, 1950, Communist North Korea crossed the thirty-eighth parallel, invading free South Korea. The Soviet Union aided North Korea, and the United States helped to defend South Korea. November, 1950, saw the world’s first all-jet air battle between a Soviet MiG and a US F-80 Shooting Star. Later, the US F-86 Sabre jet would prove an even

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tougher opponent against the MiGs. On July 27, 1953, an uneasy armistice was signed, but difficulties persisted, and US forces continued to help defend the peace. THE JET AGE In the 1950s civil aviation also entered the jet age. In May, 1952, the British De Havilland Comet commenced passenger service. Soon after entering service, three such planes seemingly came apart in midflight. By April, 1954, the plane was grounded, and the American Douglas DC-8 and the Boeing 707 came to dominate the world’s jet passenger airline market, giving their manufacturers the lead in the industry for years, until a European consortium was formed in 1970 to manufacture the Airbus aircraft. THE SPACE AGE The Soviet Union led the early space race, with a series of firsts. In 1957, the Soviet Union sent Sputnik, the world’s first Earth-orbiting satellite, into space. In 1961, the Soviet cosmonaut Yuri Gagarin became the first human in space with his Earth-orbiting mission Vostok 1. The United States followed shortly thereafter with Alan Shepard, Virgil “Gus” Grissom, and John Glenn. In the early 1960s international attention and tensions were also riveted by the work of American spy planes when the Soviets shot down an American U-2 plane and captured its pilot, Gary Powers, and spy plane photographs revealed Soviet movement to put missiles on Cuba, within range of the United States. THE VIETNAM WAR Events in Southeast Asia during the 1950s and 1960s involved the United States in another war and again tested the nation’s air power. While trying to stabilize a tenuous political situation between North and South Vietnam, the United States was drawn into a police action that would occupy two presidents and divide the American people. The Vietnam

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War was called the first television war, with fighting broadcast on the nightly news. MEMORABLE EVENTS IN FLIGHT Many people claim there are events in history that are so important that every person who was alive and sentient can remember exactly where they were and what they were doing when it happened. One such event was the assassination of President John F. Kennedy on November 22, 1963. Another three involve human flight. On July 20, 1969, the Eagle lunar landing craft set down on the surface of Moon, where US astronauts Neil Armstrong and Edwin “Buzz” Aldrin left footprints and an American flag. Another such event is the January 28, 1986, explosion of the space shuttle Challenger, which killed seven astronauts, including Christa McAuliffe, the first teacher in space. The third day is September 11, 2001, when four hijackings by Al-Qaeda terrorists resulted in the deaths of almost five thousand people at the World Trade Center and the Pentagon. OPERATION DESERT STORM By 1991, when Americans would watch another war on television, aviation had evolved significantly from the time of the Vietnam War. During the Gulf War, missile-mounted cameras delivered photographic images while delivering bombs with exactitude measurable in inches, without risking American fliers. Global positioning system (GPS) satellites had been placed in orbit and delivered with inexpensive handheld units target exactitude. One-half million American military personnel and the armament and equipment to support them were delivered to the other side of the world. The war was over within days. For the first time since the invention of the airplane, there was a war with no aces. HIGHWAY IN THE SKY The technology revealed in Operation Desert Storm helped to revolutionize the world of civilian aviation

Homebuilt and Experimental Aircraft

Principles of Aeronautics

throughout the late twentieth century. The US air traffic control (ATC) system had been pinning its hopes on a new microwave landing system, which was scrapped in favor of the astonishingly accurate GPS system. Computer flight control and navigation technology was adapted for affordable installation in small general aviation and personal aircraft, delivering military precision and failsafe computer systems for private pilots. The ability to wage war without legions of pilots convinced the US Joint Chiefs of Staff that the military should develop uninhabited aerial vehicles (UAVs) for both intelligence and weapons delivery. Lightweight and quiet jet engines and lightweight but strong composite flight structures were developed for the job. The military was not alone in realizing the value to human flight of such engines and materials. In 1977, one of the oldest obstacles to human flight was finally overcome, when Gossamer Condor, powered by pedals, made the first human-powered flight. Its designer had worked with extremely lightweight but strong materials, as had Jeana Yeager and Dick Rutan in piloting the Voyager, which in 1986 became the first plane to make a nonstop transglobal flight without refueling. The 1903 Wright Flyer and the 1988 Voyager sit side by side at the National Air and Space Museum, awaiting the next addition in the history of human flight. —Mary Fackler Schiavo Further Reading Anderson, Dale, Ian Graham, and Brian Williams. Flight and Motion: The History and Science of Flying. Taylor & Francis, 2015. Curtis, Wendy, and Evan Penn Serio. Big History in Flight. CreateSpace Independent Publishing Platform, 2016. Dawkins, Richard. Flights of Fancy: Defying Gravity by Design and Evolution. Head of Zeus, 2021. Grant, R. G. Flight, the Definitive Illustrated History of Aviation. Dorling Kindersley Ltd., 2022. Reay, D. A. The History of Man-Powered Flight. Elsevier Science, 2014.

Spitzmiller, Ted. The History of Human Space Flight. UP of Florida, 2017. See also: Aeronautical engineering; Aerospace industry in the United States; Neil Armstrong; Biplanes; Blimps; Glenn H. Curtiss; Leonardo da Vinci; Jimmy Doolittle; Amelia Earhart; First flights of note; Yuri Gagarin; German Luftwaffe; John Glenn; Robert H. Goddard; Heavier-than-air craft; Helicopters; Hindenburg; Hot-air balloons; Human-powered flight; Lighter-than-air craft; Otto Lilienthal; Charles A. Lindbergh; Ernst Mach; Billy Mitchell; Montgolfier brothers; Sir Isaac Newton; Wiley Post; Eddie Rickenbacker; Rockets; Russian space program; Igor Sikorsky; Spaceflight; Supersonic jets and commercial airfare; Wind tunnels; Wright brothers’ first flight; Chuck Yeager

Homebuilt and Experimental Aircraft Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Experimental aircraft are aircraft that are still in the experimental testing phase of development, or craft built by amateurs from kits or plans. Experimental aircraft are important for the progress of the aviation industry. Flight test of an experimental aircraft validates expected performance criteria, tests structures, evaluates handling characteristics, and more. Scientific research is dependent on flight test results from experimental aircraft. A new airplane as such is experimental, but a proven and certified aircraft is experimental when mounted with new, untried applications requiring testing. KEY CONCEPTS doped fabric: a resin-impregnated fabric that will become the waterproof “skin” of a homebuilt aircraft, providing a streamlined contour for the aircraft

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formers: structural skeleton pieces that provide the basic shape of the aircraft fuselage prototype: the first unit of a newly designed aircraft as the foundation piece for refinement and functioning of the design TYPES OF EXPERIMENTAL AIRCRAFT A distinction must be made between true experimental aircraft and amateur-built airplanes constructed from kits or plans. The former category contains aircraft involved in research and design at costs reaching into the millions of dollars, while the latter group contains aircraft constructed by individuals for fun, at a substantially smaller cost. An experimental military project or airline endeavor may reach hundreds of millions of dollars before the pro-

Principles of Aeronautics

duction run gains approval. The homebuilt or kit plane project, on the other hand, is notably less expensive. Single airplanes built for personal use have flown at a cost well below $10,000. Another difference between the two groups of experimental aircraft includes the time spent in flight test. The large company projects may spend more than a year in flight test while a homebuilt or kit plane undergoes a basic forty-hour testing period. THE DESIGN PROCESS Regardless of the group to which the airplane belongs, every aircraft flying today began in the much the same manner. Industry discovers a need for a particular design or a mission requirement. After the creation of a new concept, pilots, engineers, and

Flown in 1955, the Stitts SA-3A Playboy CF-RAD, pictured here at the Canada Aviation and Space Museum, was Canada’s first homebuilt aircraft.Photo by Ahunt at English Wikipedia. [Public domain.]

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mechanics discuss and research ideas for the production of the new craft. The next step is sketching the design idea. More than a few new ideas begin as pencil drawings on napkins in restaurants over lunchtime discussions. From the first idea sketches, aeronautical engineers further refine the drawing by use of computer-aided design software. During the initial design period, engineers exchange ideas and make concessions and compromises that are eventually lofted into the drawing process. Modelers next construct a model of the new aircraft. Models of the new airplane are necessary for many reasons. Wind-tunnel testing requires models of different sizes. Models enable designers to visualize proportional sizing. Problems not visible on a drawing board may vividly stand out in three dimensions. After studying the models in depth, technicians and engineers develop mock-ups of various sections and components of the new aircraft. These mock-ups allow others to test the airplane and offer their opinions to the designers regarding positive and negative aspects of the new craft. Mock-ups also allow pilots, engineers, and mechanics to spot problems before production of the aircraft. The earlier design changes can be made, the more economically they can be incorporated into the production schedule.

profiles are developed for the initial flight and the flight test program to follow. The most exciting event at an aircraft plant is the first flight of a new design, an event anticipated by everyone in the company. Typically, first flights do not last more than about three-quarters of an hour. The only concern the company and the pilots have with the aircraft is whether it will fly. Initial flight testing does not try to test the edge of the flight envelope. Pilots, engineers, and managers are not interested in how fast or high the airplane is capable of flying on the first flight; they only want to see it fly and see it handle the way it is expected to handle. Test crews will address other questions regarding the performance envelope later. As test pilots put the aircraft through its paces, they keep meticulous data on every aspect of each flight. The company uses the data to refine the flying qualities of the prototype and suggest changes in the production run. After completion of the flight test program, the new airplane will finally reach acceptance. If it is an airliner, company officials from the airline will either accept or reject the new craft. Military acceptance is somewhat different, in that the airplane flies through a more intense flight test program. Additionally, flight tests of military aircraft involve weapons systems compatibility.

THE PROTOTYPE AND FIRST FLIGHT At the completion of the design process, construction of the prototype begins. Construction of one or two copies of the new craft is required for testing. This is an expensive proposition; until the craft goes into mass production, the per cost of the craft can be phenomenal. The purpose of the prototype is further research and development. Changes follow in quick succession as shortcomings become evident and better construction methods become available. As the first prototypes are readied for flight, flight

HOMEBUILT EXPERIMENTAL AIRCRAFT The other type of experimental airplane is the burgeoning kit plane and homebuilt industry. In 2001, approximately twenty-two thousand homebuilt aircraft constructed by amateur builder-pilots were flying in the United States. One organization dedicated to homebuilt enthusiasts is the Experimental Aircraft Association (EAA). The national headquarters of the EAA is located in Oshkosh, Wisconsin. There are local chapters of the EAA throughout the United States that allow home-

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Principles of Aeronautics

The wooden structure of a homebuilt Pietenpol Air Camper under construction. Photo by Ahunt, via Wikimedia. [Public domain.]

building proponents to exchange technical information and discuss problems or other building concerns. Members also enjoy the camaraderie of other members, along with the encouragement offered at monthly or biweekly meetings. In Canada, the federal government branch Transport Canada Civil Aviation (TCCA) maintains a list of approved aircraft construction kits. A homebuilt aircraft must be registered and supported by a suite of documentation. However, the registration fee is currently just $110. Following registration, the aircraft is subject to all federal reg-

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ulations regarding the care and maintenance of privately owned aircraft. ADVANTAGES AND DISADVANTAGES OF THE HOMEBUILT There are many reasons pilots choose to build their own aircraft. One reason is performance. Since the 1950s, aircraft manufacturers have done relatively little in the way of increasing or improving aircraft performance. For example, a popular older model production aircraft flies at 100 knots, burns 28.5 liters of fuel per hour, and can travel up to about 500

Principles of Aeronautics

nautical miles before refueling. The latest model of the same aircraft today can fly about 106 knots, burns approximately 30.4 liters of fuel per hour, and has a range of about 500 nautical miles. There has been virtually no change over the years in its performance. However, ingenious amateur aeronautical engineers and pilots have produced airplanes using the same engines in newly designed airframes. These homebuilts are capable of 175 knots, with ranges beyond 1,000 nautical miles, using the same power plant, producing the same power at the same fuel flow. The backyard engineers have managed to attain much greater speeds over longer distances for the same amount of fuel. While an increase in performance is advantageous, there are limitations to the use of a homebuilt airplane. For instance, homebuilts or kit planes are restricted from use in commercial operations. They also require a large amount of time for construction. Some pilots complain about the handling qualities of the smaller airframes. However, in many instances the advantages far outweigh the disadvantages. One very important advantage is that of knowing the aircraft. As the owner-builder of the aircraft, the pilot is intimately familiar with all the systems of the airplane. Another benefit to constructing an airplane is that as the manufacturer of the craft, the builder is qualified to perform all maintenance and inspections on the airplane according to the Federal Aviation Regulations (FARs). In other words, each year, when it is time to inspect the airplane, the builder of the craft can save hundreds if not thousands of dollars in shop fees. Another advantage homebuilt owners have over production aircraft owners is that they can build the airplane at their own pace, according to their own budget. A homebuilder can spend $600 one month, and if funds are lacking the next month, the building process can slow down while the builder spends

Homebuilt and Experimental Aircraft

only $50, or nothing at all. For the pilot who has purchased an aircraft with a loan, however, the bank will want payment each month. BUILDING TIME The time required to build a kit plane varies. With a fast-build option, a builder can have a plane airborne in less than a year. On the other hand, some builder-pilots have dragged out a project for twenty-five years. Average build times, depending on the make and model of the airplane, is about 2,000 to 3,000 hours of work. Working part time, this equates to two to four years. After the airplane is finished, it is time for the first test flight. For this important first flight, many homebuilders opt to hire a professional test pilot familiar with their design. This is a smart choice for pilots who have allowed their flying skills to degrade during the construction process. Many would like to fly their homebuilt on the first flight, but this is a case where vanity must defer to common sense. Following the initial flight, the homebuilder is free to fly the airplane through a test program. During this time, the airplane will be restricted to one geographical area for forty flight hours. After the airplane is proven through this test period, the restrictions are lifted. Now the owner-builder-pilot is free to use the airplane for personal use as any other airplane. TYPES OF HOMEBUILTS While the production market is limited, homebuilts offer a wide selection of airplanes to the potential builder, running the gamut from very simple single-seat ultralights to highly sophisticated, six-place family airplanes. A pilot desiring a production four-place family airplane that can cruise faster than 145 knots will spend more than $400,000, plus ongoing maintenance and upkeep. On the homebuilt market, however, one can choose from among many relatively inexpensive four-seat, high-speed,

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long-range airplanes. Similar savings can be made in homebuilding seaplanes and amphibious aircraft. After choosing the group of airplanes from which to select, a pilot might research the safety aspects of particular designs. The next decision is the airplane’s appearance, which is a matter of personal preference. Not only are there a wide variety of kits available for homebuilt plane types, but builders who want more than what is available on the market also can design a new craft incorporating all the desired attributes. Most homebuilts and kit planes are smaller in size and weigh less than manufactured planes. Coupled with proportionally larger engines, this tends to increase the performance of the design. Many builders opt for two-place designs that provide opportunities for a great deal of compromise. Most pilots find themselves flying alone or with only one other person. When they have a need to carry more, they rent larger airplanes. CONSTRUCTION TECHNIQUES AND MATERIALS An exciting aspect of homebuilding is the selection of construction materials. Construction techniques vary with different airplanes. A popular airplane that uses construction techniques of the 1930s and 1940s is the Pitts Special. The fuselage of the Pitts is constructed of steel tube and wood formers covered with doped fabric. The wings are constructed of wood spars and ribs and covered in fabric. Other homebuilts constructed completely of wood are beautiful examples of artistic creation. Some use woodworking techniques that date back to World War I. Conventional construction of modern light airplanes is sheet aluminum riveted on formed aluminum structures. In the early 1970s, homebuilders began experimenting with foam, epoxy, and other composite materials. Many of the new materials, such as Kevlar fabric and carbon graphite, are

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lighter than steel and provide greater tensile strength. In addition to being stronger and lighter, some of the new composite materials are easier to work with and enable the builder to form the compound curves of aeronautical structures more easily than when working with conventional materials. ENGINES With the use of Stirling engines or engines that use other alternative fuels, pilots may be able to fly farther and faster than ever imagined. Modern engines are one reason homebuilts are capable of such great speeds. The fact that the aircraft is experimental allows owner-pilot-builders to select the engine of their choice. Selection of the engine can be just as varied as selection of the aircraft itself. The modern certified aircraft engine is a costly item. A new Lycoming or Continental aircraft engine can easily exceed costs of $20,000. Propellers and other accessories on the engine can drive that cost up another $5,000 or $6,000. Although such high prices may be discouraging, the homebuilder has many engine options. The engine on a homebuilt does not have to be a certified aircraft engine. Because the airplane does not adhere to the specifications defined by the Federal Aviation Administration (FAA), builders can use any power plant they find suitable for their design. Indeed, many homebuilts are flying using engines from Volkswagen cars, chainsaws, snowmobiles, or outboard motorboats. An automotive V-8 engine powers one of the most popular homebuilts, the Lancair IVP. The IVP can carry four passengers in pressurized comfort at altitudes above 25,000 feet, at speeds greater than 330 knots. Another example for those who doubt the use of automotive engines is that of the Volkswagen engine. Properly adapted for aerial use, this little engine has powered airplanes as fast as 370 kilometers per hour while getting more than 34 kilometers per liter.

Hot-Air Balloons

Principles of Aeronautics

While some doubt the validity of homebuilt and kit-plane flying, this class of airplane is here to stay. For individuals with some technical background and the ability to work with their hands, homebuilding is a way to acquire an airplane inexpensively. The rewards they reap flying their own creations are many; chief among them the cost savings the homebuilder will realize over the years of flying the airplane. Saving money, however, is only a part of the compensation. The greatest reward is watching people admire the airplane. Most homebuilders are very pleased to hear the comments others make regarding the craftsmanship and work invested in the airplane. —Joseph F. Clark III Further Reading Dwiggins, Don. Build Your Own Sport Plane: With Homebuilt Aircraft Directory. Hawthorne Books, 1979. Dyer, Edwin M. Japanese Secret Projects: Experimental Aircraft of the IJAS and IJN, 1939-1945. Midland, 2009. Griehl, Manfred. Luftwaffe X-Planes: German Experimental Aircraft of World War II. Pen & Sword Books, 2015. Norton, Bill. U.S. Experimental and Prototype Aircraft Projects: Fighters, 1939-1945. Specialty Press, 2008. Pace, Steve. The Big Book of X-Bombers and X-Fighters: USAF Jet-Powered Experimental Aircraft and Their Propulsive Systems. Voyageur Press, 2016. Winchester, Jim, editor. Concept Aircraft: Prototypes, X-planes and Experimental Aircraft. Thunder Bay Press, 2005. See also: Aerodynamics and flight; Aeronautical engineering; Airplane maintenance; Flight testing; Forces of flight; German Luftwaffe; Messerschmitt aircraft; Military aircraft; Model airplanes; Training and education of pilots; Ultralight aircraft; Wright brothers’ first flight; Wright Flyer

Hot-Air Balloons Fields of Study: Physics; Aeronautical engineering; Mathematics; Pilot training

ABSTRACT Hot-air balloons are large round inflatable sacks filled with hot air that rise above the ground, towing compartments for passengers or cargo. The first crewed free flight on November 21, 1783. The hot-air balloon ushered in the age of human flight by providing a means for people to leave the ground and rise to significant heights for sustained periods. This lighter-than-air craft also proved that air was still breathable and life sustaining at higher altitudes. KEY CONCEPTS parachute valve: a piece of fabric at the top of a balloon, in the shape of a parachute and used to release hot air for a controlled descent skirt: a short, cylindrical section at the base of a hot-air balloon; can be thought of as the “chimney” for the gas burner providing the hot air tethered balloon: one that is affixed securely to the ground and therefore has a limited altitude range variometer: a device that measures the rate of ascent or descent of a balloon EARLY HISTORY Hot-air balloons inaugurated the era of human flight. The hot-air balloon proved for the first time that a human being could survive at some height above the ground. This was a notable scientific achievement, since prior to the advent of balloons many people had no clear understanding of how high the breathable atmosphere extended. With the advent of the balloon, the dream of leaving the confines of the earth was realized. This was the dawn of a new era, the preamble to the space age. Two French brothers, Joseph-Michel and Jacques-Étienne Montgolfier, invented the hot-air balloon in 1783. Their inspiration was the observation that a paper bag placed over a small, smoky indoor cooking fire rose into the air. As a test, they lined a large cloth bag with paper and caused it to rise by filling it with black smoke and hot air from

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fire pot, to which the pilots frequently added straw. Although the pilots had to extinguish small fires when parts of the balloon’s flimsy material ignited from sparks, they landed safely. This flight was witnessed by thousands of Parisians. The second human free-flight balloon ascension was made by Professor Jacques Alexandre César Charles and a passenger on December 1, 1783, in a hydrogen-filled balloon. Hydrogen-filled balloons, filled with dangerously combustible gas, nevertheless had the advantage of requiring only one-third the gas volume of hot-air balloons for the same buoyancy. The larger hot-air balloons were more difficult to handle and transport. The risk of balloon material igniting from the sparks of open fires made them hazardous, and smoke from the fire choked the riders. Flight time was limited by the fuel supply. In contrast, the hydrogen balloon was well suited to long-term scientific and military observation. Its gas was not expelled until a descent was desired. For these reasons, hot-air balloons became rare during the years between 1800 and 1960, while hydrogen types flourished. Hot air balloon in flight. Photo by Bradley Lewis, via Wikimedia Commons.

an outdoor straw fire. Later, the Montgolfiers sent animals aloft first on tethered balloons and then on a free flight in order to prove that the animals could survive far above the ground. They succeeded, and the animals lived. After the free flight, King Louis XVI of France was persuaded to allow the required permission for humans to attempt a test flight. With approval, two other Frenchmen, Jean-François Pilâtre de Rozier and the Marquis François-Laurent d’Arlandes, made the first human free-flight balloon ascent on November 21, 1783. In a 21.3-meter-high, 14-meter-diameter Montgolfier balloon, they flew for 25 minutes, traveling for several kilometers over the city of Paris. The balloon carried its own grated

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TWENTIETH CENTURY A rebirth of hot-air ballooning occurred on October 10, 1960, in Bruning, Nebraska, when an American, Ed Yost, performed a free-flight test in his prototype balloon. His new design featured a polyurethane-coated nylon envelope and a propane-powered burner. This system was much safer and more rugged than that of the previous era. In 1963, Yost and a partner, Don Piccard, traveled to Britain and made the first English Channel crossing by hot-air balloon. By the 1970s, hot-air balloon manufacturers had flourished in the United States, Britain, and France. A hot-air balloon could be obtained for the price of an expensive motorcycle, thus bringing it within the budget of many sports enthusiasts. Many balloonists

Principles of Aeronautics

Hot-Air Balloons

started businesses offering one-hour chartered flights for one hundred dollars. High cost and adventurous record-breaking attempts also continued in the twentieth century. Bertrand Piccard and Brian Jones achieved the first nonstop circumnavigation of the globe in March 1999, in a combination hot-air-and-helium balloon. They launched their Breitling Orbiter 3 from Switzerland, traveled around the world in twenty days, and landed in Egypt. CONSTRUCTION The main parts of a modern hot-air balloon are the envelope, the burner-fuel system, and the basket. The envelope is the bag, or air sack, containing hot air. It is constructed of pieces of fabric, usually nylon. Each slice of fabric, called a gore, consists of panels and stretches from the top to the bottom of the balloon. Forming the gores in specific dimensions determines the overall shape of the balloon. Round, oblong, and special shapes such as those of a piggy bank, a soda can, and even Godzilla, Mickey Mouse, and other characters have been constructed, often for advertising. The envelope top, or crown, is constructed with a parachute valve, a piece of fabric in the shape of a parachute. It is attached in such a manner that a section can be pulled away when a pilot pulls a connected cord. This action releases some of the hot air through the top of the balloon, reduces the overall inside air temperature, and causes the balloon to descend. At the base, which is open, there is usually a short, cylindrical fabric section called the skirt. It is coated with fire-resistant material, because it is close to the flame. The burner is mounted in a frame attached between the basket and the skirt. Propane from a tank is ignited by the burner’s pilot light. An on-off valve allows the pilot to control fuel flow. The amount of fuel available determines the amount of time the balloon can stay aloft. Baskets, which usually hold from two to five people in

Hot air balloon in flight. Photo by Kropsoq, via Wikimedia Commons.

addition to propane fuel tanks, are still made of wicker because the shock-absorbent material helps provide a soft landing for passengers and pilots. USE In the United States, hot-air balloons must be registered with the Federal Aviation Administration (FAA). Pilots are certified in one of three classes: student, private, or commercial. In Canada, balloons and balloon pilots are registered with Transport Canada, and balloon licensing is regulated in much the same way as a regular pilot’s license with required instruction and certifications through stages. The Canadian Balloon Association is the parent group for Canadian balloon pilots. Hot-air balloons

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lift off by inflating their containing envelope with heated air. Because hot air expands, the heated air becomes lighter than the ambient, cooler surrounding air, which pushes upward against the air bag and provides the lift necessary for flight. Within the envelope, the heated, lighter air rises and displaces the cooler, heavier air, which descends. Prior to launch, the pilot checks weather conditions for local winds and any possible storm indications. Storms are hazardous for several reasons: Lightning strikes can electrocute people and damage the balloon; rain, hail, or snow can cause damage, present visibility problems, and make the balloon heavier; and high wind makes launching and landing dangerous. To check the weather conditions, a pilot can either consult a weather service or go to the launch site and send up a small party-size helium balloon. From the balloon’s changing position as it ascends, the pilot can gauge both the speed and the direction of the wind. Because the air inside the envelope of a hot-air balloon is lighter than the air outside, the relative pressure is upward, and air does not pour out of the open-ended base. To prepare for launching, the deflated envelope is laid out on the ground, then the gas-fired burner is positioned to force heated air into the envelope opening. Another method uses a powerful fan to initially provide a partial cold-air inflation. The balloon, as it inflates, gradually rises from horizontal to vertical. The balloon basket is anchored to prevent a gust of wind from blowing the balloon away prior to launch. The ground crew also holds the basket down until pilot and passengers are ready to launch. Once the balloon is fully inflated, more lift can be generated by continuing to heat the air within it. When the lift of the heated air is greater than the total weight of the balloon, basket, equipment, and occupants, the balloon rises. Just prior to this point, all tie-lines are released, and the crew releases the basket. The pilot fires the burner again, and the bal-

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loon lifts off. Whenever the burner is turned off, the air in the bag gradually cools, and the balloon slowly descends. Neither the heating nor the cooling causes an instant effect. There is a thermal time lag, usually of a half-minute or more, due to the large amount of air to heat or cool. To maintain one particular altitude, the pilot periodically turns the burner on and off. If the pilot is skilled, the balloon will neither rise nor fall to any appreciable degree during this operation. In addition to keeping the burner off, the pilot can also cause a descent by momentarily opening the parachute valve and allowing some of the hot air to escape. Because a hot-air balloon has no propulsion system, the horizontal direction and speed of the balloon are determined by the prevailing winds. Riding with the wind, passengers feel no wind except for gusts. Winds generally blow in different directions at different altitudes. The pilot seeks out the desired wind to carry the balloon in the desired direction. There is an upper limit to balloon ascension, even if the burner is left on continuously. As elevation above the ground increases, the air becomes thinner. Eventually, the air becomes so thin that it provides no further lift as the balloon’s buoyancy is equal to that of the air surrounding it. The pilot uses an onboard altimeter to determine the balloon’s altitude and a variometer to indicate the rate of ascent or descent. A global positioning system (GPS) device can be used to obtain a readout on the balloon’s latitude, longitude, and elevation. Normally, the pilot tries to land in a large, flat, open area, such as a field, meadow, flatland, or desert, with no nearby obstructions, such as power lines, telephone poles, trees, or fences. In the case of a no-wind landing, the touchdown can be very gentle. If there is wind, the basket will drag along the ground until stopped by friction. After the basket has stopped, the pilot can fully open the parachute valve, causing the balloon to collapse completely.

Howard R. Hughes

Principles of Aeronautics

The ground crew tracks the balloon’s path in a recovery vehicle and meets it at the landing site. Hot-air balloon accidents have yielded relatively few accidents in comparison to other forms of air travel. In July 2016, the deadliest hot-air balloon crash in US history took place in Texas when a balloon crashed and killed all sixteen people on board. The balloon went down in Lockhart, Texas, and caught fire before crashing to the ground. Another crash happened in January 2018 in Luxor, Egypt, however, it caused only one death; the twelve other passengers survived. In June 2021, a hot-air balloon made contact with a power line while in flight. The contact caused the balloon to catch fire and the resulting crash killed all five passengers. FESTIVALS There are thousands of hot-air balloon pilots worldwide, and periodic balloon festivals are held in many countries. These festivals usually feature competitions and mass ascensions, in which as many as five hundred balloons float in the air at the same time. The annual October festival in Albuquerque, New Mexico, is one of the largest festivals, with more than 850 balloons aloft in cooperative weather. —Robert J. Wells Further Reading Becker, Jean. Hot Air Balloons: History, Evolution and Great Adventures. White Star, 2010. Holmes, Richard. Falling Upwards: How We Took to the Air. William Collins, 2014. Kim, Mi Gyung. The Imagined Empire: Balloon Enlightenments in Revolutionary Europe. U of Pittsburgh P, 2017. Kotar, S. L., and J. E. Gessler. Ballooning: A History, 1782-1900. McFarland Inc. Publishers, 2011. Simons, Fraser. The Early History of Ballooning-The Age of the Aeronaut. Read Books Limited, 2020. See also: Aeronautical engineering; Blimps; Leonardo da Vinci; Dirigibles; First flights of note; Flight balloons; Steve Fossett; Hindenburg; History of human flight;

Lighter-than-air craft; Montgolfier brothers; Training and education of pilots; Jules Verne; Weather conditions; Wind shear

Howard R. Hughes Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT Howard Hughes was born on December 24, 1905, in Houston, Texas, and died on April 5, 1976, in an airplane en route from Acapulco, Mexico, to Houston, Texas. Hughes was a pioneer aviator, aircraft designer, builder, and multimillionaire. A world-class pilot and aircraft designer, Hughes is known, in aviation, for building and flying the Spruce Goose, by far the largest aircraft of its day and for founding the Hughes Aircraft Corporation. THE MANY FACES OF HOWARD HUGHES Howard R. Hughes was born into wealth in 1905, attended private schools in his youth, and later studied at both Rice University and the California Institute of Technology. He first flew in an airplane when he was fourteen years old. Upon the death of both of his parents in his late teens, he inherited $871,000 and the Hughes Tool Company, which held the patent on the most widely used well-drilling bit in the world. Hughes left school to operate the company, but his interests were not limited by that business. From 1926 through 1932, Hughes was active in the production of motion pictures, became a pilot, and founded the Hughes Aircraft Company, where he designed, built, and flew airplanes. In 1935, in a plane of his own design, he set a world speed record of 567.12 kilometers per hour and followed that with transcontinental records in 1936 and 1937. Following his record-breaking around-the-world flight in 1938, he was treated to a ticker-tape parade in

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New York City. By 1938, he held nearly every major aviation award. For his flying accomplishments he won the Harmon Trophy in 1938, the Collier Trophy in 1939, the Octave Chanute Award in 1940, and a Congressional Medal in 1941. By 1939, Hughes had placed the Hughes Aircraft Company at the forefront of design in experimental military airplanes. During World War II, his company was a major defense contractor. Hughes designed the eight-engine Spruce Goose, a large plywood seaplane contracted as a troop carrier in 1942. Its only flight was piloted by Hughes in 1947. Hughes’s successes placed him among the top three most wealthy Americans. At the war’s end, Hughes reentered the Hollywood scene and controlled RKO Studios from 1948 through 1955. Throughout the 1950s, he concen-

Principles of Aeronautics

trated on expanding his business empire, and by the 1960s, he was a billionaire. He owned the controlling stock in Trans World Airlines (TWA) until he was forced to sell out in 1966. Hughes had suffered a nervous breakdown in 1944 and had been critically injured in a 1946 air crash, after which he developed an addiction to morphine that led to other dependencies. Always an eccentric, he went into seclusion in 1950, becoming a reclusive shell of a person living in a rented hotel room in Las Vegas, Nevada. Almost nothing is known of this period of his life. Hughes was elected into the Aviation Hall of Fame in 1973. He died in 1976 on board a plane traveling from Acapulco, Mexico, to Houston, Texas, where he was to receive medical treatment. —Kenneth H. Brown Further Reading Barlett, Donald, and James B. Steele. Howard Hughes: His Life and Madness. W. W. Norton, 2004. Marrett, George J. Howard Hughes: Aviator. Naval Institute Press, 2007. Porter, Darwin. Howard Hughes, Hell’s Angel. Blood Moon, 2005. Richardson, Jeffrey. Howard Hughes and the Creation of Modern Hollywood. America Through Time, 2019. Wellman, Douglas, and Mark Musick. Boxes: The Secret Life of Howard Hughes. Boutique of Quality Books Publishing Company Inc., 2016. See also: Aeronautical engineering; Aerospace industry in the United States; Air transportation industry; Glenn H. Curtiss; Amelia Earhart; Charles A. Lindbergh; Wiley Post; Eddie Rickenbacker; Igor Sikorsky; Chuck Yeager

Human-Powered Flight

Hughes with his Boeing 100 in the 1940s. Photo via Wikimedia Commons. [Public domain.]

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Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics

Principles of Aeronautics

ABSTRACT Human-powered flight means sustained, level flight powered solely through the use of human muscle. The earliest attempts at human flight were powered by the pilot’s own activity. Although nonhuman engines proved to be the key to viable aircraft, experimental human-powered craft continued to be built into the twenty-first century. These craft were often the products of university students and professors or enthusiastic amateurs, spurred on as much by the intellectual challenge as by the stimulus of monetary prizes. KEY CONCEPTS carbon fiber: a modern fiber material consisting of carbon produced by processing fibers of a carbohydrate material such as linen or cotton, leaving just the carbonized base as strong, black fibers that can be used as threads and woven into a light-weight, high-strength structural fabric for advanced composite materials homebuilders: individuals who build their own aircraft at home, or “DIYers” Kevlar: a polyaramid material with very high tensile strength that can be made into high-strength threads and woven into structural fabrics for use in advanced composite materials wing-flapping propulsion: properly termed ornithoptic propulsion, flying in the manner of birds, by the flapping of wings WHY FLY? Although it will never be an efficient mode of transportation, human-powered flight satisfies an innate human desire to emulate the freedom of birds. Unfortunately, using arms to flap attached wings cannot generate adequate lift and propulsion, as bird wings do, but well-conditioned athletes can maintain fractional horsepower outputs for long periods of time using their legs, and this, in the late twentieth century, led to a series of remarkably long, controlled flights over both land and water. The earliest truly successful flights were made by entrepreneurs

Human-Powered Flight

in response to monetary prizes. Unfortunately, it does not appear that the resulting aircraft are practical flying machines for the vast majority of fliers and homebuilders, requiring too much muscle power and being far too large, too fragile, too expensive and too vulnerable to being upset by atmospheric turbulence. The earliest seekers of human-powered flight were the tower and bridge jumpers, dating from at least 1000 CE. Stability and control, as well as wing-flapping propulsion, were always in question, although some glides were at least partially successful. The key insight, as it was for motor-powered human flight, was to separate lift and propulsion: use fixed wings for lift and an engine and a propeller for propulsion. Monetary prizes eventually stimulated gifted teams of designers and enthusiasts and they have transformed the almost universal dreams of human-powered flight into reality. The first prize offered for human-powered flight, the Prix Peugeot of 1912, was won in Paris in 1921 by bicycling champion Gabriel Poulain and his Aviette when he flew 12.2 meters in a straight line, using biplane wings attached to a bicycle to glide forward after he abruptly increased his wing angle to lift him into the air. By 1937, a 70-kilogram German

Engelbert Zaschka’s human-power aircraft, Berlin 1934. Photo by Popular Science, vol. 125, no. 1, July, 1934, via Wikimedia.

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bicycle racer, who was able to generate a momentary power output of 1.3 horsepower, had upped the straight-line distance to almost 805 meters using a 34-kilogram sailplane-like airplane, called Mufli, with a pedal-powered propeller. THE KREMER PRIZE Late-blooming enthusiasm for human-powered flight in England resulted in seven men forming the Cranfield Man-Powered Aircraft Committee in 1957. Industrialist and philanthropist Henry Kremer was then inspired, in 1959, to offer a £5,000 prize for the first British human-powered aircraft that could take off and fly a figure-eight course between two turning points not less than 805 meters apart and fly over a 3-meter height marker at the beginning and end of the flight. In response, three postgraduate students of Southampton University formed SUMPAC (for Southampton University Man-Powered Air Craft) and made the first British human-powered flight of 15.24 meters in 1961. But SUMPAC was unable to exceed 610 meters in flight length and could not turn more than about 80 degrees. A second effort, backed by the famous De Havilland Aircraft Company, flew about 915 meters at an average height of over 2 meters in 1962 in Puffin, creating a world record that was to stand for ten years, but the craft could not be turned more than about 80 degrees. The distance record, still a British record, was made in 1972 by John Potter with a 1,071-meter flight in Jupiter. Meanwhile, Professor Hidemasa Kimura of Nihon University was working with his students, and in 1966 their Linnet made Japan’s first human-powered flight; the flight was only 15 meters in length but this began a long-term commitment to human-powered aircraft. By 1977 their Stork B had established a new world record of 2,093.9 meters in a flight of over four minutes and was a strong contender for the Kremer Prize. By 1967, the Kremer Prize for a figure-eight human-powered flight had been doubled and opened

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to entrants from any country. Then, in 1973, with still no winner in sight, Henry Kremer raised the award to £50,000 (about $129,000 at that time), the largest prize in the history of aviation. The human-powered movement was very slow to reach the United States. Finally, in 1973, Professor Eugene Covert and students at the Massachusetts Institute of Technology (MIT) built a two-person biplane (Burd), which apparently never even left the ground under its own power. Credit for the first human-powered flight in the United States is therefore given to Joseph Zinno, retired from the US Air Force, for his 23.5-meter flight in 1976 in Olympian ZB-1, which he had designed, built, and flown. MACCREADY’S SUCCESS It was in that same year of 1976 that fifty-one-year-old Californian Paul MacCready decided that he knew how to design a human-powered aircraft that could win the Kremer Prize. He had impressive credentials for the challenge. As a teenager, MacCready was a Junior National Champion in model airplanes; at the age of sixteen, he soloed in a Piper Cub; in 1947, he graduated from Yale with a degree in physics; in 1948 and 1949, he was the National Soaring Champion; in 1952, he received a doctorate in aeronautics from California Institute of Technology; in 1957, he decided to go into business for himself, eventually forming AeroVironment in 1971 to solve energy and environmental problems. MacCready’s initial design was inspired by observations of soaring birds and the Rogallo hang glider. He realized that the low power output from a human meant that the airplane had to have a very large wing area (around 1,000 square feet) and have a very high aspect ratio (a large span of about 30.5 meters with a chord of only about 3 meters) in order to minimize lift-induced drag. The drag of the required bracing wires for an extremely light, fragile aircraft with these huge dimensions would be acceptable if flight speeds and flight altitudes were very

Principles of Aeronautics

low. The structure would have to be designed to be easily repaired, the same rule practiced by the Wright brothers. Aerodynamicist Peter Lissaman convinced MacCready that a canard surface had to be added to his wing for pitch stability. Turning the aircraft was a major hurdle, because the outer wing always wanted to stall; wing warping and a rolling front (canard) surface eventually solved this problem. MacCready thought it would take six weeks to win the prize; it took a year. Flight control, weather, power, and structural problems kept cropping up. Finally, on August 23, 1997, the Kremer Prize was won by MacCready’s team with an official flight time

Human-Powered Flight

of 6 minutes, 22.5 seconds. Their huge airplane, the Gossamer Condor, weighed 31.75 kilograms and the pilot and engine, bicycle racer Bryan Allen, weighed 62.1 kilograms. On September 22 of that year, Maude Oldershaw piloted the Gossamer Condor. It is interesting to note that all of the principal members of the team were model aircraft builders; many were also hang-glider enthusiasts. The Gossamer Condor is now the property of the National Air and Space Museum in Washington, D.C. CROSSING THE ENGLISH CHANNEL Retired British Rear Admiral Nicholas Goodhart had developed a huge (42-meter wingspan)

MIT Light Eagle human-powered aircraft, predecessor to the MIT Daedalus aircraft. Photo by Armstrong Flight Research Center, NASA, via Wikipedia. [Public domain.]

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twin-powered airplane, Manflier, for the Kremer Prize and, beaten to the prize, he suggested that the next great project should be a human-powered flight across the English Channel. Henry Kremer responded with a doubled award of £100,000 for the first such successful human-powered flight. It would require remaining in the air for more than one hour. MacCready quickly rose to the new challenge with a new, lighter, stronger, more streamlined design, the Gossamer Albatross, using high-technology materials (carbon fiber-reinforced plastic, DuPont Kevlar, and a new, superthin DuPont Mylar for the covering), a new cruise prop designed by aerodynamicist Eugene Larabee of MIT, and new cockpit instrumentation, including a Polaroid sonar altimeter. By June, 1978, guided in his rigorous physical training by physiologist Joseph Mastropaolo, Allen was able to generate 0.31 horsepower for 2.5 hours, enough time, MacCready thought, to make a successful flight. However, two months later, the warp control jammed and the Gossamer Albatross suffered the worst crash of the program, although the pilot was only bruised. Some eight months later, on April 25, 1979, Allen flew a record flight of over one hour and the decision was made to go to England and try for the prize. After weeks of waiting on the English coast for suitable weather, at 5:51 a.m. on June 12, 1979, pilot/power plant Allen lifted off from England. Slowed by a headwind, out of his crucial water supply, and cockpit instrumentation out of battery power, Allen felt at four different times that he would have to give up the effort. Somehow, fighting cramping legs and nearing exhaustion, he struggled on and, at 7:40 a.m., touched down lightly in France, winning the second large Kremer Prize for the team. He had flown the Gossamer Albatross 36.2 kilometers in 2 hours, 49 minutes (an average speed over the water of less than 12.9 kilometers per hour).

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THE LANGFORD GROUP Meanwhile, in 1978, the MIT Model Rocket Society, led by student John Langford, decided to see if they could get the hangar-evicted MIT Burd to fly with the addition of two 1.5-horsepower model airplane engines. The attempt failed, but the society pledged to build something that would fly by springtime, a craft that would compete for Kremer’s Channel prize. The society’s Chrysalis made its first flight on June 5, 1979, just one week before Allen won the English Channel prize. They had built a real flying machine, however, one that ended up being flown by more than forty-five pilots before the end of the summer. Two years later, Langford had returned to MIT as a graduate student and led an effort to win a new Kremer prize, this one for flying around a 1,500-meter course in less than three minutes, requiring a speed of 34 kilometers per hour. Energy storage before takeoff was allowed. The group’s Monarch won the $33,000 prize on May 11, 1984, narrowly beating MacCready’s latest effort. Inspired, the Langford team vowed to pursue the ‘ultimate’ human flight challenge: to emulate the fabled flight of the exiled Daedalus and son Icarus from the island of Crete to Greece. Thus began a four-year effort that ended up requiring more than $1 million worth of corporate and institutional sponsorship. Key members of the team included builder Juan Cruz, Mark Drela (completing a thesis on low-speed aerodynamics), physiologist Ethan Nadel, and a group of highly trained and conditioned superathletes, as well as leader Langford. The result was a plane that weighed 31.75 kilograms without pilot, power plant, or fuel, 8.9 meters in length, with a wingspan of 34.1 meters, and with a cruising airspeed of 24.14 kilometers per hour. On April 23, 1988, piloted and powered by a Greek bicycle champion racer, Kanellos Kanellopoulos, they flew their Daedalus the more

Hypersonic Aircraft

Principles of Aeronautics

than 113 kilometers from Crete over the sea to Santorini in about four hours, breaking up just 27.5 meters from shore when the craft encountered a strong headwind with turbulent air. The next frontier in human-powered flight appears to be the helicopter. In 1980, a prize of $25,000 was offered by the American Helicopter Society for the first human-powered helicopter that could hover for a full minute, rising to at least 3 meters above the ground at some point during that period. Successful hovers have so far not exceeded about 24 seconds and the height requirement appears to be even more difficult. —W. N. Hubin Further Reading Allen, Bryan. “Winged Victory of Gossamer Albatross.” National Geographic, Nov. 1979, pp. 640-51. Dorsey, Gary. The Fullness of Wings: The Making of a New Daedalus. Viking, 1990. Grosser, Morton. Gossamer Odyssey: The Triumph of Human-Powered Flight. Houghton Mifflin, 1981. Herman, Irving P. Physics of the Human Body. Springer International Publishing, 2016. Langford, John S. “Triumph of Daedalus.” National Geographic, Aug. 1988, pp. 191-99. Long, Michael E. “Flight of the Gossamer Condor.” National Geographic, Jan. 1978, pp. 131-40. Nagaraj, Vengalatorre T., Inderjit Chopra, and Darryll J. Pines. Gamera, A Human-Powered Helicopter: In Pursuit of an Aviation Milestone. American Institute of Aeronautics and Astronautics Inc., 2021. Reay, D. A. The History of Man-Powered Flight. Elsevier Science, 2014. Singer, Bayla. Like Sex with Gods: An Unorthodox History of Flying. Texas A&M UP, 2003. Vogel, Steven. Prime Mover: A Natural History of Muscle. W. W. Norton, 2003. See also: Aerodynamics and flight; Aeronautical engineering; Airfoils; Flight propulsion; Forces of flight; Glider planes; Gravity and flight; Materials science; Paper airplanes; Wright Flyer

Hypersonic Aircraft Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Hypersonic refers to aircraft capable of flying at speeds greater than 3.5 times the speed of sound. Flight at hypersonic speeds is required to efficiently reach space and return from it. Aircraft flying at hypersonic speeds encounter several problems in addition to those encountered at lower speeds, including extremely high temperatures and pressures, as well as the need for control systems that react to disturbances extremely quickly. KEY CONCEPTS aerospike rocket engine: a rocket engine with an inverted structure such that the exhaust exits rearward along the outside of the engine to provide thrust rather than rearward from the inside of the engine resistive load: any property that resists the motion of the aircraft, such as compression of the air in front of the vehicle, resulting from the aircraft’s motion throttleable rocket engine: a rocket engine whose power output can be controlled and adjusted “on the fly” APPLICATIONS OF HYPERSONIC FLIGHT Although they are advertised as being able to circle the globe in less than four hours, hypersonic aircraft do not offer much promise to weary airline passengers in the near future. Most applications of hypersonic aircraft are either in the context of warfare or spaceflight. With the exception of the terminal stage of certain missiles, hypersonic flight is conducted exclusively at very high altitudes, where the air density and pressure are a fraction of their values at sea level.

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In space, any controlled maneuver requires the expenditure of fuel, whereas inside the atmosphere, aerodynamic forces can be used by deflecting control surfaces. Because the speed required for Earth orbit at low altitudes is approximately 29,000 kilometers per hour, spacecraft reenter the atmosphere at extremely high Mach numbers, ranging typically from 25 on the space shuttle to over 36 on the Apollo capsules. In reentry flight, the craft spends only a few minutes at hypersonic speed before decelerating to supersonic speeds, which allow more controlled maneuvering and gliding to selected landing sites. During ascent into space, modern hypersonic aircraft ride on rocket boosters, spending the shortest possible time in the dense lower regions of the atmosphere. This situation will have to change when aircraft use air-breathing engines for propulsion at hypersonic speeds. Air-breathing engines take the oxygen needed for combustion from the atmosphere, reducing the amount lifted from the ground. The advantage of mastering this technology may be easily seen. In a hydrogen-oxygen propulsion system, which is the most efficient known means of chemical propulsion, 89 percent of the total weight of fuel and oxidizer is oxygen. However, air-breathing hypersonic flight poses several difficult problems. FEATURES OF HYPERSONIC FLOWS The air flowing around a vehicle moving at hypersonic speeds has several interesting features. In front of the vehicle’s nose there is an extremely strong shock wave. This shock is like the blast wave from an explosion, heating the air enough to make oxygen and nitrogen molecules vibrate at high frequencies, dissociate into atoms, radiate large amounts of heat, and even ionize. The air becomes compressed to values as high as ten to one hundred times its normal density, and the extremely high pressure imposes very high resistive loads on the ve-

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hicle. Over the upper surface of the vehicle, the air accelerates to supersonic speeds, and the density and temperature fall so quickly that the dissociated air does not have time to recombine. Around the vehicle, shock waves lie very close to the surface. Air friction heats the surface and increases the drag on the vehicle. Within a thin layer, the flow changes properties through a very large range. The reliable design of such vehicles is extremely difficult, because accurate, full-scale aerodynamic prediction is difficult and expensive, and approximate methods do not provide enough accuracy. Refining the prediction methods through experimentation is also not easy because of the sheer difficulty of conducting flow experiments under hypersonic conditions. Hypersonic wind tunnels require extreme pressures, temperatures, and flow rates and can operate under steady conditions only for milliseconds. In the early 1990s, when President Ronald Reagan’s National Aerospace Plane initiative resumed the development of technology for hypersonic flight, the total experience of wind-tunnel testing at hypersonic speeds from all tests conducted to that date was estimated to be less than one second. EARLY HYPERSONIC FLIGHTS The inherent difficulties of hypersonic travel have not prevented the development of hypersonic vehicles. In 1933, German rocket expert Eugen Sänger published his concept for an “antipodal bomber” (antipodes are two points on opposite sides of the earth), a crewed hypersonic glider launched on a large rocket that would deliver bombs to distant targets across the globe, by skipping in and out of the atmosphere. This project was canceled in 1942. In February, 1949, the US Army launched the V-2 WAC Corporal rocket from the White Sands Missile Range in New Mexico. The rocket reached a speed of 5,633 kilometers per hour and an altitude of 160 kilometers before the WAC Corporal stage ignited and reached an altitude of 393 kilometers miles.

Principles of Aeronautics

The vehicle reentered the atmosphere at a speed of more than 8,050 kilometers per hour. On April 12, 1961, Soviet flight major Yuri Gagarin returned to Earth after an orbital flight during which he traveled at hypersonic speeds that charred the surface of his spherical space capsule. Since then, rockets with or without human crew have routinely flown in the hypersonic range. THE X-15 PROGRAM The first hypersonic research airplane, which used aerodynamic lift to stay aloft, was the North American X-15, developed by the National Aeronautics and Space Administration (NASA). Air-launched from a B-52 bomber, the X-15 first flew on June 8, 1959. It was 15.47 meters long with a wingspan of 6.86 meters. Its Thiokol XLR-99 throttleable rocket engine burned a mixture of anhydrous ammonia and liquid oxygen to reach Mach 6. By the end of August, 1963, the X-15 piloted by NASA’s Joseph A. Walker had reached a record altitude of 107,960.2 meters. X-15 flight tests revealed a number of interesting facts about hypersonic flight, including the existence of turbulent hypersonic boundary layers, and that turbulent heating rates were lower than predicted by theory, but that hot spots developed on the surface, causing material failures. The flights demonstrated piloted transition from aerodynamic to reaction controls and back again, including hypersonic/supersonic reentry at angles of attack up to 26 degrees and glide to precise landings. The third X-15, which set a number of records, was lost, along with its pilot, Michael J. Adams, on November 15, 1967. The program was canceled after this fatal accident. AIR-BREATHING PROPULSION The X-15A-2 vehicle was designed to pursue the idea of hypersonic flight using an air-breathing engine instead of a rocket. The plan was to test a ram-

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jet engine using supersonic combustion, although it was never flight-tested. The challenges of hypersonic air-breathing propulsion were numerous, but two fundamental problems dominated. First, theoretical research showed that drag is incurred when a supersonic flow is slowed down to subsonic speeds, as through a shock, and when heat is added to a flow at a high Mach number. At low supersonic speeds, the drag due to the shock is less than the drag that would be incurred if heat were added to a supersonic flow. Thus, ramjet engines for flight at less than Mach 4 use shocks to slow the flow to subsonic speeds before adding heat by burning fuel in the combustion chamber of the ramjet engine. At speeds above Mach 4, it is more efficient to add heat at supersonic speeds than it is to slow down the flow to subsonic conditions. Second, in supersonic combustion there is an extremely short time available in which to add fuel, mix it with the air moving at supersonic speed, and complete the combustion before the flow exits the engine. The X-30, dubbed the National Aerospace Plane, was built to develop supersonic combustion ramjets, or scramjets. In the 1990’s, this program was canceled without any test flights. NASA’s Langley Research Center at the turn of the millennium described a program called Hyper X to study hypersonics technology at speeds from Mach 5 to Mach 10. The NASA/Boeing X-43 was designed to study scramjet-powered flight at speeds from Mach 6 to Mach 10, following launch using a Pegasus booster rocket from a B-52 over the Pacific Ocean. Released at 6,100 meters, the 3.7-meter-long X-43 was designed to be accelerated by the rocket to a speed of Mach 6 and an altitude of 27,432 meters. The scramjet engine was designed to operate for seven to ten seconds, accelerating the X-43 to a speed of Mach 10. The engine had an oval-shaped air intake and burned hydrogen with air in a copper combustion chamber at supersonic speeds. Lacking landing gear, the vehicle was designed to transmit

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data before expending its energy and falling into the ocean. LIFTING BODIES The US Air Force and NASA have continued to study hypersonic lifting bodies for hypersonic reentry. The X-20 Dyna-Soar hypersonic boost glide vehicle was designed to launch into orbit on a Titan III solid-fuel rocket, reenter the atmosphere, and glide at hypersonic speeds to deliver a nuclear weapon. However, the Dyna-Soar was shelved without ever flying. The X-23 lifting body flew in 1966, demonstrating maneuvering during reentry. The X-24A and X-24B craft investigated low-speed characteristics of lifting bodies. The NASA/Boeing X-37, part of NASA’s Hyper X program, investigates technologies for orbit-on-demand, including hypersonic glide reentry. The vehicle, built by Boeing Phantom Works, is 8.4 meters long, with a wingspan of 4.6 meters, a weight of about 6 tonnes, and a payload bay 2.13 meters long and 1.22 meters in diameter. The Boeing X-40 maneuverable spaceplane integrated technology demonstrator is a predecessor to planned vehicles for flight at Mach 16 up to 91,440 meters, sending 454 to 1,361 kilograms of payload into orbit for military missions. In the 1990s, the X-38 crew return vehicle (CRV) extended the work on the X-23 and X-24, in the development of a lifting body that would be attached to the International Space Station (ISS) as an emergency escape system. Separated from the ISS using rocket thrusters and able to carry an incapacitated crew of up to seven, the X-38 was to navigate using the global positioning system (GPS) and glide through hypersonic reentry at angles of attack of up to 38 degrees, with the heating taken by thermal tiles on the vehicle. Following supersonic maneuvering using flaperons, the X-38 would deploy first parachutes and then a large parafoil. An on-board automatic control system would guide the

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parafoil-suspended X-38 to a soft landing into the wind. In 2001, the X-38 project was canceled, but a parallel project conducted by the French and the European Space Agency (ESA) aimed to develop a reusable crew taxi or crew rescue vehicle. NASA’s space shuttle used aerodynamic lift at high angles of attack and hypersonic speeds of up to Mach 25 during its reentry and descent into the atmosphere. It used heat-shield ceramic tiles to protect critical parts of the fuselage and the wings during reentry. By using aerodynamic lift in the upper atmosphere, the shuttle could stay at high altitudes, where the air is much thinner, until much of its orbital kinetic energy has been dissipated before sinking into the denser parts of the atmosphere and gliding to a runway landing. OTHER REUSABLE HYPERSONIC SPACEPLANES The NASA/Lockheed Martin X-33 reusable launch vehicle, a smaller predecessor of the Lockheed VentureStar concept, tested the idea of achieving single-stage boost to low-Earth orbit using ultra-lightweight composite fuel and oxidizer tanks and a rocket engine that used an “aerospike” external expansion nozzle. The X-33 was canceled in 2001, along with the launch-on-demand, glide-tolanding NASA/Orbital Sciences X-34 vehicle. The Buran (Snowstorm) Soviet space shuttle had its first orbital flight in November, 1988, on an Energia booster. It circled Earth twice between 247 and 256 kilometers above the surface before reentering and landing at Tyuratum. The French-European Hermes spaceplane project was conceived as a mini-shuttle, carrying four to six crew members and 4,500 kilograms of cargo into orbit atop an Ariane-5 booster. The project was canceled in 1992 but may have been replaced by the continuing ESA crew rescue vehicle project. Hypersonic air-breathing vehicle designs including a scramjet engine were reported to have been

Principles of Aeronautics

tested in the 1990s by Russia on top of a surface-to-air missile and by the Indian Space Research Organization using a solid rocket booster. The Japanese Hope-X space shuttle and the British horizontal takeoff and landing (HOTOL) concepts do not appear to have progressed beyond small-scale wind-tunnel models. As of mid-2001, there was no reusable hypersonic aerodynamic vehicle in operation other than NASA’s space shuttle. WAVERIDER CONCEPTS Aircraft configurations optimized for aerodynamic flight at hypersonic speeds are generally thin and flat, with a highly swept fuselage and short wings. At hypersonic cruise, the upper surface of the vehicle stays essentially parallel to the flight direction, minimizing the disturbance to the flow there. The oblique shock formed under the vehicle stays very close to the slanted surface, providing a lifting cushion of high pressure. Such vehicles are called hypersonic waveriders. Waverider configurations generally exhibit rudders and elevator-aileron combinations (elevons) as primary control surfaces. Tip flaps improve lift-to-drag ratio and rudder effectiveness. Such vehicles are unstable in pitch, like many modern fighter planes, and require fly-by-wire, stabil-

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ity-augmented computer control. Concepts for reducing the shock drag and heating at the nose include the Russian idea of injecting ionized gas jets into the shock, and the American idea of ionizing the gas ahead using plasma or laser beams. —Narayanan M. Komerath Further Reading Houchin, Roy F. US Hypersonic Research and Development: The Rise and Fall of “Dyna-Soar,” 1944-1963. Taylor & Francis, 2006. Jenkins, Dennis R., and Tony Landis. Hypersonic: The Story of the North American X-15. Specialty Press, 2008. Musielak, Dora. Scramjet Propulsion: A Practical Introduction. John Wiley & Sons, 2022. Viviani, Antonio, and Giuseppe Pezzella. Hypersonic Vehicles: Past, Present and Future Developments. IntechOpen, 2019. ———. Hypersonic Vehicles: Applications, Recent Advances, and Perspectives. IntechOpen, 2022. See also: Advanced propulsion; Aerodynamics and flight; Aeronautical engineering; Flight propulsion; Fluid dynamics; Forces of flight; High-altitude flight; High-speed flight; Jet engines; Ernst Mach; National Aeronautics and Space Administration (NASA); Ramjets; Rocket propulsion; Scramjet; Shock waves; Sound barrier; Space shuttle; Spacecraft engineering; Supersonic aircraft; Wind tunnels; X-Planes; Chuck Yeager

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J Jet Engines Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Thermodynamics ABSTRACT A jet engine is an internal combustion engine that converts the chemical energy of fuel into mechanical energy in the form of thrust by the high-speed exhaust gases leaving the engine nozzle. Fundamentally, a jet engine is a gas turbine. Gas turbines are used widely to generate electricity in power stations, to power boats, trains, military tanks, and to drive gas pipeline compressors. It is as a jet engine, however, that the gas turbine has had its greatest industrial impact. KEY CONCEPTS afterburner: a system that increases jet engine thrust by injecting additional fuel into the exhaust stream as it exits the combustion chamber, where the extra fuel is instantly ignited thus producing a large quantity of additional exhaust gas to augment thrust axial compressor: a compressor in which air is driven through a tapering containment housing by a series of rotating blades that sweep a successively smaller diameter centrifugal compressor: a compressor in which the rotating blades compress air by directing it outward radially from the central axis and forcing it against the containment housing ideal gas law: the relationship of pressure (P), temperature (T), volume (V), mass in moles (n), and the universal gas constant, R, as PV = nRT

DESCRIPTION A jet engine consists of several components: a compressor, a combustion chamber, a turbine, and an exhaust system. At the front of the jet engine is the compressor, driven by a shaft connected to the turbine. The compressor takes in air from the atmosphere and compresses it to produce high-pressure air. The air then enters the combustion chamber, where jet fuel is injected in fine droplets. Combustion occurs with ignition, and the hot gases exit the combustion chamber and enter the turbine, downstream of the combustion chamber. The hot gases leave the turbine through the exhaust system, exiting at high speed from the jet engine nozzle and propelling forward both the jet engine and the aircraft attached to it. The principle behind this propulsion is described by Newton’s third law of motion, which states that for every action there is a reaction equal in magnitude and opposite in direction. Jet propulsion is the movement of a small mass of gas at a very high velocity, whereas in a propeller-driven airplane, the propeller moves a large mass of air at low velocity. HISTORY The first patent for the modern gas turbine was granted in 1930 in England to Sir Frank Whittle, whose design led to the W-l turbojet engine with a centrifugal compressor. Simultaneously yet independently, German engineer Hans P. von Ohain also obtained a patent for a turbojet engine less than five years after Whittle had received his patent. Von Ohain’s engine also had a centrifugal compressor, whereas another German design, by Ernst Heinkel, had an axial compressor. A plane with von Ohain’s

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He-S3b engine made its test flight on August 27, 1939. Two years later, on April 12, 1941, a plane with Whittle’s turbojet engine was tested. By the 1940s, German turbojet engine prototypes had adopted the axial compressor, whereas British models all used the centrifugal compressor. By 1943, the two main turbojet engines were Germany’s Junkers Jumo 004 and Britain’s Rolls-Royce Welland. In the United States, General Electric Company engineers modified the Whittle engine and produced an American version called the I engine. In October, 1942, the I engine had its first test flight in the Bell P-59A. During World War II, scientists from both Allied and Axis countries worked feverishly to design and test the jet engine. By 1946, several countries had successfully developed turbojet engines. In the United States, General Electric built the I-16 and

Principles of Aeronautics

the I-40. In England, Rolls-Royce built the Welland I, the Derwent I, and the Nene. In Germany, Junkers manufactured the Jumo 004-4. By the 1950s, the turbojet had been applied to civilian aviation. Early passenger jets included the De Havilland Comet I, which first flew in 1952 but was withdrawn from service two years later because of fatal accidents. By 1954, the United States had successfully tested its Boeing 707 passenger jet, with regular flights commencing four years later. After adopting the jet engine, commercial aviation quickly developed into an international business, with most countries operating their own national airlines. International jet aircraft industries manufacture many types of planes: wide-body models that can carry hundreds of passengers; supersonic planes that can fly at Mach 2; aircraft that are capable of vertical takeoffs and landings (VTOL); and military jet air-

An afterburner glows on an F-15 Eagle engine following a repair during an engine test run November 10, 2010, at the Florida Air National Guard base in Jacksonville International Airport. Photo via Wikimedia Commons. [Public domain.]

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craft that can take off and land on the relatively short deck of an aircraft carrier. The gas-turbine engine that powers all jet aircraft is, however, basically the same engine that was designed by Sir Frank Whittle in 1930. It consists of a compressor, combustion chamber, turbine, and exhaust system. There are four major manufacturers of jet engines: Société Nationale de Construction de Moteurs Aeronautiques (SNECMA) in France, Rolls-Royce in the United Kingdom, and Pratt & Whitney and General Electric in the United States. COMPONENTS Compressor. The purpose of the compressor is to increase the pressure of the gas. In the compressor, atmospheric air is pressurized to typically ten to forty times the inlet pressure, and consequently the temperature of the air rises to between 200 and 550 degrees Celsius. The ideal gas law states the proportionality of the pressure and temperature of gases. The two basic types of compressors are the centrifugal-flow compressor and the axial-flow compressor. The centrifugal-flow compressor, preferred for smaller engines, is a simpler device that uses an impeller, or rotor, to accelerate the intake air and a diffuser to raise the pressure of the air. The axial-flow compressor is favored for most engine designs, because it is capable of increasing the overall pressure ratio. The axial-flow compressor uses rotors fitted to many differently sized discs to accelerate the intake air and stationary blades, known as stators, to diffuse the air until its pressure rises to the correct value. The type of compressor used in an engine affects the engine’s exterior appearance: An engine with a centrifugal compressor usually has a larger front area than an engine with an axial compressor. An engine with an axial compressor is longer and has a smaller diameter than an engine with a centrifugal compressor.

Jet Engines

Combustion chamber. In the combustion chamber, jet fuel, typically kerosene, is injected in fine droplets to allow for fast evaporation and subsequent mixing with the hot, compressed air. The compressed air is used for combustion, which occurs with ignition; the hot pressurized gases then reach temperatures of 1,800 to 2,000 degrees Celsius. To protect the combustion chamber walls from these high temperatures, some of the intake air, routed from the compressor, is used to cool the combustion chamber walls. The three types of combustion chambers are the multiple chamber, annular chamber, and can-annular chamber. The multiple chamber, with individual chambers, or flame tubes, arranged radially, is used on engines with centrifugal compressors and early axial-flow compressor engines. The annular chamber has one annular flame tube with an inner and outer casing. The can-annular chamber combines characteristics of the multiple chamber and the annular chamber and has several flame tubes in one casing. Turbine. In an aircraft engine, the sole function of the turbine, which is downstream of the combustion chamber, is to power the compressor. Similar to the compressor, the turbine has several large discs, though typically not as many as the compressor, fitted with many blades. Gases at temperatures between 850 and 17,000 Celsius exit the combustion chamber and enter the turbine. The hot gases impact the turbine blades, causing the discs carrying them to rotate at high speeds, averaging 10,000 revolutions per minute. The discs are mounted on a shaft that is connected at the other end to the compressor discs. The turbine blades are usually made of nickel alloys, because these materials are both strong and able to withstand the high temperatures within the turbine. The blades are fitted with many small holes through which cool air is forced to prevent the blades from melting.

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Exhaust system. The jet engine’s exhaust system is configured so as to maximize the thrust of the engine. The exhaust system consists of a nozzle and may also include a thrust reverser and an afterburner. In a basic exhaust system, the hot gases leaving the turbine are discharged through a propelling nozzle at a velocity that provides thrust. In VTOL aircraft, the nozzle can be made to swivel vertically so the aircraft can move up and down. The thrust reverser enables the aircraft to slow down and stop more quickly upon landing, allowing the aircraft to land on shorter runways without relying solely on braking devices. Thrust reversal quite simply reverses the direction of exhaust gases to decelerate the aircraft. The two main thrust reversal methods use either clamshell-type deflector doors or bucket-type deflector doors on a retractable ejector. Afterburner. An afterburner is used in some aircraft, such as supersonic jets (SSTs) and military aircraft that need to reach high speeds in a short time. Unburned oxygen from the jet engine’s exhaust system flows into an afterburner, where more fuel is injected into the hot gases to augment the thrust of the engine. The temperature of the exhaust gases increases, thereby increasing the gas velocity and the thrust of the engine. This additional thrust allows for acceleration to supersonic speeds or for faster takeoffs to accommodate combat situations or the shorter runways of aircraft carriers. TYPES OF JET ENGINES The basic types of jet engines are the turbojet, turbofan, turboprop, and turboshaft. Turbojet and turbofan engines are called reaction engines, because they derive their power from the reaction to the momentum of the exhaust gases. The turboprop and turboshaft engines, however, utilize the momentum of the exhaust gases to drive a power turbine that, in turn, drives either a propeller or an output shaft.

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Turbojet. The turbojet was the first jet engine type to be invented and flown. In a turbojet, all of the intake air passes through the compressor and is burned in the combustion chamber. The hot gases pass through the turbine and are then expelled through the exhaust nozzle to provide the thrust required to propel the engine and the aircraft attached to it forward. Examples of the turbojet appear in both civilian and military aircraft, including the Olympus 593 in the Concorde SST. Turbofan. By the end of the twentieth century, the turbofan had become the most popular choice for aircraft propulsion in both civilian and military aircraft. In a turbofan engine, a large fan is placed at the front of the compressor of the jet engine. The amount of intake air is increased up to ten times. Most of this cool intake air either bypasses the compressor, combustion chamber, and turbine and exits the fan nozzle separately, as in separate-flow turbofans, or gets mixed with the turbine exhaust and exits through a common nozzle, as in the mixed-flow turbofan. Afterburners in turbofan engines are equipped with a mixer to mix the cooler bypass air with the hot exhaust gases, thus allowing an easier burning of the bypassed air. Turbofan engines are characterized by their bypass ratio, which is the mass flow rate, in pounds per second, of air going through the fan divided by the mass flow rate of air going through the compressor. Low-bypass engines have ratios of up to two; medium-bypass engines have ratios from two to four, and high-bypass engines have ratios from five to eight. Ultrahigh-bypass engines have bypass ratios from nine to fifteen or higher. The highest bypass ratios, although providing high propulsion efficiency, likewise involve large, heavy components. The advantage of the turbofan is its greater thrust on the same amount of fuel, which results in more efficient propulsion, lower noise levels, and an improved fuel consumption. Turbofan jet engines

Principles of Aeronautics

power all modern commercial aircraft, such as the Boeing 747; business jets, such as the Gulfstream IV; and most military airplanes, such as the F-18. Future turbofans may combine various bypass features. For example, the variable-cycle engine (VCE) would have both high-bypass and low-bypass features. Such an engine would be designed for planes that travel at subsonic and supersonic speeds. The VCE would operate by a valve that would control the bypass stream, either increasing it for subsonic speeds or decreasing it for supersonic speeds. Turboprop. A turboprop engine is a turbojet engine with an extra turbine, called a power turbine, that drives a propeller. In the turboprop engine, the jet exhaust has little or no thrust. Planes powered by turboprop engines typically fly at lower altitudes and reach speeds up to 400 miles per hour (640 kilometers per hour). An example of the turboprop engine is the Rolls-Royce DART in the British Aerospace 748 and the Fokker F-27. Turboshaft. A turboshaft is a turboprop engine without the propeller. The power turbine is instead attached to a gearbox or to a shaft. One or more turboshaft engines are used on helicopters to power the rotors. The turboshaft engine has industrial applications, such as in power stations, and marine applications, such as in hovercrafts. JET ENGINE POLLUTION Because it is an internal combustion engine whose exhaust gases flow directly into the environment, a jet engine is a serious source of air pollution. Because of its high level of noise, it also causes noise pollution. AIR POLLUTION Air pollution results from the combustion process of the gas-turbine engine. Jet-engine emissions, including carbon dioxide, carbon monoxide, hydrocarbons, and nitrogen oxide gases, contribute to both the greenhouse effect and atmospheric ozone deple-

Jet Engines

tion. They also endanger the health of people especially near airports. Some regard aircraft transportation as more polluting than any other type of transportation, including the automobile. Generally, older aircraft are greater polluters than newer aircraft. The turbofan and bypass turbofan engines in particular use less fuel and therefore pollute less. A new MD-90 is about 50 percent more economical than a DC-9 or a DC-10, because the newer plane uses less fuel. Nevertheless, studies show that per passenger, an airplane uses twice as much fuel per passenger than does a car with three passengers, when the car drives the distance a jet travels in one hour (770 kilometers). Airplane fuel consumption could be improved by eliminating various classes of cabins in commercial aircraft. Business- and first-class cabins seat fewer passengers, thereby reducing the overall fuel efficiency of the aircraft. If a reduction in carbon dioxide aviation emissions is to be realized, older aircraft must be replaced with newer ones that have more fuel-efficient engines. The most environmentally friendly aircraft include the B-777 and B-767. Carbon monoxide is contained in the combustion exhaust fumes. Both carbon monoxide and hydrocarbon emissions occur at the highest rates when airplanes idle their engines on runways, where often twenty planes are lined up waiting for takeoff. Airplanes pollute hundreds of times more when idling than when flying. Nitrogen dioxide emissions contribute to acid-rain formation. The emission of hydrocarbons, especially hydrocarbon radicals, contributes to ozone formation. In terms of these emissions, the new high-bypass turbofan jet engines pollute much less than older turbofan and turbojet engines. Sulfur dioxide emissions also contribute to acid-rain formation. Nitrogen oxides have a possible role in ozone depletion, and its reduction can only be effected by less air traffic in general.

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NOISE POLLUTION Noise is measured on a logarithmic scale in decibels, a unit of audio power. The decibel range is from zero decibels to about 160 decibels. A normal conversation takes place at about 40 decibels, and a noise level of 90 decibels would make it impossible to hear a normal conversation. The noise from a nearby jet takeoff is about 110 decibels. The main source of jet-engine noise is the propulsion system and the resultant noises generated by both internal and external processes. In early turbojet engines, the noise occurred behind the exhaust nozzles when the hot exhaust gases mixed with the cool atmospheric gas. The high-bypass turbofan engines alleviated this noise problem. Nevertheless, noise issues continue with the fan noise and core noise in high-bypass turbofan engines. Fan noise can be either broadband, discrete tone, or multiple tone, depending on whether the tip speed of the fan rotor blades is subsonic or supersonic. Core noise includes the noise from the rotation of the compressor, the noise from the turbulence generated in the combustion chamber, and the noise from the turbine. Aircraft noise is regulated by federal rules that become increasingly stringent with time. Aircraft are classified as either stage one, for very noisy, 1960s-era jetliners; stage two, for moderately noisy, 1970s-era jetliners; or stage three, for more quiet, modern aircraft. Beginning in the year 2000, only stage-three aircraft may operate in the United States and Europe. Supersonic commercial aircraft, such as the Concorde, operate under different regulations and are only allowed to take off and land at certain airports because of the noise they make during takeoff. In actuality, the Concorde was not allowed to fly over North American airspace at supersonic speeds, and after the fatal crash of a Concorde, the entire fleet was grounded after

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twenty-two years of service. An updated version of the aircraft is scheduled to fly only by Emirates Airline in the Middle East, beginning in 2022. To reduce external noise, the exhaust stream velocity may be decreased by flying jets that have a turbofan engine with a bypass ratio of five or higher. Such engines reduce exhaust noise considerably. With lower levels of external noise, internal noises are more audible. To reduce internal noise, the fan tip speed can be decreased, although this would result in the necessity of more compressor stages, therefore resulting in a heavier engine. More spacing between the rotor and stator would also lessen the noise, but the larger spaces would require a larger engine. —Said Elghobashi Further Reading Crumpsty, Nicholas, and Andrew Heyes. Jet Propulsion: A Simple Guide to the Aerodynamics and Thermodynamic Design and Performance of Jet Engines. Cambridge UP, 2015. Giffard, Hermione. Making Jet Engines in World War II: Britain, Germany and the United States. U of Chicago P, 2016. Golley, John. Jet: Frank Whittle and the Invention of the Jet Engine. Datum Publishing Ltd., 2009. Hunecke, Klaus. Jet Engines: Fundamentals of Theory, Design and Operation. Crowood Press, UK, 2010. Rolls-Royce. The Jet Engine. 5th ed., Wiley, 2015. See also: Advanced propulsion; Aerodynamics and flight; Aeronautical engineering; Air transportation industry; Aviation and energy consumption; Avro Arrow; DC plane family; Firsts; Flight propulsion; Fluid dynamics; German Luftwaffe; Greenhouse gases; High-speed flight; Hypersonic aircraft; Military aircraft; Propulsion technologies; Ramjets; Rockets; Scramjet; Shock waves; Sound barrier; Supersonic aircraft; Supersonic jetliners and commercial airfare; Andrei Nikolayevich Tupolev; Turbojets and turbofans; Turboprops; Chuck Yeager

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Jet Propulsion Laboratory (JPL) Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Jet Propulsion Lab, or JPL, in Houston, Texas, is the primary National Aeronautics and Space Administration (NASA) site for control and operations of US lunar and interplanetary missions. Almost all US lunar and planetary missions have been controlled from JPL since it was transferred to NASA’s control. Prior to that time, many of the United States’ early developments in rocketry and missiles took place at JPL. JPL is also responsible for NASA’s Deep Space Network of tracking and telemetry stations. THE EARLY YEARS The Jet Propulsion Laboratory (JPL), now located in the city of La Cañada Flintridge, California, evolved from a project in rocket propulsion that began in the early 1930s. During that time, one of the leading organizations in aeronautics study and research was the Guggenheim Aeronautical Laboratory at the California Institute of Technology (GALCIT), directed by Dr. Theodore von Kármán, in Pasadena. In 1936, graduate student Frank J. Malina successfully approached von Kármán with a proposal to write his doctoral dissertation on rocket propulsion and high-altitude sounding rockets. This early work in rocket propulsion attracted the attention of two local rocket enthusiasts, John Parsons and Ed Forman. Together, the four men worked through the theories of rocket propulsion and designed a rocket engine, with some insights gained from a meeting with rocket pioneer Robert H. Goddard. Because no safe campus facilities were available at which to test the rocket engine, Malina, Parsons, and Forman drove to an isolated site a few miles from the California Institute of Technology (Cal

Jet Propulsion Laboratory (JPL)

Tech) in the Arroyo Seco wilderness area. Although their first few tests were unsuccessful, the men eventually developed a working rocket engine. Their early successes with rocket engines gained the group facilities on the Cal Tech campus in which to continue their work. In 1938, GALCIT was awarded a grant to study the possibility of using rockets to assist US Army Air Corps aircraft on takeoff from short runways. A much larger grant from the National Academy of Sciences was awarded in 1939 to continue the jet-assisted takeoff (JATO) rocket work, signaling the beginnings of a significant shift for GALCIT’s rocket project, from research for sounding rockets to research for military applications. In 1943, von Kármán produced a report, together with Malina and Qian Xuesen, on rocket research at GALCIT and proposed a significant expansion of rocket research, including a proposal to construct missiles capable of carrying explosive warheads and investigations of ramjet engines. This report contained the first usage of the term Jet Propulsion Laboratory to describe the GALCIT rocket facilities that had been constructed for the JATO work. Though the laboratory’s research was focused primarily on rocket propulsion technology, von Kármán chose the name Jet Propulsion Laboratory over Rocket Propulsion Laboratory. There may have been several reasons for this choice. Because rockets propel themselves through jets of gas, the term “jet propulsion” is more general than “rocket propulsion” and technically more accurate. Furthermore, by not limiting the scope of the laboratory to rocket research, von Kármán was leaving the door open for the laboratory to continue the original GALCIT work in other fields of aeronautical research. In addition, many military minds may have mentally associated the term “rocket” with fireworks. Jet propulsion was a new technical term that would more readily have caught their attention. Finally, due to the preponderance of poorly written science fiction

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Aerial view of the Jet Propulsion Laboratory. Photo via Wikimedia Commons. [Public domain.]

about rockets and rocket ships and the negative publicity many amateur rocket enthusiasts had garnered, the term “rocket” had come to carry an unfavorable connotation, which von Kármán may have been trying to avoid. THE ARMY YEARS In 1944, the Army authorized a $1,600,000 grant to construct a major research and development facility for rocketry and guided missile research operated under contract by Cal Tech. The new facility was officially named the Jet Propulsion Laboratory, GALCIT. The new JPL was charged with the mission of carrying forth several separate areas of research: rocket engine research, underwater solid-fueled mis-

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siles, ramjet research, and long-range heavy missile research, most of which had been mentioned in von Kármán’s proposal the year before. At about this time, von Kármán left both Cal Tech and JPL, and the directorship of the now-official Jet Propulsion Laboratory fell to his former graduate student, Dr. Frank Malina. To measure performance of their rockets, JPL’s engineers and scientists developed radio telemetry techniques to monitor their missiles in flight. Telemetry is data transmitted from a remote location by radio signals. To track its missiles, JPL also developed a series of ground radio and radar stations. By 1945, JPL had launched rockets from White Sands, New Mexico, to altitudes of nearly 50

Principles of Aeronautics

kilometers. The JPL team eventually developed the technology for two-way radio control of the rockets. In 1947, JPL launched the Bumper-Wac rocket, which first carried an American payload to the edge of space. By the late 1940s, JPL had developed the Corporal missile, the United States’ first operational surface-to-surface missile. By the early 1950s, researchers at JPL had designed the first solid-fueled antiaircraft missiles. In 1954, JPL proposed Project Orbiter, which would use a Redstone rocket as a first stage and either Loki or Sergeant rockets as upper stages, to put an artificial satellite into orbit around the earth by as early as 1957. The project was rejected. In October, 1957, however, the Soviet Union placed its Sputnik satellite into orbit. After Sputnik, JPL was given the go-ahead on its orbital project. Redstone would provide the missile, but JPL would design the payload and upper stage of the rocket to put the satellite into orbit. JPL would also handle tracking of the satellite. Finally, on January 31, 1958, the United States launched JPL’s satellite, which was named Explorer 1. With Explorer 1, JPL had once again shifted its emphasis, which now focused on the electronics and communications involved in fabricating a satellite rather than on the rocket used to launch the satellite. Following Explorer 1, JPL, under the Army’s supervision, was responsible for the development and operation of several other uncrewed Explorer spacecraft. THE NASA YEARS Prior to launch of Explorer 1, several different government agencies were involved in space-related activities. It was deemed advantageous, however, to put all space-related activities except for certain military applications, under one civilian agency’s jurisdiction. Thus, on October 1, 1958, the National Aeronautics and Space Administration (NASA) was created, and would soon attract a num-

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ber of aerospace engineers who had worked on Canada’s Avro Arrow project after it was abruptly canceled following the launch of the Soviet Union’s Sputnik satellite. On December 3, 1958, JPL was transferred from the jurisdiction of the Army to that of NASA but would continue to be operated by Cal Tech, under contract with NASA. The role of JPL under NASA would be primarily one of satellite and space probe design and operations. Although JPL was responsible for the Deep Space 1 spacecraft, which successfully tested an ion-drive propulsion engine, very little rocket propulsion work continued at JPL after this time. NASA continued the expansion programs at JPL’s Pasadena site that had begun under the Army’s administration. Additional scientists and engineers were hired, and new facilities were built, so that JPL came to be situated on 177 acres of land near where von Kármán’s team had done its original rocket propulsion experiments. Although under NASA, the Jet Propulsion Laboratory ceased research in jet or rocket propulsion, its original name remains in use. By the year 2001, JPL had been responsible for nearly 60 spacecraft missions, as well as numerous payloads flown on space shuttle missions. Although a few of these missions failed to perform as expected or were lost due to launch vehicle or spacecraft failures, most missions were successful. Several JPL missions, such as the Voyager missions to the outer solar system, many of the Mariner missions to the inner planets, the Galileo mission to Jupiter, the Magellan mission to Venus, and the Viking missions to Mars, have enjoyed spectacular successes. Under NASA, JPL has achieved dominance in the field of lunar and planetary exploration, having successfully handled missions to every planet in the solar system except for Pluto, as well as missions to several asteroids. Although studies of Earth were largely carried out by other NASA centers, JPL has also played a key role in several missions studying the planet Earth.

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THE DEEP SPACE NETWORK When Explorer 1 was to be placed into orbit in 1958, scientists immediately realized that there would be difficulty monitoring it. As Earth turned beneath the satellite, the ground track, or location on the surface of Earth underneath the spacecraft, would shift to the west with each orbit and not all of the orbits would pass over the United States. The satellite would be overhead in the United States only for a short period of each orbit that did pass over the United States. To track and monitor the satellite, therefore, JPL was responsible for deploying portable radio tracking equipment to several sites around the world. Two other Explorer spacecraft were successfully launched, and two more were lost during launch vehicle failure, before JPL was transferred from the Army to NASA. All these satellites needed remote facilities for tracking, telemetry, and control. The original equipment and sites used for Explorer 1 would suffice for the later missions, so that new facilities for each mission would not have to be built and deployed. This decision paved the way, however, for more permanent tracking and telemetry stations. After JPL was transferred to NASA, a decision was made to build permanent tracking and telemetry stations to support the large number of planned space missions, both crewed and uncrewed. These stations formed the backbone of the Deep Space Network (DSN), operated for NASA by JPL. The core of the DSN is composed of three large communications complexes located near Madrid, Spain, near Canberra, Australia, and at Goldstone, in California’s Mojave Desert. These sites are located nearly 120 degrees apart on Earth’s surface and thus can provide whole-sky coverage. Nearly any portion of the sky is above the horizon from at least one of the DSN sites. Each site has several antennas for telemetry and two-way communications with spacecraft. Because many of NASA’s space probes have traveled a long way from Earth, the signals from these

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space probes have become increasingly weak. The DSN has located at each site a 70-meter diameter parabolic dish, forming one of the most sensitive and powerful telecommunications systems on Earth. Also located at each site are 34-meter-diameter, high-efficiency dishes for use with slightly stronger signals. Each site also holds 26-meter-diameter dishes on mounts designed to track satellites in fast-moving orbits near Earth. Each site also holds a 11-meter-diameter dish, each of which can be linked together with those of the other sites for astronomical use in a technique known as long-baseline interferometry. OTHER PROJECTS Although JPL is known primarily for its roles in the early years of American rocket research and in the design and control of NASA spacecraft, JPL has also been involved in several other noteworthy projects. Many of these projects are natural spin-offs and extensions of the technologies that were developed for uncrewed spacecraft operations. JPL has played an important role in the study of solar energy as an alternate source of energy. JPL has also worked to develop an airborne, infrared fire-spotting system for the U.S. Forest Service. JPL has been involved in the advancement of robotics and automation and the development of miniature sensors and instrumentation. To deal with the enormous volume of data returning from space probes, JPL has also developed new, more powerful computer technologies, many of which have found their way into everyday non-space-related applications. —Raymond D. Benge Jr. Further Reading Anderson, Frank W., Jr. Orders of Magnitude: A History of NACA and NASA, 1915-1980. 2nd ed., Government Printing Office, 1981. Jet Propulsion Laboratory. Deep Space Network. Author, 2000. ———. Introduction to the Jet Propulsion Laboratory and Its Technology and Applications Programs. Cal Tech JPL, 1991.

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———. Missions from JPL: Fifty Years of Amazing Flight Projects. CreateSpace Independent Publishing Platform, 2010. ———. Jet Propulsion Laboratory. Author, 2000. Koppes, Clayton R. JPL and the American Space Program. Yale UP, 1982. See also: Advanced propulsion; Aeronautical engineering; Avro Arrow; Yuri Gagarin; John Glenn; Robert H. Goddard; High-altitude flight; Johnson Space Center; Rocket propulsion; Rockets; Russian space program; Scramjet; Alan Shepard; Space shuttle; Spacecraft engineering; Spaceflight; Valentina Tereshkova; Konstantin Tsiolkovsky; Chuck Yeager

Johnson Space Center Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics; Spacecraft engineering; Astronaut training ABSTRACT The Johnson Space Center, also called the Lyndon B. Johnson Space Center, was established September 19, 1961, and began operations on March 1, 1962. It is the principal facility for oversight and operations of crewed spaceflight by the National Aeronautics and Space Administration (NASA). SIGNIFICANCE The Johnson Space Center (JSC) has been responsible for astronaut training, crewed spacecraft development, and control of all US crewed space missions in flight since 1965. The JSC is also the site of the primary control center for the International Space Station (ISS) and is the location of the Lunar Receiving Laboratory, which studies Moon rocks and meteorites. HISTORY As early as 1957, engineers and scientists at the Langley Aeronautical Laboratory at Hampton, Vir-

Johnson Space Center

ginia, were collaborating on the possibility of crewed missions for the exploration of space. With the United States rapidly increasing the number of planned space missions, the National Aeronautics and Space Administration (NASA) was created on October 1, 1958, to coordinate the rapidly expanding US space program and to consolidate the various space projects under one civilian agency. Realizing that crewed space missions would be much more complex and challenging than uncrewed missions, on November 4, 1958, NASA created a special task force, called the Space Task Group (STG), to deal with the issues of crewed spaceflight. The STG was based at Langley but was charged with oversight of the crewed spaceflight. STG’s first crewed space program was Project Mercury. In May of 1959, STG was made one of six departments of the newly formed Goddard Space Flight Center, in Greenbelt, Maryland. Because of STG’s rapid growth during this period, however, STG never physically moved to Goddard. On March 1, 1961, STG was made an independent entity within NASA. On May 25, 1961, President John F. Kennedy made public a challenge and a goal for NASA to send a crewed mission to the Moon. This was an enormous jump from Project Mercury, and it required a major expansion of the roles, duties, and personnel associated with the STG. With the expansion of the STG to such levels, NASA administrator James Webb created the Office of Manned Space Flight, a special NASA division that included STG. One of the first goals of the new Office of Manned Space Flight was to secure a location for a new NASA center dedicated to crewed space missions. Cape Canaveral, NASA’s primary launch facility, was also a military missile test facility, and the Langley site was not fully suitable for expansion to include all of the facilities envisioned for the new center. The selection of a permanent site for the new NASA center near Houston, Texas, was not without contention. Many NASA personnel wanted the new

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center to be either in California, near the Jet Propulsion Laboratory, or in Florida, near the launch facilities. Most of the STG team at Langley preferred establishing a facility adjacent to or near Langley. Pressure from then-vice president Lyndon B. Johnson and Speaker of the House Sam Rayburn, both of Texas, together with several influential Texas legislators, caused NASA to consider a more centrally located site. Longtime associates of Johnson at Humble Oil finally helped the Houston site to win NASA’s favor. Humble Oil donated 1,000 acres of land to Rice University in Houston, with the stipulation that the land be made available to NASA. Although land costs were not a major issue, it would not have been prudent for NASA to turn down the offer of free land for the center, especially in light of

Principles of Aeronautics

the political pressure placed upon the agency to locate a major NASA center in Texas. Thus, on September 19, 1961, NASA announced that the new center would be located on the outskirts of Clear Lake City, a suburb of Houston. On November 1, 1961, the new NASA installation became officially designated the Manned Spacecraft Center (MSC), at which time the STG ceased to exist as a separate entity and was absorbed into the new center. Some operations began at once in leased office spaces in the Houston area, and, on March 1, 1962, Robert Gilruth, director of the new MSC and former head of the STG, moved his headquarters to Houston, officially making the MSC an operational NASA center. MSC was formally opened in February, 1964.

Mission Operations Control Room 2 at the conclusion of Apollo 11 in 1969. Photo via Wikimedia Commons. [Public domain.]

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Construction on Mission Control began soon after the Houston site was selected, and MSC’s Mission Control served as the backup command center for the Gemini 2 and Gemini 3 missions, the first crewed Gemini missions. Beginning with the Gemini 4 mission in June of 1965, Mission Control in Houston has acted as the principal mission-command center for all US crewed space missions. Astronaut training facilities were constructed at MSC to prepare astronauts for the conditions that they would face in space travel. Spacecraft were designed and tested at MSC, and the Lunar Receiving Laboratory was constructed to house and study the Moon rocks returned to Earth by the Apollo astronauts. Nearly adjacent to MSC was Ellington Air Force Base, later renamed Ellington Field after it was decommissioned by the Air Force, where training aircraft were kept for the astronauts. On February 17, 1973, the Manned Space Flight Center was formally renamed the Lyndon B. Johnson Space Center in recognition of the late President Lyndon B. Johnson, his role in the Houston site’s selection, and his support for crewed spaceflight. MISSION CONTROL The Mission Control Center (MCC) is the part of JSC with which the public is most familiar. Mission Control occupies a prominent building, designated Building 30, near the center of JSC. On the first floor of the MCC, advanced computer systems analyze the telemetry data collected during crewed space missions. The most visible part of the MCC is the Flight Control Room (FCR). There are actually several FCRs in the MCC. Nearly identical FCRs exist on the second and third floors, with the third floor FCR used primarily for military missions. Down the hall from the primary space shuttle FCR is a slightly different FCR used for the International Space Station (ISS) operations. The FCR consists of rows of flight-control consoles facing a large display at the front of the room. Each flight-control position

Johnson Space Center

has computer screens and other readouts, and the controller at that position is responsible for monitoring a specific part of the mission. The lead flight control position is that of the flight director, who is ultimately responsible for all decisions related to the mission. Although the FCRs are the most publicly visible part of the MCC, they are only a small part of Mission Control. Each flight-control position is assisted by a team of engineers and technicians, many of whom work in small rooms adjacent to the FCR. TRAINING SYSTEMS To prepare astronauts for the various situations to be encountered in spaceflight, numerous training facilities were built at the Johnson Space Center and at nearby Ellington Field. Some of these facilities, such as a large centrifuge built to simulate the high accelerations experienced on liftoff, were built for the Gemini and Apollo missions and were later dismantled to make room for space shuttle training systems. Other training systems include mock-ups of the various spacecraft used in crewed spaceflight. The mock-ups were used as simulators to train astronauts to deal with various situations that they would encounter in space travel. The space shuttle simulators are still used. A Space Environment Simulation Laboratory (SESL) was constructed at MSC/JSC. The SESL consists of several chambers that are designed to reproduce the environment experienced by astronauts and equipment in space. The atmospheric pressure within the chambers can be reduced to that of a vacuum, and high intensity lamps and other electromagnetic radiation sources can be used to simulate the radiation environment in space. These chambers were used to test spacecraft, equipment, and space suits. The SESL can also be used to train astronauts to deal with the difficulties faced by such harsh environments. To simulate the near-weightless conditions of spaceflight, JSC constructed a Neutral Buoyancy Training Facility (NBTF) at Ellington Field. The

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NBTF consists primarily of a very large tank of water. Astronauts wearing space suits and equipment are weighted to have a buoyancy equal to their weight. This simulation of weightlessness gives astronauts a chance to practice working and handling equipment in such an environment. A smaller but similar facility, the Weightless Environment Training Facility (WETF), where the centrifuge used to operate, was constructed in the early days of the space shuttle program. AIRCRAFT OPERATIONS The JSC operates several aircraft from Ellington Field. Some of these aircraft are used for astronaut training, and others are used in support of JSC’s mission as lead NASA center for crewed spaceflight operations. One of JSC’s training aircraft is a KC-135A transport aircraft, known as the “Vomit Comet,” which flies parabolic arcs that yield a few seconds of near-zero-gravity environment inside the aircraft. The KC-135A is used to train astronauts and perform experiments at very low gravity in a manner far superior to that of the NBTF. Unfortunately, it is unable to maintain a low-gravity environment for more than a few seconds at a time, so many parabolic arcs are needed per flight. Many of the astronauts act as pilots for their spacecraft. These astronauts must keep current in-flight training. The JSC maintains T-38 jet trainers at Ellington to allow the astronauts to train in high-performance aircraft. Furthermore, at least one of the T-38 trainers is fitted with control systems that mimic the very sluggish and difficult flight controls of the space shuttle. Pilot astronauts can use this trainer to practice the maneuvers needed to pilot the space shuttle to a safe landing. In addition to the training aircraft at Ellington, JSC also is the home to a very large turboprop cargo aircraft called the Super Guppy. This aircraft has a cargo bay 7.62 meters tall, 7.62 meters wide, and 33.8 meters long. It is used to transport large pieces

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of equipment, such as ISS components. A similar aircraft, nicknamed the Pregnant Guppy, carried components of the Saturn rockets used in the Apollo missions. LUNAR RECEIVING LABORATORY In 1967, construction began on a special laboratory at MSC designed to handle the Moon rocks expected to be brought back to Earth by the Apollo astronauts. Not knowing at the time whether the Moon had any indigenous life, NASA constructed the laboratory with special safeguards designed to prevent any cross-contamination of the Moon rocks with the Earth environment. Although it was soon determined that there is no life on the Moon, this sterile laboratory environment permits researchers the opportunity to analyze Moon rocks without accidentally contaminating them with Earth material. Although the last Moon mission returned to Earth on December 19, 1972, NASA maintains the Lunar Receiving Laboratory as a repository for the precious Moon rocks brought back to Earth. Facilities were constructed at the laboratory to analyze the geological properties of the Moon rocks and to study any remnants of life-forms that may exist in them. Because the Lunar Receiving Laboratory was designed to study Moon rocks without contaminating them with Earth material, it was natural for scientists to think of using the same laboratory to study meteorites found on Earth. Numerous meteorites found during the late 1970s and 1980s were sent to JSC’s Lunar Receiving Laboratory for study. Among these meteorites, one called ALH-84001 created a great deal of excitement when researchers at JSC, working with scientists at Stanford University, announced in 1996 that ALH-84001 appeared to be a piece of the planet Mars thrown loose during a giant meteorite impact on that planet long ago. Furthermore, these researchers announced findings that indicated that this meteorite may contain fossil remains of Martian life. These findings remain in doubt, but it is clear

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that the unique facilities of the Lunar Receiving Laboratory present an ideal location to study extraterrestrial samples.

Dye, Paul. Shuttle, Houston: My Life in the Center Seat of Mission Control. Hachette Books, 2020. Viator, Ray. Houston, Space City USA. Texas A&M UP, 2019.

—Raymond D. Benge Jr. Further Reading Bruns, Laura, and Mike Litchfield, editors. Johnson Space Center, The First 50 Years. Arcadia Publishing, 2013. Chladek, Jay. Outposts on the Frontier: A Fifty-Year History of Space Stations. U of Nebraska P, 2017. Dethloff, Henry C. Suddenly, Tomorrow Came: The NASA History of the Johnson Space Center. Dover Publications Inc., 2012.

See also: Aeronautical engineering; Neil Armstrong; Yuri Gagarin; John Glenn; Robert H. Goddard; Jet Propulsion Laboratory (JPL); National Aeronautics and Space Administration (NASA); Rocket propulsion; Rockets; Russian space program; Space shuttle; Spacecraft engineering; Spaceflight; Valentina Tereshkova; Training and education of pilots; Konstantin Tsiolkovsky; Unidentified aerial phenomena (UAP)

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L Landing Gear Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT The landing gear is the equipment that supports an aircraft on the ground, allows it to maneuver between runways and parking places, and supports the aircraft during takeoffs and landings. Landing gear allows aircraft to move effectively around the surface, provides for safe takeoffs and landings, and if needed can be deployed to slow an aircraft’s airspeed by increasing drag. KEY CONCEPTS ground loop: an often-disastrous maneuver in which an aircraft suddenly turns when touching down and begins to go into a rolling motion loop: an in-flight maneuver in which the aircraft is made to turn and roll in one motion tail-dragger: a landing gear arrangement in which there is a small, nonretractable wheel supporting the tail of an airplane tricycle landing gear arrangement: the arrangement common on most present-day aircraft in which there is a single wheel-and-strut supporting the nose of the aircraft and two wheel-and-strut assemblies rearward of the center of mass supporting the rest of the aircraft TO GET AIRBORNE The weight of an airplane in flight is supported by the lift force on its wings. However, the airplane must pass through two transitional stages: takeoff, when the airplane leaves the ground, and landing,

when it returns to the ground. The demands upon the landing gear during takeoffs differ from those during landings. During takeoffs, the airplane may accelerate to a speed of more than 225 kilometers per hour in a runway distance of less than 1,525 meters. Should the pilot stop the airplane during its takeoff run, the tires and brakes must sustain heavy mechanical friction loads without failure. During a routine takeoff, the landing gear must not only support the airplane but also respond to the pilot’s directional commands. During landing, the wheels must absorb the descent speed of the airplane as it contacts the runway. The tires, on first contact with the runway, spin to a rotational speed that matches the airplane’s landing speed. The brakes contained in the landing gear must then bring the airplane to a stop. In the routine landing of heavy commercial airplanes, reverse thrust is obtained from the engines, whether propeller or jet. However, in an emergency, the brakes must be capable of stopping the airplane without any engine assistance. PURPOSE Airplanes have landing gear for three reasons: to maneuver the airplane along the ground, to support and control the direction of the airplane during takeoff until the lift on the wings is able to support the weight of the airplane, and to support the weight of the airplane during landing as the wings gradually lose lift. The wheels and the connecting structure must be able to absorb the vertical or descending speed of the airplane at the instant of touchdown. During the critical landing phase, the pilot must have sufficient skill to keep the descent rate within a small enough magni-

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The retractable main landing gear of a Boeing 747. Photo by Olivier Cleynen, via Wikimedia Commons.

tude to prevent damage to the landing gear and the rest of the airplane. During takeoff and landing, the pilot must be able to control the airplane during both routine conditions and emergency conditions, such as tire blowouts. Airplanes that operate from an aircraft carrier must have very strong and resilient landing gear. The relative velocity between the wheels and carrier deck might be much higher than that experienced by a land-based airplane and the course is not entirely under the control of the pilot. In addition to the requirements of landing, takeoff, and ground maneuvering, some landing gear

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must be retracted into the airplane’s wings or fuselage. Except for low-performance general aviation airplanes, retractable landing gear is a feature of nearly all modern airplanes. The reason for retracting the landing gear is to reduce aerodynamic drag that would otherwise be caused by the extended gear. In some situations, though, deploying the landing gear to increase aerodynamic drag may be useful in reducing the airspeed of an aircraft. Because the space in either the wings or fuselage is limited, there is an incentive to limit the diameter of the wheels. To meet the airplane’s takeoff, landing, and maneuvering requirements, the tire pres-

Principles of Aeronautics

sure can be as high as 200 pounds per square inch for typical military airplanes; the tires in commercial airplanes might be as high as 140 pounds per square inch. An important part of the design of an airplane’s landing gear is the selection of the proper tire, and a significant part of the routine maintenance of an airplane is the regular inspection and replacement of tires. TYPES OF LANDING GEAR The most obvious part of the landing gear is where and how the wheels are attached to the airplane. There are many common arrangements. In the so-called conventional arrangement, two main wheels are placed near the front of the airplane and well ahead of the airplane’s center of gravity. A much smaller tail wheel is placed at the rear, just under the elevator. For the first four decades of powered flight, nearly all airplanes, both civil and military, used this arrangement. Except in some limited-production aerobatic, sport, or homebuilt airplanes, this wheel arrangement is no longer in use. The increasingly inappropriate term “conventional” has been replaced by the more descriptive term “tail-dragger.” The tricycle arrangement has become the most common form of landing gear. In the tricycle arrangement of landing gear, there are two wheel-and-strut assemblies placed forward, with the main wheel of the tail-dragger moved back past the center of gravity of the airplane. The tail-dragger arrangement, nevertheless, has certain advantages over the tricycle arrangement, one of which is that the presence of two rather than three wheels means less drag in flight. The tail-dragger arrangement also provides for better propeller clearance when the aircraft is on the ground. Because the tail-dragger lands at a higher angle relative to the wind, it can use more lift in the wing and consequently land at a lower speed and therefore require a shorter runway. Because of its lower landing speed, the

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tail-dragger might be better suited to rough-field landings. The tail-dragger’s disadvantage is the location of the center of gravity behind the main wheels, an inherently unstable condition. Unless quickly corrected, the response of a tail-dragger to a slight side motion, or drift, at landing is the ground loop, a maneuver in which the airplane turns suddenly to one side, rolling the airplane to touch down the opposite wingtip. Damage to the airplane from a ground-loop can include a crushed wingtip or a collapsed landing gear. The tail-dragger pilot must have sufficient skill to keep the airplane completely aligned with the runway during landings, even at low speeds. The advantage of the tricycle gear, used in most airplanes except heavily loaded transport aircraft or sailplanes, is the reduced likelihood of ground loops, as the center of gravity is ahead of the two main wheels. In addition, the pilot has better visibility on the ground. The cabin floor is horizontal on the ground, facilitating the loading of passengers and cargo. The bicycle, or tandem-wheel, arrangement is a specialized arrangement occasionally used on military airplanes and common on sailplanes. The advantage is the reduced weight of a third wheel. Weight reduction is especially critical on aircraft intended for vertical takeoff, such as the Harrier jet. Large transport airplanes often employ multiple-wheel arrangements to distribute the weight of the aircraft on the runway. The C-5A aircraft has a double wheel at the nose. In the rear, there are four sets of double bogies. A bogie is wheel arrangement in which the wheels are mounted one at each of the four corners of a cart. The center of the cart is strut-connected to the airplane. CONCLUSION An airplane’s landing gear permits it to take off, land, and maneuver on the ground. The landing

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gear also allows control of the airplane during the critical landing and takeoff operations and provides brake force as needed in emergency conditions. Although there are several types of landing gear arrangements depending upon the performance and weight of the airplane, the tricycle landing gear arrangement remains the most common. Further Reading Schmidt, R. Kyle. Advances in Aircraft Landing Gear. SAE International, 2015. ———. The Design of Aircraft Landing Gear. SAE International, 2021. ———. Aircraft Tires: Key Principles for Landing Gear Design. SAE International, 2022. ———. Airfield Compatibility: Key Principles for Landing Gear Design. SAE International, 2022. ———. Aircraft Wheels, Brakes, and Brake Controls: Key Principles for Landing Gear Design. SAE International, 2022. See also: Aerodynamics and flight; Aeronautical engineering; Airplane safety issues; Flight landing procedures; Forces of flight; Tail designs; Takeoff procedures; Taxiing procedures

Lighter-Than-Air Craft Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT Lighter-than-air (LTA) craft are aircraft that float in the sky because of LTA gases that make the overall density of the aircraft less than that of air, including both balloons that float with the winds and dirigibles that can propel themselves and direct their course. Although LTA craft have been supplanted by heavier-than-air (HTA) craft for most tasks, many techniques and technologies later adopted for HTA were developed with LTA craft. LTA craft provide a number of niche functions, such as weather sampling, advertising, telecommunications re-

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peaters, high-altitude science platforms, and heavy-cargo transporters. KEY CONCEPTS aerostat: a lighter-than-air vehicle that can remain in one position in the air buoyancy: the extent to which an object will float in a fluid medium on the basis of their relative densities composite materials: materials that are themselves constructed from two or more different materials Rozier balloon: a combination craft consisting of a hot-air balloon assisted by a helium-filled balloon to which it is attached, allowing flights of longer duration DESIGN PRINCIPLES A balloon is a fabric container for lighter-than-air (LTA) gas that allows the balloon to float. Usually, a balloon also lifts a payload or a gondola suspended beneath the balloon. A dirigible, which is a shortened form of the term “dirigible balloon” (meaning directable balloon), contains one or more balloons plus the propulsion system and payload. Balloons and dirigibles are called LTA craft to compare them with airplanes and helicopters, which are heavier-than-air (HTA) craft that stay in the sky because of the application of some form of propulsion causing lift. Buoyancy is the key factor for LTA craft. Archimedes (287-212 BCE) derived the principle stating that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. LTA craft use gases, including warmed air (which has expanded and is thus less dense than the surrounding air) with densities less than air. Two low-density gases widely used to provide buoyancy are hydrogen and helium. Typically, hydrogen lifts 27 kilograms per 28.3 cubic meters. Helium lifts 14 percent less (24 rather than 27 kilograms) per 28.3 cubic meters, but helium has the

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major safety advantage that it does not burn, while hydrogen can ignite explosively. Unfortunately, helium was not available until the 1920s, and even then the United States government (which had most of the world’s supply) was slow to allow exports. Consequently, most LTA craft until the late 1930s flew with hydrogen, and there were many catastrophic fires. Hot air gets only 7.7 to 9.1 kilograms of lift per 28.3 cubic meters, about one third that of hydrogen. Thus, hot-air balloons must be three times larger to lift the same payload, which makes hot-air dirigibles very inefficient. However, for balloons, the lesser complexity and cost of avoiding hydrogen or helium is a major advantage. Heating air for buoyancy is usually done by burning propane or kerosene. Heat is constantly drained away at the surface of the balloon, so hot-air balloons require frequent firings of their burners. Con-

sequently, they tend to have shorter range than balloons using low-density gas. However, the rapid changes in the buoyancy of hot-air balloons do allow their pilots to ascend or descend to catch different winds and thus get some control of their craft’s direction. LTA craft pilots can decrease buoyancy to drop lower or land by valving out some of the lifting gas. They can increase buoyancy by dropping ballast (water, sand, or other material carried along for dropping as needed). In extreme conditions, balloonists have dropped everything in the gondola and even the gondola itself. There are several more-sophisticated methods of modifying buoyancy, particularly for craft on long-duration and/or high-altitude flights, where warmth during the day causes the craft to rise too high and cold at night causes it to sink too low. Shiny upper surfaces, reflecting sunlight that would

Goodyear blimp N3A near Liberty Island, New York, NY. Photo by Till Niermann, via Wikimedia Commons.

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cause the balloon to expand and rise too high, and transparent lower surfaces, absorbing infrared radiation (heat waves) from the ground at night, often help. The Rozier balloon has a hot-air balloon, providing buoyancy variation, beneath a low-density gas balloon, providing endurance. Conversely, superpressure balloons maintain buoyancy by having an envelope strong enough to keep the same volume even if the gas inside them expands. This comes at a cost of additional weight compared to zero-pressure balloons, which expand and contract with changes in surrounding pressure. Another aspect of buoyancy is that the density and pressure of the surrounding air decreases with altitude. Hence, there is less lift available per unit volume, so LTA craft must be larger to carry a given payload to higher altitudes. Consequently, LTA craft with heavy payloads tend to be limited to low altitudes of a few thousand feet. For higher altitudes, designers can compensate for decreased lift per unit volume by using lighter payloads, such as remotely controlled instruments to operate the craft instead of people. Light materials are vital for LTA construction. The best material for the early balloons was light, strong, and expensive silk. By the mid-twentieth century, synthetic materials, such as polyester and polyethylene-coated nylon, improved on silk’s performance at a lower price. By the beginning of the twenty-first century, composites of a number of synthetic materials allowed even greater strength and lighter weight. Similarly, the electrolytic process for purifying aluminum, invented in 1886, allowed structures light enough to fly dirigible structures and pressurized gondolas carried by balloons. Composites in the late twentieth and early twenty-first centuries allowed all of these structures to become lighter still. Dirigibles are of three types: nonrigid, a streamlined balloon with the car and engines below; semirigid, the same with a strengthening keel below

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so the craft can be larger; and rigid, an enclosed structure holding any number of gas bags so the size can be very large. HISTORY OF LTA FLIGHT In 1782 and 1783, Joseph-Michel and Jacques-Étienne Montgolfier, two French brothers, flew hot-air balloons with animals as their first aeronauts. On November 21, 1783, they were ready for a human crew. Their pilot, Jean-François Pilâtre de Rozier, and another man flew over Paris for twenty-five minutes while desperately stoking their lifting fire and sponging out fires in their rigging caused by sparks from the lifting fire. Only a few days later, on December 1, 1783, Jacques-Alexander-César Charles, of the French Academy, flew a hydrogen balloon. The flight illustrated the advantages of hydrogen balloons over hot air. Because hydrogen is more buoyant than hot air, the balloon could be one-third the size of a comparable hot-air balloon. Rather than just twenty-five minutes, Charles flew for two-and-a-half hours, dropped off his passenger at sunset, and then rose high enough to be the first person to see the sun set twice in one day. Shortly thereafter, balloonists began attempting not just to fly, but to go places. Jean-Pierre Blanchard, another Frenchman, and John Jeffries, an American, were the first aeronauts to fly across the English Channel to France on January 7, 1785. However, their flight illustrated the major problem of balloons as transportation. They had to drop all their cargo to reach land, and their destination could be only roughly planned—they could have no more specific intention than to land somewhere in France. That vagueness increased as balloonists made longer flights. Inventors tried vainly for decades to make their balloons steerable, but they always failed because engines powerful enough to move a craft against strong winds were too heavy to be lifted.

Principles of Aeronautics

Still, balloon flights in the nineteenth century supplied entertainment, scientific data, and observation data for armies. Balloon rides and balloon-borne fireworks were connected with most major celebrations. For scientists, balloonists discovered that the atmosphere grew thinner and cooler with increased altitude but that the magnetic field retained its strength. For armies, tethered balloons allowed observers to see several miles beyond the enemy’s lines. Such balloons were first used during the French Revolution in 1793, and again in the American Civil War (1861-65). By the end of the nineteenth century, observation balloons were in wide use. DIRIGIBLES, BALLOONS, AND THEIR COMPETITION As with HTA aircraft, dirigibles only became practical when light and powerful internal combustion engines were developed. On September 20, 1898, Brazilian Alberto Santos-Dumont first used a 3.5-horsepower, 30-kilogram motor to propel himself and Number 1, a 25-meter nonrigid craft with 1812 cubic meters of gas volume, around Paris. Santos-Dumont made steady improvements over the next several years, inspiring many other nonrigids. Meanwhile, in Germany, Count Ferdinand von Zeppelin built a large rigid dirigible, Luftschiff Zeppelin Number 1 or LZ-1, which translates as “airship number one.” It was 128 meters long and 12.8 meters in diameter, with a gas volume of 13,025 cubic meters, sixty times greater than Santos-Dumont’s Number 1. The LZ-1, which first flew in July, 1900, had seventeen separate gas cells held together by an aluminum framework and covered with fabric. After ten more years of work, von Zeppelin had dirigibles in commercial service carrying sightseeing passengers and mail. With the beginning of World War I, rigid dirigibles did well at first, staging the first long-range bombing attacks in 1915. However, airplane tech-

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nology rapidly improved, and pilots found the rigids to be large, slow, highly flammable targets. Likewise, airplanes replaced observation balloons because the airplanes could cover more territory and also attack targets. The only dirigibles successful throughout the war were nonrigids used to guard convoys against submarines. Still, the long flights by rigid dirigibles during the war suggested that intercontinental passenger service, or even flying warships, might develop. All these dreams eventually crashed. France abandoned large rigids when the Dixmude exploded in 1923. Great Britain abandoned large rigids when the R-101 crashed and burned in 1924. In the 1920s and 1930s, the US government operated four rigids as military ships intended for long-range reconnaissance. Two of the airships, the Akron and Macon, carried their own fighter planes for defense. Because the United States had most of the world’s helium supply and used helium for its LTA gas, none of these craft exploded. However, three of them were lost in storms, and the United States abandoned the giant rigids after the third, the Macon, went down in a storm at sea in 1935. The Lufftschiffbau Zeppelin company in Germany had the best safety record because it had built more than a hundred rigids and had thoroughly worked out the design details. In 1928, the company’s Graf Zeppelin began a commercial flight life that circled the world, made regular flights to Brazil and North America, made an Arctic expedition, and flew one million miles before being retired. The last and greatest rigid was the Hindenburg: 244.75 meters long and 41.2 meters in diameter. Its 198,218 cubic meters of gas allowed it to carry fifty passengers and sixty crew in absolute luxury at a speed of 135 kilometers per hour and a range of 17,703 kilometers. Unfortunately, the Luftschiffbau Zeppelin company still flew with hydrogen, and the doped-cloth skin was also quite flammable. Lightning, leaking

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gas, or anti-Nazi sabotage caused the Hindenburg to catch fire while preparing to land at Lakehurst, New Jersey, on May 6, 1937. Within one minute, the craft was destroyed, and filmed footage of the event convinced the public that large dirigibles were unsafe. That left only nonrigids, which were again a major part of antisubmarine warfare in World War II. However, they were retired in the 1950s when helicopters provided the same hovering capability with greater dash capability and easier storage. In the last third of the twentieth century, the few working nonrigid dirigibles were limited to flying advertising billboards and carrying television cameras for overhead views of sporting events. The only new application came in the 1990s, when tethered balloons returned to service as aerostats, providing platforms at altitudes as high as ten to fifteen thousand feet for radar stations and communications repeater stations. Balloons fared better than dirigibles. Development of small radio transmitters combined with remotely operating weather instruments made possible balloon-borne radiosondes to report temperature, pressure, and relative humidity. Angle data from antennas tracking the radiosondes yielded wind speed and direction at different heights. Use of radiosonde balloons continued into the twenty-first century, helping predict weather, plot sky conditions for aircraft, and fire artillery more accurately. Larger balloons have carried science payloads and human crews to high altitudes for decades because they can reach altitudes as high as 48 kilometers, which airplanes cannot reach, carrying large payloads that would not fit in an airplane fuselage. From the 1930s through the early 1960s, balloons were the frontier of human-crewed aviation that led to higher flights by HTA craft and eventually to space capsules.

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The greatest problem at high altitudes is low pressure, which Swiss balloonist Auguste Piccard surmounted with a pressurized cabin, essentially the first space capsule, and he suggested similar pressurized cabins for high-flying transports. On May 27, 1931, Piccard and an assistant reached 15.8 kilometers, making them the first to reach the stratosphere. More importantly, they discovered that cosmic rays increased with altitude, proving that they came from somewhere in space rather than the other suggested source, radioactivity within the earth. Such flights carried personnel and instruments to steadily greater heights and developed many technologies that were later used in the space race. In fact, on May 4, 1961, the American Stratolab V balloon reached an altitude of 34.6 kilometers with an open gondola so the two pilots could test space suits in near-space conditions for the Mercury orbital-flight program. After the 1960s, scientific balloon flights using improved robotic instrumentation allowed balloons to shed the weight of the balloonists and their life-support gear. In the closing decades of the twentieth century, astronomic balloon-borne instruments conducted sky surveys in a number of frequency bands that cannot penetrate the lower atmosphere and provided valuable weather data from the lower stratosphere. By the late twentieth century, advances in fabrics allowed the US National Aeronautics and Space Administration (NASA) to begin replacing zero-pressure balloons with superpressure balloons, which do not need to vent excess helium when warmed by the sun and which consequently can fly for weeks or months. By the early twenty-first century, NASA had begun flying large superpressure balloons with several-ton payloads in a program called the Ultra Long Duration Balloon (ULDB).

Principles of Aeronautics

BALLOONING FOR FUN AND ADVENTURE Although the aviation frontier passed ballooning by, a balloon ride is still a beautiful and awe-inspiring experience. A panoramic view floats by below and sounds from the ground float up to balloonists. This was a rare experience until the renaissance of hot-air ballooning, started by American Edward Yost. While developing high-altitude balloons for the United States government in the 1950s, Yost realized that polyethylene-coated nylon is a lighter, less flammable material than that used in the Montgolfiers’ balloons. He used an acetylene welding torch as a less labor-intensive source of hot air than the Montgolfiers used. After some development, such as replacing the welding torch with a propane burner, Yost made the first modern hot-air balloon launch from Bruning, Nebraska, on October 10, 1960. Beginning in the 1960s, the new hot-air balloons radically reduced the cost and complexity of supplying buoyant gas. Thus, were born ballooning clubs, competitions, and tour services. Also, for advertising, hot-air balloons have flown in shapes varying from spark plugs to human faces, and even a mansion. For more ambitious flying, Yost’s hot-air technology (plus lightweight insulating material lining the gasbag, and helium) made the Rozier balloon practical for long-distance flights. Varying the amount of heat in the inner balloon provides altitude control for hunting favorable winds. That capability, along with worldwide weather reports, made balloon flights possible across the Atlantic and then the Pacific Oceans. In March, 1999, another Piccard, Auguste’s grandson Bertrand, and Brian Jones spent twenty days flying 48,280 kilometers to make a complete circumnavigation of the globe. For astronomical and meteorological observations, balloons are still a much cheaper alternative to spacecraft, with shorter turnaround times and without the vibration and acceleration of a rocket launch.

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DIRIGIBLE ECONOMICS AND PROSPECTS By the beginning of the twenty-first century, dirigibles were enjoying a resurgence in several niche markets. However, dirigibles will probably never recover aviation primacy from HTA’s for several reasons. First, there is a massive investment cost for building and developing dirigibles. Several factors make dirigibles more efficient as size increases. In particular, lifting volume increases by the cube while surface volume (and thus drag) only increases by the square. However, the large size makes the design and building of a dirigible as expensive as that of a ship. Large size also reduces the number of units made, so dirigibles have less chance to go down the learning curve toward lower costs and improved designs than HTA craft, which are typically made by the hundreds or thousands. Second, hangar costs are high. Dirigibles are kept inflated because their helium lifting gas is expensive and would require too much time and effort to pump back into tanks. However, inflated dirigibles can easily be swept off their parking area by winds. Consequently, dirigibles must have their own special hangars rather than be casually parked on runways, as airplanes are. Third, dirigibles are vulnerable to and limited by bad weather. The giant buoyant structures can be seized by freak gusts of wind on takeoff and landing, and they are more vulnerable than airplanes to icing. Zeppelin passenger flights were not scheduled in winter. In the sky, dirigibles are so large that winds may pull them in different directions and destroy them, as happened to the US airships Shenandoah, Akron, and Macon. Moreover, unless specially designed for high altitude, dirigibles cannot readily climb above storms as jet-propelled airplanes can. Fourth, due to the drag from the great size per unit mass of cargo, dirigibles are significantly slower than HTA competition. At best, they can ob-

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tain half the speed of propeller-driven planes and a fifth that of jets. Thus, a jet with one-fifth of the cargo capacity of a dirigible can deliver the same cumulative mass of cargo. This longer time makes dirigibles uncompetitive in the passenger market. Still, dirigibles have potential for certain markets because they can run quietly, run smoothly, linger for long periods, carry heavy and awkwardly large payloads, and land without runways. Lighter and more fireproof materials have increased these advantages. The number of advertising dirigibles increased steadily beginning in the 1980s. At the start of the twenty-first century, the Zeppelin Company was marketing sightseeing semirigids a third the size of the Hindenburg. CargoLifter in Berlin was designing a cargo-carrying rigid larger than the Hindenburg. Meanwhile, an entirely new concept was being developed: dirigibles in the lower stratosphere serving as high-altitude platforms. Such platforms could serve many functions of communications satellites and astronomical satellites at a fraction of the cost. However, as with most LTA tasks, there is competition from airplanes. —Roger V. Carlson Further Reading Charles River Editors. Famous Dirigibles: The History and Legacy of Lighter Than Air Vehicles from the Renaissance to Today. Independently Published, 2019. Crouch, Tom D. Lighter Than Air: An Illustrated History of Balloons and Airships. Johns Hopkins UP, 2009. Hissler, Sascha. Lighter Than Air Concepts. GRIN Verlag, 2010. Joshi, Mangala. Coated and Laminated Textiles for Aerostats and Airships. CRC Press, 2022. Taylor, John A. Principles of Aerostatics: The Theory of Lighter-Than-Air Flight. CreateSpace Independent Publishing Platform, 2014. See also: Aeronautical engineering; Blimps; Dirigibles; First flights of note; Flight balloons; Hot-air balloons; Montgolfier brothers; Jules Verne

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Otto Lilienthal Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Otto Lilienthal was born on May 23, 1848, in Anklam, Prussia (now in Germany), and died on August 10, 1896, in Berlin, Germany. He was an aviation pioneer and creative genius who was the first man to build and fly a successful heavier-than-air flying machine. Lilienthal was the first person to prove that flight could be achieved and sustained with the cambered, or curved surface, airfoil wing. He built and successfully flew a number of heavier-thanair flying machines before anyone else in history had done so. KEY CONCEPTS cambered wing: a wing shape having a convex-curved upper surface and a concave-curved lower surface BACKGROUND Otto Lilienthal’s passion to fly blossomed early in his life. Although there was no formal science of aviation during Lilienthal’s youth, there is evidence that Lilienthal studied birds in grammar school. At the age of twenty-five, Lilienthal joined the Aeronautical Society of Great Britain, where he gave his first lecture about the theory of avian flight. He then began systematic experiments and tests with models and kites on the force of air on human-made wings. No mere tinkerer, he was an accomplished engineer, with his own business engineering boilers and steam engines. He obtained a patent for a mining machine, the first of his twenty patents, four of which were aviation patents. Lilienthal’s ongoing experiments and studies culminated, in 1891, with his building his first heavier-than-air flying machine, which flew for a distance of 80 feet. This machine would be described

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today as a hang glider. Over the next five years, he built a total of eighteen flying machines and made more than two thousand sustained and replicable flights. In 1892, Lilienthal built a new glider with improved flight characteristics. The following year, he built a flight station near his home, where he made a number of flights with distances of up to 244 meters. Lilienthal not only designed, engineered, and built a machine that could fly, but he also taught himself to fly it. Although Lilienthal’s flying machines were difficult to control and to turn, they did accomplish sustained flight. His outstanding contribution to the science of flight was the cambered, or curved surface, wing. This wing form, with a rounded top surface and a concave or flat underside, produces the lift needed to make an airplane fly and is still used today on most airplanes.

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By 1895, Lilienthal’s flight accomplishments were widely reported, and Lilienthal was visited by flight enthusiasts from many different countries. He corresponded with and shared his ideas with other aviation pioneers, such as Octave Chanute and Orville and Wilbur Wright. He generously published and shared the results of his aviation theories and experiments. On August 9, 1896, at the age of forty-eight, Lilienthal crashed while flying one of his machines and died the next day. He is famous for the following quotation, “To invent an airplane is nothing. To build one is something. But to fly is everything.” Lilienthal was a creative genius whose ingenuity, observations, engineering, and daring laid the cornerstone for the development of aviation. —Mary Ann Turney and Robert Maxant Further Reading Lilienthal, Otto. Birdflight as the Basis of Aviation: A Contribution Towards a System of Aviation Compiled from the Results of Numerous Experiments Made by O. and G. Lilienthal. Markowski International Publishing, 2001. Raffel, Markus, and Bernd Lukasch. The Flying Man. Otto Lilienthal-History, Flights and Photographs. Springer, 2022. See also: Aerodynamics and flight; Airfoils; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Leonardo da Vinci; First flights of note; Glider planes; Human-powered flight; Montgolfier brothers; Igor Sikorsky; Ultralight aircraft; Wright brothers’ first flight

Charles A. Lindbergh Fields of Study: Aeronautical engineering; Mechanical engineering; Aeronautics

Otto Lilienthal. Photo via Wikimedia Commons. [Public domain.]

ABSTRACT Charles Lindbergh was born on February 4, 1902, in Detroit, Michigan, and died on August 26, 1974, in Hana, Maui, Hawaii. A pioneer of early aviation, he became the

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first aviator to fly an airplane nonstop from New York to Paris. While Lindbergh’s 1927 New York-to-Paris flight made him a national and worldwide hero, he was more than another flier who set a record. In the forty-seven years between the flight and his death, he contributed significantly to civil and military aviation, scientific research, and conservation. KEY CONCEPTS wing-walker: a daredevil performer who walks atop the wing of an airplane, usually a biplane, while it is in flight EARLY LIFE Charles Augustus Lindbergh, whose family moved about a great deal, was raised more by his mother, Evangeline Land Lindbergh, than by his father, Charles August Lindbergh, who served in the US House of Representatives from 1907 until 1917. During his precollege years, Lindbergh, an unimpressive student, attended eleven schools. He showed considerable mechanical ability, however, and was entranced by automobiles, motorcycles, and especially airplanes. INTEREST IN FLYING Lindbergh first saw an airplane in 1910, when a single-engine aircraft flew at treetop level up the river alongside the Minnesota farm where his family was living. From that time forward, Lindbergh thought of little but flying. He wanted to study aeronautical engineering in college, but no universities offered such programs. Finishing high school in 1918, he farmed for two years before entering the University of Wisconsin to study civil engineering. By 1920, he owned an Excelsior motorcycle, on which he rode to Lincoln, Nebraska, in 1922 when, bored by his studies, he dropped out of the university to attend flying school. Although the school closed before he earned his pilot’s license, he knew by then that he wanted to spend his life flying.

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Charles Lindbergh. Photo via Wikimedia Commons. [Public domain.]

Lindbergh apprenticed as a mechanic to a barnstorming pilot. He earned the nickname “Daredevil Lindbergh” by walking on the wings of the planes piloted by his boss, dazzling and delighting the assembled throngs below. By 1923, he was able to pilot planes himself. He traded his motorcycle for a war-surplus airplane, a Curtiss Jenny, in which he barnstormed on his own until, determined to perfect his skills as a pilot, he joined the US Army Air Service Reserve in 1924. In 1925, the US Postal Service inaugurated airmail service to the Midwest, and Lindbergh became one of its earliest pilots, flying between St. Louis and Chicago, a treacherous route because of its severe winter weather. Twice Lindbergh had to parachute from his plane. While flying this route,

Principles of Aeronautics

Lindbergh learned that an affluent Frenchman, Raymond Orteig, was offering a $25,000 prize to the first person to fly nonstop from New York to Paris. Seven people had attempted this feat and failed. Lindbergh immediately began to work toward winning the prize. He designed a plane, the Spirit of St. Louis, to be built by Ryan Airlines in San Diego with funding from both Lindbergh and a group of St. Louis businessmen. SETTING RECORDS Those who had failed to fly nonstop across the Atlantic Ocean had attempted the flight in dual-engine planes. Lindbergh designed a single-engine plane that would conserve weight. An enormous fuel tank occupied the area from the engine to the pilot’s seat, totally blocking forward vision. A periscope was installed on the left window to overcome this problem. The plane carried 1,710 liters of fuel, which so impeded its takeoffs that it barely cleared trees at the ends of runways. After battling eight days of bad weather conditions that made a takeoff impossible, Lindbergh finally was ready to fly out of New York’s Roosevelt Field on May 20, 1927, taking off at 7:52 a.m. To minimize weight, he carried only five sandwiches and a quart of water. He further lightened the plane by having no radio or parachute aboard. Lindbergh flew the great circle route over Cape Cod, Nova Scotia, Newfoundland, and, after the long Atlantic crossing, Ireland, England, and France. The most hazardous leg of the flight, the crossing of the Atlantic, occurred at night. Ice formed on the wings shortly after Lindbergh passed Newfoundland. Fortunately, it soon dissipated. Lindbergh’s chief battle now was against sleep. He would doze off and then quickly be jarred into wakefulness, realizing he was flying off course. The Spirit of St. Louis flew through rain while passing over the southern tip of Ireland, but as the plane approached southern England, the weather

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cleared. The weather over Cherbourg, France, was so good that Lindbergh finally took a first bite from one of his sandwiches. He followed the Seine to Paris’s Le Bourget Airfield where he landed on May 21 at 10:22 p.m., having flown more than 3,500 miles in 33.5 hours. Cheering crowds greeted him, and Raymond Orteig later awarded him the promised $25,000 prize. Once home, the bashful Lindbergh was lionized. He was given a ticker-tape parade down New York City’s Broadway Avenue. He became a roving international goodwill ambassador for the United States. In the course of these travels, he met Anne Spencer Morrow, the daughter of the US ambassador to Mexico, whom he married in 1929. THE DOWN YEARS Celebrity perplexed the ever-reticent Lindbergh. He tried increasingly to evade public notice. He and his wife traveled throughout the world and became ardent conservationists. Their first son, Charles Augustus Jr., born in 1930, was kidnapped and murdered in 1932. In 1935, the Lindberghs, longing for privacy, relocated to England, where Lindbergh worked with Dr. Alexis Carrel to develop an early heart pump machine for use in open-heart surgery. In Lindbergh’s later years, his pro-Nazi, anti-Semitic sentiments were condemned by his once-adoring public. He gradually withdrew from public life, spending many of his remaining years at his favorite home in Hana, on the island of Maui, Hawaii, where he died of cancer in 1974. —R. Baird Shuman Further Reading Fleming, Candace. The Rise and Fall of Charles Lindbergh. Random House Children’s Books, 2020. Hampton, Dan. The Flight: Charles Lindbergh’s Daring and Immortal 1927 Transatlantic Crossing. HarperCollins, 2017.

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Kessner, Thomas. The Flight of the Century: Charles Lindbergh and the Rise of American Aviation. Oxford UP, 2010. Lindbergh, Charles. We. ISHI Press, 2015. Lindbergh, Reeve. “Charles Lindbergh.” People of the Century. Simon & Schuster, 1999.

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See also: Aerospace industry in the United States; Aircraft icing; Neil Armstrong; Biplanes; Jimmy Doolittle; Amelia Earhart; Otto Lilienthal; Billy Mitchell; Eddie Rickenbacker; Konstantin Tsiolkovsky; Valentina Tereshkova; Manfred von Richthofen; Wright brothers’ first flight

M Ernst Mach Fields of Study: Physics; Aeronautical engineering; Mathematics ABSTRACT Ernst Mach, an Austrian physicist and philosopher, was born February 18, 1838 in Chirlitz-Turas, Moravia, Austrian Empire (now part of Brno, Czech Republic), and died February 19, 1916in Munich, German Empire. Mach was influential in both science and philosophy. In 1887 he developed the principles of supersonics and designated the Mach number as the ratio of the speed of an object to the speed of sound.

was awarded a doctorate in physics and went on to work as a tutor in Vienna for several years, addressing students of medicine on topics in physics. This resulted in his book Compendium of Physics for Medical Students (1863). In 1864, Mach joined the University of Graz as a professor of mathematics and later taught physics there. He lectured at Graz until 1867, when he took on a professorship in experimental physics at Charles University in Prague, where he would remain for nearly thirty years. During this time Mach carried

KEY CONCEPTS Mach bands: an optical effect in which the borders of regions having different light intensities are seen as being lighter or darker; the effect is seen between the two arcs of a double rainbow as Alexander’s Dark Band Mach number: the ratio of the velocity of an object to the velocity of sound in a particular fluid medium; Mach 1 = the speed of sound, Mach 2 = two times the speed of sound, etc. EARLY LIFE Ernst Waldfried Joseph Wenzel Mach was born on February 18, 1838, in what is now part of the Czech Republic. Mach lived and worked when the study of science was closely related to the study of philosophy, especially in German-speaking countries. Mach was influential in both disciplines but is perhaps best remembered for his contributions to the development of Einstein’s theories of relativity. In 1860 he

Ernst Mach. Photo via Wikimedia Commons. [Public domain.]

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out a considerable amount of original research in areas such as optics, waves, and the study of sensory perceptions. In 1887 he developed the principles of supersonics and designated the Mach number as the ratio of the speed of an object to the speed of sound. His ideas also contributed to the fields of physiology and psychology, and he is notorious for having stubbornly denied the existence of the atom. In 1895 Mach found himself back at the University of Vienna, as professor of inductive philosophy (inductive logic is a type of reasoning in which specific observations lead to more general conclusions). However, in 1898 he experienced a severe stroke that paralyzed the right half of his body, and he was forced to resign from his professional position. He did continue to speak, publish, and pursue independent scientific study, however. In 1901 the Austrian government honored him with an appointment to parliament. Throughout his life Mach was particularly interested in a field called epistemology, which refers to the study of human knowledge and the limits of understanding. One of his last two books dealt with this discipline; titled Knowledge and Error, it appeared in 1905. Mach also wrote an autobiography, published in 1910. He died six years later on February 19, 1916, at the age of seventy-eight. LIFE’S WORK One of Mach’s most important epistemological ideas was that all of our insights about the world are based on imperfect and fallible sensory information. According to him, what we call scientific laws are not really absolute laws of nature, but reflections of how human beings perceive the universe. Mach wrote about these ideas in his book The Analysis of Sensations (1897). As a result, Mach refused to accept any scientific hypothesis that could not be proven through testing. His strict insistence on being able to verify theory was somewhat revolutionary at the time, when many

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complex scientific ideas were put forward and accepted without experimental proof. (This is also the reason Mach doubted that atoms were real, since during his lifetime there were no electron microscopes that could be used to view individual atoms.) Mach’s firm belief that scientific ideas needed to be provable led him to reject Isaac Newton’s abstract notions of “absolute,” or unchanging, space and time. Mach’s book The Science of Mechanics (1883), generally considered his masterpiece, strongly criticized these concepts. It also introduced a sophisticated idea that Albert Einstein later came to call “Mach’s principle.” Part of Mach’s disagreement with Newton had to do with the question of how an object’s acceleration (the change in its speed) is measured through space. When physicists measure an object’s change in speed, they always think about what that change is relative to. In other words, the object may be speeding up or slowing down, but speeding up or slowing down compared to what? To solve this problem, Newton came up with the idea of absolute space: a theoretical constant that no one had ever seen, but that scientists could always use as a point of comparison. Mach, on the other hand, suggested that physicists should calculate an object’s acceleration not in relation to Newton’s abstract and invisible constant, but in relation to all of the other matter in the entire universe. What Einstein dubbed “Mach’s principle” is the idea that when an object has inertia (when it resists a change in its speed), the source of that inertia is the object’s relationship to all the other matter in the universe. This proposal helped Einstein think about gravity and movement in new ways. Einstein always said that the concepts in The Science of Mechanics were enormously powerful influences on him as he was working on his general theory of relativity. Despite this fact, Mach’s principle was never included in Einstein’s calculations, and there is still some disagreement among physicists about whether Newton’s ideas or Mach’s more accurately describe the

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way the universe really works. One of Mach’s first projects after he received his doctorate was an attempt to prove the Doppler effect, a theory proposed in 1842. The theory states that light and sound waves increase in frequency as their source draws nearer to an observer, and decrease in frequency as their source moves away from an observer. In 1860 scientists were still trying to design experiments that would show that the Doppler effect was real. Mach devised a simple, portable set of equipment that could be used to clearly demonstrate the effect. IMPACT Besides the principle Einstein named after him, Mach’s name is preserved in at least two other scientific concepts. The first is the “Mach number,” which describes the relationship between the speed of an object and the speed of sound in whatever medium the object happens to be traveling through. A rocket or a jet plane moving at greater than “Mach 1" is traveling at supersonic speeds, or faster than the speed of sound. Mach was the first scientist to realize that a projectile moving faster than the speed of sound actually creates shock waves around itself. He was then able to photograph the effects of the shock waves created by a bullet moving at supersonic speeds. In order to do this, Mach invented an entirely new photographic method, taking advantage of the fact that shock waves cause light to refract (bend). The resulting image he called a shadowgraph. A second well-known scientific phenomenon named after Mach is an optical illusion he discovered. Mach showed that when a person is looking at two regions of space that are side by side, and when those regions are of significantly different levels of lightness or darkness, the eye tends to see bands of either brighter or darker space at the border between them. The bands do not really exist, but our brains perceive them nonetheless. They are known

as “Mach bands.” These are seen everywhere in the phenomenon of double rainbows, in which the sky between the rainbows appears darker than the sky on the outside of the rainbows, a region known as Alexander’s Dark Band. Mach’s research into sensory awareness also included investigations of hearing, movement, and the human perception of time. —M. Lee Further Reading Blackmore, John T. “Three Autobiographical Manuscripts by Ernst Mach.” Annals of Science, vol. 35, no. 4, July 1978, pp. 401-19. ———. Ernst Mach—A Deeper Look: Documents and New Perspectives. Springer Netherlands, 2012. ———. Ernst Mach. His Life, Work and Influence. U of California P, 2021. Hoffmann, Christoph. “Representing Difference: Ernst Mach and Peter Salcher’s Ballistic-Photographic Experiments.” Endeavour, vol. 33, no. 1, Mar. 2009, pp. 18-23. Karwatka, Dennis. “Ernst Mach and the Mach Number.” Tech Directions, vol. 69, no. 5, Dec. 2009, p. 14. Preston, John, editor. Interpreting Mach: Critical Essays. Cambridge UP, 2021. Seeger, Raymond J., and Robert S. Cohen, editors. Ernst Mach: Physicist and Philosopher. Springer Netherlands, 2013. See also: High-speed flight; Mach number; Sound barrier; Supersonic aircraft

Mach Number Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT The Mach number indicates the ratio of the speed of an object in a medium to the speed of sound in the same medium. The speed of sound is the speed at which the weakest distur-

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bances in pressure propagate through a medium. In a gas, the speed of sound is close to the average speed of random thermal motion of molecules, because disturbances propagate by the collisions between molecules. Thus, the Mach number is also viewed as the ratio between the speed of organized motion of the gas, or a body relative to the gas, and the average speed of random thermal motion in the gas. KEY CONCEPTS incompressible: unable to be reduced in volume by the external application of pressure Mach angle: the angle between the central axis of the Mach cone and the surface of the Mach cone Mach cone: a conical region of space, extending rearward and expanding radially from the nose of an aircraft as it moves, defined by the compression of air CALCULATION The Mach number is named in honor of Ernst Mach, a nineteenth-century Austrian scientist who conducted experimental research on the aerodynamics of artillery shells. Speed ranges in the field of aerodynamics are classified according to Mach number. A Mach number of 1 corresponds to motion at the speed of sound. The speed of sound thus depends on the temperature of the gas. The speed of sound in air at a temperature of 0° Celsius (273.15 Kelvins) is 331 meters per second. To find the speed of sound in meters per second at other air temperatures in the range 200 to 700 Kelvins (from minus 73.15° to 426.85° Celsius), divide 5,471 (obtained as product of 331 and the square root of 273.15) by the square root of the air temperature in Kelvins. The Mach number is then the ratio of an object’s speed to this speed of sound. MACH RANGES The low-speed regime is generally identified with Mach numbers of less than 0.3. In this range, the

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maximum change in density that can occur when the flow is stopped is less than about 5 percent of the actual density. Hence, such flows are also described as being incompressible. Small propeller-driven airplanes and helicopters fly in this speed range, though their propeller or rotor tips move quickly enough relative to the air to encounter Mach numbers close to 1. The subsonic range is generally the range from Mach 0.3 to 1.0. In this range, the changes in density associated with changes in Mach number become significant. The lift coefficient and drag coefficient rise more steeply with increased angle of attack as Mach number increases. According to the Prandtl-Glauert rule, the lift coefficient associated with a given angle of attack scales with the number resulting from 1 divided by the square root of 1 minus the square of the Mach number. The drag coefficient also increases with Mach number and with thickness. Thus, aircraft flying in this range use thinner airfoils and smaller angles of attack. Turboprop aircraft reach speeds well into the subsonic regime. Much of the close air combat between fighter planes that involves sharp maneuvers occurs in this speed range. The range that includes Mach 1 and values slightly greater and less than Mach 1 is called the transonic regime, roughly taken as Mach 0.8 to 1.2. Transonic flows include both supersonic and subsonic regions. Most modern airliners cruise in the transonic regime. Although the actual flight Mach number is below 1.0, there is some supersonic flow over the wings. The critical Mach number is the lowest flight Mach number where sonic conditions, in which the Mach number equals 1, are first encountered on the airfoil. When the flow speed, or the speed of an object relative to the medium, is clearly greater than the speed of sound, the speed is said to be supersonic. Bullets, artillery shells, surface-to-air missiles, and air-to-air missiles all typically operate in the super-

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An F/A-18 Hornet creating a vapor cone at transonic speed just before reaching the speed of sound. Photo by Realbigtaco, via Wikimedia Commons.

sonic regime, with Mach numbers up to about 3.5. Most modern fighter aircraft can fly at supersonic speeds for short durations, with their engines operating on afterburners. The Lockheed Martin F-22 is capable of cruising at supersonic speeds without afterburners. Large aircraft capable of cruising at supersonic speeds are the North American B-70 Valkyrie bomber, the British Aerospace/Aerospatiale Concorde, the Tupolev Tu-144, and the Tupolev Tu-44 Backfire Bomber. Speeds greater than five times the speed of sound are described as hypersonic speeds. Spacecraft and missile warheads reentering Earth’s atmosphere fly at hypersonic speeds, with Mach numbers as high as

36 for the Apollo capsule and about 25 for the space shuttle. The importance of Mach number to flight can be seen from the Mach cone. An object moving at a Mach number of 2 through air generates pressure disturbances that propagate in all directions at the speed of sound in the medium of air. If the speed of sound were 300 meters per second, when the object reached a given point, the disturbances it had generated a second before would have spread within a sphere whose radius was only 300 meters, yet the object itself would have traveled 600 meters in the same second. Disturbances generated 0.1 seconds before would have reached a radius of only 30 me-

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ters, yet the object would have traveled 60 meters. The disturbances generated at each intermediate point propagate out to smaller and smaller distances, all lying within the Mach cone. The Mach cone has its apex at the location of the object, and its axis is the path taken by the object to reach that point. The angle made by the axis of the Mach cone surface is called the Mach angle. This is the inclination of the weakest wave front generated by an object moving at supersonic speeds through a medium. Such a weak wave is called a Mach wave. The region ahead of the Mach cone is called the zone of silence. In this region, the sound from the approaching object cannot have reached at the instant in question. The Mach angle is thus given by the inverse sine of the reciprocal of Mach number. For example, if the Mach number is 2, the Mach angle is 30 degrees, whereas for a Mach number of 3, the Mach angle is 19.47 degrees. Most disturbances created by moving objects involve large pressure differences. These disturbances raise the temperature of the air, and hence move faster, accumulating along a shock front. Shocks formed by blunt objects can reach a shock angle of 90 degrees: Such shocks are called normal shocks, and they result in subsonic flow on the downwind side of the shock. As the flight Mach number of a wing exceeds the critical Mach number, the drag caused by the occurrence of shocks rises sharply. Early theoretical analyses predicted that the drag would rise to extremely high values, preventing an aircraft from accelerating through the speed of sound. This was called the sound barrier, the existence of which was conclusively disproved when Air Force Captain Charles E. “Chuck” Yeager flew the Bell X-1 rocket plane faster than the speed of sound in 1947 and became the first person to fly faster than Mach 1. —Narayanan M. Komerath

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Further Reading Anderson, J. D. Hypersonic and High-Temperature Gas Dynamics. American Institute of Aeronautics and Astronautics, 2000. Clegg, Stewart, and William Devenport. Aeroacoustics of Low Mach Number Flows: Fundamentals, Analysis. Elsevier Science, 2017. Cummings, Russell M., Scott A Morton, William H. Mason, and David R. McDaniel. Applied Computational Aerodynamics: A Modern Engineering Approach. Cambridge UP, 2015. Kim, Kwang-Yong, Abdus Samad, and Ernesto Benini. Design Optimization of Fluid Machinery: Applying Computational Fluid Dynamics and Numerical Optimization. Wiley, 2019. Kundu, Ajoy Kumar, Mark A. Price, and David Riordan. Conceptual Aircraft Design: An Industrial Approach. Wiley, 2019. Takahashi, Timothy. Aircraft Performance and Sizing, Volume II; Applied Aerodynamic Design: Volume 2. Momentum Press, 2017. See also: Aerodynamics and flight; Differential equations; Fluid dynamics; High-speed flight; Hypersonic aircraft; Jet engines; Ernst Mach; Sound barrier; Supersonic jetliners and commercial airfare; Supersonic jets invented; Viscosity; Wind tunnels; Chuck Yeager

Materials Science Fields of Study: Physics; Chemistry; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Materials science examines the structure and properties of materials in order to create materials with useful and novel properties. Materials scientists use principles from chemistry, physics, various subfields of engineering, and applied mathematics. Depending on the material being studied and its intended applications, other fields, such as biology or even planetary science, may be relevant. Materials science has applications in every imaginable industry in which

Principles of Aeronautics

synthetic materials are used and is responsible for countless innovations in aeronautical engineering. KEY CONCEPTS accelerometer: a device that measures the change of stress on an internal component to determine the rate of acceleration or deceleration, and changes in the direction of motion advanced: indicates a process or mechanism that functions beyond the current standard levels of performance advanced polymer: a polymer that is typically of a complex molecular structure, produced by polymerization of similarly complex monomeric units, and having characteristic properties that greatly exceed those of simpler polymer systems monomeric unit: refers to a small molecular species that is joined to an untold number of other such molecules to form the corresponding polymer; for example, polyethylene can be readily seen as the head-to-tail conjoining of multiple ethylene molecules; the monomeric unit retains its essential structure within the polymer AN INTRODUCTION Materials science examines the structure and properties of materials in order to create materials with useful and novel properties. To this end, materials scientists use principles from chemistry, physics, various subfields of engineering, and applied mathematics. Depending on the material being studied and its intended applications, other fields, such as biology or even planetary science, may be relevant. Materials science has applications in every imaginable industry in which synthetic materials are used and is responsible for countless innovations in aeronautical engineering, as well as lighter and stronger automotive parts, bacteria-resistant medical supplies, superior toothpastes, and plastic bags that keep fruit fresher.

Materials Science

BASIC PRINCIPLES The advent of materials science occurred when humans first began to create tools and built structures. Its importance is exemplified by the fact that many time periods, including the Bronze Age, the Iron Age, and the Silicon Age, are named for the materials that contributed greatly to human development during those periods. Similarly, the Industrial Revolution, while not named for a specific material, refers to a period of significant development in the means of creating materials and incorporating them into devices. For much of history, knowledge of what would now be considered materials science was passed from parent to child, leading to the use of terms such as “smith” and “cartwright” as family names in addition to job titles. Later, apprenticeship became the dominant form of education, eventually evolving into the higher education system of the twenty-first century. Materials science is a common major or academic department at larger or more science-oriented colleges and universities, and the fields of chemistry, physics, biology, and engineering also encompass elements of the science. Materials science and materials engineering often overlap; however, science and engineering are distinct fields. Scientists attempt to understand phenomena, whereas engineers seek to apply scientific knowledge to problems. Although these definitions are not mutually exclusive, scientists and engineers approach their work differently: A scientist might design a device as a proof of concept, whereas an engineer would experiment with different designs to optimize the performance of a given device. Thus, materials scientists are primarily concerned with understanding why materials behave as they do and using that knowledge to devise new materials, while materials engineers focus on optimizing the application of these concepts and materials for particular uses.

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CORE CONCEPTS Metals. Metals, as materials, are composed of metallic elements that exist as positively charged ions embedded in a sea of electrons, known as “delocalized electrons” because they are not tightly bound to their source atoms. These bonds contrast with covalent and ionic bonds, in which electrons are localized between their source atom and the bonded atom. Metals conduct electricity and heat well. Metal materials may be pure metals or alloys, which are solid solutions composed of different metals. Brass (composed of copper and zinc) and bronze (copper and tin) are examples of alloys. Ceramics. Ceramics are solids that are held together by covalent or ionic bonds. Because the nature (strength, length, orientation) of these bonds depends on the identity of the constituent compounds, the properties of these materials vary much more dramatically than those of metals. Ceramics may be semiconductors (conductors of electricity at high temperatures) or superconductors (perfect conductors of electricity) or may be piezoelectric (the application of pressure influences its mechanical properties and vice versa) and pyroelectric (the application of heat influences its electrical properties and vice versa). Semiconductors. Semiconductors, the basis for modern electronics, conduct electricity at ‘high’ temperatures. The conductivity of semiconductors can be fine-tuned through a process called “doping,” which is the intentional introduction of a given impurity into an otherwise perfectly ordered material. This introduced material has a different number of electrons than the host material, which affects how readily electricity flows through the material. Doping produces either n-type or p-type semiconductors, depending on whether the impurity has more or fewer electrons than the host atoms, respectively. Polymers. Polymers are generally solids composed of chains of repeating units, or monomers. For ex-

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ample, silk is composed of repeating units of fifty-nine amino acids, and polyethylene is composed of repeating units of ethylene monomers. The properties of polymers can vary dramatically: Silk is a polymer, but amber and rubber are as well. This variation can be ascribed to the fact that both the identity of the monomer and its microstructure (how it is organized into a chain) determine how a polymer behaves. Examples of different microstructures include straight chains, chains with branches, comb polymers (many shorter chains descending from a central chain, forming a comb-like shape), and star polymers (numerous polymers all extending from the same central point, forming a star-like shape). Polymers also differ in chain length. Plastics. Plastics are polymers that may retain whatever shape they are formed into, whereas nonplastic polymers, such as rubber, return to their original shape after the deforming force is removed. This property is known as plasticity. Plastics may also contain additives used as fillers or to fine-tune their properties. Like all polymers, plastics come in a wide variety of types, as defined by the constituent monomers and microstructure, and these types have different properties. Plastics are also categorized by how they respond to heating: Thermoplastics maintain their chemical structures when melted, while thermosetting plastics do not. Thus, thermoplastics can be molded repeatedly, whereas thermosets can only be molded once. Composites. Composite materials contain numerous components with substantially different chemical and physical properties that retain these individual properties, but when combined produce a new material with very different properties. One example of a composite is concrete, which is composed of cement and an aggregate, such as sand or gravel. When combined to form concrete, neither the cement nor the aggregate undergoes physical or chemical changes, though the concrete so formed is very different from any of its individual components.

Principles of Aeronautics

Biomaterials. Biomaterials are materials with biological applications and biological materials that can serve other uses. In the former case, materials are designed to be incorporated into living organisms and must therefore exhibit biocompatibility, which refers to a material’s ability to be accepted by a biological system, without toxicity in the original material or any of the degradation products that might be produced by exposure to physiological conditions. In the latter case, biological materials are altered to serve human needs, such as when wood is pressurized to create a building material that is much more resistant to decay and deformation than the original material. Nanomaterials. Nanomaterials are materials on the nanoscale, that is, on the order of 10-9 meters, or nanometers. These materials are particularly interesting because the properties of nanoscale components are at times dictated by quantum mechanics rather than classical (Newtonian) mechanics. An example of nanomaterials is quantum dots, which are semiconductor particles with diameters of 2 to 10 nanometers. The color that these particles emit can be adjusted by changing their size, with smaller particles emitting colors toward the blue end of the visible spectrum and larger particles emitting colors toward the red end of the visible spectrum. The rich crimson color found in older stained-glass windows is due to nanodots consisting of three atoms of gold within the glass. APPLICATIONS PAST AND PRESENT Aviation. There are few areas that surpass the aviation industry for the relevance of materials science. This is particularly true in regard to advanced composite materials, the precise combinations of specialized fabric materials and advanced polymer resins. In principle, the construction of an advanced composite is the same as embedding a mat of random glass fibers in an epoxy resin and waiting for the resin to harden. In practice, however, the process of

Materials Science

preparing an advanced composite is very different; a painstaking process of fiber alignment in several layers, followed by impregnation with a thermosetting resin whose exact identity is often a proprietary secret, and completed by controlled heating of the raw resin-fiber assembly on a mold using reduced pressure to extract contaminant residual gases with the entire workpiece perhaps subjected to increased external pressure. The advantages of advanced composites are their inherent high strength and relatively light weight. The wings of high-speed fighter aircraft, for example, are rock-hard structures of compressed carbon fiber/resin weighing and costing much less than any metal structure that would be needed to provide the same strength characteristics. Medicine. The applications of materials science in medicine range from new drug delivery systems to improved prostheses and artificial organs. The discovery of new materials for use in medical equipment has led to improvements in durability, cost, and practicality. For example, dental fillings were made of gold for centuries. More recently, ceramic materials were used as cheaper alternatives, with the added advantage of being less noticeable due to their subtler, off-white color. However, ceramic fillings eventually degrade. To address this issue, researchers are working to develop new filling materials that are cheap, strong, biocompatible, and stable. One candidate material is titanium, which can be implanted into the jawbone itself in a biocompatible fashion. Materials science has similarly been key to the development of scaffolding used to grow tissue artificially. When tissues are grown in the laboratory, they require scaffolding for support, much like a vine requires a trellis. Materials scientists work to create biocompatible, effective scaffolding for this purpose. Transportation. Materials scientists are responsible for scientific advancements that affect nearly every category of vehicle. The materials in the framework of these vehicles must be both lightweight for fuel

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efficiency and strong for safety. Furthermore, a wide range of materials is needed to produce everything from flame-retardant upholstery to heat-resistant engine parts. One example of a promising category of material for use in automobiles and other engine-propelled vehicles is piezoelectrics. Piezoelectric ceramics, which are ceramics that respond to physical deformation with an electrical response and electrical stimulation with a physical deformation. Lead zirconate titanate is an example of a piezoelectric ceramic. In cars, piezoelectrics are used as passive sensors (for instance, in accelerometers or airbag impact sensors) and generators (spark plugs) due to their ability to convert movement or impact into electricity. They also function as active sensors (such as fuel level sensors) and actuators (such as those used to position mirrors) thanks to their ability to respond to electricity mechanically. Electronics. The electronics industry was born of the development of new materials that gave humans great control over the flow of electrons and the ability to create circuits, allowing scientists to build devices to serve their needs. One of the most important classes of materials in the electronics industry is semiconductors. Silicon is an example of a semiconductor, and its prevalence in electronics has given rise to terms such as “Silicon Valley” and “the silicon revolution.” The purity of silicon in electronics applications is crucial; one area of continuing research focuses on the development of better methods for creating thin films of pure silicon based on an understanding of its structure and properties. Food and Drink. Packaging is crucial to keeping food fresh, especially considering how far most food must travel to reach the consumer’s home. Containers for food such as produce must typically be transparent, allowing the buyer to check the contents for damage, and maintain the optimal levels of water vapor, oxygen, and carbon dioxide to maintain freshness and discourage rot or drying. For any food or drink container, it is crucial that the material not

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degrade or leach into the product, as such materials can harm human health. For example, bisphenol A (BPA) is present in some plastics but can disrupt the endocrine system when ingested over a period of time. Studies suggest that BPA is present in most people in detectable quantities, which has raised particular concerns about pregnant women and the potential effects of fetal exposure to BPA. Companies have begun to phase out the use of BPA in food packaging both voluntarily and in response to new regulations. In response to such concerns, materials scientists are working to develop better materials and methods for packaging foodstuffs. Energy. Materials science plays an important role in the attempt to meet ever-increasing energy demands across the globe as supplies of nonrenewable energy sources dwindle. For example, research into fuel cells, which turn fuel into electricity by means of a chemical reaction, relies on materials science to devise better anode, cathode, and electrolyte materials to improve the efficiency of these cells and best accommodate the specific fuels being used. Similarly, materials scientists are continually striving to create more efficient, sturdier, and more versatile photovoltaic systems, which convert solar energy into electricity, as well as to determine the materials that will create the cleanest, most efficient biofuels. Sensors. Sensors are used in a wide range of fields to detect specific targets, which may include tumors, pollutants in wastewater, or physical imperfections in a crucial device component. Ideally, a sensor responds with high selectivity and specificity, meaning that it responds nearly every time it encounters the target and rarely responds to anything other than the target. The advantage of a sensor is that it responds to something that is difficult to detect, such as the presence of a contaminant in parts-per-million concentrations, in a way that is much easier to detect, such as by emitting light or changing color. One sensor design involves the use of self-assembled monolayers (that is, single layers of a material) on

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gold, glass, or another substrate to detect biologically relevant molecules. The binding of the target molecule improves the fluorescence of the self-assembled monolayer, and this change can be easily detected.

waste, such as coconut husks and palm fronds, into new materials and use waste materials to generate energy to offset the energy consumed by processing raw agricultural products.

SOCIAL CONTEXT AND FUTURE PROSPECTS Materials science will likely continue to contribute to significant advances in industry and science as a whole and particularly in the medical field. For example, much of the work in the fields of cancer detection and treatment involves the creation of biocompatible materials that can target tumors, such as tumor-targeting quantum dots that fluoresce after reaching their targets. Other materials are being designed to deliver chemotherapy drugs directly to tumors, minimizing the damage done to healthy cells and allowing the patient to maintain better overall health during the treatment. As these technologies mature, the identification and treatment of cancer will become more successful and less invasive. On a broader scale, materials science offers potential treatments for a wide range of human ailments, promising to improve the overall quality of life. Another prominent example of the social significance of materials science is its relevance in addressing climate change. As the negative effects of human activity on the environment become increasingly clear, pressure is mounting to find more economical and efficient ways to use natural resources. To this end, materials science has sought to find ways to reduce dependence on nonrenewable resources and reuse “waste” material in an economically viable fashion. For example, a key area of research in materials science is the improvement of solar cells. Future solar cells will be more efficient, more versatile, and less expensive to make. Another popular research area is the use of waste material as a substitute for freshly generated material in various production and manufacturing processes. For example, materials scientists seek to incorporate agricultural

Further Reading Ashby, M. F., Michael F. Ashby, Hugh Shercliff, and David Cebon. Materials Engineering, Science, Processing and Design. Elsevier Science, 2019. Douglas, Elliot. Introduction to Materials Science and Engineering. Pearson, 2013. Gordon, J. E. The New Science of Strong Materials; or, Why You Don’t Fall Through the Floor. Princeton UP, 2006. Hosford, William F. Materials Science. Cambridge UP, 2007. Hummel, Rolf E. Understanding Materials Science: History, Properties, Applications. 2nd ed., Springer, 2004. Irene, Eugene A. Electronic Materials Science. Wiley, 2005. Lynch, Charles T. Handbook of Materials Science. Volume I. General Properties. CRC Press, 2019. “Materials Science.” ACS. American Chemical Society, n.d. Accessed 10 Sept. 2012. Mercier, Jean P., Gerald Zambelli, and Wilfried Kurz. Introduction to Materials Science. Elsevier Science, 2012. Sutton, Adrian P. Concepts of Material Science. Oxford UP, 2021.

—Richard M. Renneboog

See also: Advanced composite materials in aeronautical engineering; Advanced composite materials repair; Aeronautical engineering

Messerschmitt Aircraft Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT Messerschmitt was a major manufacturer of German aircraft. Tens of thousands of various Messerschmitt aircraft were produced and served as the foundation for the German Luftwaffe in World War II. These aircraft aided the early German victories and introduced numerous innovations into the aircraft industry.

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KEY CONCEPTS fighter-bomber: an aircraft designed to carry out delivery of explosive ordinance as well as to engage in aerial combat maneuvers against enemy fighters fueled weight: the weight of an aircraft carrying only its full load of fuel gross weight: the weight of an aircraft with its maximum load net weight: the weight of an aircraft with no load ORIGINS In 1923, engineer Willy Messerschmitt established an aircraft company in Bamberg, Germany. In 1927, he moved his firm to Augsburg, Germany, where he merged with another company and created the corporation of Bayerisch Flugzeigwerke (BFW). BFW, with Willy Messerschmitt as chief designer, initially

Principles of Aeronautics

produced gliders and sport and transport aircraft, but in 1933 it secured a contract from Adolf Hitler’s Reich air ministry and began to produce military aircraft (designated by the prefix Bf) for the Luftwaffe. In 1936, Willy Messerschmitt seized complete control of the company, renamed it Messerschmitt AG, and continued to focus production on military aircraft (now designated by the prefix Me). During World War II, Messerschmitt AG produced fifteen distinct series of aircraft, ranging from fighters and bombers, to the first jet-powered aircraft. BF-109 SERIES The most famous of the Messerschmitt aircraft is the Bf-109; a single-seat fighter used by the Luftwaffe from 1935 to 1945 and produced in greater num-

A Bf 109G-6 of JG 27 in flight, 1943. Photo by Bundesarchiv/Hebenstreit, via Wikimedia Commons.

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bers (approximately 33,000) than any other World War II aircraft except the Russian Il-2. The Bf-109 was the first “modern” German fighter. It possessed such advancements as a light alloy stressed skin construction, low cantilever wings with trailing edge flaps, a retractable tail wheel landing gear, and an enclosed cockpit. The first version of this aircraft, the Bf-109A, was produced in 1935, but met with pilot resistance due to its limited agility compared with biplanes. Newer versions of the Bf-109, the Bf-109B, Bf-109C, and Bf-109D, added greater agility, horsepower, and armament and were delivered in modest numbers to the Luftwaffe in the late 1930s. By February, 1939, however, such variants were removed from front-line duty and replaced by the Bf-109E, known as the Emil. The numerous versions of the Emil saw the plane used as a fighter, fighter-bomber, and reconnaissance fighter. The Bf-109E-4 was the most widely used of the Emil aircraft. A fighter, weighing 4,685 pounds empty, it had a wingspan of 9.75 meters, was 8.63 meters in length, and 2.47 meters in height. Powered by one Daimler-Benz DB-601 Aa inverted-V piston engine, the Bf-109E-4 had a maximum speed of 560 kilometers per hour, a cruising speed of 483 kilometers per hour, a ceiling of 10.5 kilometers, and a maximum range of 660 kilometers. It was armed with two 20-millimeter MGFF fixed, forward-firing cannons built into the leading edge of the wing and two 7.92-millimeter MG17 fixed, forward-firing machine guns in the upper part of the forward fuselage with synchronization to fire through the propeller. The Emil, however, was difficult to maneuver at high speeds, and production ceased in 1942 as the Luftwaffe sought a more aerodynamic and better handling plane. Efforts to provide such a plane resulted in the production of the Bf-109F, known as the Friedrich. The Bf-109F-2 was the best of this series. A fighter and fighter-bomber weighing 2,353.2 kilograms empty, it had a wingspan of 9.9 meters,

Messerschmitt Aircraft

was 8.9 meters in length, and stood 2.6 meters in height. Powered by one Daimler-Benz DB 601N inverted-V piston engine, the Bf-109F had a maximum speed of 600.3 kilometers per hour, a cruising speed of 561.5 kilometers per hour, a ceiling of 11 kilometers, and a maximum range of 880.3 kilometers. It was armed with one 15-millimeter MG151/15 fixed, forward-firing cannon and two 7.92-millimeter MG17 fixed, forward-firing machine guns with synchronization to fire through the propellers. The Bf-109F was used throughout 1942, but was replaced in 1943 with the Bf-109G. The Gustav, as it was known, added a more powerful engine, a pressurized cockpit, and was used solely as a fighter by the Luftwaffe throughout the remainder of the war. Later versions of the Bf-109, the Bf-109H and Bf-109K, were introduced in 1945, and although both versions added few improvements to the Bf-109 line, they were produced in significant numbers. The Bf-109 series served the Luftwaffe over Spain during the Spanish Civil War and over Poland, France, England, and North Africa during World War II. BF-110 SERIES The Bf-110 was produced by BFW on request from the Luftwaffe for a heavy fighter. As with the Bf-109, the Bf-110 was produced in several versions, with a total output of approximately six thousand aircraft. The most noteworthy version was the Bf-110C-4. This heavy fighter carried a pilot, navigator/observer, and radio operator/gunner in an enclosed cockpit. Weighing 5,150 kilograms empty, it had a wingspan of 16.2 meters, was 12.1 meters in length, and stood 4.18 meters in height. Powered by two Daimler-Benz DB 601A-1 inverted-V piston engines, the Bf-110C-4 had a maximum speed of 560 kilometers, a cruising speed of 489 kilometers, a ceiling of 10 kilometers, and a maximum range of 1,094 kilometers. It was armed with two 20-millimeter MGFF fixed, forward-firing cannons, four 7.92-millimeter

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MG17 fixed, forward-firing machine guns, and one 7.92-millimeter MG15 trainable, rearward-firing machine gun. A later version, the Bf-110G-4c/R3, was reconfigured to serve as a night fighter. Carrying the same three-man crew as the Bf-110C-4, it weighed 5,094 kilograms, had a wingspan of 16.25 meters, was 13 meters in length, and stood 4.18 meters in height. Powered by two Daimler-Benz DB 605B-1 inverted-V piston engines, the Bf-110G-4c/R3 had a maximum speed of 550 kilometers per hour, a cruising speed of 510 kilometers per hour, a ceiling of 8 kilometers, and a maximum range of 1,300 kilometers. It was armed with two 30-millimeter MK108 fixed, forward-firing cannons, two 20-millimeter MG 151/20 fixed, forward-firing cannons, and one 7.92-millimeter MG81z trainable, rearward-firing two-barrel machine gun. This plane also carried several varieties of radar which, although increasing drag and hampering performance, enabled it to enter night service. The various versions of the Bf-110 served the Luftwaffe over Poland, Norway, England, North Africa, and Russia throughout World War II. ME-163 SERIES The Me-163 Komet was the first rocket-powered aircraft used in World War II. Although it did not come on line until 1944, the Me-163B-1a Komet was the best known of these rocket aircraft. It was a single seater that weighed 1,908 kilograms empty, had a wingspan of 9.35 meters, was 5.8 meters inches in length, and stood 2.75 meters in height. Powered by one Walter HWK 109-509A-1/2 rocket motor, it had a maximum speed of 954 kilometers per hour, a climb rate of 4,862 meters per minute, and a ceiling of 12 kilometers. It was armed with two 30-millimeter MK108 fixed, forward-firing cannons or two 20mm MG151/20 fixed, forward-firing cannons. The Komet, however, had several problems. It functioned for only 7.5 minutes under power, and frequently suffered from premature engine shutdown

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at high altitude or immediately after takeoff. Such problems, combined with the late stage of the war when the Komet was introduced, resulted in only 279 of the aircraft actually reaching Luftwaffe service. ME-262 SERIES The world’s first operational jet fighter was Messerschmitt’s turbojet-powered interceptor fighter, the Me-262. The most popular of the Me-262 class was the Me-262A-1a. This was a single seater that weighed 4,419 kilograms empty, had a wingspan of 12.5 meters, was 10.6 meters in length, and stood 3.8 meters in height. Powered by two Junkers Jumo 004B-1/2/3 Orkan turbojet engines, it had a maximum speed of 869 kilometers per hour, a climbing rate of 1,120 meters per minute, and a range of 1,049 kilometers. It was armed with four 30-millimeter MK-108 fixed, forward-firing cannons located in the nose cone. In 1944, Messerschmitt produced the Me-262a-2, a fighter-bomber, nicknamed Sturmvogel (storm bird). This aircraft was basically the same as the Me-262a-1a, with the notable exception that the Sturmvogel was equipped to carry one 500-kilogram bomb or two 250-kilogram bombs. Although unstable, difficult to fly, and used in limited numbers by the Luftwaffe, the Me-262 was a nearly unstoppable aircraft that changed the course of the aircraft industry by ushering in the jet age. ME-323 SERIES The most unique of the Messerschmitt aircraft was the Me-323 Gigant (giant). The Me-323E-2 Gigant was a heavy transport plane operated by a pilot, copilot, flight engineer, and radio operator on the flight deck, plus a load master and up to six gunners in its belly. Its empty weight was 29,600 kilograms, but the Gigant had a maximum takeoff weight of 45,000 kilograms. It could carry a payload of 120 troops or freight up to 15,422 kilograms. The

Principles of Aeronautics

Gigant had a wingspan of 55 meters, was 28.5 meters in length, and stood 9.6 meters in height. It was powered by six Gnome-Rhone 14N-48/49 radial piston engines, which provided a maximum speed of 253 kilometers per hour, a cruising speed of 225 kilometers per hour, a ceiling of 4.5 kilometers, and a range of 1,300 kilometers. To protect this rather slow-moving aircraft, the Gigant was armed with one 20-millimeter MG151/20 trainable cannon in each of two power-operated EDL 151 wing turrets, one 13-millimeter MG131 trainable, forward-firing machine gun in each of two nose positions, one 13-millimeter MG131 trainable, rearward-firing machine gun in the rear of the flight deck, and one 13-millimeter MG131 trainable, lateral-firing machine gun in each of the two forward and two beam positions. The Gigant was used to support the Afrika Corps in North Africa and the Wehrmacht in Russia. ME-210/410 SERIES The Me-210 was constructed by Messerschmitt to replace the Bf-110 and to act as a dive-bomber. The Me-210, however, suffered from serious technical and aerodynamic problems from the beginning. These problems plagued the Me-210 throughout its production and it never acted as a serviceable aircraft. Messerschmitt never gave up on the craft, and in 1943, the Me-410 Hornisse (hornet), a modified version of the Me-210, entered Luftwaffe service. The best known of the Hornisse was the Me-410A-1/U2. This heavy fighter and fighter-bomber carried a pilot and radio operator/gunner, and weighed 7,518 kilograms empty. It had a wingspan of 16.3 meters, a length of 12.47 meters, and stood 4.3 meters in height. It was powered by two Daimler-Benz DB 603A inverted-V piston engines, which provided for a maximum speed of 624.4 kilometers per hour, a cruising speed of 587.4 kilometers per hour, a ceiling of 10 kilometers, and a maximum range of 1,690 kilometers. It was armed with two 20-millimeter MG151/20 fixed,

Messerschmitt Aircraft

forward-firing cannons in the nose, two 20-millimeter MG151/20 fixed, forward-firing cannons in a ventral tray, two 7.92-millimeter MG17 fixed, forward-firing machine guns in the nose, and one 13-millimeter MG131 trainable, lateral/rearward-firing machine gun. The Me-410 served the Luftwaffe over France, Russia, and Eastern Europe during the last two years of World War II. POSTWAR MESSERSCHMITT After the war, Messerschmitt AG briefly left the aircraft industry to produce products as varied as sewing machines and motor scooters. By the mid-1950a, the company had returned to the aircraft industry to produce passenger, transport, and training aircraft. The company survived Willy Messerschmitt’s death in 1978 and, as a result of mergers and reorganizations, in the 1990s Messerschmitt AG became Messerschmitt Bolkow-Blohm GmbH. The company continues to produce aircraft, but also produces missiles, parts for spacecraft, as well as railroad and highway vehicles. —Gregory S. Taylor Further Reading Forsgren, Jan. Messerschmitt Bf 109: The Design and Operational History. Fonthill Media, 2017. Forsyth, Robert. Messerschmitt Me 264 Amerika Bomber. Bloomsbury Publishing, 2016. Goss, Chris. Messerschmitt Bf 109: The Latter Years-War in the East to the Fall of Germany. Pen & Sword Books, 2019. Griehl, Manfred. Messerschmitt Aircraft Since 1925. Pen and Sword Aviation, 2015. Harvey, James Neal. Sharks of the Air: Willy Messerschmitt and How He Built the World’s First Operational Jet Fighter. Casemate Publishers (Ignition), 2011. Jackson, Robert. Messerschmitt Bf 109 A-D Series. Bloomsbury Publishing, 2015 See also: Aeronautical engineering; Airplane manufacturers; First flights of note; German Luftwaffe; Jet engines; Military aircraft

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Military Aircraft Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Pilot training ABSTRACT Although military aircraft have existed since the 1700s, their development and impact on war became more pronounced in the twentieth century. Aircraft were first used mainly for occasional battlefield observation; later, they carried nuclear weapons and directly affected conventional combat campaigns. Aviation, weapons, and electronic technologies also intertwined to create diverse, sophisticated means of fighting wars in the air. KEY CONCEPTS afterburner: a system that increases jet engine thrust by injecting additional fuel into the exhaust stream as it exits the combustion chamber, where the extra fuel is instantly ignited thus producing a large quantity of additional exhaust gas to augment thrust Cold War: a period during the 1950s and early 1960s during which the Soviet Union and the United States essentially dared each other to start a nuclear war though neither one would THROUGH WORLD WAR I Military aircraft first saw action in 1794, when the French embarked in hot-air balloons to locate enemy forces. In the 1800s, other nations used balloons for observation but little else because balloons lacked controlled mobility. Balloon-type aircraft with controlled propulsion, called dirigibles (also airships or zeppelins), appeared in the late 1800s. The invention of the airplane in the early 1900s created greater military possibilities. The first planes were slow, flimsy, and carried small loads, but they were more efficient than dirigibles. Most industrial nations had created airplane units by 1914, and in 1911, Italy first used planes in combat.

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World War I (1914-18) combat determined the best airplane types while exposing airships’ shortcomings. Germany tried bombing England with zeppelins, but they were too cumbersome and too vulnerable. The Allies used dirigibles more effectively as submarine spotters (also later in World War II), but their limitations prevented any further use. Most World War I airplanes were wooden biplanes with fixed landing gear, fabric skins, and fixed-pitch propeller engines. Combatants initially used them for observation; however, by 1915, they were building airplanes, later called fighters, to control the sky above the battlefield. The early fighters’ flimsy wings could not carry machine guns, so both sides wanted fuselage-mounted guns that could fire through the propeller without striking it. The Germans were the first to achieve success, with their Fokker E-III. Nations then exchanged the fighter performance lead through multiple generations of fighter design. The Allies eventually retained air control with such fighters as England’s Sopwith Camel and France’s SPAD XIII, though the Germans’ late-appearing Fokker D VII was considered the best such plane. Air commanders used fighters against enemy troops, but they also designed planes for ground attack. Single-engine, two-seat attack planes such as England’s DH-9 and Germany’s Halberstadt CL II struck targets on or near the battlefield. Larger, multiengine bombers such as England’s Handley-Page 0/400 and Germany’s Gotha G-IV dropped heavier bomb loads on distant targets. Aircraft provided intelligence and occasionally disrupted enemy advances and retreats. Fighter pilots received public adulation for their difficult and essential air combat missions. The successful ones were called aces, and the maneuvering between fighter opponents came to be called dogfights. The bombers lacked weapons accuracy and encountered difficulties against fighters, but their destructive potential earned attention.

Principles of Aeronautics

Military Aircraft

An F-16 Fighting Falcon, P-51D Mustang, F-86 Sabre, and F-22 Raptor fly in a formation, representing four generations of American combat aircraft. Photo by J.M. Eddins Jr./US Air Force, via Wikimedia Commons.

BETWEEN THE WARS In the years before World War II (prior to 1939), no major conflict existed to spur innovation, uncertainty existed over airplanes’ potential, and air forces struggled for independence and identity. However, by the late 1930s, most leading air armies’ planes were all-metal monoplanes with retractable landing gear and high-powered, variable-pitch propeller engines. Many airplanes had basic instruments to aid with flying in varying weather conditions. These conditions created unusual results and other military aircraft types. Cargo planes appeared, as the German air force (Luftwaffe) fielded the Junkers (Ju) 52 and the US Army Air Corps converted the successful DC-3 airliner into the C-47.

For a time, bombers flew faster than fighters, contributing to English and US air leaders’ belief that bombers alone could win wars. The best example was the United States’ four-engine B-17, with its protective armament and Norden bombsight for accurate, high-altitude, level-flight attacks. Bombsight and engine development difficulties led the Luftwaffe to favor smaller bombers and dive-bombing capability. The Royal Air Force and US Army Air Corps also purchased smaller, twin-engine bombers such as the Mosquito and B-25, but viewed them as just one part of their offensive array. By the late 1930s, fighter performance, as seen in Germany’s Messerschmitt (Me) 109 and England’s Spitfire, surpassed that of bombers.

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Navies had used floatplanes in World War I, and these still served as observation and rescue craft. However, the Japanese, US, and British navies started building fighter and attack planes specially for aircraft carrier operations. Naval warfare demanded dive-bombers such as the United States’ SBD Dauntless and torpedo-bombers such as Japan’s B5N “Kate.” WORLD WAR II An air power success watershed, this war began with some biplanes still flying and ended with the first jets in service. The Luftwaffe abetted the Nazis’ early Blitzkrieg victories with such planes as the Stuka dive-bomber. However, the Luftwaffe’s problems with target intelligence, bomber durability, fighter range, and radar surfaced during the Battle of Britain (1940-41). England’s victory revealed that bombers still suffered against concentrated daylight fighter opposition. The European air war stalemated, as England’s fighters likewise lacked long-range escort ability, forcing its bombers into less accurate night raids. Both sides developed night fighters and associated radar control procedures. In the Pacific, airplanes decided important naval battles such as Coral Sea and Midway. Japan’s Zero fighters initially triumphed against obsolete US planes, but the US Navy successfully responded with Hellcat and Corsair fighters. The US Army Air Force’s long-range B-29 bombers attacked Japan and dropped the first atomic bombs, which hastened Japan’s surrender. The United States’ European daylight bombing offensive with unescorted B-17s and B-24s faced failure until it introduced drop tank-equipped P-47s and P-51s. These fighters escorted the bombers, suppressed the Luftwaffe fighter defense, and guaranteed air control for the Normandy invasion. P-47s and other fighters assisted the ensuing ground campaign by flying air support missions. Meanwhile, Al-

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lied bombers crippled the Germans’ oil and transportation facilities. Air supply flourished, as Ju-52s carried paratroops and towed assault gliders in Germany’s Blitzkrieg. C-47s did even better with Allied paratroop and glider assaults. US cargo planes also conducted successful air-only supply operations in Asia. Rapid technological advance demanded timely aircraft development and aircrew training. These World War I issues became critical in World War II with air power’s rising importance; the Germans and Japanese paid dearly for failures in both. The Germans’ Me 262 jet fighter appeared too late to stop Allied bombers. The Soviets’ poorly trained air force suffered heavy losses, but it compensated with sheer numbers and effective aircraft designs like the Shturmovik attack plane. THE COLD WAR AND BEYOND East-West tensions and the occasional hot war sustained explosive military air progress and diversification. The United States led this process, as associated costs soared. The end of the Cold War (1945-91) raised spending questions even in the United States as developmental opportunities still emerged. Air forces sought air superiority through ever-faster and more sophisticated jets. The Korean War (1950-53) featured America’s F-86 and the Russian-built Mikoyan-Gurevich (MiG) 15, which exemplified the initial swept-wing, transonic designs. Early-1950s fighters like the US F-100 used afterburning jet engines to attain supersonic speeds. The United States’ F-4, France’s Mirage III, and the Soviets’ MiG 21 were late-1950s Mach-2 fighters that fought in various wars. Increasingly, fighters used radar-guided or heat-seeking air-to-air missiles, inspiring some belief that dogfights and gun armament were passé. Occasional success by relatively cheap, maneuverable MiGs in Vietnam revealed that dogfights were

Principles of Aeronautics

quite alive. Afterward, fighter design reincorporated dogfight maneuverability and guns, while maintaining supersonic speed and improving missile systems to help pilots handle any air combat situation. The United States’ F-14, F-15, and F-16 led this advance, though the French and Soviets attempted parity with the Mirage F1 and several MiG variants. The United States also led bomber advances while the Soviets strained to match them. The newly independent US Air Force supported the nuclear war-oriented defense policy of the 1950s with a jet bomber fleet. Most effective was the swept-wing, eight-engine B-52. Also, the Air Force developed tankers like the KC-135 to give bombers and other planes worldwide range. As air defenses improved, supersonic bombers capable of low-level attacks appeared, such as the American B-1 and Soviet Backfire. Both US and Soviet bombers also carried cruise missiles for standoff weapons delivery. In the 1990s, the United States built the radar-evading B-2 stealth bomber. Different attack plane types met varying mission and situational demands. National air arms preferred fast, sophisticated planes that could also be fighters, fly nuclear strike missions, or operate in bad weather. These included the US Air Force’s F-105 and F-111, the Soviets’ Fencer, and the Europeans’ Jaguar and Tornado. Indeed, many forces used fighters such as the F-4 and F-16 as attack planes. Conversely, the US Navy and Marines for many years used various dedicated attack planes, including the A-6 Intruder for night/poor weather missions, the Harrier vertical/short takeoff and landing (V/STOL) attack jet for air support, and the A-7 and A-4 for dive-bombing. The US Navy’s successes in the Korean and Vietnam (1961-75) Wars with the propeller-driven A-1 helped inspire US Air Force purchase of the rugged, slow A-10 attack jet. The Air Force also used its turboprop-driven C-130 transports as air support gunships.

Military Aircraft

Technological advance bred more variety and prohibitive cost for nearly all nations. Attack planes increasingly used precision-guided munitions (PGMs, or “smart weapons”) with satellite, laser, infrared, or television seekers. The Vietnam and 1973 October Wars showcased guided surface-to-air weapons’ increasing lethality and inspired F-4, F-111, A-6, and F-16 variants designed to jam or shoot enemy radars. Only the United States could afford the F-117 stealth fighter, which was used in the Gulf War’s (1991) air combat. The United States also led development of various early warning radar planes, and its SR-71 was among the fastest, most sophisticated reconnaissance planes ever built. Cargo planes were essential to major logistic operations such as the 1948 Berlin Airlift, and the American C-5 exemplified the jet cargo planes that the United States and Soviet Union later built to rapidly deploy large loads. Helicopters also gave armies their own rapid battlefield mobility and air support. In Vietnam, the US UH-1 “Huey” and AH-1 Cobra performed well as light cargo and attack helicopters, respectively. The Soviets’ Hind combined attack and cargo helicopter attributes, and provoked Western concern until its heavy Soviet-Afghan War (1979-89) losses diminished its menace. In the Gulf War and 1999 Kosovo War, the United States orchestrated its many air weapons to stifle its opponents. Enemy defenses managed only a sporadic and unsuccessful effort. —Douglas Campbell Further Reading Eden, Paul E., and Sophearith Moeng, editors. Aircraft Anatomy: A Technical Guide to Military Aircraft from World War II to the Modern Day. Amber Books, 2018. Griffin, John A., Robert H. Smith, and Kenneth D. Castle. Canadian Military Aircraft: Aircraft of the Canadian Armed Forces: Serials and Photographs, 1968-1998. Vanwell Publishing Ltd., 2005.

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Murray, Aaron R. Modern Military Aircraft. Rosen Publishing Group Inc., 2015. Newdick, Thomas. The World’s Greatest Military Aircraft: An Illustrated History. Amber Books Ltd., 2015. Skaarup, Harold A. Canadian Warplanes. Universe, 2009. Winchester, Jim, editor. Classic Military Aircraft: The World’s Fighting Aircraft, 1914-1945. Chartwell Books, 2014. See also: Aeronautical engineering; Biplanes; Glenn H. Curtiss; Jimmy Doolittle; German Luftwaffe; Jet engines; Messerschmitt aircraft; Billy Mitchell; Monoplanes; Wiley Post; Eddie Rickenbacker; Stealth bomber; Training and education of pilots; Triplanes; Chuck Yeager

Billy Mitchell Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Billy Mitchell was born on December 29, 1879, in Nice, France. He died on February 19, 1936, in New York, New York. Mitchell commanded the US air effort in World War I and thereafter was an outspoken advocate of air power and of an independent US air force. He lobbied, cajoled, and bullied the US governmental and military power structure to gain recognition of the role of air power in warfare. KEY CONCEPTS court-martial: a military court of law adjudicated by military officers rather than by publicly elected judges HISTORY William “Billy” Mitchell grew up in Milwaukee, Wisconsin, and in Washington, D.C. The son of a US senator, he was deeply steeped in patriotism and military history. In 1898, at the outbreak of the Spanish-American War, he dropped out of George Washington University to join the US Army and served in

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Billy Mitchell. Photo via Wikimedia Commons. [Public domain.]

the aviation section of the Signal Corps. The armed forces were Mitchell’s home and his love for the remainder of his life. Mitchell was an Army representative to a flight demonstration by Orville and Wilbur Wright, and he himself learned to fly in 1915. From 1917 to 1918, he commanded the Army Air Service of the US Expeditionary Forces in Europe and was promoted to brigadier general in 1920. Mitchell traveled widely, observed other countries’ increasing interest in air power, and became a strong proponent for air power and for an independent Air Force. He worked hard to develop strategic doctrines that would utilize air power in the conduct of modern warfare and gathered many supporters. In 1921, the Army and the Navy held a demonstration of air power with a captured German battleship as the target. Mitchell’s pilots sank the ship with heavy bombs, disregarding the rules set for

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Principles of Aeronautics

the demonstration. Mitchell gained support in Congress but alienated his military colleagues by regularly and publicly criticizing the military’s mismanagement of air power. At his 1925 court-martial, personally ordered by President Calvin Coolidge, he was found guilty of insubordination, reduced in rank to colonel, and suspended from active service for five years. Mitchell resigned a few months later and continued speaking out against the military command and for air power. His rank was posthumously restored, and he was decorated for his service. —Kenneth H. Brown Further Reading Burlingame, Roger. General Billy Mitchell: Champion of Air Defense. Reprint. Greenwood Press, 1978. Cohen, Lester, and Emile Gavreau. Billy Mitchell: Founder of Our Air Force and Prophet Without Honor. Lulu Press Inc., 2019. Drez, Ronald. Predicting Pearl Harbor. Billy Mitchell and the Path to War. Pelican Publishing Company, 2017. Hurley, Alfred F. Billy Mitchell: Crusader for Air Power. Reprint. Indiana UP, 1975. Mitchell, William. Memoirs of World War I: “From Start to Finish of Our Greatest War.” Random House, 1969. ———. Winged Defense: The Development and Possibilities of Modern Air Power, Economic and Military. Putnam, 1925. US Air Force, Office of Air Force History. Billy Mitchell: Stormy Petrel of the Air. CreateSpace Independent Publishing Platform, 2015. Waller, Douglas C. A Question of Loyalty. HarperCollins, 2009. Wildenberg, Thomas. Billy Mitchell’s War with the Navy: The Army Air Corps and the Challenge to Seapower. Naval Institute Press, 2014. See also: Aerodynamics and flight; Biplanes; Military aircraft; Training and education of pilots

Model Airplanes Fields of Study: Physics; Aeronautical engineering; Mechanical engineering

ABSTRACT Model airplanes are facsimiles ranging in size from a few inches to many feet in length intended to represent actual or imagined airplanes in reduced scale, for display, recreational flying purposes, or to test and modify the aerodynamics of a proposed design before construction of the actual airplane. KEY CONCEPTS dethermalizer: a mechanism that will act to arrest or slow the upward motion of a model by controlling the horizontal stabilizer to prevent the model from being carried too high by a thermal air current wing loading: the amount of weight being borne by a wing relative to its surface area SIGNIFICANCE Model airplanes were used, before full-scale airplanes had flown, to investigate the science of flight. Models are also used for aeronautical research and aerial reconnaissance. They provide youth with an aeronautical education, and many pilots have become interested in flying through modeling. Modeling is a passionately followed hobby worldwide, providing pleasure of both building and flying. For those who compete in regional, national, and world contests, flying model airplanes is a highly demanding sport. Flying models take many forms. They can be unguided after launch and known as free-flight, or FF, models; constrained and controlled by wires and known as control-line, or C/L, models; or controlled remotely by radio signals and known as radio-control, or R/C, models. From an aerodynamics standpoint, models will always suffer from what is known as scale effect, obtaining smaller maximum lift coefficients and greater drag coefficients. However, wing loadings, or weight divided by wing area, are much lower, so landing speeds are much lower than for full-scale aircraft. Modern model engines are suffi-

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ciently light and powerful that it is possible to build a model that has more thrust than its weight and can climb straight up or even hover. Structurally, models profit from a different scale effect and are less likely to suffer in-flight or landing damage. FREE-FLIGHT MODELS Free-flight, or FF, models can be the least expensive flying models, the easiest to build from raw materials, and the easiest and safest to fly by oneself. However, they are the most demanding of trim and stability because of their “launch-and-pray” nature. The smallest and lightest models can be flown indoors or on very calm mornings or evenings. Powered models are normally flown outside. They usually utilize a timer-controlled dethermalizer that

Principles of Aeronautics

tips up the leading edge of the horizontal stabilizer to prevent them from being lost if rising air and wind would otherwise take them out of sight. Contests with free-flight models usually involve trying to keep them aloft for the maximum amount of time for each of the different classes of models. CONTROL-LINE MODELS Control-line, or C/L, models are the next least expensive flying models and have the additional advantage that they cannot fly out of sight and be lost. They also provide tactile feedback to the flier, because they are flown on steel lines, mostly stranded stainless-steel cables, that range from about 10 meters to about 35 meters in length. The lines are attached to a control handle in the flier’s hand that

Boeing 747-400 scale display model. Photo by Poudou99, via Wikimedia Commons.

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operates the elevator through a bellcrank. Manipulation of this handle grants the flier the option of using a full hemisphere of space, inverted or upright. Control-line flying has a unique dependence on surface winds, because the planes are connected to the ground through the flier. Inverted flight requires extra learning, because response to control handle movements is reversed. Control-line models include sport/trainers, scale, stunt, carrier, speed, and race types. Scale control-liners feature engine power, retractable gear, “bombs,” and other realistic details. Stunt models are optimized for aerobatics and use a symmetrical wing section that enables them to make inverted and upright maneuvers. They often use a flap on the trailing edge of the wing that is mechanically linked to the bellcrank, so that it deflects oppositely to the elevator and enhances the maximum lift available for abrupt maneuvering. Carrier models are judged on the difference between their maximum and minimum speeds and for their ability to grab a wire with their tail hook for landing. Speed models are used in contests, which are won by the fastest speeds, with either piston or jet power, for a specified number of laps. Racer-type models are flown with two or more fliers in the circle, to a specified number of laps and with mandatory pit stops. Combat contests require two fliers in the circle, each trying to cut the opponent’s trailing streamer, often flying at speeds of more than 100 miles per hour. RADIO-CONTROL MODELS Radio-control, or R/C, models require a battery-powered miniature receiver with a separate channel for each servo. A servo is an electric motor that rotates a shaft one way or the other from the neutral position, based on the movement of a lever in the transmitter held by the flier. The number of channels utilized varies from two or three for trainers to six or more for sophisticated models that are

Model Airplanes

determined to fully emulate their full-scale counterparts. Radio-control models are the most popular form of model airplane flying, no doubt because of the challenge involved with flying them well and because of their good simulation of full-scale flight. They also require the most time to learn how to fly without crashing. They are the most expensive type of flying model and require the most sophisticated models. However, the best R/C models, both airplanes and helicopters, can perform all the same aerobatic maneuvers, and more, as can their full-scale counterparts. The most difficult problem to overcome in first learning to fly R/C airplanes is that the airplane apparently responds differently whether it is going away or coming toward the flier. It is also difficult for the beginner to judge the landing approach and landing. In this, computer-based simulators can be of considerable assistance. Competitive R/C events include combat, precision aerobatics, and pylon races. POWER PLANTS Gravity was the original power plant for both FF and R/C gliders. Twisted rubber strands were the next power plant, used until the 1930s, when the first miniature spark-ignition engines were commercially produced. In the 1940s, the much simpler and lighter glow-plug engine, which required a battery only for starting, appeared. Diesel and compressed-air engines are used in small models. Jet engines have been available since the 1940s. Electric motor engines are the newest type of power plant, providing quiet and clean power. —W. N. Hubin Further Reading Goethert, B. H. Transonic Wind Tunnel Testing. Dover Publications, 2007.

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Hawkey, Pat. Building and Detailing Model Aircraft. Kalmbach Publishing Company, 2009. Kundu, Ajoy Kumar, Mark A. Price, and David Riordan. Conceptual Aircraft Design, An Industrial Approach. Wiley, 2019. Mohan, Jim. RC Ground School: The Beginners Guide to Flying Electric RC Airplanes. CreateSpace Independent Publishing Platform, 2015. Russell, D. A. The Design and Construction of Flying Model Aircraft. Read Books, 2008. See also: Aerodynamics and flight; Aeronautical engineering; Avro Arrow; Biplanes; Fluid dynamics; Glider planes

Monoplanes Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Monoplanes are airplanes possessing only one primary lifting surface. Monoplanes are less expensive to build, more efficient in flight, and are capable of higher speeds than two-winged biplanes. After their early structural problems were solved, monoplanes quickly became the favored configuration for transports (in the 1920s) and for light airplanes and fighter aircraft (in the 1930s). KEY CONCEPTS banking: turning an aircraft in flight in such a way that it flies in a controlled arc either to the left or to the right as desired dihedral angle: the angle of a wing relative to horizontal radial engine: an engine in which the pistons and cylinders are arranged radially about a common crankshaft DEVELOPMENT The earliest practical airplanes were biplanes, with low-powered engines and very large wing areas. The

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braced-wing biplane design was considered the lightest and strongest aircraft configuration, especially for the thin wing sections then thought to be necessary. The first airplane to cross the English Channel, however, was Louis Blériot’s wire-braced monoplane, the Blériot XI, on July 25, 1909, six years after the Wright brothers flew their first biplane at Kitty Hawk, North Carolina. In 1912, the streamlined Deperdussin monoplane established a world speed record of more than 100 miles per hour. Biplanes continued to predominate during World War I, because their rapid climb to a fighting altitude and their maneuverability for fighting were favored over high speeds. However, a few monoplanes, such as the Fokker Eindecker, the first to have a machine gun synchronized to fire between propeller blades, were used. Early monoplane designers did not appreciate the twisting to which a wing is subjected in flight, and there were a number of structural failures due to the elimination of external bracing to reduce drag. In 1927, the greater efficiency of the monoplane was decisively demonstrated by Charles A. Lindbergh’s New York-to-Paris flight in his Ryan monoplane, the Spirit of St. Louis. In the 1930s, the development of the Douglas DC-1 and DC-2 models, using the modern configuration of a single low wing, a retractable landing gear, streamlined engine cowlings, and flaps for good low-speed performance, instantly made all biplane transports obsolete. A DC-2 carrying passengers nearly won the 1934 London-to-Australia race against specialized racing machines. The military, requiring extra strength and maneuverability from its aircraft, took longer to be convinced of the monoplane’s advantages and still maintained a few biplanes at the beginning of World War II in 1939. The 1937 Piper J-3 Cub, a strut-braced, high-wing monoplane with an inexpensive four-cylinder opposed engine, was far less expensive to produce and

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Monoplanes

Submarine Spitfire. Photo by Bryan Fury, via Wikimedia Commons.

fly than were biplanes with much more powerful radial engines. This configuration has dominated the lower end of light-plane flight ever since. DESIGN In the traditional monoplane design, a single large wing is followed by a much smaller horizontal surface containing both a stabilizing surface and a pitch-control surface, or elevator. For pitch stability with this configuration, the airplane’s center of gravity must be sufficiently forward that the horizontal tail generates a downward force, or negative lift. Pitch stability means that the aircraft will tend to maintain a constant nose attitude relative to the horizon, even when disturbed by atmospheric turbulence. However, a monoplane’s wing can be made

into a very efficient lifting surface, because it does not compete with another nearby lifting surface, as does a biplane’s wing. Different monoplane designs differ in their relative vertical locations of the wings on the fuselage. If the wing is mounted on top of the fuselage, as in high-wing aircraft, the critical upper surface of the wing is minimally disturbed by airflow around the fuselage. The placement of the primary lifting surface above the center of gravity also enhances lateral stability because of the pendulum effect, in which the airplane tends to return to wings-level flight if it is banked. High-wing aircraft give pilots and passengers a particularly good view of the ground but the upward and sideways views are typically restricted when the aircraft is banked.

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The most efficient location for a monoplane’s wing, from an aerodynamic standpoint, is considered to be in the middle of the fuselage. In this mid-wing configuration, the interference drag between the wing and the fuselage can be minimized. Therefore, many racing airplanes use this efficient design. The pilot typically must sit close to the center of gravity, about one-quarter or one-third of the way back from the leading edge of the wing to the trailing edge. The pilot’s field of view is thus severely restricted, and the pilot’s location obstructs any carry-through structure for the wing spar. The low-wing airplane is favored for most high-speed airplanes, because the wing provides a good place to house retracted gear. To minimize interference drag, both the leading and trailing edges of the wing normally require rather elaborate fairings. To obtain lateral stability, the wing of a low-wing airplane must incorporate more of a dihedral angle, the upward tilt of the wingtips, than that of a high-wing airplane. In the 1980s, a number of canard-type airplanes, efficient aircraft with a horizontal lifting surface in front of the main wing, were designed for both lowand high-speed flight. The low-speed canard-type aircraft are mainly those linked to Burt Rutan’s very successful VariEze and later designs. Canard aircraft with two nearly equal wings, the dragonfly configuration, may be thought of as either two-surface monoplanes or biplanes with a great deal of stagger. High-speed military aircraft often use a canard for extra pitch control at high angles of attack. Propeller-powered canard aircraft normally use pusher propellers, which tend to be less efficient because they operate in the wake of the wing. A few three-surface aircraft, with both a canard surface and a conventional tail surface, have also been designed and flown; they have the advantage of placing the pilot and passengers ahead of the wing. —W. N. Hubin

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Further Reading Herris, Jack. German Monoplane Fighters of World War I. Aeronaut Books, 2014. Jackman, W. J., and Thos. H. Russell. Flying Machines. Outlook Verlag, 2022. Maslov, Mikhail. King of Fighters-Nikolai Polikarpov and His Aircraft Designs, Volume R-the Monoplane Era. Helion, 2021. Ministry of Munition Aircraft Production. Report on the Junker All-Metal Single-Seater Monoplane Type D.1. July 1919. Reports on German Aircraft. Naval and Military Press, 2014. Owens, Colin. Brandenburg Aircraft of WW I, Volume 3-Monoplane Seaplanes. Aeronaut Books, 2015. Rathakrishnan, Etherijan. Theoretical Aerodynamics. Wiley, 2013. Thurston, David. The World’s Most Significant and Magnificent Aircraft: Evolution of the Modern Airplane. SAE International, 2000. See also: Aerobatics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Charles A. Lindbergh; Glider planes; Messerschmitt aircraft; Military aircraft; Model airplanes; Ultralight aircraft; Wing designs

Montgolfier Brothers Fields of Study: Fluid dynamics; Aeronautical engineering; Mechanical engineering ABSTRACT The Montgolfier brothers were aviation pioneers who first accomplished successful human flight. Joseph-Michel Montgolfier was born on August 26, 1740, in Vidalonles-Annonay, Ardeche, France, and died on June 26, 1810, in Balaruc-les-Bains, France. Jacques-Étienne Montgolfier was born on January 6, 1745, in Vidalonles-Annonay, France, and died on August 2, 1799, in Serrières, France. The Montgolfier brothers were pioneer developers of the hot-air balloon. Their work opened the way for exploration of Earth’s upper atmosphere.

Principles of Aeronautics

Montgolfier Brothers

KEY CONCEPTS density: the weight of a material per unit volume, typically grams per cubic centimeter or kilograms per cubic meter; the density of water = 1 gram per cubic centimeter; density of air = approximately 0.00129 grams per cubic centimeter (at 0ºC and 760 mm) thermal air current: a rising column of air that is warmer and thus has less density than the air around it, such as that above an open fire SIGNIFICANCE Joseph-Michel and Jacques-Étienne Montgolfier were two of sixteen children born to Pierre Montgolfier and his wife. Pierre’s success in the paper industry provided the necessary finances for Joseph-Michel and Jacques-Étienne to obtain good educations and to conduct a variety of scientific

Jacques Etienne Montgolfier, portrait. Image via Wikimedia Commons. [Public domain.]

Joseph-Michel Montgolfier, portrait. Image via Wikimedia Commons. [Public domain.]

experiments. Inspired by wood chips floating over a fire in the family fireplace, the two brothers theorized that when heated air was collected inside of a paper bag, the bag would rise. This discovery led to their invention of the first hot-air balloon in 1782. On June 5, 1783, the Montgolfier brothers made the first public demonstration of their hot-air balloon at the marketplace in their hometown. The balloon was constructed from multiple sections of cloth and lined with paper that was coated with alum to provide a form of fireproofing. The sections were held together with approximately two thousand buttons. The fuel to heat the air inside the balloon was a mixture of straw and carded wool. Once released, the balloon stayed in the air for ten minutes, reached an altitude of about 6,560 feet, and traveled a distance of more than 1 mile.

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On September 19, 1783, the Montgolfier brothers sent the first living creatures, a duck, a sheep, and a rooster, on a balloon flight in Versailles. Watched by King Louis XVI; his wife, Marie Antoinette; and some 130,000 spectators, the balloon stayed aloft for about eight minutes, reached a height of 500 meters, and safely landed 3.2 kilometers from the point of departure. This successful exhibition made the Montgolfier brothers national figures, and a gold medal was issued in their honor. In Paris, on November 21, 1783, the Montgolfier brothers conducted the first untethered human flight. It was manned by Jean-François Pilâtre de Rozier, a science teacher, and Marquis François-Laurent D’Arlandes. The balloon sailed over Paris for about twenty-five minutes and traveled approximately 11.3 kilometers from the launch site. In later life, Joseph-Michel invented a type of parachute, a calorimeter, and a hydraulic ram and press. In 1807, he was made a knight of the Legion

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of Honor. Jacques-Étienne developed a process for producing a new type of paper called vellum. Both brothers were honored by the French Academy of Sciences. —Alvin K. Benson Further Reading Anderson, Dale, Ian Graham, and Brian Williams. Flight and Motion: The History and Science of Flying. Taylor & Francis, 2015. Gillespie, Charles Coulton. The Montgolfier Brothers and the Invention of Aviation 1783-1784. Princeton UP, 2014. Kotar, S. L., and J. E. Gessler. Ballooning: A History, 1782-1900. McFarland Inc. Publishers, 2011. Lynn, Michael R. The Sublime Invention: Ballooning in Europe, 1783-1820. Taylor & Francis, 2015. Simons, Fraser. The Early History of Ballooning-The Age of the Aeronaut. Read Books Ltd., 2020. See also: Aeronautical engineering; Blimps; Dirigibles; Flight balloons; Steve Fossett; Hindenburg

N National Aeronautics and Space Administration (NASA) Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Established on October 1, 1958, NASA is the civilian space agency and aeronautical research agency for the United States of America. The National Aeronautics and Space Administration oversees all US civilian space exploration activities and coordinates the US involvement in international space cooperative ventures. It is also the primary US agency for civilian government research in aeronautics. KEY CONCEPTS space telescope: a telescope system positioned in geosynchronous orbit about Earth, capable of much finer depth perception and resolution than terrestrial telescope systems due to the absence of atmospheric interference winglet: a small vertical portion at the tips of wings, designed to counter the drag effect of wingtip vortices ORIGINS The idea of a central agency for aeronautical research in the United States dates back to 1915, when an amendment to another bill created the National Advisory Committee for Aeronautics (NACA) to help the United States catch up with European countries in aeronautical research. NACA was primarily involved in aircraft design and testing. An aeronautical research center, later to be named the Langley

Aeronautical Laboratory, was founded with NACA. In the years leading to World War II, additional aeronautical facilities were constructed at Moffett Field in California (later named the Ames Research Center) and at Lewis Field in Cleveland, Ohio. Rockets and space travel were of little interest to NACA until the 1950s. During the 1930s, however, rocketry experiments were being conducted at the Guggenheim Aeronautical Laboratory at the California Institute of Technology (GALCIT). During World War II, GALCIT became the Jet Propulsion Laboratory (JPL) and was under Army control. Besides developing missiles for the Army, such as the WAC Corporal, JPL also developed the Aerobee, a version of the WAC Corporal designed for civilian high-altitude research activities. After the war, the US Army also created a separate missile unit, which eventually became the US Army Ballistic Missile Agency (ABMA), headquartered near Huntsville, Alabama. During the early 1950s, the Navy and the Air Force began their own missile programs. Each of the separate missile and rocket programs eventually began to develop rocket boosters with a goal of launching satellites into orbit around Earth. By the mid-1950s, the Air Force and the Army were both looking at possible lunar space probes, and JPL was considering the possibility of interplanetary space probes. The Air Force was also investigating rocket-propelled aircraft, an area of research that overlapped with NACA’s mission. At this time, there was no central unified agency overseeing rocket development or space exploration. Multiple agencies, and even separate departments within each agency, were working independently of one another, often duplicating efforts and competing with one another

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for resources. Possibly as a result of the fragmented approach to space exploration and rocket development, the United States appeared to lag slightly behind the Soviet Union in these areas during the 1950s. The Soviet Union’s rocket development and space exploration activities were coordinated under one authority, largely working under the leadership of Sergei Korolev. A working group, called the Upper Atmospheric Rocket Research Panel (UARRP), consisting of representatives from the different US agencies involved in space exploration, including NACA, was formed in the mid-1950s to address some of these concerns. In January 1956, UARRP issued a report suggesting that all US space-related activities be centralized in one agency. A later report suggested that civilian space exploration be formed into an agency separate from Department of Defense space activities. Little real progress had been made in consolidating US space efforts until the Soviet Union launched Sputnik 1 on October 4, 1957, and Sputnik 2 on November 3, 1957, with Sputnik 2 carrying a dog named Laika into space. The United States tried to respond with the Navy’s Vanguard rockets, but the early Vanguards failed to launch a satellite. Finally, on January 31, 1958, the Army succeeded in launching Explorer 1, built by JPL, atop a modified intercontinental ballistic missile (ICBM) built by ABMA. The US government finally began to take seriously the need for a unified effort at space exploration. On July 29, 1958, President Dwight D. Eisenhower signed into law the National Aeronautics and Space Act of 1958. This act dissolved NACA and created the National Aeronautics and Space Administration (NASA), effective October 1, 1958. NASA was responsible not only for aeronautical research, as NACA had been, but would also be the US civilian space agency. NASA acquired all NACA facilities, including the Langley Research Center, the Lewis Research Center, and the Ames Research Center, along with two flight stations. On

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Installation of Explorer 1, the first US satellite, to its launch vehicle, Jupiter-C, January 1958. Photo via Wikimedia Commons. [Public domain.]

December 3, 1958, JPL was transferred to NASA. Much of ABMA was also transferred to NASA, becoming the Marshall Space Flight Center on July 1, 1960. Since that time, NASA has built numerous research centers and other stations throughout the country. Though space exploration gets most of the public attention, NASA has always remained active in aeronautical research, with several research centers devoted primarily to non-space-related activities. EARLY CREWED SPACEFLIGHT One of NASA’s early goals was to launch a person into space. This goal was formally stated on October 7, 1958, shortly after NASA’s formation. The first US crewed spacecraft project was named Mercury.

Principles of Aeronautics

Prior to the Mercury project, two competing ideas for crewed spaceflight had existed. Scientist Wernher von Braun and many others believed that a crewed spacecraft should take off like an aircraft, fly into space, and land again like an aircraft. Such a spacecraft would be fully reusable, and would be an extension of well-proven flight technology. Preliminary work toward such a space plane had already begun. Like the Soviet Union’s Korolev, many of the engineers in NASA were not willing to wait for the development of a safe and reliable space plane, however. Rather, they wanted to use modified ICBMs to launch a crewed capsule into space. Such an approach would yield results much faster. NASA engineers realized that the Soviet Union would likely beat the United States in sending a human into space if the United States were to wait to develop a method of flying a space plane into orbit. Thus the Mercury project aimed to launch a small capsule containing a human being atop a modified ICBM. Researchers at Ames showed that a nuclear warhead could safely survive reentry into the atmosphere with a blunted body. The Mercury capsule, therefore, would be shaped with a blunted bottom and use an ablative heat shield to prevent the capsule from burning up due to friction as it reentered Earth’s atmosphere at the high speeds required for Earth orbit. Such a craft could not land as an aircraft, so it would deploy a parachute and float down to a landing in the ocean, called a splashdown. The first launch of a Mercury capsule was an uncrewed test flight on September 9, 1959. The first crewed launch was May 5, 1961, when Alan B. Shepard was launched into space atop a modified Redstone rocket. The Redstone, however, was not powerful enough to put the Mercury capsule into orbit. Rather, Shepard’s flight, lasting only about fifteen minutes, was merely a suborbital ballistic trajectory. The first US crewed spaceflight took place on February 20, 1962, when a modified Atlas missile carried a Mercury capsule containing John H. Glenn into or-

National Aeronautics and Space Administration (NASA)

bit. The United States, however, did not beat the Soviet Union into space, for a modified ICBM had carried Yuri Gagarin into orbit around the Earth in a Vostok capsule on April 12, 1961. With the Soviet Union beating the United States in sending a human into space, President John F. Kennedy consulted his science advisers for a goal that the United States might hope to accomplish ahead of the Soviet Union. That goal was for a US crewed mission to the moon within a decade. President Kennedy made this goal public in a speech on May 25, 1961, even before the United States had put a human into orbit. NASA had to scramble to accomplish this goal. The crewed lunar mission, called the Apollo Program, was born in November 1961. In order to launch a spacecraft to the moon, NASA had to create the largest rocket America had ever known, eventually dubbed the Saturn V. Realizing that it would take a rocket bigger than they could build to launch a spacecraft to the moon’s surface and back, they opted to launch a spacecraft into orbit around the moon. Astronauts would then descend to the lunar surface in a small landing craft, and then ascend to rendezvous with the orbiting spacecraft, which would carry them back to Earth. Such a mission would involve extended missions in space, and spacecraft rendezvous. None of this was at that time possible. Thus, as work progressed on the Apollo missions, NASA created the Gemini Program to develop the skills and test the procedures needed in the upcoming Apollo missions. The Gemini Program ran from December 7, 1961, until December 23, 1966, with the first crewed flight on March 23, 1965. While the Mercury capsules held just one astronaut, the Gemini capsules each had a crew of two astronauts. The first crewed Apollo spaceflight was Apollo 7, launched October 11, 1968. Apollo 7 was an Earth orbital test flight. The first lunar landing mission was Apollo 11, launched July 16, 1969, crewed by Edwin “Buzz” Aldrin and Neil Armstrong, both of

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whom walked on the surface of the moon, and by Michael Collins, who piloted the command module that orbited the moon during the landing mission. The last of these lunar missions was Apollo 17, launched December 7, 1972. CREWED SPACEFLIGHT AFTER THE MOON The last three scheduled Apollo missions to the moon were cancelled. The hardware for these missions, however, was not wasted. The third stage of a Saturn V rocket was adapted to be used as a crewed space station called Skylab, launched May 14, 1973. Three Apollo capsules were used to ferry astronauts to and from Skylab from May 25, 1973, to February 8, 1974. Left in low-Earth orbit, Skylab eventually reentered Earth’s atmosphere and burned up on July 11, 1979, with some solid pieces striking the Indian Ocean and Australia. The final Apollo mission was the Apollo-Soyuz Test Project. This program was a rendezvous mission between the US Apollo spacecraft and a Soyuz spacecraft from the Soviet Union during July 1975.

Buzz Aldrin salutes the USflag on the Moon, July 20, 1969. Photo via Wikimedia Commons. [Public domain.]

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This rendezvous mission was primarily a political mission, designed to show good will between the two superpowers. It was one of the first space missions involving more than one nation. This project eventually became a model for later international space cooperative ventures. One of the major drawbacks of the spacecraft used in the early days of the US space program was that they could only be used one time. By the 1970s, the United States was no longer in a space race with the Soviet Union, so NASA took the time to investigate a reusable spacecraft, much as had been envisioned in the earliest days of space exploration. A compromise vehicle was eventually developed that would take off as a rocket, with strap-on solid rocket boosters and a discardable external fuel tank. The spacecraft would land, however, as a glider. Designed to transport satellites and equipment into orbit and to carry astronauts and equipment to a permanent space station, this partially reusable spacecraft was called the space shuttle. The first operational flight of a space shuttle was on April 12, 1981. The worst accident in NASA’s history involved the explosion that destroyed the space shuttle Challenger on January 28, 1986, seventy-three seconds after launching, when the shuttle’s external fuel tank ruptured after being penetrated by a plume of gas escaping from a failed solid-rocket joint seal. Nevertheless, the space shuttle program continued. On July 27, 1995, the space shuttle Atlantis launched to rendezvous with the Russian space station Mir to exchange crew. Over the next three years, there were several more missions to Mir, fulfilling some of the hopes of the Apollo-Soyuz Test Project. Beginning in 1998, the first elements of the International Space Station (ISS) were launched. This space station was a scaled-down version of the proposed space station Freedom authorized by President Ronald Reagan in 1985. The space shuttle was scheduled to have many dozens of flights through

Principles of Aeronautics

the 2000s, constructing the ISS and transporting crew and equipment to the station. The space shuttle missions to Mir and the ISS finally fulfilled some of the original design plans of the space shuttle project. Despite these successes, by 2004 President George W. Bush had announced that NASA’s missions would begin moving away from the space shuttle and the ISS and toward the development of a new manned exploration vehicle with the goal of carrying passengers back to the moon once more and, eventually, to Mars. According to his plan, NASA would aim to retire the space shuttle by 2010, when the ISS was scheduled to be completed. After continuing to conduct regular missions to the ISS over the subsequent years, the three remaining shuttles, Atlantis, Discovery, and Endeavour launched and landed for the final times between March and July 2011. The final return of Atlantis in July, after bringing remaining supplies to the ISS, marked the official end of the shuttle program. The three shuttles were then brought to museums throughout the country to be displayed as a significant part of NASA’s history. Under President Barack Obama, NASA maintained the goal of eventually bringing humans to Mars. In 2014, NASA conducted a test flight of its design for its most advanced manned spacecraft yet, Orion. Using knowledge gained from the experiences of previous manned and unmanned programs, NASA claimed to have built a craft with greater safety focus and capabilities of carrying humans into deeper space than before. Under the administration of President Donald Trump, NASA notably pivoted away from a focus on a crewed Mars mission, instead concentrating on returning astronauts to the surface of the moon. In 2017 Trump signed an official directive indicating the goal of a lunar colony; in 2019 the timeline for a crewed moon mission was accelerated to 2024, which many experts suggested was unlikely to be met. During this period NASA also increased its collaboration

National Aeronautics and Space Administration (NASA)

with the growing commercial space travel industry, continuing the Commercial Resupply Services (CRS) and Commercial Crew Program (CCP) initiatives begun under earlier administrations. In May 2020 a vehicle developed by the company SpaceX brought NASA astronauts to the ISS, a first for a private spacecraft. The subsequent administration of President Joe Biden maintained NASA’s focus on the moon through the Artemis program and proposed a budget increase for the agency. Under Biden NASA was also given more leeway to continue and expand its climate change studies, which Trump had sought to sideline. UNCREWED SPACE EXPLORATION In addition to crewed spaceflight, NASA has conducted many uncrewed space flights. Some of these missions, such as the Ranger moon probes (1961-65) and the Surveyor moon landers (1966-68) were precursors to crewed missions. Others, such as the Explorer series, which began as an Army project but was transferred to NASA after its formation, were scientific missions designed to study the sun, Earth, and space environments. NASA has been a leader in interplanetary explorations. Spacecraft in the Mariner series visited Mercury, Venus, and Mars. The Pioneer series of spacecraft were designed as small interplanetary spacecraft. Some were lunar flyby missions, others were placed into solar orbit to study the solar wind, several of which remained in operation for over thirty years. Pioneer 10 and Pioneer 11, launched in the early 1970s, were the first spacecraft to fly past Jupiter and achieve escape velocity to leave the solar system. They were joined by the two Voyager spacecraft as the first four spacecraft ever launched from Earth to leave the solar system. Voyager 1, launched September 5, 1977, flew past Jupiter and Saturn. Voyager 2, though launched on August 20, 1977, before Voyager 1, arrived at Jupiter and Saturn after

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Voyager 1 and continued on to pass Uranus in January 1986 and Neptune in August 1989. NASA launched the Galileo spacecraft to Jupiter on October 18, 1989, and the Cassini spacecraft to Saturn on October 15, 1997. On August 20 and September 9, 1975, NASA launched two Viking spacecraft to the planet Mars, both of which achieved the first successful surface landing missions on Mars. Starting on December 4, 1996, with the launch of the Pathfinder mission, NASA began a series of further missions to study Mars. The Mars Exploration Rover mission began in 2003 with the dual launch of the robotic rovers Spirit and Opportunity. While Spirit went beyond its planned mission time, it eventually became stuck in soft soil and became inactive in 2010. Opportunity exceeded its life expectancy even further, though it finally stopped transmitting back to NASA in 2018 after a major dust storm on Mars. The rover Curiosity was launched and landed on Mars in August 2012 as part of the Mars Science Laboratory mission; it outlived its original two-year mission and remained active as of 2021. Another NASA rover, Perseverance, landed in February 2021; it launched the experimental aircraft Ingenuity that April. The year 2021 additionally saw major NASA progress in its mission to increase humanity’s understanding of the sun. The Parker Solar Probe, initially launched in 2018, had become the fastest object manufactured by humans and had approached within 8.5 million kilometers (5.3 million miles) of the solar surface on its tenth approach in November 2021. In December, it was reported that the probe had “touched the sun” for the first time back in April as it flew inside of the star’s upper atmosphere. In addition to interplanetary missions, NASA has launched many astronomical satellites into orbit around Earth to study the universe. Many satellites contained telescopes or other instruments of various types to study the entire range of the electromagnetic spectrum. Perhaps the most famous of these

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orbiting observatories is the Hubble Space Telescope, deployed from the space shuttle in April 1990. Its fame is rapidly being eclipsed and surpassed by the more recent James Webb Space Telescope’s superior capabilities and farther position in space. Since its entry into service the JWST has made many startling discoveries, including the existence of several Earth-similar exoplanets, as well as the atmosphere of another. AERONAUTICAL RESEARCH When NASA was formed in 1958, it absorbed NACA, and was charged with not only space exploration but also aeronautics. Though most public attention, and much of NASA’s budget, is directed toward space exploration activities, a major portion of NASA’s activity has involved aeronautical research. Upon its formation, NASA inherited the Air Force X-15 project. The first X-15 rocket plane flew in 1959 as a NASA aircraft. X-15 flights continued until 1968. Other aeronautical research involved lifting-body aircraft designs, such as the X-24, the HL-10, and the M-2 aircraft of the 1950s. Such aircraft use the shape of the aircraft rather than wings to provide lift. In 1975, NASA began the Aircraft Energy Efficiency Program, designed to increase flight efficiency and develop less-polluting aircraft engines. The new engine designs from this program were incorporated in Boeing’s 767 and McDonnell Douglas’s MD-80 commercial aircraft. Additional designs showed that wingtip winglets also increase efficiency, and many aircraft designed from the 1980s and later have included these winglets. In addition to efficiency, NASA has also promoted aircraft safety. NASA conducts crash tests to design safety systems that maximize the likelihood of survival during an aircraft crash. NASA also works to develop improved guidance systems for both commercial and private aircraft. During the 1990s, NASA undertook a study at major commercial airports to determine the optimal spacing between ar-

Principles of Aeronautics

National Aeronautics and Space Administration (NASA)

Photo via iStock/Pgiam. [Used under license.]

riving and departing aircraft. The Lewis Research Center has had a long history of studying icing on aircraft and ways of dealing with this problem, dating back to NACA days. During the 1970s, NASA developed fly-by-wire technology, whereby aircraft control could be done electronically rather than using mechanical means. NASA has not limited itself to fixed-wing aircraft. The Ames facility oversees NASA’s helicopter research. Ames was also the lead site for the XV-15, an experimental aircraft with tilting rotors designed as a hybrid between helicopters and traditional fixed-wing aircraft. NASA also operates research aircraft designed to carry infrared and radar instruments to study the ground under the aircraft’s flight path. Additional

science aircraft include the Kuiper Airborne Observatory that flew from 1977 to 1995. The Kuiper was a modified C-141 aircraft carrying a 36-inch-diameter infrared telescope high above much of Earth’s atmosphere anywhere it was needed in the world. The Kuiper was eventually replaced with another airborne observatory called the Stratospheric Observatory for Infrared Astronomy (SOFIA), which began routine operations in 2010. The SOFIA was a modified Boeing 747SP designed to carry a 2.5-meter reflecting telescope into the lower stratosphere. Unlike the Kuiper, which was entirely a NASA project, the SOFIA was jointly operated with the Deutschen Zentrum Luft- und Raumfahrt (DLR), the German equivalent to NASA. SOFIA, like Kuiper before it, operated out of NASA’s Ames facility.

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NASA CENTERS Due to the complex and varied nature of NASA’s mission, the agency has many research and operations centers, each with its own specialty. NASA headquarters in Washington, DC, handles administrative duties. The Kennedy Space Center on Cape Canaveral, Florida, is NASA’s primary launch facility, supported by the White Sands Test Facility at Las Cruces, New Mexico, and the Wallops Flight Facility on Wallops Island, Virginia. The Jet Propulsion Laboratory in Pasadena, California, is NASA’s primary center for interplanetary spacecraft development and operations. The Johnson Space Center in Houston, Texas, coordinates all crewed spaceflight activities. The Goddard Space Flight Center in Greenbelt, Maryland, handles most Earth-orbiting satellites and oversees much of NASA’s astronomical studies. Aeronautical research is performed at the NASA Ames Research Center (at Moffett Field, California), the Neil A. Armstrong Flight Research Center (at Edwards Air Force Base, California), Langley Research Center (at Hampton, Virginia), and the Glenn Research Center (at Lewis Field, Cleveland, Ohio). Ames is also the headquarters for NASA’s astrobiology program, and Armstrong (then known as the Dryden Flight Research Center) supported space shuttle landings if the shuttle could not land at Kennedy due to weather. The Marshall Space Flight Center (at Huntsville, Alabama) and the Stennis Space Center (in southern Mississippi) are the primary centers for rocket research and development. —Raymond D. Benge Jr. Further Reading “About NASA.” NASA, 5 Jan. 2022, www.nasa.gov/about/index.html. Accessed 21 Jan. 2022. Bizony, Piers, Andrew Chaikin, and Roger D. Lannias. The NASA Archives: 60 Years in Space. Taschen, 2019.

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Chang, Kenneth. “The Shuttle Ends Its Final Voyage and an Era in Space.” New York Times, 21 July 2011, www.nytimes.com/2011/07/22/science/space/22space-shu ttle-atlantis.html. Accessed 20 Oct. 2017. Croft, Melvin, and John Youskauskas. Come Fly with Us: NASA’s Payload Specialist Program. U of Nebraska P, 2019. Hatfield, Miles. “Parker Solar Probe Completes a Record-Setting Swing by the Sun.” Parker Solar Probe, NASA, 24 Nov. 2021, blogs.nasa.gov/parkersolarprobe/ 2021/11/24/parker-solar-probe-completes-a-recordsetting-swing-by-the-sun/. Accessed 21 Jan. 2022. Johnson-Groh, Mara. “NASA Enters the Solar Atmosphere for the First Time, Bringing New Discoveries.” NASA, 21 Dec. 2021, www.nasa.gov/feature/goddard/2021/nasaenters-the-solar-atmosphere-for-the-first-time-bringingnew-discoveries. Accessed 21 Jan. 2022. Logsdon, John. The Penguin Book of Outer Space Exploration. Penguin Publishing Group, 2018. “Orion Overview.” NASA, 3 Aug. 2017, www.nasa.gov/ exploration/systems/orion/about/index.html. Accessed 24 Oct. 2017. Patel, Neel V. “The Five Biggest Effects Trump Has Had on the US Space Program.” MIT Technology Review, 26 Oct. 2020, www.technologyreview.com/2020/10/26/ 1011214/five-biggest-effects-trump-us-space-programnasa-moon/. Accessed 20 May. 2021. Pellerin, Charles J. How NASA Builds Teams: Mission Critical Soft Skills for Scientists, Engineers, and Project Teams. Wiley, 2009. Vertesi, Janet. Shaping Science Organizations, Decisions, and Culture on NASA’s Teams. U of Chicago P, 2020. Williams, Dave, and Elizabeth Howell. Leadership Moments from NASA: Achieving the Impossible. ECW Press, 2021. See also: Aerodynamics and flight; Aeronautical engineering; Aerospace industry in the United States; Neil Armstrong; Yuri Gagarin; John Glenn; Robert H. Goddard; Hypersonic aircraft; Jet Propulsion Laboratory (JPL); Johnson Space Center; National Advisory Committee for Aeronautics (NACA); Parachutes; Rocket propulsion; Rockets; Russian space program; Alan Shepard; Space shuttle; Spacecraft engineering; Spaceflight; Valentina Tereshkova; Konstantin Tsiolkovsky; Unidentified aerial phenomena (UAP); X-Planes (X-1 to X-45); Chuck Yeager

Principles of Aeronautics

National Advisory Committee for Aeronautics (NACA) Fields of Study: Aeronautical engineering; Mechanical engineering ABSTRACT NACA was a US government organization formed to promote the scientific development of aircraft and flight. It functioned from 1915 to 1958, when it was made a part of the National Aeronautics and Space Administration (NASA). It was the premier American research facility on aeronautics and rocketry, the National Advisory Committee for Aeronautics maintained US leadership in aircraft development through the mid-twentieth century. KEY CONCEPTS sound barrier: the speed of sound, once thought to be a speed too difficult for aircraft to attain (a barrier) transonic drag: drag characteristics of air flow when an aircraft is moving in the transonic speed range from about 0.8 to 1.2 times the speed of sound wind tunnel: an enclosed tunnel-like room designed to provide a space in which forced air flow can be used to test the aerodynamic characteristics of a structure such as a model of an airplane or other object HISTORY Established in 1915, the National Advisory Committee for Aeronautics (NACA) promoted the scientific advancement of aircraft at a time when US technological prowess in the field was declining. Although the Wright brothers had pioneered powered flight in 1903, both private and government research in aircraft technology declined in the United States over the following decade. Although flight was considered by many Americans to be an impressive technical achievement, many others considered flight an often-dangerous passing fad. Unreliable engines and

National Advisory Committee for Aeronautics (NACA)

haphazard construction led to many deaths, and many people believed powered flight to be a science that was ahead of its time. During this period, various European designers emerged as the leaders in aerospace research, and the United States’ early lead in aircraft development disappeared. WORLD WAR I With the outbreak of World War I in 1914, the US government, pondering the possibility of US involvement in the war, came to the realization that the United States could not produce the advanced aircraft needed to wage modern warfare. Although prewar European and US military planners had considered the use of aircraft merely as observation platforms, as the war progressed, airplanes were used for increasingly important tasks, such as air defense and bombing. Under pressure to prepare the United States for a possible war, Congress established NACA as a branch of the Smithsonian Institute on March 3, 1915. The administration of President Woodrow Wilson, afraid that American citizens would consider NACA a purely military facility at a time of neutrality, added the proposal for NACA funds as a rider on the annual naval appropriations bill. NACA was originally limited to a $5,000 annual budget and twelve unpaid staffers who directed research projects at the Smithsonian and various university facilities. However, NACA’s contributions to wartime research, most notably the development of the ubiquitous JN-4 Jenny aircraft, earned the institution a long-term future as an institution separate from the Smithsonian and the creation of a permanent research facility of its own. Constructed in the middle of swampy ground owned by the US Army north of Norfolk, Virginia, NACA’s first facility, the Samuel Langley Memorial Aeronautical Laboratory, known simply as “Langley,” opened in 1920. The new laboratory boasted four buildings and a full-time staff of eleven.

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THE INTERWAR YEARS In the post-World War I era, NACA grew at an extremely slow pace. Postwar disillusionment and pro-neutrality sentiments reduced the amount of military-related research that was conducted at Langley. Also, the low pay associated with government employment relative to that of the private sector, coupled with Langley’s remote location, led many engineers to accept jobs elsewhere. Despite its limitations, however, NACA continued to make significant scientific breakthroughs. Although military research lagged, the number of civilian aircraft boomed in the 1920s, due to the large number of surplus military aircraft, interest generated by traveling air shows and wing-walkers, and the exploits of civilian pilots, such as Howard Hughes and Charles A. Lindbergh. Driven by the growth of the civilian aircraft market, NACA made several contributions to aircraft development and technology. For instance, NACA pioneered the use of specially trained test pilots. Although other institutions had used full-time pilots, NACA was the first to employ pilots with backgrounds in engineering to identify problems in the air as well as on the ground. Langley’s labs also developed advanced wind tunnels that measured precise aircraft takeoff and landing speeds. By 1931, Langley boasted the largest wind tunnel in the United States, capable of conducting tests on full-sized aircraft instead of scale models. NACA’s wind tunnels proved particularly valuable in the development of early airliners, planes too large for their manufacturers to test themselves. These aircraft, the Boeing 247 and Douglas DC-1, pioneered civilian air travel before World War II, and the development of these two planes formed the backbone of commercial air travel after the war. NACA also developed an innovative aerodynamic engine cowl that greatly reduced drag on the early piston-powered aircraft. In the 1920s and 1930s, the biggest goal of aircraft designers was speed, and Eu-

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ropean aircraft designers opted for complex liquid-cooled engines to boost top speed. The NACA cowl, however, boosted speed by reducing drag, permitting US manufacturers to use less complex and less expensive air-cooled engines. Although NACA grew slowly during the 1920s and 1930s, the organization’s contributions during this period ensured its long-term future and greatly aided the US war effort in World War II. WORLD WAR II If World War I had provided the motivation for the creation of NACA, World War II proved the value of its research facilities. As it had in World War I, the United States began World War II with aircraft that were less capable than those of its enemies. NACA faced the task of improving the United States’ air arm as quickly as possible. Toward this end, NACA expanded its presence and roles to aid the war effort. In addition to new facilities at Langley, NACA constructed new specialized laboratories in other parts of the country. In 1939, NACA opened a laboratory at the US Army Air Corps base at Moffett Field, south of San Francisco, California. NACA’s Moffett Field facility tapped into the pool of skilled engineers on the West Coast and was situated close to the region’s developing aircraft industry. In 1940, the West Coast lab was renamed the Ames Aeronautical Research Laboratory in honor of NACA’s long-time director. In the same year, NACA opened a propulsion research lab in Cleveland, Ohio, to support research in engine development in conjunction with the major engine manufacturers in the Midwest. In 1948, the Cleveland facility became the Lewis Flight Propulsion Laboratory. The new laboratory and propulsion laboratories proved their worth by improving upon the new aircraft types introduced during the war. NACA wind tunnels allowed for improvements to new fighters, such as the P-38 Lightning, by solving serious dive-instability problems and boosted the speed of

Principles of Aeronautics

the P-51 Mustang by introducing a laminar flow airfoil that moved air over the wing at peak efficiency. NACA’s participation in the development of Boeing’s B-29 bomber, particularly in aerodynamic and wing-loading issues, helped to expedite an advanced aircraft design into an effective weapon in only three years. During the war, NACA laboratories improved the performance of eighteen different warplanes, stretching speed, bomb load, and endurance beyond the capability of their original designs. NACA also branched into the field of rocketry during World War II. Although earlier rocket pioneers such as Robert H. Goddard conducted research on their own, the widespread use of rockets during the war attracted NACA attention, both in the development of its own rocket designs and in the improvement of Army and Navy projects. Although NACA concentrated on the military applications of rockets, as bombardment weapons or air-to-air ordnance, late in the war, the agency began research in ballistic missiles that paved the way for future work. POSTWAR CONTRIBUTIONS Deserving of praise for its wartime contributions, NACA received some undeserved criticism when the United States turned to jet propulsion in the mid-1940s. NACA conducted initial research on jet engines in the mid-1930s, but found that contemporary manufacturing methods made the technology unfeasible. By World War II, however, British breakthroughs had made the jet a viable means of propulsion, and the British shared their innovation with their American allies. Instead of allowing NACA to develop the new engines, however, the US Army gave General Electric, a private corporation, the development rights. Bell Aircraft received a contract to develop an airframe for the new jet engine, an airplane that eventually emerged as the XP-59. Bell, however, lacked NACA’s research capability, and the XP-59 could not match the performance of its Euro-

National Advisory Committee for Aeronautics (NACA)

pean rivals, the British Gloster Meteor and German Me-262 aircraft. Because NACA had had a hand in the development of so many of US wartime aircraft, many Army and aviation observers incorrectly believed that NACA had developed the XP-59 and had failed in the task. However, NACA knew nothing about the XP-59 until 1943, a full year after the aircraft’s first test flight. Once involved in jet aircraft development, NACA’s facilities proved invaluable in integrating captured German data into the US Air Force’s growing arsenal of jet warplanes. NACA wind tunnels provided aerodynamic data on swept-wing configurations that resulted in the advanced F-86 Sabre, the premier US fighter of the Korean War. NACA’s wind tunnels also suggested solutions to the problem of shock waves that formed on wingtips near speeds of Mach 1, the mythical sound barrier. Using the ballistic data of a .50-caliber machine gun bullet, NACA collaborated with Bell Aircraft to build the X-1, the first in a series of legendary experimental aircraft. On October 14, 1947, test pilot Charles E. “Chuck” Yeager took the X-1 beyond Mach 1 and became the first pilot to break the sound barrier. As aircraft broke the sound barrier with increasing frequency throughout the 1950s, another problem, known as transonic drag, surfaced. Because subsonic aircraft shapes were inappropriate for supersonic flight, jet aircraft of the 1950s continually failed to meet speed and altitude expectations in supersonic flight. NACA’s solution to transonic drag was to create a design element known as area ruling. Transonic drag occurred at the wings, where the mass of the airplane, the fuselage plus the wings, suddenly increased, and the air simply could not move out of the way quickly enough. Because airplane designers could not dispense with the craft’s wings, they had to make the fuselage thinner. On aircraft designed with area ruling, the fuselage narrowed as the wings spread, resulting in an airplane with a wasp-waisted or hourglass shape. With this innovation, aircraft

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speeds continued to rise, and engineers could predict aircraft performance beyond the sound barrier. NACA’s early forays into rocketry beginning in World War II increased throughout the late 1940s and early 1950s. The United States acquired advanced rocket technology, along with jet propulsion, from the defeated Nazis, and began a series of rocket testing by several different agencies. The US Army, having secured the services of the top German rocket scientist, Wernher von Braun, began rocket testing at the Redstone Arsenal in Alabama. At the same time, the US Navy and the US Air Force began their own rocket programs with the intent of developing nuclear delivery systems. In addition, the Smithsonian and the National Academy of Sciences developed rockets for scientific research. NACA contributed to these military projects primarily by testing internal systems and lightweight materials. NACA’s role in the US rocket program became preeminent, however, after October 4, 1957, when the Soviet Union launched Sputnik 1, the first human-made Earth-orbiting satellite. Although Sputnik was a minor technical achievement, its launch created widespread public fears of Soviet atomic bombs raining down upon American cities from orbit, and the US government demanded a response from its own rocket programs. The US response to Sputnik, the first launch of the Navy’s Vanguard rocket, embarrassingly exploded on the launch pad on December 6, 1957. One month later, a smaller Army rocket known as Explorer 1 finally put a small satellite into orbit. The public demand for a response to Sputnik, coupled with the inefficient system of multiple rocket programs, generated the idea of a single space agency, which NACA, as a civilian agency with advanced research labs, was in the best position to lead. Many Americans, particularly in Congress, worried that a military-led project would create only rockets for military use. Congress also blamed the various military rocket projects for allowing the Sovi-

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ets to take the lead in rocket technology. Therefore, on July 29, 1958, President Dwight D. Eisenhower signed the National Aeronautics and Space Act into law. The law merged NACA with the various military rocket programs, scientific rocket projects, and several other government laboratories into a new entity to run the US space program. On October 1, 1958, the newly amalgamated institutions became the National Aeronautics and Space Administration (NASA). —Steven J. Ramold Further Reading Bilstein, Roger E. Orders of Magnitude: A History of the NACA and NASA, 1915-1990. CreateSpace Independent Publishing Platform, 2013. US NACA. Annual Report of the National Advisory Committee for Aeronautics. Volume 1-2. US Government Printing Office, 1915. Reprint. Arkose Press, 2015. US NACA. Report-NACA. Issues 24-50. Arkose Press, 2015. US NACA. Report-NACA. Issues 83-110. Creative Media Partners, LLC., 2019. See also: Aeronautical engineering; Air transportation industry; Flight testing; Robert H. Goddard; Military aircraft; Rockets

National Transportation Safety Board (NTSB) Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT The National Transportation Safety Board was created April 1, 1967, to be the independent US agency responsible for the investigation of civil aviation, railroad, highway, marine, and pipeline accidents within the United States and for the issuing of safety recommendations designed to prevent future accidents. The NTSB provides in-

Principles of Aeronautics

dependent crash-site analysis and offers recommendations for improving the safety of all forms of transportation. HISTORY The National Transportation Safety Board (NTSB) was established by Congress in 1967 to investigate the causes of all transportation-related accidents involving aviation, railroads, highways, marine craft, or pipelines. Although the NTSB’s funding appropriations came from the Department of Transportation (DOT), the NTSB functioned independently of the DOT. In 1975, Congress passed the Independent Safety Board Act, which formally severed all ties between the NTSB and DOT. NTSB investigators operate twenty-four hours a day, seven days a week, investigating accidents within the United States as well as accidents involving US crafts overseas. Once NTSB teams reach the crash site, they evaluate the evidence to determine the probable cause of the accident and issue safety recommendations to prevent a recurrence. Since opening its doors in 1967, the NTSB has investigated more than 110,000 aviation accidents. Although the NTSB does not have the regulatory power to enforce its recommendations, approximately 82 percent of its 11,000 safety recommendations have been implemented by the Federal Aviation Administration (FAA). RECOMMENDATIONS The NTSB is responsible for investigating all civil aviation accidents in the United States. The number of civilian takeoffs and landings exceeded 63 million in 1997, and the number of passengers flying rose from 580 million in 1995 to 630 million in 1997. By December, 2020, roughly 100,000 flights were taking off and landing each day. The safety of these commercial flights rests with the NTSB, which focuses on specific problems, such as operations, cabin safety, weather, and aircraft design, when issuing its recommendations for improved safety.

National Transportation Safety Board (NTSB)

One of the principal recommendations in the area of operations involves the addition of ground proximity warning systems (GPWS) for aircraft equipped with ten or more seats. The recommendation was issued after an Eastern Air Lines Lockheed L-1011 crashed into the Florida Everglades on December 29, 1972 and a TWA Boeing 727 crashed into a mountain on its approach to Washington Dulles International Airport in Virginia on December 1, 1974. One hundred ninety-one people died in these two crashes, and, after thorough investigations of each, the NTSB determined that the cause of both accidents was “controlled flight into terrain,” which could have been prevented if the aircraft had been equipped with warning systems. In 1975, the FAA implemented the NTSB recommendation that all large passenger aircraft be equipped with ground proximity warning systems that alert the crew if terrain is approaching, if the plane is descending too quickly, and if the landing gear is not functioning properly. In 1994, the original recommendation was expanded to include smaller aircraft capable of carrying as few as ten passengers. A second area of concern for the NTSB involves fire safety. On several occasions, fires that started in aircraft lavatories or cargo areas have resulted in fatalities. In July, 1973, the NTSB recommended that airplanes be equipped with smoke detectors after a Boeing 707 crashed near Paris, France, after a fire started on board. After several more incidents, the NTSB recommended, and the FAA mandated, that automatic-discharge fire extinguishers be installed in all aircraft trash receptacles. Airline attendants are also required to routinely check the containers. After a fatal fire occurred on board an Air Canada flight that was forced to land at Cincinnati, the NTSB recommended that all lavatories be equipped with smoke alarms, that floor-level lighting be installed for passenger safety during an emergency evacuation, and that fire-blocking materials be used in all cabin and seat material. In addition, the NTSB

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recommended that all emergency slides be equipped with a heat-resistant coating to prevent injury to passengers during a postcrash evacuation. In 1981, after a fatal fire on board a Lockheed L-1011 out of Riyadh, Saudi Arabia, the NTSB issued a recommendation for aircraft modifications aimed at preventing the spread of fires from cargo areas to the cabin. Additional restrictions on the containment of cargo fires followed the crash of a South African Airways Boeing 747 that crashed into the Indian Ocean with the loss of all 160 people on board. The most serious weather-related problem addressed by the NTSB involves wind shear. The first instance of NTSB involvement with the weather phenomena occurred in 1968, and since that time, the NTSB has issued more than sixty safety recommendations. The most serious crash involving wind shear occurred at Dallas-Fort Worth International Airport on August 2, 1986, when a Delta Air Lines Lockheed L-1011 crashed, killing 135 people on board. Investigators examined the data and suggested the need for additional pilot training specifically geared toward this type of weather condition and for the installation of low-level wind shear alert systems at all major airports. As a result, the terminal Doppler weather radar (TDWR) warns pilots and air traffic controllers allowing them to prevent possible disasters. Since 1985, only one wind shear-related accident has occurred, at Charlotte, North Carolina, where the TDWR system was not yet operational. Another potential weather-related issue that the NTSB has investigated deals with icing. The accumulation of ice on airplanes has been a problem since the early days of aviation, but it was not until the crash of a USAir Fokker F-28 at New York’s LaGuardia International Airport in 1976 that the NTSB issued specific recommendations concerning the measurement and forecasting of icing on airplanes and protection against it. The FAA implemented these recommendations. In 1994, the NTSB issued additional warnings about icing problems on

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the ATR-72 passenger planes, and the FAA ordered the modification of deicing systems the following year. As the number of aircraft operating in limited airspace multiplied, midair collisions began to increase. As early as 1967, the NTSB advocated the development of a system designed to prevent such accidents. The proposed technology would be separate from the air traffic control system and would offer the earliest possible warning of a potential crash. In 1993, the FAA ordered that all aircraft used for transport be equipped with traffic alert and collision avoidance systems (TCAS). Mode C transponders, located near major airports, analyze the altitude of airplanes equipped with the device and alert air traffic controllers, who then warn the airplanes before a disaster occurs. Since the implementation of this recommendation, the number of near-midair collisions has dramatically decreased. When evaluating the causes of crashes, the NTSB examines aircraft design and has revealed several areas where modifications were necessary. While investigating a crash that occurred when an American Airlines DC-10 attempted to take off from Dallas-Fort Worth International Airport on May 21, 1988, the NTSB discovered that the minimum specifications for the brake friction material were inadequate for a rejected takeoff that required more than twice the minimum amount of material to stop safely. As a result, the FAA increased the safety standard and ordered additional training for pilots to improve passenger safety during aborted takeoffs. Another area of concern involves the length of airport runways. The FAA requires a 1,000-foot safety area at the end of runways for emergencies. Newer airports have allowed for plenty of room, but older airports frequently have sharp drops in terrain at the ends of runways. A 1994 crash at LaGuardia International Airport prompted investigators to recommend the use of soft-ground arresting systems to slow airplanes down in the event of an emergency.

Principles of Aeronautics

Arrestor-beds have prevented accidents at many airports, including John F. Kennedy International Airport in New York. Always cognizant of the possibility of human error, the NTSB has advocated several changes that would improve the safety of passengers. One recommendation included cross-referencing pilots’ licenses with the National Driver Register (NDR) to check for alcohol-related violations that could indicate a potential problem that would adversely affect a pilot’s performance during flights. Since the late 1980s, the NTSB has also recommended random drug screening. Another area of particular concern involves the interaction of crew members. The NTSB found that on numerous occasions, because the pilot remains the final authority in the cockpit, other crew members were hesitant to warn the pilot of potential problems for fear of reprimand. On December 28, 1978, a United Air Lines DC-8 ran out of fuel and crashed on approach to Portland, Oregon, killing ten people, because the first officer had failed to communicate the problem to the pilot. The NTSB found that improved crew management would reduce potential fatalities, and the FAA ordered a crew management training program for all major airlines. Aircraft design flaws account for many fatalities, and the NTSB has issued numerous recommendations based on their investigations of accidents caused by such flaws. In 1991, the NTSB examined the wreckage of an Atlantic Southeast Airlines EMB-120 that crashed in Georgia and found that excessive wear on the propeller-control unit had rendered the aircraft uncontrollable. After the NTSB issued its report, the FAA required the installation of a fail-safe device that prevents propellers from rotating too far. In its investigation of another crash in Georgia in 1995, the NTSB found that a small crack had developed in the aircraft’s propeller, resulting from the improper installation of a propeller blade. As a result of this investiga-

National Transportation Safety Board (NTSB)

tion, the NTSB advocated the use of ultrasonic inspection techniques to detect future problems. After the crash of a Turkish Airlines DC-10 near Paris, France, in 1974, the NTSB suggested the use of blowout pressure-relief doors to prevent a recurrence of an explosion that would buckle the cabin floor and damage flight controls. In 1989, the NTSB investigated a similar incident. On February 24 of that year, a United Air Lines Boeing 747 took off from Honolulu, Hawaii, bound for New Zealand. During the airplane’s ascent, the lower cargo door flew off, but the modifications implemented as the result of the Turkish Airlines crash saved the 355 lives on board. In addition to accidents caused by faulty aircraft design, the NTSB also investigates accidents involving structural fatigue and corrosion. On April 28, 1988, the NTSB investigated the structural failure of an Aloha Airlines Boeing 737-200 that lost a portion of its fuselage during takeoff from Hilo, Hawaii. The force of decompression during the accident resulted in one flight attendant being sucked out of the plane. After examining the aircraft, the NTSB recommended numerous changes in the structure and design of similar aircraft. The NTSB offers additional recommendations in numerous areas, including the improved quality of off-wing escape slides, fuel-tank protection, and safety belts. In addition to airplane safety, the board is also interested in the safety of helicopters and investigates problems involving the in-flight loss of the main rotor control and the need for flight restrictions during adverse weather conditions. More recently, the NTSB has worked with the National Aeronautics and Space Administration (NASA) to determine the survivability of space orbiters. The NTSB was involved in the investigation of the 1986 space shuttle Challenger explosion. NTSB investigators also located a flaw in a crashed Titan 34D military launch vehicle, enabling the problem to be addressed before another accident occurred.

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Though its main focus is aviation, the NTSB also studies incidents involving other kinds of transportation, from Amtrak trains to the amphibious sightseeing vehicles known as duck boats. In the 2010s it has been heavily involved in investigating accidents relating to self-driving cars, including fatal accidents involving Uber and Tesla vehicles. The NTSB has also investigated school bus crashes, and in 2018 officially recommended that school buses be equipped with seatbelts. Since its founding, the NTSB has gained a reputation for its fair and impartial analysis of crash sites. In 2015, it was estimated that the NTSB had investigated more than 140,000 aviation incidents as well as several thousand ground transportation incidents. The recommendations made by the board have been implemented with a high degree of success. Many lives have been saved, and the board continues to improve the safety conditions on commercial aircraft, earning the confidence of the traveling public. With only approximately four hundred employees as of 2014, the agency provides an invaluable service. —Cynthia Clark Northrup Further Reading Beene, Ryan. “NTSB Advocates for Seat Belts on School Buses after Deadly Crashes.” Bloomberg, 22 May 2018, www.bloomberg.com/news/articles/2018-08-14/asianstocks-face-mixed-start-dollar-holds-gain-markets-wrap. Accessed 16 Aug. 2018. Cobb, Roger W., and David M. Primo. The Plane Truth. Airline Crashes, the Media, and Transportation Policy. Brookings Institution Press, 2004. Dismukes, R. Key, Benjamin A. Berman, and Loukia Loukopoulos. The Limits of Expertise: Rethinking Pilot Error and the Causes of Airline Accidents. CRC Press, 2017. Holleran, Robert Suerig, and Lindy Philip. Bracing for Impact: True Tales of Air Disasters and the People Who Survived Them. Skyhorse Publishing, 2015. Neuman, Scott. “NTSB: Tesla Booted from Crash Investigation for Not Following Rules.” NPR, 13 Apr. 2018, www.npr.org/sections/thetwo-way/2018/04/13/ 602081183/ntsb-tesla-booted-from-crash-investigationfor-not-following-rules. Accessed 16 Aug. 2018.

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Stich, Rodney. Unfriendly Skies: 20th and 21st Centuries. Silverpeak Publishers Inc., 2007. US Congress, Senate Committee on Commerce, Science and Transportation. Board Reauthorization Act of 2009: Report of the Committee on Commerce, Science, and Transportation on S.2768. US Government Printing Office, 2010. See also: Aerospace industry in the United States; Air transportation industry; Aircraft icing; Airflight communication; Airplane accident investigation; Airplane guidance systems; Airplane maintenance; Airplane propellers; Airplane radar; Airplane safety issues; Federal Aviation Administration (FAA); Flight instrumentation; Flight landing procedures; Flight recorder; Landing gear; National Aeronautics and Space Administration (NASA); Shock waves; Space shuttle; Taxiing procedures; Wake turbulence; Weather conditions; Wind shear; Wing designs

Sir Isaac Newton Fields of Study: Mathematics; Physics; Aeronautical engineering; Mechanical engineering ABSTRACT Sir Isaac Newton was an English physicist and mathematician. He formulated the basic laws for the branch of physics known as mechanics, identified gravitation as the force that controls the motion of bodies in space and expressed it mathematically, and invented calculus independently of Gottfried Wilhelm Leibniz. He also made lasting contributions to optics. KEY CONCEPTS differential calculus: a method of determining rates of change of a quantity or property with respect to a standard reference such as time, as a large number of infinitesimally small progressive changes integral calculus: a method of determining the overall change in a quantity or property from the change rates determined by differential calculus natural philosophy: the study of natural phenomena, what today is called “science”

Principles of Aeronautics

Sir Isaac Newton

scientific method: a method of study by which one posits a theory, designs an “experiment” in which all other variables are eliminated, and interprets the results of that experiment with regard to the original theory CALCULUS Sir Isaac Newton invented the method of “fluxions,” which is known today as infinitesimal calculus. This mathematical process would prove to have many practical uses. For example, consider an empty bowl on a weight scale in which one drops coins one at a time. The total amount of money collected would not change continuously with time; rather, it would do so in “jumps” that represent the value of the coins added. If one pours water instead of coins into the bowl, the total weight would change continuously. The mathematical techniques that are needed to determine the rates of changes in these two cases are very different. In the case of coins, the changes are finite; one could use a stopwatch to measure the time increment between two consecutive coin drops and a weight scale to register the change in the weight of the bowl over that time interval. Dividing the change in weight by the increment of time would give an estimate for the time rate of change of the weight over that interval. In the case of water, the changes are infinitesimal, however. Here, to perform a similar operation, one would need to imagine the existence of very small amounts of water being poured into the bowl over very tiny time intervals and subsequently take their ratios. How does one do this? Differential calculus teaches how to determine such ratios, and integral calculus shows how to backtrack. That is, if one knows what the ratio of two infinitesimal quantities is equal to, one can determine one of the quantities in terms of the other. Prior to the invention of calculus by Newton, no one knew how to determine the changes of quantities that vary

Sir Isaac Newton, portrait, c. 1689. Image via Wikimedia Commons. [Public domain.]

continuously. Calculus is used extensively in modern science and engineering. Although Newton did his work on calculus in 1666 and shared it with friends and colleagues, he would not publish it until 1693. The German mathematician Gottfried Wilhelm Leibniz (1646-1716) also worked on calculus and published his work in 1684. This nine-year difference in publication dates led to a bitter dispute between supporters of Newton and those of Leibniz regarding who should get historical credit for the invention of calculus. Members of the Royal Academy, of which Newton was a member, officially accused Leibniz of plagiarism in 1699, claiming, correctly, that Leibniz had seen Newton’s papers when the former visited London in 1676.

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Friends of Leibniz thought it was exactly the opposite, claiming, correctly, that Leibniz had not taken notice of Newton’s work on this subject. Leibniz died before the dispute was settled, and these tensions lasted about a century. Today, however, it is agreed that Newton and Leibniz invented calculus independently. However, that does not mean that the agreement led to a tie: Newton is given credit for having invented calculus before Leibniz, and Leibniz is given credit for having published his work before Newton’s. Being reticent and late to publish his work caused Newton grief in life, modest loss of credit after death, and considerable stress and strain on his friends and foes alike. EARLY LIFE Isaac Newton was born three months after his father died. He was born prematurely, and he is believed to have suffered from Asperger’s syndrome, a form of autism. His mother, Hannah Ayscough, remarried when Newton was three years old, and she moved in with her new husband, the Reverend Barnabus Smith. The toddler was left to be cared for by his maternal grandmother. Ayscough had three children with her second husband, was widowed for the second time, and returned to Newton’s birthplace after eight years. Newton started school in the hamlet. When he was twelve, he went to King’s College in Grantham, where he excelled in his studies. He was removed from school when he was seventeen and returned home. His mother wanted him to become a farmer, but Newton did not like farming; in fact, he was not good at it. Henry Stokes, a headmaster at King’s College, convinced Newton’s mother to send him back to the college to complete his studies. Newton did so, completed his work brilliantly, and was admitted to Trinity College at the age of nineteen, in June, 1661. Newton’s mother still wanted him to devote his life to farming, but his maternal uncle, a clergyman who had attended Cambridge University,

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persuaded her to send Newton to the university. He was granted financial assistance for three years because he was a student with limited means. Such students were known as “sizars”; they earned money by serving the faculty and the wealthy students of the college. In 1664, he was elected a scholar, guaranteeing him four years of financial support. He obtained his degree in August, 1665, but the university was closed for about a year shortly after that because of concerns about the great plague. As a result, Newton went back to Woolsthorpe, where he continued his scientific work on calculus, optics, and universal gravitation. LIFE’S WORK Newton discovered the generalized binomial theorem and started working on calculus, although he would not publish this work until 1693. On July 5, 1687, he published a three-volume work entitled Philosophiae natural is principia mathematica (The Mathematical Principles of Natural Philosophy, 1729). This title is usually abbreviated as Principia. The work was prepared during the 1665-66 period, while Newton was in Woolsthorpe. It contains Newton’s laws of motion, the law of universal gravitation, and a derivation of Johannes Kepler’s laws for the motion of the planets. Starting about 1668, Newton studied optics and discovered that the phenomenon of color had mathematical patterns that could be measured. He found that white light was a mixture of colored rays that manifested in the color spectrum, as exemplified by the rainbow. He also postulated that light consisted of streams of tiny particles. However, when he expressed his ideas in public in 1672 and 1675, he encountered hostile opposition. The reason was that his views on the nature of light were contradicted by the then accepted notions that considered colors to be modified, but distinct, versions of white light. Newton formulated a theory of sound, but the speed he derived was too low and did not agree with

Principles of Aeronautics

his experimental results. This discrepancy was due to the fact that the concept of propagation without heat loss, the so-called adiabatic propagation, had not been proposed yet, and, hence, he did not take the heat capacities of air into account. Newton believed that matter consisted of different arrangements of atoms and devoted much time to the study of alchemy later in his life. While his burial place was being moved, exhumation revealed that Newton’s remains contained large quantities of mercury. It is speculated that this was due to his alchemical experiments. Newton was sensitive to criticism of his work and did not want to publish it. Indeed, he had many contemporary critics; among them were Christiaan Huygens (1629-95) and Edme Mariotte (1620-84). That may be why he held onto Principia for twenty years before publishing it, and why he waited until Huygens and Mariotte were dead before publishing Opticks in 1704. Newton’s friends recognized his genius, and their persistent support and encouragement made it possible for Newton to publish his work and achieve recognition. His friend and mentor Isaac Barrow helped communicate Newton’s discoveries to the mathematics community of London. Another friend, the English astronomer Edmond Halley, was the one who finally persuaded Newton to publish Principia. Despite the criticism and the controversies, Newton’s work was well received, even in his lifetime, and he was recognized for it. He was elected a fellow of Trinity College in 1667 and Lucasian Professor of Mathematics in 1669. He became a fellow of the Royal Society of London in 1672, its president in 1703, and held that position for twenty-four years (1703-27). Unfortunately, Newton suffered a mental breakdown between 1675 and 1679. He was appointed warden of the British Mint in 1695 and was knighted by Queen Anne in 1705. Newton is still being honored for his work today. The Newton crater, a lunar feature, was named for

Sir Isaac Newton

him by American scientists. Rue Newton, located in the sixteenth arrondissement of Paris, is named for him. In physics, the laws of mechanics are named for him, and the standard unit of force is called the Newton: one pound-force is equal to 4.444 Newtons. IMPACT Specialists in the history of science believe that Newton contributed more to the development of science than any other individual in history. His work is thought to have ushered in the Age of Reason. There are three major reasons for this: First, Newton produced a scheme that helped humankind understand how the universe worked; that scheme far surpassed those that preceded it in intuitiveness, consistency, elegance, universality, and mathematical predictability. Second, in the process of devising his scheme, he developed the scientific method by formulating four rules of investigation that were revolutionary, concise, and universal—where “universal” means that, unlike the methods that preceded Newton’s, his applied to all branches of science. Last, Newton not only outlined his rules for reasoning but also described how they might be applied to the solution of a given problem, and he actually applied them with great success to formulate some universal laws of nature. The method he invented was more scientifically based than the philosophical approaches of Aristotle (384-322 BCE) and Thomas Aquinas (1225-74). When it comes to the development of modern science, Newton is considered to be the most important individual contributor. His ideas form the basis of modern technological civilization. His principles, although originally conceived for the physical sciences, were applied to the social sciences and influenced, for example, the economic theories of Adam Smith (1723-90). —Josué Njock Libii

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Further Reading Christianson, Gale E. Isaac Newton. Oxford UP, 2005. Feingold, Mordechai. The Newtonian Moment: Isaac Newton and the Making of Modern Culture. Oxford UP, 2004. Goldish, M. Judaism in the Theology of Sir Isaac Newton. Springer Netherlands, 2013. Johnson, George. The Ten Most Beautiful Experiments. Alfred A. Knopf, 2008. Leny, Joel. Newton’s Notebook: The Life, Times and Discoveries of Sir Isaac Newton. History Press, 2009. Martinich, Aloysius, Fritz Allhoff, and Anand Vaidya, editors. Early Modern Philosophy: Essential Readings with Commentary. Blackwell, 2007. Newton, Isaac. Sir Isaac Newton’s Mathematical Principles of Natural Philosophy and His System of the World. U of California P, 2022.

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Newton, Isaac. The Mathematical Principles of Natural Philosophy. Flame Tree Publishing, 2020. ———. Newton[‘s] Revised History of Ancient Kingdoms: A Complete Chronology. Master Books, 2009. Newton, Isaac, J. Edleston, and R. Cope. Correspondence of Sir Isaac Newton and Professor Cotes. Routledge, 2014. Outram, Dorinda. Panorama of the Enlightenment. J. Paul Getty Museum, 2006. See also: Daniel Bernoulli; Differential equations; Fluid dynamics; Forces of flight; Gravity and flight; Ernst Mach; Materials science

P Paper Airplanes Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT A paper airplane consists of a sheet of paper folded and creased into the shape of an airplane. Learning to fold and fly a paper airplane effectively is a basic study of aerodynamics. KEY CONCEPTS Bernoulli principle: the pressure exerted by a moving fluid such as air is inversely related to the speed of the fluid flow center of gravity: the point within an aircraft, or any other body, about which the entire mass of that body is equally distributed center of lift: the point at which the lift force appears to function against the weight of an airplane pitch: the tendency of the nose of an aircraft to move up or down vertically as it moves through a fluid medium HISTORY OF PAPER AIRPLANES Human experimentation with flying did not begin with the Wright brothers. People have been fascinated by flight since ancient times. The first flying devices made from paper were kites, constructed by the Chinese around 1 CE. Even Leonardo da Vinci tried to devise a way for humans to fly. It is said he used parchment folded into winged flyers during his experiments. At the beginning of the twentieth century, paper airplanes were used as a common tech-

nique to study aerodynamics. During World War I, flying paper airplanes became a popular activity with children. In the 1940s, the General Mills Company offered a series of fourteen paper model warplanes. Even to the present day, paper airplanes continue to be an object of fascination for many people, and practically everyone has a personal favorite model that they can fold and fly. There are serious competitions among people to create the ultimate paper airplane, one that flies the farthest, or that is the most aerobatic, or that is the most unique flyable design. HOW AIRPLANES FLY The wings of an airplane share a shape with those of insects, bats, and birds, called an airfoil. An airfoil is curved on top and flat on the bottom. Air rushing over the wing travels faster than the current going under the flat bottom of the plane. The eighteenth-century Swiss scientist Daniel Bernoulli discovered that when air speeds up, its pressure is reduced. When air slows down, its pressure is increased. Therefore, the faster air under the wing pushes upward. This is what causes lift. During level flight, the effects of lift and weight are equal. If lift exceeds weight, the plane will rise. If weight exceeds lift, the plane will fall. The center of lift on a paper airplane is the point at which the force of lift seems to be working. The center of gravity is the balance point of the plane, the point at which the force of gravity seems to be working. On paper airplanes, the center of gravity needs to coincide with the center of lift. If the center of lift is in front of or behind the center of gravity, the nose of the plane will pitch up or down accordingly.

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Another set of opposing forces present during flight are drag and thrust. These two forces are what propel the plane forward or back. Real planes get their thrust from a propeller or engine. Paper airplanes get their thrust from being launched or thrown by a person. A throw gives a plane its initial speed, and inertia moves it along against the force of drag. Gravity also works to maintain the forward motion of the plane as downward motion due to gravity is converted to forward motion by the lift and the aerodynamic character of the plane. When a plane flies level, drag is what functions to slow its forward motion. Most of drag comes from air resistance. As a plane flies, air sticks to it, creating turbulence, or resistance to motion. If the nose of a plane points down, gravity will add thrust and the plane will crash. Any surface not parallel to the

Paper plane. Photo by Dietmar Rabich, via Wikimedia Commons.

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flow of air adds drag. Sharp creases and accurate folding will reduce drag and increase time aloft. Lift also contributes to drag by pushing up and a little back. A typical paper airplane’s drag is one-fifth of its weight. Differences in wing loading, the specific amount of weight a standard size area of the wing lifts in flight, will create difference in speeds. Wing loading is how many grams per square centimeter the wing is lifting. The larger the wing area, the less wing loading and more slowly the plane will glide. BUILDING A STABLE CRAFT Another factor that affects flight is stability, which helps an airplane return to steady flight after a bad throw or a strong gust of wind. There are three basic types of stability: pitch, directional, and spiral.

Parachutes

Principles of Aeronautics

FLYING TECHNIQUES Getting a good flight out of a paper airplane requires a good throw. To get the most out of a throw, the plane should be held on the bottom near the front, using the forefinger and thumb. How a plane is thrown depends on the type of flying intended. The types of flying include slow flight, fast flight, and high (world record) flight. Slow flight is achieved by holding the plane in front of the shoulder and pushing the plane forward and slightly downward. For fast flying, the plane should be held in front of the shoulder for short flights and above the shoulder for long flights. The high throw is mainly used for competition and is achieved by throwing the plane straight up as hard as possible. If done properly, the plane will spiral up, level off, and glide slowly forward. —Maryanne Barsotti

Folding instructions for a traditional paper dart. Image via Wikimedia Commons. [Public domain.]

Pitch stability keeps the airplane flying at a constant speed. If the nose of a plane pitches up, the plane will slow down. If it pitches down, the speed will increase. There is a small distance along the length of a plane where it must balance to provide optimum pitch stability. On a paper airplane, this distance is less than one-inch long. If the balance point is too far forward, the plane will dive; too far back, and it will spin out of control. Directional stability can be maintained by creating a fin on the back of the plane to counteract the tendency to spin. On most paper airplanes, the body acts as the fin. If most of the plane’s body is behind the balance point, it will be directionally stable. Bending the wing tips up will add to its stability. Spiral stability is when the plane flies straight and smooth. A spirally unstable plane will circle, turning tighter and tighter, until it spins into a dive. To correct a spirally unstable plane, the wings, as viewed from the nose, should be bent up slightly so that they make a Y shape with the body.

Further Reading Blackburn, Ken, and Jeff Lammers. The World Record Paper Airplane Book. Workman, 1994. Morris, Campbell. The Best Advanced Paper Aircraft, Book 1: Long Distance Gliders, Performance Paper Airplanes, and Gliders with Landing Gear. CreateSpace Independent Publishing Platform, 2011. Schmidt, Norman. Great Paper Fighter Planes. Sterling Publishing Co. Inc., 2005. See also: Aerodynamics and flight; Ailerons, flaps, and airplane wings; Daniel Bernoulli; Flight roll and pitch; Fluid dynamics; Forces of flight; Glider planes; Model airplanes; Parachutes; Stabilizers; Tail designs; Wing designs

Parachutes Fields of Study: Physics; Fluid dynamics ABSTRACT Parachutes are large, umbrella-like devices called shrouds attached to people or other objects by ropes, and are used to

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slow down falls to the ground from aircraft or any other great height. Parachutes are the best-known devices for emergency escape and descent from endangered aircraft. Their other uses include deploying paratroops; distributing airborne food and other supplies, weapons, and assault or other vehicles; decelerating aircraft for landing; recovering space vehicles; and skydiving, or sport parachuting. They are also used in high-speed racing to assist the deceleration of the vehicle when a race has ended. KEY CONCEPTS drag chute: a type of parachute used to decelerate a racing vehicle at the end of a race surface area resistance: the resistance to motion through a fluid primarily by the portion of an object’s surface area that faces the direction of motion and secondarily by the drag exerted by the remaining surface area terminal velocity: the maximum speed that can be attained by an object falling through a fluid medium under the force of gravity alone DEVELOPMENT A parachute is a very light, flexible device that is intended to retard the passage of an object through Earth’s atmosphere to the ground. Parachutes resemble huge umbrellas. They are most frequently used to slow the fall of a human or of other objects from high-flying aircraft or from any other great height, most often ensuring a safe landing. The term “parachute” derives from a French term meaning “against fall,” in the sense of to protect one from a fall or a bad tumble. The theory of the parachute is credited to the fifteenth-century Italian genius Leonardo da Vinci. However, the first practical application of a parachute occurred in the late eighteenth century. At that time, parachutes were used for exhibition purposes in France to allow aeronauts quick descent from gas-filled balloons. By the beginning of World War I, this use had evolved into the application of

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parachutes as lifesaving devices for emergency jumps from damaged aircraft. By the 1920s, parachutes had become familiar devices with widespread military uses, including the dropping of airborne troops (paratroopers), weapons, vehicles, and supplies. Parachutes have developed many additional uses related to peacetime aircraft recovery and to spaceflight. In the popular automobile sport of “drag racing,” in which racers pair up against each other in an attempt to reach the end of a quarter-mile distance from a standing start, a type of parachute called a “drag chute” is used to help slow a vehicle from its high attained speed. Typically, the drag chute is released at the finish line, providing an immediate braking force to the vehicle as it enters the deceleration and recovery zone of the track. Speeds attained in a race lasting as little as six seconds can be several hundred kilometers per hour. Drag chutes are used to reduce the demand on physical braking systems. They are also used on vehicles attempting to set land speed records, and in other such applications. OPERATION Parachute operation is based upon several simple principles of physics. Two forces act on falling objects. These are gravity (or Earth’s gravitational force) and air resistance. Gravity pulls any object initially suspended in the atmosphere downward, toward Earth’s surface. Air resistance, due to the atoms and molecules that comprise air, and other particles of matter in the air, slows a falling object’s movement. The pull of gravity is so much stronger than air resistance that the downward speed of a falling object, whether a rock or a human, is only slowed very slightly, and will become balanced when the falling object attains a speed called its “terminal velocity,” after which the speed at which it falls will not increase. With two objects of the same weight, air resistance is much greater for the one which has the larger sur-

Principles of Aeronautics

face area. This is because objects of the same weight with large, flat surfaces, such as clay saucers, offer greater areas of resistance to the air than those with small surfaces, such as a clay brick of the same weight as the saucer. Therefore, when an object is shaped like a saucer, it falls more slowly than a sphere of the same weight. DESIGN AND CONSTRUCTION Parachutes designed for human use are all oblate hemispheres 2 to 3 meters across when open, and of weights ranging between 10 to 15 kilograms. Parachutes used to drop cargo are often 33 meters across or larger, and heavier. Parachutes used to decelerate aircraft or spacecraft for landing and recovery are even larger than this. They are also most often used in assembled groups of three or more parachutes. The most common parachute used by humans is the seat pack model associated with a seat in an aircraft. The other kinds of parachutes attach directly to the chest or the back of a wearer. All parachutes are worn on harnesses. Each parachute harness is made up of a group of straps fitting around the shoulders and the legs of a parachutist. The parachute harness straps connect parachute and parachutist, also supporting the parachutist during descent to the ground. Straps called risers are attached to the shoulder portions of the parachute harnesses to hold the lines, called shrouds, that attach to the parachute canopy. The canopy is the umbrella-like part of the parachute. A rip cord is also attached to a harness strap, usually on the parachutist’s left side. It terminates in a ring that the parachutist pulls soon after jumping. Pulling a rip cord causes the parachute canopy and its shrouds to leave their enclosing pack. This process is accomplished by the ejection of a small parachute from the pack. The small chute opens and pulls the larger one out after it. As each canopy leaves the pack, air enters it and causes it to open. All parachutes are carefully folded before insertion into their carry

Parachutes

packs. This careful treatment makes sure that the parachute will open properly when the rip cord is pulled by a parachutist. The initial opening of the canopy can slow down the descent through the air so quickly that the parachutist is jerked sharply upward in “opening shock.” To reduce the extent of this opening shock and to stabilize the parachutist’s descent, manufacturers use several canopy modifications that lead to a planned canopy air porosity. Often, ribbon canopy material, having planned holes (slots), is used. These slots allow enough airflow through the canopy to reduce air resistance and minimize opening shock. They also help to minimize parachutist sway and maximize

British airborne forces training alongside NATO counterparts. Photo by Corporal Andy Reddy RLC, via Wikimedia Commons.

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comfort during the descent. Another type, the vortex-ring parachute, is composed of four sections that rotate during the descent, functioning like a helicopter rotor to produce maximum parachute stability. A parachute is most often made of one type of material, usually nylon, silk, cotton, rayon, or a plastic film, although mixed materials are used in some cases. The fiber is turned into cloth for canopies, cord for shrouds, and webbing for harnesses. The most important parachute construction factors include proper air porosity, adequate material strength, good aerodynamic behavior, the lightest weight possible, and easy operation. The materials experimented with and used increase as new fabrication techniques, new artificial polymers, and new fabrics develop. PARACHUTE JUMPING AND PARACHUTE USES Parachutes are decelerators (or air brakes) that allow parachutists to descend toward Earth at rates of 14 to 8 kilometers per hour, depending on the parachutist’s weight and the canopy’s diameter. All parachute jumps made from under 153 meters above ground level are very dangerous because this height does not allow enough distance and time for complete parachute opening. Even safe jumps can lead to parachutists landing with great force, due to the excessive rate of their decelerated fall, and spraining their ankles or breaking bones. This is most often true of jumps over rough terrain. Winds also add to landing dangers, because they engender sideways parachute motion through the air. The addition of this motion to air-braked fall speed causes some landings to seem like jumps from fast-moving automobiles and can cause similar injuries. It is therefore crucial that the parachutist be well trained in how to control a parachute. The other skills needed include a well-honed ability to judge the current wind speed, the altitude, the direction of sideways motion, and potential ground speed. Parachute jumping, or skydiving, nonetheless has become a popular sport

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that has a great many enthusiasts in Europe and America. In skydiving, a slow-moving aircraft, cruising at a 3.2-kilometer altitude, is used as a jumping platform and skydivers often perform stunts while falling. Sport parachutes are unlike those used for simple descent. Many safety features are removed for ease in maneuvering. Also, the sport parachutes are often designed to be rotated by a control that regulates the direction of air passing through the canopy. In addition, skydivers do not pull their rip cords quickly after leaving the plane. Rather, they use an altimeter, which notes the rate of descent and indicates the last instant when the parachute can be opened safely. In addition to the classical application of parachutes as devices to carry humans, parachutes are used to deploy paratroops in military assaults; to distribute supplies from aircraft; to slow, as needed, the rates of descent of bombs or flares; to decelerate jet airplanes during their landing; and to recover space vehicles and weather or flight recorders. —Sanford S. Singer Further Reading Castleden, Rodney. Inventions that Changed the World. Canary Press Books, 2020. Federal Aviation Administration (FAA). Parachute Rigger Handbook. Mepcourt Media LLC, 2018. Kuntz, Jerry. A Leap from the Clouds: The Balloon-Parachute Act and the Daredevil Heritage of Aviation. McFarland Inc. Publishers, 2022. Lynn, Michael R. The Sublime Invention: Ballooning in Europe, 1783-1820. Taylor & Francis, 2015. Rodriguez, Mark. Parachuting: Clear and Unbiased Facts About Skydiving. Lulu.com, 2016. Theotokis, Nikolaos. Airborne Landing to Air Assault: A History of Military Parachuting. Pen & Sword Books Ltd., 2020. See also: Aerodynamics and flight; Federal Aviation Administration (FAA); Fluid dynamics; Forces of flight; Gravity and flight; Wind shear

Principles of Aeronautics

Plane Rudders Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT The rudder is a large, vertical, movable, flap-like device attached to the vertical stabilizer on most aircraft, or movable vertical fins on a missile. The rudder is the primary device used to yaw, or steer the nose of the aircraft to the left or right, in a turn or to counteract the yaw resulting from aileron use in certain cross-control maneuvers. KEY CONCEPTS cross-control: use of both rudder and ailerons in the opposite sense of their regular use to control the approach of an airplane to a runway, producing a slip lift: the upward pressure experienced by wings due to differential pressure between upper and lower surfaces of the wing as it moves through a fluid medium as determined by its airfoil camber and thickness slip: a maneuver in which an aircraft is made to turn sideways slightly while maintaining the same direction of motion, thus presenting a greater surface area in the wind direction and acting to slow the aircraft somewhat yaw: the tendency of an aircraft to turn horizontally about its center of mass as it moves through a fluid medium FUNCTION OF A RUDDER An aircraft’s or a missile’s rudder, a flap or a wing-shaped surface mounted at or near the craft’s rear, serves a purpose similar to that of a rudder on a ship. When the rudder is deflected to one side or the other, it produces a force and a resulting moment, or yaw, about the vehicle’s center of gravity. The force rotates the vehicle in the same direction as the deflection of the rudder.

Plane Rudders

Because rudders have been used for centuries to steer ships, early airplane designers naturally assumed that they could be used to steer airplanes. However, these designers often failed to anticipate the roll of the aircraft that resulted from the use of the rudder. When the rudder causes an airplane to yaw, it causes one wing to travel slightly more quickly through the air than the other and, hence, to produce more lift, which subsequently causes the airplane to roll in the direction of the turn. This roll was a problem with early airplanes, which flew very close to the ground, and required the use of ailerons and similar devices to control the resulting roll. Through experimentation, early aviators learned that the most successful turns are coordinated turns, made using a combination of rudder and ailerons. On wingless missiles, the rudder is the only device used to make the vehicle turn. A missile’s rudder yaws the missile such that it flies at an angle to the airflow and develops a side-force on its body, or fuselage. This side-force produces the needed acceleration along the turn radius to carry the missile through the desired turn. TURNS Airplane turns are more complex and require more than the use of a rudder. As noted above, when the rudder is deflected, the fuselage yaws, and the wings develop different lift forces. The wing on the outside of the turn develops a larger lift than does the wing pointing into the turn. The difference in lift between the wings results in a roll of the fuselage, which tilts or rotates the lifting force of the wings into the direction of the turn. Because the lifting force of the wings is much greater than the forces on any other part of the airplane, it is the tilted lift that provides the force to turn the airplane. When the turn is properly coordinated, the combination of yaw caused by the rudder, roll caused by the ailerons, and the slight increase in thrust will produce just the right amount of lift to balance the weight of the air-

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craft, so that the aircraft can make the turn without losing altitude. ENGINE LOSS The rudder must also be used to keep the airplane from yawing or turning when a multiengine airplane loses one of its engines. When a multiengine plane encounters an engine-out situation, the rudder must be used to produce enough yaw to counteract the effect of having more thrust on one side of the airplane than on the other. For this reason, multiengine airplanes have much larger rudders than do single-engine airplanes. LANDINGS Another common use of the rudder is to cross-control an airplane, especially in its approach to landing. In an ideal landing, the atmospheric wind would be blowing straight down the runway. In the real world, the wind is often at an angle to the runway and, when landing or taking off, the pilot must adjust the flight of the plane to account for the crosswind. On takeoff, this is done by allowing the plane to yaw into the wind as soon as it leaves the ground and by flying away in a straight line extending from the runway centerline with the airplane turned somewhat into the wind in a slightly sideways motion. The approach to landing can be made in the same manner, with the plane yawed into the wind; at some point, the pilot must align the fuselage with the runway before the wheels touch down, so the aircraft can be properly controlled on the ground. To do this, the pilot uses the rudder to yaw the airplane until it is parallel to the runway and uses the ailerons to keep the wings level. This use of rudder and aileron is the opposite of that used in a turn and is referred to as cross-control. The rudder is controlled on most aircraft by cables or hydraulic lines connected to pedals on the floor of the cockpit. The pilot presses the right rudder pedal to move the rudder and, thus, the nose of

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the aircraft, to the right, or presses the left rudder to rotate left. Modern airliners and fighters use power-augmented hydraulic or electrical systems to connect the rudder pedals to the rudder, and the rudder is often connected to an automated control system, which will allow control of the airplane by a computer system. —James F. Marchman III Further Reading Pamadi, Bandu N. Performance, Stability, Dynamics, and Control of Airplanes. American Institute of Aeronautics and Astronautics, 2004. Williamson, Hank, editor. Air Crash Investigations: Jammed Rudder Kills 132, The Crash of US Air Flight 427. Lulu.com. 2011. Wrigley, Sylvia. Why Planes Crash Case Files: 2001-2003. Fear of Landing, 2018. See also: Aeronautical engineering; Ailerons, flaps, and airplane wings; Daniel Bernoulli; Flight roll and pitch; Forces of flight; Glider planes; Paper airplanes; Rockets; Tail designs; Wing designs

Wiley Post Fields of Study: Aeronautical engineering; Mechanical engineering; Physiology ABSTRACT Wiley Post was born on November 22, 1898, in Grand Saline, Texas, and died in a plane crash with his friend Will Rogers on August 15, 1935, near Point Barrow, Alaska. He was a famous and colorful aviator of the 1920s and 1930s. Post twice held speed records for transglobal flights, discovered the jet stream, and worked to develop the first pressure suit for stratospheric flight. POST HISTORY During the 1920s, Wiley Post worked in the Oklahoma oil fields. After losing his left eye in an

Principles of Aeronautics

oil-field accident, which led him to adopt his signature eye patch, he used money from an insurance settlement to buy his first airplane. He then performed as a parachute jumper and barnstormer. In 1925, the American humorist Will Rogers hired Post to fly to a rodeo, and the two became lifelong friends. During the late 1920s, Post, flying a TravelAir biplane, was the personal pilot for wealthy Oklahoma oilmen F. C. Hall and Powell Briscoe. Hall bought for Post’s personal use a Lockheed Vega 5-C, which Post named Winnie Mae, after his daughter. In the Vega, a streamlined, single-engine plane known for its ruggedness and airworthiness, Post won the 1930 National Air Derby, a Los Angeles-to-Chicago race that made him a national figure. Although the plane’s cruise speed was 140 miles per hour, Post’s winning time approached 200 miles per hour. In 1931, Post flew around the world in the Winnie Mae with Australian-American aviator Harold Gatty. Traveling a northern route of some 15,000 miles, they set a world record of eight days and sixteen hours, breaking the speed record of twenty-one days set in 1929 by the German airship the Graf Zeppelin. Post received the Distinguished Flying Cross in 1932. In July, 1933, Post, flying alone with navigational instruments and an automatic pilot, reduced the time to seven days and eighteen hours, an achievement that earned him the solo record for around-the-world flight and the Harmon International Trophy. Post took up the challenge of high-altitude flight in 1934, funded by Frank Phillips of the Phillips Petroleum Company. The Winnie Mae could not be pressurized, so Post asked the B. F. Goodrich Company to help him devise a pressurized flying suit made of rubberized parachute material, with pigskin gloves, a helmet made of plastic and aluminum, and a liquid-oxygen breathing system. Post first used the suit in a September, 1934, flight over Chicago, in

Wiley Post

Wiley Post. Photo via Wikimedia Commons. [Public domain.]

which he also used a supercharger on Winnie Mae’s engine to set an unofficial height record of 16,000 meters. In his high-altitude test flights, Post was the first flier to encounter the jet stream, which he used to his advantage in a May, 1935, flight from Burbank, California, to Cleveland, Ohio. At times, the ground speed on this flight approached 250 miles per hour, and the average ground speed was about 179 miles per hour. However, he failed in four attempts at making a stratospheric flight across the entire continental United States. Ever the visionary innovator, Post predicted the development of supersonic transports and even

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space travel. He conducted secret experiments in a high-altitude chamber owned by the US Army and researched the biological rhythms related to pilot fatigue. In 1935, Post explored flight routes from the West Coast of the United States to Russia. With funding from US airlines, he combined the parts of two planes: the wings of a Lockheed Explorer and the fuselage of a Lockheed Orion. Pontoons were necessary to land in Alaskan and Siberian lakes, and when the desired pontoons did not arrive, Post used a heavier set from a much larger plane. In July 1935, Post and Rogers left Seattle, Washington, in this heavy plane, further weighted down with fishing and hunting equipment. Lost in bad weather, they landed in a lagoon near Point Barrow, Alaska. When they tried to take off, the engine failed, and the plane plunged back into the lagoon, killing both men. Post’s famous Winnie Mae was subsequently sold by his widow to the Smithsonian Institution. —Niles R. Holt Further Reading Jenkins, Dennis R. Dressing for Altitude: US Aviation Pressure Suits, Wiley Post to Space Shuttle. National Aeronautics and Space Administration, 2012. Johnson, Bobby H., Stanley R. Mohler, and Smithsonian Air and Space Museum. Wiley Post; His Winnie Mae and the World’s First Pressure Suit. MilitaryBookshopco.uk, 2011. Penenberg, Adam L. Sky Rivals. Two Men, Two Planes, An Epic Race Around the World. Wayzgoose Press, 2016. Sterling, Bryan B., and Frances H. Sterling. Forgotten Eagle: Wiley Post, America’s Heroic Aviation Pioneer. Carroll & Graf, 2001. Stinson, Patrick M. Around-the-World Flights, A History. McFarland Inc. Publishers, 2011. See also: Aeronautical engineering; Airplane accident investigation; Airplane safety issues; Atmospheric circulation; Glenn H. Curtiss; Jimmy Doolittle; Amelia Earhart; First flights of note; Forces of flight; John Glenn; Gravity

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and flight; Charles A. Lindbergh; Billy Mitchell; Montgolfier brothers; Eddie Rickenbacker; Igor Sikorsky; Valentina Tereshkova; Manfred von Richthofen; Weather conditions; Wind shear

Pressure Fields of Study: Physics; Aeronautical engineering; Fluid mechanics; Biomechanics; Physiology; Mathematics ABSTRACT Pressure is the force exerted by a solid, liquid, or gas on its container or another object. Pressure is typically measured in pascals (Pa) or in atmospheres (atm). A pascal is equal to one newton (1 kg·m/s2) applied over an area of one meter. One atmosphere is the amount of atmospheric pressure exerted on an object at sea level and is equal to 101,325 pascals. KEY CONCEPTS Bernoulli principle: the pressure exerted by a moving fluid such as air is inversely related to the speed of the fluid flow Boyle’s Law: when pressure or volume are changed at constant temperature, the product of volume and pressure before the change is equal to the product of pressure and volume after the change Charles’s or Gay-Lussac’s Law: the temperature of a gas is directly proportional to the pressure lift: the upward pressure experienced by wings due to differential pressure between upper and lower surfaces of the wing as it moves through a fluid medium as determined by its airfoil camber and thickness EXPRESSING PRESSURE The concept of pressure can be expressed in either a scalar or a vector quality. Scalar measurements only measure force, whereas vector measurements in-

Principles of Aeronautics

clude both force and direction. Pressure is the scalar quality, whereas stress is the vector quality. The equation for pressure is P = F/A, where P is pressure, F is force, and A is the area upon which the force is applied. BACKGROUND The earliest known experiments with pressure involved buoyancy and date back to ancient Greece. The mathematician and scientist Archimedes (ca. 287-212 BCE) was the first to write about the use of pressure in experiments, and Archimedes’s principle discusses the effects and implications of pressure displacing a liquid. This displacement effect can be used to determine the density and volume of an irregularly shaped object. Two English scientists, astronomer Richard Towneley (1629-1707) and physician Henry Power (1626-68), mathematically determined the relationship between pressure and volume in 1660 and 1661. Drawing on their work, Irish chemist and physicist Robert Boyle conducted experiments in pressure that led to the formulation of Boyle’s law of pressure in 1662. Using mercury and air in a series of connected tubes and beakers, Boyle found that the relationship between pressure and volume is such that the two quantities will multiply to equal a constant value. Boyle’s law can be expressed as either PV = k or P1V1 = P2V2, where P is the pressure exerted, V is the volume of the system, and k is a known constant. Independently of Boyle, French physicist Edme Mariotte (ca. 1620-84) achieved the same results using similar experiments. Although Mariotte did not deduce Boyle’s law until 1676, the law is sometimes referred to as Boyle-Mariotte’s law. In 1738, Swiss physicist and mathematician Daniel Bernoulli (1700-1782) calculated the relationship between pressure, gases, and the environment. This resulted in the development of Bernoulli’s principle, which is the basis of the theory of flight. The princi-

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ple states that when pressure underneath the wing is higher than the pressure above the wing, lift force is generated, allowing the aircraft to take off. In 1802, French chemist and physicist Joseph Louis Gay-Lussac (1778-1850) drew on the earlier work of fellow French scientists Jacques Alexandre César Charles (1746-1823) and Guillaume Amontons (1663-1705) in his definition of the relationship between pressure and temperature. Neither scientist had accurate enough equipment to fully explain their results, so the equation was theoretical until more precise thermometers could be produced. The law of this relationship is named after Gay-Lussac, although it is also referred to as Charles’s law. Gay-Lussac’s law is expressed by the equations P/T = k and P1T2 = P2T1, where P is the pressure exerted, T is the temperature of the system, and k is a known constant. PRESSURE TODAY In the twenty-first century, pressure is an integral part of natural and industrial processes. For example, pressure is used to make both natural and synthetic diamonds and other jewels. An understanding of pressure is critical in numerous other activities and events, including scuba diving, flight, the use of explosives, and osmosis. According to Gay-Lussac’s law, as pressure increases, so too does temperature, if mass and volume remain constant. This law is the principle behind the creation of synthetic gems such as diamonds. It is possible to turn carbon into diamonds or graphite through intense heat and pressure. Man-made forges are capable of producing the high pressure and temperatures that are needed to make this conversion. While the size and quality of such gems often make them unsuitable for jewelry or metallurgical purposes, the diamonds produced are frequently used in mining and tool production. Scuba divers must understand the role Boyle’s law plays in diving. As a diver descends, increasing

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amounts of pressure will be exerted on him or her by the water. As the diver goes deeper, the air in his or her tanks will become denser, causing less air to be available for breathing. The same process also occurs in the diver’s bloodstream. If a diver swims to the surface too quickly, gases will build up in his or her blood, causing a medical condition known as decompression sickness or “the bends.” Decompression sickness occurs when gases in an individual’s bloodstream come out of solution as a result of depressurization. In severe cases of the bends, gas bubbles may form so rapidly that divers have reported the sensation of their blood actually fizzing like a carbonated soft drink. The condition is very painful and dangerous and is treated with a decompression chamber. Flight is possible because of pressure. An aircraft is able to obtain flight during takeoff as a result of the air pressure above the wings being lower than the air pressure beneath them. This is an application of Bernoulli’s principle. While the theory was developed in the early eighteenth century, it was not until 1903 that American inventors Orville (1871-1948) and Wilbur Wright (1867-1912) successfully completed the first airplane flight in Kitty Hawk, North Carolina. Explosives cause damage with a combination of shrapnel, heat, and pressure. Shrapnel are fragments that can cause damage to an object upon impact. The pressure comes from the energetic waves that emanate from the explosion. These waves, known as shock waves, can cause internal damage to a body as well as physical damage to structures and other surroundings. Osmosis is a natural process by which molecules of a solvent filter through a membrane from a solution of lower solute concentration to one of higher concentration, diluting the more concentrated solution so that the two concentrations become equal. This process was observed by French physicist

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Jean-Antoine Nollet (1700-1770) in 1748. Reverse osmosis is a water-filtration technique developed in the mid-twentieth century that uses pressure to force impurities from the water. This technique is used throughout the world, most commonly to clean sewage water (in conjunction with chemical treatment) and for the purpose of desalination, which is the removal of salt from seawater to make it potable. —Douglas R. Jordan Further Reading Anderson, Marlow. The Physics of Scuba Diving. Nottingham UP, 2011. Archimedes. The Works of Archimedes. Trans. Thomas Little Heath Sr. 1897. Cambridge UP, 2010. Cutnell, John D., and Kenneth W. Johnson. Physics. 9th ed., Wiley, 2012. Kucera, Jane. Reverse Osmosis: Design, Processes, and Applications for Engineers. Wiley, 2010. Galdai, Giovanni P., Tomáš Bodnár, and Šárka Neèasová, editors. Fluids Under Pressure. Birkhäuser, 2020. Jenkins, Dennis R. Dressing for Altitude: U.S. Aviation Pressure Suits, Wiley Post to Space Shuttle. US National Aeronautics and Space Administration (NASA), 2012. Johnson, R. S. Fluid Mechanics and the Theory of Flight. Johnson & Ventus Publishing ApS, 2012. Levine, Ira N. Physical Chemistry. 6th ed., McGraw, 2009. Loveday, John, editor. High-Pressure Physics. CRC Press, 2012. Sidharth, Burra G., Marisa Michelini, and Lorenzo Santi, editors. Frontiers of Fundamental Physics and Physics Education Research. Springer, 2014. US Department of the Army. Fundamentals of Flight: FM 3-04.203. Department of the Army, 2007. Von Mises, Richard. Theory of Flight. Dover Publications, 2012. Wegener, Peter P. What Makes Airplanes Fly? History, Science, and Applications of Aerodynamics. Springer New York, 2012. See also: Aerodynamics and flight; Ailerons, flaps, and airplane wings; Airfoils; Fluid dynamics; Forces of flight; Rocket propulsion

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Propulsion Technologies Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Propulsion deals with the means by which aircraft, missiles, and spacecraft are propelled toward their destinations. Subjects of development include propellers and rotors driven by internal combustion engines or jet engines, rockets powered by solid- or liquid-fueled engines, spacecraft powered by ion engines, solar sails or nuclear reactors, and matter-antimatter engines. Propulsion system metrics include thrust, power, cycle efficiency, propulsion efficiency, specific impulse, and thrust-specific fuel consumption. Advances in this field have enabled humanity to travel across the world in a few hours, visit space and the moon, and send probes to distant planets. KEY CONCEPTS grain: a shaped charge of solid rocket fuel designed to undergo oxidation combustion at a predetermined rate to provide a desired amount of thrust Law of Conservation of Energy: the total energy of a system remains constant as the sum of the energies of the individual components of the system throughout any change of the system LOX/LH2 engine: an engine that uses liquid hydrogen as fuel and liquid oxygen as the oxidizer for the combustion reaction of the fuel momentum: a characteristic expressed as the product of mass and velocity that is conserved in accord with the law of conservation of energy such that the total momentum of the components of a system remains constant throughout any change of the system specific impulse: thrust developed per second per weight of propellant consumed under standard gravity thrust: the force or pressure exerted on the body of an aircraft in the direction of its motion

Propulsion Technologies

DEFINITION AND BASIC PRINCIPLES Propulsion is the science of making vehicles move. The propulsion system of a flight vehicle provides the force to accelerate the vehicle and to balance the other forces opposing the motion of the vehicle. Most modern propulsion systems add energy to a working fluid to change its momentum and thus develop force, called thrust, along the desired direction. A few systems use electromagnetic fields or radiation pressure to develop the force needed to accelerate the vehicle itself. The working fluid is usually a gas, and the process can be described by a thermodynamic heat engine cycle involving three basic steps: First, do work on the fluid to increase its pressure; second, add heat or other forms of energy at the highest possible pressure; and third, allow the fluid to expand, converting its potential energy directly to useful work, or to kinetic energy in an exhaust. In the internal combustion engine, a high-energy fuel is placed in a small closed area and ignited under compression. This produces expanding gas, which drives a piston and a rotating shaft. The rotating shaft drives a transmission whose gears transfer the work to wheels, rotors, or propellers. Rocket and jet engines operate on the Brayton thermodynamic cycle. In this cycle, the gas mixture is compressed adiabatically (no heat added or lost during compression). Heat is added externally or by chemical reaction to the fluid, ideally at constant pressure. The expanding gases are exhausted, with a turbine extracting some work. The gas then expands out through a nozzle. BACKGROUND AND HISTORY Solid-fueled rockets developed in China in the thirteenth century achieved the first successful continuous propulsion of heavier-than-air flying machines. In 1903, Orville and Wilbur Wright used a spinning propeller driven by an internal combustion engine to accelerate air and develop the reaction force that

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propelled the first human-carrying heavier-than-air powered flight. As propeller speeds approached the speed of sound in World War II, designers switched to the gas turbine or jet engine to achieve higher thrust and speeds. German Wernher von Braun developed the V2 rocket, originally known as the A4 for space travel, but in 1944, it began to be used as a long-range ballistic missile to attack France and England. The V2 traveled faster than the speed of sound, reached heights of 83 to 93 kilometers, and had a range of more than 320 kilometers. The Soviet Union’s 43-tonne Sputnik rocket, powered by a LOX/RP2 engine generating 3.89 million Newtons of thrust, placed a 500-kilogram satellite in low Earth orbit on October 4, 1957. The United States’ three-stage, 111-meter-high Saturn V rocket weighed more than 2,280 tonnes and developed more than 33.36 million Newtons at launch. It could place more than 129,300 kilograms into a low-Earth orbit and 48,500 kilograms into lunar orbit, thus enabling the first human visit to the moon in July, 1969. The reusable space shuttle weighed 2,030 tonnes at launch, generated 34.75 million Newtons of thrust, and could place 24,400 kilograms into a low-Earth orbit. In January, 2006, the New Horizons spacecraft reached 57,600 kilometers per hour as it escaped from Earth’s gravity. Meanwhile, air-breathing engines have grown in size and become more fuel efficient, propelling aircraft from hovering through supersonic speeds. HOW IT WORKS Rocket. The rocket is conceptually the simplest of all propulsion systems. All propellants are carried on board, gases are generated with high pressure, heat is added or released in a chamber, and the gases are exhausted through a nozzle. The momentum of the working fluid is increased, and the rate of increase of this momentum produces a force. The reaction to

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this force acts on the vehicle through the mounting structure of the rocket engine and propels it. Jet Propulsion. Although rockets certainly produce jets of gas, the term “jet engine” typically denotes an engine in which the working fluid is mostly atmospheric air, so that the only propellant carried on the vehicle is the fuel used to release heat. Typically, the mass of fuel used is only about 2 to 4 percent of the mass of air that is accelerated by the vehicle. Types of jet engines include the ramjet, the turbojet, the turbofan, and the turboshaft. Propulsion System Metrics. The thrust of a propulsion system is the force generated along the desired direction. Thrust for systems that exhaust a gas can come from two sources. Momentum thrust comes from the acceleration of the working fluid through the system. It is equal to the difference between the momentum per second of the exhaust and intake flows. Thrust can also be generated from the product of the area of the jet exhaust nozzle cross section and the difference between the static pressure at the nozzle exit and the outside pressure. This pressure thrust is absent for most aircraft in which the exhaust is not supersonic, but it is inevitable when operating in the vacuum of space. The total thrust is the sum of momentum thrust and pressure thrust. Dividing the total thrust by the exhaust mass flow rate of propellant gives the equivalent exhaust speed. All else being equal, designers prefer the highest specific impulse, though it must be noted that there is an optimum specific impulse for each mission. LOX-LH2 rocket engines achieve specific impulse of more than 450 seconds, whereas most solid rocket motors cannot achieve 300 seconds. Ion engines exceed 1,000 seconds. Air-breathing engines achieve very high values of specific impulse because most of the working fluid does not have to be carried on-board. The higher the specific impulse, the lower the mass ratio needed for a given mission. To lower the mass ratio, space missions are built up in several

Principles of Aeronautics

stages. As each stage exhausts its propellant, the propellant tank and its engines are discarded. When all the propellant is gone, only the payload remains. The relation connecting the mass ratio, the delta-v, and specific impulse, along with the effects of gravity and drag, is called the rocket equation. Propulsion systems, especially for military applications, operate at the edge of their stable operation envelope. For instance, if the reaction rate in a solid propellant rocket grows with pressure at a greater than linear rate, the pressure will keep rising until the rocket blows up. A jet engine compressor will stall, and flames may shoot out the front if the blades go past the stalling angle of attack. Diagnosing and solving the problems of instability in these powerful systems has been a constant concern of developers since the first rocket exploded. APPLICATIONS AND PRODUCTS Many kinds of propulsion systems have been developed or proposed. The simplest rocket is a cold gas thruster, in which gas stored in tanks at high pressure is exhausted through a nozzle, accelerating (increasing momentum) in the process. All other types of rocket engines add heat or energy in some other form in a combustion (or thrust) chamber before exhausting the gas through a nozzle. Solid-fueled rockets are simple and reliable, and can be stored for a long time, but once ignited, their thrust is difficult to control. An ignition source decomposes the propellant at its surface into gases whose reaction releases heat and creates high pressure in the thrust chamber. The surface recession rate is thus a measure of propellant gas generation. The thrust variation with time is built into the rocket grain geometry. The burning area exposed to the hot gases in the combustion chamber changes in a preset way with time. Solid rockets are used as boosters for space launch and for storable missiles that must be launched quickly on demand.

Propulsion Technologies

Liquid-fueled rockets typically use pumps to inject propellants into the combustion chamber, where the propellants vaporize, and a chemical reaction releases heat. Typical applications are the main engines of space launchers and engines used in space, where the highest specific impulse is needed. Hybrid rockets use a solid propellant grain with a liquid propellant injected into the chamber to vary the thrust as desired. Electric resistojets use heat generated by currents flowing through resistances. Though simple, their specific impulse and thrust-to-weight ratio are too low for wide use. Ion rocket engines use electric fields or, in some cases, heat to ionize a gas and a magnetic field to accelerate the ions through the nozzle. These are preferred for long-duration space missions in which only a small level of thrust is needed but for an extended duration because the electric energy comes from solar photovoltaic panels. Nuclear-thermal rockets generate heat from nuclear fission and may be coupled with ion propulsion. Proposed matter-antimatter propulsion systems use the annihilation of antimatter to release heat, with extremely high specific impulse. Pulsed detonation engines are being developed for some applications. A detonation is a supersonic shock wave generated by intense heat release. These engines use a cyclic process in which the propellants come into contact and detonate several times a second. Nuclear-detonation engines were once proposed, in which the vehicle would be accelerated by shock waves generated by nuclear explosions in space to reach extremely high velocities. However, international law prohibits nuclear explosions in space. Ramjets and turbomachines. Ramjet engines are used at supersonic speeds and beyond, where the deceleration of the incoming flow is enough to generate very high pressures, adequate for an efficient heat engine. When the heat addition is done without slowing the fluid below the speed of sound, the engine is called a scramjet, or supersonic combustion ramjet. Ramjets cannot start by themselves from rest. Turbojets add a

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turbine to extract work from the flow leaving the combustor and drive a compressor to increase the pressure ratio. A power turbine may be used downstream of the main turbine. In a turbofan engine, the power turbine drives a fan that works on a larger mass flow rate of air bypassing the combustor. In a turboprop, the power is taken to a gearbox to reduce revolutions per minute, powering a propeller. In a turboshaft engine, the power is transferred through a transmission as in the case of a helicopter rotor, tank, ship, or electric generator. Many applications combine these concepts, such as a propfan, a turboramjet, or a rocket-ramjet that starts off as a solid-fueled rocket and becomes a ramjet when propellant consumption opens enough space to ingest air. Gravity assist. A spacecraft can be accelerated by sending it close enough to another heavenly body (such as a planet) to be strongly affected by its gravity field. This swing-by maneuver sends the vehicle into a more energetic orbit with a new direction, enabling surprisingly small mass ratios for deep space missions. Tethers. Orbital momentum can be exchanged using a tether between two spacecraft. This principle has been proposed to efficiently transfer payloads from Earth orbit to lunar or Martian orbits and even to exchange payloads with the lunar surface. An extreme version is a stationary tether linking a point on Earth’s equator to a craft in geostationary Earth orbit, the tether running far beyond to a counter-mass. The electrostatic tether concept uses variations in the electric potential with orbital height to induce a current in a tether strung from a spacecraft. An electrodynamic tether uses the force that is exerted on a current-carrying tether by the magnetic field of the planet to propel the tether and the craft attached to it. Solar and plasma sails. Solar sails use the radiation pressure from sunlight bounced off or absorbed by thin, large sails to propel a craft. Typically, this works best in the inner solar system where radiation is more intense. Other versions of propulsion sails, in which la-

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sers focus radiation on sails that are far away from the Sun, have been proposed. In mini magnetospheric plasma propulsion (M2P2), a cloud of plasma (ionized gas) emitted into the field of a magnetic solenoid creates an electromagnetic bubble around 30 kilometers in diameter, which interacts with the solar wind of charged particles that travels at 300 to 800 kilometers per second. The result is a force perpendicular to the solar wind and the (controllable) magnetic field, similar to aerodynamic lift. This system has been proposed to conduct fast missions to the outer reaches of the solar system and back. CAREERS Propulsion technology spans aerospace, mechanical, electrical, nuclear, chemical, and materials science engineering. Aircraft, space launcher, and spacecraft manufacturers and the defense industry are major customers of propulsion systems. Workplaces in this industry are distributed over many regions in the United States and near many major airports and National Aeronautics and Space Administration centers. The large airlines operate engine testing facilities. Propulsion-related work outside the United States, France, Britain, and Germany is usually in companies run by or closely related to their respective governments. Because propulsion technologies are closely related to weapon-system development, many products and projects come under the International Traffic in Arms Regulations. Machinery operating at thousands to hundreds of thousands of revolutions per minute requires extreme precision, accuracy, and material perfection. Manufacturing jobs in this field include specialist machinists and electronics experts. Because propulsion systems are limited by the pressure and temperature limits of structures that must also have minimal weight, the work usually involves advanced materials and manufacturing techniques. Instrumentation and diagnostic techniques for propulsion systems are constantly pushing the boundaries of tech-

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Principles of Aeronautics

nology and offer exciting opportunities using optical and acoustic techniques. SOCIAL CONTEXT AND FUTURE PROSPECTS Propulsion systems have enabled humanity to advance beyond the speed of ships, trains, balloons, and gliders to travel across the oceans safely, quickly, and comfortably and to venture beyond Earth’s atmosphere. The result has been a radical transformation of global society since the early twentieth century. People travel overseas regularly, and on any given day, city centers on every continent host conventions with thousands of visitors from all over the world. Jet engine reliability has become so established that jetliners with only two engines routinely fly across the Atlantic and Pacific oceans. However, jet engines are not very energy-efficient, which makes them expensive to operate and detrimental to the environment. As such, addressing this problem is a major area of jet engine research in the twenty-first century. Propulsion technologies are just beginning to grow in their capabilities. As of the early twenty-first century, specific impulse values were at best a couple of thousand seconds; however, concepts using radiation pressure, nuclear propulsion, and matter-antimatter promise values ranging into hundreds of thousands of seconds. Air-breathing propulsion systems promise specific impulse values of greater than 2,000 seconds, enabling single-stage trips by reusable craft to space and back. As electric propulsion systems with high specific impulse come down in system weight because of the use of specially tailored magnetic materials and superconductors, travel to the outer planets may become quite routine. Spacecraft with solar or magnetospheric sails, or tethers, may make travel and cargo transactions to the moon and inner planets routine as well. These technologies are at the core of human aspirations to travel far beyond their home planet.

Further Reading De Luca, Luigi T., Max Calabro, Toru Shimada, and Valery P. Sinditskii, editors. Chemical Rocket Propulsion: A Comprehensive Survey of Energetic Materials. Springer International Publishing, 2018. Faeth, G. M. Centennial of Powered Flight: A Retrospective of Aerospace Research. American Institute of Aeronautics and Astronautics, 2003. Gohardani, Amir S. Distributed Propulsion Technology. Nova Science Publishers Inc., 2014. Hunley, J. D. The Development of Propulsion Technology for U.S. Space-Launch Vehicles, 1926-1991. Texas A&M University Press, 2013. Kucinski, William. So You Want to Design Engines: UAV Propulsion Systems. SAE International, 2018. Martin, Richard. “The Race for the Ultra-Efficient Jet Engine of the Future.” MIT Technology Review, 23 Mar. 2016, www.technologyreview.com/s/601008/the-race-forthe-ultra-efficient-jet-engine-of-the-future/. Accessed 31 Aug. 2018. Musielak, Dora. Scramjet Propulsion: A Practical Introduction. Wiley, 2022. Norton, Bill. STOL Progenitors: The Technology Path to a Large STOL Aircraft and the C-17A. American Institute of Aeronautics and Astronautics, 2002. Peebles, C. Road to Mach 10: Lessons Learned from the X-43A Flight Research Program. American Institute of Aeronautics and Astronautics, 2008. Sabry, Foud. Plasma Propulsion: Can SpaceX Use Advanced Plasma Propulsion for Starship? One Billion Knowledgeable, 2021. Schaberg, Christopher. “The Jet Engine Is a Futuristic Technology Stuck in the Past.” The Atlantic, 11 Feb. 2018, www.theatlantic.com/technology/archive/2018/02/ engine-failure/552959/. Accessed 31 Aug. 2018. See also: Advanced propulsion; Aerodynamics and flight; Aeronautical engineering; Flight propulsion; Fluid dynamics; Forces of flight; High-altitude flight; High-speed flight; Hypersonic aircraft; Jet engines; Materials science; National Aeronautics and Space Administration (NASA); Pressure; Ramjets; Rocket propulsion; Scramjet; Shock waves; Space shuttle; Spacecraft engineering; Supersonic aircraft; X-planes (X-1 to X-45)

—Narayanan M. Komerath

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R Ramjets Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics; Mathematics ABSTRACT A ramjet engine is a jet engine in which the working fluid is compressed solely by the deceleration of the fluid entering the engine. Ramjet engines represent the simplest type of air-breathing engines. They are used to power long-range guided missiles, and they also offer the potential to improve the payload and reusability of space launch vehicles. KEY CONCEPTS combustor: the part of a ramjet engine in which fuel is burned deceleration: the reduction of speed, the opposite of acceleration diffuser: a structure within a ramjet engine that serves to diffuse or randomize the movement of the air/fuel mixture entering the engine, thus slowing it and allowing it to burn without decreasing the pressure inside the combustor PRINCIPLES OF RAMJET ENGINES Jet engine designs can be understood in terms of the gas turbine cycle. The fluid is first compressed, heat is added at constant pressure, and then work is extracted as the fluid expands. Heat addition is more efficient at high pressure. At high flight speeds, the pressure rises due to the deceleration of air entering the engine is high enough for engine performance, without mechanical compressors. This also removes the need for a turbine to drive the compressor.

Since compression depends on a high flight speed, ramjets cannot accelerate from rest, nor produce useful levels of thrust below Mach 0.6. Thus, ramjets are used on vehicles where there is some other propulsion device for the takeoff stage, with ramjet startup occurring at supersonic speeds. In the theoretical case of the ideal ramjet, air entering at a supersonic Mach number is decelerated through a loss-less diffuser. Fuel is added and mixed with the air before it enters the combustor, and then ignited, to complete the fuel-air reaction at constant pressure (no pressure losses) inside the combustor. The heated gas then expands out through a frictionless nozzle, the exhaust Mach number equaling the Mach number ahead of the inlet. This exhaust velocity is higher than the inlet velocity because the exhaust temperature and speed of sound are higher than the inlet values. The thrust of the ideal ramjet is limited by two factors. Firstly, the thrust becomes zero at the Mach number where deceleration of air raises the temperature to the material limits of the engine, preventing further heat addition. Secondly, when the flow velocity reaches the local speed of sound anywhere inside the engine duct, the mass flow rate of air and the amount of heat addition are maximized. In practice, four other major factors limit ramjet efficiency. The first is that decelerating a supersonic flow usually produces shocks. Drag due to shock losses can be minimized by careful inlet design, but operation over a range of conditions requires variable-geometry inlets, which add weight and complexity. Second, there is a compromise in the burner design. Without flame holders to create zones of slow-moving fluid and turbulence, it is difficult to

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get the fuel and air to mix and react within the short distance available for a combustor. Increasing the distance usually increases the engine weight, but flame holders and turbulence increase drag. Third, heat addition in any form to a moving fluid entails an irrecoverable loss in the work available from the fluid. The higher the Mach number at heat addition, the greater this Rayleigh line loss. Fourth, the nozzle can rarely be made large enough to enable full expansion of the exhaust to the outside pressure. Solutions to each of these problems can be seen in the various designs of ramjets.

Principles of Aeronautics

HISTORY French engineer René Lorin is credited with inventing the ramjet in 1913. Practical applications had to wait until the 1940s. Small Lorin-type ramjets were tested atop a Luftwaffe Dornier Do-17Z-2 in early 1942. The Skoda-Kauba SK-P.14 ramjet-powered fighter (early 1945) was built around a 1.5-meter diameter, 9.5-meter-long Sanger ramjet. The ramjet duct and two forward fuel tanks occupied much of the fuselage, with the pilot lying prone atop the ramjet in a cockpit located in the aircraft nose. The small unswept wings carried fuel tanks. Booster

A Bloodhound on display at the RAF Museum, Hendon, London. Photo via Wikimedia Commons. [Public domain.]

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rockets on a tricycle undercarriage that could be jettisoned enabled takeoff and acceleration to ramjet startup speed. Germany also used ramjet engines to augment the V-1 Doodlebug rocket bombs sent over Britain. Sanger ramjets were also tested with the Messerschmitt Me-262 turbojet fighter and other Luftwaffe aircraft. Studies using subsonic ramjets at the University of Southern California (USC) in late 1943 led to the 1945 contract to the Glenn Martin company to develop the Gorgon 4 guided ramjet missile. The Gorgon test vehicles had swept wings and tails, designed for Mach 0.7 flight with a range of 80 to 140 kilometers, with the engine firing for 270 seconds. The full-scale USC supersonic ramjet was tested in August, 1945. The Marquardt Company delivered the first engines for testing to the US Navy, with the first free flight of a supersonic ramjet-powered vehicle on November 14, 1947, off Point Mugu, California. The National Advisory Committee for Aeronautics (NACA) used the F-23 Ramjet Research Vehicle in tests at their Wallops Island facility from 1950 to 1954. The two 1,000-pound-thrust engines of the F-23 used acetylene fuel, reaching Mach 3.12 and an altitude of 48,463 meters. In 1959, a French experimental aircraft set a speed record of 1,641.5 kilometers per hour using ramjet engines. Meanwhile Soviet designer Mikhail Bondaryuk developed a kerosene-fueled ramjet stage for the EKR launch vehicle in 1953 and 1954, producing 567 kilograms of thrust, with a specific impulse (Isp) of 1,580 seconds. This engine was studied for an experimental winged cruise missile, which formed the basis for the later Burya missiles. On August 29, 1947, the McDonnell XH-20 “Little Henry” helicopter first flew, powered by ramjet engines at its rotor tips. While this concept eliminated the need for a countertorque system such as a tail rotor, it was too noisy to be a practical helicopter propulsion device. At the turn of the twenty-first century, a ramjet-powered spinning disc was being

Ramjets

developed as an efficient power-generation device. With these two exceptions, all ramjet applications have been for high-speed flight. Ramjets are thought to be useful for flight at up to Mach 18, with advances in materials and fuels. RAMJET-POWERED MISSILES The British Bloodhound and SeaDart series, the US Navy’s Mach 2.7 Talos, which could carry a 5-kiloton-nuclear warhead, the Soviet SA-6, and the Indian Akash are examples of surface-to-air missiles which use a solid-fueled rocket boost, followed by ramjet-powered acceleration. The BAe Meteor beyond visual range air-to-air missile (BVRAAM) uses a solid-fueled variable-flow rocket-ramjet engine. The ramjet engine enables the thrust to be distributed and controlled over a longer duration, widening the range of parameters within which the missile has a high probability of destroying its targets. Ramjet air-to-surface missiles include the Russian KH-31/ AS-17 Krypton. In 1955, the US Navy launched and then canceled full-scale development of the Triton, a ramjet-powered, Mach 3.5, 21,600-kilometerrange, submarine-launched cruise missile. France has deployed the ramjet-powered, air-launched, nuclear-armed, Mach 3.5, 300-kilometer-range ASMP cruise missile. Newer programs are the US Fasthawk Mach 4 booster-ramjet cruise missile to replace the Tomahawk, and the CounterForce Mach 4-6 surface-to-air missile (SAM). TURBORAMJETS AND RAMROCKETS Most missiles which use ramjets are actually rocket-ramjets or ramrockets. They use a rocket booster either as a separate stage or as an integral part of the engine. At liftoff, the intake is closed or blocked by fuel, and the vehicle operates as a rocket. As the rocket propellant grain burns down, the intakes are opened, and a combustion chamber formed for the ramjet to start operating. In some missiles, the ramjet engines are separate strap-ons

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that do not operate fully until the rocket booster stage is expended. High-speed aircraft use engines which operate partially as ramjets. For example, the SR-71 Blackbird has engines which start as turbine engines. At high altitudes and speeds, larger air intakes open, allowing air to bypass the fan and operate as a ramjet. The Japanese ATREX project developed an expanding air turboramjet engine. In this concept, liquid hydrogen fuel was used to precool the incoming air before sending it through a fan (at takeoff) or around the fan at high speeds. Combustion was conducted in subsonic flow. A tip-turbine operated in the high-speed bypass flow to recover work to be used to run the liquid hydrogen turbopump. A plug nozzle was used, where the flow adjusted itself to be optimally expanded as the external conditions changed. HYPERSONIC RAMJETS The vehicles discussed above are mostly limited to publicized Mach numbers below 3.5. The ramjet also offers several advantages as a propulsion system for space launch vehicles and hypersonic missiles. Without complex turbomachinery, the engine can be quite light, offer an unobstructed airflow path, and can use a wide variety of fuels, ranging from cryogenic hydrogen to storables like kerosene and methane. However, major problems face engine designers. Above Mach 4, shock losses suffered in decelerating the flow to subsonic speeds for combustion may exceed the Rayleigh line losses of heat addition to a supersonic stream. The pressure rise incurred in deceleration to subsonic speeds would demand heavy casings, and the temperature rise is such that further heat addition would melt the burner. Improvements in materials can yield only limited gains, because most fuels would decompose and not release heat at very high temperatures. For these reasons, supersonic-combustion ramjets (scramjets) are being developed in several countries, including the United States, Russia, Britain, Europe,

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Japan, and India. In these designs, the fuel is mixed into a supersonic airstream and the heat added by reaction until the Mach number comes down close to unity. The technology for air liquefaction, where oxygen is recovered from air at the lower altitudes and stored in liquid form for rocket flight at high altitudes, appears to be key to making these into viable space launch engines. In the 1960s, scramjet research produced a few designs, such as those by Aerojet General, which showed positive net thrust (more thrust than drag) at hypersonic Mach numbers in wind tunnel tests. Such engines injected the fuel in jets perpendicular to the supersonic airstream, enabling fast mixing, albeit with high drag. Antonio Ferri’s “thermal compression” idea removed the need for variable geometry. The X-15 project, intended to study scramjet operation, was canceled before testing full-scramjet mode. In the mid-1980s, the National Aeronautics and Space Administration (NASA), the US Air Force, the US Navy, Britain, France, Germany, and Japan each conducted large programs directed toward different vehicle concepts. Best-known among these was the National Aerospace Plane (NASP) project announced by President Ronald Reagan, with the French Hermes, German Sanger, and British HOTOL springing up concurrently. When American funding for NASP dried up in the mid-1990s, citing difficulties with supersonic fuel-air mixing, all these programs dropped from public view, citing high cost. Scramjet engines have since been developed for missile applications. A November, 1991, test lasting 130 seconds near Baikonur Cosmodrome in Kazakhstan is reported to have taken a scramjet on a SAM booster to Mach 8. The Russian GELA hypersonic experimental flying testbed, believed to be an air-launched strategic cruise missile, was shown at Moscow in 1995. The Mach 6-10 Hyper-X program, the Boeing/NASA X-43, and a DARPA scramjet program are examples. The Johns Hopkins Applied Physics Lab reported success with a dual-combustor ramjet which proved

Eddie Rickenbacker

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operation of a scramjet engine up to Mach 6 with JP-10 storable liquid hydrocarbon fuel. NUCLEAR RAMJETS The heat addition in the ramjet need not be chemical. In the 1950s, the US Air Force’s Project Pluto developed a Mach 3 ramjet-powered missile where the flow was heated to over 2,500 degrees Fahrenheit by a fast neutron nuclear reactor. The missile would carry nuclear weapons and loiter around the periphery of the Soviet Union in tense times. In a nuclear war, these 150,000-pound “Doomsday Missiles” were to dash supersonic at low altitudes (152 meters) and deliver their 22,680-kilogram payloads to their targets. After dropping bombs, the missiles were to cruise back and forth across the Soviet Union indefinitely, destroying property with the shock waves created by their passage, and contaminating everything with radiation from their engines. The nuclear ramjet engine was tested in the Nevada desert. The danger of the missile going out of control during flight testing and cruising back and forth across the United States ensured the project’s cancellation. Robert W. Bussard described an interstellar ramjet. The vehicle would create a magnetic field and capture hydrogen ions (protons) occurring in space. Nuclear fusion of these protons would heat the gas and propel them through a nozzle. The critical speed needed for ramjet startup was estimated to be about 6 percent of the speed of light, and the inlet diameter was of the order of 6,000 to 10,000 kilometers. Lasers were proposed to ionize hydrogen ahead of the inlet. There is debate whether the protons would actually enter the engine, and would sustain fusion. —Narayanan M. Komerath Further Reading Brewer, G. Daniel. Hydrogen Fuel Technology. CRC Press, 2017. Farokhi, Saeed. Future Propulsion Systems and Energy Sources in Sustainable Aviation. Wiley, 2020.

Ingenito, Antonella. Subsonic Combustion Ramjet Design. Springer International Publishing, 2021. Myhra, David. Focke-Wulf’s Proposed “Ta-283" 2-Man Twin Ramjet Powered Bomber. RCW Technology & Ebook Publishing, 2013. Rurner, Martin J. L. Rocket and Spacecraft Propulsion Principles, Practice and New Developments. Springer Berlin Heidelberg, 2008. See also: Advanced propulsion; Aerodynamics and flight; Aeronautical engineering; Flight propulsion; Fluid dynamics; Forces of flight; German Luftwaffe; High-speed flight; Hypersonic aircraft; Jet engines; Ernst Mach; Mach number; National Aeronautics and Space Administration (NASA); Propulsion technologies; Rocket propulsion; Scramjet; Shock waves

Eddie Rickenbacker Fields of Study: Aeronautical engineering; Mechanical engineering ABSTRACT Eddie Rickenbacker was born on October 8, 1890, in Columbus, Ohio; he died on July 23, 1973, in Zurich, Switzerland. Rickenbacker was a decorated World War I American air ace who returned home to enter business, founding an automobile company and, later, an airline. As manager of Eastern Air Lines for many years, he helped develop many of the features of air travel now taken for granted. KEY CONCEPTS flight training: instruction and practice in operating a specific type of aircraft efficiently; military flight training also includes aerial combat techniques stewardess: the original term for a female flight attendant Edward Vernon “Eddie” Rickenbacker, the third of eight children, entered the world of work as a boy, first by selling newspapers and then by moving to jobs in a glass factory, a foundry, a brewery, a

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shoe factory, and a monument works. He became interested in automobiles, and, at age sixteen, he was hired by Lee Frayer, a race-car driver and auto company executive who introduced him to the world of automobile racing. By 1912, Rickenbacker was working with auto designer Fred Dusenburg and entering races on his own. In 1914, he set a world speed record at Daytona Beach, Florida. Rickenbacker became interested in aviation after an aircraft-designer friend, Glenn Martin, took him on a flight in 1916. He was further intrigued by flying after meeting some Royal Air Force (RAF) fliers on a trip to England later that year. When the United States entered the war in 1917, Rickenbacker volunteered for service, becoming a driver for General William “Billy” Mitchell. Soon he was able to persuade Mitchell to assign him to flight training, and he joined the Ninety-fourth Aero Pursuit Squad-

Eddie Rickenbacker. Photo via Wikimedia Commons. [Public domain.]

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ron near Toul, France. He had spent fewer than three weeks in training. WORLD WAR I FIGHTER ACE During 1918, Rickenbacker’s flying skill steadily improved, and he began to shoot down enemy planes with increasing frequency. In October of that year, Rickenbacker scored fourteen victories. Although there is controversy over his exact wartime total of downed aircraft, it was certainly at least twenty-four, including four balloons. Rickenbacker survived 134 aerial battles and logged more combat hours than any other American pilot. These achievements made him famous when he returned home, promoted to the rank of major. Rickenbacker was awarded the French Croix de Guerre in 1918 and the Congressional Medal of Honor in 1930. BUSINESS CAREER After the war, Rickenbacker declined numerous offers to endorse products or to go act in motion pictures and returned to the automobile industry as president of the Rickenbacker Motor Company. After a bold start in 1922, the company went bankrupt in 1925, leaving its namesake deep in debt. Undaunted, Rickenbacker bought a controlling interest in the Indianapolis Motor Speedway, wrote a book about his war experiences, and even authored a syndicated comic strip. His primary occupation was as a sales manager for General Motors. Even with all these activities, he still found time to travel the country giving speeches on aviation and its future. He urged many city governments to consider building municipal airports. In 1934, Rickenbacker became general manager of Eastern Air Lines. Under his management the airline added routes and became the first profitable airline in the United States. Stewardesses tended to passengers during each flight, and pilots were provided with up-to-date navigational instruments. Eastern also started its own meteorology division

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and instituted regular medical checkups for pilots. Maintaining a close relationship with Donald Douglas, Rickenbacker bought planes from Douglas Aircraft, and made a record-breaking flight from California to New Jersey in the new Douglas airliner, the DC-1. WORLD WAR II ACHIEVEMENTS Always an advocate for air travel and military air power, Rickenbacker traveled all over the country to give talks. Early in 1941, on a trip to Atlanta, he was seriously hurt in a plane crash and required months of surgery and physical therapy. The United States was now involved in World War II, and Rickenbacker, when he was well, was sent by the War Department on special missions. He gave inspirational talks to pilots and recommended improvements in aircraft and procedures. He traveled to England, where he met with Prime Minister Winston Churchill and was entrusted with supreme commander of the Allied Expeditionary Force Dwight D. Eisenhower’s planning documents for the invasion of North Africa, which he brought back to Washington, D.C. In October, 1942, on a mission to New Guinea, the plane carrying Rickenbacker and a crew of seven ran out of fuel and crashed in the Pacific Ocean. Using three small rubber rafts, the men managed to survive for twenty-four days with virtually no shelter, water, or provisions. Only one man died; the others kept alive by drinking rainwater and by eating small fish and a gull they caught with their bare hands. At the end of this unprecedented ordeal, the survivors were spotted by a navy pilot and rescued. Prodded by Rickenbacker, the Navy made many modifications to the survival gear carried in planes, increasing the size of the rafts and providing for sails and solar water stills. During the remainder of the war, many other servicemen benefited from these steps. After the war, Rickenbacker rejoined Eastern Air Lines, but was never as successful in business as he

had previously been. He was gradually eased out of management and retired in 1964, but he continued to speak and write until his death in 1973, on a trip to Switzerland. —John R. Phillips Further Reading Charles River Editors. Eddie Rickenbacker: The Life and Legacy of America’s Top World War I Fighter Ace. CreateSpace Independent Publishing Platform, 2017. Groom, Winston. The Aviators: Eddie Rickenbacker, Jimmy Doolittle, Charles Lindbergh, and the Epic Age of Flight. National Geographic Society, 2013. Lewis, W. David. Eddie Rickenbacker: An American Hero in the Twentieth Century. Johns Hopkins UP, 2005. Rickenbacker, Edward V. Fighting the Flying Circus: Frederick Stokes, 1919. Read Books Limited, 2013. ———. Rickenbacker. Prentice Hall, 1967. ———. Seven Came Through. Doubleday, 1943. Ross, John F. Enduring Courage: Ace Pilot Eddie Rickenbacker and the Dawn of the Age of Speed. St. Martin’s Publishing Group, 2014. See also: Air transportation industry; Neil Armstrong; Biplanes; DC plane family; Jimmy Doolittle; Amelia Earhart; Yuri Gagarin; John Glenn; Charles A. Lindbergh; Billy Mitchell; Wiley Post; Alan Shepard; Valentina Tereshkova; Andrei Nikolayevich Tupolev; Manfred von Richthofen; Wright brothers’ first flight; Chuck Yeager

Rocket Propulsion Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Rocket propulsion causes the movement of an aerodynamic device by the ejection of matter as hot, rapidly expanding combustion gases, without the need for taking on ambient air for combustion. Rocket propulsion permits the application of rockets both inside and outside the atmosphere, in space, or under water.

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KEY CONCEPTS airframe: the essential structure of an aircraft design without the additional items required for its intended use mass flow rate: the rate of change of mass with respect to time payload: the personnel, objects, and materials being transported by a vehicle, generally not as integral parts of the vehicle propellant: the high-temperature gases being expelled from the exhaust nozzles of a rocket following combustion of the rocket’s fuel propulsion: causing movement of an object or material by the application of a force PROPELLANT AND PROPULSION Most rockets exhaust propellant gases at high velocities and temperatures. The propellant is produced at high pressure through the release of chemical, nuclear, or electrical energy to the working fluid. In outer space, the propellant escapes from the rocket chamber to a high-exit kinetic energy in proportion to its available energy per unit mass. This explains why the reaction of low molecular weight fuel, such as liquid hydrogen, with liquid oxygen can produce an effective exhaust velocity almost twice that of rockets using solid fuel mixed with oxidizer crystals. Newton’s second law shows that rocket thrust is the product of the effective exhaust velocity multiplied by the mass flow rate. Thus the higher the “effective” exhaust velocity, the lower the mass of propellant required for a given mission. Future long-range space missions may be based on electric, nuclear, or solar energy to increase the effective exhaust velocity up to ten times, thereby reducing required propellant mass by a factor of ten. A quick estimate of a rocket thrust is to multiply the rocket chamber pressure by the smallest flow area of the exhaust nozzle, called the throat. Common rockets contain at least the following components: engine, nozzle, propellant fuel storage,

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payload, airframe, and guidance and control devices. Payloads vary widely and include spaceships, instrument packages for upper atmosphere observations, warheads on missiles, artillery projectiles, and fireworks. HISTORY As early as 600 CE, the Chinese manufactured black powder, a mixture of charcoal, sulfur, and saltpeter, for use as rocket propellant. In the year 1232, the Chinese used rocket-propelled fire-arrows successfully to defend their towns against hordes of invading Mongols. In the early eighteenth century, Sir William Congreve developed a sophisticated military missile, known as the Congreve rocket, which provided the “rocket’s red glare” observed by Francis Scott Key in 1812 at Fort McHenry. The first liquid fuel rocket, propelled by hydrogen and oxygen, was designed in 1903 by the Russian scientist Konstantin Tsiolkovsky. Physicist Robert H. Goddard was the first in the United States to succeed in launching a liquid fuel rocket using oxygen and gasoline on March 16, 1926. Within 2.5 seconds, Goddard’s rocket gained an altitude of about 15 meters and a speed of 100 kilometers per hour. The Soviet Union was the first nation to achieve spaceflight, with the Sputnik 1 satellite on October 4, 1957, and with the pilot Yuri Gagarin’s flight on Vostok 1 on April 12, 1961. The next crewed spaceflight was made by the United States one month later, on May 5, 1961, in Mercury capsule Freedom 7, piloted by astronaut Alan Shepard. In the 2-by-2-meter capsule, Shepard experienced a gravity load of 7 g’s (seven times his weight) during launch and a gravity load of 1 g during recovery. Rocket flight has remained a continual challenge to crews. At the end of the twentieth century, rockets had already launched the space shuttle more than one hundred times. Rocket technology development in the last half-century has made a major impact on modern space exploration and warfare strategies.

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Principles of Aeronautics

The launch of NASA’s STS-1 in 1981. Photo via Wikimedia Commons. [Public domain.]

Examples of such technology include the nearly 1,360,777-kilogram-thrust solid booster rockets developed for the space shuttle and the approximately 100,000 horsepower required by the turbine-driven pumps to pressurize the liquid hydrogen and oxygen for each of the three shuttle main engines. Improving materials and thrust-level control through-

out the burn period is an ongoing technical challenge. ROCKET SCIENCE Rockets have been developed for many different purposes and therefore differ widely in dimension, takeoff weight, thrust, range, propellant type, pres-

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sure, and temperature. The combustion process itself pressurizes solid fuel rockets, making solid fuel rockets more simple than liquid fuel rockets and capable of higher thrust levels. In the case of the space shuttle, two solid booster rockets are used to launch the vehicle, each with almost 1,360,777 kilograms of thrust. Their function is to accelerate the shuttle as quickly as the crew can tolerate to near orbital velocity. From then on, the liquid fuel space shuttle main engines (SSME) continue to provide thrust with almost twice the effective exhaust velocity but only about one-seventeenth the thrust level. Typical fireworks rockets have burn times of only seconds and therefore require exceptionally high acceleration rates or thrust-to-weight ratio. Air-pressure-driven, water-type toy rockets have low exhaust velocities, similar in magnitude to the nozzle velocity of a garden hose. The most important difference between rockets and jet engines is that rockets do not need to take in air, whereas jet engines require air for combustion and temperature control with mass-flow rates of up to one hundred times that of their fuel-flow rate. Because jet engines must ingest air, they can only operate below 2,540-meter altitude. However, rockets can operate anywhere inside and outside the atmosphere, even under water. The rocket nozzle is used to accelerate the propellant to high exit velocity. To keep this nozzle small and therefore lightweight, the propellant must be generated at high pressure inside the rocket chamber. The corresponding smaller exit area also minimizes the nozzle thrust loss from ambient air pressure. The use of a low molecular weight propellant at high temperature increases the effective exhaust velocity, thus minimizing the required propellant mass-flow rate. The most energetic chemical propellants are produced by the combustion of liquid hydrogen with liquid fluoride as the oxidizer. This mixture generates a combustion temperature of 3,982 degrees Cel-

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sius and nozzle gas velocities of up to 4,694 meters per second. Less corrosive is a combination of liquid hydrogen with liquid oxygen, which produces a combustion temperature of 2,982 degrees Celsius and nozzle gas velocities of up to 4,450 meters per second. To pressurize these liquid propellants, very high-horsepower turbopumps are used. For example, the SSME requires both fuel and oxidizer pumps of a delivery pressure of around 3,629 kilograms per square inch. With their combined-flow rate of approximately 454 kilograms per second, this requires almost 100,000 horsepower for pumping per engine. In contrast, a solid fuel rocket is much simpler to operate and therefore less expensive, as it does not need a pump to pressurize its combustion chamber. To understand the pressurization process, one must realize that the maximum amount of mass-flow which can escape through a rocket nozzle is directly proportional to gas density or pressure inside the chamber. Prior to ignition, the rocket chamber is at ambient pressure. The solid propellant inside is typically a mixture of oxidizer crystals, such as ammonium perchlorate, combined in a synthetic rubber binder, which serves as fuel. The fuel inner surface geometry is designed to adjust the combustion rate and thereby provide the desired thrust/time characteristics. When this solid surface is ignited, the amount of hot gas produced exceeds that escaping out the nozzle. Therefore, gas mass accumulates inside the rocket chamber and increases the gas pressure. This pressure rise continues until it is high enough to allow as much gas to escape out of the nozzle as is being generated inside the combustion chamber. In a fireworks rocket, this operating pressure is reached within a fraction of a second. Some small liquid-propellant rockets can be operated without a pump, if the fuel and oxidizer are pressurized by a container of high-pressure inert gas. An even more basic liquid rocket uses a

Principles of Aeronautics

monopropellant, such as hydrogen peroxide. This liquid, when brought into contact with a catalyst, transforms into a 538-degree-Celsius steam-and-oxygen gas that makes a good propellant. Liquid propellants have the advantage over solids in that the exhaust velocity is higher and that thrust is controllable with a valve. Thrust control can also be obtained by combining a liquid oxidizer with a solid fuel, which is termed a hybrid engine. Hypergolic propellants such as nitric acid and hydrazine spontaneously combust upon contact, thereby eliminating the need for an igniter. Rocket staging is an important technology used to increase payload capacity. The dropping off of empty fuel and oxidizer containers reduces weight and drag, which, in turn, reduces the thrust required in the subsequent stage. The disadvantage of rocket staging in launching the space shuttle is that retrieving and refurbishing the solid booster rocket casings adds several months to the launch turnaround time. This means many units are needed for a frequent launch schedule. The cost associated with this arrangement is the main reason for current research efforts to develop a space shuttle replacement in the form of a single-stage-to-orbit vehicle. In such a vehicle, the weight savings normally achieved by the staging process must be replaced by reducing the amount of oxidizer carried on board, necessitating takeoff with an air-breathing jet engine instead of a rocket. The switch to rocket propulsion cannot occur until after the vehicle reaches a speed of Mach 10. Then, a nonstaged rocket is sufficient to continue into orbit. Such an air-breathing jet engine is called a supersonic combustion ramjet, or scramjet, and the technology as yet does not meet those requirements. Long-range space missions in orbital trajectories represent flight in a zero-gravity environment for a majority of the mission. In those cases, a very small thrust supplied over a long time period can be made more fuel-efficient than can the use of chemical

Rocket Propulsion

rockets. The energy for this type of rocket is supplied by either nuclear or solar energy. Electric energy can be used to heat gas to temperatures of up to 5,538 degrees Celsius in an electric arc and produce a very high exit velocity. Ion rockets are even more propellant efficient. They accelerate charged particles in an electric field to exhaust velocities up to ten times those possible in chemical rockets. International cooperation in rocket launch systems expanded at the end of the twentieth century. Near Moscow, the Russians have built more than 10,000 rocket engines. Based on the Russians’ experience, Lockheed Martin placed an order for 101 type RD-180 rocket engines. The RD-180 is a single-engine rocket, producing up to 423,383 kilograms of thrust and using two combustion chambers, each with its own steerable nozzle. It burns a kerosene-oxygen mixture, with up to 1 ton of oxygen per second at maximum thrust. It can be throttled back to 40 percent thrust level for accurate trajectory control. These rockets were planned for use in the US Atlas III Program, designed to put 4,082 kilograms of payload in Earth geosynchronous orbit. Other applications of rocket technology are in the airbags used for passenger safety in modern automobiles. These bags are filled with the exhaust gases from what is essentially a small solid rocket, which is ignited when the vehicle experiences a collision and instantly fills the airbag with nitrogen gas and some carbon dioxide. —John L. Loth Further Reading Davenas, A. Solid Rocket Propulsion Technology. Elsevier Science, 2012, De Luca, Luigi T., Max Calabro, Toru Shimada, and Valery P. Sinditski, editors. Chemical Rocket Propulsion: A Comprehensive Survey of Energetic Materials. Springer International Publishing, 2017. El-Sayed, Ahmed F. Fundamentals of Aircraft and Rocket Propulsion. Springer London, 2016.

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Heider, Stephen D., William E. Anderson, Timothée L. Pourpoint, and Joseph Cassady. Rocket Propulsion. Cambridge UP, 2019. Mishra, D. P. Fundamentals of Rocket Propulsion. CRC Press, 2017. Sutton, George P., and Oscar Biblarz. Rocket Propulsion Elements. Wiley, 2016. See also: Advanced propulsion; Forces of flight; Robert H. Goddard; Jet Propulsion Laboratory (JPL); Johnson Space Center; National Aeronautics and Space Administration (NASA); Propulsion technologies; Ramjets; Rockets; Russian space program; Space shuttle; Spacecraft engineering; Konstantin Tsiolkovsky

Rockets Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT A rocket is an entirely self-contained projectile or vehicle that is self-propelled by jets of gases from the combustion of fuel carried onboard. Because they are self-propelled and self-contained, rockets are capable of operating independently of any outside support equipment. Furthermore, because rockets do not need to take in air to operate, they are capable of operations outside of Earth’s atmosphere. KEY CONCEPTS grain: a shaped charge of solid rocket fuel designed to undergo oxidation combustion at a predetermined rate to provide a desired amount of thrust Law of Conservation of Energy: the total energy of a system remains constant as the sum of the energies of the individual components of the system throughout any change of the system Law of Conservation of Momentum: the total momentum of a system, as the sum of the momentum of all individual parts of the system, remains constant unless the system is acted upon by a force

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LOX/LH2 engine: a rocket engine that uses liquid hydrogen as fuel and liquid oxygen as the oxidizer for the combustion reaction of the fuel monopropellant: a self-igniting, self-contained rocket fuel that does not require a separate oxidizer for combustion multistage rocket: a rocket essentially consisting of two, three or more individual rocket motors in a stacked conformation comprising the single rocket, each stage being used up in sequence for different thrust characteristics NATURE AND USE OF ROCKETS A rocket is propelled forward by a jet of material coming from an exhaust structure, typically at one end of the rocket. This jet is composed of hot gases resulting from burning fuel that is carried in the rocket. The fuel is burned in a combustion chamber and the exhaust gases are expelled under pressure from the rocket. Frequently the exhaust gases are directed using a nozzle. Causing the gases to be expelled in one direction pushes the rocket in the other direction, in accord with Newton’s third law of motion. The amount of force pushing on the rocket as a result of expelling the jet of gas is called thrust. An important consideration for a rocket is its thrust-to-weight (TTW) ratio. The higher a rocket’s TTW, the greater its acceleration. If a rocket has a TTW of less than one, it cannot lift off vertically from the surface of a planet or moon, though it can still fly horizontally with the aid of wings. As a rocket burns fuel, it has less mass and, thus, less weight. As the mass of the rocket decreases, its TTW increases. Rockets, therefore, tend to accelerate faster the longer they burn, unless the thrust is reduced. The Saturn V rocket that carried the Apollo missions to the Moon had an initial TTW of 1.25. The space shuttle was designed with a TTW of approximately 1.5. A rocket is an entirely self-contained system. Jet engines also rely on the expulsion of hot gases in order to achieve thrust; however, jet engines must take

Principles of Aeronautics

in air that mixes with the fuel and burns to provide the exhaust gases. Rockets, in contrast, carry everything they need with them and do not need to take in air to mix with the fuel. Depending on the fuel and the rocket design, rockets sometimes carry oxygen or another chemical that acts as an oxidizer to combine with the fuel in order to make it burn. The fuel and oxidizer taken together are generally called the rocket propellant. Other rockets contain self-oxidizing fuel, sometimes called a monopropellant, which does not need to be mixed with anything in order to burn. A few advanced rocket designs are able to expel gases without burning fuel at all and, thus, do not need an oxidizer to mix with the propellant. Different propellants burn with different efficiencies and release different amounts of energy. As a consequence, not all combinations of propellants yield the same thrust even when used in the same rocket. Rocket engineers characterize the efficiency of a rocket propellant by its specific impulse. The specific impulse of a rocket propellant is determined by dividing the thrust provided by the propellant by the weight of propellant consumed per second. The amount of thrust produced depends not only on the propellant used, but also on the rocket design. Thus, the specific impulse of a propellant is valid only for that propellant used in a particular rocket, hence it is specific to the rocket. APPLICATIONS Rockets have many uses. Some rockets are used in conjunction with other propulsion sources to provide additional thrust. Such rockets are called booster rockets. Some rockets are designed to carry cargo or scientific instruments. Anything carried by the rocket that is not part of the rocket itself is called the payload of the rocket. Many military rockets carry a payload of a bomb or other explosive weapon. In such cases, the rocket is called a missile, and the payload is called a warhead.

Rockets

A Soyuz-FG rocket launches from Baikonur Cosmodrome, Kazakhstan, 2006. Photo via Wikimedia Commons. [Public domain.]

Rockets have been used with aircraft, either as the sole propulsion system or as strap-on boosters used to achieve extra thrust needed for heavy aircraft to lift off on short runways. The main use of rocket-assisted takeoff is for military transports that need to take off from airfields with runways that are too short to allow for conventional takeoff with engines alone. Because rockets can continue to accelerate for as long as they have propellant, they are useful for achieving very high speeds. Such high speeds are needed to achieve orbit around Earth or to leave the vicinity of Earth. Rockets used to launch vehicles into space are called launch vehicles. Furthermore,

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because rockets are self-contained, they can operate outside of Earth’s atmosphere, including under water. All spacecraft use rockets for propulsion. Sometimes rockets are used to propel other rockets. In such cases, the combined rockets are called multistage rockets. The first rocket is known as the first stage and is often called the rocket booster. TYPES OF ROCKETS Rockets are often classified by the type of fuel that they use. Early rockets used a fuel composed of a paste made from gunpowder. Many modern rockets use a fuel that is a solid chemical. These are called solid-fueled rockets or, occasionally, simple solid rockets. Solid-fueled rockets have an advantage in that the fuel is often easy to manufacture and can be cast into the rocket casing itself. A single piece of solid fuel is called a grain or charge. The shape of the grain can be adjusted to yield different specific impulses as needed. The shape can even be adjusted to yield a different thrust at different times after the grain is ignited. Furthermore, solid fuel is often stable at normal environmental temperatures, and the rocket can be left fully fueled until needed. A disadvantage, however, is that once ignited, solid fuel burns by itself and cannot be easily controlled. A solid-fueled rocket is nearly impossible to turn off once it is ignited, and the amount of thrust cannot be much changed from the initial design considerations taken into account during rocket construction. Once the propellant of a solid-fueled rocket is ignited, it generally has to continue burning until it is completely consumed. In 1926, the American physicist Robert H. Goddard designed and built a rocket that used a liquid rather than a solid propellant. The initial liquid propellant consisted of gasoline and liquid oxygen. Since that time, many other liquid propellants have been used. A major advantage of liquid-fueled rockets is that the amount of thrust can be easily controlled by simply adjusting valves that govern the

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amount of propellant that goes into the combustion chamber. Furthermore, liquid-fueled rockets can be turned off at any time by simply shutting off the propellant control valves. Most liquid-fueled rockets can even be turned on again by opening the valves again after they have been shut off. Although some liquid propellants need an ignition source to start burning the fuel, a few mixtures of fuel and propellant simply begin to burn on contact. These self-igniting fuels are called hypergolic propellants. Although liquid-fueled rockets have some clear advantages over solid-fueled rockets, there are some serious disadvantages. Many liquid propellants are cryogenic liquids that must be kept at extremely low temperatures. These cryogenic fluids generally cannot be stored for extended periods of time in the rocket. The rocket, therefore, can only be fueled shortly before use. Furthermore, most liquid propellants are extremely dangerous to transport and to store. Liquid-fueled rockets tend to have complex valve and control systems and, thus, are usually more complicated to design and more expensive to construct than are solid-fueled rockets. Most rocket designs require the burning of either solid or liquid fuels to provide the source of hot gas and energy that expels the jet of gas that powers the rocket. A few rocket designs do not require burning of the propellant to provide the exhaust jets needed to power the rocket. One such design would employ a reservoir of compressed gas, which would be released and expelled from the rocket. Alternately, the compressed gas could force another propellant, such as water, from the rocket. Some toy rockets are of this extremely simple and inexpensive design. A major disadvantage of this design, however, is that it is very inefficient and cannot provide very much thrust for very long. Another rocket design calls for the use of a nuclear reactor to heat gases to cause them to be expelled from the rocket. Such nuclear-powered rockets are extremely efficient and powerful. A few

Principles of Aeronautics

nuclear-powered rocket motors have been constructed and tested on the ground or in very short flights, but, due to safety concerns, none have been used in extended flight. Another technology used to provide the jet of gases is the acceleration of charged atoms or molecules, called ions, with electric fields. The ions are ejected from the rocket in the direction in which the electric field accelerated them. Ion-drive rockets are quite economical. The electric fields can be generated using solar power, and almost any gas can be used as a propellant. The major disadvantage of an ion rocket is that the gas must be very diffuse for the system to work, and this results in very low thrust. The thrust, however, can be sustained for extended periods, resulting in very high speeds after long periods of operation. Because ion-driven rockets have a very low thrust, they cannot be used to launch a vehicle into space, but they can be used once a rocket is already in space. PARTS OF A ROCKET Different rocket designs obviously require different components. Some of the most complicated and diverse rockets are liquid-fueled rockets. There are general similarities among most liquid-fueled rockets, however. The bulk of the rocket’s volume holds tanks storing the propellant. For most propellants, there must be separate tanks for the fuel and the oxidizer. The location of the tanks does not really matter, but generally the oxidizer tank is located forward of the fuel tank. This placement allows for a shorter path for the fuel to travel from the tank to the rocket motor. If the rocket is designed to operate within the atmosphere, the rocket body may have fins that help stabilize the rocket in flight, but a true rocket does not use the fins as wings to fly. Some rockets are guided rockets, able to steer while in flight. The mechanisms for guidance and steering are also located within the rocket body. Generally, a rocket’s control mechanism is located away from the

Rockets

rocket motor in order to minimize any damage to the guidance system from the rocket motor’s heat or vibration. The rocket motor consists primarily of the combustion chamber, which is where the propellant is burned. The rest of the rocket motor is generally composed of valves and plumbing to deliver the propellant and to mix the fuel and oxidizer as efficiently as possible. Frequently, because the fuel and oxidizer are cryogenic fluids, the fuel lines carry the propellant past the combustion chamber before injecting the fuel into the chamber. This has the advantage of helping to cool the combustion chamber and warm the fuel, generally resulting in a more efficient burning process. Most rockets contain a nozzle to help direct the exhaust gases from the combustion chamber. A nozzle is not strictly necessary, because a properly designed combustion chamber tends to direct the exhaust gases away from the chamber as a jet through an opening at one end of the chamber. The exhaust gases, however, tend to expand as soon as they are out of the combustion chamber and the jet of exhaust gas becomes less directional after leaving the combustion chamber. A nozzle will direct the gas jet in the desired direction. The more directional the jet of gas leaving the rocket motor, the higher the thrust that the rocket motor will have. Thus, a properly designed rocket nozzle is an important part of a rocket motor. The most efficient type of nozzle is the bell-shaped Venturi nozzle, which is narrow at the point where it connects to the combustion chamber and flares out to a much larger diameter farther from the combustion chamber. Some nozzles are fixed in position, whereas others are capable of tilting slightly, thus changing the direction of the jet of gases and, consequently, the direction of the rocket’s thrust. Such movable rocket nozzles are important in guided rockets. For rockets designed to operate outside Earth’s atmosphere, the motion of the rocket nozzle is the chief mechanism for steering the rocket.

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Solid-fueled rockets are simpler in design than are liquid-fueled rockets. Both types of rocket have similar rocket bodies and nozzles; the main difference between the two types is in the design of the combustion chamber and the propellant storage. Often, the solid-fueled rocket contains a single grain of fuel. The combustion chamber is often located inside the grain, with the grain itself forming the walls of the combustion chamber. As the grain burns, the combustion chamber expands outward. Alternately, the grain fills the combustion chamber and burns from one end to the other. This is generally a less efficient design. The design and shape of the grain can be adjusted to yield variable thrust according to a predetermined formula, but the thrust variations are determined by the manufacture of the grain and cannot be changed after the rocket is ignited. THE PHYSICS OF ROCKETS One of the fundamental laws of physics is the law of conservation of momentum. Momentum is defined as the product of mass and velocity. The conservation of momentum law says that the total momentum of a system does not change unless an external force acts on the system. Rockets operate using this principle. Jets of material streaming away from one end of the rocket carry momentum. This may be thought of as negative momentum since it is in a direction opposite to the direction of the rocket’s flight. As a consequence, the rocket must have momentum in the opposite, or positive, direction. The sum of the two yields zero. As the jet of gas leaves the rocket carrying negative momentum, the rocket must have an increase of positive momentum. This means that the rocket must increase its forward speed as the rocket’s mass decreases. Thus, the more mass that is expelled from the rocket, the more momentum it carries. Likewise, the faster the mass leaves the rocket, the more momentum that it carries. The rate at which negative momentum is carried from the rocket determines the

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thrust of the rocket. Real rockets are generally not ideal, and a complex set of equations describes their behavior relating mass flow rate, exit Mach number, exit temperature, exit pressure and exit velocity of the propellant gases. The calculated thrust of a rocket is directly related to the complex variable term mdot, which is itself composed of six other terms, and is given by the equation F = (mdot)Ve + (pe - p0)Ae where Ve is the exit velocity of the propellant exhaust, pe is the exit pressure of the propellant exhaust gases, p0 is the free stream pressure of the propellant exhaust gases, and Ae is the area of the exhaust port from which the propellant exhaust gases exit. (A more complete summary is available online at grc.nasa.gov/WWW/K-12/rocket/rktthsum.html.) Much of a rocket consists of storage space for the propellant. After propellant has been used, the portion of the rocket used in storing the propellant becomes dead weight. If the final mass can be reduced, then the final velocity will be increased. This mass reduction can be accomplished by jettisoning the now-vacant propellant storage spaces. Rather than building rockets that jettison used propellant storage areas, rockets are often designed to propel other rockets, called stages. The first rocket, or first stage, fires until it has used up its propellant. After the first stage has finished using its propellant, it drops off, and the second stage begins operation. A rocket can have as many stages as needed. However, the difficulty and expense of designing and constructing multiple stages, each with its own rocket motors, makes it generally economically unfeasible for a rocket to have more than two or three stages. —Raymond D. Benge Jr. Further Reading Barrows, Timothy M., and Jeb S. Orr. Dynamics and Simulation of Flexible Rockets. Elsevier Science, 2020.

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Edberg, Donald L., Guillermo Costa, and Willie Costa. Design of Rockets and Space Launch Vehicles. American Institute of Aeronautics and Astronautics, Inc., 2020. Gorn, Michael H. Spacecraft: 100 Iconic Rockets, Shuttles, and Satellites That Put Us in Space. Voyageur Press, 2018. Longmate, Norman. Hitler’s Rockets: The Story of the V-2s. Skyhorse Publishing, 2009. Siddiqi, Asif A. The Red Rocket’s Glare: Spaceflight and the Russian Imagination 1857-1957. Cambridge UP, 2010. Van Riper, A. Bowdoin. Rockets and Missiles: The Life Story of a Technology. Johns Hopkins UP, 2007. See also: Advanced propulsion; Aerodynamics and flight; Aeronautical engineering; Robert H. Goddard; Jet Propulsion Laboratory (JPL); Johnson Space Center; Materials science; National Aeronautics and Space Administration (NASA); Rocket propulsion; Russian space program; Space shuttle; Spacecraft engineering; Konstantin Tsiolkovsky; Jules Verne

Rotorcraft Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid mechanics; Mathematics ABSTRACT A rotorcraft, or rotary-wing aircraft, is any aircraft that uses a rotor, or rotating wings, to provide the craft’s lifting force. Rotorcraft are the first aircraft able to perform short and vertical takeoffs and landings, and continue to comprise the majority of all short takeoff and landing (STOL) and vertical takeoff and landing (VTOL) aircraft. The autogyro was the forerunner of modern rotorcraft and used an unpowered rotary wing system to provide lift. KEY CONCEPTS counterrotation: rotation in opposite directions; occurs in helicopters as the angular momentum of the rotating blades imparts an equal and opposite angular momentum in the aircraft, thus requiring a secondary rotor in the tail section to provide a balancing torque

Rotorcraft

lift: the upward pressure experienced by wings due to differential pressure between upper and lower surfaces of the wing as it moves through a fluid medium as determined by its airfoil camber and thickness rotary wing: a wing that generates lift by rotating rather than by linear motion through a fluid medium such as air ROTARY- VERSUS FIXED-WING AIRCRAFT All aircraft can be divided into two general categories: fixed-wing aircraft and rotary-wing aircraft, or rotorcraft. The principal difference between the two types of aircraft is the method used to provide the lifting force that allows the aircraft to fly. Fixed-wing aircraft use relatively large, stationary wings to provide the required lift. The lift on the wings is generated by driving the entire aircraft through the air with some type of propulsion system, such as propellers or jet engines. Rotorcraft, on the other hand, are equipped with at least one rotor, made up of a set of two or more rotating wings, that provides the lift that the aircraft needs to stay aloft. The lift on each of the rotating wings is generated as the rotor spins through the air, around the rotor shaft. As a result, the rotor can generate lift without the entire aircraft necessarily having to be in motion. TYPES OF ROTORCRAFT The two most common types of rotorcraft are the helicopter and the autogyro. Helicopters and autogyros are similar in that they both have large-diameter rotors that provide lift for the aircraft in all flight conditions. The rotors of a helicopter are powered and provide lift while the vehicle is stationary. The rotors of an autogyro, however, are not powered but can spin freely and provide lift only after the vehicle has been driven forward like an airplane. Helicopters are used in a large number of civilian and military applications. Civilian applications include airborne ambulance, police surveillance, news

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gathering, firefighting, logging, heavy construction, intracity passenger transportation, tourism, cargo transportation, and search and rescue. Military applications include troop transport, logistical support, combat air support, and combat search and rescue. Autogyros, in contrast, have essentially no commercial or military applications and are flown mainly by sport aviators. Virtually all other rotorcraft can be broadly grouped into a category called tilting proprotor aircraft. These aircraft include the tilt-shaft/rotor, tilt-prop, tilt-wing, and tilt-rotor aircraft. All have two or more rotors, which provide either lift or propulsive force by tilting to a vertical position for vertical flight and to a horizontal position for forward flight. In vertical flight, the rotors provide all the lift

Principles of Aeronautics

necessary to keep the aircraft aloft in takeoff, landing, and hover. In forward flight, wings perform that function, and the rotors provide propulsive force. With the exception of the V-22 Osprey, a tilt-rotor aircraft, tilting proprotor aircraft have never progressed beyond the prototype stage. HELICOPTERS Helicopters can be distinguished from other rotorcraft by the fact that their rotors have a fixed orientation relative to the aircraft fuselage and simultaneously provide lift and propulsion. On a helicopter, engines provide the power that drives the rotation of the rotor. Most modern helicopters have either one or two rotors that provide lift and propulsive force. The maximum forward speed of a

A modern, closed-cabin autogyro in flight. Photo by Airwolfhound from Hertfordshire, UK, via Wikimedia Commons.

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helicopter is limited by the fact that the rotor (or rotors) must provide both propulsion and lift. Under high-speed flight conditions, the vibratory forces on the rotor blades become extreme, thereby limiting the top speed of the helicopter. In order to increase the top speed, some helicopters, known as compound helicopters, have been equipped with auxiliary propulsion, such as propellers or jet engines. Helicopters have been built in a variety of configurations. The most common configuration is the single-rotor helicopter, which has a single main rotor for thrust and pitch and roll control and often has a smaller tail rotor that acts to control counterrotation and provides directional control. Another common configuration is the tandem helicopter, which has two large rotors, one near the forward end of the helicopter and the other near the aft end. This configuration is particularly well suited for the transport of heavy cargo, because the two rotors can accommodate large changes in the aircraft’s center of gravity. Less common configurations include the coaxial and side-by-side helicopters. The coaxial helicopter has two counterrotating rotors that share a common mast. Side-by-side helicopters also have two rotors, but one is located on the right side of the aircraft and the other is located on the left side. A variant of the side-by-side helicopter is the synchropter, on which the two rotors are placed close together and synchronized so that their blades interleaf without contacting each other. The concept for the helicopter has been around since the Chinese top, which predates the Roman Empire. Leonardo da Vinci also considered the possibility of vertical flight. However, like the airplane, the helicopter did not become a practical concept until the invention of the internal combustion engine. Significant developments in the direction of a practical helicopter began to be achieved not long after Orville and Wilbur Wright flew their first airplane in 1903. Men such as Emile and Henry Berliner, Raoul Pescara, Louis-Charles Breguet, Hein-

Rotorcraft

rich Focke, and Anton Flettner made major contributions to the development of the helicopter. Although credit is given to Igor Sikorsky for building the first successful helicopter, the VS-300, others argue that the Focke-Wulf Fw-61 was the world’s first practical helicopter. At the same time, other individuals, including Arthur Young, Frank Piasecki, and Stanley Hiller, were developing their own designs. After World War II ended, a number of companies began designing and building helicopters. The most successful were Sikorsky, Bell, Piasecki (later Boeing), Kaman, and Hiller. During the Korean War, the use of helicopters for medical evacuation showcased the usefulness of helicopters and spurred further developments. Among the most important developments was the introduction of the turboshaft engine. Later, during the Vietnam War, the role of helicopters was expanded to include troop and cargo movement and attack missions. The expanded uses for the helicopter led to more developments that resulted in modern helicopters. AUTOGYROS Autogyros look very much like helicopters, except that they typically have only one rotor with either two or four blades. The rotor on an autogyro also has a fixed orientation relative to the fuselage, but, in contrast to that of the helicopter, the auto gyro’s rotor provides only lift. Propulsive force is provided by an auxiliary power source, such as a propeller. In addition, the rotation of the rotor is driven not by an engine, but by the air that passes through the rotor disk as it is dragged through the atmosphere by the aircraft. This behavior is similar to that of a maple seed, which spins as it falls from the tree. Because the rotor requires the aircraft’s forward motion in order to rotate, the autogyro can neither take off nor land vertically, nor can it hover. It is a short takeoff and landing (STOL) vehicle. Autogyro development began in about 1920 and was considered a viable alternative to the helicopter

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until the development of Sikorsky’s helicopter. The father of the autogyro is Juan de la Cierva of Spain. After building two unsuccessful aircraft, Cierva flew his first successful autogyro in 1923. In the United States, the Pitcairn and Kellett Aircraft Companies were principally responsible for the development of the autogyro. Operating under a license from Cierva, they cooperated with Cierva in developing his original design. Cierva used conventional aircraft controls to fly his autogyro. By 1932, control was achieved by tilting the rotor with respect to the fuselage, eliminating the need for all aircraft controls except the rudder. Also, early autogyros started their rotors by taxiing around on the ground. Later models were equipped with a geared connection to the engine. TILTING PROPROTOR AIRCRAFT The principal reason for the development of tilting proprotor aircraft was to overcome the inability of helicopters to fly at high forward speeds, while retaining the ability to hover, take off, and land vertically. A comparison of the rotors of helicopters and autogyros to those of tilting proprotor aircraft shows that the latter have much smaller diameters. Tilting proprotor aircraft also have wings and vertical and horizontal tail surfaces like those of conventional fixed-wing aircraft. Tilt-shaft/rotor aircraft were the predecessors of the modern tilt-rotor aircraft. This rotorcraft was able to take off and land vertically, as well as hover, by virtue of the fact that its rotors could pivot to provide lift for takeoff, landing, and hover, as well as propulsive force in forward flight. Stationary wings provided the required lift in forward flight. The first tilt-shaft/rotor aircraft was the Model 1-G, which was built by the Transcendental Aircraft Corporation and became the first such aircraft to successfully make the transition from hover to forward flight in December, 1954. The rotors on the Model 1-G were located at the ends of the wings, and the engine that provided power to them was located in the fuselage. The Model 2 fol-

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lowed the Model 1-G and was tested from 1956 to 1957. Bell Helicopter Company, which had been working on a similar concept since 1951, introduced the XV-3 in 1955. Two prototype aircraft were eventually built and flew many test flights. The XV-3 was similar in design to the Model 1-G, which was not surprising because aircraft engineer Robert Lichti played a major role in the design of both aircraft. The concept for the tilt-prop aircraft was similar to that of the tilt-shaft/rotor aircraft, except that the diameter of the tilt-prop rotors was smaller, like a large propeller. Only two aircraft of this type were ever built, both of which were built by Curtiss-Wright and called the X-100 and the X-19. The X-100 had two tilting props at the ends of a stationary wing. The X-19 had four tilting props, two at the ends of the main wing and two at the ends of a smaller wing at the tail of the aircraft. The tilt-wing aircraft was another variant of the tilting proprotor aircraft. As with the others, the orientation of the rotors could be changed from horizontal to vertical, so that the aircraft could both hover and fly at high forward speeds. However, on a tilt-wing aircraft, the rotors were rigidly attached to the wing, and the entire wing pivoted. The first tilt-wing aircraft was the Vertus 76 (VZ-2), which first flew in 1958. Hiller Aircraft then built and flew the X-18 in 1959. The two most successful tilt-wing aircraft, the LTV-Hiller-Ryan XC-142 and the Canadair CL-84 Dynavert, appeared in the mid-1960s. In 1964, the XC-142 became the largest vertical takeoff and landing aircraft to fly. Five prototypes were built, but all eventually crashed or were otherwise accidentally destroyed. Similar in configuration to other tilting proprotor aircraft, the tilt-rotor appears to be a fixed-wing aircraft with large propellers attached to nacelles at the tips of the wings. The engines that provide the power to turn the rotors are located in the nacelles. In 1975, Bell Helicopter Company began the development of the XV-15 tilt-rotor research aircraft.

Russian Space Program

Principles of Aeronautics

Two prototype aircraft were built, and the first successful flight with conversion to forward flight took place in 1977. Since that time, the XV-15 has performed hundreds of hours of research flight testing and flight demonstrations. The unprecedented success of the XV-15 led directly to the development of the Bell-Boeing V-22 Osprey, the first operational tilt-rotor aircraft.

space. Starting in 1945, the Soviet Union developed sophisticated scientific and technological expertise that allowed it to make significant accomplishments in space exploration. The program played an important role in extending humankind’s knowledge of outer space. The technological and scientific accomplishments of this great endeavor also had a significant impact on the international struggle known as the Cold War.

—Donald L. Kunz Further Reading Cai, Guowei, Ben M. Chen, and Tong Heng Lee. Unmanned Rotorcraft Systems. Springer, 2011. Federal Aviation Administration (FAA). Rotorcraft Flying Handbook (FAA-H-8083-21). CreateSpace Independent Publishing Platform, 2013. Federal Aviation Administration (FAA). Flying Handbook (FAA-H-8083-21b). CreateSpace Independent Publishing Platform, 2019. Johnson, Wayne. Rotorcraft Aeronautics. Cambridge UP, 2013. Masarati, Pierangelo, and Giuseppe Quaranta. Adverse Aeroelastic Rotorcraft-Pilot Couplings, from Analysis to Prevention. Springer International Publishing, 2023. Tischler, Mark Brian, and Robert K. Remple. Aircraft and Rotorcraft System Identification: Engineering Methods with Flight Test Examples. American Institute of Aeronautics and Astronautics, 2012. See also: Aeronautical engineering; Airfoils; Autogyros; Leonardo da Vinci; First flights of note; Flying wing; Forces of flight; Helicopters; Igor Sikorsky

Russian Space Program Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT The Russian space program was one of two successful attempts in the mid-twentieth century to travel into outer

RUSSIAN SPACE ASPIRATIONS Russia has had a long and significant role in the history of space exploration. Most historians of science designate Konstantin Tsiolkovsky as the father of modern spaceflight. In the first decade of the twentieth century, Tsiolkovsky produced a groundbreaking theoretical study on the possibilities of traveling in space. The essay, “Issledovanie mirovykh prostanstv reaktivnymi priborami” (1903; “Exploration of Cosmic Space with Reactive Devices”), published in the journal Naootchnoye Obozreniye (scientific journal), described the methods to be employed to develop vehicles that would carry human beings into outer space. Tsiolkovsky was both a technological visionary and a social utopian. He perceived spaceflight as the instrument to free humankind from the drudgery of earthly existence. He viewed the power to conquer the law of gravity as a metaphor for the human race’s ability to liberate itself by embarking on a new historical epoch of limitless possibilities. The connection among science, technology, and political and social philosophy within Russian culture played an important role in the development of Soviet technological policy. Unfortunately, Tsiolkovsky’s ideas were constrained by the autocratic regime of Czar Nicholas II and the economic, political, and social instability it fostered. This cultural turmoil led to Russia’s disastrous defeat in World War I and the subsequent Bolshevik Revolution. The modern history of Russian spaceflight begins in this politically explosive

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The Hall of Space Technology in the Tsiolkovsky State Museum of the History of Cosmonautics, Kaluga, Russia. The exhibition includes the models and replicas of the following Russian/Soviet inventions: the first satellite, Sputnik 1 (a ball under the ceiling); the first spacesuits (lower-left corner); the first human spaceflight module, the Vostok 3KA (center); the first Molniya-type satellite (upper right corner); the first space rover, Lunokhod 1 (lower right); the first space station, Salyut 1 (left); the first modular space station, Mir (upper left). Photo via Wikimedia Commons. [Public domain.]

era. From the ascension of Lenin to the construction of the Soviet space station Mir, the Russian space program would be linked to and directed by changes in the accepted political doctrine of Communist totalitarianism. EARLY COMMUNISM AND SPACE THEORY The intellectual foundation of Communism was laid on the philosophy of Karl Marx, who did not consider himself a political philosopher in the clas-

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sical sense but insisted that his ideas were based upon scientific principles. Technology would be the instrument used to establish Marx’s new utopian society. In 1917, Vladimir Ilyich Lenin, the leader of the Communist Party, accepted these ideas in the abstract, but the practical problems of reconstructing a war-torn nation drove Lenin to compromise his adherence to strict Marxist-Leninist theory in favor of economic recovery. Lenin’s famous statement, “Electrification plus Soviet power equals

Principles of Aeronautics

socialism,” set the tone for his national recovery program. From this emphasis on science and technology, a technological elite developed whose expertise was used to create a new socialist order. Many of these technologists were influenced by the works of Tsiolkovsky, especially by his utopian vision based upon space travel. The Soviet scientific community during the 1920s adopted a research and development program focusing upon the possibilities of space exploration, and two influential works were published during this decade. Yuri Kondratyuk’s book Zavovevanie mezhplanetnykh prostorov (1929; The Conquest of Interplanetary Space, 1997) and Nikolai A. Rynin’s work Mezhplanetye Soobschchicheniia (1927-32; Interplanetary Flight and Communication, 1970-71) had a significant impact on the technologists around the world who were working on the possibilities of spaceflight. TECHNOLOGY UNDER STALIN Soviet society drastically changed with the death of Lenin and the ascension of Joseph Stalin to power. In Stalin’s purges, technological expertise became secondary to ideological purity, and he launched a nationwide attack against the “elite experts”; many of them suffered the same fate as their military and political counterparts. Stalin’s concentration on making socialism safe in Russia had an important impact on Soviet space research. The utopian vision of a socialist cosmos was declared unimportant at a time when the Soviet Union needed to construct a competitive industrial and military sector in order to protect its borders from both its fascist and democratic rivals. The aeronautical expertise that had been focused on spaceflight during the 1920s was now directed toward the construction of a world-class air force. During the 1930s, the Soviet Union made great strides in aeronautical engineering, generating a confidence among Russia’s military leadership that its air

Russian Space Program

force was among the best in the world. This optimism was shown to be unfounded when the German Luftwaffe soundly defeated the Soviet Air Force during the Spanish Civil War. Stalin reacted with reprisals against the Russian aeronautical engineering establishment. Many of the Soviet Union’s finest rocket scientists were sent to the gulag (a series of camps for political prisoners) and released only after the German invasion of 1941. Among these prisoners was Sergei Korolev, who became the driving force behind the postwar Soviet space program, working on the development of military rockets for the defense of the Soviet Union. WORLD WAR II AND THE EARLY COLD WAR Two major scientific developments of World War II had a lasting impact on the Russian space program. In the last months of the war in the European theater of operations, the Nazis attempted to change the strategic direction of the conflict by introducing a new super weapon, the V-2 rocket. The German industrial sector was too damaged to mass-produce this weapon in the numbers needed to change the outcome of the war, but all of the Allied nations, including the Soviet Union, recognized the potential of this revolutionary new delivery system. The Russians expended considerable resources and energy to capture as many German rocket scientists as possible. The new technology became even more important after the United States successfully used two atomic bombs to force the Japanese to surrender in August, 1945. The breakdown of the wartime alliance due to Soviet expansion in Eastern Europe brought on the Cold War. Once again, Stalin focused upon the defense of the “Motherland,” but this time he accepted the connection between rocket science and the protection of the Soviet Union. A new generation of Soviet rockets was produced through the combined efforts of German and Russian scientists. With the

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successful detonation of an atomic bomb in 1949 and a hydrogen bomb in 1953, the Soviets accelerated their research in an attempt to create an accurate, uncrewed delivery system for these new weapons of mass destruction. After Stalin’s death in 1953, the direction of Russian rocket technology once again focused on space travel. THE SPUTNIK ERA The Khrushchev era catapulted the Soviet Union into a position of prominence in the area of space exploration. Nikita Khrushchev was a true intellectual child of Marxist-Leninist thought and believed in the compatibility of socialism and scientific truth. Like Tsiolkovsky, he envisioned a utopian state that would reap the benefits of increased productivity based upon science and technology. He extended this idea of universal brotherhood to the entire universe when the Soviet Union successfully launched Sputnik, the first artificial satellite, on October 4, 1957. Khrushchev believed this great scientific and technological accomplishment confirmed both the power of Russian science and the inevitability of Communism because it showed that the Communist system had created the conditions and the environment for great scientific advancement. Sputnik had an impact on Khrushchev’s foreign policy that went far beyond the technological strategic implications of United States-Soviet relations. This dramatic event also captured the attention of the newly independent nations of Africa, Asia, and the Middle East. An important aspect of the Cold War was the struggle between the democratic and Communist camps to win the allegiance of this important segment of the world community. When Sputnik went into orbit, most of the leading nations of the Third World issued press communiqués praising the achievements of the Soviet scientific community. Many seemed convinced that the socialist model, based upon the universal ideal of a one-world community sharing equally the benefits of

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human knowledge, was responsible for such great accomplishments. Khrushchev also used the image of Soviet scientific prowess to challenge the theory that war was inevitable between the capitalist and Communist nations. Russia’s seeming ability to accurately target the United States helped to create the reality of mutual assured destruction, which Khrushchev believed would reduce the likelihood of a third world war. Khrushchev’s confidence in this new strategic doctrine established a sense of security among the nations of Western Europe that bordered the Soviet Empire, and it upset an already strained relationship between the Soviet Union and the ultraradical People’s Republic of China. Mao Zedong embraced the Leninist doctrine that power would have to be taken from the capitalist nations through the use of force. As a result of Sputnik, the Chinese believed that the Soviet Union had the ability to bring down the capitalist West. Mao was not deterred by the possibility of widespread death and destruction. He believed a new socialist order would rise from the dust and inaugurate a utopian epoch. He had no concept of the fact that the dust of the old civilization would contain deadly levels of radiation with a half-life of ten thousand years. Khrushchev refused to adopt Mao’s radical strategy, an attitude that helped create the Sino-Soviet split. The success of the Russian space program also caused considerable tension between Khrushchev and the Soviet military establishment. Khrushchev believed that a new strategic doctrine that reflected recent accomplishments in space technology was necessary if the Soviet Union was to reach the ultimate economic goal of universal material prosperity. Khrushchev desperately wanted to reduce the size of the military in order to redirect money and resources into the domestic economy. He created a Seven-Year Plan that proposed increasing both agricultural and industrial output. The military perceived these cuts as unwarranted and dangerous,

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and it vigorously opposed his plan. At the same time that the Soviet leader proposed massive cuts in conventional forces, he approved a large budget for important research into the development of spy satellites. Khrushchev knew the United States was far more advanced in this field; he recognized that if the Soviet Union hoped to maintain some sort of military parity, significant progress would have to be made in this all-important area. This action exacerbated his problems with the military, which recognized that introducing this new technology could also mean a further reduction in the military budget. THE SPACE RACE Khrushchev’s plan to reduce both world tensions and the size of the Russian military rested upon the image of Soviet scientific and technological superiority. A potentially dangerous aspect of this situation was the absolute importance of staying one step ahead of the accomplishments of the United States. On November 4, 1957, the Soviet Union launched Sputnik 2; this spacecraft carried Laika, a Russian dog that was the first living creature to be placed into orbit. These successes set the stage for the greatest era of human space exploration. Russia’s first crewed project, Vostok, had to reflect both Soviet scientific strength and the proposed egalitarian nature of the Communist system. Yuri Gagarin had all the attributes necessary for this space spectacular. He was a highly intelligent, handsome test pilot from one of Russia’s elite units. Politically, Gagarin was made to order. He was born in the Russian hinterland, grew up in a log cabin, and was the son of a poor artisan. The success of his magnificent flight on April 12, 1961, seemed once again to validate the inherent strength of the Soviet system. The Russian space program soon scored another propaganda victory on June 16, 1963, by launching the first woman into space, and like Gagarin, she fit the Marxist model perfectly. Valentina Tereshkova

Russian Space Program

was a simple factory worker whose lack of scientific training and expertise would be emphasized to show once again the power of Soviet science. Soviet propaganda would describe how the innate strength of the socialist model based upon the power of technology would one day create a utopian society. When intelligence reached the Soviet Union that the United States was planning to launch two astronauts into space, Khrushchev reacted by pressuring Sergei Korolev to strike first by launching a capsule containing three men. The Russian space program had already started to develop plans for a vessel that could carry more than one cosmonaut. Initially the program was designated Soyuz, but in 1961 it was only in the earliest stages of development. To meet the deadline set by Khrushchev, the Russians had to modify the Vostok capsule at great risk to the three cosmonauts. All but essential equipment was removed, and they had to fly without the protection of their outer spacesuits as well, in order for three men to fit inside what was supposed to be a one-person vehicle. On October 12, 1964, Voskhod 1 was launched and placed into orbit. It returned the three cosmonauts safely to earth in what was perceived to be the next example of Soviet dominance of outer space. On March 18, 1965, the crew of Voskhod 2 again impressed the world when Aleksei Leonov made the first spacewalk, remaining outside his capsule for twelve minutes while orbiting 128 miles above the surface of Earth. THE SOVIET MOON PROGRAM Sergei Korolev had developed a plan to land cosmonauts on the lunar surface that consisted of three major stages. The Vostok and Soyuz programs were to provide the Soviets with the necessary experience and information concerning both the effect of spaceflight on human beings and the skills needed to successfully complete a sophisticated lunar mission. This would be followed by a program designated

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Luna, which would consist of a series of reconnaissance missions to familiarize the cosmonauts with the surface of the moon. Finally, the N-Program would be the Russian equivalent of the American Apollo Program, which would transport three cosmonauts to the moon. Two important events occurred in the mid-1960s that would forever change the direction of the Soviet lunar program. On January 14, 1966, Sergei Korolev died of complications resulting from his years as a prisoner in Stalin’s gulag. Korolev’s great intelligence, formidable power, and universal respect among Russia’s scientific elite had enabled him to push his fellow space scientists to achieve at levels unmatched by any other members of the space establishment. The problems that resulted from his death were compounded by the political demise of Nikita Khrushchev. In pursuit of his new socialist order, Khrushchev had alienated too many powerful interest groups, especially the Soviet military. When widespread agricultural and industrial failure was combined with the military and political embarrassment of the Cuban Missile Crisis, Khrushchev was removed from office. Khrushchev was replaced by Leonid Brezhnev, a Stalinist hardliner whose political philosophy was far more practical than that of Khrushchev. He inherited a very inefficient economy that already had to balance the military expenditures of the world’s largest army with the growing consumer expectations of Soviet society. Brezhnev’s strategic view differed significantly from that of Khrushchev. He believed that if the Soviet Union continued an extensive military buildup, the United States by the early 1980s would find it necessary to begin to accommodate to Russian international demands. THE SOVIET SPACE PROGRAM IN DECLINE On September 12, 1970, after the success of the Apollo Program, the Russians attempted to salvage

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some international respect by landing an uncrewed vehicle on the lunar surface. Luna 16 extracted soil samples to be studied back on Earth. A second moon mission on November 17, 1970, saw a Soviet Lunokhod lunar rover explore the surface of the Moon. However, these two missions actually reflected the underlying weakness of the Russian space program. In the 1980s, the United States established its clear supremacy in outer space. The year 1981 saw the successful flight of the space shuttle that displayed a level of space technology decades beyond the capabilities of the Soviet Union. The Soviet Union attempted to maintain some respectability by concentrating its resources on an extensive space station program. Instead of competing against the United States in the arena of space travel, the Soviets decided to focus on creating a permanent working environment that would provide space-based laboratories for scientific research. Soviet premier Mikhail Gorbachev attempted to institute a series of reforms that would revitalize the Soviet economy and provide an economic foundation for the development of a new generation of technology that would allow the Soviet Union to compete in space once again with the United States. Instead of reinforcing the Communist system, glasnost and perestroika set in motion a chain of events that brought down the Soviet Union. Initially there was great optimism about a future democratic Russia operating within a structure where both material goods and ideas flowed freely. Unfortunately, this dream was not realized, and Russia fell into economic and political chaos. In 1996, the new Russia ranked eighteenth out of the top twenty nations in expenditures on space technology. By the turn of the century, a series of disasters ravaged the space station Mir and in the end turned the broken spacecraft into a metaphor for the collapse of the Russian space program.

Burt Rutan

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THE REBIRTH OF RUSSIAN SPACE ASPIRATIONS In the twenty-first century, Russia’s space program is alive and well, with its permanent space station having developed into the International Space Station (ISS). The ISS has been augmented with modules from other nations and has been host to numerous cooperative ventures with astronauts from several different nations including the United States, Japan, China, France, and India. Following the end of the United States’ space shuttle program, Russian rockets lifting off from Baikonur Cosmodrome became the United States’ primary means of sending astronauts to the ISS. The Russian space program also includes plans and explorations for the establishment of permanent stations or colonies on both the Moon and Mars. —Richard D. Fitzgerald Further Reading Burgess, Colin. Soviets in Space: Russia’s Cosmonauts and the Space Frontier. Reaktion Books, 2022. Gerovitch, Slava. Voices of the Soviet Space Program: Cosmonauts, Soldiers, and Engineers Who Took the USSR into Space. Palgrave Macmillan, 2014. Harvey, Brian. The Rebirth of the Russian Space Program: 50 Years After Sputnik, New Frontiers. Springer, 2007. ———. European-Russian Space Cooperation from De Gaulle to ExoMars. Springer International Publishing, 2021. Reichl. Eugen. The Soviet Space Program: The Lunar Mission Years: 1959-1976. Schiffer Military, 2019. Zak, Anatoly. Russia in Space: The Past Explained, the Future Explored. Griffin Media, 2014. See also: Yuri Gagarin; Rocket propulsion; Rockets; Valentina Tereshkova; Konstantin Tsiolkovsky

Burt Rutan Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics

ABSTRACT Burt Rutan was born June 17, 1943, in Portland, Oregon. He is the best known, most creative, most prolific, and most influential late-twentieth-century aircraft designer, having designed and built at least forty new and revolutionary types of aircraft designs, as well as other machines and devices. KEY CONCEPTS canard: an aircraft design having small stabilizing wings near the front of the airplane, well ahead of the main wings composites: materials that are themselves constructed from two or more different materials such as sheets of woven fiberglass and a thermosetting plastic resin; advanced composites use more exotic materials such as polyamide fibers (Kevlar) and carbon fibers homebuilt: typically assembled by the purchaser of a kit of ready-to-assemble parts rather than in an aircraft manufacturing facility THE GENIUS OF BURT RUTAN Born into an airplane-involved family, Elbert Leander “Burt” Rutan began to design and build award-winning model airplanes while still a teenager. He made his first solo flight at age sixteen, and his ability to look at aircraft design from a pilot’s viewpoint has been an important factor in the success of his many airplane designs. Rutan revolutionized aircraft design with his tail-first, canard airplanes and his all-composite homebuilt and commercial aircraft. His best-known design, the Voyager, was the first aircraft to fly around the world without refueling, in December 1986. Mostly through his Scaled Composites firm, he has designed forty new types of aircraft as well as a catamaran, a space-load launcher, a gondola, and a car body. His futuristic-looking prototypes have been used in a number of Hollywood motion pictures.

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In 1965, Rutan received a BS in aeronautical engineering from California Polytechnic University, where his thesis won a national award from the American Institute of Aeronautics and Astronautics. After graduating from college, he took a job as a civilian flight test project engineer at the Air Force Flight Test Center at Edwards Air Force Base, California, and began working on his first homebuilt, the VariViggen, inspired by the canard XB-70 bomber and the canard Saab Viggen fighter. In 1972, Rutan left the Air Force to work in development and flight testing for a homebuilt kit manufacturer. Two years later, in June 1974, he established the Rutan Aircraft Factory to develop and sell

Burt Rutan. Photo by D Ramey Logan, via Wikimedia Commons.

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homebuilt aircraft plans. Rutan’s second homebuilt design, the VariEze, introduced in 1975, was a very efficient canard homebuilt that revolutionized homebuilding. The VariEze’s moldless composite construction of fiberglass-covered foam did not require specialized skills or tools to build and produced smooth, sculpted surfaces. The longer-range follow-up, the Long-EZ, set many distance records, including for around-the-world flights, and remains one of the most popular homebuilt aircraft. A powered glider, the Solitaire, and a push-pull, twin-engine canard, the Defiant, were his last designs for homebuilders. In April 1982, Rutan founded the Scaled Composites firm to develop research prototypes for government and industry. Scaled Composites has produced such well-publicized aircraft as the Voyager, the Pond Racer, the AD-1 skew-wing aircraft for the National Aeronautics and Space Administration (NASA), the Beechcraft Starship prototype, the Advanced Technology Tactical Transport, the Triumph business jet, the Ares close air support airplane, the Proteus high-altitude aircraft, and the asymmetrical twin-engine Boomerang. The firm competed in the first private race to space, the $10 million Ansari X Prize: a race to develop a practical, reasonably inexpensive, reusable flight vehicle for short flights out of the atmosphere for future space tourists. In 2004 the firm won the prize with Rutan’s SpaceShipOne, the world’s first private spaceship. Virgin Galactic commissioned Rutan to create a version of the spaceship for tourist flights, starting in 2005. By the time Rutan retired from Scaled Composites in 2011, he had developed thirty-eight piloted craft, half a dozen unpiloted craft, and other vehicles. In 2015 he worked on developing the SkiGull, an amphibious plane that runs on regular gasoline and can land on ocean swells, grass fields, snow, beaches, and other unimproved terrain. With a projected range of 2,500 miles between refuelings and

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no need to stop at airports, Rutan eventually hopes to fly with his wife to remote places around the world. Over the course of his career, Rutan has received many awards, including Outstanding Design Awards from the Experimental Aircraft Association, the Presidential Citizen’s Medal, the Collier Trophy, the Chrysler Award for Innovation in Design, and the British Gold Medal for Aeronautics. In 1995, he was inducted into the National Aviation Hall of Fame. In March 2012 he received the lifetime achievement trophy from the National Air and Space Museum. —W. N. Hubin Further Reading Adams, Eric. “X Prize Victory, Redemption.” Popular Science, 4 Oct. 2004. Accessed 29 Mar. 2016. Alef, Daniel. Burt Rutan: Aeronautical and Space Legend. Titans of Fortune Publishing, 2016. Atherton, Kelsey D. “Legendary Plane Designer Burt Rutan Tests Weird Seaplane.” Popular Science, 24 Nov. 2015. Accessed 29 Mar. 2016.

Downie, Don, and Julia Downie. The Complete Guide to Rutan Aircraft. 3rd ed., Tab, 1987. Grady, Mary. “A Legendary Airplane Designer Hints at His Next Creation.” Wired, 12 Mar. 2015. Accessed 29 Mar. 2016. Lennon, Andy. Canard: A Revolution in Flight. Aviation, 1984. Linchan, Dan. Burt Rutan’s Race to Space: The Magician of Mojave and His Flying Innovations. Zenith Press, 2011. Rollo, Vera A. Foster. Burt Rutan: Reinventing the Airplane. Maryland Historical, 1991. Seedhouse, Erik. Virgin Galactic: The First Ten Years. Springer International Publishing, 2015. Yeager, Jeana, and Dick Rutan, with Phil Patton. Voyager. Alfred A. Knopf, 1987. See also: Advanced composite materials in aeronautics; Aerodynamics and flight; Aeronautical engineering; Aerospace industry in the United States; Richard Branson; Glenn H. Curtiss; Steve Fossett; Homebuilt and experimental aircraft; Howard R. Hughes; Otto Lilienthal; Ernst Mach; Igor Sikorsky; Spacecraft engineering; Konstantin Tsiolkovsky; Andrei NikolayevichTupolev; Wright brothers’ first flight

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S Scramjet Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT A scramjet (supersonic combustion ramjet) is an air-breathing jet engine that relies on the compression of air taken in at supersonic speeds for propulsion. The scramjet relies on the speed of the aircraft itself to forcefully compress the air it takes in before combustion. Although development of the scramjet began as early as the 1950s and 1960s, the first successful flight of a scramjet vehicle did not take place until 2002. Researchers continue working on the scramjet in hopes of one day turning it into a practical technology. KEY CONCEPTS combustion chamber: a structure as the second stage of a jet engine within which the fuel-air mixture is combusted, producing rapidly expanding hot exhaust gases compressor: a rotating structure at the first stage of a jet engine that compresses air taken in sound barrier: the speed of sound, once thought to be a speed too difficult for aircraft to attain (a barrier) supersonic: at speeds greater than the speed of sound (Mach 1 to Mach 3.5) turbine: a structure as the third stage of a jet engine that is driven by the high-speed gases flowing past from the combustion chamber, that in turn powers the first-stage compressor RAMJETS AND SCRAMJETS A scramjet (supersonic combustion ramjet) is an air-breathing jet engine that relies on the compres-

sion of air taken in at supersonic speeds for propulsion. Scramjets are a variant of the standard ramjet air-breathing engine. The key difference between the two is that while the ramjet engine decelerates the air to subsonic velocities prior to combustion, the scramjet relies on the speed of the aircraft itself to forcefully compress the air it takes in before combustion. This means that the airflow within the engine in a scramjet is supersonic at all times. The unique nature of the scramjet is reflected in the term “scramjet” itself, which is an abbreviation of “supersonic combustion ramjet.” Supersonic refers to speeds greater than that of sound. Although development of the scramjet began as early as the 1950s and 1960s, the first successful flight of a scramjet vehicle did not take place until 2002. Researchers continue working on the scramjet in hopes of one day turning it into a practical technology. BACKGROUND Scientists and engineers have been pursuing the possibility of supersonic flight since the early twentieth century. The idea of the sound barrier, an imaginary boundary at the speed of sound that seemingly represented the maximum rate of speed at which an aircraft could travel, was first suggested in 1935. While it had long since been proven that bullets and other projectiles could exceed the speed of sound, no one yet knew whether an aircraft could survive the pressures involved in supersonic flight. In the 1940s, the US Air Force decided to find out. Its efforts eventually led to the creation of the Bell X-1, an experimental aircraft designed to fly at supersonic speeds and absorb eighteen times the force of

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Artist’s conception of the NASA X-43 with scramjet attached to the underside. Image via Wikimedia Commons. [Public domain.]

gravity. The X-1, which was dropped in-flight from the belly of a B-29 Superfortress instead of taking off from the ground, was flown by famed test pilot Chuck Yeager out of what is now Edwards Air Force Base in southern California. After slowly inching closer to the speed of sound in a series of preliminary flights, Yeager officially broke the sound barrier for the first time on October 14, 1947. In the years that followed Yeager’s historic flight, scientists worked steadily to find a way to make supersonic flight a practical reality. As it happened, this effort coincided with an already ongoing push to create a working ramjet supersonic engine. The concept of a ramjet engine was first proposed by French aerospace engineer René Lorin in 1913. While at least two engineers were issued patents for ramjet designs early in the century, no attempt was

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made to actually build a ramjet-propelled aircraft until a French company called Nord-Aviation began work on such a project in 1953. That project led to the creation of the Griffon II, an aircraft equipped with a combination turboramjet engine. The Griffon II set a world speed record of approximately 1,640 kilometers per hour on February 24, 1959. Meanwhile, the first experimental scramjet engines were built between the mid-1950s and the late 1960s. Still, it took decades of work for a scramjet-powered aircraft to take flight. In fact, the first successful test flight of a scramjet vehicle did not take place until August 16, 2002. OVERVIEW At the most basic level, a scramjet is a type of jet engine. What makes a scramjet unique is how it differs

Principles of Aeronautics

from traditional jet and ramjet engines. Traditional jet engines are combustion engines that generate thrust through jet propulsion. A typical jet engine is composed of a compressor, a combustion chamber, and a turbine. All three parts are mounted on a shaft that runs the length of the engine. Located at the front of the jet engine and consisting of a series of blades that rotate at high speed, the compressor serves to compress incoming air. In the combustion chamber, fuel is sprayed into the compressed air and the resulting mixture is ignited by a spark from a spark plug. This causes the resulting mixture of combustion exhaust gases to expand spontaneously to create a material referred to as a jet. The reaction force that results from the jet attempting to blast out of the nozzle generates the necessary thrust to move the aircraft forward. As the jet makes its way out of the engine, it passes through and turns the turbine blades. The energy generated by this action also turns the compressor at the front of the engine. Although a traditional jet engine is very effective, its heavy components add a great deal of weight to the vehicle to which it is attached. In addition, jet engines have many moving parts that can break down and cause a catastrophic failure at any time. Ramjets and scramjets are specifically designed to overcome these very problems. Ramjets and scramjets are alternative types of jet engines that do away with rotary compressors and turbines. Instead, they use the ramming of air to produce thrust. In short, ramjets and scramjets are designed to convert the kinetic energy of incoming supersonic or hypersonic air (more than five times the speed of sound) into pressure energy. Air that flows at such high speeds also has high dynamic pressure. Because the dynamic pressure of air moving at subsonic speeds is too low to produce useful thrust, ramjets and scramjets are unable to provide the necessary thrust for an aircraft to take off. As such, they can only operate effectively when the surrounding air is already moving at supersonic speeds.

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Ramjet and scramjet engines consist of a diffuser and a propelling nozzle. The diffuser reduces the velocity and increases the pressure and temperature of incoming air. As in a traditional jet engine, fuel is added in the combustion chamber and the mix is ignited by a spark plug. The nozzle subsequently decreases the pressure and increases the velocity of the exhaust jet to create thrust. The difference between ramjets and scramjets is simple. In ramjets, incoming air is slowed to subsonic speeds before undergoing combustion. In scramjets, the combustion chamber is modified so as to be able to operate using air flowing at supersonic speeds, and the propelling nozzle is designed to further accelerate the exhaust jet. All of this allows scramjets to generate more thrust than ramjets. —Jack Lasky Further Reading Choubey, Gantam, and Manvendr Tinari. Scramjet Combustion: Fundamentals and Advances. Elsevier Science, 2022. Czysz, Paul A., Claudio Bruno, and Bernd Chudoba. Future Spacecraft Propulsion Systems and Integration: Enabling Technologies for Space Exploration. Springer Berlin Heidelberg, 2017. “Explainer: What Is a Scramjet?” SBS News, 26 Aug. 2013, www.sbs.com.au/news/explainer-what-is-a-scramjet. Accessed 6 Jan. 2020. Gerdroadbary, Mostafa Barzegar. Scramjets: Fuel Mixing and Injection Systems. Elsevier Science, 2022. Musielak, Dora. Scramjet Propulsion: A Practical Introduction. Wiley, 2022. Nardi, Tom. “Scramjet Engines on the Long Road to Mach 5.” Hackaday, 16 Jan. 2019, hackaday.com/2019/ 01/16/scramjet-engines-on-the-long-road-to-mach-5. Accessed 6 Jan. 2020. Patel, Piyush. “What Is a Scramjet?” Science ABC, 12 Oct. 2019, www.scienceabc.com/innovation/what-is-ascramjet-engine.html. Accessed 6 Jan. 2020. Redd, Nola Taylor. “Breaking the Sound Barrier: The Greatest Moments in Flight.” Space.com, 30 Sept. 2017, www.space.com/16709-breaking-the-sound-barrier.html. Accessed 6 Jan. 2020.

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Segal, Corin. The Scramjet Engine: Processes and Characteristics. Cambridge UP, 2009. Sheedy, Chris. “The Scramjet Is a Super-Fast, Experimental Engine with No Moving Parts.” Create, 1 Nov. 2019, www.createdigital.org.au/scramjet-superfast-experimental-engine-no-moving-parts. Accessed 6 Jan. 2020. “What’s a Scramjet?” NASA, 30 Jan. 2004, www.nasa.gov/ missions/research/f_scramjets.html. Accessed 6 Jan. 2020. See also: Advanced propulsion; Aerodynamics and flight; Aeronautical engineering; Fluid dynamics; High-speed flight; Mach number; National Aeronautics and Space Administration (NASA); Propulsion technologies; Ramjets; Sound barrier; Chuck Yeager

Alan Shepard Fields of Study: Aerospace history; Astronautics

Principles of Aeronautics

Academy in Toms River, New Jersey, prior to his acceptance into the United States Naval Academy. Shepard pursued flight training after World War II, earning his wings in 1947. Completing US Naval Test Pilot School training in 1950, he remained at the school, participating in high-altitude research, flight operations development for a naval in-flight refueling system, F2-H3 Banshee testing for carrier deployment, and angled carrier-deck development. After two tours of duty in the Pacific aboard the USS Oriskany, Shepard returned to the Naval Test Pilot School to fly F-3H Demon, F-8U Crusader, F-4D Skyray, F-11F Tiger, and F-5D Skylancer aircraft. Shepard achieved instructor status there, but five months later entered the Naval War College in Newport, Rhode Island, graduating in 1958. After the National Aeronautics and Space Administration (NASA) began screening military files for potential astronauts, Shepard’s test-pilot career sin-

ABSTRACT Alan Shepard was born on November 18, 1923, in East Derry, New Hampshire. He died on July 21, 1998, in Monterey, California. He is remembered as the first American astronaut to fly in space. Shepard flew the first US-manned space flight in 1961 and became the only Mercury astronaut to walk on the Moon. KEY CONCEPTS astronaut: personnel specially trained to undertake missions in space, the word translates to “star sailor” test pilot: a pilot whose job is to fly newly developed aircraft to determine their performance and capabilities THE MAKING OF AN ASTRONAUT Alan Bartlett Shepard Jr. was born to Colonel Alan B. Shepard and his wife in East Derry, New Hampshire. He graduated from Pinkerton Academy in Derry and spent a year studying at Admiral Farragut

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Alan Shepard. Photo via Wikimedia Commons. [Public domain.]

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gled him out as a prime candidate. In April, 1959, Shepard was one of the seven astronauts selected for Project Mercury. The Mercury astronauts worked cooperatively on all aspects of Project Mercury but competed for flight assignments. In 1960, NASA picked Shepard, John Glenn, and Virgil “Gus” Grissom to train for a suborbital Mercury-Redstone mission. Although the Soviets had beaten the Americans into space by launching Yuri Gagarin into orbit on April 12, 1961, NASA moved forward with Project Mercury. Shepard lifted off on May 5, 1961, inside his Freedom 7 spacecraft strapped atop a Redstone missile. Shepard was exposed to high g-forces and five minutes of weightlessness. The spacecraft achieved a 116-mile altitude before splashing down in the Atlantic 302 miles from Cape Canaveral. Three years later, Shepard was grounded by Meniere’s syndrome, an inner-ear condition capable of inducing nausea, ringing ears, and vestibular disturbances. He was reassigned to Astronaut Office management. In 1969, Shepard underwent surgery that corrected his condition, and, within six months, he had gained command of Apollo 14. The Apollo 14 mission launched on January 31, 1971, with Shepard, Stuart Roosa, and Edgar Mitchell. Shepard and Mitchell touched down Lunar Module Antares in the Fra Mauro region, deployed scientific instruments on the lunar surface, and collected samples during two moonwalks. Shepard hit two golf balls before leaving the surface. After thirty-three hours, Shepard and Mitchell lifted off the Moon to rejoin Roosa. Apollo 14 splashed down in the Pacific on February 9, 1971. In 1974, Shepard retired from the Navy and NASA. He enjoyed subsequent success in the business world, helping found the Astronaut Scholarship Foundation. Diagnosed with leukemia in 1997, Shepard valiantly fought the disease and expected to join the other surviving Mercury astronauts to witness their colleague Glenn fly aboard space shuttle

Discovery in late 1998. However, Shepard’s condition worsened, and Shepard died on July 21, 1998, three months before Glenn’s mission. —David G. Fisher Further Reading Benge, Janet, and Geoff Benge. Alan Shepard: Higher and Faster. Emerald Books, 2007. Burgess, Colin. Freedom 7: The Historic Flight of Alan Shepard, Jr. Springer International Publishing, 2013. Carpenter, Scott M., et. al. We Seven. Simon & Schuster, 1962. Hamilton, John. Project Mercury. Abdo Publishing, 2018. Koli, Monica. 7 Astronauts Who Changed the World. Prabhat Prakhashan, 2021. Milkyway Media. Summary of Alan Shepard and Deke Slayton’s “Moon Shot.” Milkyway Media, 2022. Shepard, Alan, and Deke Slayton, with Jay Barbree and Howard Benedict. Moon Shot: The Inside Story of America’s Race to the Moon. Open Road Media, 2011. Thompson, Neal. Light This Candle: The Life and Times of Alan Shepard. Crown, 2007. See also: Neil Armstrong; Yuri Gagarin; John Glenn; National Aeronautics and Space Administration (NASA); Russian space program; Valentina Tereshkova; Chuck Yeager

Shock Waves Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics; Mathematics ABSTRACT A shock wave is a very sharp, thin, compression wave front generated when chemical, nuclear, electrical, or mechanical energy is suddenly released or deposited in a gas, liquid, or solid occupying a limited space. Shock waves travel at supersonic speeds, and as they propagate, they raise the pressure, density, and temperature of the medium in which they travel. These changes to the medium often have annoying or destructive results.

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KEY CONCEPTS Doppler effect: a change in the observed frequency of electromagnetic or sound waves, caused by relative motion between the source and the observer Hugoniot elastic limit: the greatest stress that can be developed in a material without permanent deformation remaining when the stress is released Mach number: the ratio of the velocity of an object to the speed of sound in the surrounding medium particle velocity: the velocity of a small area of the medium (smaller than a wavelength but larger than an atom) that is alternately accelerated and decelerated as a wave passes rarefaction: the process by which high pressure in a shock wave is relieved by the propagation of release waves from free surfaces into the shocked material shock front: a supersonic shock pulse abrupt to the point where it is often represented as a discontinuous jump of pressure, density, internal energy, and particle velocity shock metamorphism: a term used to describe changes in rocks and minerals resulting from the passage of transient, high-pressure shock waves sonic boom: a loud transient explosive sound caused by a shock wave preceding an object traveling at supersonic speeds sound barrier: the point of sharp increase in aerodynamic drag experienced by an object approaching the speed of sound OVERVIEW When a source of sound moves at subsonic speeds, the frequency of the sound is altered by the Doppler effect. If a source of sound moves faster than the speed of sound, a shock wave occurs. When a shock wave is formed, the source producing it is actually “outrunning” the waves it creates. When the source is traveling at the speed of sound, the waves it emits in the forward direction “pile up” directly in front of it. When the source moves at supersonic speed, the

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waves pile up on one another. The different wave crests overlap one another and form a single large crest, which is the shock wave. A shock wave is essentially the result of constructive interference of a large number of waves. A shock wave in air is analogous to the bow wave of a boat traveling faster than the speed of the water waves it produces. A shock wave, then, is a disturbance moving through a medium at supersonic speed accompanied by an extremely rapid rise in pressure, density, and temperature. Shock waves arise from a sudden release of chemical, nuclear, electrical, or mechanical energy in a limited space. Such waves can be generated by the detonation of high explosives (chemical energy), the passage of an aircraft traveling at supersonic speeds (mechanical energy), or the discharge of lightning bolts (electrical energy). Unsustained shock waves lose energy through viscous dissipation and are reduced to sound waves. Examples of such sound waves include thunder generated by lightning and sonic booms generated by jet aircraft. A shock wave is a specialized type of stress wave. The most familiar type of stress wave is the pressure, or sound wave. It is a progressive wave that travels through homogeneous fluids (liquids or gases) at a constant speed. The amplitude of the wave can be described as a pressure fluctuation or as a variation in particle velocity. In pressure waves, the particle velocity is much slower than the propagation velocity of the wave, and the direction of the particle velocity is longitudinal, that is, parallel to the direction of propagation. A key aspect of pressure waves is that they generate no shear stress (stress that causes deformation without any change in volume) or differential stress (a condition in which the stress components are not the same in every direction). In homogeneous solids, there are two principal types of stress waves: longitudinal and transverse. While longitudinal waves move in the same direction as the direction of energy transfer, transverse waves

Principles of Aeronautics

travel in a direction perpendicular to the direction of propagation. Transverse waves arise because solids resist compression, distortion, and change in their shape. Transverse waves travel more slowly than longitudinal waves. Because the speed of transverse waves is less than that of longitudinal waves, the magnitude of stress (force per unit of area) in a transverse wave is less than in a longitudinal wave. Stresses generated by transverse waves are pure shear; thus, the strength of a transverse wave is limited by the shear strength of the medium through which it is traveling, whereas the strength of longitudinal waves has no limit. As liquids have no shear strength, and transverse waves are pure shear, transverse waves do not propagate through fluids. Another type of stress wave is the elastic wave. Elastic waves are more complex than pressure waves because, unlike gases or liquids, elastic materials can support differential stresses. The particle velocity and stress in an elastic wave increase as the strength of an initial disturbance increases. Ultimately, the stress reaches a limiting value beyond which plastic, or irreversible, distortions occur in the medium through which the wave is propagating. This plastic yielding affects both the shape and speed of the stress wave. The onset of this plastic wave behavior occurs at a pressure value for the medium known as the Hugoniot elastic limit. Once the wave passes the Hugoniot elastic limit for the medium, its pressure increases dramatically. Elastic waves are linear, meaning that they arrange themselves in a line-like manner; the result is that they can be superimposed upon themselves. Yet, plastic waves are nonlinear and as a result cannot be superimposed. The speed at which a wave is propagating drops precipitously once it exceeds the Hugoniot elastic limit. In an elastic wave, the shear modulus (a value relating rigidity) and bulk modulus (a value relating compressibility—pressure changes on a body) contribute to the longitudinal wave velocity. When the

Shock Waves

speed of the wave passes the Hugoniot elastic limit, only the bulk modulus contributes significantly, dropping the overall wave speed to nearly that of the bulk wave speed. Because of the high pressure in waves that exceed the Hugoniot elastic limit, and because the bulk modulus increases with pressure, the bulk wave speed in a high-pressure wave is greater than that in a low-pressure wave. In very strong stress waves (shock waves), this effect is so large that the bulk wave speed rises above the longitudinal wave velocity. Since the propagation speed of a plastic wave is less than that of an elastic wave, any disturbance powerful enough to produce a wave exceeding the Hugoniot elastic limit actually produces two waves: an elastic precursor that travels at the longitudinal wave velocity and in which the longitudinal stress equals the Hugoniot elastic limit, and a slower-moving “plastic” stress wave that travels slightly faster than the bulk wave speed. When a disturbance produces one of these strong “double pulse” compression wavefronts, the stress in the wave first jumps to the Hugoniot elastic limit in the faster elastic precursor, then rises to its final value in the slower plastic wave. In extremely strong compression waves, the plastic wave actually travels faster than the elastic precursor; at this point, the plastic wave is supersonic. The result of the plastic wave’s velocity exceeding the elastic precursor’s velocity is a single sharp rise in pressure: a shock front. The fundamental equations describing abrupt shock fronts were derived in 1887 by French physicist Pierre Henri Hugoniot. Shock waves, like plastic waves, are nonlinear. Since a shock wave travels faster than sound, a propagating shock wave can overrun elastic waves and add the elastic wave’s energy to its own. The double-pulse structure of plastic waves is eliminated by the development of a single-pulse supersonic shock front. Shock fronts are usually abrupt and represent a discontinuous jump of pressure, density, particle velocity, and internal energy.

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Behind the shock front, where pressure tails off rapidly from its peak value to its pre-shock ambient value, is the wave’s rarefaction phase. Rarefaction relieves the high pressure generated in a shock wave by propagating release waves from free surfaces into the medium undergoing shock. This release wave is a pressure wave, not a shock wave, and travels at the speed of sound in the medium. At any instant, immediately ahead of the shock front, the medium through which the shock is propagating remains undisturbed. Yet, an infinitesimal distance behind the shock front, the medium is in a shocked state: It is compressed to a higher density, and its particles are accelerated. This additional particle velocity behind the shock, added to the wave’s propagation speed, permits the rarefaction portion of the shock wave to travel faster than the shock front itself. Material engulfed by the shock wave is rapidly accelerated by the sharp pressure gradient at the shock front. At the same time, material affected by rarefaction is accelerated down a more gradual pressure gradient, allowing the compressed material to expand to a low pressure. Gradually, the rarefaction part of the wave overtakes the shock front, and the entire shock wave simultaneously lengthens and decreases in amplitude as it travels. The strength of the shock declines rapidly when the rarefaction catches up with it. In a sense, the shock front, by accelerating particles as it passes, sets in motion its own destruction. The ultimate cause of this difference in particle velocities produced by a passing shock wave and its accompanying release wave is thermodynamic (the relation of heat to energy). A shock wave conserves mass, energy, and momentum as it compresses the material through which it is traveling. The rarefaction, release wave, conserves all of these as well as entropy (heat absorbed divided by the thermodynamic temperature). Shock compression is thermodynamically irreversible, while rarefaction is reversible and adiabatic (occurs without gain or loss of heat).

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Release from high pressure, besides accelerating material down the pressure gradient established by rarefaction, may also result in a change of state for the medium shocked. The state of the shocked medium after release depends on the peak shock pressure it experienced. Shock compression deposits a large amount of internal energy into the medium, and since a shock is not thermodynamically reversible, much of this energy remains as heat even after decompression. If a solid undergoes modest shock pressures, it becomes a hot solid. Materials released from progressively higher shock pressures and higher heat produce first liquid, then vapor. APPLICATIONS Shock waves encountered on Earth are generated both naturally and artificially. Naturally occurring shock waves result from lightning, volcanic eruptions, earthquakes, and meteorite impacts. Shock waves are generated artificially by supersonic jet aircraft, bull whips, spacecraft re-entering the atmosphere, chemical explosives, and nuclear weapons. In many of these examples, the resulting shock wave phenomena cause severe damage to natural and man-made objects. Materials that have experienced the passing of a shock wave often undergo a change of state. Passing shock waves can permanently alter the electrical and electronic properties of matter. Gases compressed to high temperatures by shock waves may become luminescent. Shock pressures can bond dissimilar metals and also increase the hardness of many metals. A common application of shock waves is the simple volume compression of materials. One of the most interesting by-products of shock waves is the effect they have on rocks and minerals. When a rock or mineral is exposed to a transient, high-pressure shock wave, its state is often altered by a process known as shock metamorphism. Natural shock metamorphism is produced by the nearly instantaneous transfer of kinetic energy by means of

Principles of Aeronautics

intense shock waves directly into the rocks. The results of this quick energy transfer have distinct and dramatic effects on the density, crystal structure, and composition of the rock. The only known natural method of producing such effects is the hypervelocity impact of a large meteorite. The two most common byproducts of shock waves encountered on Earth are thunder and sonic booms. A sonic boom is the result of an observer sensing the passage of the pressure or shock wave that an aircraft causes when it travels through the atmosphere at supersonic speeds. The sonic boom is simply the noise generated by air displaced by the aircraft as it travels faster than the speed of sound. Physically, the sonic boom is analogous to the bow wave created by a moving boat. A boat moving over still water compresses surface waves before it, forming a wake. A sonic boom is generated when an aircraft, traveling through the still atmosphere, compacts the pressure waves it is forming in front of itself. All aircraft produce pressure waves, but when they are flying at subsonic speeds (Mach numbers less than one) the pressure waves move away in all directions at the speed of sound and are too weak to be heard. These pressure waves propagate like concentric wavelets generated when an object is dropped into still water. When the aircraft’s speed is supersonic (Mach numbers greater than one), or hypersonic (Mach numbers greater than five), the pressure waves cannot get away ahead of the vehicle, as their natural speed is slower than that of the aircraft. Slower means slightly more than 1,200 kilometers per hour at sea-level conditions (15 degrees Celsius). Because they cannot get away, a shock front develops across which significant and discontinuous changes in air density and temperature occur. Above Mach 1, the aircraft outruns the speed at which pressure waves can travel, and the pressure disturbances coalesce and lag behind the aircraft, which is in effect traveling at the apex of a conical shock wave. The entire

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atmosphere in front of the main shock remains undisturbed. Ultimately, the pressure waves form two conical shock waves, the main shock wave (compression shock) being generated by the nose of the aircraft, and the tail shock (collapse shock) at the aircraft’s rear. The conical main shock wave trails the aircraft like the bow waves of a boat. Any observer contacted by these shock waves would hear the sonic boom. Actually, the sonic boom is a double boom, since a shock wave forms at both the front and the rear of the aircraft. Even the most modest sonic boom can startle humans and animal life. A common impression is that sonic booms occur only when an aircraft “breaks the sound barrier” as it travels from subsonic to supersonic speeds. In reality, the main shock wave is created continually and propagates as long as the aircraft remains at supersonic speeds. Several observers on the ground will each hear a loud “boom” as the shock wave passes. Thunder is probably the most common example of a shock wave that people encounter. During electrical storms, lightning releases heat energy into the atmosphere, displacing air in a manner similar to a supersonic aircraft. The shock waves generated by a lightning bolt, traveling at speeds far greater than that of sound, produce thunder. The sharp crack of a bull whip is also attributed to a small sonic boom. As the whip is thrown forward and then snapped quickly backward, the tip exceeds the speed of sound, displaces the surrounding air, and produces a weak shock wave. Minor high speed shock waves can also occur on aircraft propellers. At high speeds, the ends of the propeller blades may actually have velocities that exceed the speed of sound, particularly in turns. This can have unfortunate effects of the structural integrity of the propeller. CONTEXT Naturally generated shock waves have always played a role in the history of humankind. The clap of

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thunder, the concussion from an exploding volcano, and the rumble of an earthquake are all the result of shock waves and are a constant reminder that humans inhabit a dynamic planet. With the invention of gunpowder, and eventually chemical explosives, humankind has been able to generate shock waves of ever-increasing magnitude. When such powerful shock waves travel through an undisturbed medium (gas, liquid, or solid), the pressure, temperature, and density of the disturbed state are increased many times over. As a result, when living things or structures are hit by such a strong shock wave, they are destroyed by the violent pressure change they experience. Historically speaking, it is this knowledge and the eventual use of shock waves as a destructive weapon that changed the way in which warfare is conducted. With the invention of nuclear weapons, humankind has increased its ability to generate shock waves artificially to a horrific level. Despite the self-destructive use to which humankind has put its knowledge of shock waves, there are also a number of peaceful applications. Explosives are necessary for building roads, tunnels, and mines; forming metals; and, under controlled circumstances, propelling spacecraft. A number of universities and government-sponsored laboratories exist that have mechanical, electric, and chemical-driven shock tubes; supersonic wind tunnels; hyperballistic firing ranges; and chemical explosives all used to study both the peaceful applications and destructive effects of shock waves. The study of shock waves is important because a clear understanding of shock wave-associated phenomena aids in the construction of earthquake-resistant buildings, the efficient and safe design of jet aircraft and space vehicles, the detection of underground nuclear explosions, a number of specific applications in medicine, and the development of protective coverings and armors. Shock waves provide a

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clearer understanding of the physical forces that shape the planet, solar system, and universe. —Randall L. Millstein Further Reading Ben-Dor, Gabi. Shock Wave Reflection Phenomena. Springer New York, 2013. Forbes, Jerry W. Shock Wave Compression of Condensed Matter: A Primer. Springer Berlin Heidelberg, 2015. Krehl, Peter O. K. History of Shock Waves, Explosions and Impact: A Chronological and Biographical Reference. Springer Berlin Heidelberg, 2008. Panaras, A. G. Aerodynamic Principles of Flight Vehicles. American Institute of Aeronautics and Astronautics, 2012. Sasoh, Akihiro. Compressible Fluid Dynamics and Shock Waves. Springer Nature Singapore, 2020. See also: Aerodynamics and flight; Aeronautical engineering; Airplane propellers; Fluid dynamics; Ernst Mach; Mach number; Pressure; Ramjets; Scramjet; Sound barrier; Space shuttle; Supersonic aircraft; Chuck Yeager

Igor Sikorsky Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Igor Ivanovich Sikorsky was born on May 25, 1889, in Kiev, Ukraine, and died on October 26, 1972, in Easton, Connecticut. He was a Russian-American aeronautical engineer, aircraft manufacturer, and inventor best known for developing the helicopter. Sikorsky’s introduction of controlled-pitch rotor blades was instrumental to the development of the modern helicopter. KEY CONCEPTS autogyro: forerunner of the helicopter, an aircraft with short wings and an unpowered overhead rotor for lift

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Igor Sikorsky

swashplate: a mechanical device on the rotor hub of a helicopter that translates control movements from the pilot’s control stick into various pitch adjustments of the rotor blades variable pitch rotors: a system by which the pitch of a helicopter’s rotor blades can be changed in flight through the action of a swashplate FROM RUSSIA TO AMERICA Igor Sikorsky was born in 1889 to educated parents who were both physicians, although his mother did not practice professionally. His formal training began in 1903, when he enrolled at the Russian Naval Academy in St. Petersburg. His interest in education led him to leave the service in 1906 and to enroll at the Polytechnic Institute in Kiev. At the age of twenty, Sikorsky built his first helicopter, which would not leave the ground. In 1911, he set a record by flying for thirty minutes at 113 kilometers per hour in the S-5, a plane he had designed himself. In 1913, at the request of the Russian Army, he designed and built the world’s first four-engine, dual-controlled airplane, which served as a bomber in World War I. A strong anti-Bolshevist, Sikorsky left Russia after the Revolution of 1917 and made his home in the United States. He became a US citizen in 1928. He originally joined in business with a group of Russian immigrants building airplanes, but the business failed. In 1923, he started over, forming the Sikorsky Aero Engineering Corporation, which built “flying boats” for Pan American’s transoceanic flights and fifty-six “aerial yachts” for wealthy clients. After the stock market crash of October, 1929, his once-wealthy clients could no longer meet their payments, ending his independent company, which became a division of the United Aircraft Corporation. Sikorsky continued to head the division until his retirement. In 1939, Sikorsky developed the VS-300, the first helicopter with controlled-pitch blades. This innova-

Igor Sikorsky. Photo via Wikimedia Commons. [Public domain.]

tion turned out to be instrumental in making helicopters practical. Although early helicopter models were used in World War II after 1944, they came of age during the 1950s. Sikorsky also designed a patrol bomber, known as the Flying Dreadnought, for the US Air Force’s use in World War II. Sikorsky continued to be a vital part of the company even after his 1957 retirement at the age of 68. He consulted on design and on business matters and was at his desk the day before his death at 83 years of age. —Kenneth H. Brown Further Reading McKenna, John A., and Ned Allen. Skycrane: Igor Sikorsky’s Last Vision. American Institute of Aeronautics and Aerospace, 2010. Prouty, Ray. Helicopter Aerodynamics, Volume 1. Eagle Eye Solutions, 2009.

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Prouty, Raymond W. Helicopter Aerodynamics, Volume II. Eagle Eye Solutions, 2009. Sikorsky, Sergei I., and Igor I. Sikorsky Historical Archive. The Sikorsky Legacy. Arcadia Publishing, 2007. See also: Aeronautical engineering; Aerospace industry in the United States; Autogyros; First flights of note; Flight propulsion; Flight roll and pitch; Forces of flight; Helicopters; Rotorcraft

Sound Barrier Fields of Study: Physics; Aeronautical engineering; Mathematics ABSTRACT The sound barrier is a wall of superimposed compression waves beginning just ahead of the nose of an aircraft traveling at or above the speed of sound in air. When an aircraft meets or exceeds the sound barrier and travels at supersonic speeds, it creates a continuous pressure wave that reaches the ground as a sonic boom. KEY CONCEPTS Doppler shift: the amount by which an observed frequency differs from its natural frequency according to the relative velocities of the source and the observer shock front: a supersonic shock pulse abrupt to the point where it is often represented as a discontinuous jump of pressure, density, internal energy, and particle velocity DOPPLER EFFECT The Doppler effect, discovered by Austrian physicist Christian Johann Doppler in 1842, is the change in the observed frequency of a wave, of sound or light, for example, due to relative motion between the observer and the wave source. When observer and source approach each other, the emitted frequency of the waves is measured to be higher, due to the ve-

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locity of approach; the greater the relative speed, the greater the frequency shift. When the source and observer are receding from each other, the emitted frequency is measured to be lower, in direct proportion to the velocity of recession. The magnitude of this difference in frequencies is known as the Doppler shift, and is proportional to relative velocity and direction of motion. Although the Doppler effect applies to all types of waves, it is particularly noticeable for sound waves. When an ambulance speeds by, for example, the pitch, or frequency, drops noticeably. The effect is most easily explained by considering water waves on a placid pond created by a small insect jiggling its legs. The insect’s movement creates a pattern of equally spaced concentric rings; each ring represents the crest of a wave traveling outward from the insect at constant speed. If the insect is traveling toward the left while jiggling its legs, the wave pattern is distorted; the rings are no longer concentric, but the centers of consecutive waves are displaced in the direction of motion. Although the insect has not changed the frequency with which it jiggles its legs, an observer at a position toward the insect’s left encounters a higher frequency of waves because the waves are compressed in the direction of motion. An observer at a position to the insect’s right perceives a corresponding lower frequency of waves. The waves are spread farther apart because the insect is moving away from the observer. Although sound waves are invisible and spread into three dimensions, the same principle applies. When a source of sound approaches, the perceived frequency, or pitch, is higher than the emitted frequency, and the opposite is true for a receding source. WAVE BARRIER If the insect discussed above were to swim across the water at the same speed as the velocity of water waves while continuing to jiggle its legs at a constant

Principles of Aeronautics

frequency, the wave crests would be superimposed on one another directly in front of the insect rather than moving ahead of it. This wall of water may be considered a wave barrier, because the insect would have to exert considerable effort to swim over this barrier in order to travel at a speed greater than the wave velocity. However, after the insect had surmounted the barrier by exceeding the wave velocity, the water ahead would be smooth and undisturbed. When an aircraft travels at the speed of sound in air, it also encounters a barrier of superimposed sound waves. The compression waves are stacked up along the leading edge of the aircraft, requiring some additional thrust for the aircraft to punch through. After the plane exceeds the velocity of

Sound Barrier

sound, however, there are no further barriers to inhibit additional acceleration; the airplane may travel at supersonic speed. SHOCK WAVES As the jiggling insect travels through water with a speed greater than the wave velocity, it produces the pattern in which each consecutive wave crest, represented by a circle, is located outside the previous crest. The wave crests overlap to form larger crests. This small wall of water, called a bow wave, has a solid “V” shape. When an aircraft flies at a supersonic speed, the overlapping spherical sound waves form a shock front, a cone of air pressure that grows in size until

U.S. Navy F/A-18 transonic pushing into the sound barrier. The supersonic white cloud is formed by decreased air pressure and temperature around the tail of the aircraft. Photo via Wikimedia Commons. [Public domain.]

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intercepted by the ground. This thin conical shell of compressed air is termed a shock wave. Just as a person floating in a tranquil lake will be hit by the bow wave of a speed boat traveling faster than the speed of water waves, people on the ground will be struck by the shock wave of a supersonic aircraft. This wave, called a sonic boom, is heard as a sharp cracking thunderclap. SONIC BOOMS The sound of a subsonic aircraft is perceived by a listener on the ground as a continuous tone. The shock wave produced by a supersonic airplane, consisting of many superimposed waves, occurs like an explosion in a single burst. Both processes consist of a burst of high-pressure air that creates a loud, unpleasant noise. In actuality, the shock waves produced by supersonic aircraft create a double sonic boom; the shock wave from the bow of the plane is a pulse of increased pressure that is followed a fraction of a second later by a negative-pressure pulse from the trailing edge of the aircraft. Overall, the pressure wave has the general appearance of the letter “N.” This pressure shock wave is produced during the entire course of a supersonic flight and not only during the time when it passes the sound barrier, as is mistakenly believed. Because the width of the sonic boom trail is about 20 miles, and its length is the flight path, sonic booms can create considerable problems. First, there is the annoyance factor of people being startled or awakened by the loud, explosive noise. Because of sonic booms’ intense and rapid pressure changes, sonic booms can destroy property in inhabited areas. Broken windows and structural damage are not uncommon. Finally, sonic booms can be problematic even in uninhabited regions; they have been known to topple rock structures in national parks. BRIEF HISTORY OF SUPERSONIC FLIGHT The speed of sound at sea level is 1,223 kilometers per hour. The speed of sound decreases with

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increased altitude, so that at 15,240 meters, sound travels at 1,062 kilometers per hour. Because the wall of pressure termed the sound barrier differentiates subsonic from supersonic flight, the speed of sound is defined as a velocity of Mach 1. Mach 2, then, would be twice the speed of sound, and so on. Although several attempts were made in the early 1940s to exceed the sound barrier, early jet planes of the period were neither powerful nor sturdy enough to succeed. When an aircraft reaches Mach 1, strong local shock waves form on the wings, and the flow of air around the plane becomes unsteady. As a result, the airplane is subjected to severe buffeting that interferes with the plane’s stability and renders it difficult to control. In 1943, U.S. aeronautical engineers began working on the first airplane specifically designed to surmount these problems and withstand the tremendous air pressure of Mach 1 in order to obtain supersonic flight. This goal was realized on October 14, 1947, when Captain Charles E. “Chuck” Yeager of the US Air Force smashed through the sound barrier in a Bell X-1 rocket plane. Although many supersonic flights at ever-increasing speeds were made over the next decade, the speed never exceeded Mach 2.5, because friction caused by the rapidly moving air overheated the outer shell of the airplanes. Using jet engines specifically designed for supersonic flight, the North American F-100 Super Sabre jet fighter became the first jet capable of flying at supersonic speeds in level flight. The first supersonic bomber, the Convair B-58 Hustler, became operational in 1956. By 1963, the X-15 rocket plane was able to fly 67 miles above the earth’s surface at a speed exceeding Mach 6. The world’s first supersonic transport (SST) plane, the Tupolev Tu-144, was tested by Soviet pilots in 1968. Britain and France jointly constructed the Concorde SST, which was designed to fly at Mach 2 and began commercial service in 1969. Since that time, however, the num-

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ber of supersonic flights has been limited due to the high cost of fuel and the problems of sonic booms. In the United States, commercial supersonic flights are now restricted to transoceanic flights. —George R. Plitnik Further Reading Agarwal, Ramesh K. Environmental Impact of Aviation and Sustainable Solutions. IntechOpen, 2020. Benson, Lawrence R. Quieting the Boom: The Shaped Sonic Boom Demonstrator and the Quest for Quiet Supersonic Flight. National Aeronautics and Space Administration (NASA), Aeronautics Research Mission Directorate, 2013. Gunston, Bill. Faster Than Sound: The Story of Supersonic Flight. Haynes, 2012. Hampton, Dan. Chasing the Demon: A Secret History of the Quest for the Sound Barrier, and the Band of American Aces Who Conquered It. HarperCollins, 2018. Petty, Chris. Beyond Blue Skies: The Rocket Plane Programs That Led to the Space Age. University of Nebraska Press, 2020. See also: Aerodynamics and flight; Aeronautical engineering; Avro Arrow; Fluid dynamics; Forces of flight; High-speed flight; Hypersonic aircraft; Ernst Mach; Mach number; Materials science; National Aeronautics and Space Administration (NASA); Pressure; Propulsion technologies; Ramjets; Scramjet; Shock waves; Chuck Yeager

Space Shuttle Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Astronautics ABSTRACT The space shuttle was a reusable space launch vehicle developed by the United States to launch astronauts and large satellites into Earth orbit. It was the first reusable launch vehicle to carry humans into space. The space shuttle program began on January 5, 1972, and carried on until the final shuttle landing on July 21, 2011.

KEY CONCEPTS O-ring: a circular, flexible seal typically made of silicone rubber impervious to most solvents and able to resist high heat thrust: the force or pressure exerted on the body of an aircraft in the direction of its motion PLANNING THE SPACE SHUTTLE The space shuttle program was initially conceived in the 1960s, when the National Aeronautics and Space Administration (NASA) began planning a comprehensive program for a permanent American presence in space. The plan included three components: a permanently crewed space station, a reusable vehicle to carry astronauts from Earth to orbit and back, and a space tug to move satellites around in orbit. However, the need to fund other national priorities resulted in cuts to the NASA budget at the end of the Apollo program. Because this new space effort’s cost far exceeded its budget, it was scaled back to include only the reusable launch vehicle, which was called the space shuttle. On January 5, 1972, President Richard M. Nixon officially announced the inauguration of the space shuttle program. NASA’s ambitious schedule called for suborbital tests by 1977 and the first orbital tests by 1979. The shuttle was scheduled to begin regular launches by 1980. THE SHUTTLE VEHICLE The space shuttle consisted of three major components: a reusable, winged orbiter that carries the crew; a large external tank that holds fuel for the main engines; and two solid rocket boosters that provide most of the shuttle’s lift during the first two minutes of flight. The space shuttle was designed to reach orbits ranging from about 185 kilometers to 645 kilometers high. Normally, space shuttle missions ranged from five to sixteen days in orbit. The smallest crew to fly on the shuttle was composed of two people, on the first few test flights, but the shuttle normally carried crews ranging from five to eight

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Space Shuttle Discovery launch, 2007. Photo via Wikimedia Commons. [Public domain.]

people, depending on the flight objectives. At liftoff, the space shuttle weighed about 2,041,166 kilograms. The orbiter, manufactured by the Space Division of Rockwell International, carried the crew, the payload, and the main propulsion system. The empty weight of the orbiter is about 68,039 kilograms, approximately the same as that of a DC-9 jet aircraft. The crew compartment of the orbiter has three levels: the flight deck, the middeck, and a lower level equipment bay. The crew compartment is pressurized to one standard atmosphere with a mixture of 80 percent nitrogen and 20 percent oxygen, similar

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to the air pressure and composition at Earth’s surface. The volume of the crew compartment is 65.84 cubic meters, about the equivalent of a 4.5-by-4.5by-3-meter room. The uppermost level of the crew compartment is the flight deck. The mission commander and the pilot were seated side-by-side in the forward portion of the flight deck. The mission commander and the pilot sat at workstations that contain the controls and displays used to guide the orbiter throughout the flight. Two seats for mission specialists were located directly behind the seats of the mission commander and the pilot. At the rear of the flight deck there were two overhead- and aft-viewing windows for observing orbital operations. The middeck, which is directly beneath the flight deck, provided accommodations for additional crewmembers and contained three avionics equipment bays. Depending on the mission requirements, bunk sleep stations and a galley could be installed in the middeck. In addition, three or four seats of the same type as the mission specialists’ seats on the flight deck could be installed in the middeck. An airlock, located in the rear of the middeck, provided access to the payload bay. Normally, two extravehicular mobility units (EMUs) were stowed in the airlock. The EMU is an integrated spacesuit assembly and life-support system that enabled flight crew members to leave the pressurized orbiter crew cabin and work outside the cabin in space. Removable panels in the middeck floor provided access to the equipment bay that housed the major components of the waste-management and air-cleaning and recirculating systems. This compartment had space in which to stow lithium hydroxide canisters, used to clean the air, and five separate spaces for crew equipment stowage. When on the ground, astronauts entered and exited the crew compartment through a side hatch in the middeck. The payload bay, measuring 4.5 meters wide and 18.3 meters long, held large payloads being carried

Principles of Aeronautics

to orbit. Two payload bay doors, each 18.3 meters long, were hinged at each side of the fuselage. The payload bay doors exposed the payload bay to space when they were opened along the centerline. The back surface of the doors, which had a combined area of approximately 149 square meters, contained radiators that exhausted the heat generated by equipment on the orbiter. Seals on the doors provided a relatively airtight payload compartment when the doors were closed and latched. The crew compartment of the orbiter did not contain sufficient space for experiments, and the payload bay was not pressurized, so astronauts working in the payload bay had to wear spacesuits. To provide space for experiments, the European Space Agency (ESA) designed the Spacelab, a large, pressurized module that could be carried in the orbiter’s payload bay. Astronauts entered the Spacelab through the airlock at the rear of the middeck of the crew compartment. The Spacelab provided electrical power and a pressurized working environment for astronauts to perform a variety of experiments. The orbiter also contained the three liquid-fueled main engines. These engines burned liquid hydrogen and liquid oxygen, which is carried in the external tank attached to the orbiter. The top surface of the orbiter is covered with white silica ceramic material that protected the surface during reentry from temperatures of up to 650° Celsius. The bottom of the orbiter and the leading edge of the tail are covered with black silica ceramic heat-shield tiles, having very low thermal conductivity to protect those surfaces from temperatures of up to 1260° Celsius. The orbiter’s external tank, which was designed by Martin Marietta and built at NASA’s Michoud Assembly Facility in New Orleans, Louisiana, contained all of the fuel, liquid hydrogen, and the oxidizer, liquid oxygen, for the orbiter’s main engines. At the top of the external tank there was a conical nose cone that reduced the air drag on the vehicle and served as a lightning rod. The oxygen tank, located beneath the

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nose cone, had a volume of 554 cubic meters. A 43.2-centimeter-diameter fuel line carried the oxygen to the orbiter with a maximum flow rate of 66.6 cubic meters per minute. The liquid hydrogen tank, which was located below the liquid oxygen tank, had a volume of 1,515.5 cubic meters. The 43.2-centimeter fuel line connecting the hydrogen tank to the orbiter has a maximum flow of 1,341.2 cubic meters per minute. Just before the shuttle reached orbital velocity, the external tank was jettisoned, and it burned up on atmospheric entry. The two solid-fueled rocket boosters were attached to the main tank. The solid rocket boosters were the largest solid-propellant motors ever flown and the first that were designed to be reused. The propellant mixture in each motor consists of ammonium perchlorate as the oxidizer, aluminum as the fuel, iron oxide as a catalyst, and a polymer binder that held the mixture together. The fuel was shaped so that each rocket provided a high thrust at ignition and then reduced the thrust by approximately one-third after 50 seconds to prevent overstressing the vehicle during the time when it experienced maximum dynamic pressure. Because the solid boosters were too long to manufacture as a single unit, each booster consisted of four segments. These segments were joined together using a system of clamps and O-ring seals, which were made of compressible material that filled the space in the joints to prevent leakage of high-pressure gas through the joints. Each solid booster weighed 589,670 kilograms at liftoff and 87,090 kilograms after the fuel had been burned. At liftoff, each of the solid boosters developed approximately 1,496,855 kilograms of thrust. The solid rocket boosters also contained a parachute system, which allowed them to descend into the Atlantic Ocean after use. They were recovered by ship and returned to the manufacturer for refurbishment and reuse. The orbiter returned to Earth as a glider, using conventional flight controls and wings that provided

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lift. The wingspan was 23.8 meters. Each wing, constructed of aluminum alloy with a multirib-and-spar arrangement, had a maximum thickness of 1.5 meters and is approximately 18.3 meters long where it was attached to the fuselage. The main landing gear was stored in the wings and was extended only a few seconds before landing. THE SPACE SHUTTLE FLIGHT PROFILE The space shuttle was launched vertically from a transporter-launching pad that was modified from a Saturn V launching pad after the end of the Apollo program. The shuttle could carry a crew of up to eight astronauts and could deliver a payload of up to 29,483.5 kilograms into low-Earth orbit. The liquid-fueled main engines ignited about seven seconds before the planned liftoff. A computer checked the performance of the main engines, which could be shut down if a problem was detected. If no problems were detected, the solid rocket boosters, which must burn until their fuel is exhausted, were ignited. At liftoff, the three main engines and the two solid-fueled booster rockets developed a total of more than 3,084,428 kilograms of thrust. The solid rocket boosters, which provided most of the thrust to lift the space shuttle off the pad and up to an altitude of about 45.72 kilometers, burned for approximately two minutes. At an altitude of about 45 kilometers, just after they burned out, the solid boosters were jettisoned from the external tank by pyrotechnic separation devices. Eight small rockets on the solid boosters fired to carry them well clear of the orbiter. A parachute system slowed the descent of the solid boosters, which were recovered from the ocean, about 274 kilometers from the launch site. The external tank continued to provide fuel for the orbiter’s three main engines until about eight minutes after liftoff. The main engines shut down at a speed just below orbital speed, and the external tank was jettisoned. After a short period of coasting, two small maneuvering engines, fueled from tanks

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on the orbiter, fired to place the orbiter in Earth orbit. Environmental control and life-support system radiators, used to cool the orbiter’s systems, were located on the interior of the payload bay doors. Once the orbiter achieved orbit, the payload bay doors were opened to allow proper cooling of the spacecraft. During the mission, the path of the orbiter could be adjusted using the maneuvering engines. Once the mission was completed, the maneuvering engines served as retrorockets, firing opposite the direction of the orbiter’s motion and slowing the orbiter so that it reentered Earth’s atmosphere. SPACE SHUTTLE MISSIONS The first space shuttle orbiter, named Enterprise, was unveiled to the public on September 17, 1976, when it was rolled out of the Rockwell International hangar in Palmdale, California. Initially, the Enterprise was used for a series of ground tests. During 1976 and 1977, the Enterprise was carried aloft by a specially modified Boeing 747 aircraft, allowing engineers to study the aerodynamics of the orbiter. On August 12, 1977, the Enterprise separated from the Boeing 747 at an altitude of 7 kilometers, allowing the flight crew, Gordon Fullerton and Fred Haise, to perform approach and landing maneuvers at Edwards Air Force Base in California. After a series of unpowered flight tests, the Enterprise, which was never intended for powered flight, was retired. A fleet of four shuttles, Columbia, Challenger, Discovery, and Atlantis, was built for orbital operations. The space shuttle Columbia was launched from NASA’s John F. Kennedy Space Center at Cape Canaveral, Florida, on its first flight on April 12, 1981. John W. Young, a veteran of NASA’s Gemini and Apollo programs, was the commander, and Robert L. Crippen was the pilot. This was a test flight, and the only payload carried on the mission was a Development Flight Instrumentation Package,

Principles of Aeronautics

which contained sensors and measuring devices to record orbiter performance and the stresses that occurred during launch, ascent, orbital flight, descent, and landing. Postflight inspection of Columbia revealed that an overpressure wave that occurred when the solid rocket boosters ignited resulted in the loss of 16 heat-shield tiles and damage to 148 others. However, Columbia‘s first flight demonstrated that the shuttle could perform a safe ascent into orbit and return to Earth for a safe landing. The first five space shuttle missions were flown by Columbia, while the other space shuttle orbiters were under construction. The sixth shuttle flight, launched on April 4, 1983, was the first mission of Challenger. This mission deployed the first tracking and data relay satellite (TDRS), part of the satellite network used to relay shuttle communications to the ground. A malfunction of the inertial upper stage booster, which moves the satellite from the low orbit of the shuttle into the higher orbit required for global communications, resulted in an improper but stable orbit. Propellant aboard the satellite was used over the next several months to move the TDRS into the proper orbit. Between April 1981 and January 1986, the space shuttles completed twenty-four missions. On June 18, 1983, the shuttle Challenger carried the United States’ first woman astronaut, Sally K. Ride, into orbit and deployed two communications satellites, Anik C-2 for Telesat Canada and Palapa-B1 for Indonesia. During the Challenger mission launched on February 3, 1984, the first untethered spacewalk took place. Astronauts Bruce McCandless II and Robert L. Stewart used the manned maneuvering unit (MMU) to fly in space unconnected to the orbiter. This mission also launched three satellites, but two, the Westar-VI and Palapa-B2, were placed into a low, elliptical orbit when the payload assist module rocket motor, which should have boosted them into a high, circular orbit, failed. The shuttle Challenger carried the long duration exposure facility (LDEF) into orbit on

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April 6, 1984. The LDEF, whose purpose was to expose various materials to the space environment to monitor their stability or degradation in space and to determine the flux of micrometeorites and orbital debris, was supposed to be retrieved and returned to Earth after about two years. The orbiter Discovery made its first flight on August 30, 1984, on a mission that launched three communications satellites. This mission also deployed a 31-meter-long, 4-meter-wide solar wing, which tested several different types of solar cells being considered for future space missions and demonstrated that very large structures could be deployed in space. The Spacelab space laboratory flew three times: carried into orbit by Columbia on November 28, 1983, and by Challenger on April 29, 1985 and July 29, 1985. The shuttle Atlantis made its first flight on September 20, 1985. On January 28, 1986, the twenty-fifth space shuttle mission was launched from the Kennedy Space Center. The space shuttle Challenger, after a night of below-freezing temperatures, lifted off on its tenth mission into space at about 10:40 a.m. eastern standard time, carrying a crew of seven astronauts: Francis R. Scobee, the commander; Michael J. Smith, the pilot; Judith A. Resnik, Ellison S. Onizuka, and Ronald E. McNair, all mission specialists; Gregory B. Jarvis, a payload specialist; and America’s first teacher in space, Sharon Christa McAuliffe. Seventy-four seconds after the launch, a sealing O-ring in one of the boosters failed and Challenger was destroyed, killing all seven crew members. A subsequent investigation established that the previous night’s low temperature had hardened the O-ring seals between the segments of the solid-fueled rocket boosters. One joint in the right solid rocket booster had developed a leak, and the hot gases cut through metal on the shuttle to cause the disaster. NASA immediately suspended the space shuttle program while the shuttle’s overall safety was evaluated. The solid-fuel rocket booster joints were rede-

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signed and the shuttle orbiter underwent more than two hundred modifications before the shuttle fleet returned to service. An escape system was added to the orbiter to allow astronauts to escape from a crippled shuttle and parachute to Earth. Construction began on a new shuttle, named Endeavour, to replace Challenger. Shuttle flights resumed on September 29, 1988, when the shuttle Discovery carried a crew of five astronauts, commanded by Frederick H. Hauck, into orbit. This mission placed another TDRS communications satellite into orbit. Subsequent shuttle flights performed a variety of functions. On April 24, 1990, the shuttle Discovery placed the Hubble Space Telescope (HST) into orbit. The Hubble was designed to be serviced in orbit by future space shuttle missions. The first Hubble-servicing mission, flown by Endeavour and launched on December 2, 1993, accomplished its three primary objectives: restoring the planned scientific capabilities of the Hubble by installing a corrective lens designed to compensate for the incorrect shape of the mirror; restoring the reliability of Hubble’s guidance system; and validating the concept of servicing while in orbit. The shuttle Columbia was launched on January 9, 1990, to place the SYNCOM IV-F5 defense communications satellite in orbit and to retrieve the long duration exposure facility (LDEF), which had been stranded in orbit after the Challenger accident. The space shuttles launched several interplanetary spacecraft. On May 4, 1989, the shuttle Atlantis launched the Magellan spacecraft, which went into orbit around Venus and performed radar mapping of its surface. On October 18, 1989, the shuttle Atlantis launched the Galileo spacecraft, which went into orbit around Jupiter, exploring that planet and its moons. On October 6, 1990, the shuttle Discovery launched the joint ESA/NASA Ulysses spacecraft, which was placed on a trajectory to pass Jupiter, where its orbit was altered to explore polar regions of the Sun.

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In 1984, the US government decided to build a space station, similar to the one that had been in the original 1960s plan. In preparation for this new space station, NASA began a series of space shuttle missions to Mir, the Russian space station. As part of this project, Russian cosmonaut Sergei Krikalev, the first Russian to be a crew member on an American spacecraft, flew on the space shuttle Endeavour in March 1995. In June 1995, the space shuttle Atlantis carried out the first mission to dock with the Mir space station. On October 11, 2000, the shuttle Discovery flew the one-hundredth space shuttle mission, carrying a large truss, the Pressurized Mating Adapter-3, four large gyroscopes, and two heat pipes to the International Space Station (ISS). Before the launching of the shuttle Discovery in March 2001, on a mission to bring the second crew to the ISS, James Kelly, the pilot, noted that, twenty years after the inception of the space shuttle program, the shuttle had finally realized its initial goal of transporting people to and from a permanent workplace in low-Earth orbit. During its first twenty years of operation, NASA’s space shuttle fleet carried more than 600 astronauts and placed more than 1,360,777 kilograms of cargo into orbit. Tragedy struck the space shuttle program once more on February 1, 2003, when Columbia disintegrated upon reentry, killing all seven crew members. It was later discovered that the cause of the disaster had occurred upon the shuttle’s launch, when a piece of foam insulation broke off of a propellant tank and struck the edge of the craft’s left wing. Information suggesting that members of Mission Control had flagged the issue generated further debate about NASA’s approach to safety issues. The shuttles were grounded until July 2005, when Discovery launched on a successful mission. END OF THE SPACE SHUTTLE PROGRAM In 2004, President George W. Bush announced that once the ISS was completed, which was planned for

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2010, the space shuttle would be retired, bringing the program to an end. As part of a new scope for NASA, he also outlined a plan to develop a new manned exploration vehicle designed to carry passengers back to the moon and, eventually, to Mars. While Atlantis, Discovery, and Endeavour had continued to conduct regular missions, mainly to continue delivering parts for the construction of the ISS, in adherence with this new initiative, the retirement of the space shuttles began with the final landing of Discovery in March 2011. Endeavour landed at Kennedy Space Center, completing its final mission, in June of that year, and Atlantis touched down for the final time in July after carrying supplies to the ISS, marking the official end of the shuttle program. The three shuttles were then distributed to be displayed at museums throughout the country. Overall, the program had led to the completion of more than one hundred missions over approximately thirty years. —George J. Flynn Further Reading Baker, David. NASA Space Shuttle Manual: An Insight Into the Design, Construction and Operation of the NASA Space Shuttle. Voyageur Press, 2011. Bizony, Piers. NASA Space Shuttle 40th Anniversary. Motorbooks, 2021. Chang, Kenneth. “The Shuttle Ends Its Final Voyage and an Era in Space.” The New York Times, 21 July 2011, www.nytimes.com/2011/07/22/science/space/22spaceshuttle-atlantis.html. Accessed 20 Oct. 2017. Leinbach, Michael D., Jonathan H. Ward, and Eileen Collins. Bringing Columbia Home: The Untold Story of a Lost Space Shuttle and Her Crew. Arcade Publishing, 2018. Sivolella, David. To Orbit and Back Again: How the Space Shuttle Flew in Space. Springer New York, 2013. ———. The Space Shuttle Program: Technologies and Accomplishments. Springer International Publishing, 2017. “Space Shuttle Program Fast Facts.” CNN, 26 July 2017, www.cnn.com/2013/10/21/us/space-shuttle-program-fastfacts/index.html. Accessed 20 Oct. 2017.

White, Rowland. Into the Black: The Extraordinary Untold Story of the First Flight of the Space Shuttle Columbia and the Astronauts Who Flew Her. Atria Books, 2017. See also: Aerodynamics and flight; Aeronautical engineering; Forces of flight; Yuri Gagarin; Glider planes; High-speed flight; National Aeronautics and Space Administration (NASA); Propulsion technologies; Rocket propulsion; Rockets; Russian space program; Alan Shepard; Spacecraft engineering; Spaceflight; Valentina Tereshkova; Konstantin Tsiolkovsky

Spacecraft Engineering Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Spacecraft engineering is an interdisciplinary engineering field concerned with the design, development, and operation of unmanned satellites, interplanetary probes, and manned spacecraft. Unmanned satellite missions include commercial communications and remote sensing (including meteorological satellites), scientific research, military communications, navigation, and reconnaissance. Interplanetary probes are confined to scientific and exploratory missions. Manned spacecraft missions are confined to assignments at the International Space Station (ISS) and to flights in support of the ISS. KEY CONCEPTS encryption and decryption: generally, a system that alters the nature of a signal to prevent unwanted intrusion or pirating of data in the signals, essentially converting the signal into a “secret code”; decryption is the opposite process of encryption, enabling recovery of the original signal data modulators and demodulators: electronic systems that function to stabilize and isolate transmission frequencies

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vibrational load: the frequency and amplitude of vibration experienced by a spacecraft relative to the frequency and amplitude of vibrations the craft is designed to withstand SUMMARY Spacecraft engineering is an interdisciplinary engineering field concerned with the design, development, and operation of unmanned satellites, interplanetary probes, and manned spacecraft. Unmanned satellite missions include: commercial communications and remote sensing (including meteorological satellites), scientific research, military communications, navigation, and reconnaissance. Interplanetary probes are at present exclusively confined to scientific and exploratory missions. Manned spacecraft missions are currently confined to long-duration assignments at the International Space Station (ISS) and to short duration flights in support of the ISS on either the United States’ Space Transportation System (STS) or the Russian Federation Soyuz spacecraft. However, planning and operations for manned missions returning to the Moon or manned flights to Mars began in the 2020s. The National Aeronautics and Space Administration’s (NASA’s) Artemis program, which aims to return astronauts to the moon, undertook its first full mission in December, 2022, as an unmanned test flight around the Moon. DEFINITION AND BASIC PRINCIPLES Spacecraft engineering is the process of designing, constructing, and testing vehicles for deployment and operation in the full expanse of space above the Earth’s atmosphere, generally regarded as beginning at an altitude of fifty miles. Launch vehicles and rocket propulsion are considered part of the related but separate field of rocketry. Spacecraft have to be robust enough to survive several harsh environments. The vibrational loads generated by the launch vehicle will damage or de-

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stroy weak or poorly designed structures. The electronic components must function reliably in a high-radiation environment. All parts of the spacecraft cycle from extreme heat to extreme cold each orbit as the spacecraft moves into and out of Earth’s shadow. Outgassing in a vacuum can contaminate solar panels and camera optics. Motion through the plasma of the ionosphere creates potentially damaging charges of static electricity. Energy is expensive in space: It must either be collected from sunlight using solar panels of limited area or generated from onboard fuel supplies, the exhaustion of which will end a mission. Batteries wear out through repeated charging and discharging and must be managed carefully to last as long as possible. Electrical components must operate with high efficiency and draw as little power as possible; inactive components have to be kept off or in a low-power standby state. Everything must be designed with the knowledge that repairs or replacements will be impossible at worst and difficult, dangerous, and expensive at best. Batteries, fuel cells, pressure vessels, propulsion systems, and nuclear power modules are all inherently hazardous devices; a launch failure can be catastrophic. All launch ranges operate under rigidly enforced safety standards for the protection of the spacecraft, the launch vehicle, and the personnel working around them. Reliability, survivability, and safety have to be designed into the spacecraft from conception. Rigorous testing throughout the development and manufacturing process plays a major role in spacecraft engineering. BACKGROUND AND HISTORY Spacecraft engineering did not exist before the development of the A-4 (more popularly known as the V-2) rocket by German scientists during World War II. Both the United States and the Union of Soviet Socialist Republics (USSR) integrated technology obtained from captured German scientists,

Principles of Aeronautics

engineers, and rocket hardware into their domestic long-range ballistic-missile development efforts. The primary goal was the development of intercontinental-range ballistic missiles for the delivery of nuclear warheads in case of war. The A-4, however, flew very poorly without the ton of high explosives it was originally designed to carry. The ordnance payload was replaced with scientific instruments for exploration of atmospheric conditions at high altitude and astronomical observations unimpeded by atmospheric interference. The extreme conditions of rocket flight and the harsh environment of space posed new challenges to the instrument developers. Techniques that evolved to meet those challenges laid the foundation for the new field of spacecraft engineering. In the early years, spacecraft were of necessity small and lightweight. The main challenges were miniaturizing components and operating on small amounts of electrical power. The newly invented transistor and its packaging into integrated circuits were adopted by spacecraft engineers immediately in spite of their initially high cost. Power came from compact high-performance batteries supplemented by recently developed silicon solar cells to produce electricity from sunlight. The initiation of manned spaceflight added the challenges of providing a livable environment for the crew and returning them safely to Earth. Spacecraft engineering had to confront the biological issues of providing air, water, and food while disposing of waste products. Long-duration manned missions raised quality-of-life issues such as comfort, privacy, personal hygiene, and physical fitness. In the 1970s, interplanetary spacecraft ranged far from the sun and strained the capabilities of solar panels to provide sufficient electrical power. Spacecraft engineers harnessed the energy of radioactive decay to power deep-space missions and to keep the spacecraft warm so far from the sun. Missions to Mercury and Venus, so near the sun, posed the op-

Spacecraft Engineering

posite challenge of keeping the spacecraft from overheating. Command, control, and data acquisition from remote platforms with limited power available for broadcasting has always been and continues to be a major communications challenge for the spacecraft engineer. HOW IT WORKS Spacecraft. Spacecraft are composed of a payload and a bus. The bus is designed as a major system composed of seven or more subsystems. The electrical power system (EPS) provides the power to operate the active components. The communications system (Comms) maintains contact with ground control. The command and data handling system (C&DH) issues electronic commands to all onboard units and collects data from each unit for transmission to the ground. The thermal control system (TCS) regulates the temperatures of all onboard units to keep them within acceptable operating ranges. The attitude control system (ACS) controls the rotational dynamics of the spacecraft to achieve and maintain the required orientation in space. The propulsion system (PS) makes necessary changes in the trajectory of the spacecraft to keep it on course. The structure holds all of the spacecraft components and provides the mechanical support necessary during manufacture, transport, and launch. Manned spacecraft include additional environmental control and life-support systems (ECLSS). Redundant (duplicate) components and subsystems are used as much as possible to maximize reliability. Electrical power. The electrical power system is responsible for power generation, capture, or storage, plus delivery of conditioned electrical power to all parts of the spacecraft. Power may be generated by onboard reactors such as fuel cells, or captured from sunlight by solar panels. Power from sunlight is stored in rechargeable batteries for later use when the spacecraft is in eclipse, or for times when power

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demand temporarily exceeds the total available from solar panels alone. Power-conditioning circuits are necessary to provide electricity at the voltage, current, and stability required by individual components. Communications (comms). Communications consists of the antennas, transmitters, receivers, amplifiers, modulators, and demodulators necessary for communications with the ground. Comms may also include equipment for encrypting and decrypting the signal to prevent interception and to block attempts at illegally seizing control of the spacecraft with false messages. Communication must be maintained across distances that may stretch billions of miles in the case of interplanetary probes using signals of modest strength because of the limited amount of electrical power available. Static-free frequency modulation (FM) signals are preferred to minimize errors in transmission. High frequencies, on the order of billions of cycles per second, allow large amounts of data to be moved quickly. Spacecraft engineers are also experimenting with internet-type communications protocols, as well as laser communication systems that encode data onto a beam of light. Command and data handling. The command and data handling system centers on the flight computer. The computer monitors the status of all components: turns them on and off in accordance with schedules transmitted up from the ground, collects housekeeping data from all units, and science data from any science packages onboard. Attitude control system. High-gain antennas must be accurately pointed toward Earth to maintain communications with ground control. Remote-sensing instruments and science packages need to be pointed at their study targets. Manned spacecraft must maintain proper attitude for safe reentry. To achieve all of these, the altitude control system senses the orientation of the spacecraft relative to the fixed stars and reorients the spacecraft as necessary to fulfill the mission.

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Spacecraft orientation is determined by reference to the Sun, Earth, and the brightest stars. The Sun and the bright stars can be located optically; Earth can be sensed even during eclipse by the infrared radiation emitted by its warm surface. Interplanetary probes can locate Earth by homing in on the radio signal coming from ground control. Some spacecraft require three-axis stability where rotation about any axis must be rigorously suppressed. If the spacecraft begins to rotate in an undesired manner, onboard gyroscopes are spun up to absorb the additional rotational momentum and, by reaction, leave the spacecraft as a whole stationary. Many other spacecraft maintain stability by rotation about a fixed axis. Control of the altitude control system is the responsibility of the command and data handling system. When total rotational momentum of the spacecraft gets too large, the excess is eliminated by firing the attitude thrusters in the propulsion system. Propulsion system. The propulsion system performs occasional maneuvers required to keep Earth-orbiting satellites on station or interplanetary probes on course. The propulsion system consists of rocket thrusters, propellant, pumps, valves, and pressure vessels. Attitude control thrusters control the rotational dynamics of the spacecraft. Course correction thrusters change the speed or direction of motion of the spacecraft. The propellant must be storable for long periods of time under the harsh conditions of space. Special pressurization techniques are necessary to move liquid propellants from tanks to thrusters in zero gravity. The spacecraft must carry enough propellant for the planned mission lifetime plus a reserve necessary for deorbit at end of life. Structure. Spacecraft structures must satisfy the competing requirements of strength and low weight. Spacecraft structures must not bend or sag under the acceleration loads experienced during launch. Nuts and bolts must contain a locking feature to prevent them from loosening under vibrational

Principles of Aeronautics

loads. The mass of the structure plays a passive role in thermal control by conducting heat from warmer to colder parts of the spacecraft. Thermal control system (TCS). The TCS uses active and passive methods of moving heat to maintain normal operating temperatures for critical components. Active methods include the use of heaters to warm cold objects and pump-driven fluids to move heat from hot to cold areas. Passive devices include reflective coatings and insulation. Environmental control and life-support systems (ECLSS). Life-support systems manage air, water, food, and waste. Humans require a pressurized atmosphere that provides oxygen for respiration and humidity for comfort, while removing carbon dioxide. Too little oxygen leads to hypoxia, while too much leads to oxygen toxicity. Too much carbon dioxide leads to carbon dioxide poisoning. Too little humidity leads to extreme crew discomfort and possibly dangerous electrostatic discharges; too much humidity leads to condensation, which can interfere with electrical systems and nurture the growth of bacteria and fungi. Humans need about three kilograms of water a day for proper hydration plus an additional three kilograms for hygiene and housekeeping. Active adults require 2,500 to 5,000 calories a day to function without losing body mass. Corresponding amounts of waste are generated. Captured waste products must either be recycled, returned to Earth, or dumped overboard. Short-duration flights can be open-loop, that is to say, all the required consumables can be onboard at launch and the waste products dumped overboard or disposed of after landing. The mass involved is prohibitive for long-duration flights. Open-loop systems can continue to be used if regular resupply flights are possible, as is done for the ISS. For manned missions to Mars, by contrast, resupply will be difficult if not impossible, driving the ECLSS design toward full regenerative recycling, where carbon dioxide is taken up by plants that provide food and oxygen and

Spacecraft Engineering

wastewater is purified and reused. The ISS has an important role as a test bed for these emerging technologies. APPLICATIONS AND PRODUCTS Commercial spacecraft. To date, all commercial spacecraft have been communications satellites or remote-sensing satellites. Geosynchronous satellites appear stationary in the sky to an observer on Earth and make radio contact through fixed antennas possible. Television and radio programming sent up from the ground are amplified and rebroadcast to anyone with a line of sight to the spacecraft. Because the satellite never sets, the signal is uninterrupted. These satellites also provide radio and telephone communications for ships at sea and people living in remote locations. The market for these satellites supports a number of spacecraft manufacturers worldwide. Remote-sensing satellites observe the surface of Earth across a broad range of the electromagnetic spectrum that stretches from radio waves, through the infrared and optical bands, and into the ultraviolet. The oldest and most mature application of remote sensing is in the field of weather forecasting. Infrared and optical photography from space is used in land-use planning, mapping, crop surveys, and pollution monitoring. In 2021, the first private passenger-carrying suborbital spacecraft carried groups of tourists to the threshold of space. The flights were carried out by SpaceX, Virgin Galactic—owned by British billionaire Richard Branson—and Blue Origin—owned by Amazon founder Jeff Bezos—all took paying customers on flights into space. Scientific spacecraft. The first US spacecraft, Explorer I, made the first major scientific discovery of the space age when it discovered belts of protons and electrons trapped by Earth’s geomagnetic field. Discoveries by scientific spacecraft of all nations have profoundly changed mankind’s knowledge of

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Earth and its place in the Universe. Interplanetary probes have mapped almost every significant body in the solar system and discovered dozens of planetary moons undetectable from Earthbound telescopes. The Hubble Space Telescope has photographed stars at birth, stars at death, and a black hole at the center of the Milky Way. Energy that does not penetrate Earth’s atmosphere such as X-rays, gamma rays, and high-energy ultraviolet rays can be studied only from space platforms. The James Webb Space Telescope was launched on December 25, 2021, from Guiana Space Centre by the European Space Agency, NASA, and the Canadian Space Agency. It reached its final destination at the Sun-Earth L2 Lagrange point in January, 2022. Its larger size and enhanced capabilities allow it to capture clear images of celestial objects too old, too distant, or simply too faint for the Hubble telescope, and in its relatively short period of operation to date it has returned a stunning amount of new knowledge of the Universe. Military spacecraft. Military spacecraft include communications satellites, reconnaissance and surveillance satellites, missile-attack early-warning satellites, and navigation-support satellites such as the global positioning system (GPS) constellation. Military spacecraft are considered force-enhancement or force-support assets. They do not directly engage in hostilities. SOCIAL CONTEXT AND FUTURE PROSPECTS Spacecraft and the services they provide are now part of everyday life. Airliners navigating across oceans and pedestrians navigating city streets rely on GPS devices to find their destinations, and embedded GPS chips in cell phones allow parents to track their children anywhere. Satellites bring television straight to the home and radio straight to the automobile. Public databases allow anyone connected to the Internet to acquire a satellite photo of almost any spot on Earth. Accurate long-range

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weather forecasting exerts a daily influence on almost all business and personal planning. Spacecraft are now an indispensable part of the global economic and social infrastructure. This infrastructure must be replaced as it ages and wears out, creating a continuing demand for the services of the spacecraft engineer. Spacecraft have become so numerous in low-Earth orbit that disposal of spacecraft at end of life is a major design challenge for the modern spacecraft engineer. On February 10, 2009, Iridium-33, a US communications satellite, collided with Cosmos-2251, a defunct Russian military communications satellite. The debris generated by the collision, as well as other debris, threatens other satellites at the same orbital altitude. Future collisions are certain to happen more often as the population of spacecraft and “space junk” increases: They are best prevented by deliberately deorbiting nonoperational spacecraft so that they burn up in the atmosphere in reentry or by moving them to orbits at seldom-used altitudes. Tougher regulation by United States and international agencies is highly probable. In 2011, NASA shut down the space shuttle program, which provided manned spaceflights to the ISS, so that commercial firms would take over this function. Companies such as Boeing and SpaceX received funding from NASA for this purpose, and in 2018, SpaceX provided the first private spacecraft to reach the ISS. In 2020, SpaceX for the first time brought a NASA crew to the ISS. —Billy R. Smith Jr. Further Reading “Aerospace Engineering and Operations Technologists and Technicians.” US Bureau of Labor Statistics, 18 Apr. 2022, www.bls.gov/ooh/architecture-and-engineering/ aerospace-engineering-and-operations-technicians.htm. Accessed 8 June 2022. Barrera, Thomas P. Spacecraft Lithium-Ion Battery Power Systems. Wiley, 2023.

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Chen, Qi, Zhigang Liu, Xiaofeng Zhang, and Liying Zhu. Spacecraft Power Systems Technologies. Springer Singapore, 2020. Edwards, Bradley C., and Eric A. Westling. The Space Elevator: A Revolutionary Earth-to-Space Transportation System. B.C. Edwards, 2003. Howel, Elizabeth. “Space Tourism Took a Giant Leap in 2021: Here’s 10 Milestones from the Year.” Space.com, 27 Dec. 2021, www.space.com/space-tourism-giant-leap2021-milestones. Accessed 8 June 2022. Reid, Corey. Spacecraft Engineering: Systems and Design. CLANRYE International, 2022. Reid, Corey. Spacecraft Engineering. New York Research Press, 2022. Seedhouse, Erik. Tourists in Space: A Practical Guide. Chichester, England: Praxis, 2008. Swinerd, Graham. How Spacecraft Fly: Spaceflight Without Formulae. Copernicus Books, 2008. Swinerd, Graham, John Stark, and Peter Fortescue, editors. Spacecraft Systems Engineering. Wiley, 2011. Whitcomb, Dan, and Steve Gorman. “NASA Pushes Back Time Frame for Human Moon Mission to 2025.” Reuters, 9 Nov. 2021, www.reuters.com/lifestyle/science/ nasa-says-wont-send-manned-mission-moon-until-2025nyt-2021-11-09/. Zhang, Hua, Yuting Zhang, Chengbo Huang, Yanxing Yuan, and Lili Cheng. Spacecraft Electromagnetic Compatibility Technologies. Beijing Institute of Technology Press, 2020. See also: Aeronautical engineering; Richard Branson; Robert H. Goddard; Gravity and flight; Johnson Space Center; Materials science; National Aeronautics and Space Administration (NASA); Propulsion technologies; Rocket propulsion; Rockets; Russian space program; Space shuttle; Spaceflight; Konstantin Tsiolkovsky

Spaceflight Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Spaceflight, or space exploration, or space travel is flight beyond Earth’s atmosphere using artificial satellites, space

Spaceflight

probes, or crewed spacecraft. Spaceflight is considered the greatest scientific, technological, and human adventure of the twentieth and twenty-first centuries, allowing humans to explore what is considered by many to be the final frontier. KEY CONCEPTS command module (CM): the segment of a lunar mission rocket dedicated to the overall operational control of the mission’s operations and functions landing module (LM): the segment of a lunar mission rocket used to set the lunar excursion module down on the surface of the Moon and return the astronauts and recovered samples to lunar orbit lunar excursion module (LEM): the segment of a lunar mission rocket that supports the functions of astronauts on the lunar surface after they have landed multistage rocket: a rocket essentially consisting of two, three or more individual rocket motors in a stacked conformation comprising the single rocket, each stage being used up in sequence for different thrust characteristics satellite: an object either natural or artificial that orbits a larger or more massive body in space BACKGROUND Humans have long dreamed of leaving Earth to explore extraterrestrial worlds. Ancient writers told stories of trips beyond Earth, and natural philosophers speculated that heavenly bodies were made of an element completely different from terrestrial elements. In the sixteenth century, Polish astronomer Nicolaus Copernicus vastly expanded humanity’s knowledge of the space containing these heavenly bodies by locating the Sun, instead of Earth, at the universe’s center. As astronomical knowledge increased, storytellers imagined spaceflights of increasing sophistication. In the nineteenth century, writers such as Jules Verne, H. G. Wells, E. R. Burroughs, and others de-

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picted space travel in elaborate technical detail. In the twentieth century, science-fiction writers described spaceflight with scientific accuracy, and their stories became more popular than they ever had been, as practical means of going into space became a reality. HISTORY Just as Orville and Wilbur Wright had to solve several basic problems before achieving success in their first airplane, so, too, did spaceflight pioneers need to solve such problems as discovering a way to escape Earth’s gravity. Rockets were first proposed for spacecraft propulsion in the twentieth century. The Russian engineer Konstantin Tsiolkovsky wrote extensively on the theory of spaceflight, including the

Principles of Aeronautics

need for multistage rockets, where two or more rockets are ignited in turn. The American physicist Robert H. Goddard designed, built, and launched the first liquid-fueled rockets. During World War II, the German rocket pioneer Wernher von Braun led a team of scientists who developed the first rocketpowered ballistic missile. Although the Germans designed this V-2 as a weapon, it became the model for all rockets—military, scientific, civilian, and commercial—that followed it. Toward the end of World War II, the US and Soviet military captured German scientists and engineers who had worked on the V-2 project. These scientists formed the core of postwar rocket-research programs. The United States launched more than fifty captured V-2 rockets and began using two-stage

SpaceX’s Crew Dragon capsule approaching the International Space Station in Earth orbit. Photo via Wikimedia Commons. [Public domain.]

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rockets for upper-atmosphere studies. Some of these vehicles achieved spaceflight, reaching the point where space begins, about 100 kilometers above Earth’s surface. However, they did not have enough speed to go into orbit. The Soviet Union was the first country to achieve orbital spaceflight when Sputnik 1 began to circle Earth on October 4, 1957. This first artificial satellite ushered in the age of spaceflight. A month later, the Russians launched Sputnik 2, which contained the dog Laika, the first space traveler from Earth. These first Soviet spaceflights created a sensation around the world and especially in the United States, where it had long been assumed that Americans would be the first to achieve spaceflight. The Sputnik flights did much to change the nature of the Cold War from a political conflict between the United States and the Soviet Union to a comprehensive competition involving science, technology, and economics. Spaceflight became a symbol of the achievements of two different societies, capitalist and communist. After civilian rockets failed to launch American satellites, President Dwight D. Eisenhower turned to the military for assistance. Five days after Sputnik 2 entered orbit, the US Army used a Jupiter C rocket to orbit Explorer 1, whose instrumentation had been developed by University of Iowa physics professor James Van Allen. This Explorer mapped a doughnut-shaped region of high radiation surrounding Earth that was later named the Van Allen radiation belts. After the Soviet Union launched Sputnik 3 on May 15, 1958, US leaders realized that the United States was falling behind in the space race. An acrimonious debate between Congress and the Eisenhower administration ensued, with the final resolution that the US space program needed an effective legislative foundation. This legislation, the National Aeronautics and Space Act of 1958, created a civilian agency to explore space: the National Aeronautics

Spaceflight

NASA astronaut Bruce McCandless II using a Manned Maneuvering Unit outside Space Shuttle Challenger on shuttle mission STS-41-B in 1984. Photo by Askeuhd, via Wikimedia Commons.

and Space Administration (NASA). The act made no mention of crewed spaceflight, but its broad charter gave NASA the responsibility for the scientific, but not military, exploration of space. During the first few years of the space age, uncrewed spaceflight characterized both US and Soviet programs. These uncrewed spaceflights ranged from satellites in low-Earth orbit to probes aimed at interplanetary space. The first successful lunar probe was the Soviet Union’s Luna 1, which flew by the moon in January 1959. In March of that year, the United States Pioneer 4 glided by the moon, and in September, the Soviet’s Luna 2 became the first human artifact to land on the moon. A month later, the Russians used their circumlunar probe Luna 3 to photograph the far side of the moon. Soon after these uncrewed satellites and probes were launched, both Soviet and American scientists began work on crewed space vehicles. Because of their lead in large rockets, the Soviet Union was able, on April 12, 1961, to launch the world’s first

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crewed spacecraft, Vostok 1, a 3-tonne sphere with a 2-tonne service module. Soviet cosmonaut Yuri Gagarin thus became the first person to orbit Earth. After Gagarin’s single orbit and safe return to Earth, the Soviet Union achieved several firsts and set several records. It launched several endurance record-setting crewed spaceflights and had a cosmonaut take the first spacewalk. Furthermore, Vostok 6 was piloted by Valentina Tereshkova, the first woman to make a spaceflight. The Soviets were also the first to orbit a spacecraft containing three cosmonauts. The initial US program for crewed spaceflight was called Mercury, and it became the responsibility of the newly formed NASA. A few months into John F. Kennedy’s presidency and less than one month after Gagarin’s flight, Alan Shepard became the first American astronaut launched into space, though his suborbital flight in a Mercury capsule lasted only about fifteen minutes. The first American orbital flight was made by astronaut John Glenn on February 20, 1962. Other Mercury flights stretched the spacecraft’s orbital time to more than one day, and scientists and astronauts gained much valuable experience and information from the program, including the fact that humans should be active pilots rather than passive passengers during the missions. While the United States and the Soviet Union developed their crewed spaceflight programs, both countries continued to develop uncrewed satellites and probes. For example, Americans launched the Television Infrared Observations Satellite (TIROS), the first weather satellite, in 1960, and it recorded more than 23,000 cloud images. Mariner 2, sent off by US scientists in 1962, became the first spacecraft to explore another planet, Mars. From 1962 to 1965, the United States sent a series of Ranger probes to the moon to take close-up photographs of its surface. The first successful soft landing on the moon was that of the Soviet Union’s Luna 9 on February 3, 1966. The United States achieved a success-

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ful soft landing on June 2, 1966, with its Surveyor 1. On April 3, 1966, the Soviet’s Luna 10 became the first probe to successfully orbit the moon. The first American lunar orbiter went around the moon on August 14, 1966. With these and other lunar projects, it seemed obvious to many that the United States and the Soviet Union were engaged in a race to land humans on the moon. CREWED LUNAR SPACEFLIGHTS The early Soviet successes in spaceflight placed intense political pressure on the US president and lawmakers to find some accomplishment by which the United States could pull ahead of the Soviet Union. President Kennedy’s advisors suggested a crewed landing on the moon as such an achievement, and on May 25, 1961, Kennedy stood before Congress to ask the nation to “set the goal of landing a man on the Moon, before this decade is out, and safely returning him to Earth.” To attain this goal, NASA officials first had to decide how to get to the moon. Eventually NASA scientists chose a lunar orbit rendezvous method, and consequently astronauts practiced rendezvous and docking techniques as part of the Gemini Program, a series of increasingly demanding missions with a two-person spacecraft. The Gemini missions had three phases. In the earliest missions, which began in 1965, astronauts tested the spaceworthiness of the Gemini spacecraft. They also performed the first American extravehicular activities (EVAs) and made the first-ever use of a personal propulsion unit. The middle Gemini missions, Gemini 4, Gemini 5, and Gemini 7, explored human endurance in space by progressively extending stays to two weeks, the maximum time that an Apollo lunar trip was expected to take. The final Gemini missions allowed astronauts to master the techniques of chasing a target vehicle and docking with it. Apollo was the name of the mission to land men on the moon. Tragically, before its first orbital trial,

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the Apollo Program came to an abrupt halt when, on January 27, 1967, a fire killed three astronauts, Roger Chaffee, Virgil “Gus” Grissom, and Edward White, in the command module (CM) during a countdown exercise. The spacecraft had a pure oxygen atmosphere and much flammable material, and a spark caused by an electrical short circuit ignited flames that rapidly engulfed the astronauts, who died of asphyxiation. Until the fire, the Apollo Program had proceeded without major difficulties, but these deaths delayed the first missions. NASA scientists redesigned the CM by minimizing flammable materials and changing the prelaunch cabin atmosphere to a mixture of 60 percent oxygen and 40 percent nitrogen. The success of the Apollo missions depended on the gigantic Saturn V rocket that had been developed by Wernher von Braun. The initial missions in the Apollo series were uncrewed tests of the Saturn and CM engines. For example, on April 4, 1968, the CM and the lunar module (LM) were tested on Apollo 8. The first crewed test, which began on October 11, 1968, was Apollo 7, the objective of which was to test the safety and reliability of all the spacecraft’s systems. The first spaceflight involving humans leaving Earth orbit and traveling to the moon was Apollo 8. This flight began on December 21, 1968, and the spacecraft went into lunar orbit on December 24, when the astronauts described the moon’s surface and read a passage from the first book of the Bible. The Apollo 9 mission in March 1969, tested the command and service module (CSM) and the LM in Earth orbit, and the Apollo 10 mission in May tested the CSM and LM in a lunar orbit. The lunar landing mission, Apollo 11, took place between July 16 and July 24, 1969. On July 20, Neil Armstrong, after adeptly piloting the LM to a safe landing, became the first person to step onto the surface of the moon, and he was later joined by Edwin “Buzz” Aldrin. Armstrong and Aldrin spent about two and one-half hours collecting rocks and

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setting up scientific experiments. Several hours later, their capsule, the Eagle, rocketed from the moon to rendezvous with the CSM, the Columbia, which was piloted by Michael Collins. All three astronauts returned safely to Earth, where they received a jubilant reception. From 1969 through 1972, six other Apollo missions traveled to the moon, although Apollo 13 was unable to land on the lunar surface because of an explosion in one of its oxygen tanks. The Apollo 13 crew had to use the life-support systems of the LM Aquarius to help them survive the long trip back to Earth. NASA engineers consequently redesigned the oxygen tanks, and the final four Apollo lunar missions were able to explore the moon safely and extensively. The hundreds of pounds of moon rocks that were returned to Earth have given scientists a deeper understanding of the origin and evolution both of the moon and of the entire solar system. SPACEFLIGHT AFTER APOLLO Travel to the moon was a risky and expensive enterprise, and neither the United States nor the Soviet Union made the trip in the 1980s and 1990s. Critics of crewed spaceflight pointed out that science was much better and more inexpensively served by space satellites and probes. In the three decades after Apollo, robotic explorers such as Viking, Voyager, and Galileo proved to be highly efficient knowledge-gatherers. In 1976, two Viking spacecraft arrived at Mars: an orbiter that photographed the planet from above and a lander that analyzed rocks on the surface. In 1977, two Voyagers were launched by NASA to start their twelve-year journey to the outer reaches of the solar system. The scientific instruments and cameras on the Voyagers sent back highly detailed information about the giant planets of Jupiter, Saturn, Uranus, and Neptune, along with the planets’ fifty-seven moons. Voyager highlights included dramatic pictures of the turbulent storms of Jupiter’s

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complex atmosphere, revelations of the complexities of Saturn’s many rings, active volcanoes on Jupiter’s moon Io, Neptune’s Great Dark Spot, and nitrogen geysers on Neptune’s moon Triton. In 1997, Galileo became the first spacecraft to orbit an outer planet, and it has gathered much useful information about Jupiter’s moons. The Soviets, too, used robotic probes in their scientific studies of the solar system. For example, in 1975, Venera 9 landed on the surface of Venus and returned the first pictures of its rocks and soil. These uncrewed missions did not mean the end of crewed explorations of space. In 1971, Soviet scientists launched Salyut, the world’s first space station. The Americans later launched their own space station, Skylab, which was visited by three-person crews in the 1970s, during which time astronauts made detailed studies of Earth’s continents, oceans, and atmosphere. In 1975, the United States and the Soviet Union cooperated in the first international docking in space, when astronauts and cosmonauts performed an orbital rendezvous between an Apollo and a Soyuz capsule. To make crewed spaceflight less expensive and more frequent, NASA developed the Space Transportation System (STS), commonly known as the space shuttle. Because landings at airfields are much less expensive than splashdowns at sea, and because it makes economic sense to reuse rockets, NASA engineers designed the space shuttle as a winged vehicle that was launched as a rocket, with two recoverable rocket boosters, and landed as an airplane. In 1981, the space shuttle Columbia made its first flight. The other orbiting space shuttles of the 1980s and 1990s were Challenger, Atlantis, and Discovery. The missions of these shuttles included launching artificial satellites and retrieving them for servicing and repairs; performing scientific experiments in space; conducting secret military missions; and launching commercial communication satellites.

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Despite NASA’s aim for routine trips to space, the shuttle was plagued with problems, most notably the Challenger explosion on January 28, 1986. All seven crew members, including Sharon Christa McAuliffe, a New Hampshire schoolteacher, were killed. NASA stopped all shuttle missions while a special commission appointed by President Ronald Reagan studied the accident in order to determine the cause of the accident and the prevention of future such tragedies. The cause was a failure of a rubber O-ring that sealed the joint between two segments of one of the rocket boosters. To prevent any recurrence of this disaster, NASA engineers redesigned the booster joints and added a bailout system that improved chances for crew survival in a crisis. The space shuttle resumed flying on September 28, 1988, with the liftoff of a redesigned Discovery. One of the successes of the revamped STS was the Hubble Space Telescope (HST), which was launched from an orbiting shuttle in 1990. The HST was an uncrewed observatory far above the atmosphere of the earth, whose haze, clouds, and turbulence hampered telescopes on the ground. Unfortunately, after the HST was in orbit, astronomers discovered a problem with its mirror that seriously hindered its effectiveness. A shuttle repair mission in 1993 helped the HST achieve its astronomical potential. The HST was then able to take dramatic photographs of star births in the Eagle nebula and of galaxies 10,000,000,000 light-years away. It also measured an unimaginably gigantic burst of gamma rays in a distant galaxy that is the most powerful explosion ever observed. The 1980s and 1990s were also characterized by an increasing number of spaceflights from countries other than the United States. On February 20, 1986, the Soviets launched the large Mir space station, which remained in orbit until 2001, when it was manipulated to fall harmlessly into the Pacific Ocean. During Mir‘s fifteen-year existence, cosmonauts set endurance records and learned much

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about how humans can live for long periods in space. After the Soviet Union ceased to exist in 1991, Russia took over the operation of Mir. Space cooperation between America and Russia resumed in 1995, when the space shuttle began to dock with Mir, which was periodically occupied by astronauts from various countries, including the United States. With the end of Mir, crewed spaceflight centered on the International Space Station (ISS). The idea behind the ISS was to share among several nations the cost of the construction and operation of a large space station. However, the structure and timetable of the ISS was continually changed during the presidencies of Ronald Reagan and Bill Clinton. The United States, Canada, Japan, Russia, and the European Space Agency (ESA) agreed to cooperate in building the redesigned ISS, whose construction actually began in outer space in 1998. The space station, which was planned to be the size of a football field, was the focus of spaceflights in the early twenty-first century and was ultimately completed with a final mission in 2011. With that final mission, NASA also announced the official retirement of its space shuttle program due to the craft’s failure to deliver on the promise of less expensive spaceflight through reusability. The last three shuttles that had been in use, Discovery, Endeavour, and Atlantis were moved to different facilities for exhibition. The participation of several nations in the ISS was but another indication of the increasing involvement in spaceflight of countries around the world. Although the United States and the Soviet Union monopolized the early history of spaceflight, France launched its first satellite in 1965, and Britain its first in 1971. Fourteen nations founded the ESA in 1975 to combine their economic and scientific resources to develop new spacecraft for various missions. One of ESA’s achievements was the space probe Giotto, sent to study Halley’s comet in 1986.

Spaceflight

Japan also sent a probe to Halley’s comet, and the nation’s Advanced Earth Observing Satellite, launched in 1995, has gathered important information on Earth’s lands, seas, and atmosphere. Other nations that have become actively involved in spaceflight are China, India, Canada, Israel, Australia, Brazil, Sweden, and South Africa. Another trend of the late twentieth and early twenty-first centuries has been the commercialization of spaceflight. Various communications satellites have proved to be successful moneymakers for several companies. Some companies and governments have begun research on commercial crewed spaceflight, but these efforts have encountered serious difficulties because of the high cost of spaceflight. Similar problems have hindered plans for interplanetary travel, such as a crewed voyage to Mars. Critics of crewed spaceflight argue that it redirects funds from useful uncrewed programs and from important social and medical programs. In contrast, enthusiasts of crewed spaceflight cite the medical and technical knowledge that has been derived from previous space ventures and emphasize the dreams that have energized scientists and engineers throughout history and the ineradicable desire to explore other worlds. By the second decade of the twenty-first century, following years of robotic rovers and probes examining the surface of Mars, the planet believed to have once had an atmosphere most suitable for life aside from Earth, plans were being made in earnest to send a crewed mission to land on and possibly even colonize the planet. While NASA had announced a goal to land humans on Mars’s surface by the 2030s, independent missions had also sprung up with a goal to get humans there even sooner. As of 2016, entrepreneur Elon Musk had introduced a controversial proposal for his company Space Exploration Technologies Corp. (SpaceX) to use several reusable rockets to send crewed spacecraft into orbit about Mars within ten

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years. At the same time, the Dutch nonprofit project Mars One announced that it was accepting applications for volunteers for a one-way trip to live and die on Mars within a decade as well. However, at that point, both projects had failed to adequately explain how the missions would be funded. Regardless, at least the SpaceX mission has proceeded apace. —Robert J. Paradowski Further Reading Dick, Steven J., and Roger D. Launius. Societal Impact of Spaceflight. NASA, Office of External Relations, History Division, 2022. “NASA’s Journey to Mars.” NASA, 14 Sept. 2015, www.nasa.gov/content/nasas-journey-to-mars. Accessed 2 Nov. 2016. Neufeld, Michael J. Spaceflight: A Concise History. MIT Press, 2018. Pasztor, Andy. “Elon Musk Outlines Plans for Missions to Mars.” The Wall Street Journal, 27 Sept. 2016, www.wsj.com/articles/elon-musk-outlines-plans-for-manne d-missions-to-mars-1475011627. Accessed 2 Nov. 2016. Pyle, Rod. Space 2.0: How Private Spaceflight, a Resurgent NASA, and International Partners Are Creating a New Space Age. BenBella Books, 2019. Seedhouse, Erik. SpaceX: Making Commercial Spaceflight a Reality. Springer New York, 2013. Smith, Andrew. “Can Mars One Colonise the Red Planet?” The Guardian, 30 May 2015, www.theguardian.com/ science/2015/may/30/can-mars-one-colonise-red-planet. Accessed 2 Nov. 2016. Sparrow, Giles. Spaceflight: The Complete Story from Sputnik to Curiosity. DK Publishing, 2019. Zubrin, Robert. The Case for Space: How the Revolution in Spaceflight Opens Up a Future of Limitless Possibility. Prometheus Books, 2019. See also: Aeronautical engineering; Neil Armstrong; Yuri Gagarin; Robert H. Goddard; Jet Propulsion Laboratory (JPL); Johnson Space Center; National Aeronautics and Space Administration (NASA); Propulsion technologies; Rocket propulsion; Rockets; Russian space program; Space shuttle; Spacecraft engineering; Valentina Tereshkova; Konstantin Tsiolkovsky; Jules Verne

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Stabilizers Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics; Mathematics ABSTRACT Small wings that are placed at positions forward or aft of an aircraft’s wings to provide balance in pitch and yaw during flight. On a missile, stabilizers may also be known as fins. Without stabilizers, it would be very difficult or even impossible to control the orientation of an aircraft or missile in flight. KEY CONCEPTS canard: a small stabilizer wing system set ahead of the main wings of an airplane center of gravity: the point within an aircraft, or any other body, about which the entire mass of that body is equally distributed center of lift: the point at which the lift force appears to function against the weight of an airplane stall: the loss of the ability of wings to provide lift due to ascending at an angle that causes the pressure difference between the upper and lower surfaces of a wing to decrease and equalize PLAIN WINGS Early attempts to achieve gliding flight used only wings, occasionally adding the shifting of weights beneath the wing to keep the wing balanced in its motion. At any given angle of attack, there is some point on the wing where the forces are in balance, but this position, sometimes known as the “center of lift” or “center of pressure,” moves forward or aft as the wing’s angle to the flow changes. The location of this point and its distance from the center of mass or gravity of the wing or vehicle will determine its pitching moment, that is, the tendency of its nose to move up and down, rotating around the center of

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gravity. It is considered desirable to have the center of lift behind the center of gravity for positive stability, that is, to create a natural tendency for the vehicle to return to level flight after any disturbance. For example, in a stable aircraft, a gust-induced increase in lift will cause the airplane to rotate nose downward and automatically reduce its lift in correction. Because of this, a stable airplane will always tend to rotate nose down in flight unless that rotation is counteracted by another force or moment. This correction is the purpose of the horizontal stabilizer. HORIZONTAL STABILIZERS The horizontal stabilizer is normally placed on the rear or tail of the fuselage, somewhat like the tail feathers of a bird. This placement requires the stabilizer to have a downward load or negative lift to counteract the nose-down moment of the wing. The common horizontal stabilizer is usually a small wing

placed toward the rear of the fuselage and mounted at a negative angle of attack so that it will cause a downward force. The stabilizer is usually equipped with flaps known as elevators, which can be moved up and down to alter the force on the stabilizer, allowing the pilot to rotate the aircraft nose up or nose down in pitch. This allows control of the angle of attack of the wing and, hence, control of the lift produced by the wing. When larger control forces are needed, the whole stabilizer is designed to be moveable or to rotate about a pivot point, and it is then known as a stabilator or elevon. Some airplane designers believe that the horizontal stabilizer should be in front of the wing, where it can correct the nose-down pitch of the stable wing with an upward force, thus increasing the lifting capability of the aircraft instead of decreasing it, as may happen with a tail-mounted stabilizer. The Wright brothers and other early aviators used this

Vertical and horizontal stabilizer units on an Airbus A380 airliner. Image by Olivier Cleynen, via Wikimedia Commons.

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A Boeing 737 uses an adjustable stabilizer, moved by a jackscrew, to provide the required pitch trim forces. Generic stabilizer illustrated. Image via Wikimedia Commons. [Public domain.]

arrangement on their primitive designs but, like most others, eventually built airplanes with the horizontal stabilizers at the tail. When the horizontal stabilizer is in front of the wing, it is called a canard. There are special circumstances, such as transonic and supersonic flight, where canards may have advantages, but most analyses show that the best place for the horizontal stabilizer is near the tail of the aircraft. VERTICAL STABILIZERS The vertical stabilizer is almost always mounted above the tail of the airplane. It is designed to limit the rotation of the aircraft in yaw, operating as a sort of weathervane, much like the feathers at the aft end of an arrow. Attached to the vertical stabilizer or fin is the rudder, which acts as a flap on the winglike stabilizer to move left or right and create forces which will yaw the airplane when desired.

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The vertical stabilizer on most single-engine airplanes is mounted on the fuselage at a slight angle to counteract the torque of the engine, which tends to make the fuselage try to roll in a direction opposite to the turning of the propeller. Some aircraft have two vertical stabilizers where larger control surfaces are needed or where at very high angles of attack, part of the stabilizer may be in the wake of the fuselage. Vertical and horizontal stabilizers are placed on an airplane in many different arrangements, depending on the control needs of the design. Sometimes the horizontal stabilizer is mounted on the vertical stabilizer, either at its top in a T-tail arrangement or part of the way up in a cruciform design. Often the vertical/horizontal tail arrangement is dictated by the need to control the airplane in stall and to make sure that, in that situation, the vertical stabilizer and rudder are not in the wake of the

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horizontal stabilizer, where their usefulness would be very limited.

and made its first flight on July 17, 1989. It was the first US military bomber with stealth capability.

MISSILE STABILIZERS The stabilizers on a missile are often simply referred to as fins. These small wings are mounted at the tail of the missile and are often fully moveable and do not have attached flaps. These moveable fins provide both balance or stability in flight and the control forces needed to maneuver.

KEY CONCEPTS elevons: wing components that serve the combined functions of ailerons and elevators as the means of steering control radar: an electronic detection system that uses detection of a reflected radio frequency signal from a specific source UAP: the acronym for unidentified aerial phenomena, commonly referred to as “UFOS” or “flying saucers,” many reports of which can be directly tied to the undisclosed operation of stealth aircraft

—James F. Marchman III Further Reading Bibel, George, and Robert Hedges. Plane Crash: The Forensics of Aviation Disasters. Johns Hopkins UP, 2018. Federal Aviation Administration (FAA). Aircraft Weight and Balance Handbook (FAA-H-8083-1A). Skyhorse Publishing, 2011. Yedavalli, Yeda K. Flight Dynamics and Control of Aero and Space Vehicles. Wiley, 2019. Young, Trevor M. Performance of the Jet Transport Airplane: Analysis Methods, Flight Operations, and Regulations. Wiley, 2019. See also: Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Biplanes; Federal Aviation Administration (FAA); Flight roll and pitch; Fluid dynamics; Forces of flight; Glider planes; Monoplanes; Plane rudders; Rockets; Tail designs; Triplanes; Wing designs

Stealth Bomber Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT The B-2, also known as the stealth bomber, Spirit bomber and Flying Wing, was the first strategic bomber that could penetrate enemy air defenses, attack enemy sites, and return without the assistance of fighter jets by using stealth technology. It was first introduced in November, 1988,

THE NEED FOR AN INVISIBLE PLANE During the Cold War, one of the difficulties facing the US military was how to penetrate the Soviet Union’s complex air defense systems in case of war. In the event of a military conflict with the Soviets, US planes would have to fly extended distances, evade Soviet defenses, bomb their targets, and return to US airspace. This would require fighter escort for protection and would result in losses from antiaircraft systems. The B-2 or “stealth” bomber became the answer to this problem. Jack Northrop, founder of the aircraft company that bore his name, had developed the idea of a plane that would lack the usual wings and tail used to control and stabilize the aircraft as early as 1949. Northrop’s design became known as the flying wing. Starting in the 1960s and continuing through the 1980s, the plane would be the most researched and most secret weapon developed by the US military. The stealth bomber was preceded by a prototype, the SR1-7, which was known for slanted and treated surfaces that made it less visible to radar. Continued development of the stealth bomber continued into the 1970s outside of public view. Only President

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Jimmy Carter’s slip of the tongue, that the military was developing a “stealth” bomber, brought the aircraft to the attention of the public. By the 1980s, the US military build-up included continued secret development of the stealth bomber. Its unveiling on November 22, 1988, gave the public its first view of the plane that was to change the way aircraft were built. THE “STEALTHY” PLANE The B-2 bomber was different from its predecessors in its ability to evade radar and other air defenses. There are several features of the B-2 that define it as the first stealthy plane of its type. The curved surfaces and relatively flat exterior of the stealth bomber, which stands only 17 feet high, serves to

Principles of Aeronautics

confuse radar, which tends to pass over and around it. The aircraft is coated with a special paint that scatters radar beams when they hit the plane. The graphite composition of the plane’s exterior—the exteriors of most planes are made of aluminum— also absorbs radar signals. The design and composition of the B-2 baffles the main detection devices used by air defense systems and allows the bomber to penetrate them without being detected. Spotting the stealth bomber with the naked eye is also difficult. Its slender shape and dark color can make it hard to see in daylight or dark. Even in the unlikely event it is seen, the stealth bomber has the capability to evade attack from the ground. One of the worst dangers to bombers is a heatseeking missile. These missiles are programmed to

F-117 Nighthawk stealth aircraft. Photo via Wikimedia Commons. [Public domain.]

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locate aircraft using the plane’s heated exhaust as a guide. The B-2 is protected from such attacks. The plane’s engines are located inside the plane, and the exhaust system, located at the top of the plane, cools the air from the engines before releasing it. This confuses the detection equipment on heat-seeking missiles and complicates efforts to shoot down the bomber from below or from the air. The B-2s design also makes the bomber fuel efficient, giving it a range of more than 9,600 kilometers. This allows it to fly a mission without being refueled, a process that makes bombers vulnerable to attack. The technology has also been applied to the development of the F-117A stealth fighter. Interestingly, the frontal profile of the F-117 is remarkably like many of the UFOs people have reported sighting. That this aircraft is undoubtedly responsible for several UFO reports is given credence by the fact that the F-117 was flown for many years before its existence was revealed publicly. In many respects, the top view of the F-117 bears many similarities to the deign of the B-2. HOW THE B-2 FLIES The plane’s stealth capabilities caused difficulties for its designers in creating a plane that could fly and be controlled. Because the B-2 lacks the standard wings and tail section with flaps and rudders to control direction and ascent and descent, they had to create an entirely new design. The lack of conventional wings, which control ascent and descent, forced designers to place all control items on the rear of the plane and to turn over many pilot decisions to onboard computers. At the outside rear of the plane there are two rudders, used to keep the plane from yawing off course. Minute adjustments to the direction of the plane are made by the B-2s computers. Also at the rear but nearer the center of the plane are three pairs of elevons, devices used to steer the plane and adjust it for de-

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scent and ascent. These act in the same way as the flaps on the wings of a standard plane. Once again, their use is controlled by onboard computers. Finally, at the center rear of the plane is the gust loud alleviation system (GLAS). The system takes the place of stabilizers, with computer adjustments to steady the plane when it hits air turbulence. THE FUTURE OF THE B-2 The collapse of the Soviet Union and its elimination as a military threat to the United States called into question the need for a bomber invisible to a nonexistent radar system. The high cost of building the B-2 in times of limited defense budgets also threatened the system. The original plans for 132 planes that would carry out the bulk of US air attacks during a war was reduced to 21 planes. This decision had the additional consequence of doubling the cost of each plane, to approximately $1.2 billion. This raised complaints that the B-2 was taking up funds needed for other military projects. Although the B-2 will never see action against the type of air defenses it was built to penetrate, the development of stealth technology produced a dramatic shift in military aircraft design. Instead of the large jet designs developed after World War II, the sleek, curved design has become the model for a new generation of military aircraft. The production of the B-2 ceased in 2004. In 2020, Russia began building its own stealth bomber, known as the PAK Da strategic bomber. News sources from the country announced that trials for the plane’s engine were scheduled to begin later that year, while final assembly of the entire plane was scheduled for 2021. China also began developing a stealth bomber, called the Xian H-20, in 2020. —Douglas Clouatre Further Reading Goodall, Jim, and Big Apple Agency. A Pictorial History of the B-2A Stealth Bomber. Schiffer Publishing Ltd., 2016.

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Hansen, Ole Steen. The B-2 Spirit Stealth Bomber. Capstone Press, 2005. “Russia Begins Construction of the First PAK Da Strategic Bomber—Sources.” TASS, 15 May 2020, tass.com/defense/1160253. Accessed 10 June 2020. “Russia Starts Building Its First Stealth Bomber: TASS.” Reuters, 26 May 2020, www.reuters.com/article/ us-russia-airplane/russia-starts-building-its-first-stealthbomber-tass-idUSKBN2322EZ. Accessed 10 June 2020. Westwick, Peter. Stealth": The Secret Contest to Invent Invisible Aircraft. Oxford UP, 2020. See also: Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Flying wing; Forces of flight; Materials science; Unidentified aerial phenomena (UAP); Wing designs

Supersonic Aircraft Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Supersonic aircraft, also called SSTs for supersonic transports, are aircraft that can fly faster than the speed of sound. The speed of sound (Mach 1) is a milestone in the range of aircraft speeds, and aircraft that can fly faster than this speed are different in many ways from those that are not designed to exceed the speed of sound. KEY CONCEPTS Bernoulli principle: the pressure exerted by a moving fluid such as air is inversely related to the speed of the fluid flow lift: the upward pressure experienced by wings due to differential pressure between upper and lower surfaces of the wing as it moves through a fluid medium as determined by its airfoil camber and thickness SST: acronym for supersonic transport

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AIRCRAFT AND SPEED The first fifty years of aviation was characterized by ever-increasing aircraft speeds, as aerodynamic research found ways to reduce airplane drag and as engine and fuel improvements made possible continual improvements in thrust and power. By the 1940s, highly streamlined airplanes with modern wing designs and powerful piston engines were capable of approaching the speed of sound in a full-power dive. When this occurred, as it did scores of times in some fighter aircraft of World War II, unexpected things began to happen which made the aircraft difficult, if not impossible, for the pilot to control, and sometimes caused the airplane to fail structurally. These problems aroused investigations in aerodynamic theory and led to experiments that verified high-speed flight experience. All combined to reveal huge increases in drag as the speed of sound. or Mach 1, was approached, leading to the definition of this speed as the “sound barrier.” As a high-speed aircraft flies through the atmosphere, the surrounding air must accelerate as it moves around the plane. It is this acceleration that creates lift on the wing. The wing is shaped to cause more flow acceleration over its upper surface than its lower surface, and this results in lower pressures on top of the wing and, hence, lift from the higher pressure below the wing. As aircraft speeds of 70 to 80 percent of the speed of sound are reached, portions of the flow over the wing will have accelerated to speeds greater than the speed of sound. Contrary to popular belief, it is not the existence of supersonic flow over the wing which causes the drag to rise. It is, rather, the normal inability of that supersonic flow to gradually slow back to subsonic speed that causes the problem. When a supersonic flow over a wing tries to slow to subsonic speed it will usually do so in a very sudden speed drop, with an accompanying jump to a higher pressure. This sudden change in speed and pressure is known as a shock wave, and the pressure

Principles of Aeronautics

Supersonic Aircraft

Using the schlieren photography technique, NASA was able to capture the first air-to-air images of the interaction of shockwaves from two supersonic aircraft flying in formation, 2019. Photo via Wikimedia Commons. [Public domain.]

jump often causes the flow to separate or break away from the surface of the wing, resulting in a large, drag-producing wake. When this occurs, often at speeds between 70 and 80 percent of the speed of sound, the drag on the wing and, hence, the drag on the airplane begins to increase. The drag rises sharply as speed is increased beyond that point. At the same time, the new flow patterns over the wing change the way lift is produced on its surface and alter the way the aircraft tends to pitch nose up or down. Similar patterns of flow and separation over the stabilizer surfaces may result in an inability to use these to control the airplane, especially in view of the changed pitching moments. These changes, which can occur rather suddenly at the point of shock wave formation (the “critical” Mach number), can easily lead to loss of airplane control, along with increases in drag which quickly exceed the thrust of the engine. A plane not designed to handle these

loading changes can suffer structural failure and even lose a wing or tail surface. BREAKING THE SOUND BARRIER The combination of drag increase and stability changes that occurred on a wing as the speed of sound was approached created a very real fear of the sound barrier among pilots of aircraft capable of reaching these speeds in high-speed dives. Fortunately for many of these pilots, the speed of sound is a function of air temperature and, hence, of altitude. As the diving plane descended to lower altitudes, the speed of sound increased, the plane’s Mach number (its speed divided by the speed of sound) decreased, and the shock waves disappeared, if the aircraft could hold together that long. Aerodynamicists and others who studied the flows around wings at speeds near that of sound learned to design wings and control surfaces that could han-

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dle the changes in this transonic speed range and, in the 1940s, they began to look at airplane designs which would allow flight beyond the sound barrier to supersonic speeds. The development of new jet and rocket engines also offered the hope of producing enough thrust to overcome the large increase in drag that occurred as Mach 1 was approached. One such design was the Bell X-1. Despite the belief that drag became infinite at the speed of sound, a misinterpretation of aerodynamic theory, engineers at the National Advisory Committee for Aeronautics (NACA) knew that bullets and artillery shells flew at supersonic speeds. They realized that if an airplane could be supplied with enough thrust and if wings and stabilizers could be designed for transonic operation and control, it would be possible to break the so-called sound barrier. To help ensure their success, they modeled the shape of their new experimental airplane on an artillery shell. The X-1 was equipped with a rocket engine and designed for air launch, in which it was dropped from beneath the wing of a B-29. Its rocket engine ignited for longer periods of time in subsequent flights, gradually pushing its speed toward Mach 1. Finally, on October 14, 1947, with Air Force Captain Charles E. “Chuck” Yeager at the controls, the X-1 reached and exceeded the speed of sound. At 1,126.5 kilometers per hour, Yeager had reached Mach 1.06 before cutting off the plane’s rocket engine. This X-1, which Yeager nicknamed “Glamorous Glennis,” after his wife, now hangs in the National Air and Space Museum in Washington, D.C. The X-1 and other experimental planes explored ever-higher supersonic speeds and evaluated aerodynamic and control theories related to transonic and supersonic flight. The Bell X-5, a small aircraft with variable sweep wings, tested the theory that increasing the wing’s sweep would reduce the transonic drag rise. The Douglas Skyrocket reached Mach 2 in November, 1953, with Scott Crossfield as

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pilot. The Bell X-2 reached Mach 3 on September 27, 1956, but then tumbled out of control, crashing and killing the Air Force pilot, Captain Milburn Apt. SUPERSONIC DESIGN The theory that sweeping the wings would decrease the drag rise that occurs as Mach 1 is approached was developed independently in Germany and the United States well before the flight of the X-1, but it was confirmed by the X-5. Sweeping the wing lowers the effective Mach number of the flow perpendicular to the wing, and it is this component of the flow that influences the increase in drag at transonic speeds. Tests confirmed that as the wing is swept aft, the transonic drag rise is both delayed and reduced. This allows an aircraft with a swept wing to fly faster before experiencing the transonic increase in drag and allows it to fly through the speed of sound with less thrust. Nothing is free, however, and the reduction in drag is accompanied by a reduction in the lifting capability of the wing and requires more wing area for a given amount of lift. One way to accomplish this is by the use of a triangular or delta-shaped wing, a wing shape used on many supersonic aircraft. One of the first supersonic designs for a US fighter aircraft using a delta wing was the Convair F-102. In early tests in 1953, however, it was determined that this aircraft could not reach the supersonic speeds for which it had been designed. This led to a redesign of the aircraft based on the area rule concept developed by NACA engineer Richard Whitcomb. Whitcomb realized that when the air moving around an aircraft had reached the speed of sound, it had been compressed or squeezed together as much as it could be compressed and that the only way the flow could move around the airplane was to push the surrounding air out of the way, creating more drag. His theory said that this drag could be decreased if the airplane body or fuselage could be squeezed in enough to make room for the flow that

Principles of Aeronautics

had to go around the wing. The resulting fuselage shape became known as the “Coke-bottle” or “wasp-waist” design and its use in the redesign of the F-102 proved his theory. When Whitcomb’s area rule is applied to modern high-speed aircraft, the dramatic necking-in of the fuselage is not as evident as on the F-102. Designers have learned to blend wing and fuselage to give the required ideal variation of cross-sectional area or volume in more subtle ways. Richard Whitcomb went on to develop other improvements in transonic and supersonic aircraft design, such as his supercritical airfoil of the 1960s. The supercritical airfoil is shaped in such a way as to produce a weaker shock wave than older wing designs as it accelerates toward Mach 1 and places that shock closer to the airfoil’s trailing edge, thus reducing the transonic drag rise. This development is used on almost all high-speed subsonic and supersonic aircraft designed since the 1970s and allows subsonic aircraft to fly closer to the speed of sound with less thrust and fuel consumption than was possible with older airfoil shapes. THE SST Many supersonic aircraft have been designed and flown since the flight of the X-1, and supersonic flight is now commonplace. The Concorde, developed jointly by British Aerospace and Aerospatiale of France in the late 1960s, flew in prototype form in 1969 and went into passenger service in 1976, allowing anyone who could afford its premium-priced ticket to cross the Atlantic Ocean at Mach 2. The United States and the Soviet Union also had programs to develop supersonic transports, with Boeing selected as the firm to build the American SST. Boeing later canceled the project, believing that there was insufficient demand to allow either an airline or a manufacturer to make a profit on such a plane. The Tu-144 resulted from the Soviet SST project, but only a few were built and the aircraft was withdrawn from

Supersonic Aircraft

service after several crashes. The Concorde was also withdrawn from service after almost twenty-five years of continual service after a crash on a takeoff from Paris on July 25, 2000, which was caused by debris on the runway piercing its fuel tanks. Supersonic airliner flight has been limited by international agreement to travel over the oceans, limiting their usefulness to transatlantic and Pacific routes, and the Concorde does not have the range required for nonstop flights across the Pacific. At supersonic speeds, the twin shock waves coming from the leading and trailing edge of an SSTs wings can result in loud and destructive sonic booms at ground level due to the sudden pressure change across the shock. As a result, flight of SSTs over land has been forbidden and military supersonic flight is restricted to defined training areas. The National Aeronautics and Space Administration (NASA) and companies such as Boeing and Airbus have continued to explore designs for supersonic passenger planes of the future but, as of 2001, none is on the way to production. There have also been explorations by several companies of the possibility of building a commercially successful supersonic business jet. —James F. Marchman III Further Reading Brown, Eric. Miles M.52: Gateway to Supersonic Flight. History Press, 2012. Gunston, Bill. Faster Than Sound: The Story of Supersonic Flight. Haynes, 2012. Hans-Reichel, Michael. Subsonic Versus Supersonic Business Jets-Full Concept Comparison Considering Technical, Environmental and Economic Aspects. Diplom.de, 2012. Schuermann, Martin. Supersonic Business Jets in Preliminary Aircraft Design. Bod Third Party Titles, 2016. Torenbeek, Egbert. Essentials of Supersonic Commercial Aircraft Conceptual Design. Wiley, 2020. See also: Aerodynamics and flight; Aeronautical engineering; Aviation and energy consumption; Avro Arrow; Flight

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propulsion; Flight roll and pitch; Fluid dynamics; Forces of flight; High-speed flight; Jet engines; Ernst Mach; Mach number; Propulsion technologies; Ramjets; Rocket propulsion; Scramjet; Shock waves; Sound barrier; Supersonic jetliners and commercial airfare; Supersonic jets invented; Training and education of pilots; Wing designs; Chuck Yeager

Supersonic Jetliners and Commercial Airfare Fields of Study: Aeronautical engineering; Mathematics; Accounting ABSTRACT Supersonic jetliners are large passenger jets designed to fly safely at speeds faster than the speed of sound. Supersonic jetliners seemed to represent a major advance in commercial air travel, but their costs—financial and environmental—proved to outweigh their benefits, and they never replaced slower airplanes, even for transatlantic flights. KEY CONCEPTS sonic boom: a loud transient explosive sound caused by a shock wave preceding an object traveling at supersonic speeds sound barrier: the point of sharp increase in aerodynamic drag experienced by an object approaching the speed of sound, once thought to be a speed too difficult for aircraft to attain (a barrier) SST: acronym for supersonic transport supersonic: at speeds greater than the speed of sound (Mach 1 to Mach 3.5) SUPERSONIC FLIGHT BEGINS On October 14, 1947, Chuck Yeager piloted an experimental plane, the Bell XS-1, which had been dropped from a Boeing B-29, to a speed that surpassed Mach 1 (the speed of sound). His flight proved that it was possible to “break” the sound

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barrier. Traveling at such high velocity was a milestone, but before reliable supersonic aircraft could be designed, a great deal of further research and development remained to be done, particularly in the areas of aerodynamics and control. Physics required different model development at such speeds, and more research was needed to provide for land takeoffs. Early supersonic research and development was generally conducted for military applications, but during the late 1950s and 1960s researchers began investigating the technology for long-distance commercial travel. The US government selected Boeing and General Electric to participate in a federally funded program for commercial supersonic transport (SST). Meanwhile, British Aerospace and French Aerospatiale joined forces to produce the Concorde, and the Soviet Union began developing the Tupolev, TU-144. The TU-144 flew its maiden voyage on December 31, 1968, and a few months later, in March, 1969, the Concorde prototype took to the skies. In July, the TU-144 hit Mach 2, twice the speed of sound. Nearly two years later, on March 24, 1971, amid concerns regarding supersonic jetliners’ effects on the environment, high noise level, and extreme cost, the US Congress rejected further funding for the Boeing supersonic model. Strong public opposition to the SST and a faltering economy effectively canceled the American program. Two years later, at the 1973 Paris Air Show, the Concorde showed its capabilities. Tragically, at the show, the TU-144 crashed, killing everyone on board and eight people on the ground. In December, 1975, the TU-144 began limited commercial service, but the Soviet transport did not sell outside the country. Poor economic returns and a fatal accident on October 27, 1979, helped seal the TU-1 fate. The Concorde became a technical success, providing flawless international service across the

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Atlantic Ocean until July 25, 2000, when debris on a Paris runway caused a crash on takeoff. While the technology of the Concorde cut long-distance travel times in half, its costs in comparison to everyday flights were extravagant. In November, 2003, Concorde services were discontinued. Once thought to be the answer to commercial travel, the supersonic jetliner proved to have economic, environmental, and political problems that limited the market for SSTs. Laws that limited supersonic commercial flight to overseas flights restricted the routes SSTs could fly. Concerns for the ozone layer and high ticket prices also limited demand for supersonic travel. Supersonic transport continues to be researched, but until the technology becomes both economically and environmentally feasible, new production is improbable. —Cynthia J. W. Svoboda Further Reading Glancey, Jonathan. Concorde: The Rise and Fall of the Supersonic Airliner. Atlantic Books, 2015. Laurence, Philip K., and David W. Thornton. Deep Stall: The Turbulent Story of Boeing Commercial Airplanes. Taylor & Francis, 2017. Orlebar, Christopher. Concorde. Bloomsbury Publishing, 2017. Schuermann, Martin. Supersonic Business Jets in Preliminary Aircraft Design. BoD Third Party Titles, 2016. Simons, Graham M. Boeing 737: The World’s Most Controversial Commercial Jetliner. Pen & Sword Books Ltd., 2021. Torenbeek, Egbert. Essentials of Supersonic Commercial Aircraft Conceptual Design. Wiley, 2020. See also: Aerodynamics and flight; Aeronautical engineering; Aerospace industry in the United States; Air transportation industry; Airplane safety issues; Aviation and energy consumption; Federal Aviation Administration (FAA); High-speed flight; Jet engines; Mach number; Materials science; National Transportation Safety Board (NTSB); Shock waves; Supersonic aircraft; Supersonic jets invented

Supersonic Jets Invented Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics; Mathematics ABSTRACT Supersonic jets are airplanes capable of sustained level flight faster than the speed of sound. Until 1969, when the prototypes of two supersonic airliners, the Tu-144 and the Concorde, first exceeded the speed of sound, the only airplanes that reached such speeds were experimental and military aircraft. KEY CONCEPTS prototype: a preproduction example of an aircraft or other designed object built to test the functioning and operational feasibility of the proposed design ORIGINS AND HISTORY The first airplane to fly at supersonic speed in level flight, on October 14, 1947, was the rocket-powered Bell X-1, an experimental aircraft designed in the United States to test the practicability of supersonic flight, which some theorists had thought impossible. During the 1950s, a growing number of jet-powered aircraft, built in the United States and elsewhere, were capable of still higher speeds. The United States Air Force (USAF) received its first supersonic interceptor, the North American F-100 Super Sabre, in 1954. During the 1960s, the air forces of the United States and several other major nations put into service many supersonic aircraft that had been developed or designed in the 1950s. Outstanding examples in the USAF were the Republic F-105 Thunderchief, a fighter-bomber (brought into service in 1960); the Convair B-58 Hustler, a four-engine nuclear bomber capable of twice the speed of sound (1960); the Northrop T-38 Talon, a trainer

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(1961); the MacDonnell F-4C Phantom, a two-seat tactical fighter (1963); and the Lockheed SR-71A, a reconnaissance plane capable of more than 3,400 kilometers per hour (1966). In 1967, the USAF received its first “swing-wing” airplane, the General Dynamics F-111, a supersonic fighter-bomber that had first flown in 1964. With the incorporation of these and other types of aircraft during the 1960s, the USAF became largely supersonic in its fighter and bomber divisions. The United States Navy (USN) flew aircraft of similar performance. Jet airplanes were not the fastest winged flying machines in the 1960s. The rocket-powered North American X-15A-2 reached 7,297 kilometers per hour in October, 1967. However, it flew at such

Principles of Aeronautics

great altitude, once reaching 108 kilometers, that it was less an airplane than a spacecraft. In the 1960s, design of supersonic airliners advanced quickly, with the United States, the Soviet Union, Britain, and France competing to supply what promised to be an enormously profitable future market. The first supersonic transports to fly were the Soviet Tupolev Tu-144 (December, 1968) and the British-French Aerospatiale Concorde (March, 1969). Prototypes of both flew faster than sound in 1969. In the United States, the Boeing 2707 seemed superior in design to both the Concorde and the Tu-144. However, just as construction of the prototype was starting, Congress canceled government

The #46-062 Bell X-1 rocket-powered experimental aircraft (known for becoming the first piloted aircraft to fly faster than Mach 1, or the speed of sound, on October 14, 1947) photographed during a test flight. Photo via Wikimedia Commons. [Public domain.]

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Principles of Aeronautics

The D-558-II, which in 1953 became the first aircraft to exceed twice the speed of sound. Photo via Wikimedia Commons. [Public domain.]

funding of the project in March, 1971. Without that help, Boeing could not proceed with the 2707. IMPACT The importance of supersonic jets for the United States in the 1960s was largely military and strategic. The high speed (both horizontal and in rate of climb) of these powerful airplanes helped maintain the United States’ superiority over potential foes (although speed in itself provided little advantage in the peculiar conditions of the Vietnam War). The Soviet Union tried to match US advances in airplane performance. The high cost of doing so contributed to the later collapse of the Soviet system.

Further Reading Conway, Erik M. High-Speed Dreams: NASA and the Technopolitics of Supersonic Transportation, 1945-1999. Johns Hopkins UP, 2005. Gunston, Bill. Faster Than Sound: The Story of Supersonic Flight. Haynes, 2012. Nahum, Andrew. Frank Whittle (Icon Science): The Invention of the Jet. Icon Books, 2017. See also: Aeronautical engineering; Air transportation industry; Avro Arrow; Fluid dynamics; High-speed flight; Jet engines; Mach number; Military aircraft; National Aeronautics and Space Administration (NASA); Propulsion technologies; Sound barrier; Supersonic aircraft; Supersonic jetliners and commercial airfare; Chuck Yeager

—Peter Bakewell

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T Tail Designs Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics ABSTRACT Tail designs are the various arrangements of horizontal and vertical stabilizing surfaces at the rear part of an airplane. An airplane’s tail design is important because it stabilizes and controls the airplane in both up-and-down move- ments of roll and pitch, and side-to-side movements of yaw. KEY CONCEPTS cruciform: in the shape of a cross empennage: the term for the tail structure of an airplane pitch: the tendency of the nose of an aircraft to move up or down vertically as it moves through a fluid medium roll: the tendency of the body of an aircraft to rotate about its central axis as it moves through a fluid medium yaw: the tendency of an aircraft to turn horizontally about its center of mass as it moves through a fluid medium THE PARTS OF AN AIRPLANE’S TAIL The tail of an airplane is called by various names, such as “empennage” and “stabilizer.” The preferred term is “stabilizer,” because it is at least partially descriptive of the component’s function. However, the stabilizer provides not only stability but also some of the airplane’s control. The tail of an airplane is designed to provide both stability and control of the airplane in pitch, roll and

yaw. There are many different forms an aircraft tail can take in meeting these dual requirements of stability and control. Most tail designs have a horizontal winglike structure and one or more vertical or near-vertical structures. Whenever practical, these structures are identified as the horizontal and vertical stabilizers, although some designs do not conveniently fit such a description. The many types of airplane tail design include, but are by no means limited to, the conventional, T-tail, cruciform-tail, dual-tail, triple-tail, V-tail, inverted V-tail, inverted Y-tail, twin-tail, boom-tail, high boom-tail, and multiple-plane tail designs. CONVENTIONAL TAIL DESIGN The conventional tail design is the most common form. It has one vertical stabilizer placed at the tapered tail section of the fuselage and one horizontal stabilizer divided into two parts, one on each side of the vertical stabilizer. For many airplanes, the conventional arrangement provides adequate stability and control with the lowest structural weight. About three-quarters of the airplanes in operation today, including the Airbus A380, the Boeing 777 and 747, and the Beech Bonanza A-36, use this arrangement. THE T-TAIL DESIGN In the T-tail design, a common variation of the conventional tail, the horizontal stabilizer is positioned at the top of the vertical stabilizer. The horizontal stabilizer is then above the propeller flow, or prop wash, and the wing wake. Because the horizontal stabilizer is more efficient, it can therefore be made both smaller and lighter. The placement of the horizontal stabilizer on top of the vertical stabilizer can

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Photo via iStock/Maxian. [Used under license.]

also make the vertical stabilizer more aerodynamically efficient. By making the vertical stabilizer more effective, its size may be reduced. However, the horizontal stabilizer in the T-tail layout imposes a bending and twisting load on the vertical stabilizer, requiring a stronger, and therefore, a heavier, structure. These loads are avoided in the conventional design. There is also the possibility that at the high pitch angle usually associated with landing the airplane, the horizontal stabilizer of the T tail will be immersed in the slower and more turbulent flow of the wing wake. In some cases, it is possible to compromise severely the control function of the horizontal tail. Nevertheless, the T tail is the second-most common tail design after the conventional. Both major American transport plane builders, Boeing and McDonnell-Douglas, use the T-tail de-

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sign. The Boeing 727, with its three fuselagemounted engines, has a T-tail design, as do the variants of the McDonnell Douglas MD-90, formerly the Douglas DC-9. Other aircraft that employ the T-tail design are the Lockheed C-5A, the Gates Learjets 23 and 35A, the Cessna Citation CJ1, the Piper Lance II, and the Beech Skipper 77. CRUCIFORM-TAIL DESIGN The cruciform tail is an obvious compromise between the conventional and T-tail designs. In the cruciform design, the horizontal stabilizer is moved part of the way up the vertical stabilizer. In this position, the horizontal stabilizer is moved up and away from the jet exhaust and wing wake. The lifting of the horizontal stabilizer also exposes the lower part of the vertical stabilizer, as well as the rudder, to un-

Principles of Aeronautics

disturbed airflow. Undisturbed airflow on the rudder is important, particularly in the recovery from spins. A military example of the cruciform tail is the North American Rockwell B-1B supersonic bomber. Other aircraft that use the cruciform-tail design are the Dessault Falcon 100 and the Commander. DUAL-TAIL DESIGN The dual-tail design, in which the two vertical stabilizers are placed at the ends of the horizontal stabilizers, was at one time fairly common on large flying boats and twin-engine propeller-driven bombers such as the North American B-25. In some cases, this arrangement is attractive, because it places the vertical stabilizers in the prop wash of wing-mounted propellers. The result is the maintenance of good directional control during low-speed operations. The positioning of the two vertical stabilizers at the ends of the horizontal stabilizers allows for a smaller, lighter, and more aerodynamically efficient horizontal stabilizer. However, the overall weight of a plane with a dual-tail design is greater than that of a plane with the single conventional-tail design. The dual tail is part of the design of the Republic Fairchild A-10 ground-attack airplane, in which the plane’s two jet engines are mounted to the rear of the fuselage. When this airplane is viewed from the rear and slightly to either side, the engine exhausts, blocked by the vertical stabilizer, are not easily visible. If a heat-seeking missile is launched at a departing or escaping A-10, the main heat source, the engine exhausts, are at least partially blocked by the vertical stabilizer. The Ercoupe, a private light airplane developed in the late 1940s and still seen at small airports, uses a dual tail to keep the vertical stabilizer out of the wake from the fuselage and the wing-fuselage junction. The Ercoupe is unique in that it is the only commercial light airplane ever produced with the dual-tail design. Other craft that use the dual-tail

Tail Designs

design include the Consolidated B-24, the Short Skyvan, and the Martin PBM Mariner flying boat. TRIPLE-TAIL DESIGN The triple-tail design, with two vertical stabilizers placed at the ends of the horizontal stabilizers and one mounted on the fuselage, is attractive when the height of the vertical stabilizer must meet certain restrictions, such as hangar-door height. Certainly this was the important consideration in the design of the Lockheed Constellation, one of the most significant passenger airplanes of the late 1940s. Another well-known example of the triple-tail design is the Grumman E-2 Hawkeye. V-TAIL DESIGN The V-Tail, sometimes called the “butterfly” tail, has had limited application in airplane design, the most significant of which has been by the Beech Company in the Beechcraft Bonanza V-35. Clearly, the usual definition of horizontal and vertical stabilizers has no application to the V tail. The intended advantage of the V-tail design is that two surfaces might serve the same function as the three required in the conventional tail and its variants. Removal of one surface then would reduce the drag of the tail surfaces as well as the weight of the tail region. However, wind tunnel studies by the National Advisory Committee on Aeronautics (NACA) have shown that for the V tail to achieve the same degree of stability as a conventional tail, the area of the V tail would have to be about the same size as that of the conventional tail. Another disadvantage of the V tail has to do with turning the airplane. To turn left, for example, the pilot would press the left rudder pedal and bank the airplane with the left wing down. In V-tail aircraft, the right side of the V (as viewed from the rear) deflects upward, and the left surface deflects downward. This arrangement drives the nose to the left but also causes the airplane to roll away from the

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turn. Although this tendency to roll is overcome by the wing control provided by the ailerons, it is clear that one control of the airplane produces a secondary effect that opposes the primary effect of another control. This secondary effect of opposing the primary purpose of another control is called adverse coupling. Adverse coupling is one reason that the most recent Bonanza design, the A-36, uses the conventional tail. The undesirable rolling motion caused by the V tail might be avoided by inverting the butterfly tail. However, except for a few small homemade glider-sail planes, this design has been avoided because of ground clearance problems. INVERTED Y-TAIL DESIGN The inverted Y tail is actually a conventional tail with a noticeable droop to the horizontal stabilizers. In other words, the outer ends of the horizontal stabilizers are lower than the ends attached to the fuselage. The F-4 Phantom, originally a mainstay of the McDonnell Company, used the inverted Y tail to keep the horizontal surfaces out of the wing wake at high angles of attack. It is interesting to note that the tips of the horizontal stabilizers on the first McDonnell Navy fighter, the F-2H Banshee, were bent decidedly upward. TWIN-TAIL DESIGN The twin tail is a feature of various air superiority fighters used by both the US Navy (the F-14 Tomcat) and the US Marine Corps (the F/A-18 Hornet). Although both the F-14 and F/A-18 designs have a superficial resemblance, they also have important differences. The tilt angle of the vertical stabilizer of the F-14 is more pronounced than that of the F-18, so much so that it approaches that of the V tail on the Beech model V-35 Bonanza. With two vertical stabilizers, the twin tail is more effective than the conventional single tail of the same height.

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BOOM-TAIL DESIGN Boom tails are used when an aircraft’s fuselage does not extend entirely back to the horizontal stabilizer. In both the Lockheed P-38 Lightning fighter of World War II and the Fairchild C-119 cargo plane, engines were mounted on the booms. In the case of the C-119, the twin boom allowed easy access to the rear of the fuselage for loading and removing cargo. The twin boom has also been used for an airplane with engines mounted in the fuselage, with one engine, known as the tractor, in the nose of the airplane and one engine, known as the pusher, in the rear of the airplane. Because the thrust of both engines is along the centerline of the airplane, it is much easier in this arrangement to compensate for the loss of one engine than it is in the wing-mounted engine installation. Both the Cessna Skymaster and the Adam 309 have fuselage-mounted engines. In the case of the Adam 309 the horizontal stabilizer is raised to avoid propeller wake from the pusher, or rear-mounted, engine. MULTIPLE-PLANE TAIL DESIGN Finally, the obsolete multiple-plane tail design has two or more horizontal stabilizers. This layout was used extensively in bombing airplanes of World War I and even in a few early British passenger and freight-carrying airplanes. It may be seen again on the recently constructed replica of the Vickers Vimy airplane. —Frank J. Regan Further Reading Barker, Sean. Aircraft as a System of Systems: A Business Process Perspective. SAE International, 2018. DeLaurier, James. Aircraft Design Concepts: An Introductory Course. Taylor& Francis Group, 2022. Keane, Andrew J., András Sóbester, and James P. Scanlan. Small Unmanned Fixed-Wing Aircraft Design. Wiley, 2017. Kundu, Ajoy Kumar. Aircraft Design. Cambridge UP, 2010. Sforza, P. M. Commercial Airplane Design Principles. Elsevier Science, 2014.

Principles of Aeronautics

Torenbeek, Egbert. Synthesis of Subsonic Airplane Design: An Introduction to the Preliminary Design of Subsonic General Aviation and Transport Aircraft, with Emphasis on Layout, Aerodynamic Design, Propulsion and Performance. Springer Netherlands, 2013. See also: Aerodynamics and flight; Aeronautical engineering; DC Plane family; Flight roll and pitch; Forces of flight; Monoplanes; Stabilizers; Types and structure of airplanes; Wake turbulence; Wing designs

Takeoff Procedures Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Pilot training; Mathematics ABSTRACT The activities required as part of the process of launching an aircraft from the surface into the air for the purpose of controlled flight. Takeoff procedures are necessary because all aircraft must become airborne safely. Their challenge lies in how quickly takeoffs occur under widely varied conditions. KEY CONCEPTS fixed-wing aircraft: aircraft having immovable wings attached to the fuselage in the traditional sense of an airplane movable-wing aircraft: aircraft having wings that can change configuration, although remaining attached to one position on the fuselage rotary-wing aircraft: autogyros, helicopters, and other rotorcraft seaplane: an airplane designed to take off and land on water rather than solid ground EARLY HISTORY Every flight begins with a takeoff. Takeoffs come in a variety of styles, and to the untrained person, they seem simpler than they truly are. The takeoff pre-

Takeoff Procedures

sented the greatest challenge to early aviators in their design of controllable airplanes. Unsuccessful takeoffs held little evidence of their causes, because any one of numerous details, from poor planning to faulty design, could ruin a takeoff. Aviators had not amassed a database of accident reports from which they could glean knowledge, and they did not have the experience or insight to pinpoint the problem or sequence of problems causing each failed takeoff. Successful takeoffs require the right combination of aircraft design, favorable conditions, and skillful piloting. Takeoffs also require power. Early aviators such as Otto Lilienthal and Orville and Wilbur Wright first built hand- or foot-launched gliders that required only sufficient wind and downward-sloping terrain. Later, when the Wrights made a gasoline engine for their Flyer, they faced the same challenges that had kept Samuel Pierpont Langley from success. Because early engines were underpowered, takeoffs demanded much room and planning. The Wright brothers, for example, launched their Flyer not from a field or a runway, but from a monorail track laid in a slight depression. They depended entirely on the engine to provide power for takeoff. The Wrights later supplemented their engine’s meager takeoff power with a weighted catapult that required several men to raise and set. Within a decade, engine power began to grow, and by 1910, the Wrights had abandoned their monorail-takeoff system, allowing wheels to reduce the drag and absorb the stresses of launches without rails. The Wrights also sought greater engine power to accelerate the Flyer‘s mass and drag to the point that aerodynamic lift overcame gravity. TAKEOFF TECHNIQUES Although engine power for takeoff has increased dramatically since the time of the Wright brothers, takeoffs still need planning. The fixed wings of airplanes use airflow to produce lift, and power builds forward momentum to achieve flying speed. After

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the plane reaches flying speed, the pilot uses training and skill to leave the runway and enter the sky at the correct speeds, pitch attitudes, and recommended power settings. Simple airplanes have simple procedures: The pilot must point the nose skyward and maintain full power. More complex airplanes have different considerations. Complex airplanes require a specific time or height above the surface to make the first power change. Some engines have limits on the amount of time full power may be maintained, as pressures within the engine become very high. In the United States, the Federal Aviation Administration (FAA) certifies airplane engines with slight tolerance to overboost. Necessary for takeoff, overboost allows pilots to run the engine at very high power settings for a specified amount of time, after which the pilot must reduce the power to the recommended setting, usually called maximum except takeoff (METO), and maintain a prudent airspeed. Pilots may choose between three basic takeoff techniques, normal, short-field, and soft-field takeoffs. Their use depends on both the circumstances and the pilot’s decision. NORMAL TAKEOFFS Pilots use a normal takeoff when there is no need to employ either of the other two types of takeoff. For a normal takeoff, the runway length must pose no challenge, its surface must be firm and dry, and no appreciable obstructions should interfere with the airplane’s climb path. Although normal takeoffs may seem routine to the observer, they require much skill on the part of the pilot. All takeoffs must be carefully planned. Normal takeoffs call for pilots to make certain there is no conflicting airplane traffic, to taxi onto the runway with the wing flaps properly set, and to apply takeoff power. At the right airspeed, the pilot raises the nose and allows the airplane to fly smoothly off the runway while making small adjustments to the pitch attitude to maintain the best rate-of-climb airspeed.

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SHORT-FIELD TAKEOFFS When pilots have a less-than-normal amount of runway available, they use a short-field takeoff technique. Details vary with airplane type, but pilots have basic techniques upon which they rely. The first involves using the aircraft’s short-field takeoff charts to determine the minimum runway distance required by current conditions. These include wind direction and speed, air temperature, the airplane’s weight, and the condition of the runway surface. Some takeoff tables or graphs include mention of the runway’s slope or of the presence of tall grass, if the runway is not paved. In any case, aircraft manuals assume that the airplane is properly maintained, that its engine is producing full power as it did when new, and that its exterior is clean and free of drag-producing dents. The charts also assume that the pilot performs the takeoff procedure smoothly and skillfully, exactly as outlined by the manufacturer. A pilot must use the runway’s full length by taxiing to the very edge of the runway’s end and then carefully aligning the airplane’s nose with the runway’s centerline. As with all takeoffs, the pilot must quickly ensure that engine oil pressure and oil temperature are proper. Holding the airplane stationary by applying and holding the brakes, the pilot adds power until reaching the manufacturer-specified power setting. When the engine sustains that power, the pilot releases the brakes so the airplane accelerates to the airspeed that the pilot determined when planning the takeoff. At that airspeed, the pilot notes how far down the runway the airplane has traveled, analyzes the airplane’s acceleration, how much runway remains, and the airplane’s ability to fly. Once airborne, the pilot raises the nose to a climb angle that results in the exact airspeed that the aircraft manual demands. The pilot maintains that climb angle and airspeed until no obstacles, such as trees, powerlines, or buildings, threaten the airplane’s climb path. When safely above any obsta-

Principles of Aeronautics

cles, the pilot then lowers the airplane’s nose, increasing the airspeed to one that provides a more efficient climb. This efficiency considers engine cooling, flight visibility, and other safety considerations. SOFT-FIELD TAKEOFFS A third takeoff technique, the soft-field takeoff, involves runways that are poorly maintained, strewn with small debris, or are covered with snow or grass, muddy, or otherwise not hard, clean, and dry. Pilots use soft-field takeoffs to reduce the chance of damage to the nosewheel and to allow the airplane to leave the surface at an airspeed lower than that of either normal or short-field takeoffs. Soft-field takeoffs require a well-developed judgment, because soft fields often pose several challenges, some severe, usually at the same time. Unimproved airstrips are common in rural and remote areas, as are livestock and wildlife. Numerous accidents occur yearly in North America when airplanes collide with deer, coyotes, and even cattle, to name just a few wildlife hazards. Information as basic as the runway’s dimensions are easily found for hard-surfaced runways, but these are often mere guesswork for grass and dirt strips. Compared to concrete runways, which are reasonably consistent in firmness along their entire length, soft fields can vary tremendously in a short distance. Grass causes drag, and long grass at unkempt, idle airstrips can retard an airplane enough to prevent takeoff. Grass that is wet from dew or rain is even more of a hindrance. Pilots sometimes cannot define the runway edges at soft fields, because there are no painted markings, nor any contrast between the runway and its surroundings, as there is on concrete or asphalt runways. Even those airstrips that have well-maintained grass and defined edges and are free of wildlife may still have drainage problems that are invisible to pilots. Some or all of an airstrip may have a very porous soil, which quickly drains away water. Another part of the strip might consist of soil

Takeoff Procedures

that retains water below the surface, into which an airplane’s wheels may sink. The nosewheel-equipped airplane has no advantage over tailwheel-type aircraft in this environment. A pilot making a soft-field takeoff must handle the flight controls smoothly, because at the low speed and high nose angle demanded by a soft-field takeoff, roughly handled flight controls could force the airplane back onto the runway. Accidents involving the mishandling of the flight controls in such situations have resulted in damage to property and injury to persons. In the United States, applicants for pilot certificates must not only demonstrate as much skill as is practicable during the flight test, but also demonstrate knowledge relating to the various elements of soft-field takeoffs under various conditions. Safe and efficient takeoffs demand good planning and skill. TAKEOFF TESTING STANDARDS Because events during takeoffs happen so quickly as to seem automatic, flight instructors must carefully ensure that their students consider takeoffs an extreme low-altitude maneuver requiring good planning. Accident statistics consistently show mishaps occurring during takeoffs and landings. The aviation industry has worked to improve takeoff planning in different ways. In the United States, the Federal Aviation Administration (FAA) took steps in the mid-1990s to help flight instructors in their teaching by changing their Practical Test Standards requirements for all takeoffs. Applicants for US pilot certificates must, before takeoff, verbally review the available runway assigned for takeoff, stating its length, the distance required for the takeoff, the airspeeds required for the technique to be used, and the departure procedure. SEAPLANE TAKEOFFS Seaplane takeoffs have their own considerations. Although seaplane operations are statistically less com-

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mon than landplane operations, they are a vital part of aviation and require specialized knowledge. A land plane’s pitch attitude, or the relationship of the nose to the horizon, is governed by the landing gear and remains constant until the moment a pilot rotates for takeoff. On the water, a seaplane or amphibian will change its pitch attitude with the rising and falling of the water, or of the taxiing speed, or with a shift in airplane loading. Aileron control is more critical in a flying-boat takeoff, because the craft’s fuselage is a single-keel hull, and it rolls left and right just as in flight. Land planes’ wheels, as do twin floats on some seaplanes, prevent such rolling. Seaplane takeoffs promise excitement as water pounds the hull while the pilot gives full attention to getting the airplane “on the step.” This means that the bottom of the hull or the floats are mostly out of the water as the wings increase their lift, and water pressure on the V-shaped hull or float bottoms releases its suction and allows the seaplane to fly. Seaplane pilots must watch for boats, buoys, and such, but give extra care to partially or near-fully submerged logs or other obstacles. Even the water itself may become an obstacle, as seaplanes may strike a large wave that sends the craft airborne too soon for it to fly. As the craft settles onto the water again, a second wave may strike the aircraft in such a manner as to engulf the craft’s nose. Over the decades, pilots have shared their experience to amass a pool of knowledge from which pilots may draw. —David R. Wilkerson Further Reading Federal Aviation Administration. Airplane Flying Handbook. (FAA-H-8083-3A). Skyhorse Publishing, 2011. ———. Instrument Procedures Handbook ASA FAA-H-8083-16B. Aviation Supplies and Academics Inc., 2017. Kurt, Franklin. Water Flying. Macmillan, 1974. See also: Air transportation industry; Flight landing procedures; Flight recorder; Landing gear; Monoplanes;

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Taxiing Procedures Fields of Study: Aeronautical engineering; Mechanical engineering; Pilot training ABSTRACT Taxiing procedures are required to assist in the process of self-propelled aircraft movement on land. Taxiing procedures are essential in moving aircraft to and from runways. Despite its apparent ease, taxiing demands pilot vigilance, as many mishaps occur during taxi. KEY CONCEPTS landplane: an airplane designed to take off and land from a solid surface seaplane: an airplane designed to take off and land on water rather than solid ground tail-skid: essentially a type of short ski used in place of a wheel to support the tail of an airplane tricycle landing gear arrangement: the arrangement common on most present-day aircraft in which there is a single wheel-and-strut supporting the nose of the aircraft and two wheel-and-strut assemblies rearward of the center of mass supporting the rest of the aircraft HISTORY AND TECHNIQUES Before airplanes may take off for flight, they must travel from the parking ramp to the runway. Although most accomplish this by taxiing, that was not always so. The first flying machines, such as Otto Lilienthal’s foot-launched gliders, Samuel Langley’s houseboat-launched Aerodrome, and Wilbur and Orville Wrights’ rail-launched Flyer, were not designed with taxiing in mind. Soon, however, aircraft designers abandoned landing skids and foot-launches for humanity’s oldest con-

Principles of Aeronautics

venience, wheels. Even so, early wheeled airplanes could not taxi, the small and inefficient primitive engines used to power them were woefully insufficient for any task other than turning the propeller. Aircraft such as Louis Blériot’s famous monoplane required handlers to trundle the airplane into the takeoff position or roll it to its parking spot after landing. Blériot and other early designers included main wheels to trundle their flying machines into takeoff position, and to the parking area after landing, but placed skids at the tail to retard movement on landing. Taxiing moves an airplane under its own power, using the airplane’s control systems to steer. By 1918, airplanes had grown large, and engines had sufficiently powerful to make the labor-intensive trundling method inefficient. Eventually, the tail-skid was replaced with wheels all around, one of which was steerable. It was extremely difficult to use the trundle method of ground movement to move the huge bombers built by manufacturers in Germany and England. Certainly, after World War I, taxi techniques had become well established. A 1930s development was the tricycle landing gear. Tricycle landing gear places the third wheel under the nose, bearing the weight of the engine while the aircraft is parked or taxiing. That extra weight provided traction during taxi and reduced nose-over accidents when pilots applied brakes too enthusiastically. Tricycle landing gear lowered the nose permanently for taxiing, so pilots could look over engine cowlings to enjoy approximately the same view automobile drivers would see over the hoods of their vehicles. By the 1950s, the term “conventional landing gear” denoted the tailwheel type, because the word “tricycle” had become standard for the increasingly popular nosewheel system. By the end of the twentieth century, few manufacturers built tailwheel airplanes, although the tailwheel remained popular with amateur builders. Designers learned quickly that flying machines, though at rest on the ground, remain subject to the

Taxiing Procedures

wind. Because winds can be unpredictable, pilots taxi airplanes slowly, so that moving the throttle to idle allows a prompt stop. When taxiing into the wind, wings produce lift, which reduces brake and steering effectiveness. Because strong winds have overturned taxiing airplanes, pilot training since the 1940s has included positioning the flight controls so as to keep the wind flowing over the top of the airplane structure. For new pilots, automotive driving habits slow their learning to taxi. Taxiing pilots steer by using foot controls, or rudder pedals, and most brakes are activated by pressing the top of these pedals. This takes some acclimation. Landing gear design also affects pilot taxi technique. Tricycle airplanes are easy to taxi. Tailwheel airplanes require greater attention. Wide landing gear makes taxiing easier than does narrow gear. The wide-landing-gear design saw an extreme expression in the 1980s, when one amateur-built airplane appeared with wheels at each wingtip. The popular airplane was stable but could not negotiate very narrow taxiways. Taxiing surfaces also affect taxiing procedures. Not all taxiing surfaces are clean and strong. They range from well-maintained concrete at major airports to narrow dirt strips at ranches and even to riverbanks and sandbars. Taxiing techniques must accommodate each of these. Generally, tailwheel-type airplanes are more suitable to rugged taxiing surfaces than are nosewheel-type aircraft. Modern pilots learn to control their airplanes from first movement. Taxiing demands alertness, and pilots must keep track of the movement of everything else along the taxi path. Right-of-way issues and individual pilot habits make taxiing on busy airports an effort in safety. To observe the entire area, pilots must look around in a complete circle, which few airplane structures allow. Airport space is extremely valuable, so parking areas place airplanes closely together. Pilots must avoid people, other aircraft, and obstructions when taxiing. During the

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1990s, pilot training in the United States began to require that pilots verbally announce “clear right” or “clear left” when taxiing past taxiways, runways, and parked aircraft or other obstructions. WATER TAXIING Seaplanes also taxi, and taxiing on water demands nautical, as well as aviation, skills. Water operations allow pilots to taxi with about two-thirds power, in a technique called step taxiing. Seaplane pilots use a step taxi when the takeoff or landing area is distant from where the seaplane will moor, or anchor. When not step taxiing, seaplane pilots taxi slowly when they are near the shore or at any time when speed would be hazardous. Because water is fluid, seaplane pilots must give even more attention than landplane pilots to the wind while taxiing. TAXIING ACCIDENTS Accidents do occur during taxiing. The two most common reasons for taxi accidents are pilot distraction or inattention and taxiing too fast for conditions. It is difficult to set rules for safe taxiing speed, because circumstances and conditions continually change. What is prudent one day may be rash or reckless the next. Traditionally, pilots have used the phrase “a brisk walk” to describe a prudent speed. However, if a pilot needs to stop immediately, even that speed may be too fast. An additional cause for taxi accidents or incidents is a pilot’s holding the flight controls improperly for the wind conditions. Taxi accidents tend not to be newsworthy events, but are dangerous nonetheless. In all, taxiing, like flying, can be safe and enjoyable when responsible pilots follow procedures. —David R. Wilkerson Further Reading Pilot’s Manual Editorial Board. The Pilot’s Manual: Flight School: How to Fly Your Airplane Through All the FAR/JAR Maneuvers. Aviation Supplies & Academics Inc., 2009

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Federal Aviation Administration (FAA). Flight Instructor Practical Test Standards for Airplane Single-Engine Land and Sea: FAA-S-8081-6D. Ravenio Books, 2016 See also: Airflight communication; Federal Aviation Administration (FAA); Flight landing procedures; Flight schools; Flight testing; Forces of flight; Landing gear; Monoplanes; Plane rudder; Takeoff procedures; Training and education of pilots

Temperature Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Thermodynamics; Fluid mechanics; Measurement; Mathematics ABSTRACT Quantification of temperature is necessary for many reasons, including scientific experiments, weather prediction, and many manufacturing processes. Scientists and mathematicians have developed and investigated a variety of principles and scales associated with the measurement and definition of temperature. KEY CONCEPTS molecular motion: the idea that atoms and molecules are in a constant state of vibration that depends on the amount of thermal energy they contain, such that the higher their temperature the more energetic and rapid is their vibration; thus, materials expand and contract according to temperature temperature: an artificially defined thermal state of a particular object or matter; all temperatures are relative to the defined state of absolute zero on any of the four standard temperature scales TO KNOW TEMPERATURES Quantification of temperature is necessary for many reasons, including scientific experiments, weather prediction, and many manufacturing processes.

Principles of Aeronautics

Temperature, by its formal definition, measures the movement of molecules in an object. Greater movement results in higher temperatures; conversely, less movement results in lower temperatures. The byproduct is heat, so temperature is often thought to measure the heat of an object. Mathematicians, many of whom are also physicists, have made significant contributions in quantifying heat and developing the temperature scales widely used in the twenty-first century. HISTORY Joseph Fourier began heat investigations in the early nineteenth century. His work “On the Propagation of Heat in Solid Bodies” was controversial at the time of its publication in 1807. Joseph Lagrange and Pierre-Simon Laplace argued against Fourier’s trigonometric series expansions; however, Fourier series are widely used in a variety of theories and applications in the twenty-first century. Jean-Baptiste Biot, Simone Poisson, and Laplace objected at various times to Fourier’s derivation of his heat transfer equations. In 1831, Franz Neumann formulated the notion that molecular heat is the sum of the atomic heats of the components. Studying mixtures of hot and cold water, which did not produce water that was the average of the two temperatures, he concluded that water’s specific gravity increases with temperature. This relationship was later shown by other researchers to be true only for a certain range of temperatures. In the late nineteenth century, James Maxwell and Ludwig Boltzmann independently developed what is now known as the “Maxwell-Boltzmann kinetic theory of gases,” showing that heat is a function of only molecular movement. Their equations have many applications, including estimating the temperature of the sun. Around the same time, Josef Stefan proposed that the total energy emitted by a hot body was proportional to the fourth power of the temperature, based on empirical observations. In the twentieth and

Temperature

twenty-first centuries, scientists continued to study heat and have developed mathematical and statistical models to estimate heat. These models are used in areas like astronomy, weather prediction, and the global warming debate. MEASURING TOOLS AND TEMPERATURE SCALES Heat can be difficult to quantify. Scientists and mathematicians developed many methods and instruments to measure and describe perceived temperature. Some of the earliest were called thermoscopes, often attributed to Galileo Galilei. In the early 1700s, Gabriel Fahrenheit created mercury thermometers and marked them with units that became known as “degrees Fahrenheit.” He empirically calibrated his thermometer using three values. Icy salt water was assigned temperature zero. Pure ice water was labeled 30. A healthy man would show a reading of 96 degrees Fahrenheit. Later, Fahrenheit would measure the temperature of pure boiling water as 212 degrees Fahrenheit, adjusting the freezing point of water to be 32 degrees Fahrenheit so there was 180 degrees between the freezing and boiling point of water. Anders Celsius created a different temperature scale in the mid-1700s. The Celsius temperature scale was numerically inverted with respect to Fahrenheit. He used 100 to indicate the freezing point of water and 0 for the boiling point of water. Because there were 100 steps in his temperature scale, he referred to it as a “centigrade” (centi means “a hundred” and grade means “step”). A few years later, Carolus Linnaeus allegedly reversed the scale to make zero the freezing point and 100 the boiling point. About a century after Celsius created his scale, William Thomson, Lord Kelvin, is given credit for the idea of an absolute zero, a temperature so cold that molecules do not move. The Kelvin scale was precisely defined much later after scientists and mathematicians better understood the concept of

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conservation of energy. Near-absolute zero conditions produce many interesting problems in mathematics and science. For example, clumping of atoms as they approach an unmoving state can be studied as a classic packing problem, which has extensions in areas like materials science and digital compression. The Kelvin temperature scale uses the same scale as centigrade, with absolute zero about 273 degrees below the freezing point of pure water (actually 273.15 degrees). Converting from degrees centigrade to Kelvin is as simple as shifting the scale by adding 273 or 273.15, depending on the precision required. A fourth temperature scale related to the Fahrenheit scale was, and still is, used only in thermodynamic engineering calculations involving Fahrenheit temperatures. It is analogous to the Celsius scale in that it uses degrees of the same magnitude as Fahrenheit degrees, and begins at absolute zero. Thus, the ice point of water is 491.69 degrees Rankine and the boiling point is 671.69 degrees Rankine. In the mid-twentieth century, the centigrade scale was replaced with the Celsius scale. The changes were relatively minor, so one estimates the freezing and boiling points of water to be 0 degrees Celsius and 100 degrees Celsius. In actuality, 100 degrees Celsius (the boiling point of water) is now 99.975 degrees Celsius. Converting from degrees Celsius to degrees Fahrenheit, or degrees Fahrenheit to degrees Celsius, involves multiplicative rescaling, not just translation, since 1 degree Celsius is 1.8 times larger than 1 degree Fahrenheit, and the point of the zero temperature is different. Celsius and Fahrenheit temperature scales coincide at just a single point, at which both scales read a temperature of -40 degrees.

Gaskell, David R., and David E. Laughlin. Introduction to the Thermodynamics of Materials. CRC Press, 2017. Helrich, Carl S. Modern Thermodynamics with Statistical Mechanics. Springer, 2009. Luscombe, James. Thermodynamics. CRC Press, 2018. Marzek, Andrew, and Anthony Meijer. Statistical Thermodynamics. Oxford UP, 2017. Rao, Y. V. C. An Introduction to Thermodynamics. Universities Press, 2004. Valsaraj, Kalliat T., and Elizabeth M. Melvin. Principles of Environmental Thermodynamics and Kinetics. CRC Press, 2018. Zimmerman, Kim Ann. “Temperature: Facts, History & Definition.” Live Science, 20 Sept. 2013, www.livescience.com/39841-temperature.html. Accessed 22 Feb. 2017. See also: Advanced composite materials in aeronautical engineering; Advanced composite materials repair; Aeronautical engineering; Aircraft icing; Airplane safety issues; Hot-air balloons; Materials science; Viscosity; Weather conditions

Valentina Tereshkova Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Physiology; Mathematics ABSTRACT Valentina Nikolaeva Tereshkova became the first woman in space as a Russian cosmonaut in 1963. She was born March 6, 1937, in Bolshoye Maslennikovo, Yaroslavl Oblast, Russia, Soviet Union (now in Russia). During her seventy-one-hour flight, she achieved an altitude of more than 143 miles and traveled a distance of l,222,020 miles. Upon her return to the Soviet Union, she became a national hero and traveled the world extolling the virtues of the Communist system. She later served as a Soviet politician.

—Chad T. Lower Further Reading Chang, Hasok. Inventing Temperature: Measurement and Scientific Progress. Oxford UP, 2004.

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EARLY LIFE Valentina Tereshkova was born on a collective farm about two hundred miles from Moscow. Her father,

Principles of Aeronautics

a former tractor driver on a commune, was lost in the Finno-Russian War sometime during 1939 or 1940, when Tereshkova was just two years old. Her mother and older sisters worked in the Krasnui Perekop (Red Canal) textile factory during Tereshkova’s school years. At age seventeen, Tereshkova began working at the Yaroslavl tire factory while continuing her studies for Young Communist Workers. In 1955, she joined her mother and sisters as a spindler in the textile factory. At the same time, she completed studies during evening classes at the polytechnic institute in Yaroslavl. By 1961, Tereshkova had become a cotton-spinning technologist. She also headed the Textile Mill Workers’ Parachute Club, making her first jump at age twenty-two, and eventually had 126 jumps to her credit. She joined the Communist Party and became secretary of the local Komsomol (Young Communist League). In world politics, the Cold War began to intensify. On April 12, 1961, Soviet cosmonaut Yuri Gagarin successfully orbited the earth. US astronaut Alan Shepard’s suborbital flight followed on May 5. Gherman Titov flew aboard Vostok 2 on August 6 and was the first to experience space sickness. On August 11 and 12, 1962, Soviet premier Nikita S. Khrushchev arranged for two spacecraft to launch within twenty-four hours of each other for the world’s first group flight. Vostok 3 contained Andriyan G. Nikolayev, whom Tereshkova would marry in 1963, while Vostok 4 held Pavel R. Popovich. Though the two craft flew within four miles of each other, they could not maneuver without expending a great deal of fuel, so an actual rendezvous was not possible. Neither cosmonaut experienced space sickness during his three- to four-day journey, and Soviets began to feel better about the safety of longer flights. Space sickness apparently depended more on the individual than on the length of the flight.

Valentina Tereshkova

Tereshkova’s space mission was celebrated on this 1963 USSR postage stamp. Photo via Wikimedia Commons. [Public domain.]

Following Gagarin’s successful flight of Vostok 2, many women wrote to Moscow asking about spaceflight training. Air force majors Vera Sokolova and Marina Popovicha were rejected. Sokolova was a flight surgeon with many hours of flying experience; Popovicha was a qualified jet test pilot with an engineering degree, and she was married to cosmonaut Pavel Popovich. Khrushchev, however, decided that the woman selected to fly into space would not be an elite, highly trained specialist but rather an ordinary

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factory worker. He wanted to demonstrate that under socialism, any woman could fly into space. LIFE’S WORK In November 1961, five selected trainees—Tatiana Kuznetsova, Valentina Ponamareva, Irena Solovyeva, Zhanna Yorkina, and Valentina Tereshkova—secretly reported to training camp. All were under thirty years old, stood less than five feet seven inches in height, and weighed less than 155 pounds. They were also required to have parachuting experience because the Vostok spacecraft design required the pilot to bail out just before it made a hard-earth landing, as opposed to the soft-water landing US spacecraft made. Tereshkova was the only one without higher education. The five women trained as copilots and later progressed to piloting jets. The women received physical training, classroom lectures, and further jet-flight orientation. Following their preflight training courses in jet aerobatics in a two-seater MIG trainer, the women received commissions as second lieutenants in the Soviet Air Force. Nikolayev, the pilot of Vostok 3, had become a flight cadet in 1951. He was the only bachelor among the cosmonauts. The women were all single. Nikolayev was assigned as their training coach. Some reports claim that Tereshkova was chosen from the other candidates because of her subsequent engagement to Nikolayev. Other journalists wondered how much of a part Khrushchev played in matching the pair. Tereshkova’s flight, Vostok 6, was launched at 12:30 p.m. Moscow time on June 16, 1963, following Valeri Bykovsky’s Vostok 5 launch by forty-five hours and thirty minutes. She was the tenth person to enter space. Both spacecraft were launched from the Baikonur space center in the central Asian republic of Kazakhstan. Tereshkova’s radio call name was Chaika (seagull), while Bykovsky’s was Yastreb (hawk). Both frequently sent greetings to other na-

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tions while in orbit and appeared on Moscow television. Water from each cosmonaut’s home area had been chemically treated to kill bacteria and was installed in each spacecraft. When the cosmonauts were thirsty, they pushed a button and water entered a mouthpiece through a rubber tube. For washing, they used pieces of cheesecloth soaked with a soap-and-water mix. Both were on a schedule of four meals per day. Meat, caviar sandwiches, and fruit were cut into small pieces to make eating easier. Vitamins were added to the rations to help normalize metabolism in the weightless state. Aviation Week and Space Technology reported in its July 1963 issue that perhaps the objective of Vostok 5 and Vostok 6 had been to make visual, optical, radio, and radar observations of their vehicles upon crossing courses. This exercise would give future cosmonauts an opportunity for inspection or even neutralization, if necessary, of a target vehicle. The Soviets agreed it was a wonderful space feat, but the flight mostly celebrated the fact that a woman had entered the space arena and that it had been the Soviet Union that had achieved it. On the second day of Tereshkova’s flight, Bykovsky became alarmed when he failed to rouse his travel companion. Tereshkova finally responded that she had overslept, and she reported promptly thereafter. Once again, both spacecraft flew within three miles of each other, but a true rendezvous was still not possible. Tereshkova was permitted to take manual control of her spacecraft for one orbit. Years after her mission, Tereshkova revealed that shortly after launch, she discovered a problem with her spacecraft’s automatic orientation system, which meant that initiating the landing sequence would take her farther from Earth, not closer to it. Ground control had to send and install a new program while Tereshkova was in orbit, which it fortunately was able to do with no further complications. After completing forty-eight orbits over the course of two days, twenty-two hours, and fifty minutes, Tereshkova

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landed during her forty-ninth orbit. She touched down at 11:20 a.m. in an area 386 miles northeast of Karaganda, a coal-mining and industrial city in Kazakhstan. Bykovsky completed eighty-one orbits and landed about 360 miles northwest of Karaganda. Following their successful landings, Tereshkova and Bykovsky were warmly greeted by Premier Khrushchev at Moscow’s airport. In his welcoming speech, Khrushchev compared Tereshkova’s courage to that of Marina Raskova, the World War II hero buried in the Kremlin wall. Despite rumors of disorientation and space sickness, Tereshkova became an instant celebrity and began touring different countries, extolling the virtues of her country and its values. In November, Tereshkova and Nikolayev married in a simple ceremony at the Griboyedov Street Palace. Gagarin, Bykovsky, and the two men’s wives were witnesses for the couple. Acting as surrogate father for both bride and groom, Premier Khrushchev and his wife hosted a gala reception. Khrushchev reeled off a twenty-one-toast salute during the four-hour-long state splurge. Following her honeymoon, Tereshkova continued her travels. She assumed dozens of ceremonial posts and moved into the office of chair of the Committee of Soviet Women in Moscow. She also received the title of Hero of the Soviet Union and continued her studies at the air force academy. On June 10, 1964, Tereshkova gave birth to daughter Elena Andrianovna, the first child of two spacefarers. There had been some popular concern that the birth might be complicated and the health of the baby might be impaired since both parents had been in space. However, neither the child nor the parents seemed to suffer any ill effects from exposure to elements of space. Tereshkova continued with her studies at the air force academy. A published account of the First USSR Conference on Space Physics in June 1965 an-

Valentina Tereshkova

nounced that Tereshkova had determined the existence of a dust layer around Earth at about twelve miles from its surface that seemed to play a major role in the development of enigmatic nacreous clouds. The crew of the Voskhod 1 confirmed Tereshkova’s findings. In 1966, she received the rank of major in the Soviet Air Force. In September 1969, she earned a diploma from the Zhukovsky Aircraft Engineering Academy, which qualified her as a full-fledged engineer. By April of 1977, Tereshkova had earned her doctorate in aeronautical engineering. After the overthrow of Khrushchev, speculation that the marriage of Tereshkova and Nikolayev may have been orchestrated by the Soviet premier was fueled when the couple stopped living together and no longer stood near each other in official photographs of the cosmonauts. In 1982, their marriage ended in divorce. Tereshkova later married again, to Yuliy Shaposhnikov, a physician, who died in 1999. Afterward she moved to a dacha near Star City, the cosmonaut enclave northeast of Moscow. Even though the first group of women cosmonauts was disbanded in 1969, Tereshkova stayed on the payroll of the cosmonaut corps as an instructor at the Yuri Gagarin Cosmonaut Training Center until April 1997, when, having reached the age limit, she was retired with the rank of major general by the decree of Russian president Vladimir Putin and became a senior staff scientist at the Gagarin Cosmonaut Training Center. In the meantime she was involved in a wide variety of political, social, and scientific activities, many of them unpaid. Her record as a model Communist earned her admission to the Supreme Soviet as a deputy from 1966 to 1989 and a member of the Presidium beginning in 1974. She also joined the World Peace Council in 1966, became vice president of the Women’s International Democratic Federation in 1969, was a member of the Central Committee of the Communist Party from 1971 to 1989, and was an official

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representative at the United Nations’ World Conference on Women in Mexico City in 1975, held in honor of International Women’s Year. In 1994 the Russian Federation made Tereshkova chair of the Russian Center for International Scientific and Cultural Cooperation, and beginning in 1998 she served on the editorial board of Polyot (flight), a Russian popular-science magazine. She was also known to support orphanages and help out fellow Russians. Her achievement as a cosmonaut and work for peace brought her international recognition and many honors. The Soviet Union awarded her two Orders of Lenin, the Order of the October Revolution, the Order of the Red Banner of Labor, the Tsiolkovsky Gold Medal of the Academy of Sciences, a bronze bust in Moscow’s Space Heroes Alley, and honorary citizenship of six cities. Tereshkova also received honorary doctorates from three foreign universities, the Joliot-Curie Gold Medal, the Simba International Women’s Movement Award, a gold medal from the British Interplanetary Society, and the United Nations Gold Medal of Peace. A crater on the far side of the moon is named for her, as is an asteroid. In 2000 she was named Greatest Woman Achiever of the Century by the International Women of the Year Association. President Putin invited Tereshkova to his home to celebrate her seventieth birthday in 2007. At the celebration, she volunteered for any planned mission to Mars, even if that mission were to be one way. In June 2013, Tereshkova again visited Putin at his residence, this time to commemorate the fiftieth anniversary of her historic spaceflight. During the celebration, Putin awarded her the Order of Alexander Nevsky in honor of her decades of public service. Tereshkova carried the Olympic flag at the 2014 Winter Olympics in Sochi. SIGNIFICANCE Tereshkova’s feat as the first woman in space stands as a remarkable legacy on its own. However, aside

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from biomedical information, her record flight produced at least two other practical benefits. Her flight demonstrated that a healthy person can, with proper training, withstand the strains of launch, weightlessness, and reentry without having the highly specialized professional background of a military test pilot, and it also opened the way for the training of scientists for later, more complex missions. She also became one of the first space voyagers to enter politics and turn scientific notoriety into a stepping stone for sociopolitical causes, in her case peace and women’s issues. —Lillian D. Kozloski Further Reading Cavallaro, Umberto. Women Spacefarers: Sixty Different Paths to Space. Springer International Publishing, 2017. Eidelman, Tamara. “A Cosmic Wedding.” Translated by Nora Favorov. Russian Life, Nov./Dec. 2013, pp. 22-25. Gerovitch, Slava. Voices of the Soviet Space Program: Cosmonauts, Soldiers, and Engineers Who Took the USSR into Space. Palgrave, 2014. Ghosh, Pallab. “Valentina Tereshkova: USSR Was ‘Worried’ about Women in Space.” BBC News, 17 Sept. 2015. Accessed 18 Sept. 2015. Gibson, Karen. Women in Space: 23 Stories of First Flights, Scientific Missions, and Gravity-Breaking Adventures. Chicago Review Press, 2014. Griswold, Robert L. “’Russian Blonde in Space’: Soviet Women in the American Imagination, 1950-1965.” Journal of Social History, vol. 45, no. 4, 2012, pp. 881-907. Harrison, Albert A. Spacefaring: The Human Dimension. U of California P, 2001. Isachenkov, Vladimir. “First Woman in Space, Valentina Tereshkova, Honored 50 Years after Historic 1963 Flight.” Huffington Post. TheHuffingtonPost.com, 14 June 2013. Accessed 4 Dec. 2013. Kevles, Bettyann Holtzmann. Almost Heaven: The Story of Women in Space. Basic, 2003. ———. “Fifty Years of Women in Space.” Daily Beast, 14 June 2013. Accessed 4 Dec. 2013. Llinares, Dario. The Astronaut: Cultural Mythology and Idealised Masculinity. Cambridge Scholars Publishing, 2011. Menzio, Maria Rosa. The Secrets of Soviet Cosmonauts. Springer International Publishing AG, 2020.

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Shayler, David, and Ian Moule. Women in Space following Valentina. Springer, 2005. Siddiqi, Asif A. Sputnik and the Soviet Space Challenge. UP of Florida, 2003. Tereshkova, Valentina Vladimirovna Nikolaeva. Valentina Tereshkova, In Her Own Words: The First Lady of Space. Spacehistory101.com Press, 2015. See also: Aeronautical engineering; Neil Armstrong; Glenn H. Curtiss; Amelia Earhart; First flights of note; Yuri Gagarin; John Glenn; Robert H. Goddard; Otto Lilienthal; Charles A. Lindbergh; Wiley Post; Rockets; Russian space program; Alan Shepard; Space shuttle; Spacecraft engineering; Spaceflight; Jules Verne; Wright brothers’ first flight; Chuck Yeager

Training and Education of Pilots Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Physiology; Mathematics ABSTRACT Working to become a qualified, licensed pilot involves all flight and ground-based training and education for personnel who wish to be involved with the operation of aircraft. Due to the complex nature of modern aircraft and their operation, the training and education of aviation personnel is essential. KEY CONCEPTS airframe: the essential structure of an aircraft design without the additional items required for its intended use air traffic control system: monitored radar stations on which aircraft signals appear accompanied by unique identifying information, course prediction, and altitude positive control: maintaining the operation of an aircraft by the actions and commands of the pilot rather than flying free without human control

Training and Education of Pilots

test pilot: a pilot who undertakes the dangerous job of flying repaired or experimental aircraft to determine their flight capabilities INTRODUCTION The US aviation training industry is both highly structured and multifaceted, involving the training of pilots, mechanics, avionics technicians, air traffic controllers, and airline dispatchers, along with a variety of other engineers and technicians who make it possible for people and materials to be transported worldwide by air for military, commercial, or other civil service purposes. HISTORY OF AVIATION TRAINING Aviation training began in the United States, beginning in the nineteenth century with the US Army’s training of men to operate hot-air balloons used in aerial observation, as had been a practice in Europe. Training for flight in heavier-than-air vehicles began in the summer of 1908. On August 1, 1907, the US Army Signal Corps had established the Aeronautical Division under the direct command of Captain Charles deForest Chandler. Having been awarded a bid to provide one aircraft and two trained pilots for the Army, Orville Wright, who with his brother, Wilbur, had pioneered flight in a heavier-than-air craft, began providing flight instruction at Fort Myer, Virginia, that same year. Although neither would solo, the first two students were Lieutenant Thomas E. Selfridge and Second Lieutenant Benjamin D. Foulois. On September 17, 1908, Selfridge was tragically killed in an accident in which Orville Wright was severely injured. Foulois was transferred to Europe, and training was halted until the following year. Two subsequent students, Lieutenant Frank Lahm and Second Lieutenant Frederic Humphreys, were selected, and both soloed their first aircraft under Orville Wright’s supervision at College Park, Maryland, on October 26, 1908.

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CIVILIAN FLIGHT TRAINING Most aviation training in the United States is civilian flight training, in which ordinary citizens are trained to be pilots. Pilots may learn to fly a variety of different aircraft for recreation or airline transportation, or for more specialized purposes, such as aerial crop dusting, pipeline patrol, law enforcement, or sight-seeing operations. Those wishing to fly for purely personal transportation may choose to pursue either a Recreational Pilot Certificate or a Private Pilot Certificate. Either of these certificates requires that the pilot be at least seventeen years of age (although in order to fly solo in training, a pilot need be only sixteen years of age), have a mastery of the English language, and pass a basic physical examination.

Photo via iStock/yacobchuk. [Used under license.]

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A Recreational Pilot Certificate allows the holder to fly aircraft of up to 180 horsepower and to carry no more than one passenger into and out of smaller airports within fifty miles of the pilot’s home airport during daylight hours only. The recreational pilot may fly into larger airports or venture farther than fifty miles from home only with the permission of a certified flight instructor (CFI). Training for the Recreational Pilot Certificate involves a minimum of thirty hours of flight training, including dual instruction and supervised solo operations. A Private Pilot Certificate enables the holder to avoid the restrictions of the recreational certificate and involves a minimum of forty hours of flight instruction and supervised solo flights.

Principles of Aeronautics

Those seeking careers in flying in which they will be paid for their services as pilots must possess either a Commercial Pilot Certificate or an Airline Transport Pilot (ATP) Certificate. In order to qualify for a Commercial Pilot Certificate, the pilot must have acquired at least 250 hours of flight time and be trained to fly in aircraft that are slightly larger and more complex than those aircraft required for private or recreational pilot training. A pilot wishing to be an airline transport pilot must have accumulated at least 1,500 hours of flight time and be at least twenty-three years of age. Holders of Private Pilot Certificates and higher-level certificates may choose to add an Instrument Rating to their certificates, which enables them to operate an aircraft without visual reference to the ground in what is called “instrument meteorological conditions.” This additional rating involves an additional forty hours of instruction in instrument flying procedures. Holders of Airline Transport Pilot Certificates are required to have an Instrument Rating. In addition, those pilots wishing to fly aircraft with more than one engine must add a Multiengine Rating to their certificate. These certificates allow pilots to operate aircraft with piston or turboprop engines weighing up to 5,670 kilograms. For jet-powered aircraft or aircraft weighing more than 5,670 kilograms, an additional certificate is required in the specific aircraft to be flown. Pilots certified in the previous categories may choose to be certified to fly airplanes, helicopters, gliders, gyroplanes, or seaplanes, or any combination thereof. Each certificate and rating requires the applicant to pass both a knowledge exam, administered in a computer-based testing format, and a practical flight exam given by authorized representatives of the Federal Aviation Administration (FAA). TRAINING REGULATIONS Civilian flight training is regulated under Title 14 of the Code of Federal Regulations (CFR), either Part

Training and Education of Pilots

U.S. Army test pilot stands in front of a Douglas A-20 Havoc, 1942. Photo via Wikimedia Commons. [Public domain.]

61 or Part 141. The basic difference between the two parts is that Federal Aviation Regulations (FARs) Part 141 training requires that the training be conducted under a greater degree of structure than that of Part 61. Under Part 141 of training, the lessons are structured under a standardized curriculum in which pilots must pass through various stages of training, each requiring an evaluation by a chief or assistant CFI separate from the student’s primary flight instructor. This arrangement is intended to note and correct any of a student’s problem areas prior to taking the final practical flight test that determines whether the individual will be awarded the pilot certificate sought. This test is accomplished at the end of a pilot’s training for a particular certificate and is, again, given by a representative of the FAA. Provided that the student passes the check

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ride, the student is then awarded the certificate and is entitled to all of the privileges associated therewith. If the student’s check ride is not satisfactory, the student must return for additional training before again taking the practical flight test. Under FAR Part 61, the required training time is reduced slightly, with a Private Pilot Certificate requiring a minimum of 35 hours and a Commercial Pilot Certificate requiring 190 total flight hours. There is no difference for the ATP Certificate. Although there is less required structure under FAR Part 61 training, the required subject matter is the same. Training conducted under FAR Part 61 ranges from individual flight instructors and students engaging in instruction in privately owned aircraft to

Principles of Aeronautics

many larger and more complex organizations that offer flight training. It is important to note that many flight training organizations and individuals operating under Part 61 have as much or more structure built into their curriculum as do Part 141 organizations, although some have less. When choosing where to undergo flight training, a prospective student would do well to make a site visit to observe and compare several organizations and individuals before making a final decision. For those wishing to obtain a degree in aviation, there are many two- and four-year institutions around the country offering degrees in aviation flight, aircraft maintenance, air traffic control, aviation electronics, or aviation management. For those wishing to pursue careers as airport administra-

Picture of an astronaut going through pilot training exercise. Photo via Wikimedia Commons. [Public domain.]

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tors or airline pilots, a four-year degree is almost a necessity. MILITARY FLIGHT TRAINING Individuals who wish to fly in the armed services must first qualify for selection as a pilot candidate. This involves a series of interviews by a pilot selection board, psychological and physiological aptitude evaluations, and fitness examinations. In addition, applicants must meet the criteria to serve as a military officer or warrant officer, which most often involves, among other criteria, a four-year academic degree, preferably in a technical field. Exceptions to this requirement are made in certain branches depending upon need and job requirements. In addition, serving as a pilot in the military obligates the individual to several years of military service, the length of which depends upon the military’s current and projected needs for flight officers. Upon selection, pilots are sent through a pilot screening program, which involves several hours of flight in a light aircraft in order to assess an individual’s suitability for pilot training. After candidates pass this phase, they are sent on to primary flight training for several months of intensive ground school and flight training. When primary flight training is complete, the pilots are then routed to aircraft-specific training geared toward specific craft, such as fighters, transport, or bombers, based on their preferences, their primary flight training performance, and the needs of the military. During their aircraft-specific training, pilots undergo additional training in the aircraft in which they will serve prior to being assigned to a specific wing or squadron, where they will be fully qualified to fly their specific type of aircraft for specific types of missions. AIRCRAFT MAINTENANCE TRAINING An individual wishing to pursue a career in the maintenance of aircraft as a civilian must choose from one of two options. The first option is to be-

Training and Education of Pilots

come employed as an aircraft repairperson. An Aircraft Repairman Certificate is earned through on-the-job training for a predetermined amount of time with an employer engaged in aircraft maintenance and repair. This certificate is given for a specific type of work performed, and the privileges of the certificate are forfeited upon termination of employment. The second option is to pursue an Aircraft Mechanic Certificate. The individual may choose to pursue an Airframe Mechanic Certificate, a Powerplant Mechanic Certificate, or, more commonly, the combined Airframe and Powerplant Mechanic (A&P) Certificate. The mechanic certificate allows the holder to engage in the maintenance and repair of certified aircraft—all aircraft, other than ultralight aircraft weighing less than 254 pounds, for which no certification is needed—whether independently or as part of a larger organization. This certificate is awarded to the individual regardless of employment. In order to be awarded the airframe, the powerplant, or the A&P Certificate, an applicant must either complete an FAA-approved FAR Part 147 course of study at an aviation training institution, undergo thirty-six months of on-the-job training for both the airframe and powerplant certificates with an organization engaged in the maintenance and repair of certified aircraft, or study under the supervision of a previously certified mechanic. The training for the A&P Certificate involves 1,900 hours of classroom and hands-on laboratory training consisting of such topics as engine overhaul, airframe inspection, hydraulics, welding, and sheet metal, as well as basic math, physics, and electricity. After the required training has been completed, individuals are required to pass a series of knowledge examinations, including a practical exam that demonstrates their competency to exercise the privileges of the certificate. An authorized FAA representative conducts this test. As with the pilot certificate test, individuals who fail the examination may be allowed

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to retake the exam, provided they undergo additional training or complete additional study. If mechanics so choose, they may pursue an inspection authorization certificate after they have been actively engaged in aircraft repair for three years. This certificate allows for additional inspection privileges beyond those allowed by the mechanic certificate. An additional knowledge test is required for the inspection authorization. Aviation mechanics may also pursue specializations requiring advanced training and certifications from outside agencies and organizations. Mechanics may decide to pursue training in nondestructive testing or in avionics, such as aircraft radios and other flight electronics, in which case the applicant must pass a Federal Communications Commission (FCC) knowledge exam. Upon successful completion of this exam, applicants are awarded FCC certification allowing them to function as avionics repair technicians. Several institutions of higher learning in the United States have degree programs giving the student much broader and more in-depth training in avionics repair. With the ever-increasing use of advanced composite materials in all aspects of the aerospace industry, it is also wise that certified training in accord with FAA or Transport Canada regulations be undertaken by those either seeking or who are already with Airframe or A&P certification. Such training can be had at any of the available licensed training facilities in the United States and Canada, such as Abaris Training in Reno, Nevada, or Renaissance Aeronautics Associates in London, Canada. MILITARY AIRCRAFT MAINTENANCE TRAINING For those wishing to serve as aircraft mechanics in the military, the process first involves enlistment and aptitude assessment along with routine physical and other suitability exams. After trainees complete basic training and boot camp, they then move on to

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job-specific training, which can take from a few weeks to many months of additional training, depending upon the specialty. Individuals who leave the military and wish to work as civilian aircraft mechanics must first be assessed by the FAA to determine whether they meet the requirements to begin a series of tests leading to the airframe mechanics’ license, the powerplant license, or both. AIR TRAFFIC CONTROL TRAINING Although aircraft pilot and mechanic training comprise the majority of aviation training, there are many other professions within aviation that require a high degree of specialized training. Air traffic controllers, for instance, spend years becoming qualified and learning various nuances of their profession. In order to become air traffic controllers, individuals must have a mastery of the English language, be at least twenty-one years of age but no older than thirty years of age prior to beginning training, and possess certain cognitive abilities consistent with the air traffic control profession. In addition, they must successfully complete the Control Tower Operator’s Exam leading to the issuance of the Control Tower Operator’s Certificate, a basic air traffic control (ATC) certification. Successful certification also involves a series of practical examinations. There are three basic avenues individuals can take in order to become air traffic controllers. The first involves enlisting in the armed services, primarily the Air Force and Navy, and choosing air traffic control as a specialization area if aptitude testing proves this is a viable option. The ability to perform many tasks at once in a highly dynamic and often hectic environment while maintaining a three-dimensional geometric orientation is a skill that will serve the aspiring air traffic specialist well. After leaving the service, military controllers are given preferential hiring treatment by the FAA to work as civilian air traffic controllers.

Principles of Aeronautics

The second option is to attend one of the colleges and universities associated with the FAA’s Collegiate Training Initiative (CTI) program. Students in these programs take specialized coursework in air traffic control and are then given preferential hiring treatment by the FAA as air traffic controllers. The third option is to apply directly to the FAA as an individual and proceed through the official application process. This will involve a series of interviews and examinations in order to determine suitability for the job of controller. If a person is selected, the FAA will train that individual in accordance with the individual’s strengths and the FAA’s needs. Depending upon the type of specialization chosen, it may take several years before a person becomes fully qualified to perform normal job duties. Typically, a controller whose job involves handling a relatively low volume of traffic in the control tower of a less busy airport might be qualified in a few months, whereas a controller specializing as a radar controller might take several years to be fully qualified. An individual who chooses the military follows a slightly different training route. After completing basic training or boot camp, a trainee is then routed along a training track consistent with the needs of the military and the aptitude of the trainee. The trainee might specialize as a tower operator dealing primarily with takeoff, departure, and landing clearances for a specific location or may specialize in radar control, dealing with and directing aircraft flying between airports or airbases. Depending upon the type of specialization, a trainee may need to be trained for up to two years or more before becoming fully qualified to perform normal job duties as a controller AIRCRAFT DISPATCHER TRAINING Aircraft dispatching is another technical aviation job that requires a regimented training routine. Aircraft dispatchers are employed by United States and foreign passenger and cargo airlines to assist flight

Training and Education of Pilots

crews with the details of flight planning and management. Dispatchers are jointly responsible, with the flight captain, for the safe outcome of a given flight. Dispatchers assist the pilots in obtaining timely weather information, performing fuel calculations, and ensuring that the aircraft is loaded in a manner consistent with safe flight. In order to qualify as an aircraft dispatcher, an individual must be at least twenty-three years of age, have a command of the English language, and complete an FAA-certified training program requiring at least 198 hours of classroom instruction. The formats for these courses vary from six-week intensive courses in which the student attends all day, every day to courses in which training is spread out over the course of six months to a year or more and students attend in the evenings or on weekends. At the end of the training, there is a comprehensive knowledge exam and a practical exam, administered by a representative of the FAA. Several colleges and universities around the country have specialized aircraft dispatcher training certification programs. —R. Kurt Barnhart Further Reading Federal Aviation Administration (FAA). Aviation Instructor’s Handbook. Skyhorse Publishing, 2009. Kearns, Suzanne. E-Learning in Aviation. CRC Press, 2016. Kearns, Suzanne K., Timothy J. Mavin, and Steven Hodge. Competency-Based Education in Aviation: Exploring Alternate Training Pathways. Taylor & Francis, 2017. Olsen, Austin L. The Education of a Fighter Pilot. Lulu.com, 2021. Pilot’s Manual Editorial Board. The Pilot’s Manual: Ground School: All the Aeronautical Knowledge Required to Pass the FAA Exams and Operate as a Private and Commercial Pilot. Aviation Supplies and Academics Inc., 2020. Telfer, Ross A., and Phillip J. Moore. Aviation Training: Learners, Instruction and Organization. Taylor & Francis, 2017. See also: Advanced composite materials in aeronautical engineering; Advanced composite materials repair; Air-

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flight communication; Aerospace industry in the United States; Air transportation industry; Airplane maintenance; Avionics; Federal Aviation Administration (FAA); Flight instrumentation; Flight landing procedures; Flight recorder; Flight roll and pitch; Flight schools; Flight simulators; Flight testing; Fluid dynamics; Forces of flight; Hot-air balloons; Jet engines; Landing gear; Materials science; Propulsion technologies; Rotorcraft; Stabilizers; Takeoff procedures; Taxiing procedures; Types and structure of airplanes; Weather conditions; Wind shear

Triplanes Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT A triplane is an aircraft designed with three sets of wings in a vertically stacked arrangement one atop another. The triplane was an unconventional airframe design built during World War I, providing superior lift, rate of climb, and maneuverability compared to its monoplane and biplane contemporaries. The triplane is considered to have been one of the most radical designs in aviation history. KEY CONCEPTS lift: the upward pressure experienced by wings due to differential pressure between upper and lower surfaces of the wing as it moves through a fluid medium as determined by its airfoil camber and thickness DEVELOPMENT AND DESIGN CHARACTERISTICS By the end of 1915, aerial combat had defined itself enough to give aircraft designers a working knowledge of combat aircraft requirements. Combat aircraft needed the best possible combination of fast rate of climb, tight maneuverability, strength, and as much speed as available engines could provide.

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In 1916, in an attempt to counter highly effective German aircraft designs, the British Sopwith Company decided to try a radical new design: the three-winged airplane. Though a departure from conventional designs, the three-winged triplane proved to be light, fast, and a good climber. Pilots found the Sopwith Triplane to have phenomenal maneuverability, and its narrow-chord wings gave pilots a good field of view, while the combined area of the three wings gave enough lift to outclimb and outturn any aircraft in production by either side. The Sopwith Triplane was also 24 kilometers per hour faster than its nearest competitor, and was the first Allied fighter plane to have two forward-mounted, synchronized machine guns. Raymond Collishaw, a Canadian pilot flying with the Royal Naval Air Service, used a Sopwith Triplane to down sixty enemy aircraft, ranking him third on Britain’s ace list. Only 150 Sopwith Triplanes were built, and by the end of 1917 they were replaced by the Sopwith Camel biplane. THE FOKKER TRIPLANE The Fokker Dreidecker (three-winger) was designed by Reinhold Platz in response to the Sopwith Triplane. The Fokker was lightly loaded, fast-climbing, and highly maneuverable and first saw service in August, 1917, by Lieutenant Werner Voss, one of Germany’s leading aces and a member of Jagdgeschwader Nr 1, nicknamed the Flying Circus. Apart from the Sopwith Camel, the diminutive Fokker Dr-I triplane was the only other World War I fighter plane to hold the public’s imagination. Its fame was due largely to the exploits of another member of the Flying Circus, Germany’s leading ace, Baron Manfred von Richthofen, the “Red Baron,” who fought and died flying his crimson Fokker Dr-I. The Dr-I had a relatively short combat life. Only about 320 Dr-I’s were built before it was withdrawn from service due to structural failures.

Triplanes

Principles of Aeronautics

Sopwith Triplane 3. Photo by Tony Hisgett from Birmingham, UK, via Wikimedia Commons.

Other fighter triplane designs that were less successful during the war were the British Blackburn triplane, which never saw production; and the German two-seater Pfalz Dr.1, of which only ten were manufactured. A successful triplane not put to fighter use was the Italian Caproni Ca.42 heavy bomber. Introduced in 1918, thirty-two Caproni Ca.42s were built, six of which saw service with the British. The airplane had a wingspan of 29.9 meters and stood nearly 6.4 meters tall. Its slow top speed of only 125.5 kilometers per hour made it vulnerable to fighter attack, limiting its use to nighttime bombing raids. —Randall L. Milstein

Further Reading Franks, Norman. The Red Baron: A History in Pictures. Pen & Sword Books, 2016. Herris, Jack. Germany’s Triplane Craze: A Centennial Perspective on Great War Airplanes. Aeronaut Books, 2013. Jarrett, Philip. Trials, Troubles, and Triplanes: Alliott Verdon Roe’s Fight to Fly. Ad Hoc Publications, 2007. Wilkins, Mark C. German Fighter Aircraft in World War I: Design, Construction, and Innovation. Casemate Publishers (Ignition), 2019. Wilkins, Mark C. British Fighter Aircraft in World War I. Casemate Publishers (Ignition), 2021. Winchester, Jim. Biplanes, Triplanes & Seaplanes. Thunder Bay Press, 2004. VanWyngarden, Greg. Aces of Jagdgeschwader Nr III. Bloomsbury Publishing, 2016.

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Konstantin Tsiolkovsky

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See also: Aerobatics and flight; Aerodynamics and flight; Aeronautical engineering; Airfoils; Biplanes; Forces of flight; Otto Lilienthal; Billy Mitchell; Eddie Rickenbacker; Types and structure of airplanes; Manfred von Richthofen

Konstantin Tsiolkovsky Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Konstantin Tsiolkovsky was born on September 17, 1857, in Izhevskoye, Russia, and died on September 19, 1935, in Kaluga, Russia, Soviet Union. He was a Russian scientist who is considered the founding father of rocket theory. Tsiolkovsky was the first scientist to discover the mathematical theories of rocketry and astronautics upon which modern space travel is based. EARLY LIFE Born in 1857 in the remote village of Izhevskoye, Russia, Konstantin Eduardovich Tsiolkovsky would become the founding father of modern rocketry. Growing up in a modest family with a Polish father and Russian mother, he was an avid reader and had an early interest in mathematics and science. As young child, he contracted scarlet fever, which resulted in his near deafness. When he was sixteen, he was sent by his father 600 miles away to Moscow, where he combined university studies with a self-taught education. He later moved to Kaluga to become a teacher and remained there until his death in 1935. Tsiolkovsky first became interested in both aeronautics and astronautics, or cosmonautics as it is known in Russia, in Moscow, but he did not follow up on his interests until he accepted a teaching post in Borovsk. At this time, he began a life-long investigation of the theory of gases and reactive motion. In 1897, he built the first Russian wind tunnel in

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Konstantin Tsiolkovsky. Photo via Wikimedia Commons. [Public domain.]

Kaluga to examine the aerodynamic forces on dirigibles, and he completed extensive wind-tunnel tests on various dirigible and aircraft designs. His aviation work did not meet with much scientific interest, however, and it was only after he began publishing his thoughts on space travel in the 1890s that he seriously turned his attention to rocketry. Tsiolkovsky developed his first rocket design in 1903. It was a liquid-fueled design using one liquid for fuel and another as the oxygenator. His favored propellant mixture, combining liquid hydrogen and liquid oxygen, is still used today on the space shuttle. Tsiolkovsky’s design included several advanced concepts, such as regenerative cooling, whereby the fuel was ducted around the hot exhaust nozzle prior

Andrei Nikolayevich Tupolev

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to combustion to both cool the nozzle and preheat the fuel, and directional control, using exhaust guide vanes and gyroscopes. Tsiolkovsky’s later work included theoretical calculations of launch trajectories, escape velocities, travel times to planetary bodies, multistage rockets, and atomic- and solar-powered satellites. This work included the derivation of the rocket equation, in which Tsiolkovsky showed that rocket velocity was a function of exhaust velocity and change in rocket mass due to expended fuel. He designed extensive systems for crewed spaceflight, including living quarters and greenhouses for extended trips. He also published many science fiction stories focusing on space travel. Although he continued to develop theories for rocket design and spaceflight throughout his lifetime, he never actually built a rocket himself. Although Tsiolkovsky was acknowledged by the Soviet Union after his death as the father of cosmonautics, his theories were not widely publicized and his contributions not well known, particularly because Hermann Oberth and Robert H. Goddard had arrived at similar derivations independently in Germany and the United States, respectively. However, for his many theories on rocket design and his realistic but creative concepts of spaceflight, Tsiolkovsky is widely attributed as the first of the three founding fathers of modern rocketry. —Jamey D. Jacob Further Reading Charles River Editors. Soviet Russia’s Space Program During the Space Race: The History and Legacy of the Competition That Pushed America to the Moon. CreateSpace Independent Publishing Platform, 2015. Kosmodemyansky, A. Konstantin Tsiolkovsky, His Life and Work. University Press of the Pacific, 2000. McDougall, Walter A. The Heavens and the Earth: A Political History of the Space Age. The Johns Hopkins UP, 1985.

Shubin, Daniel H. Konstantin Eduardovich Tsiolkovsky: The Pioneering Rocket Scientist and His Cosmic Philosophy. Algora Publishing, 2016. Tsiolkovsky, Konstantin. The Science Fiction of Konstatin Tsiolkovsky. University Press of the Pacific, 2000. ———. Selected Works of Konstantin E. Tsiolkovsky. University Press of the Pacific, 2004. ———. The Aims of Astronautics. CreateSpace Independent Publishing Platform, 2004. See also: Aerodynamics and flight; Aeronautical engineering; Dirigibles; Forces of flight; Robert H. Goddard; Hindenburg; Propulsion technologies; Rocket propulsion; Rockets; Russian space program; Space shuttle; Spacecraft engineering; Wind tunnels

Andrei Nikolayevich Tupolev Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Chemical engineering; Mathematics ABSTRACT Andrei Tupolev was born on November 10, 1888, in Pustomazovo, Russia, and died on December 23, 1972, in Moscow, Soviet Union. Tupolev was the foremost aircraft designer of the Soviet Union. He was the first to design all-metal aircraft in the Soviet Union. During World War II, his team developed several medium and heavy bombers that contributed to the defeat of Germany. After the war, he developed Soviet jet engines and the world’s first supersonic plane, the Tu-144. KEY CONCEPTS aerodynamicist: one who studies the science of aerodynamics Stalin’s purges: actions undertaken at the command of Josef Stalin to “purge” the Soviet Union of dissidents and “undesirables,” resulting in the deaths of as many as 1.5 million Soviet citizens

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LIFE Born in 1888 to a middle-class provincial family, Andrei Nikolayevich Tupolev entered the Imperial Moscow Higher Technical School in 1908, where he studied under famed Russian aerodynamicist Nikolai Zhukovsky. While a student there, Tupolev and a group of friends formed a small syndicate and built a series of gliders. In the 1917 Russian Revolution, Tupolev sided with the Bolsheviks, who seemed to embrace modernity and technological progress more than other political groups. In 1918, Tupolev graduated and, with his mentor Zhukovsky, set up the Central Aerohydrodynamic Institute (abbreviated TsAGI). From 1918 to 1935, Tupolev served as the assistant director of TsAGI and as its chief designer. His ANT-2, produced in 1924, was the first Soviet all-metal aircraft. In the 1920s, Tupolev was the main Soviet proponent of heavy long-range aircraft and, during the 1930s, Tupolev’s design bureau developed the long-range ANT-25, a plane that was used to set several long-distance aviation records. In 1936, Tupolev traveled to both Germany and the United States in order to study their aircraft industries. On October 21, 1936, Tupolev was arrested as part of Stalin’s purges. He was charged with selling blueprints to Germany for the Messerschmitt Bf-110

Principles of Aeronautics

and, from 1939 to 1941, Tupolev worked in a special prison aviation workshop. The prison team developed several airplanes that eventually played a great role during World War II, such as the Pe-2 and the Tu-2 attack bomber. He was freed when Nazi Germany invaded the Soviet Union in 1941 and in 1943 his Tu-2 airplane was awarded a Stalin Prize. After World War II, Tupolev was charged with developing a copy of the American B-29 Superfortress. The resulting Tu-4 was the first Soviet strategic bomber. In the 1950s, Tupolev designed a series of jet bombers as well as large civilian passenger aircraft for the Soviet airline, Aeroflot. He was also responsible for the Tu-144, the world’s first supersonic transport. Tupolev died on December 23, 1972, and in 1988 he was inducted into the International Aerospace Hall of Fame. —Alison Rowley Further Reading Duffy, Paul, and Andrei Kandalov. Tupolev: The Man and His Aircraft. SAE International, 1996. Gunston, Bill. Tupolev Aircraft Since 1922. Putnam Aeronautical Books, 1995. Kerber, L. L. Stalin’s Aviation Gulag: A Memoir of Andrei Tupolev and the Purge Era. Edited by Von Hardesty, Smithsonian Institution Press, 1996. See also: Aeronautical engineering; Glenn H. Curtiss; Steve Fossett; Howard R. Hughes; Ernst Mach; Burt Rutan; Igor Sikorsky; Wright brothers’ first flight

Turbojets and Turbofans Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics

Andrei Nikolayevich Tupolev. Photo via Wikimedia Commons. [Public domain.]

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ABSTRACT Turbojets are jet engines that are turbocharged. Turbofans are turbojets onto the front end of which a large fan is

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added. Turbojets are used in most airplanes that are large and travel for long distances. They are also used in military applications such as bombers and special surveillance aircrafts. Turbofans generate more thrust than turbojets. For this reason, they are used to power jumbo jets. KEY CONCEPTS combustion chamber: a structure as the second stage of a jet engine within which the fuel-air mixture is combusted, producing rapidly expanding hot exhaust gases compression-ignition: an engine that uses the heat produced by the compression of air inside a combustion chamber as the ignition source for combustion of fuel compressor: a rotating structure at the first stage of a jet engine that compresses air taken in spark-ignition: an engine that uses an electrical spark inside a combustion chamber as the ignition source for combustion of fuel thrust: the force or pressure exerted on the body of an aircraft in the direction of its motion POWERFUL ENGINES Propeller engines are suited for small airplanes that travel for short distances. Turbojets provide transport capabilities that propeller engines cannot. The McDonnell FH-1 Phantom was the first all-jet airplane ordered by the US Navy and the Navy’s first airplane to fly 500 miles per hour. Its first flight took place on January 26, 1945. On July 21, 1946, an FH-1 Phantom became the first jet-propelled combat aircraft to operate from an American aircraft carrier. The Phantom weighed 4,552 kilograms, could accommodate one crew member, had a range of 1,119 kilometers, and was powered by two turbojets, the Westinghouse J30-WE-20, each of which could deliver 272 kilograms of thrust. A year later, the thrust delivered by turbojets had doubled. By the end of the 1950s, the turbojets could deliver thrusts that were twenty times that delivered by the J30-WE-20.

Turbojets and Turbofans

BASIC OPERATION OF A JET ENGINE An appreciation of how a jet engine works requires understanding Bernoulli’s principle, Sir Isaac Newton’s third law of motion, and how these two ideas from physics come into play in the operation of a jet engine. For ordinary commercial and pleasure flights, air is treated as an incompressible substance that has no viscosity, and the level flight is treated as steady. Under these circumstances, a principle of physics called Bernoulli’s principle states that the sum of three forms of energy remains constant. These three forms are kinetic energy (energy associated with the motion of air), potential energy (that associated with weight and elevation above or below a reference level), and the energy associated with pressure. Air is a very light substance; it is conventional to neglect its potential energy because its contribution is typically very small compared to those of the other two forms of energy. Accordingly, when kinetic energy increases, pressure energy must decrease by the same amount, and vice versa. Sir Isaac Newton formulated three laws of motion. Propulsion generated by a jet engine operates according to Newton’s third law of motion. It states that for every action there is a reaction. The reaction is equal to the action in magnitude but opposite to it in direction. This law can be seen in operation in many ordinary ways. For instance, walking involves planting a foot and pushing backward (action). The ground provides the reaction by resisting against the foot in order to create the thrust that makes walking possible. When a surface is slippery, this really means that its ability to provide a reaction is limited and that walking on that surface has become treacherous. As another example, if one inflates a small balloon but does not tie the open end shut before releasing the balloon, after release, the air inside the balloon will want to rush out of it. As it does so, a thrust will be created, helping the balloon sail forward for some time until most of the air has es-

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caped. A jet airplane works a little bit like the inflated balloon. In a jet, a mechanism must be created to introduce air into the plane and release it in such a way that thrust is continuously created. A jet engine utilizes jet propulsion, which is a kind of propulsion in which the force needed to move the body of the aircraft comes from discharging a jet of fluid from the body at high speed. As the fluid jet leaves the body, it produces a reaction force against the body and it is this force that propels the body forward, according to Newton’s third law of motion. Jet engines include turbojets, turboprops, ramjets, scramjets, and rockets. Rockets carry all they need with them to generate combustion. Therefore, they can operate in space because they do not need air to function. The other jet engines require air to operate properly. It is for this reason that they belong to a class called air-breathing propulsion, or simply air-breathing machines. They utilize the mechanical behavior of air in their operation. The propulsion system of a jet engine consists of an inlet diffuser, a compressor, a fuel injection system, a combustion chamber (also called a combuster), a turbine, and an exhaust nozzle. A jet engine slows down incoming air as it enters the engine; that is, it causes a decrease in kinetic energy. Thus, the air pressure increases according to Bernoulli’s principle. The air goes through a series of compressor blades that look somewhat like fan blades. These blades help push the air forward, giving it new energy by doing work on them. This work increases the pressure energy of the air and thereby adds to the total energy. This high-pressure air enters the combustion chamber, where it is mixed with fuel, ignited, and burned. The hot gases resulting from this combustion want to expand and they leave the combustion chamber at very high speed and pressure. On their way out of the engine, the hot gases go through the blades of a turbine. They drive the turbine by pushing against its blades like a high wind

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blowing past a windmill. The turbine drives the compressor because the two units are connected to each other by a shaft. The movement of the compressor compresses the air that enters the engine. As the exhaust gases leave the engine, they exert a thrust that propels the airplane forward while the gases travel backward, again in accordance with Newton’s third law of motion (action equals reaction). These exhaust gases leave the airplane in a fast-moving stream commonly called a jet. That is why this is called a jet engine. DIFFERENT KINDS OF ENGINES There are many different kinds of engines. They are classified a number of different ways. Classification hinges upon how the essential components are designed and the role they play. For example, if combustion takes place inside an enclosure, the engine is called an internal combustion engine. However, if it occurs in the open, the engine is called an external-combustion engine. Internal combustion engines are very common. They are used in planes and are designed to produce work at high efficiencies. The two main types of internal combustion engines are spark-ignition engines and compression-ignition engines. In spark-ignition engines, the fuel-air mixture is ignited using an electrical spark. In compression engines, there is no spark at all and ignition of the fuel-air mixture is achieved by increasing the temperature and pressure of the air inside the combustion chamber. Propeller engines are well suited for low-speed flights, but they do not work as well for high-speed flights for two important reasons: as the forward speed of an aircraft increases, the thrust that propellers provide to move the craft forward decreases and the drag resistance associated with their operation increases. Jet engines do not have these limitations. They were introduced to provide power to aircraft that move at high speeds because they work better at these speeds. Ideally, internal combustion engines

Principles of Aeronautics

should have high efficiency, high output and rapid combustion, generate minimal pollution, and be very quiet. It is very challenging to design engines that would achieve all of these goals. In compression-ignition engines, volume requirements are higher but the combustion process is slower than in spark-ignition engines. Thus, the maximum speeds of compression-ignition engines are much lower than those of spark-ignition engines. SUPERCHARGING AND TURBOCHARGING Two effective techniques were found to increase the output of compression-ignition engines: supercharging and turbocharging. An engine is supercharged by supplying pressurized air to it. When the air at the inlet to an engine is pressurized, the mass flow rate of air into the engine increases. Typically, this is associated with an increase in the flow rate of fuel to the engine. These two factors lead to increases in power output and efficiency. Compressors need power to do their work. When a compressor is driven from the crankshaft of an engine, the arrangement is called a supercharger. However, when it is driven by the turbine, the arrangement is called a turbocharger, and the affected engine is said to be turbocharged. Thus, turbocharging is a particular form of supercharging in which a compressor is driven by an exhaust gas turbine. There are thermodynamic advantages to turbochargers over superchargers, principally because the former utilize exhaust gas energy during the so-called blow-down. Turbocharging also reduces the weight per unit output and increases fuel economy. Work done by the turbine is just sufficient to drive the compressor. Hot gases enter the turbine where they are expanded to a pressure that allows the work done on the turbine to equal that done by the compressor. The pressure of the exhaust gases for the turbine is greater than that of the surrounding air, so the hot gases are expanded further in a nozzle so that their pressure will be lowered to that of the sur-

Turbojets and Turbofans

roundings. The gases leave at high velocity, and thrust is generated by the change in momentum that the gases undergo. CHARACTERISTICS AND PERFORMANCE OF TURBOJETS Nine characteristics are used to describe a turbojet: its weight, its length, its diameter, the number of stages its compressor has, the number of stages its turbine has, the thrust it can deliver at takeoff, the best thrust it can deliver at cruising speeds, its speed range, and its specific fuel consumption. The specific fuel consumption of a turbojet is the amount (in weight) of fuel it consumes per unit weight of thrust delivered, per hour. The characteristics of the latest models of turbojets are protected carefully by manufacturers because these machines are used for purposes of defense and surveillance. However, older models are available in museums of flight. For example, in the late 1950s, Pratt & Whitney designed the J-58 turbojet engine to be used by the US Navy. Its axial compressor had nine stages, its turbine had two stages, it had an afterburner, and it could provide a takeoff thrust of 14,742 kilograms. It weighed 2,721.6 kilograms and operated above 26 kilometers at speeds three times that of sound (Mach 3). The J-58 is currently in the museum of flight at Wright-Patterson Air Force Base. TURBOFANS The turbojet is very successful at increasing the speed of the air that enters the engine, which increases its kinetic energy, but another way to increase the total energy of the air is to increase the amount of air that enters the engine. A turbofan achieves this by mounting a large fan at the inlet to the engine. The design of the turbojet is modified accordingly. The addition of the fan creates two different paths that the incoming air can use to travel through the engine before leaving it. Some of the air

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follows the path that it would use in a conventional turbojet: from the inlet diffuser, it goes successively through the compressor, or compressors, the combustion chamber, the turbine, and the exit nozzle. The remaining air bypasses the compressor altogether: it goes from the inlet diffuser, around the engine, and directly to the back of the plane. In doing so, it converts the pressure that it stored in the inlet diffuser directly into kinetic energy. Here, again, air leaves the engine traveling faster than when it entered it, and as this air leaves the fan, it exerts thrust on it. This mechanism provides a second thrust that is added to that due to the operation of a turbofan as a conventional turbojet. If one looks at modern jumbo jets at airports, their engines look like huge fans. Chances are very high that these engines are turbofans. —Josué Njock Libii Further Reading Babu, V. Fundamentals of Propulsion. Springer International Publishing, 2021. Barsoum, Michel, et al. “The MAX Phases: Unique New Carbide and Nitride Materials.” American Scientist 89, no. 4, July-Aug. 2001. Decker, Reiner. Powering the World’s Airliners: Engine Developments from the Propeller to the Jet Age. Pen & Sword Books, 2020. El-Sayed, Ahmed F. Fundamentals of Aircraft and Rocket Propulsion. Springer London, 2016. Hunecke, Klaus. Jet Engines: Fundamentals of Theory, Design and Operation. Crowood Press UK, 2010. Linke-Diesinger, Andreas. Systems of Commercial Turbofan Engines: An Introduction to Systems Functions. Springer Berlin Heidelberg, 2010. Richter, Hanz. Advanced Control of Turbofan Engines. Springer New York, 2014. See also: Advanced propulsion; Aeronautical engineering; Air transportation industry; Airplane propellers; DC plane family; First flights of note; Flight propulsion; Fluid dynamics; Forces of flight; High-altitude flight; High-speed flight; Hypersonic aircraft; Jet engines; Mach number; Pressure; Propulsion technologies; Ramjets; Rocket pro-

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pulsion; Rockets; Scramjet; Sound barrier; Supersonic aircraft; Turboprops

Turboprops Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Also known as propjets or jetprops, a variation of the gas turbine engine that develops thrust with propellers instead of the exhaust duct. The turboprop is a very popular aviation engine designed to be fuel-efficient and to propel airplanes at moderately high speeds and altitudes. KEY CONCEPTS aerodynamic efficiency: the measure of the airfoil shape of a propeller blade to produce thrust relative to the forward speed of the aircraft drag: the resistance to motion through a fluid due to friction between the moving object and the fluid medium HISTORY The idea for using a gas turbine engine to drive an airplane’s propeller was first thought of by Sir Frank Whittle. His graduate thesis, written at the Royal Air Force (RAF) Staff College at Cranwell in 1928, predicted the use of turboprops even before the first gas turbine engine had been built. Turbojet engines were developed first and then adapted by adding more turbine blades, a reduction gearbox, and a propeller to become turboprops. The first flight of a turboprop airplane occurred in England on September 20, 1945. The engine was a Rolls-Royce RB50 Trent. It was installed in a modified Gloster Meteor aircraft. The first flight of the Vickers Viscount was made on December 6, 1948. The Viscount became the first turboprop-powered airplane to go into passenger service. The Tupolev Tu-95 was the former

Principles of Aeronautics

Turboprops

Turboprop engine diagram. Image by Emoscopes, via Wikipedia.

Soviet Union’s primary nuclear bomber from the mid-1950s into the early 1990s. This Bear bomber set the speed record for a turboprop airplane by flying at 925 kilometers per hour at 11.6 kilometers. Many different models of turboprop engines and airplanes are still being manufactured, such as the Raytheon King Air and the Lockheed C-130. THEORY OF OPERATION Gas turbine engines consist of a compressor, a combustion chamber, and a turbine. These components are collectively referred to as a gas generator, a name arising from the purpose of the parts, which is to generate a stream of high-velocity gas. A turbojet generates all of its thrust from having high-pressure and high-velocity gases leaving the exhaust duct at the back of the engine. A turbojet only has enough turbine blades to extract energy to drive the com-

pressor. A turboprop converts most of the exhaust pressure and velocity into rotational energy by adding extra turbine blades. The extra turbine blades extract energy to drive a reduction gearbox. This reduction gearbox is required so the large propeller will spin at a slower, more efficient speed. The reduction gearbox reduces the revolutions per minute (RPM) of the turbines, typically ten thousand to forty thousand RPM, down to two thousand. The turboprop engine’s thrust produced by the exhaust is about 10 percent of the total thrust of the engine. The other 90 percent comes from the propeller. Turboprop and turboshaft engines are nearly identical except for their purpose. Turboprop engines drive propellers to provide forward thrust. Turboshaft engines drive helicopter transmission gearboxes which provide horsepower to drive rotor blades.

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ADVANTAGES The greatest advantage of the turboprop engine is its high fuel efficiency. It is more fuel-efficient than the two other types of gas turbine engines that propel airplanes: turbojets and turbofans. A turboprop acquires this efficiency from accelerating a large mass of air to only a slightly higher velocity. Turbojets accelerate a small mass of air to a much higher velocity. Less energy from fuel is required to affect mass than to affect velocity. Several advantages for using a turboprop engine to propel an airplane come from the propellers themselves. Turboprops have the shortest takeoffs of any airplane. This capability is provided by the variable pitch of the propeller blades. The use of variable-pitch blades is similar to the use of low gears in a car at slow speeds and the use of higher gears when reaching higher speeds. Unlike a car, this blade pitch is infinitely variable, so the propeller always has the perfect gear ratio. Turboprops can also put the propeller blades into reverse pitch. Although the blades continue to turn in the same direction of rotation, the pitch or angle of the blades change from blowing air backward to blowing air in a forward direction, thereby dramatically slowing the aircraft during landing. When the blade pitch is reversed, more fuel is sent to the engine to increase the amount of reversed airflow. This causes the engine to produce more noise during landing when the blades are reversed. This increase in noise and reversed airflow is similar to thrust reversers used during landing with turbojet and turbofan engines. DISADVANTAGES A turboprop airplane’s greatest disadvantage is being limited in forward airspeed. This limit is imposed by the use of a propeller. By the time the airplane has reached Mach 0.6, which is 60 percent of the speed of sound, the propeller blades have lost much of their aerodynamic efficiency. This effi-

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ciency is lost because there is much greater drag on the blade tips at high forward speeds than at slow forward speeds. The combination of speed of the rotating blade with the forward speed of the airplane produces speeds at the blade tips that are close to Mach 1.0. Aerodynamic drag when approaching Mach 1.0 becomes extremely high. To keep the propeller blade tips at slow enough speeds to be efficient, turboprops typically fly at speeds of less than Mach 0.6 (663 kilometers per hour at 7.62 kilometers altitude and temperature of -35 degrees Celsius). Since turbojets and turbofans do not have propellers, most commercial and corporate jets can easily fly at Mach 0.8 to 0.9. In the late 1990s, some smaller airlines started replacing their turboprop-powered airplanes with turbofan-powered airplanes, even though the fuel efficiency of turbofans is not quite as good. This replacement was for two major reasons: passengers feel more comfortable flying in “jets” than in “props,” and more revenue-generating flights can be accomplished per day if the airplanes can travel faster. —John C. Johnson Further Reading Cercerále, Jiri. Whirl Flutter of Turboprop Aircraft Structures. Elsevier Science, 2015. Morales, João Paulo Zeitoun. EMB-312 Tucano: Brazil’s Turboprop Success Story. Harpia Publishing, 2018. Stroud, Nick. The Vickers Viscount, The World’s First Turboprop Airliner. Pen & Sword Books Ltd., 2017. Van Soest, Ton. Business Jets and Turboprops Quick Reference Biz QR 2017. Air Britain (Trading) Ltd., 2017. Zichek, Jared A. Goodyear GA-28A/B Convoy Fighter: The Naval VTOL Turboprop Tailsitter Project of 1950. Retromechanix Productions, 2015. See also: Aeronautical engineering; Airplane propellers; Fluid dynamics; Forces of flight; High-speed flight; Jet engines; Propulsion technologies; Shock waves; Turbojets and turbofans

Principles of Aeronautics

Types and Structure of Airplanes Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT An airplane is a means of air transportation that is propelled by an internal combustion, turboprop, or jet engine. The invention and use of airplanes defined the twentieth century, during which the world witnessed two world wars and the development of aircraft from propeller-driven airplanes to supersonic jets. Each year, airplanes are used around the world for transportation, commerce, and recreation. KEY CONCEPTS empennage: the vertical structure comprising the tail of an aircraft fixed-wing aircraft: aircraft having the traditional structure of a fuselage and nonmoving wings on either side of the fuselage movable-wing aircraft: aircraft having wings that are able to change configuration, although remaining attached to one position on the fuselage passenger plane: aircraft designed and put into service solely for the transport of commercial travelers; passenger plane designs are often modified to remove passenger-carrying capability to make room for material goods NATURE AND USE Airplanes fly by the laws of physics. They come in all shapes and sizes and serve different purposes. Some aircraft are used for training; others are used for transporting goods and freight. Military aircraft are used in waging warfare and supporting other military operations. Passenger airliners are used for the daily transportation of travelers. Although airplanes have different designs and functions, all airplanes share common traits. The fuselage, or body of the aircraft, carries people, cargo,

Types and Structure of Airplanes

and baggage. Attached to the fuselage are the wings, which provide the lift to carry the aircraft and its payload. To balance the airplane in flight, the tail, or empennage, is very important. The landing gear allows the airplane to operate on the ground. The flight controls are used to maneuver the aircraft in flight. Flaps provide additional lift and drag for takeoffs and landings. FUSELAGE The primary job of the fuselage is to provide space for the flight crew and passengers, as well as for the things the airplane is transporting. The attachment of the wings and other load-bearing structures is also an important function of the fuselage. Depending on the size and function of the aircraft, the fuselage provides a safe haven for those inside the craft. For large airplanes that fly at high altitudes, these compartments are pressurized and air-conditioned. In smaller general aviation airplanes, the cockpits can be drafty, noisy, and either cold or hot, depending on the time of the year. In airliners, seats are arranged to allow the greatest number of paying passengers to ride inside the fuselage in relative comfort. In airliners that have been repurposed to serve as cargo carriers, the fuselage is a cavernous hold without seats in the cabin. WINGS Wings are as varied as other parts of the airplane. They come in different shapes and sizes, depending on the aircraft’s speed and weight requirements. A slower airplane may have a rectangular wing or a tapered wing. A rectangular wing is one in which the chord line, or cross section, of the wing, remains constant from the root of the wing near the body of the aircraft to the wingtip. A tapered wing is one that becomes narrower toward the tip. High-speed aircraft, such as jet transports, airliners, or fighter aircraft, have swept-wing designs. The purpose of

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the swept wing is to allow the airplane to fly at higher airspeeds. The size of an airplane’s wing in relation to the airplane’s size is important. The larger the airplane, the bigger the wing must be to support it in flight. Many factors determine how the wings work in lifting an airplane. The first factor is that of the wingspan. This is the distance from one wingtip to the other. Small general aviation airplanes typically have wingspans from 11 to 13 meters. Larger airplanes, such as the Boeing 747, have wingspans that easily exceed 35 meters. The second factor is that of chord. The wing chord is the distance as measured from the leading edge of the wing, or front, to the trailing edge. In a rectangular wing, the chord is constant and, as such, is a constant-chord wing. On tapered, elliptical, or

A Lockheed U-2 in flight. Photo via Wikimedia Commons. [Public domain.]

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other odd-shaped wings, the chord is not constant. On these wings, the average chord, or mean aerodynamic chord (MAC), is required in equations dealing with the wing. One important equation in aircraft wing design involves the load the wing will bear while in flight. Wing loading directly relates to the size, or the wing area, of the airplane wing. The first mathematical step in determining wing loading is to determine the wing area by multiplying the wingspan by the chord, or MAC. After the wing area has been determined, the wing loading can be determined, using the weight of the airplane. The gross weight, or GW, is the operational weight of the airplane. To determine the wing loading of a particular aircraft, the weight of the airplane is divided by wing area. For the lightest of civilian airplanes, wing loading may reach values as low as 2.5 kilograms per thousand square centimeters, whereas a tactical jet bomber will have a wing loading of more than 170 kilograms per thousand square centimeters. As the wing flies through the air, it does so at a particular angle. This angle, measured by the relationship between the relative wind and the chord line of the wing, is directly related to the speed of the aircraft. An airplane flying at high airspeeds will have a small angle of attack, whereas one flying slowly will have a large angle of attack. The lift equation aptly expresses the relationship between the speed, angle of attack, and weight of the aircraft. An airplane’s lift must at least equal its weight in order for the airplane to remain in flight. Pilots are unable to change either the density of the air or the area of the wing. However, they are in control of the other two variables, the airplane’s speed and angle of attack. Because lift must always equal weight in level flight, if the airplane slows down, the angle of attack must increase. Accordingly, an increase in speed will require a decrease in the angle of attack.

Principles of Aeronautics

EMPENNAGE The empennage is the tail structure of the aircraft, which includes the vertical stabilizer and rudder, along with the horizontal stabilizers and elevator. These essential components provide stability for the airplane in flight. The vertical stabilizer stands straight up, like a fin, from the aft portion of the airplane’s fuselage. It is important to the stability of the aircraft in that it helps the airplane track a straight path. The larger the vertical fin is in area, the more stable the aircraft is around the vertical axis. Attached to the trailing edge of the vertical stabilizer is the rudder. By way of the pedals at the pilot’s feet, the rudder controls movement about the vertical axis of the airplane. Acting in concert with the

Types and Structure of Airplanes

vertical stabilizer are the horizontal stabilizers. Located on each side of the fuselage and near the vertical stabilizer, they provide longitudinal stability to the airplane about the craft’s lateral axis. The combination of the horizontal stabilizers and elevators resembles the main wing in shape. However, the function of the horizontal stabilizers and elevators is totally different from that of the wing. Whereas the wing lifts up in force, the horizontal stabilizer provides a downward force that provides longitudinal stability to the airplane. LANDING GEAR In order to move around on the ground, all aircraft have landing gear. The most common arrangement of the landing gear is the tricycle landing gear, in

A Canadair CL-215T amphibian with retractable wheels. Photo by Javier Rodríguez from Palma de Mallorca, España, via Wikimedia Commons.

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Principles of Aeronautics

Convair XFY-1 Pogo in flight, 1951. Photo via Wikimedia Commons. [Public domain.]

which the aircraft has two main wheels that extend from either the wing or the fuselage and a third wheel that extends from the nose of the aircraft. The brakes are located on the main wheels, or mains, whereas the steering is the function of the nose gear. Depending on the size and model of the aircraft, nose-gear steering maneuvers the airplane on the ground. The nose wheel can be freewheeling, with the maneuvering done by differential braking. Aircraft steering can be actuated by rods, cables, or hydraulics systems. Another arrangement of the landing gear is the conventional landing gear, typically seen on older aircraft. In the conventional arrangement, there are two main wheels in the front of the fuselage, with a smaller tailwheel located on the aft end. Conventional gear was the norm in the early period of aviation, but it fell out of fashion in the 1950s and

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1960s, in favor of the inherently more stable “tricycle” landing gear arrangement. Another type of arrangement, found on the B-52, is the bicycle landing gear, which has two sets of main landing gear centered on the fuselage, one behind the other along the centerline of the fuselage. Because there are no supporting landing gear outside the body of the craft, devices known as outriggers keep the wingtips from striking the ground. FLIGHT CONTROLS The flight control system controls the aircraft in flight and comprises the devices that command movement of the aircraft around all three axes: longitudinal, lateral, and vertical. The elevator controls the airplane’s longitudinal movement about its lateral axis. In other words, it

Principles of Aeronautics

causes the airplane’s nose to go up or down. In this manner, combined with the power output of the engine, the elevator adjusts the wing’s angle of attack. This adjustment will have a direct influence on the airspeed of the craft. Ailerons are small airfoils located on the outer portion of the wing. When one deflects up, the other deflects down. The down aileron increases the lift on that wing while the opposite aileron spoils the lift on the opposite wing. This starts a rolling movement about the longitudinal axis. Finally, the rudder controls the airplane about the vertical axis. Actuated by the pedals at the pilot’s feet, the trailing edge of the vertical stabilizer moves the airplane’s nose either left or right, depending on which pedal is depressed. FLAPS Airplanes have flaps for both takeoffs and landings. Located on the inboard portion of the wing at the rear, flaps change the shape of the wing in a way that creates both lift and drag. The first half of travel, after takeoff, creates more lift than drag, whereas the last half of travel, before landing, creates more drag without a noticeable increase in lift. With the flaps partially extended for takeoff, the wing will generate more lift at lower airspeeds, allowing for shorter and safer takeoffs. On the other end of the spectrum, an aircraft approaching a landing with full flaps extended is generating more drag. This will allow the pilot to fly a steeper approach, land more slowly, and stop in a shorter distance. There are four types of flaps: plain flaps, split flaps, slotted flaps, and Fowler flaps. Each has its own characteristics, with the first three found typically on general aviation airplanes. The fourth type, the Fowler flap, is typically found on larger air transports. The Fowler flap system is heavier and more complex than the other three types of flap, necessitating a larger aircraft.

Types and Structure of Airplanes

THE POWER PLANT The internal combustion engine powers many of today’s light airplanes. The most popular arrangement of the engine is in the horizontally opposed configuration. The engine is air-cooled and typically arranged in a flat four- or six-cylinder configuration, allowing the best cooling for all of the cylinders. Unlike the aircraft engines built before World War II, the modern aircraft engine is highly engineered and very reliable. Although modern engines may still fail, the likelihood of complete power loss is minimal. Most aircraft engines are four-stroke engines, which means each cylinder has an intake stroke, a compression stroke, a power stroke, and, finally, an exhaust stroke. The amount of power the engine puts out depends on the engine’s size. Essentially, an engine’s power increases with its size. Aircraft four-stroke engines come in all sizes, from one of the smallest, the 65-horsepower Continental A-65, to the 350-horsepower TSIO-540. As horsepower requirements reach higher than 350, many aircraft manufacturers opt to equip their high-end models with turboprop engines. The advantages of a turboprop engine over an internal combustion engine are increased power output, smoother operation, and the ability to operate at higher altitudes. At higher altitudes, a pilot can take advantage of winds that are more favorable and realize better specific fuel consumption. Typical cruise speeds for airplanes equipped with turboprops are in the 230-knot to 350-knot range. For faster cruise speeds, a jet engine is required. Jet engines are relatively simple devices. The thrust of a jet engine is determined in kilograms of force rather than in horsepower, as are reciprocating engines and turboprop power plants. The heart of the jet engine is the compressor and turbine. Linked together by a common shaft, the turbine and compressor spin at rates as high as 20,000 revolutions per minute. As the fuel and air mixture burns in the combustion chamber, the exhaust gases escape through

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the turbine, spinning it at high speeds. The turbine, by way of the common shaft, spins the compressor, ingesting more air into the engine. The potential power available from a jet engine is phenomenal. More phenomenal than jet engines are future engine possibilities. Presently under development, the Stirling engine may be the most significant innovation for aviation in the near future. The Stirling engine, an external combustion engine originally invented in 1896, is on the verge of becoming the power plant of choice not only for airplanes but also for cars, boats, and many other applications. TYPES OF AIRPLANES There are as many airplanes as there are reasons for their existence. Small, privately owned aircraft such as Cessna, Beechcraft, and Piper aircraft are used for transportation and recreation. Most privately owned airplanes are single-engine, one- or two-seaters that have a range of about 650 kilometers and a speed of 160 kilometers per hour. Higher-end privately owned airplanes are turboprops and jets that are rather expensive to acquire and maintain. Powered by two engines, these more complex airplanes require more training and certification for their operation than do smaller craft. The turboprops are capable of 230- to 275-miles-per-hour cruise speeds, whereas some privately owned jet aircraft can reach a speed of 966 kilometers per hour. The cost of the privately owned aircraft varies. In 2001, small two-seaters in flying condition could be purchased by bargain hunters for less than $12,000. Such airplanes are rudimentary but capable of flight and cost effective for flight training. In 2022, an airworthy eleven-year-old Cessna 162 carried as price of US$79,000, while newer units of similar airplanes cost buyers US$125,00 and more. The price tag for all types of private aircraft have experienced similar increases in the same quarter-century period. The cost of a typical four-seater family airplane in 2001 began at $25,000 for an older, used craft and

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could reach as high as $150,000 for a new model. Still more sophisticated models could cost as much as $500,000. The smaller light twin-engine craft cost $50,000 to $75,000, on the low end of the market for used craft. On the high side, a newer aircraft cost as much as $750,000. In 2001, the cost of turboprop aircraft and small corporate jets started at well over the $1 million mark. Depending on the make and model of the corporate jet, the cost can reach as high as $40 or $50 million. Typically used in commercial endeavors, these aircraft are a strain for one or two individual owners to manage financially. The airliner, the type of airplane with which most passengers are familiar, flies at high speeds and altitudes. Smaller commuter airliners carry an average of fifty passengers and a crew of five or six, including the two pilots. As the airline industry moves into the twenty-first century, there is a desire to move away from the turboprop aircraft of the 1980s and 1990s, as passengers prefer the smoother, higher, and seemingly safer ride of jet aircraft. The final category of aircraft is military aircraft. The armed forces use different types of airplanes for different jobs. The task of protecting the nation from intruders falls to fighter planes, jets that can fly at twice the speed of sound and faster. Fighters carry one or two crew members, and their mission is to stop any unannounced intruder into national airspace. The military branches also operate airline-type aircraft to move personnel and cargo throughout the world. —Joseph F. Clark III Further Reading Donaldson, Bruce K. Analysis of Aircraft Structures: An Introduction. Cambridge UP, 2008. Federal Aviation Administration (FAA). Crew Qualification and Pilot Type Rating Requirements for Transport Category Aircraft Operated Under FAR Part 121. US Department of Transportation, FAA, 2013.

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Gatewood, B. E., and M. Gatewood. Virtual Principles in Aircraft Structures. Springer Netherlands, 2012. Johnson, E. R., and Lloyd S. Jones. American Military Training Aircraft: Fixed and Rotary-Wing Trainers Since 1916. McFarland Inc. Publishers, 2015. Peery, David J. Aircraft Structures. Dover Publications, 2013. Sofuoglu, Mehmet Alper, Mellih Cemal Kuºhan, and Selim Gürgen, editors. Materials, Structures, and Manufacturing for Aircraft. Springer International Publishing, 2022.

Types and Structure of Airplanes

Sun, C. T., and Ashfaq Adnan. Mechanics of Aircraft Structures. Wiley, 2021. See also: Aeronautical engineering; Biplanes; Blimps; DC plane family; Dirigibles; Flight balloons; Flying wing; Glider planes; Helicopters; Hindenburg; Hot-air balloons; Human-powered flight; Military aircraft; Monoplanes; Paper airplanes; Rockets; Rotorcraft; Space shuttle; Stealth bomber; Triplanes; Unidentified aerial phenomena (UAP); Uninhabited aerial vehicles (UAVs)

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U Ultralight Aircraft Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Pilot training ABSTRACT An ultralight aircraft is a minimal aircraft, requiring minimum power and with slow cruise and landing speeds. Minimal aircraft, whether purchased or homebuilt, are the least expensive entry into the pleasures of flight, an affordable option for experiencing the sheer exhilaration of birdlike freedom in the air. ULTRALIGHTS BY DEFINITION In the United States, the Federal Aviation Administration (FAA) in Part 103 of the Federal Aviation Regulations (FARs) has defined a basic powered ultralight vehicle as one that has a single seat, an empty weight of less than 215 kilograms (with exemptions for pontoons and an emergency parachute), has a fuel capacity of 19 liters or less, has a maximum calibrated airspeed of 55 knots (101 kilometers per hour), and a power-off calibrated airspeed of 24 knots (45 kilometers per hour) or less. For training purposes only, Exemption 3783 of July, 1983, permitted the use of two-place ultralights for in-flight instruction. The structure, control system, and stability of an ultralight are the responsibility of the designer and builder because an ultralight is not a certificated aircraft. In Europe, this type of minimal aircraft is called a microlight. A microlight is defined as a one-or two-seat airplane with a specified takeoff weight, wing loading (number of kilograms of weight per square meter of wing area), and fuel capacity.

In the United States, an operator of an ultralight is not required to possess either a medical certificate or a pilot’s license, and this has opened up the possibility of flight for thousands of pilots who either cannot afford the expense of certificated aircraft or who are no longer able to pass the medical exam. Because they have such short takeoff and landing capabilities, ultralights can be stored and flown from small grass strips in the country, further reducing the cost of owning and flying them. In England and in Europe, on the other hand, a microlight pilot must possess a valid Declaration of Health and must have passed an examination on applicable regulations, flight procedures, navigation, and weather. Tens of thousands of ultralights and microlights have been sold and are being flown. ORIGINS Perhaps history’s first ultralight was Alberto SantosDumont’s 1909 Demoiselle; it had an empty weight of about 100 kilograms and a 35-horsepower engine. Plans for the Demoiselle were offered in the 1910 issue of Popular Mechanics, but the machine required a very light pilot to get off the ground because of its very small wing. A little later, in the Great Depression, the United States could point to Bernie Pietenpol’s 1929 homebuilt Air Camper, powered by a Model A Ford engine, as a practical minimal aircraft. However, the modern ultralight/microlight movement had its origins in a reinvention of the airplane, repeating in the early 1970s the hang gliding pioneered by Germany’s Otto Lilienthal in the 1890s. (Hang gliding relies on weight shift by the pilot hanging below the wing for control of its flight

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path.) The revival was started by the invention of the Rogallo wing, a flexible-wing glider intended for space or military applications, by Francis Melvin Rogallo of the National Aeronautics and Space Administration (NASA). It was quickly adopted for sport flying in Australia and came to the United States as a towed glider behind a speedboat. Hang gliding became very popular and thousands were sold, especially on the West Coast where winds flowing up seaside hills generated lift and the seashore sand-cushioned hard landings. However, fliers soon tired of lugging their gliders back up the hill after what sometimes were very short flights; the ultralight movement can be said to have begun with John Moody’s flight demonstrations at the July, 1976, Experimental Aircraft Association Fly-In of an Icarus II biplane hang glider powered by a 12-horsepower engine strapped to his back. When the December, 1976, issue of Popular Science magazine featured this powered hang glider, the revolution was on. The Icarus II became the Easy Riser when an engine was added, and thousands were sold. Initially, the FAA decided to exempt powered hang gliders from the certification and pilot requirements of the FARs so long as they were foot-launched, or at least foot-launchable. That was

An ultralight Pterodactyl Ascender. Photo by Ahunt, via Wikimedia. [Public domain.]

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not very practical, as powered gliders inevitably became more sophisticated and used larger engines (mostly adapted chain saw and snowmobile engines). In October, 1982, Part 103 became effective and established the basic ultralight definition, requirements, and flight restrictions, and these continue in effect. The early ultralight pilots taught themselves to fly much as did pioneer pilots; they gradually increased taxi speeds, then made short straight-ahead hops, and then made their first tentative turning flights. The slow speeds involved usually minimized injuries. However, the FAA realized that instruction by a competent instructor in a two-place ultralight would certainly increase safety, and authorized the production and sale of uncertificated, two-seat ultralights so long as only authorized instructors used them for instruction or proficiency and their weight did not exceed 159 kilograms and their stall speed did not exceed 29 knots (79 kilometers per hour). By 1982, the hang-glider manufacturer Quicksilver was selling two thousand ultralights a year and had introduced a model that used three-axis control (the MX) in place of the much less effective weight-shift control used by unpowered hang gliders. Chuck Slusarczyk had introduced his CGS Hawk with a fully enclosed cockpit, standard flight controls, and flaps, and was selling an average of forty ultralights every month. In the United States, however, the bubble burst in November, 1983, when the television program 20/20 aired an exposé of ultralight aircraft in which an ultralight suffered a structural failure, resulting in the death of the pilot. That caused ultralight sales to plummet by about a factor of eight. However, it also caused marginal designs to go out of business, and since then ultralights have continued a healthy growth. In addition to original “lawn chair under a wing” designs such as the Quicksilver, many airplanelike designs such as the Mini-Max are available. Ultralights can be built from plans (least ex-

Principles of Aeronautics

pensively), from kits, or (most expensively) purchased ready-to-fly. STRUCTURE A very low wing loading (the ratio of weight to wing area) is the key to the ultralight’s very slow stall speed, and therefore its ability to land in very short, unpaved fields. A legal ultralight, with a 79-kilogram pilot and full fuel, requires a wing area of around 15 square meters, about the same as a much heavier, certificated two-seat trainer. This very low wing loading means that even moderate air turbulence and surface winds can make for unpleasant and potentially hazardous ultralight flying, and so most ultralight pilots prefer to fly in the early morning or late evening. Excellent short takeoff performance is added to this short-field landing capability by matching the low wing loading with a lightweight, powerful engine, yielding a low power loading. This has made the two-stroke engine the engine of choice for most ultralights, but engines more powerful than about 35 horsepower quickly put an ultralight over the weight limit. Two-stroke engines are more sensitive to the fuel mixture, the installation may not be well engineered, and deterioration can occur during winter storage, causing many ultralight pilots to become familiar with forced landings. These should not be a problem so long as the pilot is ready to transition to a steep nose-down attitude for gliding and a suitable short landing area is available. The steep glide angle is required because ultralights have a great deal of airframe drag from wing bracing and unstreamlined cockpits. SAFETY Many thousands of ultralights are now flying in the United States, but the absence of registration means that the exact number is not known. Similarly, because there is no reporting requirement, it is not known how many ultralight accidents occur. Some

Ultralight Aircraft

ultralights, perhaps especially those with engines behind the pilot, may not provide very good protection for the pilot in the event of a sudden stop. However, there is no doubt that a careful ultralight pilot with a carefully maintained flying machine, flying in good weather with light winds and always within gliding distance of a suitable landing area, can fly safely and with a great deal of personal pleasure and satisfaction. A good helmet and a ballistic parachute also enhance safety. Good hearing protection is vital because the engine is very close to the pilot. The FAA’s 19-liter fuel limitation was intended to insure only local flying by ultralights, so they and their pilots would not be exposed to the navigation and weather challenges of cross-country flight. However, challenge is just what stimulates and inspires some fliers, and by 1979 two ultralights had successfully been flown from coast to coast. Short cross-country trips have become very common. Ultralights need to be extra alert for the faster certificated aircraft because they often are not very visible from the air. The greatest legal threat to ultralight flying is probably the liability problem, because most are not insured. Furthermore, almost all single-seat ultralights are much heavier than allowed by Part 103 and the more popular two-place ultralights are mostly used for passenger carrying rather than for the instruction of students. Some of these problems may be solved by sport pilot and sport airplane proposals that promise to extend low-cost flying into the two-place, lightweight, four-stroke powered, relatively slow planes, such as the first Cubs, while retaining minimal aircraft and pilot requirements. TRIKES AND POWER PARACHUTES Trikes are flexible-wing, delta-shaped, powered hang gliders that are controlled by weight shift, using a bar that extends in front of the pilot. The operation of a control bar works opposite to that of a

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Principles of Aeronautics

control stick: The nose is raised by forward motion (“pushing it up”). There are even fewer easy attitude references for the pilot than in the usual ultralight, and the pilot’s senses can provide initially conflicting information. Using a pusher engine, trikes are particularly vulnerable to anything that gets blown back from the structure and into the propeller. Handling turbulence is a considerably different and demanding task. Trikes are particularly popular in Europe but are also becoming more common in the United States. Latecomers in the ultralight field, powered parachutes, have established themselves as a popular form of recreational flying. They are very sensitive to atmospheric turbulence, requiring smooth air and very light winds for safe operation. With such a lightweight “wing,” the parachute “cart” for the pilot can be extra strong and still be legally ultralight, although the high drag of the parachute requires a relatively large engine. When ready to fly, the parachute is behind the cart and must be carefully inflated by a blast of air from the propeller before a commitment to a takeoff is made. OTHER APPLICATIONS Thanks to their ability to fly very slowly and safely at low altitudes, ultralights have proved to be very useful for photography and for law enforcement all around the world. They are also the only piloted flying machine that flies as slowly as birds. In a well-publicized experiment (dramatized in the film Fly Away Home), Canada goose goslings were raised alongside an ultralight, taught to fly behind it, and then led on a desired migration pattern from Toronto, Canada, to Virginia in 1993. The same experiment has been successfully accomplished with sandhill cranes, and it is hoped that the population of the rare whooping crane can be increased through similar imprinting techniques. —W. N. Hubin

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Further Reading Carpenter, Carol, and Brian Carpenter. A Professional Approach to Ultralights: A Comprehensive and Realistic Guide for the Purchase and Operation of Ultralight Aircraft. Rainbow Aviation Services, 2003. Federal Aviation Administration (FAA). Amateur-Built Aircraft and Ultralight Flight-Testing Handbook FAA: AFS350. FAA, 2018. Gericke, Christoph. Braking Systems in Microlight Air Planes. GRIN Verlag, 2006. Hamilton, Paul. Sport Pilot: Choosing the Light-Sport Aircraft That’s Right for You. Aviation Supplies and Academics, 2005. US Department of Transportation, Federal Aviation Administration. Amateur-Built Aircraft and Ultralight Flight-Testing Handbook: Advisory Circular No. 90-89a. CreateSpace Independent Publishing Platform, 2013. See also: Aerodynamics and flight; Aeronautical engineering; Airplane safety issues; Homebuilt and experimental aircraft; Forces of flight; Human-powered flight; Training and education of pilots

Unidentified Aerial Phenomena (UAP) Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT Animal flight refers to sustained and powered airborne travel by birds, insects, or mammals through the use of wings. Animal flight, particularly that of birds, is important to humans, who first learned and dreamt of flight by studying flying animals. The study of animal flight remains a source of information for understanding and design of flying vehicles. Technically, an unidentified flying object (UFO) is any moving object spotted in the sky that cannot be identified by the observer. However, the term’s meaning has shifted since it was first used in the

Principles of Aeronautics

1950s. Today, many people equate the term UFO with the sighting of an alien spacecraft. The term has become so synonymous with the idea of extraterrestrial visitation that the US government has reclassified such sightings as unidentified aerial phenomena (UAP) to avoid the association. Scattered accounts of people witnessing strange objects in the sky date back centuries, but sightings of such phenomena truly exploded in the years following World War II (1939-45). Widespread media reports of so-called flying saucers fueled public interest in UFOs, but the objects were not considered to be extraterrestrial at first. However, by the 1950s, the association between UFOs and alien spacecraft had been cemented in the public consciousness. Since then, the US government has made several attempts to answer the mystery behind UFO sightings, with the vast number of cases found to have natural explanations. The few that remain unexplained continue to capture public imagination. Some UFO investigators believe the reports contain evidence that would confirm the existence of alien visitation. Skeptics doubt that UFOs are of extraterrestrial origin and insist that the lack of an explanation means that more research is needed. In 2022, however, the US military declassified both document files and video recordings from military sources, and admitted publicly that the events in question demonstrate the existence of mystery craft that are potentially of extraterrestrial origin. OVERVIEW Ancient civilizations relied on a knowledge of the sky for their survival, as the movement of the sun, moon, and stars told them when to plant and harvest crops. Any celestial object that was out of the ordinary would surely have been noticed, although observers were far more likely to consider it a divine omen or harbinger of doom. Some accounts from ancient Egypt tell of “stars” falling to earth and de-

Unidentified Aerial Phenomena (UAP)

Photograph of a purported UAP in Passaic, New Jersey, taken on July 31, 1952. Photo via Wikimedia Commons.

feating the pharaoh’s enemies, or of talking “flying disks,” but these are likely to be exaggerations from the time or possibly modern hoaxes. A fourth-century CE Roman text called the Book of Prodigies features historical accounts of strange objects burning through the sky and the sighting of mysterious celestial figures. However, the book was written centuries after the sightings occurred and the author could easily be referring to meteors or other common astronomical events. In 1561, residents of the German city of Nuremberg reported witnessing a sky full of strange flying cylinders and globes that seemed to them like a celestial battle. The event was recorded by a

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Nuremberg printer on a broadsheet illustration. Modern experts dismiss any extraterrestrial involvement, saying the sightings could have been an astronomical event known as a sun dog. This occurs when sunlight is reflected by ice crystals in the atmosphere, giving the appearance of multiple suns in the sky. During the nineteenth century, reports of lights in the sky or strange flying objects increased across the globe. For example, in 1878, a farmer near Dallas, Texas, reported seeing a circular object in the sky. He referred to the object’s shape as a “large saucer.” In 1896 and 1897, residents across the United States reported seeing mysterious airships in the skies. However, none of these reports were considered to be of alien origin. The Texas farmer likely saw a hot-air balloon, while people believed the airship sightings were just inventors testing out a new technology. It was only near the start of the twentieth century that the idea of extraterrestrials was attached to the phenomena in some circles. “FLYING SAUCER” CRAZE During World War II, American pilots in both Europe and the Pacific reported seeing fast-moving balls of light following their planes. The pilots named these lights “foo fighters,” a name taken from the popular comic strip Smokey Stover. Some pilots assumed they were German or Japanese weapons, but the objects never threatened their planes. The US military investigated the sightings but did not reach a conclusion on the matter. Some experts theorize that because the objects never showed up on radar, they could have been a form of ball lightning or optical illusions caused by atmospheric conditions or pilot fatigue. In 1947, a pilot named Kenneth Arnold was flying his small plane near Washington State’s Mount Rainier when he claimed to see a group of nine objects in the distance. Arnold reported the objects moved extraordinarily fast and swerved around the

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terrain as they flew. He described them as looking flat from the edge but crescent-shaped from the top. He also said they moved “like a saucer if you skip it across the water.” Arnold’s account is viewed today as the first modern UFO sighting. The media picked up on the story and soon reports of the mysterious objects had captured imaginations among the public. However, reporters misquoted Arnold’s words and called the objects “flying saucers.” Within weeks, hundreds of reports of flying saucers began to spring up across the United States. At the time, most people believed the witnesses had mistaken natural objects for the mysterious phenomena, or had seen some sort of top secret American or Soviet technology. Very few thought the objects were of alien origin. Modern researchers believe Arnold misjudged the speed and distance of the objects, and that they were likely birds flying in formation. A few months after Arnold’s report, a farmer near Roswell, New Mexico, discovered rubber, paper, and tin foil-like metal wreckage in one of his fields. The farmer reported his find to the local authorities and the story soon made headlines nationwide. Officials from a nearby military base came to investigate and initially reported that they had recovered the wreckage of a “flying disk.” However, the next day, Army officials denied that story and said the debris was from a crashed weather balloon. The event remains shrouded in mystery as reports from people who were present at the site claimed to have witnessed far more than just a wrecked weather balloon and were sworn to secrecy about what they had seen. With the nation immersed in a flying saucer craze, the Army’s quick debunking of the initial story led to the conspiracy theory that the military was covering up the truth in Roswell. In the 1990s, the US government admitted as much, saying the object was not a weather balloon but a top-secret, high-altitude balloon developed to monitor Soviet nuclear weapons tests.

Principles of Aeronautics

As the 1940s ended and the 1950s began, the public attitude toward flying saucers started to change. The subject became popular with writers who published several best-selling books speculating that the objects were of extraterrestrial origin. Hollywood added fuel to the fire by rolling out a series of alien invasion films with titles such as Earth vs. the Flying Saucers. Flying saucer enthusiasts in the United Sates began to form groups to discover the truth—alien or otherwise—behind the sightings. Sometime between 1953 and 1956, the term unidentified flying object (UFO) was first used by the US military in place of flying saucer. The term was considered to be more accurate and left out the implication that the “saucers” were controlled by some form of intelligence. Over the next decade, UFO became the standard term to refer to any unexplained object observed in the sky. However, its widespread use in referring to flying saucers eventually made the two terms interchangeable in public usage. Today, most English dictionaries define UFO as an unidentified flying object that is assumed to be from another planet or of alien origin. INVESTIGATING UFOS After the sightings of the late 1940s, the US military began to consider the potential hazards unidentified flying objects could pose to aircraft or national security. Since that time, the military has conducted several investigations of the UFO phenomena. In 1947, the Air Force launched Project Sign; a year later, it commissioned Project Grudge. However, the most comprehensive Air Force investigation was Project Blue Book, which examined more than 12,600 UFO sightings from 1952 to 1969. Of those sightings, investigators found only 701 that could not be explained. The project found most of the UFO reports were actually natural phenomena or misinterpreted sightings of clouds, stars, or bright planets such as Venus or Jupiter. For example, in 1951, residents of Lub-

Unidentified Aerial Phenomena (UAP)

bock, Texas, claimed to have seen a semicircle of fast-moving lights in the night sky. The Project Blue Book investigators found the lights were low-flying birds reflected in the city’s new streetlights. Project Blue Book’s findings also noted that of the 701 unexplained sightings, none were a threat to national security. The sightings were not evidence of superior technology and showed no signs of extraterrestrial origins. The investigators said they simply did not have enough evidence to decide. In 1998, the Central Intelligence Agency (CIA) claimed that many of the UFO reports from 1954 to 1974 were sightings of a top-secret, high-altitude spy plane. Project Blue Book investigators did speak to the CIA in the process of their inquiry; however, the CIA did not always reveal the requested information. TOPIC TODAY Although Project Blue Book seemed to debunk most UFO sightings, it did not fully answer the questions surrounding the phenomena, nor did it put a stop to UFO sightings. For example, in 1980, two US Air Force members stationed at a British military base reported witnessing colored lights flying above a forest northeast of London. One of the men claimed to have seen what looked like a spacecraft; later investigation found damage to the trees and unusual radiation readings. In 1989 and 1990, more than 13,000 people in Belgium reported seeing a triangular-shaped flying object. The nation’s air force enlisted the help of the British to investigate, but found no credible explanation. However, they did determine the object was not a threat. In 2004, two veteran US Navy pilots off the California coast reported encountering an aircraft that seemed to accelerate impossibly fast. A decade later, pilots on the East Coast of the United States reported similar encounters. In some instances, the pilots were able to record the object on their onboard cameras.

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The 2004 incident prompted the US Congress to fund a secret military program to investigate what were then being called unidentified aerial phenomena (UAP). The term was chosen to differentiate the sightings from the extraterrestrial association of the term UFO. The program, which began in 2007, was eventually defunded in 2012. However, in 2020, the US military announced a new project known as the Unidentified Aerial Phenomena Task Force (UAPTF). Its mission was to investigate and catalogue unidentified objects that could pose threats to national security. The UAPTF investigated reports from US government personnel from the years 2004 to 2021. In a preliminary report issued in June of 2021, investigators said they looked at 144 UAP sightings during the time period. The report was able to positively identify only one of the 144 objects, although it did state that the original investigation lacked “sufficient specificity.” It went on to say that with further study, the majority of the incidents will likely fall into five categories: airborne clutter, natural atmospheric phenomena, US-based technology, foreign-based technology, or a catchall “other” source. The report did state that many of the UAP sightings interrupted planned military operations or training exercises. Eighty of the 144 objects were observed with multiple sensors, meaning they were physical objects and not light sources or optical illusions. Of the 143 unidentified sightings, 18 displayed “unusual flight characteristics.” The report pointed out that this did not mean they were necessarily of alien origin; it said the 18 reports would need further examination to rule out the possibility of sensor errors, hoaxes, cyberattack, or pilot misperception. The report recommends additional government funding to develop a better way to collect and analyze data. It is interesting to note that the frontal profile of the F-117A Stealth Fighter is remarkably like the profile of many of the UFOs reported by many wit-

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nesses. The F-117 had been flying around for twenty years before its existence was revealed to the public, and this could account for any number of sightings. But it is a telling detail with respect to witness reliability that many reports of UAPs and UFOs were made by well-trained commercial and military pilots with absolutely nothing to gain from making such a report. Indeed, reporting such an event has often subjected a pilot to ridicule by his or her peers, and even dismissal from their employment as a pilot. Reports are not limited to the terrestrial realm, either, as several high-profile reports were made by astronauts in space. These are all matter of record. —Richard Sheposh Further Reading Bordman, Adam Allsuch. An Illustrated History of UFOs. Nobrow Ltd., 2020. Crookes, David. “Roswell UFO Crash: What Is the Truth Behind the ‘Flying Saucer’ Incident?” LiveScience, 6 May 2020, www.livescience.com/roswell-ufo-crash-whatreally-happened.html. Accessed 8 Feb. 2022. Dolan, Richard M. UFOs and the National Security State: The Cover-Up Exposed, 1973-1991. Keyhole Publishing Company, 2009. ———. UFOs for the 21st Century Mind: A Fresh Guide to an Ancient Mystery. CreateSpace Independent Publishing Platform, 2014. Donderi, D. C. UFOs, ETS, and Alien Abductions: A Scientist Looks at the Evidence. Hampton Roads Publishing Company Inc., 2013. Eghigian, Greg. “UFOs, UAP—Whatever We Call Them, Why Do We Assume Mysterious Flying Objects Are Extraterrestrial?” Smithsonian, 5 Aug. 2021, www.smithsonianmag.com/air-space-magazine/ufos-uaps whatever-we-call-them-why-do-we-assume-mysteriousflying-objects-are-extraterrestrial-180978374/. Accessed 8 Feb. 2022. Kean, Leslie. UFOs: Generals, Pilots, and Government Officials Go on the Record. Crown, 2011. Krasney, Zoe. “What Were the Mysterious ‘Foo Fighters’ Sighted by WWII Night Flyers?” Smithsonian, Aug. 2016, www.smithsonianmag.com/air-space-magazine/whatwere-mysterious-foo-fighters-sighted-ww2-night-flyers180959847/. Accessed 8 Feb. 2022.

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Maloney, Mack. UFOs in Wartime: What They Didn’t Want You to Know. Penguin Publishing Group, 2011. Masters, Michael P. Identified Flying Objects: A Multidisciplinary Scientific Approach to the UFO Phenomenon. Masters Creative, 2019. “Preliminary Assessment: Unidentified Aerial Phenomena.” Office of the Director of National Intelligence, 25 June 2021, www.dni.gov/files/ODNI/documents/ assessments/Prelimary-Assessment-UAP-20210625.pdf. Accessed 8 Feb. 2022. “Project BLUE BOOK—Unidentified Flying Objects.” US National Archives, www.archives.gov/research/military/ air-force/ufos. Accessed 8 Feb. 2022. Swords, Michael D. UFOs and Government: A Historical Inquiry. Anomalist Books, 2012. “UFO History.” How Stuff Works, science.howstuffworks.com/ space/aliens-ufos/ufo-history.htm. Accessed 8 Feb. 2022. Vallee, Jacques, and Chris Aubeck. Wonders in the Sky: Unexplained Aerial Objects from Antiquity to Modern Times. Penguin, 2009. Waldek, Stefanie. “History’s Most Infamous UFO Sightings of the Modern Era.” History.com, 8 Dec. 2020, www.history.com/news/historys-most-infamous-ufo-sighti ngs. Accessed 8 Feb. 2022. Wilkins, Jacob. “The Nuremberg UFO Sighting of 1561.” Medium, 18 Nov. 2020, medium.com/lessons-fromhistory/the-nuremberg-ufo-sighting-of-15614078ecfcd946. Accessed 8 Feb. 2022. See also: Aerodynamics and flight; Air transportation industry; Blimps; Contrails; Dirigibles; Flight balloons; Flying wing; Hot-Air Balloons; Stealth bomber; Weather conditions

Uninhabited Aerial Vehicles (UAVs) Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Mathematics ABSTRACT Known variously as uninhabited or uncrewed aerial vehicles, unmanned aerial vehicles, and drones, UAVs are airborne vehicles that operate without a pilot by either a

Uninhabited Aerial Vehicles (UAVs)

preprogrammed system or an overriding command system operated from the ground. UAVs were first used by Germany during World War II, and have been regularly used by the United States beginning in the 1950s. KEY CONCEPTS aerial photography: photography carried out by airborne cameras and recording devices Cold War: a period during the 1950s and early 1960s during which the Soviet Union and the United States essentially dared each other to start a nuclear war though neither one would reconnaissance: visual or electronic monitoring of a situation or conditions in an area WHAT ARE UAVS FOR? Uninhabited aerial vehicles (UAVs), such as the Tagboard, Ryan BQM-34, Pioneer, Predator, Global Hawk Tier II+, DarkStar Tier III-, and Cypher, provide defense capabilities such as reconnaissance, aerial photography, signals intelligence collection, and laying decoy chaff corridors for striking aircraft while eliminating the risk of loss of life. Advanced UAVs provide offensive capabilities such as the deployment of missiles. Used for intelligence gathering behind enemy lines, uninhabited aerial vehicles allow the US Air Force to obtain near-real-time information that assists ground commanders and allows them to implement effective tactical maneuvers. UAVs have both military and civilian applications. DEVELOPMENT OF UNINHABITED AERIAL VEHICLES During World War II, Germany developed and deployed the first uninhabited aerial vehicle (UAV) known as the V-1. Built to attack a specific target, the V-1 was used once and was destroyed along with its target. The United States began developing uninhabited aircraft during the 1950s to supplement crewed aircraft under Strategic Air Command (SAC). Capable of achieving intercontinental-range flight,

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the UAV, like its predecessor, also destroyed itself on contact with the target. After the Soviet Union launched Sputnik and then shot down a U-2 spy plane in the early 1960s, the United States embarked on a reconnaissance program developed around the use of uninhabited aircraft. Ryan Aeronautical Company flew the first of these remote-controlled aircraft under the code name of “Red Wagon” and effectively demonstrated the ability to use UAVs for aerial photographic missions. In addition to being flown over the Soviet Union and Cuba, these vehicles were also deployed over China. By 1962, Lockheed and the Central Intelligence Agency (CIA) developed the supersonic reconnaissance D-21, known as the Tagboard. Launched from

Principles of Aeronautics

the A-12 or the B-52H airplane, the Tagboard was capable of achieving a speed of Mach 3.3 and could fly at high altitudes over 27 kilometers for a distance of 4,828 kilometers. After photographs were taken of the targeted area, the film was released and recovered by another aircraft. The Tagboard was retired in 1971 due to several mission failures and high costs. During the Vietnam conflict, the United States Air Force relied on the Ryan BQM-34 Lightning Bug for both high- and low-altitude missions. High-altitude flights operated at heights of 18 kilometers and above, while low-altitude sorties flew at an altitude below 150 meters. Capable of flying for 7.8 hours without landing, the Lightning Bug was used for

Winston Churchill and the Secretary of State for War waiting to see the launch of a de Havilland Queen Bee radio-controlled target drone, June 6, 1941. Photo via Wikimedia Commons. [Public domain.]

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photographic reconnaissance, leaflet dropping, picking up enemy signals, and aiding crewed aircraft by releasing chaff corridors as a decoy maneuver. On an average, each Lightning Bug flew just over seven missions, for a total of 3,435 missions between 1964 and 1975. In 1971 and 1972, Ryan Aeronautics experimented with the use of the Lightning Bug as an armed vehicle capable of dropping an electro-optically guided bomb. Several of these modified UAVs flew with crewed aircraft on missions over Vietnam. After the United States withdrew from Vietnam, interest in the use of drone aircraft diminished but other countries, such as Israel, continued to develop the technology. The rebirth of the UAV in the United States occurred during the Persian Gulf War of 1991 when the Navy and the Army deployed the Pioneer uninhabited aerial vehicle. Based on the designs of the Israeli Scout and the Mastiff UAV, six Pioneers were deployed during Desert Storm. The three systems used by the Marines assisted AV-8B Harriers during air strikes while the systems deployed by the Navy provided gunfire spotting for the battleships, located mines in the Persian Gulf, and pinpointed antiship missile sites in Kuwait and Iraq. Used for targeting, reconnaissance, and battle damage assessment, the Pioneer flew 533 missions and logged over 1,688 hours in the air. After the war, the Pioneer was used for reconnaissance in Bosnia and Kosovo beginning in 1993. In the Western Hemisphere, the Pioneer has been used during Operation Uphold Democracy in Haiti and in conjunction with the US Customs Service to spot illegal immigrants crossing the border and to combat the smuggling of illegal drugs. The Pioneer provides real-time video within line-of-sight and flies low- to medium-altitude missions. The aircraft is capable of operating on totally preprogrammed instructions or can receive commands while in flight. After the mission is over, the Pioneer can land in one of three different ways: by using an arresting cable across the runway, much

Uninhabited Aerial Vehicles (UAVs)

like the type used on an aircraft carrier; flying the aircraft into a recovery net; or a traditional landing. Measuring 4.27 meters in length and weighing 209 kilograms, the Pioneer can fly for five hours at speeds of up to 100 knots with a payload of 25 kilograms. ENDURANCE UAVS The usefulness of the uninhabited aerial vehicles prompted the United States to enhance the technology, making the aircraft capable of longer flight times for increased coverage during periods of conflict. The Predator is a medium-range altitude endurance uninhabited aerial vehicle (MAE UAV) capable of remaining in flight at loitering speed for twenty-four hours at a time at its 500-nautical-mile (926 kilometer) range. The cost per unit is $3.5 million. Each Predator system consists of four aircraft, one ground station, and one Trojan Spirit II SATCOM system. The aircraft is designed like the Gnat 750, with a slender fuselage 8.1 meters in length and 1.8 meters high, with a span of 14.75 meters. Manufactured by General Atomics, the Predator operates within line-of-sight of GSC (ground station control) and virtually anywhere by satellite. The midwing monoplane, powered by a four-cylinder Rotax engine, carries an electro-optic/infrared (EO/IR) Versatron Skyball Model 18 and a zoom and spotter lens, as well as a Westinghouse 783R234 synthetic aperture radar (SAR). The Predator is easily recognized because of its inverted-V tails. Transportation of the Predator by C-5, C-141, and C-130 aircraft allows the system to be highly mobile and its easy assembly allows the UAV to be operational within six hours of arrival. Data is gathered through the Trojan Spirit II (TS II) SATCOM system that transmits and receives messages simultaneously from a variety of sources including satellites, joint surveillance target attack radar system (Joint-STARS), U-2s, Rivet Joint, and airborne warning and control system (AWACS).

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The first deployment of the Predator occurred in Gjader, Albania, in 1995. After Serb air-defense gunners shot down one of the aircraft and another was destroyed as a result of ground fire, the Predators were removed from the European theatre. The second deployment occurred in Taszar, Hungary, in March, 1996, when US European Command engaged in Operation Nomad Endeavor. That same year, the Air Force ordered thirteen Predator systems with four aircraft each, scheduled for delivery between 1996 and 2002 at a cost of approximately $118 million. All of the systems are operated by the Eleventh and Fifteenth Reconnaissance Squadrons located at Indian Springs Air Force Auxiliary Field, 65 kilometers northwest of Las Vegas. The Joint Requirements Oversight Council has issued a requirement for several system upgrades, which will be retrofitted on existing Predators, including a deicing capability, an ultra high frequency (UHF) radio link, and Mode IV IFF transponders. In addition, a contract has been signed with General Atomics Aeronautical Systems to expand operations from line-of-sight to beyond-line-of-sight capability, using a Predator as a communications relay. The effectiveness of the Predator for intelligence gathering and the reduction of human risk led the United States military to explore the possibility of arming the Predator. General John P. Jumper, Air Combat Command’s top commander, proposed equipping Predator with weapons, and in February, 2001, the first successful live missile test took place at Nellis Air Force Base Range, Nevada. The armed Predator, controlled through a Ku-band satellite link and traveling beyond the controllers’ line of sight, flew to 2,000 feet before identifying its target and launching a live Hellfire missile, blowing a track off a tank. Air Combat Command and Aeronautical Systems Center continue to evaluate and analyze the test results with the goal of achieving the capability of firing missiles from the Predator at heights between 3 to 4.6 kilometers.

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Although the Predator fulfilled medium-range altitude requirements, the Air Force required an uninhabited aircraft that could conduct high-altitude reconnaissance for longer periods of time. Teledyne Ryan Aeronautics developed the conventional high-altitude endurance uninhabited aerial vehicle (CHAE UAV) and flew the aircraft for the first time in 1997. The aircraft, commonly referred to as the Tier II+ or the Global Hawk, flies at altitudes above 18 kilometers at speeds of up to 340 knots. The Global Hawk performs high-resolution reconnaissance over a 103,600-square-kilometer area in twenty-four hours at loitering speed at a range of 4,828 kilometers. With a payload of 907 kilograms, the Global Hawk can operate from conventional 1,500-meter runways. Cost per unit is $10 million. Rounding out the endurance vehicles is the DarkStar. Manufactured by Lockheed Martin and Boeing, the DarkStar flies low observable high-altitude endurance (LOHAE) missions above 13.7 kilometers and is capable of loitering for eight hours and can travel 8,000 kilometers from its base of operation. Equipped with either high-resolution SAR or EO sensors, the DarkStar images well-protected, essential targets. The first flight of a DarkStar occurred in March, 1996, and each aircraft costs $10 million. Operation uses runways under 1,219 meters in length and the aircraft is able to fly in all types of weather conditions. The aircraft is half the span of the Global Hawk and one-third the length. The development program of the DarkStar, administered by Defense Advanced Research Projects Agency (DARPA), was cancelled in 1999 due to budget cuts, with the Department of Defense opting for longer range as opposed to the stealth of the DarkStar. The latest development of an uninhabited aerial vehicle is the Cypher aircraft, manufactured by Sikorsky Aircraft Corporation. Designed to meet military needs such as reconnaissance, communication relay, and countermeasure missions, the Cypher also meets civilian needs in areas such as

Principles of Aeronautics

counternarcotics, ordinance disposal, forestry, utilities, law enforcement, and search-and-rescue. The use of a differential global positioning system (GPS) allows the Cypher to navigate autonomously. The aircraft is capable of vertical takeoff or landing (VTOL), is 2 meters in diameter, carries a payload of 22.7 kilograms, and cruises at 80 knots. The rotor system is shrouded to prevent damage to the high-speed rotors and possible risks to personnel. Equipped with a 50-horsepower engine, the Cypher has a ceiling altitude of 2.5 kilometers and can fly for three hours without landing. The onboard sensors relay information to control stations on land, sea, or in the air via digital telemetry uplink. Information about flight status, data gathered during the mission, as well as video images are downloaded simultaneously. The development of uninhabited aerial vehicles continues to advance with new capabilities and systems continuously being added or modified to increase the performance of the aircraft. The intelligence gathering designed to assist tactical commanders remains the primary function of the uninhabited aerial vehicles, but as the industry de-

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velops lower-cost units, the number and variety of applications have increased. —Cynthia Clark Northrup Further Reading Austin, Reg. Unmanned Aircraft Systems: UAVS Design, Development and Deployment. Wiley, 2011. Calafate, Carlos Tavares, and Mauro Tropea. Unmanned Aerial Vehicles: Platforms, Applications, Security and Services. MDPI AG, 2020. Elngar, Ahmed A., K. Martin Saguzam, and Mary Bella I. Thusmanis, editors. Unmanned Aerial Vehicles and Multidisciplinary Applications Using AI Techniques. IGI Global, 2022. Fahlstrom, Paul G., and Thomas J. Gleason. Introduction to UAV Systems. Wiley, 2012. Galar, Diego, Uday Kumar, and Dammika Seneviratne. Robots, Drones, UAVs and UGVs for Operation and Maintenance. CRC Press, 2020. Huang, Hailong, Chao Huang, and Andrey V. Savkin. Autonomous Navigation and Deployment of UAVs for Communication, Surveillance and Delivery. Wiley, 2022. Kealey, William. Unmanned Aerial Vehicles (UAVs) in Combat. GRIN Verlag, 2014. See also: Aeronautical engineering; Autopilot; Avionics; Flight propulsion; Ramjets

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V Jules Verne Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Literature ABSTRACT Jules Verne was born on February 8, 1828, in Nantes, France, and died on March 24, 1905, in Amiens, France. He was a prolific author of novels, short stories, plays, and essays. Verne was considered by many to be the father of science fiction. He was one of the first authors to write about rockets and the possibility of spaceflight. He also popularized the concept of hot-air ballooning with such novels as Le Tour du monde en quatre-vingts jours (1873; Around the World in Eighty Days, 1873). INTRODUCTION Jules Verne was born into a prosperous French family with deep occupational traditions on both sides. His father, Pierre Verne, came from a long line of lawyers, and his mother, Sophie Allotte de la Fuye, came from a family with a strong military history. Verne’s formal education began in 1838 at College Saint-Stanislas, where he excelled in geography, Greek, and Latin. Between 1841 and 1846, Verne attended Petit Seminaire and later the Lycée Royal de Nantes, where he began writing short essays and prose pieces. While studying law in Paris, he wrote his first play, Alexandre VI, in 1847. Verne remained in Paris and received his law degree in 1849. He worked as a stockbroker and served as secretary at the Théatre Lyrique from 1852 to 1854, all the time continuing to write. In 1856, he met Honorine de Viane, a widow with two children, and married her the following year. To-

gether they had a son, Michael, who was born in 1861. Verne’s initial foray into adventure stories came when “Un Voyage en ballon” (1851; “Voyage in a Balloon,” 1852) appeared in a children’s magazine in 1851. Twelve years later, Verne published Cinq semaines en ballon (1863; Five Weeks in a Balloon, 1876), the first of what would be called his scientific romances. Other books in this vein included Voyage au centre de la terre (1864; A Journey to the Centre of

Jules Verne, c. 1884. Photo via Wikimedia Commons. [Public domain.]

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the Earth, 1872), De la terre á la lune (1865; From the Earth to the Moon, 1873), Vingt mille lieues sous les mers (1869-70; Twenty Thousand Leagues Under the Sea, 1873), Around the World in Eighty Days, and L’åle mystérieuse (1874-75, 3 vols., including Les Naufrages de l’air, L’Abandonné, and Le Secret de l’Ile; The Mysterious Island, 1874). Using a blend of fantasy and science, Verne introduced the concept of space travel using rockets, as well as such futuristic products as television, the submarine, the Aqua-Lung, and even nuclear energy as the mysterious power source for his fantastic submarine, the Nautilus. He continued writing up until his death in 1905 from the complications of diabetes. For the next ten years, his son continued the publication of his remaining manuscripts. —P. S. Ramsey Further Reading Butcher, William. Jules Verne: The Definitive Biography. Da Capo Press, 2007. Costello, Peter. Jules Verne: Inventor of Science Fiction. Charles Scribner’s Sons, 1978. Evans, Idrisyn Oliver. Jules Verne and His Work. Aeonian Press, 1976. Lamy, Michel. The Secret Message of Jules Verne: Decoding His Masonic, Rosicrucian, and Occult Writings. Inner Traditions/Bear, 2007. Lottman, Herbert R. Jules Verne: An Exploratory Biography. St. Martin’s Press, 1996. Malone, John. Predicting the Future: From Jules Verne to Bill Gates. M. Evans, 1997. Taves, Brian, and Stephen Michaluk, Jr. The Jules Verne Encyclopedia. Scarecrow Press, 1996. Verne, Jules. Paris in the Twentieth Century: The Lost Novel. Random House Worlds, 1997. See also: Advanced propulsion; Aeronautical gravity and flight; Hot-air balloons; Lighter-than-air propulsion technologies; Rocket propulsion; Rockets; Space shuttle; Spacecraft engineering; Spaceflight; Unidentified aerial phenomena (UAP)

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Viscosity Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Physiology; Mathematics ABSTRACT The internal friction of fluids is called viscosity. Viscosity causes resistance to flow of a fluid and resistance to the movement of an object through a fluid. It is a fundamental aspect of fluid dynamics and is important in every problem involving motion of a fluid from painting to the action of a curve ball in baseball. KEY CONCEPTS boundary layer: a layer of viscous fluid immediately adjacent to a solid; in it, the velocity of fluid rapidly approaches zero relative to the solid Newtonian fluid: a fluid in which the viscous stress is proportional to the velocity gradient Poiseuille flow: the steady flow of viscous fluid in a tube driven by an external pressure difference velocity gradient: the rate at which the flow velocity changes spatially viscous stress: the internal frictional force per unit contact area between two parts of a fluid in nonuniform flow OVERVIEW Viscosity is a property of fluids (gases and liquids) by which the flow motion is gradually damped (slowed) by internal friction and dissipated into heat. Viscosity is a familiar phenomenon in daily life. An opened bottle of wine can be easily poured; the wine flows easily under the influence of gravity into a glass. Maple syrup, on the other hand, cannot be poured so easily; under the action of gravity, its flow is sluggish compared to that of wine. The syrup has a higher viscosity than the wine. Almost all fluids have viscosity; the only exceptions are the two isotopes of helium, helium-3 and helium-4, which at

Principles of Aeronautics

extremely low temperatures exhibit no viscosity at all. (In such a state, they are known as superfluids; quantum mechanics is required to understand superfluids.) A viscous fluid has internal friction. This frictional force exists between the flowing fluid and the surface of the container, such as a tube (the boundary effect), as well as between different parts of the fluid (the bulk effect). Quantitatively, the bulk effect can be illustrated in the following way. Imagine a fluid flowing east. If the flow is uniform, that is, if different parts of the fluid flow with the same velocity, then there can be no frictional force between different parts of the fluid. This results from the fact that if the flow is uniform, then the fluid looks static from the point of view of an observer moving with the fluid. Sir Isaac Newton’s first law of motion states that it should stay that way. Consequently, the flow will continue and no viscous damping is possible, at least in bulk. This is called laminar flow. To observe viscous effect in the bulk, it is essential that the flow be nonuniform. If the easterly flow is assumed to be nonuniform, the magnitude of the flow velocity increases in the northern direction; the flow is more rapid in the north than in the south. To determine how nonuniform the flow is, it is measured in terms of a velocity gradient, which measures the change in flow velocity (meter per second) per unit distance (meter) in the direction of that change (north in the example). In a viscous fluid, such a nonuniform flow generally implies a nonzero force due to viscosity, which in the example is exerted by the relatively slow fluid to the south on the relatively fast fluid to the north to slow down the latter. Following Newton’s third law of motion, there is a force equal in magnitude and opposite in direction, which is exerted by the fast fluid to the north to speed up the fluid to the south. This force is proportional to the area of contact between the two parts of the fluid. The frictional force per unit area is known as the viscous stress, which causes the flow to be

Viscosity

more uniform. In the example, it slows down the fast flowing fluid and speeds up the slower flowing fluid. The boundary effect of viscosity forces the fluid that is in direct contact with the boundary to have the same velocity as the boundary. If one considers the flow of a fluid in a tube, and if the tube is stationary, then the viscosity has the effect of slowing down the flow. In the absence of any externally applied pressure (gravity, pump), this will stop the flow completely; with applied pressure the flow may continue, but the viscosity causes resistance to the flow. Exactly how the viscous stress varies as a function of the velocity gradient depends on the fluid. In many simple fluids, such as water and air, the viscous stress is simply proportional to the velocity gradient. The proportionality coefficient is called the viscosity of the fluid. Thus, viscous stress equals viscosity times velocity gradient. Such a fluid is known as a Newtonian fluid. Viscosity is measured in units of poise, in honor of the French physician, Jean-Louis-Marie Poiseuille. In a fluid with a viscosity of 1 poise, if the velocity gradient is 1 (meter/second/meter), the viscous stress will be 0.1 newton per square meter. Under normal conditions, water has a viscosity of 0.01 poise and air has about 0.00017 poise. Many polymers are non-Newtonian fluids in that their viscosity depends on the velocity gradient (so that the viscous stress is not simply proportional to the velocity gradient—the functional relationship is nonlinear) or even how long the velocity gradient has been maintained, among other things. Paint, for example, is a non-Newtonian fluid whose viscosity decreases (the fluid becomes thinner) after it has been stirred. Usually, the viscosity of a fluid depends sensitively on the ambient temperature. The viscosity of gases generally increases with increasing temperature. In contrast, most liquids become less viscous at high temperatures. The viscosity of some substances can

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vary by a factor of many billions as a function of temperature. Pitch, for example, has a viscosity in excess of 10 billion poises at room temperature but is poured quite easily at elevated temperatures. In the process of glass formation, the viscosity increases from less than several hundred poises at high temperatures to more than 1014 poises below the glass transition temperature, which is the temperature at which the viscosity is equal to 1013 poises. One can get a sense of where viscosity comes from by considering the following microscopic model. Imagine many molecules forming a gas. Initially, it is assumed that the gas is at rest. Although the macroscopic velocity of the gas is zero, the molecules are in constant random motion at any nonvanishing temperature. The velocity is zero only as an average. Imagine such a gas is set up in a flow. Microscopically, the molecules are still undergoing random motion. The difference is that now the average velocity is no longer zero but rather equals the flow velocity. One can now assume that the flow is nonuniform, with the flow velocity being higher on the left. The molecules on the left have a higher average velocity than the molecules on the right. At this point, the random motion of the molecules will tend to mix the fast and the slow. Some of the fast-moving molecules on the left may be carried by random motion to the right, where they will collide with the molecules already there and that originally had a lower average velocity. In this process, the molecules exchange momentum; the fast ones give up some of their momentum to the slow ones, causing the latter to speed up. A similar process, in which the slow molecules on the right are carried by random motion to the left, causes the fast ones to slow down. This manifests itself on the macroscopic scale as viscosity. The viscosity of a gas can be understood on the basis of this model. The thermal random motion of the molecules mixes the velocity of molecules in a nonuniform flow. The farther a molecule can travel

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between two collisions, the more effective this mixing will be. At higher temperatures, the random motion of molecules is faster and can carry a molecule farther between two collisions. This is the reason that the viscosity of a gas increases with temperature. A simple model is not possible for liquids. In a liquid, the molecules are very close together and the motions of individual molecules are highly correlated. This presents a formidable theoretical problem. The microscopic calculation of the viscosity of a liquid has met with only limited success. APPLICATIONS With the exception of superfluids, all fluids have viscosity. It is impossible to understand the flow of fluids without a proper understanding of viscous effects. Because of the viscous effect, the velocity of fluid in direct contact with the wall is zero. In a tube, the flow velocity is not uniform but, rather, has a parabolic distribution, going to zero at the boundary and reaching a maximum at the center. The steady flow of viscous fluids in a tube is known as Poiseuille flow. The total flux of fluid is proportional to the fourth power of the tube radius. All valves depend on viscosity. Closing the valve reduces the opening and increases the resistance, which slows down the flow. Without viscosity, devices ranging from a water faucet to the gas pedal in a car would fail. To drive the flow in a tube, the viscous resistance of the fluid must be balanced by a pressure difference on the two ends of the tube, whether it is the Alaskan oil pipeline or the human artery system. The viscosity slows down the falling raindrops, as it does with all objects, causing them to reach some terminal velocity. Without the viscosity of the air, the rain drops would shoot down from the sky with the velocity of a bullet. The thin layer of fluid near a solid object where the flow velocity rapidly decreases to zero relative to

Principles of Aeronautics

the solid is known as the boundary layer. In the boundary layer, the fluid is largely dragged by the object. This is important in a number of sports. A spinning ball drags the air around it and causes the air to circulate. If such a ball is thrown through the air, the circulation and the flow of the air rushing past the ball will be in the same direction on one side of the ball and in opposite directions on the other side. Consequently, the net air velocity on the two sides of the ball will be different. Using Bernoulli’s principle, this leads to a difference in pressure on the two sides, which drives the ball to one side of the trajectory. This phenomenon is familiar in baseball and volleyball. To pitch a curve ball, the pitcher gives the ball a spin around a vertical axis. The pressure difference will therefore be between the left and the right side of the trajectory, causing the ball to be deflected to one side. The surface of a golf ball is intentionally roughened with dimples. This enhances the dragging effect and gives the ball a significant lift to an upward spinning ball, greatly increasing its range. Non-Newtonian fluids are important technologically. For example, paint must be easy to spread with a brush and, at the same time, sufficiently viscous once it is applied to avoid dripping. These two conditions can both be satisfied because of its non-Newtonian viscous properties. The viscosity of a paint depends on how fast the paint is stirred. It decreases when the paint is stirred and sheared by the brush but increases once the stir ceases. CONTEXT Viscosity can be studied macroscopically and microscopically. The macroscopic study of viscosity belongs to hydrodynamics. In hydrodynamics, the value of the viscosity is assumed to be known; the theory concentrates on working out the dynamical consequences of viscous flow. The significance of viscosity was known in the early days of hydrodynamics. Not until the twentieth century, however, were

Viscosity

effective mathematical tools developed to treat viscous flow problems of any complexity. The complicated problem of turbulent flow with viscosity is only beginning to be understood in the last quarter of the twentieth century. The application of hydrodynamics to the viscous flow of unconventional fluids, such as water and oil, in sediments and in biological systems such as human organs and the body, will continue to be of great interest for many years. The microscopic study of viscosity belongs to the field of statistical mechanics. Here, one is interested in understanding the origin of viscosity, the non-Newtonian behavior, and viscoelastic properties of a fluid on the basis of its molecular properties, such as the intermolecular forces and the deformation of the molecules under stress. Extensive work was done in the last one hundred years. The understanding of the viscosity of gases under normal conditions is essentially complete. One can now calculate the viscosity quantitatively with considerable confidence. The theory for the viscosity of liquids is at a more primitive stage; quantitative results can be expected only for simple liquids. Still less well understood are the viscous properties of concentrated polymer systems. These fluids are often non-Newtonian. Despite their technological importance, no fundamental theory is available. It is likely that qualitatively new ideas are needed before any progress can be made on this problem. The fascinating behavior of fluids, the ubiquity of viscosity, its technological significance, and the theoretical challenge of its understanding from a microscopic point of view will continue to be a major driving force in statistical mechanics and hydrodynamics. —Yautia Fu Further Reading Braithwaite, Jonathan. Essential Fluid Dynamics for Scientists. Morgan & Claypool Publishers, 2018.

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Kundu, Ajoy Kumar, Mark A. Price, and David Riordan. Conceptual Aircraft Design: An Industrial Approach. Wiley, 2019. Ower, E., and R. C. Pankhurst. The Measurement of Air Flow. Elsevier Science, 2014. Touloukian, Y. S., S. C. Saxena, and P. Hestermans. Viscosity. Springer US, 2013.

German military school at Wahlstatt. Von Richthofen was not a good student, but he proved to be athletically gifted. After passing cadet school at Wahlstatt, he went to the Royal Military Academy in Lichterfelde, near Potsdam, an important military center.

See also: Aerodynamics and flight; Aeronautical engineering; Airfoils; Airplane propellers; Daniel Bernoulli; Differential equations; Fluid dynamics; Forces of flight; Materials science; Pressure; Sound barrier; Temperature; Wake turbulence; Weather conditions; Wind shear; Wing designs

THE FIGHTER PILOT In 1911, von Richthofen became a lieutenant in the First Uhlan Cavalry Regiment of the Prussian Army, fighting in Russia during World War I and participating in the invasion of Belgium and France. After the cavalry lost its importance as a fighting force in the era of trench warfare, von Richthofen joined the infantry. He then transferred to the Imperial Air Service and entered combat as a fighter pilot in September, 1916. An important role model and teacher to von Richthofen was Captain Oswald Boelcke, who, until he was overtaken by von Richthofen, was Germany’s greatest ace, with forty victories in aerial combat. It has been said that Boelcke was the father and teacher of combat pilots, whereas von Richthofen developed his mentor’s methods to the highest degree of mastery. On October 28, 1916, von Richthofen was present in his fighter when Boelcke was killed in an aerial collision with another plane.

Manfred von Richthofen (Red Baron) Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT Manfred von Richthofen was born on May 2, 1892, in Breslau, Germany (now Wroc³aw, Poland), and died on April 21, 1918, near Vaux-Sur-Somme, France. He was the most famous fighter ace of World War I. Von Richthofen, best known as the “Red Baron,” and leader of the “Flying Circus” air fighter group, was the most famous fighter pilot of World War I. Most of his air-combat operations manual, written shortly before his death at age twenty-five, remains valid. VON RICHTHOFEN’S EARLY YEARS Born in 1892, Manfred Freiherr von Richthofen was the eldest son of a family of the lesser nobility of Silesia and heir to a Prussian military tradition. He grew up at the turn of the twentieth century in an atmosphere comparable to that of an English country squire. The young von Richthofen did not choose a career, but rather had one chosen for him. His father packed the boy off at the age of eleven to the

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THE FLYING CIRCUS Eventually, von Richthofen became commander of Fighter Group I, known officially as Jagdgeschwader I (JG I). JG I was officially chartered on June 26, 1917, by the Kogenluft, the German Air Service Headquarters. Because of its fancifully decorated triplanes, JG I came to be known as the “Flying Circus.” “Baron” von Richthofen’s own triplane was painted red, a color he had favored for his previous fighter planes, thus he became known as the “Red Baron.” The JG I comprised four fighter units. To weld his group into what became the most notoriously feared air-fighting formation in history, von

Principles of Aeronautics

Richthofen chose his subordinate leaders with great care. He was a shrewd judge of character and chose men whom he felt were capable of leadership yet could follow his instructions and orders. With his subordinates’ assistance, he would coordinate the motions and mass the forces of the JG I at whatever target he deemed appropriate. Under von Richthofen’s leadership, the Flying Circus became a very successful fighter group. One of the most successful days in JG I’s history was March 27, 1918. During that day, JG I carried out 118 sorties, and had 39 inconclusive air combats and 13 successful combats. The Red Baron had his seventy-first, seventy-second, and seventy-third victories. He was eventually credited with shooting down a total of eighty enemy aircraft, making him the top ace of World War I. THE LEGEND The precise circumstances of von Richthofen’s death remain unclear. He was reputedly shot down on April 21, 1918, by Captain A. Roy Brown, a Canadian ace flying in the Royal Air Force (RAF). It has been said that von Richthofen disobeyed one of the basic tenets of his air combat operations manual and stayed in pursuit of an enemy plane too long, while Captain Brown’s plane came up behind him. However, it is possible that von Richthofen may have been killed instead by ground fire from Australian troops. Brown died in Ontario, Canada, in March, 1944, without ever categorically claiming that it was he who was responsible for shooting down von Richthofen and his subsequent death.

Manfred von Richthofen (Red Baron)

After the end of World War I, von Richthofen’s remains were first transferred to a large German military cemetery at Fricourt. In 1925, the remains were exhumed, and a formal state funeral was held in Berlin with President von Hindenburg present. Von Richthofen was then interred with some of Germany’s greatest heroes in the Invalidenfriedhof in Berlin. In 1976, von Richthofen was once again exhumed and reinterred, this time in a family plot in Mainz in western Germany. His memory lives on in the present day as the archnemesis of Snoopy, Charlie Brown’s pet beagle of the “Peanuts” comic strips. —Dana P. McDermott Further Reading Charles River Editors. The Red Baron: The Life and Legacy of Manfred von Richthofen. CreateSpace Independent Publishing Platform, 2018. Smith, Paul J., editor. The Red Baron’s Autobiography: The Red Fighter Pilot. CreateSpace Independent Publishing Platform, 2010. Treadwell, Terry C. The Red Baron: A Photographic Album of the First World War’s Greatest Ace, Manfred von Richthofen. Pen & Sword Books Ltd., 2021. von Richthofen, Manfred Freiherr, and Manfred von Richthofen. The Red Baron. Pen & Sword Books Ltd., 2009. von Richthofen, Manfred Freiherr, and Oswald Boelcke. Richthofen and Boelcke in Their Own Words. Leonaur Ltd., 2011. Wilberg, James W. Rittmeister: A Biography of Manfred von Richthofen. 1st World Publishing Inc., 2007. See also: Aerobatics and flight; Biplanes; Jimmy Doolittle; Billy Mitchell; Eddie Rickenbacker; Triplanes

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W Wake Turbulence Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics; Mathematics ABSTRACT Wake turbulence, also known as vortex hazard and Kármán’s vortex street, is the disturbed fluid motion occurring in the region following an object moving through a fluid. The different types of wake turbulence can disrupt the flow of air moving over an aircraft, posing flight hazards such as loss of lift. KEY CONCEPTS turbulence: a region within a fluid medium where the matter moves in a random or chaotic manner vortex: a region within a fluid medium where the matter has angular momentum as it moves in a circular pattern wake: the region of disturbed air marking the passage of an aircraft through the air TYPES OF WAKE TURBULENCE Two forms of wake turbulence are significant in flight. The first is the presence of vortices in the wake of aircraft. A vortex is defined as a region within a body of fluid in which the fluid elements have an angular velocity. Wake vortices are regions of spiraling or circulating fluid left behind in a medium after an object producing lift, or experiencing changes in lift, passes through the medium. Wake vortices created by aircraft pose hazards to following aircraft. This hazard of spiraling air limits the spacing between aircraft takeoffs or landings at many airports.

A second form of wake turbulence is the turbulence found in the wake of mountains or tall buildings that is encountered by low-flying aircraft under windy conditions as the air passes by, around or over those stationary obstructions. WAKE VORTICES Every aircraft leaves behind a region of disturbed air, called its wake. The disturbances that are due to the flow accelerated by the propeller or jet engines are called prop-wash or thrust-stream turbulence. The largest disturbances are due to the effects of lift. When an aircraft generates lift on its wings, the pressure below the wings is higher than the pressure above. At the wingtips, the flow from below rolls up and over the wings. This rotation forms a vortex, which is a region resembling a long, thin tube, where the air moves in a spiral path. A vortex generally has a small core region of high rotation speed, surrounded by a much larger region of slower-moving fluid. Thus each aircraft typically leaves behind a pair of vortices, rotating in opposite directions and also moving downward at speeds of a few hundred feet per minute. The strength of these vortices is a function of the aircraft’s weight, speed, wing shape and aspect ratio, and acceleration or deceleration. Peak tangential velocities encountered in a vortex can be in excess of 100 meters per second. In addition to the strong wingtip vortices, there are several other vortices, and often a continuous sheet of vortices trailing behind aircraft. These vortices originate wherever there is a change in the distribution of lift across the aircraft. Leading-edge vortices are seen over swept wings and tails. Inboard vortex sheets are seen behind most wings and rotor

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blades. When lift changes suddenly, as during takeoff or a sharp maneuver, a starting vortex is left behind, with its core perpendicular to orientation of the trailing vortex. Sufficiently far behind the aircraft, all these vortices are swept up into the wingtip vortices. Wake vortices originate when the aircraft rotates off the runway at takeoff and end when the aircraft touches down on the runway. The vortices generated near the runway can persist near the ground, generally spaced a little less than a wingspan apart and generally within a height of about one wingspan from the ground. They can then drift with the wind and cross over to adjacent runways. An aircraft’s wake vortices persist in the air for several minutes after the aircraft has passed. Because large airliners travel at nearly 960 kilometers per hour, persistence of vortices for three minutes means that strong vortices can be encountered as far as 50 kilometers behind the airliner in the upper atmosphere. Helicopter rotors, whose tip speeds can exceed the speed of sound, also generate very strong vortices, which persist for substantial periods. Interaction of these vortices with the ground can kick up clouds of dust and small stones, posing hazards to people standing on unprepared surfaces when a helicopter hovers close to the ground. Pilot training manuals generally recommend that people remain at least three rotor diameters away from helicopter rotors to avoid this hazard. HAZARDS Aircraft encountering another aircraft’s wake vortices are in danger of rolling out of control. Strong upward or downward air motion may be encountered as many as 16 meters from the central core of a vortex. The danger is greatest for small aircraft with short wings following a large, heavy aircraft with a clean configuration, that is, with a minimum of flaps and other controls deflected, flying at a slow

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speed. General guidance for avoiding the wake vortex hazard includes: flying above the flight path of the airplane ahead, touching down on the runway beyond the touchdown point of the previous aircraft, spacing takeoffs on the same runway by three minutes, maintaining a vertical separation of at least 325 meters between airplanes. Because wake vortices limit the spacing between aircraft in flight, there is strong interest in the aviation community to find ways to alleviate their hazards. One method involves the alteration of wingtip shapes to generate several vortices that interfere with each other. Others include blowing air out of the wings near the tips and deflecting various small control surfaces. A phenomenon called the Crow instability has been observed, in which the counterrotating pair of tip vortices left behind by an aircraft develop sudden bursts and dissipate shortly thereafter. Some research efforts attempt to accelerate the instability by suitably modifying the vortices. Other approaches to the alleviation of wake vortex hazards include the placement of sensing devices near airports that identify vortices drifting onto active runways and warn approaching aircraft. Researchers also attempt to place sensors on aircraft that sense such vortices in the aircraft’s flight path. WAKE TURBULENCE BEHIND BLUFF BODIES Although aircraft are streamlined, mountains and buildings rarely are, and winds blowing across them cause large regions of turbulence downstream. Theodore von Kármán analyzed the flow patterns behind cylinders and described a phenomenon that became known as Kármán’s vortex street, a series of vortices, of opposite directions, left behind alternately from each side of the cylinder. Such patterns can be observed in the clouds moving across islands and mountains. Aircraft flying into such conditions can encounter strong turbulence. —Narayanan M. Komerath

Principles of Aeronautics

Further Reading Ginevsky, A. S., and A. I. Zhelannikov. Vortex Wakes of Aircraft. Springer Berlin Heidelberg, 2011. National Research Council, Aeronautics and Space Engineering Board, Division of Engineering and Physical Sciences. Wake Turbulence: An Obstacle to Increased Air Traffic Capacity. National Academies Press, 2008. Olsen, John. Aircraft Wake Turbulence and Its Detection: Proceedings of a Symposium on Aircraft Wake Turbulence Held in Seattle, Washington, September 1-3, 1970. Springer US, 2012. Pao, Yi-Ho. Detection: Proceedings of a Symposium on Clear Air Turbulence and Its Detection. Springer US, 2013. Raizer, Victor. Remote Sensing of Turbulence. CRC Press, 2021. See also: Aerodynamics and flight; Aeronautical engineering; Airfoils; Airplane accident investigation; Airplane safety issues; Daniel Bernoulli; Fluid dynamics; Forces of flight; Pressure; Training and education of pilots; Viscosity; Weather conditions; Wind shear

Weather Conditions Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Pilot training; Meteorology ABSTRACT The weather refers to the changing local physical conditions in Earth’s atmosphere. The amount and the location of heat, cold, wind, clouds, and precipitation affects the ability of all forms of flight to perform safely in the atmosphere. KEY CONCEPTS instrument landing system (ILS): an electronic system that can guide a pilot through a landing procedure turbulence: a mass of air moving chaotically that can cause an aircraft to respond in kind, causing passengers to be buffeted about, and potentially damaging the aircraft

Weather Conditions

wind shear: a condition that exists when wind abruptly changes direction over a short distance INTRODUCTION TO WEATHER Weather is necessary to life on earth. The significant force that creates and drives weather is energy from the sun. The bulk of the world’s water is contained in its oceans and seas. This water contains salt and is not potable for humans and other animals. The sun’s heat energy warms this salty water, causing water vapor (in the form of a gas) to rise above the surface. When the water changes from a liquid to a gas, the salt is left behind. Wind blows this freshwater vapor over land, where it can condense and fall as rain. Weather conditions are the processes and forces involved in causing this cycle of life-giving water to transfer from the sea to the sky and then to the ground. A key force in this process is the wind. Air movements and cloud formations are complex in nature, primarily due to uneven heating of land and water in different regions. Other factors are the rotation of the earth, the tilt of its axis, and its changing distance from the sun. Both land and water start heating when the sun rises. The sun heats ocean surface water more slowly than it heats the land. This is partly due to the circulation of ocean water, whereby heat is transferred down to deeper levels, as opposed to the stability of solid land. Therefore, the ocean stores more heat energy than the land, and it gives up its heat energy more slowly than land. All land that is dark, including farmland, forests, and paved cities, readily absorbs heat energy from the sun. Light-colored regions of clouds, sand, ice and snow mostly reflect the sun. When the sun goes down, land surfaces cool more quickly than water surfaces. Because of gravity, the atmosphere is densest nearest the ground. Air molecules are more compacted at the surface of the globe and air becomes less dense at higher altitudes. The density of the air

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at ground level, however, varies due to the uneven heating effects around the surface of Earth. This is very important, because air will flow from a high density (high-pressure) region to a low density (low-pressure) region. This pressure difference between any two areas results in an attempt to balance out the different air densities. The movement of this equalizing air is the wind, and the more the pressure differences, the stronger the wind. The important mechanism that allows clouds to form involves the dew point. This is the temperature at which water vapor in the air will condense. The heat of the land that has received energy from the sun will warm moist air at ground level. When moist air rises, it expands due to the lesser pressure at higher altitudes. This expansion causes it to cool. Often it cools to the dew point and condenses. When a lot of water vapor condenses, clouds form. A huge number of tiny water particles float in the air, held up by air pressure and air movement. These miniature bits of water reflect light and give the cloud its appearance. These water particles can combine, forming larger water droplets. Eventually, they can become heavy enough to fall as rain (if the temperature is above freezing) or as snow or hail (if below freezing). Some clouds will form precipitation, and some will eventually dissipate as the water particles evaporate. The two basic movements of wind are vertical and horizontal. Ground air that has warmed and expanded moves upward, traveling to areas of less pressure. This air rises vertically because the warmer air is less dense than the surrounding air and essentially floats upward on the denser air. When this warm air also contains a large quantity of water vapor, it can condense when conditions are right. When condensing, it releases heat, which additionally warms the surrounding air, causing the upward rising process to continue. An opposite effect occurs when liquid water particles in a cloud start evaporating. When the particles

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change from a liquid to a gas they absorb heat energy. This cools the surrounding air, causing it to become denser. This heavier air cannot be supported by the overall lighter pressure of thin air high up, and so it moves downward, pulled by gravity. These upward and downward vertical air movements can occur at the same time in different parts of a storm system. Two basic types of horizontal air movement are low winds affected by friction forces and higher winds moving without friction. Near the ground, air can be swirling and turbulent as it moves around obstacles such as buildings, trees, hills, and mountains. When moving horizontally, the forward portion of a warm or cold moving air mass is called a front. The continental United States is situated between a subpolar region that brings cold air into the country from the north (heading southward), and a tropical region that sends hot, humid air from the Gulf of Mexico northward. When these two air masses occur at the same time and collide, violent thunderstorms can result. The northern climate of Alaska and the southern climate of Hawaii add to the range of weather possibilities for the United States. WEATHER CONDITIONS AFFECTING FLIGHT Atmospheric conditions affect anything that flies, from birds to spacecraft. Prime weather conditions are clouds, precipitation, wind, lightning, heat, cold, and visibility. These are basic characteristics of weather that can combine to form events such as dust storms, thunderstorms, hurricanes, tornadoes, and blizzards. Clouds. There are many cloud types and combinations. The range of possible air temperatures, pressures, air motion, and amount of moisture can combine in complex ways. The main cloud types of interest to pilots are cumulus and stratus. Cumulus clouds, created in unstable air, are typically fluffy and often rise to great heights. While they may originally develop in blue sky and look harmless, they

Principles of Aeronautics

can transform into dark thunderstorms. They tend to produce heavy precipitation. Stratus clouds are created in stable air and produce smooth clouds that are layered and usually flat. They tend to produce steady, long-duration light rain over a widespread area. A pilot flying through stratus clouds will experience low ceilings, meaning the area near the ground can remain cloudy, with no visibility for navigating or landing. Precipitation and ice. The basic forms of precipitation are rain, snow, hail, and sleet. While all these conditions can cause problems of visibility for the pilot, the greatest hazard is freezing rain. Sleet (partly frozen rain) can form into ice when it falls on the metal surface of an aircraft that is at or below freezing temperature. This is called icing, and it is most hazardous to flying when it forms on wings and propeller blades and interferes with fuel systems and sensing devices. The icing condition can also occur when there is no precipitation; a light plane can experience carburetor icing when flying in cold, moist weather, for example. This is a serious condition, as it can cause engine malfunction. Ice can build up very rapidly. When it covers a wing, the lifting ability decreases, drag friction increases, and the weight of the plane increases. Ice on wings or propeller blades will cause a decrease in power. There is a decrease in stall speed, the minimum speed at which a plane’s wing will lose lift and proceed into an unwanted nose-down dive. In general, a deterioration of aircraft performance results when icing occurs. The solution to icing is to immediately increase thrust, activate anti-ice and deicing equipment, and leave the area producing the icing. Wind and turbulence. Conditions vary from no wind to extremely high wind. A headwind is the situation of flying directly into the wind, and a tailwind occurs when the flight is in the same direction as the wind. A crosswind is wind coming from an angle, between a headwind and a tailwind. Flying into a large low-pressure area allows easier altitude gain due to

Weather Conditions

the rising air. When flying near a large high-pressure area, it is more difficult to gain altitude due to the downward-flowing air mass. Turbulence is a general term for air movement that is characterized by irregular or violent motion, unlike a constant-speed wind staying in one direction. Turbulence can be exhibited between ground level and 23 kilometers (the cruising altitude of a U-2 spy plane). Turbulence is strongly associated with thunderstorms, where its most dangerous form is a condition known as wind shear. This condition occurs when wind abruptly changes direction or speed or both over a very short distance. Wind shear can occur along the boundaries of thunderstorm activity, and in the downdraft under the storm cell. The most dangerous situation for aircraft is when wind shear occurs relatively near the ground (e.g., at an altitude of 65 meters) while the plane is making a landing approach or a takeoff. The direction change can be 180 degrees; the speed change can be 80 kilometers per hour. That is extremely hazardous for an airplane at low altitude because it can place the craft in a position where there is no time for recovery from decreased lift and misaligned attitude before impact with the ground. Another form of turbulence is wind that changes direction near ground level. This is due to the friction between moving air and ground objects. A pilot looking at a weather chart of winds aloft may observe the prevailing wind direction. This may be a headwind to a landing pilot, for example. However, at ground level, due to friction with ground objects such as buildings, fences, and control towers, the head wind can change into a crosswind. These changes affect the attitude and speed of a landing aircraft. Dust devils are another factor in landing or departing. These are small, whirling, circular air currents, normally seen at or near ground level by the dust they kick up. A cruising aircraft can also encounter turbulence due to larger-scale objects. Any change in terrain el-

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evation produces wind turbulence. This is due to air being pushed up over mountains and any type of rising land from gradual inclines to abrupt cliffs. On the downwind side of mountains, air mostly descends and can be choppy and rough. Wind shear aloft can be caused by the same system that produces the near-ground-related wind shear problems. While dangerous and potentially lethal, it is not considered to be as hazardous as at ground level due to more time for corrective action before encountering the ground. Jet streams are fast moving, high-altitude, narrow bands of air moving globally between cold arctic regions and hot tropic regions. These jet streams often travel in a serpentine manner, generally traveling from west to east, although direction varies. They

Photo via iStock/guvendemir. [Used under license.]

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range in location from about 60 degrees latitude in the summer, down to about 20 degrees latitude in winter, with travel excursions between these locations. Their speed varies, sometimes as high as 320 kilometers per hour, for example, at altitudes above 9,500 meters. Strong winds exist for hundreds of miles next to the jet streams. These winds interact with slower-speed wind to cause turbulence. Thunderstorms, tornadoes, hurricanes, and warm and cold air fronts all produce turbulence at varying altitudes. Lightning. Lightning is caused by a difference in electrical charge between the ground and the sky, and even between clouds. It is most often generated when there is heavy cloud activity, as in thunderstorms. It can burn small holes in aircraft outer parts

Principles of Aeronautics

and affect sensitive instruments. However, it does not seem to strike people within a plane. Heat. The temperature of air affects how dense it is—warmer air is less dense and cooler air is denser. The denser the air, the better an aircraft will perform. Engines are more efficient, wings have more lift, and control surfaces provide more control. Much more runway is needed when taking off on a very hot day than is required on a very cold day. Cold. Cold temperatures are primarily a problem for the pilot flying in freezing temperatures in areas of high moisture content. Even with the best anti-icing and deicing equipment, it is not always possible to prevent the effects of ice on an aircraft. Cold temperatures in the presence of moisture can also cause icy runways. Visibility. Besides flying in clouds, precipitation, and darkness, other atmospheric conditions can cause visibility problems. Smog is a condition created when sunlight reacts with a combination of smoke and various gaseous pollutants and then combines with fog. Smog, smoke, dust, haze, fog, and sandstorms all describe conditions that are usually low-lying and can interfere with a pilot’s visibility, especially when landing. Thunderstorms. Thunderstorms can generate several adverse conditions for pilots of all types of craft. A thunderstorm is a cloud system containing very unstable air moving in all directions. Winds can batter and rock an aircraft, causing airsickness and aircraft attitude problems. Violent wind can also stress components of the craft, such as the wings and control surfaces. Wind can alter the course of an airplane, causing more fuel use. Strong turbulence in and around a thunderstorm may include ground-level wind shear that can make takeoffs and landings difficult or impossible. Hail can damage parts of the plane, and freezing rain can cause dangerous icing. Lightning discharges are usually frequent. The extent of a thunderstorm depends on its type. Ordinary individual storms usually form rap-

Weather Conditions

idly, reach a peak of activity, and expire in about an hour. A line of thunderstorms is called a squall line. A supercell thunderstorm is one that can last for hours and travel more than 450 kilometers. The supercell type can develop strong tornadoes as it travels. A tornado is a large, violent, swirling wind funnel that can easily destroy an airplane. The tops of thunderstorms range from about 10 to 23 kilometers. This means that small aircraft are not able to fly over the storm. Larger aircraft may not want to fly over the system, since rough air can extend far into the clear air above the main storm. PILOT AIDS RELATED TO WEATHER CONDITIONS A weather map or chart is a drawing showing continuous lines indicating highs, lows, and fronts. The lines are called isobars, which are plot locations of similar air pressure. A weather satellite photograph is an image printed from a picture taken by a television type of camera. The image, showing the land and tops of cloud formations, is transmitted from the satellite to the ground. Radar in an aircraft sends out a beam that bounces off heavy moisture and rain particles, thus showing a pilot the location of the nearest intense storm cell activities. Spherics in an aircraft is a system that detects and displays the electrical discharges of a thunderstorm. Lightning locations are shown as tiny dots of light on a screen. Autopilot systems automatically control the aircraft’s attitude, direction, and speed. This relieves the pilot of hands-on flying and allows more time for obtaining weather reports and communicating with air traffic controllers and flight crew. Ground-based wind shear detecting devices have been developed that aid in monitoring and predicting this activity in the vicinity of airports. In the 1990s, jet transports in the United States were re-

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Principles of Aeronautics

quired to be outfitted with wind shear detection devices. Accurate and timely predictions, while improving, are not yet an exact science in the year 2001. The storm system producing the wind shear can be more accurately monitored. An instrument landing system (ILS) aids in organizing a landing through use of a ground radio beacon. The pilot uses cockpit instruments and a radio receiver to guide the plane to the runway. —Robert J. Wells Further Reading Buck, Robert H., and Robert O. Buck. Weather Flying. 5th ed., McGraw-Hill Education, 2013; Flannery, Patrick S. Weather in the World of Aviator. CreateSpace Independent Publishing Platform, 2014. Ison, David. Navigating Weather: A Pilot’s Guide to Airborne and Datalink Weather Radar. Aviation Supplies and Academics Inc., 2021. Morris, Doug. Canadian Aviation Weather. Lulu Enterprises Inc., 2015. Piggott, Derek. Understanding Flying Weather. Bloomsbury Publishing Plc, 2020. See also: Aerodynamics and flight; Aircraft icing; Airfoils; Airplane accident investigation; Airplane maintenance; Airplane propellers; Airplane safety issues; Atmospheric circulation; Fluid dynamics; Forces of flight; National Transportation Safety Board (NTSB); Pressure; Shock waves; Sound barrier; Temperature; Training and education of pilots; Viscosity; Wake turbulence; Wind shear

Wind Shear Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics ABSTRACT Wind shear is a change in wind direction or speed, either vertically or horizontally, within a short distance in the atmosphere. Wind shear is a concern in all phases of flight. Strong wind shear close to the ground can be especially hazardous to aircraft.

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KEY CONCEPTS airmass: a bulk quantity of air in a location, differentiated from the surrounding air by temperature and density or humidity downdraft: a current of relatively cold air moving downward toward the ground gradient: a continuous change in a property over time or distance jet stream: a high-altitude current of air that travels at consistently high speed around Earth in the north and the south, also known as the polar vortex updraft: a current of relatively warm air moving upward away from the ground THE FUNCTIONING OF WIND SHEAR Wind shear is a gradient, which means it exhibits a specific change of wind velocity over a given distance. Wind shear can be horizontal, vertical, or both. If the wind direction changes as well as the wind speed, the actual wind shear will be greater than the wind speed shear alone. The most critical wind shears for pilots occur over horizontal distances of 1.6 kilometers or less and over vertical distances of 330 meters or less. LOW-LEVEL WIND SHEARS Wind shear that occurs below 660 meters above ground level, or along the aircraft’s final approach or takeoff and climb path is called low-level wind shear (LLWS). Low-level wind shear is most hazardous when the wind shifts from a headwind to a tailwind. This shear will cause a decrease in airspeed equal to the decrease in wind velocity and can adversely affect the performance of the aircraft. These shears are considered significant when they cause airspeed changes of 15 knots or more. In recent years, an effective LLWS alert system has been developed. These detectors are installed at airports to warn pilots of the possibility of LLWS in the vicinity. Three of the most common types of LLWSs are

Principles of Aeronautics

airmass wind shears, thunderstorm-associated wind shears, and topographical wind shears. AIRMASS WIND SHEARS Airmass wind shears commonly develop during calm, fair nights, during which the ground may become cooler than the overlying air mass, creating a surface temperature inversion. This nocturnal inversion is very stable and impedes mixing of the air masses above and below the inversion. As a result, the surface air remains calm, while the winds aloft increase, because they are not slowed by surface friction. Vertical wind shear can be remarkably strong through the inversion. Wind shears should be expected whenever wind speeds at 660 to 1,320 meters above the surface are 25 knots or greater. After daybreak, the sunlight heats the ground, and the inversion and the wind shear dissipate. In cold-winter climates, however, when the ground is covered with snow and ice, inversions, with their accompanying wind shears, can persist throughout the day and night. THUNDERSTORMS AND MICROBURSTS Thunderstorms produce strong updrafts and downdrafts throughout their lifecycles. The cumulus stage of thunderstorms is characterized by updrafts. Upon reaching the mature stage, the storm has both updrafts and downdrafts. Dissipating thunderstorms have mostly downdrafts. These updrafts and downdrafts can cause severe and even extreme turbulence that can cause an aircraft to experience structural damage. The most severe form of wind shear produced by a thunderstorm is a microburst. A microburst is a core of cool, dense air that descends rapidly from the thunderstorm. When the column of air reaches the ground, it spreads out in all directions and rolls back over itself, forming a vortex ring. Microbursts usually do not last longer than fifteen minutes, although some can linger as long as thirty minutes. There is usually heavy rain in the microburst, al-

Wind Shear

though in drier climates, the rain can evaporate before reaching the ground, creating a dry microburst. Microbursts pose a danger to aircraft because of their strong downdrafts and because the wind direction shifts 180 degrees across the center of the microburst. In a microburst with wind speeds of 45 knots, the shear across the microburst will be 90 knots. The wind shear created by the microburst can exceed the operating capabilities of even heavy transport jets. On February 8, 1985, a Delta Air Lines Lockheed L-1011 Tristar encountered a severe microburst while attempting to land in a thunderstorm at Dallas-Fort Worth, Texas. The aircraft crashed 2 kilometers short of the runway, broke up, and burst into flames, killing 134 of the 163 passengers and crew on board. TOPOGRAPHICAL WIND SHEAR Air moving across hills, mountains, and valleys can create wind shears. As air moves across the ridges of hills or mountains, the airstream is compressed and deflected upward. The compressed air speeds up. The change in direction and speed of the air creates wind shear on the tops of the hills and mountains. Higher mountains and faster winds produce stronger wind shears. Air moving across a valley with gently sloping sides will produce downdrafts on one side and updrafts on the other side. These drafts can produce significant shears if the wind is strong. In canyons with steeply sloped sides, there may be sharp downdrafts on the leeward side of the canyon. In addition, wind may travel through the canyon at right angles to the airflow above it. Several aircraft have crashed in canyons due to the unpredictable wind shears they encountered. HIGH-ALTITUDE WIND SHEAR Wind shear can be associated with jet streams, high-level frontal activity, and the tropopause, the region at the top of Earth’s troposphere, the lowest,

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densest part of the atmosphere. When high-level stable layers of air are displaced vertically, they produce atmospheric waves. These waves can develop crests, much like ocean waves. The air is usually cloudless at these altitudes, so there are no visual clues to alert pilots to the presence of these shears, although if clouds are present, they will often reflect the wave pattern of the air. These wind shears are a common source of clear-air turbulence. —Polly D. Steenhagen Further Reading Angelino, Gianfranco, Luigi De Luca, and William A. Sirignano, editors. Modern Research Topics in Aerospace Propulsion. Springer New York, 2012. Lebel, Luke. An Analysis of the Impact of Vertical Wind Shear on Convection Initiation Using Large Eddy Simulators. Pennsylvania State U, 2022. Manwell, James F., Jon G. McGowan, and Anthony L. Rogers. Wind Energy Explained: Theory, Design and Application. Wiley, 2010. Sharman, Robert, and Todd Lane, editors. Aviation Turbulence: Processes, Detection, Prediction. Springer International Publishing, 2016. Soekkha, Hans M., editor. Aviation Safety, Human Factors-System Engineering-Flight Operations-EconomicsStrategies-Management. CRC Press, 2020. See also: Aerodynamics and flight; Aeronautical engineering; Air transportation industry; Airplane accident investigation; Airplane safety issues; Atmospheric circulation; Fluid dynamics; Forces of flight; Pressure; Temperature; Weather conditions

Wind Tunnels Fields of Study: Physics; Aeronautical engineering; Aerodynamics; Mechanical engineering; Fluid dynamics; Mathematics ABSTRACT Wind tunnels are flow channels through which air or other gases are passed over a model of an aircraft or other object

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to study the effects of the airflow on the aerodynamic forces acting on the model. Wind tunnels are vital to the design and development of any aircraft. They permit the measurement of pressures and forces acting on a scale model of an aircraft to predict the flight characteristics of the aircraft even before it is built. KEY CONCEPTS aeroelastic: refers to the tendency of surfaces to deform elastically under the force of moving air closed-loop wind tunnel: a wind tunnel designed to recirculate the air or other gas system flowing through it open-loop wind tunnel: a wind tunnel designed to flow fresh air through it and exhaust it out at the end of the tunnel yawing moment: the torque required to pivot an airplane or its model horizontally about its center of mass PRINCIPLE OF OPERATION Wind tunnels are based on the principle of relative motion, which states that the forces acting on an aircraft or aircraft model are dependent only on the relative motion between the aircraft and the air. It does not matter whether the aircraft is moving at a certain velocity through still air, or whether the aircraft is fixed and the air is moving over it at an equal and opposite velocity to the aircraft speed. DESIGN Although there are many types of wind tunnels, most of them share common characteristics. They all typically have an inlet section, which is a contracting passage called a venturi, or contraction cone, to speed up the flow of air. The air then enters the test section, where an aircraft model is mounted and where the effects of the air flowing over the model are measured. The most common measurements are of the forces acting on the model. The air then enters an expanding section called a diffuser, where it

Principles of Aeronautics

slows down again. One or more fans are located at the end of the diffuser to draw the air through the tunnel. At the entrance to the tunnel, flow straighteners are mounted to reduce the swirl imparted to the air by the fan, and to damp out most of the turbulence in the air. These flow straighteners consist of screens, or short sections of honeycomb material not unlike a cross section of cardboard box material. Tunnels of this design are called open-circuit, closed-test-section, or National Physical Laboratory (NPL) tunnels. The test section dimensions may vary in size, ranging from several square centimeters to 24 meters by 36 meters. WIND-TUNNEL MODELS Airplane models used in wind tunnels vary greatly in type and size, ranging from several centimeters long to full-scale airplanes in the largest tunnels. Scales from one-twenty-fourth to one-third actual size are often used. Most commonly, a scale model of a complete airplane is tested in the wind tunnel. Sometimes components of the airplane, such as the wing, fuselage, or engine nacelles with scaled engines operating, are tested. Wind tunnels are also used by automotive engineers to test automobiles for their flow characteristics and by civil engineers to test wind effects on buildings and other structures. Numerous other things are tested in wind tunnels: parachute opening dynamics, effects of control surfaces on aircraft, helicopter rotor behavior, propellers, human ski jumpers, golf balls, the flight characteristics of birds and insects, and even the manner of air flow about scale models of large buildings in model cityscapes. MEASUREMENTS MADE The most common measurements made in a wind tunnel are of the forces and moments, or torques caused by forces that tend to rotate the model, acting on the model. The model is usually mounted from below on a strut-type mount or from behind on

Wind Tunnels

a sting mount. The strut or sting mounting is connected to a force measuring balance mounted outside the wind-tunnel test section, where the forces and moments are electronically or mechanically measured. The forces measured are usually relative to the airstream direction. The upward component of force is called the lift force; the rearward force, which must be overcome by the aircraft propulsion system, is called the drag force; and the sideward component of force is called the yaw force. The turning tendency, or torque, about the vertical axis through the aircraft’s center of gravity is called the yawing moment; the torque about the lateral axis through the airplane wings is called the pitching moment; and the torque about the airplane’s longitudinal axis, through the fuselage from front to rear, is called the rolling moment. All these forces and moments must be measured. The rolling, pitching, and yawing moments measured are important for predicting the airplane’s response to deflections of its aerodynamic control surfaces. In addition to forces and moments, other types of measurements are often made in wind tunnels. An important one is the pressure distribution across various sections of an airplane or other model. Measurements are also made for the local flow field in the vicinity of the model, or in other words, the air velocity and direction along various portions of the model and very close to the model. These measurements are often made as part of a flow visualization study, where laser beams, injected smoke, or streamers or tufts are used to study local flow directions. Wind tunnels are also used for aeroelastic studies, where control surface flutter may be examined or aeroelastic twisting or bending of helicopter rotor blades may be investigated. The list of variables that can be studied is endless. For this reason, when a new aircraft is being designed, it is not unusual for several different types of models to be built and tested in different tunnels to determine all the necessary characteristics of the airplane’s aerodynamic and structural response.

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PROBLEMS WITH TESTING There are a number of problems that occur with wind tunnel testing, which engineers must constantly strive to overcome. One of these is the scale effect, which occurs because the model is usually much smaller than the full-scale prototype. The difference in size causes some important differences in flow between the model and the prototype. These flow differences must be accounted for using various techniques at the engineer’s disposal, before the model data can be used to predict the prototype behavior. The air temperature, pressure, and velocity used in the tunnel test may have to be somewhat different from actual flight conditions. Again, there are ways to compensate for this, but they must be carefully evaluated. Another reason for differences that occur between measurements made on a wind-tunnel model and what may occur in flight is due to tunnel wall effect. An airplane flies through open air that is free to move out of the way as the airplane passes through it. Such is not usually the case with a wind-tunnel model. The walls of a wind tunnel provide a partial constraint to the motion of the air as it passes over the model, somewhat altering the forces that would occur in free flight. This effect is especially important in transonic and supersonic aircraft that are generating shock waves. In actual flight, shock waves are free to extend from an actual aircraft as far as necessary. In a wind tunnel model test, these shock waves will strike the tunnel walls and be reflected in complicated patterns that reach the aircraft model or model support system. These interactions are accounted for by complex techniques that ensure the validity of the data measured. OTHER TYPES OF WIND TUNNELS The most common variation to the open-circuit tunnel is the closed-circuit design, which has a return flow path that recirculates the air from behind the fan back around to the inlet section. Thus the same

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air continually recirculates through the tunnel. This type of tunnel is called a Prandtl- or Göttingen-type tunnel. The main advantage of this design is a reduction in the power required for a given test-section velocity, because the air is not wasted as it leaves the diffuser, but much of its kinetic energy is recovered by recirculating it back to the inlet. Other advantages are lowered noise, due to elimination of the open exhaust, and the ability to control tunnel test-section conditions better by heating, cooling, or pressurizing the air. In a closed-circuit, closed-testsection tunnel, it is even possible to change the test gas from air to some other gas to simulate certain conditions. For example, extremely low-temperature nitrogen is sometimes used simulate high-density air, and helium or Freon have been used for other kinds of simulations. Sometimes, open-test-section tunnels are used to eliminate the tunnel wall effects. This can only be done in tunnels using air. Other types of tunnels are specially designed for testing aircraft in supersonic or hypersonic flight where aerodynamic heating effects may be important. Free flight tunnels are sometimes used to check aircraft response to control surface inputs or recovery characteristics from unusual attitudes. Propulsion tunnels are used to study airframe-engine integration. These tunnels may use actual operating jet or rocket engines, or devices designed to simulate the intake or exhaust flow of a propulsion system. Finally, civil engineers often do studies in what is known as a boundary-layer wind tunnel. This type of tunnel usually has a test section that is very long, compared to its width and height. The long test section is used to simulate the growth of Earth’s atmospheric boundary layer, the region near the ground where wind velocity increases with height. Boundary-layer wind tunnels are used to study wind effects around buildings or skyscrapers, the dispersion of smoke plumes from smokestacks, and the interaction of smoke with buildings located downwind.

Principles of Aeronautics

EXAMPLES OF WIND TUNNELS Wind tunnels are usually operated by research organizations such as universities, aerospace companies, or government research laboratories. Prominent university wind tunnels include those of the Georgia Institute of Technology, which operates a test-section tunnel, and the Massachusetts Institute of Technology, which operates the Wright Brothers Wind Tunnel with an elliptically shaped test section. Other noteworthy wind tunnels are located at the Texas A&M University, the University of Michigan, and the University of Washington. Well-known company wind tunnels include the Boeing Research Wind Tunnel, with a 1.5-meterwide-by-2.5-meter-high test section, the McDonnell Douglas 2.5-meter-by-3.6-meter tunnel, and the General Motors Automotive Wind Tunnel with a 5.4-meter-by-10.2-meter test section. The US government operates wind tunnels at many of the National Aeronautics and Space Administration (NASA) facilities, predominately the NASA Langley Research Center in Hampton, Virginia, and the NASA Ames Research Center in Mountain View, California. Much of the airframe-engine integration testing is done in propulsion wind tunnels located at the United States Air Force Arnold Engineering and Development Center (AEDC) in Tullahoma, Tennessee. One of the most famous wind tunnels in the world is the 24-meter-by-36-meter cross-section wind tunnel located at the NASA-Ames Research Center. The largest wind tunnel in the world, it was originally built as a closed-return wind tunnel with a 12-meterby-24-meter oval cross section capable of a test speed of 368 kilometers per hour. In the early 1980s, the tunnel was modified also to include a 24-meter-high-by-36-meter-wide test section operating as an open-circuit tunnel. In this mode, the maximum speed drops down to 180 kilometers per hour, but the test section is large enough to fit a full-size Boeing 737 airliner inside it. The six fans

Wind Tunnels

that drive the tunnel are powered by electric motors totaling 135,000 horsepower. With this power, the original 12-meter-by-24-meter test section can now operate at 555 kilometers per hour. Virtually every fighter plane and transport plane produced in the United States since the 1970s has had one of its models tested in the NASA-Ames 12-meter-by-24meter tunnel. Another unique wind tunnel facility is the National Transonic Facility (NTF) located at the NASALangley Research Center. This is a cryogenic wind tunnel that uses very low temperature nitrogen gas at -253 degrees Fahrenheit. This tunnel can simulate flight speeds as much as 20 percent greater than the speed of sound, which is 1,223 kilometers per hour at sea level. The use of the high-density nitrogen gas in the tunnel’s 2.5-meter-by-2.5-meter cross section makes it possible to simulate flight conditions very close to those encountered by full-scale craft with a relatively low power requirement compared to other tunnels. —Eugene E. Niemi Jr. Further Reading Butcher, Colin, and Drew Landman. Wind Tunnel Test Techniques: Design and Use at Low and High Speeds with Statistical Engineering Applications. Elsevier Science, 2022. Chambers, Joseph R., and Mark A. Chambers. Radical Wings and Wind Tunnels: Advanced Concepts Tested at NASA Langley. Specialty Press, 2008. Driss, Zied. Wind Tunnels: Uses and Developments. Nova Science Publishers, 2019. Goethert, B. H. Transonic Wind Tunnel Testing. Dover Publications, 2009. Mikel, Russell. Wind Tunnels: Models, Aerodynamics and Applications. Clanrye International, 2015. Šabi2, Abdusselam. Wind Tunnels: A Detailed Approach to Analysing and Comparing Wind Tunnel Theory. GRIN Verlag, 2022. See also: Aerodynamics and flight; Aeronautical engineering; Airfoils; Flight roll and pitch; Fluid dynamics; Forces of flight; Model airplanes; National Aeronautics and Space

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Wing Designs

Administration (NASA); Shock waves; Spacecraft engineering; Wake turbulence; Wind shear; Wing designs

Wing Designs Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Fluid dynamics; Mathematics ABSTRACT Wing designs are any of a variety of wing shapes that will provide the lift needed for an airplane to fly and ensure optimum performance in the designated functioning of a particular aircraft. The most significant part of an airplane, wings generate the lift an airplane needs to overcome its weight and maneuver in flight. Wings can be long and thin or short and stubby. They can be angled or straight. The design of the wing must suit the purpose of the airplane: long-distance flight, aerobatic flight, high- or low-speed flight, or even space flight. KEY CONCEPTS quarterline chord: the line extending from the base of a wing at its center to a point at the tip of the wing one-quarter of the distance from its leading edge to its tailing edge sweep: the angle at which a wing extends from the fuselage of an aircraft transonic drag rise: a sudden increase in the drag on an aircraft as any part of it exceeds the speed of sound, due to the sudden air pressure collapse that occurs as supersonic air decelerates to subsonic speeds winglet: a small wing extension with a vertical aspect designed to control wingtip vortex formation and reduce drag BACKGROUND Wings and the concept of flight go hand-in-hand. From the beginning of time, humans have watched

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birds and insects fly and dreamed of somehow making their own wings and soaring into the skies. Early designs for flying machines, from those of Leonardo da Vinci to those of Otto Lilienthal, copied the wing shapes of birds and bats for the simple reason that these were proven designs. However, the first successful powered aircraft had wings that were nearly rectangular in shape and stacked one above the other, nothing like the designs of nature. Modern airplane wings take almost any shape imaginable, from the long, slender wings of sailplanes to the sleek, highly swept wings of the Concorde, and adjustable wings that can vary from one to the other as desired. Many things define the shape or design of a wing, including its span, sweep, taper, dihedral, twist, and its airfoil section or sections. All of these elements can be varied to provide an optimal design for a particular type of airplane. In designing wings, engineers must look first at the designated function and desired characteristics of the aircraft and determine whether it is to be designed for low-speed, high-speed, subsonic, supersonic, or even hypersonic flight. They must know whether the aircraft will travel long distances or remain close to home. They must know whether it will be used for stunt flying or aerial combat or for slow and comfortable flight. They must determine the aircraft’s aerodynamic constraints, structural constraints, and even simpler limitations, such as the width of hangar doors. SWEEP The sweep of a wing is the angle at which the wing extends from the fuselage of the particular aircraft. One can define several different sweep angles for a wing, including those for the wing’s leading and trailing edges. This angle is defined as the angle between the relevant part of the wing and the free-stream or oncoming airflow. To an aerodynamicist or a wing designer, the important definition of

Principles of Aeronautics

the sweep is that of the quarter-chord line, a line that would run along the span from the wing root, or centerline, to the wingtip, one-fourth of the way back between the wing’s leading and trailing edges. In other words, a straight or unswept wing would have a quarter-chord line that is perpendicular to the free-stream flow, whereas a swept wing would have its quarter-chord line at an angle other than 90 degrees to the flow. By this definition, a tapered, unswept wing, one with its chord at the tip different from the chord at the center or root, would still have some sweep of its leading or trailing edges. Wings may be swept for several reasons, but the usual reason is to reduce the drag rise that occurs when a wing nears the speed of sound. Because the airflow accelerates over the top of a wing as an airplane approaches the speed of sound, a region of supersonic flow appears above the wing before the plane actually reaches Mach 1. As that flow moves further back over the wing’s upper surface, it must decelerate back toward the free-stream air speed. However, supersonic flow has a strong tendency suddenly to decelerate through a shock wave. This small shock wave, which often occurs over the wing of an airplane flying at speeds just below the speed of sound, causes a sudden pressure change in the flow over the wing and can result in the flow breaking away from the wing at the shock location. The shock wave itself and the resulting separation of the flow over the wing cause an increase in drag, known as the transonic drag rise, which may double or triple the drag of a wing from its subsonic value. The onset and magnitude of this drag rise are functions of the Mach number of the flow normal, or perpendicular, to the quarter-chord line of the wing. If this line is swept, the normal component of the Mach number will be lower than the free-stream Mach number. Sweeping the wing will therefore delay and reduce the transonic drag rise, allowing an airplane to fly closer to the speed of sound with less engine thrust or to accelerate past Mach 1 and fly at

Wing Designs

supersonic speeds with smaller engines. Hence, supersonic aircraft, such as fighters and supersonic transports, have highly swept wings with sweep angles of 45 degrees or more and high-speed, subsonic transports, such those made by Boeing and Airbus, have sweep angles of around 30 degrees to allow them to fly economically at Mach speeds of 0.8 or higher. Although rearward and forward sweep will provide this drag benefit, aft sweep is usually the design choice, because forward sweep introduces a unique structural problem. The wingtip of a forward-swept wing tends to twist to a higher angle of attack than the rest of the wing, and this can lead to structural failure if the wing is not designed to resist the resulting forces. Thus, the forward-swept wing is often heavier and more expensive to produce than its aft-swept counterpart. Some airplanes have wing sweep for other reasons such as to have the lift of the wing centered at some point behind or ahead of where the wing root attaches to the fuselage. The Douglas Aircraft DC-3, the world’s first commercial airliner, which cruised below 320 kilometers per hour, had swept wings. The designers needed to move the wing’s lift a little aft from their original design and wanted to do this without changing the place where the wing mated with the fuselage. ASPECT RATIO The ratio of a wing’s average chord to its span is known as the aspect ratio. Aerodynamic theory says that a wing with a high aspect ratio will have a very good lift-to-drag ratio, which will, in turn, make it very efficient for both long-range cruising and gliding. The aerodynamic cause of the aspect ratio effect is the flow around the tip of the wing from the higher-pressure area on the lower surface into the region of lower pressure on top. This has the effect of reducing the wing’s lift and increasing its drag. If two wings have the same area, but one has a larger

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span and smaller chord, these losses near the tip will not affect as much of the wing area on the wing with the higher aspect ratio. Therefore, the wing with the higher aspect ratio will have a higher lift and lower drag than will a lower aspect-ratio wing with the same area and angle of attack. In contrast, a wing with a high aspect ratio will require a stronger structure than one with low aspect ratio and will make the airplane more sluggish in roll. This leads to tradeoffs in the design of aircraft. Wing designs with a very high aspect ratio, such as 15 to 20, are used for sailplanes that depend solely on glide for flight. In these planes, the added structural weight is more than offset by the improved gliding ability, and there is no need for fast roll rates. Commercial and military transports and general aviation airplanes designed for longer flights have wings with aspect ratios on the order of 6 to 10. This ratio is sufficient to give excellent long-range cruise capability, a lightweight structure with room for fuel in the wing, and the moderate roll rates needed for comfortable and controllable flight. Aerobatic and fighter aircraft have wings with smaller aspect ratios of 5 or less, because their ability to roll and do other maneuvers at high rates is more important than efficient, long-range flight. Later variations of the famous Spitfire airplane used by the British in World War II had the span of their efficiently designed wings clipped, or shortened, in order to improve the planes’ roll capability in combat with enemy aircraft. TWIST Many wings are twisted to make the angle of attack near the wingtip lower than it is at the base. Although wings may be twisted for several reasons, such as to obtain the most efficient aerodynamic loading along the wing’s span or to create effective structural loading, they are often twisted to make sure the inboard part of the wing stalls before the wingtips do.

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It is important that the pilot be able to control the roll of an aircraft after the wing begins to stall. Otherwise, the stall can easily turn into a dangerous spin. Because the ailerons, near the wingtips, are the devices that provide roll control, the wing should be designed such that stall will begin on the inboard portion of the wing and progress outward, allowing the pilot to feel the stall while the ailerons are still effective. Although an untwisted wing will have some tendency to stall in this desired manner on its own, due to the three-dimensional flow around the tips, twist is often added to provide an extra degree of certainty of control during a stall. DIHEDRAL When looking at an airplane from the front or rear, one may see that the wings are not horizontal but are rather swept slightly upward or even sometimes downward. This angle is called the dihedral, and it is used to improve the roll stability of the airplane. Two factors in the design of a wing will influence the airplane’s stability in roll. One is the vertical placement of the wing on the fuselage, and the other is its dihedral. Stability in roll means that if the airplane were disturbed from its wings-level position, it would automatically tend to roll back to level. For most aircraft, this stability is achieved by the use of dihedral. In a slight roll, the wing that moves downward toward a level position generates more lift than the opposite wing. The added lift then automatically helps restore the aircraft to equilibrium. If the aircraft has a high wing placed above the fuselage, the fuselage’s weight hanging below the wing will cause a pendulum-like effect, giving the craft some roll stability. Therefore, less wing dihedral will be needed than for a low-wing design. In fact, if the high-wing plane is a heavy transport, the wing may need to be built with negative dihedral, or anhedral, to ensure that the aircraft is not too stable in roll. Excessive stability would make the airplane resist the roll required during turns or other maneuvers.

Principles of Aeronautics

PLANFORM The shape of a wing when viewed from above is known as its planform shape. The area inside this shape, the planform area, is used as a reference area when calculating wing lift, drag, and pitching moment coefficients. An examination of airplane designs since the beginning of flight will reveal almost every planform shape imaginable. Planform shapes include simple rectangles, basic trapezoids, smooth curves, bird- or bat-wing contours, triangles, and wings with swept leading edges and sawtooth trailing edges. The simplest wing planform to build is probably the rectangular shape, and sometimes this is the designer’s choice when the cost of construction is more important than other factors. From a structural-efficiency perspective, a wing that is tapered to give a smaller chord at the tip than at the root is a good choice. The tapered planform can give a reasonable aspect ratio while placing the major portion of the lift on the wing’s inboard sections where it is less likely to bend the wing-support spar. Aerodynamic theory holds that, in addition to the benefits of high aspect ratio, the drag on a wing can be further minimized by optimizing the way that lift acts along the wingspan. The best low-drag lift distribution over the wingspan is an elliptical one, in which the lift tapers from a base value at the center, or root, of the wing to zero at the wingtip in a shape like an ellipse. One way to try to achieve such a lift distribution is to actually vary the chord of the wing elliptically along its span, and many airplane wings have been designed this way. Such designs were particularly prevalent in World War II-era fighter aircraft, with the best-known example being the British Spitfire. Elliptically shaped wing planforms are, however, more expensive to build than straight tapered wings, and the wing designer must compare the cost of a purely elliptical wing shape to that of a tapered wing that may approximate the same aerodynamic effi-

Wing Designs

ciency. Because it is the wing’s lift distribution and not actually its shape that should be elliptical, there are other alternatives available to the designer. One can design a wing with the right combination of taper, twist, and sweep to give an elliptical load distribution. Another alternative is to employ variations of airfoil section along the wingspan to tailor the lift distribution. In fact, a calculation of the lift distribution on the B-2 flying wing stealth bomber, with its swept wing and unusual sawtooth trailing edge, will reveal a near-elliptical lift distribution. WINGTIP DESIGNS Over the years, wing designs have included some interesting variations in wingtip shape. Many of these have been accompanied by claims of improved performance due to the reduction or elimination of the drag-producing flow around the wingtip. Although some wingtip shape variations may be capable of slightly altering the structure of the swirling vortex that trails behind a wing, none have ever demonstrated any significant effect. The trailing vortex is a consequence of the lift on a wing, and the only way to reduce or eliminate it is to lower or eliminate the lift on the wing. The winglet is, however, a wingtip device that is designed to use rather than alter the developing wingtip vortex to produce a thrust. It has been used successfully to improve the performance of many aircraft. Some wing shapes have beneficially altered the wing dihedral at the tip to enhance stability or handling or, as add-on devices, to slightly increase the wing’s aspect ratio and, thus, its aerodynamic performance. COMBINATIONS AND OTHER VARIATIONS Sometimes there is a strong desire to optimize the performance of a wing in seemingly conflicting ways or to design a wing for good flight performance while meeting a nonflying requirement. Variable sweep wings have been used on several fighter and

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bomber designs to take advantage of the properties of an unswept, high aspect ratio wing and of a swept wing with lower aspect ratio on the same airplane. This increases the cost of manufacturing and maintenance, as well as the weight of the aircraft, factors that must be weighed carefully against the aerodynamic and other performance gains. Over the history of aviation, several designs have employed wings that folded in various ways or extended and retracted either in flight or on the ground in order to optimize wing area or aspect ratio in the air or to fit into tight spaces on the ground. There have also been designs with interesting combinations of sweep. The scissor-wing concept has a single wing that rotates on an axis, with one side of the wing rotating to a forward sweep and the other moving aft. This design provides a simple but somewhat strange-looking way to achieve variable sweep. The joined wing, with fuselage-mounted aft-swept wing joined at its tips to a forward-swept wing mounted on the vertical tail, claims structural and aerodynamic benefits. Biplanes, with their wires and struts, are usually considered World War I-era designs. However, modern aerobatic biplanes provide plenty of wing area and lift with a short span for ease of roll. Tandem-wing designs place one wing in front of the other, and at least one past design proposed including a sliding section to fill the space between the tandem wings for added area on takeoff. The channel-wing design wraps part of the wing around the lower half of a propeller supposedly to enhance both wing and propeller performance. Radar reflectivity or stealth considerations may also lead to strange shapes for both the planform and the airfoil sections. Flexible or inflatable wings are often used for “flyable” parachutes and hang gliders and in military applications where wings need to be stored in small places for deployment on demand. —James F. Marchman III

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Further Reading Concilio, Antonio, Fabio Semperlotti, Ferri M. H. Aliabadi, Agnazio Dimino, Leonardo Lecce, Rosario Pecora, Ruxandra Botez, and Sergio Ricci, editors. Morphing Wing Technologies: Large Commercial Aircraft and Civilian Helicopters. Elsevier Science, 2017. Dulikravich, George, and Kozo Fujii. Recent Development of Aerodynamic Design Methodologies: Inverse Design and Optimization. Vieweg + Tenbree Verlag, 2013. Ferman, M. A. A Wing Design Method for Aerospace Students and Home Builders. Trafford Publishing, 2011. Kundu, Ajoy Kumar, Mark A. Price, and David Riordan. Conceptual Aircraft Design: An Industrial Approach. Wiley, 2019. Myhra, David. Conversations with Dr. Reimer Horten and His All-Wing Designs Ho-1 to Ho-229, Part 1 and Part 2. RCW Technology & Ebook Publishing, 2013. Yang, Lung-Jieh, and Balasubramanian Esakki. Flapping Wing Vehicles: Numerical and Experimental Approach. CRC Press, 2021. See also: Aerobatics and flight; Aerodynamics and flight; Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Biplanes; Boomerangs; DC plane family; Flight roll and pitch; Fluid dynamics; Flying wing; Forces of flight; Glider planes; High-altitude flight; High-speed flight; Hypersonic aircraft; Otto Lilienthal; Monoplanes; Plane rudders; Rockets; Shock waves; Space shuttle; Stabilizers; Stealth bomber; Supersonic aircraft; Tail designs; Triplanes; Ultralight aircraft; Wind tunnels; Wright Flyer; X-planes (X-1 to X-45)

Women and Flight Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Physiology ABSTRACT In the early days of aviation, women faced many more obstacles than just finding a way to fly. Original notions of human flight implicitly involved the image of men. Though individual women pilots—or aviatrixes, as they were then known—have been involved in powered flight

Principles of Aeronautics

since its inception, they faced both societal and legal obstacles that male pilots did not. WOMEN CAN FLY, TOO! The contributions made by women in flight have been extensive, from aviation to the space program. In the early period of human flight, women constantly had to prove themselves worthy of a chance to enter the cockpit. Even after overcoming external obstacles to flying, many early women applicants were unable to find a flight school or an instructor that would accept a woman student. Prevailing prejudices held that women had neither the physical nor the mental capability to handle an aircraft; British aviation pioneer Claude Grahame-White once proclaimed that “women lack qualities which make for safety in aviation” and “are temperamentally unfitted for the sport.” When famed aviator Amelia

Women and Flight

Earhart wanted to learn to fly, she sought out a woman instructor, feeling uncomfortable with male pilots’ attitudes toward the idea of women flying. Luckily, she found Neta Snook, the first woman ever admitted to the Curtiss Flying School in Virginia and the first to operate a commercial airfield. FLIGHT, THE ULTIMATE ADVENTURE Women have been a part of aviation from its very beginnings. Six months before the Wright brothers’ historic 1903 flight, American socialite Aida de Acosta became the first woman to pilot a powered aircraft (a lighter-than-air dirigible with a three-horsepower engine) unaccompanied; her flight was not reported in the press because her parents were afraid that it would ruin her chances of marriage. Six years later, French actor Raymonde de Laroche became the first woman to fly a

The United States Air Force’s first African American female fighter pilot, Shawna Rochelle Kimbrell. Photo via Wikimedia Commons. [Public domain.]

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heavier-than-air craft solo; soon after, she received the thirty-sixth pilot’s license issued by the AéroClub de France (the first thirty-five having gone to men). Women were piloting aircraft as early as 1798, when Jeanne-Geneviève Labrosse made a solo balloon flight over France. Margaret Graham of England dedicated a thirty-year career to the sport of balloon flying by charging a fee for carrying passengers for a ride, most likely becoming the first woman charter pilot. Mary Myers set altitude records in balloons, including one ascent to 6.4 kilometers over Pennsylvania in 1886, which she accomplished without the aid of oxygen. In 1911, Amelie “Melli” Beese became Germany’s first woman aviator, participating in a flight display at the first airfield in Berlin. She successfully obtained her pilot’s license soon after the display, despite her male colleagues attempting to sabotage her by tampering with her plane’s steering mechanism and draining gas from the fuel tank on the day of her exam. WOMEN IN EARLY COMPETITION It was not unusual in the late 1920s for women pilots to go into business for themselves. Women established passenger-carrying operations in several cities, but these ventures did little to promote women in the field of aviation. If they were to receive national recognition, they would have to compete, as male pilots were already doing, in the record-breaking arenas of distance, altitude, and speed. Viola Gentry is credited with setting the first women’s flying endurance record (without refueling), staying aloft for eight hours and six minutes on December 20, 1928. Two weeks later, her record was broken by Evelyn “Bobbi” Trout, who managed to stay in flight for more than twelve hours. The following spring, Elinor Smith astounded fliers everywhere by staying in the air for more than twenty-six hours.

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The first Women’s Air Derby, the first womenonly air race in the United States, was held in 1929. The race began in Santa Monica, California, and ended in Cleveland, Ohio. The winner was Louise Thaden, who defeated nineteen other competitors, among them Trout and Earhart. When World War II began, it looked as though American women pilots would be grounded or forced to fly only in civilian capacities. Thanks to people such as test pilots and racers Jacqueline Cochran and Nancy Harkness Love, however, American women did have an opportunity to pilot and help with the war effort. Before the United States entered the war, many women, Cochran included, went to England to join the Air Transport Auxiliary (ATA), a British civilian organization established to ferry military aircraft to and from active service squadrons and airfields. The women of the ATA flew with great valor, facing many obstacles. Both Cochran and Love independently submitted proposals to US Army Air Forces (USAAF) commander General Henry H. Arnold suggesting a similar organization be established in the United States. Arnold initially turned down both proposals, but after the Japanese attack on Pearl Harbor in December 1941 and the subsequent US entrance into the war, he reconsidered. In 1942, the Women’s Auxiliary Ferrying Squadron (WAFS) and the Women’s Flying Training Detachment (WFTD) were established under the commands of Love and Cochran, respectively; they merged the following year to form the Women Airforce Service Pilots (WASP), with Cochran as director and Love in charge of ferrying operations. Meanwhile, the Soviet Union established three Soviet Air Forces regiments—the 586th Fighter Aviation Regiment, the 587th Bomber Aviation Regiment, and the 588th Night Bomber Regiment (members of which were nicknamed “Night Witches” by the Germans)—composed of women who gladly utilized their skills as pilots to help the Allies win the

Principles of Aeronautics

war. Many of these pioneer women pilots enlisted in war efforts against the wishes of their husbands and families. BREAKING INTO THE FIELD Modern aviation is totally different from the early days of flight. In the earliest aircraft, it was pilot judgement and skill as well as the aircraft itself that made for a successful flight. The sky was all but free from other aircraft; rules and regulations were just about nonexistent. There were only a few types of aircraft available. Famed pilot Dorothy Hester would often recount that it was a delight to fly in 1927: “Everything looked so neat from up there and you feel so free. You were your own boss and you could get up there alone.” Dorothy Hester started to fly in 1927, one day after her seventeenth birthday. She found a place where she could go for an airplane ride; she spent her birthday money and her adventure began. The biplane was a Waco 9 with a Curtiss OX-S engine. She sat in an open cockpit and felt the rush of air sweep over her when the engine started. “When we lifted off the ground, my heart swelled and I felt like I was in heaven,” she later recalled. “It was the most wonderful feeling I had ever had, and I decided right then that I had found my calling.” After landing, she said to a salesman, “If I were a boy, I would certainly learn to fly”; the salesman said that the Rankin School of Flying, owned by barnstormer and flight instructor Tex Rankin, would teach her anyway. The only problem was obtaining the $250 she needed to pay for the ground course, a large sum at the time. Rankin mentioned that if she were male, she could earn the money parachute jumping in his air shows. Indignant at the implication that women could not parachute merely because of their gender, Hester convinced Rankin through forcible argument to break the established men-only barrier in the air. Hester’s determination has since become legendary. She was the first woman in Oregon to parachute

Women and Flight

during an aerial show. She got $100 per jump, and she earned her pilot’s license and flew. Dorothy Hester also held the world record for most inverted snap rolls (fifty-six) and most consecutive outside loops (sixty-two) for more than half a century (the latter record was broken by stunt pilot Joann Osterud in 1989). Many factors have changed over the years, as the airplane itself has changed. In the first days of woman pilots, women were considered good if they could fly like men. It meant that they had the physical strength to manhandle the airplane. As the airplanes improved, so did the chances for woman pilots, as it became no longer necessary to muscle an airplane around. GROUNDBREAKERS AND PIONEERS Beryl Markham, born in 1902, was a famous adventurer and bush pilot who is most widely known for her record-breaking solo flight from east to west across the Atlantic in 1936, and for her best-selling memoir West with the Night (1942). Markham was possibly the best pilot to fly out of Kenya, and certainly the boldest. Some likened her courage to that of a lion. In April 1932, with only 127 hours of flying time, she set off alone from Kenya in a single-engine Avro Avian headed for England. She first headed for Lake Victoria, then over Uganda and down the Nile River, crossing the seemingly endless expanses of marsh and swamp known as the Sudd. She then crossed the Mediterranean Sea and Europe and arrived in England. Markham repeated this trip several times in the early 1930s. While in Kenya, she worked as a bush pilot, transporting people and supplies. She worked for safari companies and even became a flying elephant-herd spotter. All of her daring escapades culminated in one flight that topped them all. On September 4, 1936, she began a twenty-two-hour flight, mostly at night and mostly on instruments, headed across the Atlantic, west with the night. Beryl Markham was an inspiration for

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millions as a true woman flying pioneer. She died in 1986. Ruth Law became the first woman to fly at night in 1912. She was also the first to loop-the-loop. She made a living by taking Florida tourists on joyrides for $50. Law was renowned as an inventor as well as for her solution to the problem of keeping a map readily accessible. She cut the map of her route into eight-inch-wide strips and affixed them to cloth, creating a cloth map that she could tie to her knee during the flight and roll out one section at a time, thus keeping her hands free to operate the controls. In 1917, after breaking the cross-country record, Law commanded a salary of nearly $9,000 a week for her exhibitions. Even earlier, Matilde Moisant, born in 1886, was the second licensed woman pilot in the United States and the first woman to fly to an altitude of 366 meters, breaking the record previously set by her competitor Hélène Dutrieu. Moisant qualified for her license after only thirty-two minutes of in-flight instruction. In so doing, she established the record for the shortest time spent learning to fly, a record that has never been broken. The first American woman to earn a pilot’s license was Harriet Quimby in 1911, who started her plane manually by turning the propeller. Amelia Earhart was the best-known woman pilot of the early twentieth century. She flew across the Atlantic with two companions in June 1928 and became the first woman to make a solo crossing in 1932. She piloted a Lockheed Vega on the west-east solo flight from Honolulu to California on January 11-12, 1935. Earhart was lost in an attempt to fly around the world with pioneer aerial navigator Fred Noonan. Florence Lowe “Pancho” Barnes was a record-breaking stunt pilot who eventually headed the Women’s Air Reserve in 1931. She was the first woman to fly into Mexico. Fran Bera held the record of most wins for the Women’s Air Derby. She learned to fly at sixteen, got her commercial license,

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taught aviation, and worked for the Federal Aviation Administration (FAA) issuing licenses to pilots. It was at the Women’s Air Derby, however, that she gained the greatest fame, winning five second-place trophies and seven first-place trophies. In June 1966, Bera set a new world altitude record for flying to a height of 12,251 meters over Long Beach, California. Anne Morrow Lindbergh, wife of Charles A. Lindbergh, was the first American woman to earn a glider pilot’s license. She served as her husband’s navigator when he set a transcontinental speed record in 1930, at which time she was seven months pregnant. She was also a best-selling author. In 1964, Geraldine “Jerrie” Frederitz Mock became the first woman to fly around the world, twenty-seven years after Amelia Earhart’s disappearance in 1937. Joan Merriam Smith was flying with the same goal in mind at the same time and completed her trip successfully, but Mock had registered first with the FAA and therefore won the title. Her plane was named Spirit of Columbus. Katrina Mumaw was the first child, male or female, to pilot a plane through the sound barrier. FAA regulations do not allow anyone under the age of seventeen to be issued a pilot certificate, but training can begin at any age. She fell in love with aviation at the age of three. She took her first plane ride at the age of five and began training with a flight instructor when she was eight. In 1994, Mumaw broke the sound barrier at the age of eleven. At thirteen, she was both competing and speaking at air meets and aviation events around the state. WOMEN AVIATORS IN THE MILITARY AND IN SPACE The fight to be allowed into the military has been a long and hard struggle for women. Their first toehold came during World War II, when the need to use all qualified male pilots in battle opened opportunities for woman pilots in noncombatant roles. Nancy Harkness Love, already a commercial airline

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pilot, was one of the first women to head a military unit of female pilots, the WAFS, which ferried military planes from manufacturing sites to air force bases. Cornelia Fort, a flight instructor who was the first Tennessee woman to qualify for her commercial pilot’s license, was the second woman to volunteer for the WAFS; she was in the air when the Japanese bombed Pearl Harbor in December 7, 1941. Fort died in a 1943 midair collision while ferrying a plane. She was the first woman to die in US military duty. Mildred McAfee was the first director of the Women Appointed for Volunteer Emergency Service (WAVES). On July 30, 1942, the WAVES were created by an act of Congress. When she was appointed to lead them, McAfee became the first woman ever commissioned as an officer in the US Navy. She retired from the WAVES as a full captain in December 1946. For her service she received the Distinguished Service Medal. In the years since World War II, many women have broken gender barriers to become successful military pilots. Trish Beckman was one of the first women trained to be a United States Navy test pilot. Sarah Deal was the first woman aviator in the US Marine Corps. Troy Devine was the first woman captain in the US Air Force U-2 program. Kelly Flinn was the first woman to pilot a B-52 bomber for the US Air Force. Patricia Fornes was first woman to lead a US Air Force ICBM Unit. In June 1993, Fornes took command of the 740th Missile Squadron at Minot Air Force Base, North Dakota. She also became the first woman to take over the command of a squadron once commanded by her own father. Colleen Nevius was the US Navy’s first woman test pilot and the first woman to graduate from the Naval Test Pilot School in Patuxent River, Maryland. In 2002, Amy McGrath became the first woman to fly a combat mission for the US Marine Corps, the combat flight restriction on women having been lifted nine years earlier.

Women and Flight

In space aviation, Russian cosmonaut Valentina Tereshkova was the first woman in space, launching in the Vostok 6 spacecraft on June 16, 1963. Sally K. Ride was the first American woman in space as well as the youngest American astronaut to orbit Earth, launching in the space shuttle Challenger on June 18, 1983. Judith Arlien Resnik was the second American woman to go into space and the first Jewish astronaut. In 1996, Linda M. Godwin performed the first ever spacewalk while docked to an orbiting space station. Shannon W. Lucid has logged more continuous time in space than any other American astronaut, male or female. Lucid spent seven months on the Mir space station in 1996. She was also the first American woman to go into space five times. —Lori Kaye and Maureen Kamph Further Reading Cooke, Julia. Come Fly the World: The Jet-Age Story of the Women of Pan-Am. Houghton Mifflin Harcourt, 2021. Douglas, Deborah G. American Women and Flight Since 1940. UP of Kentucky, 2021. Gibson, Karen Bush. Women Aviators: 26 Stories of Pioneer Flights, Daring Missions, and Record-Setting Journeys. Chicago Review Press, 2013. Kennedy, Kelly. “What It Was Like to Be One of the First Female Fighter Pilots.” The New York Times Magazine, 2 May 2018, www.nytimes.com/2018/05/02/magazine/ women-pilots-military.html. Accessed 23 May 2018. Lebow, Eileen F. Before Amelia: Women Pilots in the Early Days of Aviation. Brassery’s Inc., 2014. Pearson, P. O’Connell. Fly Girls: The Daring American Women Pilots Who Helped Win WWII. Simon & Schuster, 2018. Roussel, Mike. “Amelia Earhart and Neta Snook: Pioneering Aviators.” The History Press, www.thehistorypress.co.uk/articles/amelia-earhart-andneta-snook-pioneering-aviators/. Accessed 23 May 2018. Smith, Sally. Magnificent Women and Flying Machines: The First 200 Years of British Women in the Sky. History Press, 2021. Spring, Joyce. The Sky’s the Limit: Canadian Women Bush Pilots. Dundurn Press, 2006.

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Staats, Ann McCallum. High Flyers: 15 Inspiring Women Aviators and Astronauts. Chicago Review Press, 2022. See also: Amelia Earhart; Valentina Tereshkova

Wright Brothers’ First Flight Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT Wilbur and Orville Wright’s first successful flight at Kitty Hawk, North Carolina, on December 17, 1903, marked the beginning of a new age for air transportation and technology. SUMMARY OF EVENT On December 15, 1906, Scientific American editorialized, “In all the history of invention, there is probably no parallel to the unostentatious manner in which the Wright brothers of Dayton, Ohio, ushered into the world their epoch-making invention of the first successful aeroplane flying machine.” The periodical did not exaggerate. Wilbur and Orville Wright succeeded in their first powered flight near Kitty Hawk, North Carolina, on December 17, 1903, in front of five witnesses. Yet, for three years following that day, few seemed to realize that humankind’s dream of flying had been accomplished. As two of the seven children of Bishop Milton Wright, originally an itinerant minister from a midwestern Protestant sect, and Susan Koerner Wright, Wilbur and Orville were reared modestly. As the result of their family’s move from Indiana to Dayton, Ohio, neither boy finished high school. In 1892, the Wright brothers opened a bicycle shop in Dayton. They had always been interested in mechanical and scientific matters and were devoted tinkerers. In 1895, they began to build their own bicycles in a workshop above the store. They also experimented

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Orville Wright, 1905. Photo via Wikimedia Commons. [Public domain.] with numerous inventions. Their interest in flying was aroused by reports of the initial development of the automobile and of Otto Lilienthal’s experiments with gliders in Germany in the 1890s. The Wright brothers started their work as self-made aeronautical engineers in 1899 by writing to the Smithsonian Institution in Washington, DC, for suggestions about reading materials on the subject of human flight. They discovered that little was known about the subject, despite long interest in it. Octave Chanute and Dr. Samuel Pierpont Langley, recognized as a foremost scientist, were the leading US experimenters, but the Wrights were especially influenced by aeronautical pioneer Lilienthal. The early experimenters had met a number of setbacks. Lilienthal and Percy Pilcher, a Scottish glider pioneer, were killed in similar accidents. Chanute had given up, and Langley was to see his first manned flying machine crash after takeoff.

Principles of Aeronautics

Unlike their better-educated predecessors, the two bicycle mechanics—with their gift for visualizing abstract principles and then bringing them into operation—discovered three essential elements of flight that proved to be immutable aerodynamic principles. These involved lift, drag, and, especially, control for balance and stability in the face of shifting wind patterns and drift. The brothers’ use of movable wingtips, a three-axis rudder, and other devices addressed the difficulty of control, which had caused the other pioneers grief. After experimenting with kites beginning in 1899, the brothers decided to build a glider and sought a site for test flights. They wrote to the US Weather Bureau, which informed them that Kitty Hawk, North Carolina, on a treeless and isolated barrier beach between the Atlantic Ocean and the coastline and dotted with sandy dunes, had suitable wind currents. The Wrights transported their glider from Dayton to Kitty Hawk. In 1900, they glided for a few minutes in this first man-carrying device. Then they refined their theoretical aerodynamic calculations, realizing that they had to reexamine every earlier finding. Chanute helped them with some advice and money. In 1901, the brothers built their first wind tunnel to test wings; this enabled them to draw up accurate tables of air pressures on curved surfaces. Successful glider experiments in 1901 and 1902 inspired the Wrights to create a powered flying machine, but no one could supply an engine to their specifications. Their determination led them to decide to build one themselves, and in this endeavor they were greatly and ably assisted by Charles E. Taylor, a mechanic. Taylor machined every component of the engine, except for the crankcase, in the bicycle shop. This was the only aspect of the Wrights’ invention that someone else had a significant role in creating. The engine was crude even by the standards of the day, but it worked. The water-cooled, four-cylinder, thirteen-horsepower gasoline engine weighed less than 180 pounds and pro-

Wright Brothers’ First Flight

Wilbur Wright, 1905. Photo via Wikimedia Commons. [Public domain.] vided more power with less weight than any previous model. Its lightness enabled the brothers to keep the total weight of their 1903 Wright Flyer to 625 pounds of wood, fabric, and metal. Most important, the engine and the two chain-driven propellers enabled the craft to move through the air fast enough to generate lift on the wings to keep the machine airborne. On September 23, 1903, the Wright brothers transported their biplane, unassembled, from Dayton to Kitty Hawk. By December 14, a cold and clear day, they were ready to test the reassembled aircraft. They tossed a coin to decide who should try first, and Wilbur won. On leaving the sixty-foot wooden launching rail, the machine climbed a few feet before the engine stalled, and the craft landed after only three and a half seconds. The brothers worked on repairs until December 17. Orville then made an epochal flight of twelve seconds’ duration,

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Principles of Aeronautics

Seconds into the first airplane flight, near Kitty Hawk, North Carolina, on 17 December 1903. Photo via Wikimedia Commons. [Public domain.]

covering a distance of 120 feet. Three more tries were made that day. Wilbur flew 195 feet in thirteen seconds. Orville covered 200 feet in fifteen seconds. Finally, shortly after noon, Wilbur flew 852 feet, lying prone at the controls, as usual, for fifty-nine seconds at thirty-one miles per hour against a twenty-one-mile-per-hour wind. Subsequently, a gust of wind damaged the plane while it was parked, and no more flying was possible that year. SIGNIFICANCE Only a few American newspapers reported the Wrights’ flight on the day after their historic achievement. The skepticism that existed toward their success could be traced to several sources. For one thing, Simon Newcomb, a highly respected US

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scientist, had published “proof” earlier that year that it would be impossible for heavier-than-air planes to fly. In addition, Lilienthal’s fatal crash in 1896 was still fresh in the public’s memory, and Langley’s prestigious machine had failed just nine days earlier. Even the brothers underestimated the value of their achievement, which they reported to have cost them more than a thousand dollars. For many years, the Smithsonian Institution refused to recognize theirs as the first powered flight, preferring to honor Samuel Langley’s attempt. Not until 1942 did the Smithsonian finally publish an unequivocal statement crediting the Wrights with having invented the airplane. In 1904, the Wrights designed a new aircraft capable of sustained flight and of turning and banking

Principles of Aeronautics

maneuvers. The 1905 Wright Flyer could hold the air for thirty-nine minutes, covering as many kilometers over Huffman Prairie near Dayton, a more convenient test site than Kitty Hawk and one at which the brothers now continued their experiments. In 1908, the US War Department finally gave the Wrights a contract to build the first military aircraft. By the end of 1909, the brothers had won every flying competition around, and the Wright Company had received enough production orders to make them wealthy and famous men. Parades and medals greeted them everywhere they went. In 1932, a sixty-foot granite commemorative pylon was erected on the dune near Kill Devil Hills village, from whose slopes the initial flights had been launched. The Wright brothers’ story—of two high school dropouts with extensive intellectual curiosity, self-education, and a supportive environment who conquered the air for humankind—epitomizes the American legend of gifted amateurism and the rewards of hard work. Their achievement opened the door to a new era for transportation and technology. —Richard H. Collin and Peter B. Heller Further Reading Goldstone, Lawrence. Birdmen: The Wright Brothers, Glenn Curtiss, and the Battle to Control the Skies. Random House Publishing Group, 2015. Hazelgrove, William. Wright Brothers, Wrong Story: How Wilbur Wright Solved the Problem of Manned Flight. Prometheus Books, 2018. Howard, Fred. Wilbur and Orville: A Biography of the Wright Brothers. Dover Publications, 2013. Jakab, Peter L. Visions of a Flying Machine: The Wright Brothers and the Process of Invention. Smithsonian Press, 2014. Kelly, Fred C. The Wright Brothers: A Biography. Dover Publications, 2012. McCullough, David. The Wright Brothers. Simon & Schuster, 2016. See also: Aeronautical engineering; Glenn H. Curtiss; Forces of flight

Wright Flyer

Fields of Study: Physics; Aeronautical engineering; Mechanical engineering ABSTRACT The Wright Flyer, also known as Flyer 1, Aerostat, and the Flying Machine, was the first heavier-than-air plane flown under its own power by a human being in controlled flight from 1899 to 1903. The Wright Flyer, the most honored airplane in history, revolutionized modern aviation. Through their scientific and engineering research, Orville and Wilbur Wright solved the problems of lift, propulsion, and control that had defeated other aviation pioneers. KEY CONCEPTS drag: the resistance to motion through a fluid due to friction between the moving object and the fluid medium lift: the upward pressure experienced by wings due to differential pressure between upper and lower surfaces of the wing as it moves through a fluid medium as determined by its airfoil camber and thickness EVOLUTION OF THE Flyer Long treated as tinkerers who were lucky to invent the airplane, Orville and Wilbur Wright are now seen as highly creative scientists who helped found aeronautical engineering by designing, constructing, and test-flying a series of increasingly sophisticated aircraft, from kites and gliders to the Wright Flyer. Modern scholars are amazed by the Wrights’ collaboration, versatility, and ability to master the many fields involved in controlled flight. Without formal scientific or technical training, the Wright brothers were able to design movable wings, rudders, and elevators; a light and powerful motor; and a proper propeller. Because of these accomplishments, they came to embody an ideal as heroic American inventors.

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The brothers, Wilbur, elder by four years, and Orville, became interested in flight when their father gave them a gift of a toy helicopter. As young men, they established the Wright Cycle Company in Dayton, Ohio, where they not only repaired and sold bicycles but also manufactured their own models. The death of German aviator Otto Lilienthal in an 1896 glider accident rekindled the brothers’ interest in flight. Based on information they received from the Smithsonian Institution, they built a double-winged kite whose movements could be manipu-

Principles of Aeronautics

lated by using tethers to alter the angles of its wings. According to Wilbur, he and his brother discovered this wing-twisting mechanism by observing the flights of birds. The Wrights then moved from kites to human-carrying gliders. To test their gliders, they needed a place with consistently high winds, and the Weather Bureau informed them that Kitty Hawk, North Carolina, was one of the windiest locations in the country. Furthermore, at Kill Devil Hill, near Kitty Hawk, there were long, wide beaches. Begin-

Wright Flyer patent plan. Photo via Wikimedia Commons. [Public domain.]

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ning in 1900 and continuing in 1901 and 1902, the Wright brothers tested their gliders from this spot. They discovered that their human-carrying glider could be controlled with their wing-warping apparatus, but that their aircraft had less lift and more drag than they had expected. They realized that they needed scientific data on the lift pressures of various wing structures. Back in Dayton, they built wind tunnels in which to determine the angles at which differently shaped wings would cause lift. With the data generated by these tests, the Wrights made a double-winged glider with a vertical tail. However, in their 1902 tests at Kill Devil Hill, they found that their glider behaved erratically in high winds. They solved this problem by making the aircraft’s tail movable. They also installed a movable panel, known as an elevator, to the front of the glider to control its ascent and descent. This movable elevator and tail rudder, along with their wing-warping mechanism, gave the Wrights’ glider a system of control in three dimensions. THE WRIGHT FLYER Having created a controllable glider, the Wright brothers realized that their next step should be to equip it with a motor and propeller to achieve powered flight. Because commercial motors did not suit their needs, they designed a powerful, lightweight engine that was built by Charlie Taylor, a worker in their bicycle shop. This 4-cylinder, 12-horsepower water-cooled, fuel-injected engine was constructed of cast aluminum and steel. Because this motor would add weight and cause vibrations, the brothers lengthened the glider’s wings and added stay wires to strengthen the glider’s structure. This new system of control restricted the warpability of the 12-meter wings to the wingtips, but it worked well nonetheless. To provide thrust for their new machine, which they dubbed “the Flyer,” they used propellers, de-

Wright Flyer

signed in the shape of a twisted wing, which spun at 350 revolutions per minute. In the fall of 1903, the Wrights returned to Kitty Hawk, where they reassembled their Flyer, installed its motor, and waited for good weather. On December 17, Orville took off from the beach in the Flyer, rose to a height of about 3 meters, and flew a distance of more than 33 meters in 12 seconds. The age of powered human flight had begun. For the rest of the day, the brothers alternated runs, the longest of which was flown by Wilbur, who remained in the air for nearly one minute and covered a distance of 260 meters. THE WRIGHT FLYER AFTER KITTY HAWK Upon returning to Dayton, the Wright brothers decided to abandon their bicycle business and devote all their time to improving the Flyer. Over the next three years, they transformed their experimental aircraft into the world’s first practical airplane. Because they no longer needed the winds of Kitty Hawk, they were able to test their flyers near Dayton. In 1904, the Flyer made its first complete circle, and by 1905, Wilbur stayed aloft for 38 minutes and traveled 38.6 kilometers. On May 22, 1906, the brothers were granted US patent 821,393 for their flying machine. They continued to increase the reliability, maneuverability, and range of their planes by constant technical improvements. Although they formed a company to manufacture aircraft, Wilbur, who died in 1912, did not live to experience the full glory of the airplane’s evolution. One of Orville’s chief concerns after his brother’s death was securing the Flyer‘s place in the history of aeronautics. For many years, the Smithsonian Institution refused to acknowledge the Wright Flyer‘s role in initiating the age of aviation, and Orville kept the Flyer on display in England until the Smithsonian finally conceded that the 1903 Flyer was indeed the world’s first genuine airplane. The

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Wright Flyer was installed in the Smithsonian eleven months after Orville had died. —Robert J. Paradowski Further Reading Anderson, John D., Jr., John David Anderson, National Air and Space Museum, and M. J. Anderson, Jr. The Airplane: A History of Its Technology. American Institute of Aeronautics and Astronautics, 2002. Gaffney, Timothy R. The Dayton Flight Factory: The Wright Brothers and the Birth of Aviation. Arcadia Publishers Inc., 2014.

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Hallion, Richard P. The Wright Flyers 1899-1916: The Kites, Gliders, and Aircraft That Launched the “Air Age.” Bloomsbury Publishing, 2019. Jakab, Peter L. Visions of a Flying Machine: The Wright Brothers and the Process of Invention. Smithsonian Press, 2014. See also: Aeronautical engineering; Ailerons, flaps, and airplane wings; Airfoils; Airplane propellers; Glenn H. Curtiss; Fluid dynamics; Forces of flight; Heavier-than-air craft; Otto Lilienthal; Wind tunnels; Wright brothers’ first flight

X X-Planes (X-1 to X-45) Fields of Study: Physics; Aeronautical engineering; Mechanical engineering; Biomechanics; Physiology; Mathematics ABSTRACT X-Planes are experimental aircraft intended to test new configurations or unexplored aerodynamics. The US X-planes were the first to fly faster than the speed of sound, the first to test a variable-sweep wing in flight, the first to fly at altitudes greater than 30.5 kilometers, and the first to fly three to six times the speed of sound. Lessons learned from these research aircraft have been applied to all the high-speed aircraft and spacecraft that followed them. THE X-PLANE PROGRAM In the United States, the federal government, under the direction of the National Advisory Committee for Aeronautics (NACA), its successor the National Aeronautics and Space Administration (NASA), the US Air Force, or the US Navy, has sponsored a number of dedicated research aircraft, starting with the X-1 in 1944. As of 2001, the most recent X-Plane in progress was the X-43. Many of these aircraft were initially top secret. Additionally, new fighter and bomber prototypes are often initially given an “X” designation (e.g., the XP-59 and XB-70) in the United States as well as in other countries. The X-1 program was initiated in response to difficulties World War fighter aircraft were experiencing as they approached the speed of sound in dives. These included especially Lockheed’s P-38 Lightning fighter and Republic’s P-47 Thunderbolt fighter planes. Even at flight speeds of about 75 per-

cent of the speed of sound (i.e., at a Mach number of 0.75), regions around the fuselage and around the thickest part of wing were experiencing supersonic flow and causing shock waves to form there. The consequences were a pitch-down trim that tended to increase the airspeed even more, separated flow behind the shock waves, which caused control surface buffet and ineffectiveness, and a great increase in drag. Recovery, if possible, often relied on a reduction in Mach number produced by an increase in the speed of sound as the air temperature increased at lower altitudes. A specialized research aircraft seemed to be the only available approach because it was not known at that time how to build a wind tunnel with a supersonic test section. Since its rocket engine had fuel for only 2.5 minutes, the only option was to attach the X-1 to the bomb bay of a B-29 bomber and release it at the maximum altitude and speed, and then glide to a landing on Muroc Dry Lake in California (now Edwards Air Force Base). The X-1 made its first powered flight in August, 1947, and in October, with test pilot Chuck Yeager, made the world’s first supersonic flight. In the following year, Yeager reached Mach 1.45. He also flew the second series X-1A; in 1953 he achieved Mach 2.44 in it but nearly lost the plane and himself when it tumbled out of control due to what was later recognized as inertia coupling. Roll or inertia coupling occurs when a rolling motion creates a pitch moment because the aerodynamic axis is not aligned with the inertial axis. It caused the X-1 to tumble out of control on one flight and later caused the loss of an X-2. A solution to inertia coupling was found with the X-3 in time to solve the similar problem with the F-100 Super Sabre.

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Starting from its first powered flight in 1955, the X-2 used a two-chamber rocket engine and, in 1956, set a new world speed record of 3,058 kilometers per hour and a new altitude record of 38.4 kilometers. SUPERSONIC EXPERIMENTATION Throughout the 1950s and 1960s, one of the main focuses of X plane research was the exploration of supersonic flight. The Douglas X-3 Stilleto was designed to explore the transonic speed range, where mixed subsonic and supersonic flow exits around a plane in the range from about Mach 0.8 to about Mach 1.2, using two afterburner-equipped jet engines. (An afterburner uses raw fuel in the exhaust,

Bell X-1-2, 1949. Photo via Wikimedia Commons. [Public domain.]

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after the compressor and combustor and turbine that powers the compressor.) Its first flight was in 1952. The Northrop X-4 Bantam, a small piloted research aircraft, was a test of the semi-tailless configuration (that is, it had no horizontal tail surface). Its first flight was in 1948. A porpoising instability was found to limit flight speeds to less than Mach 1. Beginning in 1951, the Bell X-5 tested in-flight variable sweep of the wing, from 20 degrees to 60 degrees. (The later Navy F-14 Tomcat uses variable wing sweep.) For the Convair X-6, two standard ten-engine B-36 bombers were to be modified to carry a nuclear-powered turbojet, which was ex-

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pected to yield almost indefinite range. However, only the modified aircraft that tested the shield of the nuclear reactor ever flew. Six Lockheed X-7s made 130 flights from 1951 to 1960, and many speed (Mach 4.3) and altitude (32.3 kilometers) records were set. The knowledge obtained was applied to the Lockheed F-104 Starfighter aircraft. The Aerojet General X-8 Aerobee was an uncrewed aircraft, recovered via parachute, designed to measure properties of the air above the atmosphere. With flights that began in 1947, it eventually reached Mach 5.96 and an altitude of 109.7 kilometers above Earth’s surface. MISSILE, ROCKETS, AND OTHER RESEARCH Many X planes were designed to perfect their missile capabilities. Beginning in 1950, the uncrewed Bell X-9 Shrike provided valuable information relevant to the guidance and control of an air-launched air-to-ground missile. The uncrewed North American X-10, with a first flight in 1953, provided needed information for the design of supersonic cruise missiles. A canard surface was shown to be useful. The uncrewed Convair X-11demonstrated the successful use of vectored thrust to control the trajectory of rocket-powered ballistic missiles. The first flight was in 1948. The X-12, a follow-on to the X-11, became the prototype for the B-65 Atlas intercontinental ballistic missile (ICBM). First flight was in 1957. The Ryan X-13 Vertijet was a tail-sitting, delta-winged, jet-powered vertical takeoff and landing (VTOL) aircraft. It made its first full transition from vertical takeoff to fully horizontal flight and back in 1957. The complexity of the system reduced payload, but jet-powered VTOL was shown to be possible. The Bell X-14 was a VTOL aircraft that took off and landed in a horizontal attitude by diverting the jet thrust downward and then reducing the deflection in stages to transition to horizontal flight. Start-

X-Planes (X-1 to X-45)

ing with preliminary tests in 1954, it ended up making research flights for nearly twenty-five years. The North American X-15 was perhaps the most valuable high performance research aircraft of the X planes. The rocket-powered X-15 used a bullet-shaped fuselage with dorsal and ventral fins along with an all-moving slab-type horizontal tail. First flying in 1959, it eventually reached Mach 6.33 (considered to be hypersonic flight since it was greater than Mach 5) and 108 kilometers above Earth’s surface. The full pressure suit worn by the pilots was the basis for later astronaut pressure suits. The Bell X-16 was intended to be a long-range, subsonic reconnaissance aircraft flying at or above 21.3 kilometers. The “X” designation was a cover-up. The project was canceled before the first flight could take place. Beginning in 1955, twenty-six Lockheed X-17 multistage rockets were used to test the reentry requirements of long-range missiles. It showed that a blunt nose-cone shape reduced aerodynamic heating upon reentry because it forced the shock wave to form well in front of the nose. Bell Aerospace Textron X-22 explored VTOL using four turboprop engines utilizing ducted fans (that is, there was a channel or duct around each of the four propellers). It was able to demonstrate sustained hover at over 8,000 feet. First flight was in 1966. A complex transmission system insured that an engine failure would only result in a reduction in total power without any tendency to roll. Making its first flight in 1966, the uncrewed Martin Marietta X-23A proved the feasibility of a hypersonic lifting body for an orbiting, reentering space vehicle, playing a key role in the design of the space shuttle. (A lifting body is a fuselage that is broadened enough to generate a sufficiently large lift-to-drag ratio that it can be landed safely after a steep glide through the atmosphere.) The single Martin Marietta X-24A was a Mach 2, rocket-powered, air-launched aircraft that was de-

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signed to explore the low-speed characteristics of a maneuverable lifting body; its first flight was in 1969. The X-24B was a modified X-24A with a new nose and a different tail configuration and made its first flight in 1973. The X-26 aircraft, based on a Schweizer sailplane, were intended to perform ultraquiet reconnaissance over enemy territory. The X-26A was made by Schweizer and the X-26B by Lockheed. A low radar cross section was also part of the design. One aircraft was used in the Vietnam War. Two aircraft were used by the Navy to train pilots in inertia, or roll, coupling. Forward wing sweep had been recognized for many years as conveying the same benefits as rearward wing sweep, as well as potentially better low-speed and maneuverability characteristics because the stall begins at the root and progresses to the wingtips, leaving the ailerons effective as long as possible. The problem with forward sweep is aeroelastic divergence: When the forward swept wing is loaded, it will try to bend so as to increase the angle of attack, which causes more loading, and this can quickly lead to structural failure. Advanced composite materials in which stiffness and strength can be tailored for the directions in which it is needed were used to overcome this problem in the Grumman X-29A. X-PLANES AT THE TURN OF THE MILLENNIUM The Rockwell/MBB X-31 was a multinational enhanced fighter maneuverability test aircraft that used vanes on the exhaust to direct the thrust over a wide range of angles. It was able to demonstrate a 180-degree turn from a deep stall at an angle of attack of 70 degrees after a dynamic entry (the Herbst maneuver). Its first flight was in 1990. The first of two X-31s was lost on its two hundred ninety-second flight in 1995 when the airspeed probe iced up, but the pilot ejected safely.

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Boeing X-32 was designed as a joint strike fighter (JSF) concept demonstrator aircraft, rather than as a pure research or experimental vehicle. It was in competition with the Lockheed Martin X-35 to become a successor (as the F-24) to the early models of the F-16 for the United States Air Force, replace early models of the F-18 for the Navy, and (in its short takeoff and vertical land, or STOVL, version) replace the Harrier and the F-18 for the US Marine Corps and the British Royal Navy and Air Force. Its first flight was in September, 2000. The STOVL version first flew in March, 2001. The first flight of Lockheed Martin’s non-V/STOL version was in October, 2000. Lockheed Martin X-33, a liquid hydrogen/oxygen-powered space launcher using a new aerospike engine, was designed to test a particular single-stage-to-orbit configuration as part of a future reusable launch vehicle (RLV). Payload was projected at 25,000 pounds or more, and hypersonic speeds of greater than Mach 15 were anticipated. Its lifting-body design was expected to yield touchdowns at 306 kilometers per hour (about 96 kilometers per hour slower than the space shuttle). Lockheed Martin’s contract with NASA expired on March 31, 2001. Assembly continued, but the future of the vehicle was placed in doubt. The uncrewed Orbital Sciences X-34 was intended to provide design information for next-generation spacecraft. A powered version was expected to reach Mach 8. By 2001, three had been built. McDonnell Douglas/Boeing X-36, a 28-percent-scale prototype jet, was designed to test the agility of future fighter aircraft that lack the traditional tail surfaces. A forward canard, split ailerons, and thrust-vectoring were used for directional control. Designed to be unstable in both pitch and yaw, it used a fly-by-wire (computer-controlled) control system. Thirty-one flights were made in 1997. The Orbital Sciences X-37 was expected to validate new propulsion, thermal protection materials,

Principles of Aeronautics

and lightweight structures that would eventually lead to less expensive access to space. The Boeing X-40A was an 85-percent-scale version of the X-37. Its first flight was on August 11, 1998. Three lifting-body, uncrewed Scaled Composites X-38s, intended to provide a rescue vehicle for astronauts, had been built. The first flight of an X-38 was in 1998. A parafoil deployed to slow the final descent to Earth. Micro Craft X-43, an uncrewed scramjet-powered hypersonic research vehicle, also known as Hyper-X, was intended to reach Mach 10, becoming the first air-breathing (jet-powered) aircraft to achieve hypersonic speeds (above Mach 5). The first flight attempt in 2001 ended in the loss of the first X-43 because of a launch vehicle failure. The two Boeing X-45s that had been built by 2001 were intended to acquire data leading to future uncrewed combat air vehicles (UCAVs), building on the success of uninhabited aerial vehicles (UAVs) for reconnaissance. IMPRACTICAL EXPERIMENTS A number of X-Planes proved impractical for a number of reasons—they were too expensive, they were good ideas that could not be translated into realistic designs, or they simply fell afoul of congressional funding agendas. For instance, the Hiller X-18, a test bed for a vertical/short takeoff and landing (V/STOL) large cargo transport, made twenty flights, starting in 1959, before a failed prop control caused an unrecoverable spin. The whole wing, along with the turboprop engines, tilted from horizontal to vertical. Curtiss-Wright X-19 was a tandem-winged aircraft with fully tilting turboprops at each of its four wingtips. However, on its only flight, the aircraft and its crew were lost when a propeller separated from the aircraft. The project was terminated in 1965. Boeing X-20 Dyna-Soar (for “dynamic soaring”) was intended to be a prototype of a piloted, maneu-

X-Planes (X-1 to X-45)

verable hypersonic research aircraft that would go into orbit and then glide in for landing, much as the space shuttle does now. However, the success of the Mercury and Gemini suborbital and orbital missions caused this project to be terminated in 1963. Designed to explore the feasibility of producing extensive laminar flow on a large aircraft by using suction through tiny holes in the wing surface, the Northrup X-21 demonstrated the ability to obtain close to 75 percent laminar flow over the wings, with a potential increase of perhaps 50 percent in its range. However, maintenance costs (keeping the holes free of water and dust and insects) and production costs made it appear to be too expensive to be practical. The X-21s first flight was in 1963. Three versions of the Benson X-25 autogyro were built and tested. It was intended as a stowable emergency flight vehicle for pilots who were forced to abandon their aircraft behind enemy lines. (An autogyro, or autogiro, obtains lift from rotating blades rather than from a wing, and the rotor blades can be folded to minimize the space required.) The concept proved to be feasible but not practical. The first flight of the X-25A was in 1968. The Lockheed X-27 Lancer was intended as a low-cost, advanced lightweight fighter that would appeal to countries using the company’s F-104 Starfighter, but it was never funded. The Pereira X-28A Osprey I was a homebuilt design, a low-cost, single-place seaplane that the government thought could be usefully employed in Southeast Asia for civil police patrols. It was successfully tested in 1971, but no production contract resulted. The X-30 Spaceplane, announced by President Ronald Reagan in 1986 as the “Orient Express,” was intended to be a crewed, single-stage-to-orbit vehicle that could take off and land on conventional runways. It was expected to use turbojets at low speeds, ramjets at the lower supersonic speeds, scramjets

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(supersonic combustion ramjets) above Mach 4, and rocket engines while in orbit. The program was canceled in 1995 with no planes built.

Merlin, Peter W., and Tony Moore. X-Plane Crashes. Specialty Press, 2008. Pace, Steve. X-Planes: Pushing the Envelope of Flight. MBI Publishing Co. LLC, 2003.

—W. N. Hubin Further Reading Gorn, Michael H., and Guiseppe De Chiara. X-Planes from the X-1 to the X-60: An Illustrated History. Springer International Publishing, 2021. Griehl, Manfred. X-Planes: German Luftwaffe Prototypes 1930-1945. Pen and Sword Books, 2012. ———. Luftwaffe X-Planes: German Experimental Aircraft of World War II. Pen and Sword Books, 2015. Libis, Scott. Skystreak, Skyrocket & Stiletto: Douglas High-Speed X-Planes. Specialty Press, 2005.

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See also: Advanced composite materials in aeronautical engineering; Aerodynamics and flight; Aeronautical engineering; Atmospheric circulation; Avionics; Avro Arrow; Flight roll and pitch; Flight testing; Fluid dynamics; Forces of flight; High-altitude flight; High-speed flight; Hypersonic aircraft; Propulsion technologies; Ramjets; Rocket propulsion; Scramjets; Shock waves; Sound barrier; Space shuttle; Spacecraft engineering; Spaceflight; Supersonic aircraft; Training and education of pilots;;Uninhabited aerial vehicles (UAVs); Wing designs; Chuck Yeager

Y Chuck Yeager Fields of Study: Aeronautical engineering; Mechanical engineering; Aerodynamics ABSTRACT Yeager was born February 13, 1923, in Myra, West Virginia. He died December 7, 2020, in Los Angeles, California. One of the best-known American pilots of all time, Yeager was an accomplished wartime aviator during World War II, survived several crashes as a test pilot afterwards, and became the first pilot to break the sound barrier in level flight.

aged airplane make an emergency landing near his home, leaving the future pilot unimpressed with the prospects of flight. With the United States at war, however, he enlisted in the US Army Air Forces (AAF) and became an airplane mechanic at a base in California. In 1942 he enrolled in the Flying Sergeant Program, a course designed to attract skilled enlisted men into the pilot ranks. Bored with repair work and attracted by the higher rank and monthly pay, Yeager entered the program, graduating in early 1943. Initially, the AAF trained Yeager and the other pilots of his first unit, the 357th Fighter Squadron, to

KEY CONCEPTS Mach number: a multiple of the speed of sound; Mach 1 = speed of sound, Mach 2 = twice the speed of sound, etc. test pilot: a pilot who undertakes the dangerous job of flying repaired or experimental aircraft to determine their flight capabilities EARLY LIFE Born into a rural family, Chuck Yeager spent his early years working with his father in the family gas-drilling business near Hamlin, West Virginia. Like his father, Yeager was mechanically inclined, and he developed the skill to repair and maintain complex machinery, an advantage that later served him well. Yeager lived a normal Depression-era existence, graduating from high school in Hamlin just as the United States entered World War II in December, 1941. Yeager’s first experience with aviation was rather inauspicious. As a teenager, he had observed a dam-

Chuck Yeager. Photo via Wikimedia Commons. [Public domain.]

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fly the P-39 Aircobra fighter. The P-39 proved an unsuccessful aircraft, however, and Yeager was reassigned to the 363rd Fighter Squadron equipped with the more successful P-51 Mustang fighter. In November 1943, Yeager arrived at his first combat base, a British Royal Air Force facility at Leiston, England. LIFE’S WORK Yeager flew his first combat mission in early 1944 in his P-51 that he nicknamed the Glamorous Glenn for his girlfriend back home, Glennis Dickhouse. On March 4, on his seventh combat mission, Yeager downed his first enemy aircraft, a German Messerschmitt Me-109 fighter. The next day, however, Yeager tangled with three German fighters in combat over Bordeaux, France, and was shot down. Evading capture by the Germans, Yeager contacted the Maquis, an underground group resisting the German occupation of France. The Maquis hid Yeager from the Germans for nearly three weeks until late-winter snows melted and the group could move Yeager across the Pyrenees Mountains into neutral Spain. Yeager repaid the Maquis by instructing them how to fuse plastic explosives, one of the many mechanical skills Yeager had acquired as a youngster. Once returned to England, Yeager found himself in a dilemma. Army rules forbade pilots rescued by the Maquis or other resistance groups from again flying combat missions. The Army feared that if a pilot was shot down a second time, the Germans might be able to extract information about the Maquis. Faced with losing his flight status, Yeager and another escaped pilot, Frederick Glover, pled their case to General Dwight D. Eisenhower, supreme commander of Allied forces in Europe. While Eisenhower pondered Yeager’s fate, Yeager downed his second aircraft, a German Junkers Ju-88 bomber, over the English Channel. Eisenhower eventually returned both Yeager and Glover to duty, and Yeager returned to his squadron, now with the rank of lieutenant.

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Once back in the war, Yeager added to his total of destroyed German planes. On October 12, 1944, he shot down five German fighters in a single day. A month later, he became one of the few Allied pilots to shoot down a fearsome German Me-262 fighter. The first jet fighter to enter combat, the Me-262 could fly 240 kilometers per hour (kph) faster than Yeager’s P-51, but Yeager caught the German fighter in its final airfield approach and shot the vulnerable German aircraft out of the sky. With the air war starting to wind down, Yeager flew his final combat mission on January 14, 1945, and returned to the United States to report for duty at Wright Field, Ohio. Yeager, now a captain, married Glennis Dickhouse on February 26. Uncertain about his future, Yeager assumed his duties at Wright Field. His primary task was to test-fly repaired aircraft, and he had the opportunity to demonstrate his flying skills in a number of different aircraft. His versatility and mechanical skills caught the attention of Colonel Albert Boyd, head of the Aeronautical Systems Flight Test Division at Wright Field, who invited Yeager to become a test pilot. Yeager accepted the invitation and transferred to Muroc Airfield (now Edwards Air Force Base) in California. Among Yeager’s early projects was the X-1, an experimental aircraft built by Bell Aircraft Corporation to study high-speed flight. The goal of the X-1 project was to break the sound barrier and fly beyond the speed of sound (supersonic speed). Bell Aircraft and its test pilot, Chalmers Goodlin, were initially in charge of the project, but the Air Force (the Army Air Force became the Air Force in 1947) believed the project was moving too slowly. When Goodlin demanded hazard pay for the risky flights, the Air Force took over the project and made Yeager the main test pilot. The Air Force planned a flight to break the sound barrier on October 14, 1947. Two days before the flight, however, Yeager broke two of his ribs in a

Principles of Aeronautics

horseback-riding accident. Afraid the Air Force would find another pilot, Yeager told only his wife, and project engineer-pilot Jack Ridley, about the injury. On the day of the flight above the Mojave Desert in Southern California, Ridley helped Yeager into the X-1 and provided part of a broom handle for Yeager to use as a lever to close the inside aircraft hatch. Yeager was able to secure himself into the aircraft and complete the flight, reaching a top speed of Mach 1.07 (700 mph). Due to national security concerns, the facts of Yeager’s history-making flight did not become public for several months. Once the flight became public, Yeager became a national celebrity and was awarded both the MacKay and Collier Trophies for his achievement. Although less newsworthy than breaking the sound barrier, Yeager had many other achievements during his Air Force career. During the Korean War, Yeager was one of a handful of pilots to test-fly a Soviet-built MiG-15 fighter that had been flown to South Korea by a defecting North Korean pilot. In 1953, Yeager participated in a number of research flights that continuously set new speed records, culminating with a flight that reached Mach 2.44 in October. For the rest of the 1950s, Yeager commanded US fighter squadrons based in Germany, France, and Spain. Yeager’s wife, Glennis, died in 1999. Yeager married his second wife, Victoria Scott D’Angelo, in 2003. In 2012, at the age of eighty-nine, he celebrated the sixty-fifth anniversary of breaking the sound bearing by doing it again, taking off from Nellis Air Force Base in Nevada in an F-15 Eagle. He lived in Penn Valley, California, near Sacramento, for the rest of his life. SIGNIFICANCE With the US-Soviet space race just beginning during the 1950s, Yeager was primed to become the leading adviser to the National Aeronautics and Space Administration (NASA) and the space program it-

Chuck Yeager

self. Although he was not an astronaut, he did serve as the first commanding officer of the Air Force Aerospace Research Pilot School, which provided trained pilots for NASA and the Air Force. Yeager’s test-flight days would end in 1963, when he was badly burned in an accident while flying an NF-104 aircraft, but he recovered enough from his injuries to fly once again during the Vietnam War. He commanded the 405th Fighter Wing in 1966, flying 127 combat missions over Vietnam with the rank of brigadier general. He then served in several administrative posts until his retirement from the Air Force in 1975. In 1986, US president Ronald Reagan appointed Yeager to the investigative team looking into the explosion of the space shuttle Challenger. Yeager, whose very name has become synonymous with risky, but necessary, flight, excelled at meeting challenges and accomplishing great feats. Moreover, he achieved notoriety by repeatedly risking his life in incredibly dangerous situations. Even when given the option of taking the safe path, such as the option of going home after being shot down in World War II, Yeager saw challenges as jobs to be done. By accomplishing his great tasks, Yeager furthered the causes of aviation and space travel in the late twentieth century, becoming a legend in the process. —Steven J. Ramold Further Reading Caygill, Peter. Sound Barrier: The Rocky Road to Mach 1.0+. Pen & Sword, 2006. Courtwright, David. Sky as Frontier: Adventure, Aviation, and Empire. Texas A&M Press, 2005. Darling, David J. The Rocket Man: And Other Extraordinary Characters in the History of Flight. Oneworld, 2013. Freeze, Di. In the Cockpit with Chuck Yeager. Freeze Time Media, 2013. “Get the Stuff Right.” Executive Leadership, vol. 28, no. 3, 2013, p. 6. (Business Source Complete.) Accessed 12 Dec. 2013.

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Hallion, Richard P. “The Air Force and the Supersonic Breakthrough.” Technology and the Air Force: A Retrospective Assessment. Edited by J. Neufeld, G. M. Watson, and D. Chenoweth, Air Force History and Museums Program, 1997. Hallion, Richard P., and Michael H. Gorn. On the Frontier: Experimental Flight at NASA Dryden. Smithsonian, 2003. “Legends of Aviation.” Aviation History, vol. 22, no. 2, 2011, p. 21. (Academic Search Complete.) Accessed 12 Dec. 2013. Marrett, George J. Contrails Over the Mojave. The Golden Age of Jet Flight Testing at Edwards Air Force Base. Naval Institute Press, 2014. Pisano, Dominick A., F. Rober van der Linden, and Frank H. Winter. Chuck Yeager and the Bell X-1: Breaking the Sound Barrier. Harry N. Abrams, 2006.

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Yeager, Chuck. The Quest for Mach One: A First-Person Account of Breaking the Sound Barrier. Penguin, 1997. Yeager, Chuck, with Leo Janos. Yeager: An Autobiography. Bantam, 1985. See also: Aerodynamics and flight; Neil Armstrong; Jimmy Doolittle; Amelia Earhart; Flight testing; Forces of flight; John Glenn; Robert H. Goddard; Gravity and flight; High-speed flight; Otto Lilienthal; Charles A. Lindbergh; Mach number; Messerschmitt aircraft; Military aircraft; Billy Mitchell; Eddie Rickenbacker; Burt Rutan; Alan Shepard; Sound barrier; Space shuttle; Supersonic aircraft; Valentina Tereshkova; Konstantin Tsiolkovsky; Wright brothers’ first flight

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Glossary accelerometer: a device that measures the change of stress on an internal component to determine the rate of acceleration or deceleration, and changes in the direction of motion

aerospike rocket engine: a rocket engine with an inverted structure such that the exhaust exits rearward along the outside of the engine to provide thrust rather than rearward from the inside of the engine

accident rate: the number of accidents per 100,000 hours of flying time, applies to individual aircraft and aircraft types, pilots and air carriers

aerostat: a lighter-than-air vehicle that can remain in one position in the air

actuator: a device driven by an electric motor or by hydraulic or pneumatic pressure to move something to which it is attached, such as a control surface on an aircraft

afterburner: a system that increases jet engine thrust by injecting additional fuel into the exhaust stream as it exits the combustion chamber, where the extra fuel is instantly ignited thus producing a large quantity of additional exhaust gas to augment thrust

advanced: indicates a process or mechanism that functions beyond the current standard levels of performance

aileron: a small secondary structure at the outer end of a wing, used to alter the lift of a wing for control of the roll and pitch of an aircraft in motion

advanced polymer: a polymer that is typically of a complex molecular structure, produced by polymerization of similarly complex monomeric units, and having characteristic properties that greatly exceed those of simpler polymer systems

airfoil: the cross-sectional profile of an aerodynamic wing

aerial reconnaissance: viewing ground-level activities from a vantage point above the ground born aloft by balloon or other type of aircraft aerodynamic efficiency: the measure of the airfoil shape of a propeller blade to produce thrust relative to the forward speed of the aircraft aerodynamic flutter: a vibration of wing or fuselage surfaces or of other structural components, caused by disruption of the proper flow of air over those surfaces, referred to as “buzz” by pilots

airframe: the essential structure of an aircraft design without the additional items required for its intended use airmass: a bulk quantity of air in a location, differentiated from the surrounding air by temperature and density or humidity air traffic control system: monitored radar stations on which aircraft signals appear accompanied by unique identifying information, course prediction, and altitude altimeter: a device that indicates the aircraft’s altitude

aerodynamic forces: lift, drag, gravity and angular momentum for rotating objects in flight

anemometer: a device that interacts with moving air to show the direction and speed of the wind

aerodynamicist: one who studies the science of aerodynamics

angle of attack: the angle at which airflow encounters the leading edge of a wing or propeller relative to its chord line

aeroelastic: refers to the tendency of surfaces to deform elastically under the force of moving air

anthropogenic: caused by or existing as a result of human activity

631

Glossary

astronaut: personnel specially trained to undertake missions in space, the word translates to “star sailor” autogyro: forerunner of the helicopter, an aircraft with short wings and an unpowered overhead rotor for lift autorotating: rotating without being driven by a motor axial compressor: a compressor in which air is driven through a tapering containment housing by a series of rotating blades that sweep a successively smaller diameter banking: turning an aircraft in flight in such a way that it flies in a controlled arc either to the left or to the right as desired Bernoulli principle: the pressure exerted by a moving fluid such as air is inversely related to the speed of the fluid flow biomechanics: the scientific study of how animals, including man, move as a function of muscles, tendons and skeleton coordination biplane glider: a glider bearing two pairs of wings blade pitch: the angle between a propeller’s chord line and its plane of rotation, also called pitch angle Blitzkrieg: a World War II German war strategy of rapid air attacks on selected targets; the word means “lightning war” bomber: an aircraft designed for the delivery of bombs, water and other ordinance to be dropped on a target boundary layer: a layer of viscous fluid immediately adjacent to a solid; in it, the velocity of fluid rapidly approaches zero relative to the solid Boyle’s Law: when pressure or volume are changed at constant temperature, the product of volume and pressure before the change is equal to the product of pressure and volume after the change

632

Principles of Aeronautics

buoyancy: the extent to which an object will float in a fluid medium on the basis of their relative densities buzz: vibration experienced by an aircraft in flight as a result of forces exerted by the air against the aircraft as it moves camber: the curvature of the cross-section of a wing cambered wing: a wing shape having a convex-curved upper surface and a concave-curved lower surface canard: an aircraft design having small stabilizing wings near the front of the airplane, well ahead of the main wings carbon fiber: a modern fiber material consisting of carbon produced by processing fibers of a carbohydrate material such as linen or cotton, leaving just the carbonized base as strong, black fibers that can be used as threads and woven into a light-weight, high-strength structural fabric for advanced composite materials carbon sink: any process or mechanism by which carbon dioxide is removed from the atmosphere and sequestered cargo plane: aircraft designed and put into service solely for the transport of material goods; cargo plane designs generally are modified passenger planes with all passenger seating removed ceiling: the maximum altitude at which an aircraft is designed to fly centerline: the longitudinal axis of an aircraft in the direction it is designed to fly center of gravity: the point within an aircraft, or any other body, about which the entire mass of that body is equally distributed center of lift: the point at which the lift force appears to function against the weight of an airplane

Glossary

Principles of Aeronautics

centrifugal compressor: a compressor in which the rotating blades compress air by directing it outward radially from the central axis and forcing it against the containment housing centrifugal force: the force felt by an object undergoing rotation about a central axis, directed outward from the center of rotation Charles’ or Gay-Lussac’s Law: the temperature of a gas is directly proportional to the pressure chassis: the basic frame of an airplane (or other type of vehicle) closed loop wind tunnel: a wind tunnel designed to recirculate the air or other gas system flowing through it combustion chamber: a structure as the second stage of a jet engine within which the fuel-air mixture is combusted, producing rapidly expanding hot exhaust gases command module (CM): the segment of a lunar mission rocket dedicated to the overall operational control of the mission’s operations and functions composites: materials that are themselves constructed from two or more different materials such as sheets of woven fiberglass and a thermosetting plastic resin; advanced composites use more exotic materials such as polyaramide fibers (Kevlar) and carbon fibers

control yoke: the modern “joystick” with which a pilot controls the operation of an aircraft convection cell: a closed loop system of fluid movement in which warmer, less dense fluid rises and cooler, denser fluid descends, with both upper and lower levels exhibiting lateral displacement to replace the fluids that have ascended and descended counterrotation: rotation in opposite directions; occurs in helicopters as the angular momentum of the rotating blades imparts an equal and opposite angular momentum in the aircraft, thus requiring a secondary rotor in the tail section to provide a balancing torque court-martial: a military court of law adjudicated by military officers rather than by publicly elected judges Coanda effect: the tendency of a jet of air to adhere to a curved surface and to entrain adjacent air thus creating a region of low pressure Cold War: a period during the 1950s and early 1960s during which the Soviet Union and the United States essentially dared each other to start a nuclear war though neither one would combustor: the part of a ramjet engine in which fuel is burned crewed flight: air travel requiring the presence of a pilot and other personnel to maintain the aircraft’s flight

compression-ignition: an engine that uses the heat produced by the compression of air inside a combustion chamber as the ignition source for combustion of fuel

cross-control: use of both rudder and ailerons in the opposite sense of their regular use to control the approach of an airplane to a runway, producing a slip

compressor: a rotating structure at the first stage of a jet engine that compresses air taken in

crosswind: wind blowing across the direction of an aircraft’s direction of motion

condensation: the change of the physical state of water from gas to liquid

cruciform: in the shape of a cross

control surfaces: the surfaces of wings, ailerons, flaps and tail rudders used to control the flight characteristics of an aircraft

decalage: the difference between the angles of the upper and lower wings of a biplanes deceleration: the reduction of speed, the opposite of acceleration

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Glossary

delamination: a separation of the layers of material within the body of a composite structure density: the weight of a material per unit volume, typically grams per cubic centimeter or kilograms per cubic meter; the density of water = 1 gram per cubic centimeter; density of air = approximately 0.00129 grams per cubic centimeter (at 0ºC and 760 mm) deregulation: elimination of many of the government controls over aspects of the air transportation industry, essentially removing the possibility of favoritism dethermalizer: a mechanism that will act to arrest or slow the upward motion of a model by controlling the horizontal stabilizer to prevent the model from being carried too high by a thermal air current differential calculus: a method of determining rates of change of a quantity or property with respect to a standard reference such as time, as a large number of infinitesimally small progressive changes differential equation: a relationship between the derivatives of one or more functions and the functions themselves diffuser: a structure within a ramjet engine that serves to diffuse or randomize the movement of the air/fuel mixture entering the engine, thus slowing it and allowing it to burn without decreasing the pressure inside the combustor dihedral angle: the angle of a wing relative to horizontal dirigible: a lighter-than-air craft whose direction of motion can be controlled discus: a circular weighted disk of wood with a metal rim, constructed so that the radial cross-section resembles an airfoil, making the entire shape rather like a circular wing when spinning through the air dissipative forces: forces that function to reduce the amount of energy an object possesses

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Principles of Aeronautics

doped fabric: a resin-impregnated fabric that will become the waterproof “skin” of a homebuilt aircraft, providing a streamlined contour for the aircraft Doppler effect: wave impulses of a particular frequency moving toward a detector become compressed and are detected as a higher frequency, while the same wave impulse moving away from a detector becomes elongated and is detected as a lower frequency Doppler radar: a type of radar that is able to differentiate the motion towards and the motion away of any object being scanned, often used to identify the rotation of storm systems Doppler shift: the amount by which an observed frequency differs from its natural frequency according to the relative velocities of the source and the observer downdraft: a current of relatively cold air moving downward toward the ground downstroke: the downward motion in the imitation of the flapping of wings downtime: the time in which an aircraft is not able to fly due to mandated inspections and necessary repair or maintenance drag: the resistance to motion through a fluid due to friction between the moving object and the fluid medium drag chute: a type of parachute used to decelerate a racing vehicle at the end of a race echo: an electromagnetic, sonic or physical impulse that has reflected from a surface and returned to its source elevons: wing components that serve the combined functions of ailerons and elevators as the means of steering control empennage: the vertical structure comprising the tail of an aircraft

Principles of Aeronautics

encryption and decryption: generally, a system that alters the nature of a signal to prevent unwanted intrusion or pirating of data in the signals, essentially converting the signal into a “secret code,” decryption is the opposite process of encryption, enabling recovery of the original signal data ergonomics: a design principle that seeks to harmonize mechanical function with the shape and mechanics of the human body in order to minimize physical discomfort and stress error chain: The sequence of potential causative factors that have combined to result in an accident or crash FARs: Federal Aviation regulations, a set of regulations designed for the safe operation of aircraft fighter: an aircraft designed for engaging other aircraft in aerial combat fighter-bomber: an aircraft designed to carry out delivery of explosive ordinance as well as to engage in aerial combat maneuvers against enemy fighters fire-control system: an electronic system that controls the functioning of an aircraft’s armaments fixed-wing aircraft: aircraft having the traditional structure of a fuselage and non-moving wings on either side of the fuselage flap: a secondary structure of a wing near the fuselage, used to increase or decrease the lift of the wing for speed control during take-offs and landings flashpoint: the lowest temperature at which combustible vapors at the surface of a liquid will ignite flight training: instruction and practice in operating a specific type of aircraft efficiently; military flight training also includes aerial combat techniques fluid: a form of matter that has the ability to flow and adapts its shape to its container fluid dynamics: the science of the motion and properties of fluids

Glossary

flushed rivets: rivets whose heads are made flush with the surface of the aircraft’s fuselage rather than protruding, thus providing a uniformly smooth surface and reducing drag formers: structural skeleton pieces that provide the basic shape of the aircraft fuselage Fourier analysis: application of mathematical principles that simplify a complex waveform as a single expression in sines and cosines free fall: unobstructed and uncontrolled motion from an altitude toward Earth’s center of gravity fueled weight: the weight of an aircraft carrying only its full load of fuel fuselage: the longitudinal central body of an airplane general solution of a differential equation: a formula involving arbitrary constants such that, by assigning values to the constants, one gets all the solutions of the differential equation geosynchronous orbit: an orbit in which an object remains in the same location relative to a location on Earth’s surface glider: an aircraft that has no engine or other power source to provide forward motion, relying instead on the pilot’s skill in maneuvering the aircraft in existing air currents to remain aloft global warming potential: a measure of the extent to which a particular atmospheric gas is able to increase the average temperature of the atmosphere gondola: the passenger and cargo carrier suspended or affixed below a balloon or other type of floating airship gradient: a continuous change in a property over time or distance grain: a shaped charge of solid rocket fuel designed to undergo oxidation combustion at a predetermined rate to provide a desired amount of thrust

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Glossary

greenhouse gas emissions: emissions of carbon dioxide, water vapor, hydrocarbons and soot from combustion, especially at higher altitudes in the troposphere greenhouse gases: atmospheric gases that are able to absorb infrared radiation emanating from Earth’s surface, then re-emit a large percentage of that heat energy back into the atmosphere and towards the surface while the remainder goes out into space gross weight: the weight of an aircraft with its maximum load ground loop: an often disastrous maneuver in which an aircraft suddenly turns when touching down and begins to go into a rolling motion ground proximity warning system: a system designed to provide a warning that the aircraft has become to close to the ground or ground-based obstacle for safe flight gyrocompass: a device that uses a combination of gyroscopes to maintain a stable indication of “north” Hertz (Hz): the unit of frequency; 1 Hz = 1 cycle per second homebuilders: individuals who build their own aircraft at home, or “DIYers” homebuilt: typically assembled by the purchaser of a kit of ready-to-assemble parts rather than in an aircraft manufacturing facility Hugoniot Elastic Limit: the greatest stress that can be developed in a material without permanent deformation remaining when the stress is released hydrodynamics: the science of the motion and properties of water, the forerunner of the broader science that is fluid dynamics hyperbaric atmosphere: a high-pressure atmosphere hypersonic: at speeds greater than 3.5 times the speed of sound (Mach 3.5)

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hypobaric atmosphere: a low-pressure atmosphere ideal gas law: the relationship of pressure (P), temperature (T), volume (V), mass in moles (n), and the universal gas constant, R, as PV = nRT inclinometer: a device that indicates the angle at which an aircraft is ascending or descending relative to level flight incompressible: a material that cannot be reduced in volume by the application of pressure and maintains its characteristic density or mass-to-volume ratio induction icing: formation of ice on internal surfaces affected by induced airflow that results in a lowering of temperature inertial navigation system: a system that uses three gyroscopes spinning on mutually orthogonal axes inflammable air: the early chemists’ name for hydrogen before it was identified chemically instrument landing system (ILS): an electronic system that is able to guide a pilot through a landing procedure instrument rating: a certification for the ability to take off, fly and land an aircraft without visible cues using only instrument readings integral calculus: a method of determining the overall change in a quantity or property from the change rates determined by differential calculus interplane struts: structural components, typically of wood, between the upper and lower wings of a biplane or triplane to provide structural strength and stability jet aircraft: aircraft that are propelled by thrust from the exhaust of a continuous combustion of fuel jet stream: a high-altitude current of air that travels at consistently high speed around Earth in the north and the south, also known as the polar vortex

Principles of Aeronautics

jig: a kind of tool designed for the placement of components that are to be joined together in a precise orientation

Glossary

lay-up: the process of assembling a composite stack according to design parameters

jump-takeoff: a takeoff in which the blades of an autogyro are spun at a high speed then pitched to provide immediate lift, instantly “jumping” the aircraft off the ground

lift: the upward pressure experienced by wings due to differential pressure between upper and lower surfaces of the wing as it moves through a fluid medium as determined by its airfoil camber and thickness

kerosene: a distilled blend of 10 different hydrocarbons with 10 to 16 carbon atoms per molecule

loop: an in-flight maneuver in which the aircraft is made to turn and roll in one motion

Kevlar: a polyaramid material with very high tensile strength that can be made into high-strength threads and woven into structural fabrics for use in advanced composite materials

LOX/LH2 engine: an engine that uses liquid hydrogen as fuel and liquid oxygen as the oxidizer for the combustion reaction of the fuel

killer stick: essentially a heavy hardwood stick shaped to an airfoil cross section that could be thrown horizontally with deadly effect sufficient to decapitate an adult kangaroo kinetic energy: energy due to any kind of motion, be it rotation, vibration, or translation.

lunar excursion module (LEM): the segment of a lunar mission rocket that supports the functions of astronauts on the lunar surface after they have landed Mach angle: the angle between the central axis of the Mach cone and the surface of the Mach cone

Kollsman window: a secondary window at the 3 o’clock position on the face of an altimeter dial, showing a small dial with which the pilot can calibrate the altimeter with the current local pressure

Mach bands: an optical effect in which the borders of regions having different light intensities are seen as being lighter or darker; the effect is seen between the two arcs of a double rainbow as Alexander’s Dark Band

landing module (LM): the segment of a lunar mission rocket used to set the lunar excursion module down on the surface of the Moon and return the astronauts and recovered samples to lunar orbit

Mach cone: a conical region of space, extending rearward and expanding radially from the nose of an aircraft as it moves, defined by the compression of air

landplane: an airplane designed to take off and land from a solid surface

Mach number: the ratio of the velocity of an object to the velocity of sound in a particular fluid medium; Mach 1 = the speed of sound, Mach 2 = two times the speed of sound, etc.

Law of Conservation of Energy: the total energy of a system remains constant as the sum of the energies of the individual components of the system throughout any change of the system Law of Conservation of Momentum: the total momentum of a system, as the sum of the momentum of all individual parts of the system, remains constant unless the system is acted upon by a force

magnetometric: by measurements acquired using a magnetometer magnetron: an electronic device that generates microwaves mass: the amount of matter that an object comprises as an absolute intrinsic quantity

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Glossary

mass flow rate: the rate of change of mass with respect to time mean sea level (MSL): true altitude, as the average height above standard sea level where atmospheric pressure is measured in order to calibrate altitude microburst: a sudden intense downward flow of air that can cause an aircraft to lose altitude suddenly modulators and demodulators: electronic systems that function to stabilize and isolate transmission frequencies

Principles of Aeronautics

multistage rocket: a rocket essentially consisting of two, three or more individual rocket motors in a stacked conformation comprising the single rocket, each stage being used up in sequence for different thrust characteristics natural philosophy: the study of natural phenomena, what today is called “science” negative stagger: the condition in which the lower wing of a biplane is mounted farther toward the nose than is the upper wing net weight: the weight of an aircraft with no load

molecular motion: the idea that atoms and molecules are in a constant state of vibration that depends on the amount of thermal energy they contain, such that the higher their temperature the more energetic and rapid is their vibration; thus, materials expand and contract according to temperature momentum: a characteristic expressed as the product of mass and velocity that is conserved in accord with the law of conservation of energy such that the total momentum of the components of a system remains constant throughout any change of the system monocoque: an aircraft in which the chassis is integrated with the body monomeric unit: refers to a small molecular species that is joined to an untold number of other such molecules to form the corresponding polymer; for example, polyethylene can be readily seen as the head-to-tail conjoining of multiple ethylene molecules. The monomeric unit retains its essential structure within the polymer. monopropellant: a self-igniting, self-contained rocket fuel that does not require a separate oxidizer for combustion movable-wing aircraft: aircraft having wings that are able to change configuration, although remaining attached to one position on the fuselage multiengine rating: a certification allowing a pilot to fly aircraft with two or more engines

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Newtonian fluid: a fluid in which the viscous stress is proportional to the velocity gradient nonstop: a flight that does not require landing part-way to its final destination in order to refuel or change flight personnel open loop wind tunnel: a wind tunnel designed to flow fresh air through it and exhaust it out at the end of the tunnel order of the differential equation: the order of the derivative of highest order appearing in the differential equation O-ring: a circular, flexible seal typically made of silicone rubber impervious to most solvents and able to resist high heats ornithopter: aircraft that were designed to imitate birds by using the principle of flapping wings as the means of propulsion ornithoptic propulsion: flying in the manner of birds, by the flapping of wings parachute valve: a piece of fabric at the top of a balloon, in the shape of a parachute and used to release hot air for a controlled descent parameters: variables that do not enter in the differential equation but that correspond to magnitudes which can be set from the exterior

Principles of Aeronautics

particle velocity: the velocity of a small area of the medium (smaller than a wavelength but larger than an atom) that is alternately accelerated and decelerated as a wave passes passenger-hours: the number of hours of flying time multiplied by the number of passengers passenger plane: aircraft designed and put into service solely for the transport of commercial travellers; passenger plane designs are often modified to remove passenger-carrying capability to make room for material goods payload: the personnel, objects and materials being transported by a vehicle, generally not as integral parts of the vehicle

Glossary

positive control: maintaining the operation of an aircraft by the actions and commands of the pilot rather than flying free without human control positive stagger: the condition in which the upper wing of a biplane is mounted farther toward the nose than is the lower wing potential energy: the energy an object possesses due to its relative position within a frame of reference pressure gradient: the continuous change of pressure over the distance between two regions having different pressures propellant: the high-temperature gases being expelled from the exhaust nozzles of a rocket following combustion of the rocket’s fuel

pectoral muscles: a set of muscles on the upper torso of animals that are responsible for the lateral motion of arms, wings and legs

propulsion: causing movement of an object or material by the application of a force

phraseology: the standard use of words and phrases in a particular language to facilitate efficient communication

prototype: the first unit of a newly designed aircraft as the foundation piece for refinement and functioning of the design

piston aircraft: aircraft that are propelled by the action of propellers driven by an internal combustion piston engine

pushback: the movement of an aircraft from its hangar prior to a flight

pitch: the tendency of the nose of an aircraft to move up or down vertically as it moves through a fluid medium pitch angle: the angle between a propeller’s chord line and its plane of rotation pitch, roll, yaw: the three natural motions of an aircraft in flight that must be controlled by the pilot to maintain stable flight pitot tube: a tube that is open to the front of an aircraft to transmit the static air pressure outside to various instruments inside the aircraft Poiseuille flow: the steady flow of viscous fluid in a tube driven by an external pressure difference

quarterline chord: the line extending from the base of a wing at its center to a point at the tip of the wing one-quarter of the distance from its leading edge to its tailing edge radar: an electronic detection system that uses detection of a reflected radio frequency signal from a specific source radial engine: an engine in which the pistons and cylinders are arranged radially about a common crankshaft rarefaction: the process by which high pressure in a shock wave is relieved by the propagation of release waves from free surfaces into the shocked material regulation and deregulation: establishment and removal of federal rules governing the operation of the aviation industry

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Principles of Aeronautics

resistive load: any property that resists the motion of the aircraft, such as compression of the air in front of the vehicle, resulting from the aircraft’s motion

scramjet: an engine that uses its supersonic speed to compress air for combustion of hydrogen; it has no moving parts and must be launched from a supersonic carrier before it can fly on its own

resistojet: an engine that uses electrical resistance to heat exhaust gases that provide thrust as they are ejected

seaplane: an airplane designed to take off and land on water rather than solid ground

restraints: seat belts and shoulder harnesses employed to secure the pilot and other aircraft personnel in their respective seats when needed Reynolds number: defined as the product of the density of the fluid, the flow speed and an object’s length, divided by the dynamic viscosity of the fluid; the Reynolds number represents the ratio of inertial forces to viscous forces for an object moving through a fluid

servo: a device that converts electrical energy or pneumatic or hydraulic pressure into mechanical motion through an actuator shock front: a supersonic shock pulse abrupt to the point where it is often represented as a discontinuous jump of pressure, density, internal energy, and particle velocity shock metamorphism: a term used to describe changes in rocks and minerals resulting from the passage of transient, high-pressure shock waves

roll: the tendency of the body of an aircraft to rotate about its central axis as it moves through a fluid medium

sidestick: a pilot’s control “joystick” mounted to the side rather than directly in front of the pilot

rotary wing: a wing that generates lift by rotating rather than by linear motion through a fluid medium such as air

simulation: the recreation of a real or imagined event in an artificial environment rather than in a real environment

rotary-wing aircraft: aircraft in which the function of wings and propellers has been replaced by the operation of spinning blades on a rotor

sink rate: the ratio of a glider’s rate of descent to the distance covered during a flight

Rozier balloon: a combination craft consisting of a hot air balloon assisted by a helium-filled balloon to which it is attached, allowing flights of longer duration rudder: a vertical aileron on the pennage of an aircraft satellite: an object either natural or artificial that orbits a larger or more massive body in space scientific method: a method of study by which one posits a theory, designs an “experiment” in which all other variables are eliminated, and interprets the results of that experiment with regard to the original theory

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skirt: a short, cylindrical section at the base of a hot air balloon; can be thought of as the “chimney” for the gas burner providing the hot air slip: a maneuver in which an aircraft is made to turn sideways slightly while maintaining the same direction of motion, thus presenting a greater surface area in the wind direction and acting to slow the aircraft somewhat solo: a first flight by a trainee pilot without the accompaniment of an instructor pilot solution of a differential equation: a function or group of functions whose derivatives are related to the functions in the way prescribed by the differential equation

Principles of Aeronautics

sonic boom: a loud transient explosive sound caused by a shock wave preceding an object traveling at supersonic speeds sound barrier: the point of sharp increase in aerodynamic drag experienced by an object approaching the speed of sound, once thought to be a speed too difficult for aircraft to attain (a barrier) space telescope: a telescope system positioned in geosynchronous orbit about Earth, capable of much finer depth perception and resolution than terrestrial telescope systems due to the absence of atmospheric interference spark-ignition: an engine that uses an electrical spark inside a combustion chamber as the ignition source for combustion of fuel specific impulse: thrust developed per second per weight of propellant consumed under standard gravity

Glossary

supersonic: at speeds greater than the speed of sound (Mach 1 to Mach 3.5) surface area resistance: the resistance to motion through a fluid primarily by the portion of an object’s surface area that faces the direction of motion and secondarily by the drag exerted by the remaining surface area swashplate: a mechanical device on the rotor hub of a helicopter that translates control movements from the pilot’s control stick into various pitch adjustments of the rotor blades sweep: the angle at which a wing extends from the fuselage of an aircraft symmetrical: having equal dimensions on either side of a bisecting plane or axis tail-dragger: a landing gear arrangement in which there is a small, non-retractable wheel supporting the tail of an airplane

SST: acronym for supersonic transport Stalin’s purges: actions undertaken at the command of Josef Stalin to “purge” the Soviet Union of dissidents and “undesirables,” resulting in the deaths of as many as 1.5 million Soviet citizens stall: loss of the ability of wings to provide lift due to ascending at an angle that causes the pressure difference between the upper and lower surfaces of a wing to decrease and equalize stewardess: the original term for a female flight attendant streamline: the continuous flow of matter such as air or water in its direction of motion relative to a surface

tail-skid: essentially a type of short ski used in place of a wheel to support the tail of an airplane tailwheel: a wheel assembly located under the tail section of an aircraft, thus preventing the tail of the aircraft from dragging on the ground tanker: an aircraft designed to transport fuel for mid-air refueling temperature: an artificially defined thermal state of a particular object or matter; all temperatures are relative to the defined state of absolute zero on any of the four standard temperature scales terminal velocity: the maximum speed that can be attained by an object falling through a fluid medium under the force of gravity alone

structural icing: ice that forms on exterior surfaces of an aircraft when water droplets contact surfaces that are below the temperature at which water freezes

test pilot: a pilot who undertakes the dangerous job of flying repaired or experimental aircraft to determine their flight capabilities

struts and wires: the supporting and strengthening connections between the wings of a biplane

tethered balloon: one that is affixed securely to the ground and therefore has a limited altitude range

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Glossary

thermal air current: a rising column of air that is warmer and thus has less density than the air around it, such as that above an open fire thermosetting resin: a mixture of resinous chemicals that undergoes polymerization when heated, forming a solid product that can not be melted (thermoplastic polymers soften and melt when heated) throttleable rocket engine: a rocket engine whose power output can be controlled and adjusted “on the fly” thrust: the force or pressure exerted on the body of an aircraft in the direction of its motion tooling: the devices used in the formation of individual components and structures

Principles of Aeronautics

turbine: a structure as the third stage of a jet engine that is driven by the high-speed gases flowing past from the combustion chamber, that in turn powers the first-stage compressor turbulence: a mass of air moving chaotically that can cause an aircraft to respond in kind, causing passengers to be buffeted about, and potentially damaging the aircraft UAP: unidentified aerial phenomena; more recent term for UFOs. UFOs: the acronym for Unidentified Flying Objects, commonly referred to as “flying saucers,” many reports of which can be directly tied to the undisclosed operation of stealth aircraft unaccelerated: having a constant velocity in one direction only

total mechanical energy: the sum of all the kinetic and potential energies of an object in a closed system.

updraft: a current of relatively warm air moving upward away from the ground

transonic drag: drag characteristics of air flow when an aircraft is moving in the transonic speed range from about 0.8 to 1.2 times the speed of sound

variable pitch rotors: a system by which the pitch of a helicopter’s rotor blades can be changed in flight through the action of a swashplate

tricycle landing gear arrangement: the arrangement common on most present-day aircraft in which there is a single wheel-and-strut supporting the nose of the aircraft and two wheel-and-strut assemblies rearward of the center of mass supporting the rest of the aircraft

variometer: a device that measures the rate of ascent or descent of a balloon

trimotor: an airplane with three motors, typically each driving a propeller

V-12 engine: an engine with twelve cylinders and pistons arranged in two parallel banks of six in a V configuration relative to each other and attached to a common crankshaft velocity gradient: the rate at which the flow velocity changes spatially

triplane glider: a glider bearing three pairs of wings tropopause: the upper portion of the troposphere where it meets and transitions into the stratosphere troposphere: the lowest portion of the atmosphere, about 15 kilometers in thickness, extending upward from Earth’s surface

venturi: a device, particularly as a part of a carburetion system, in which the Venturi principle induces a flow of fuel into the carburetor by the action of air flowing through the carburetor body venturi principle: the movement of a fluid through a channel such as a pipe will induce additional fluid from an outside source to enter into the flow vertical acceleration: the rate at which an aircraft gains or loses altitude

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vibrational load: the frequency and amplitude of vibration experienced by a spacecraft relative to the frequency and amplitude of vibrations the craft is designed to withstand

Glossary

wind shear: a condition that exists when wind abruptly changes direction over a short distance

viscous stress: the internal frictional force per unit contact area between two parts of a fluid in nonuniform flow

wind tunnel: an enclosed tunnel-like room designed to provide a space in which forced air flow can be used to test the aerodynamic characteristics of a structure such as a model of an airplane or other object

void: an opening or hole within the body of a composite structure

wing chord or chord line: a straight line from the leading edge to the trailing edge of an airfoil

vortex: a region within a fluid medium where the matter has angular momentum as it moves in a circular pattern

winglet: a small vertical portion at the tips of wings, designed to counter the drag effect of wingtip vortices

wake: residual disturbance or turbulent air currents such as vortices caused by the movement of a body through a fluid medium

wing loading: the amount of weight being borne by a wing relative to its surface area

warp clock: a simple guide diagram that shows the angles at which fabrics are to be aligned when constructing a composite stack water/methanol injection: a fuel delivery system that injects a solution of methanol and water directly into the cylinders of an engine as a means of increasing power output as the methanol/water mixture flashes into steam and combustible methanol vapor

wing-tip vortex: a turbulent swirling motion of the air behind the wing-tip of an aircraft in flight wing-walker: a daredevil performer who walks atop the wing of an airplane, usually a biplane, while it is in flight yaw: the tendency of an aircraft to turn horizontally about its center of mass as it moves through a fluid medium

wavelength: the distance from one peak of a waveform to the next peak of that waveform

yawing moment: the torque required to pivot an airplane or its model horizontally about its center of mass

weight: the perception of the amount of matter an object comprises within a gravitational field, as the product of the mass and the acceleration due to gravity (or gravitational force)

zeppelin: a self-propelled airship having a rigid body framework and using hydrogen gas as its lifting agent

wheel sideload: force exerted perpendicularly to a wheel parallel to its axis of rotation rather than in the direction in which the wheels rotate

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Organizations Academy of Model Aeronautics 5161 E. Memorial Dr. Muncie, IN 47302 765-287-1256 modelaircraft.org Aerospace Industries Association 1000 Wilson Boulevard Suite 1700 Arlington, VA 22209-3928 703-358-1000 aia-aerospace.org American Institute of Aeronautics and Astronautics 12700 Sunrise Valley Drive Suite 200 Reston, VA 20191-5807 800-639-2422 www/aiaa.org Commemorative Air Force Dallas Executive Airport PO Box 764769 Dallas, TX 75376 commemorativeairforce.org Experience Aviation 14850 NW 44th Court Suite 203 Miami FL 33054 303-537-9291 [email protected] Experimental Aircraft Association 3000 Poberezny Rd. Oshkosh, WI 54902

800-564-6322 [email protected] eaa.org Federal Aviation Administration 800 Independence Ave., SW Washington, DC 20591 866-835-5322 www.faa.gov National Aeronautics and Space Administration 300 E Street SW #2Q66 Washington, DC 20546-0001 [email protected] National Aeronautics and Space Administration Aeronautics Research Mission Directorate nasa.gov/aeroresearch National Aeronautics and Space Administration Goddard Institute for Space Studies Columbia University Armstrong, 2880 Broadway New York, NY 10025 212-678-5500 giss.nasa.gov The Ninety-Nines, Inc. International Organization of Women Pilots www.ninety-nines.org Society of Experimental Test Pilots 44814 North Elm Ave. Lancaster, CA 93534 661-942-9574 [email protected] www.setp.org

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Principles of Aeronautics

Subject Index accelerometer, 61, 63, 64, 108, 207, 365, 368 acoustics, 121 actuator, 106, 107, 109, 201, 368 advanced propulsion, 6-12 aerial photography, 286, 553 aerial reconnaissance, 194, 197, 287, 379 aerobatics, 12-18 aerodynamic efficiency, 282, 534, 536, 583 aerodynamic flutter, 53, 57 aerodynamic forces, 26, 129, 131, 324, 528, 576 aerodynamics, 18-23 aeroelasticity, 23-26 aeronautical engineering, 26-30 aerospace industry, 30-37 aerospike rocket engine, 6, 8, 323 afterburner, 217, 329, 330, 332, 363, 374, 533, 598 ailerons, 37-39 air flight communication, 39-43 air traffic control system, 173, 175, 178, 400, 519 air transportation industry, 44-48 aircraft icing, 48-51 aircraft maintenance, 65-69 airfoil, 51-53 airframe, 33, 65, 67, 68, 69, 71, 120, 217, 231, 233, 234, 235, 241, 311, 397, 432, 519, 523, 524, 526, 547, 578, 579 airmass, 574, 575 airplane accident investigation, 53-59 airplane cockpit, 59-61 airplane manufacturers, 69-76 airplane propellers, 76-80 airplane radar, 80-85 altimeter, 83, 90, 192, 193, 194, 203, 205, 207, 316, 322, 412 anemometer, 91, 145, 149 animal flight, 92-98 Apollo missions, 341, 342, 389, 390, 436, 485 Armstrong, Neil, 98-100 astronaut training, 99, 339, 341, 342 astronautics, 263, 458, 469, 528 atmospheric circulation, 100-102 atmospheric science, 268 autogyros (gyroplanes), 102-106 autopilot, 106-109 autorotating, 102, 103 aviation fuel consumption, 109

avionics, 114-118 Avro Arrow, 118-120 axial compressor, 216, 329, 330, 331, 533 balloon flight, 182-185, 189-192, 194-200 Bernoulli principle, 122, 407, 416, 494 Bernoulli, Daniel, 121-124 biomechanics, 92, 94, 102, 114, 124, 132, 142, 145, 200, 217, 222, 231, 236, 249, 253, 280, 287, 300, 307, 318, 335, 339, 354, 378, 398, 407, 416, 445, 451, 475, 481, 564, 597 biplane, 124-126 blade pitch, 102, 104, 106, 536 Blanchard, Jean, 182 Blériot, Louis, 178, 180, 209, 279, 303, 382, 511 blimps, 127-129 blitzkrieg, 250, 376 boomerangs, 129-131 Boyle’s Law, 416, 417 Branson, Richard, 132-135 buoyancy, 127, 158, 159, 195, 196, 197, 295, 314, 316, 341, 342, 348, 349, 350, 417 business administration, 150, 247 business management, 109 carbon sink, 268, 269, 270 cargo plane, 150, 151, 153, 375, 376, 377, 506 centrifugal compressor, 216, 329, 330, 331 centrifugal force, 11, 13 Challenger (shuttle), 35, 99, 306, 390, 401, 473, 483, 589, 605 Charles’s or Gay-Lussac’s Law, 416 chemical engineering, 6, 30, 37, 44, 48, 53, 80, 254, 529 classical mechanics, 137, 156 climatology, 100 closed-loop wind tunnel, 576 Coanda effect, 280, 284 combustion chamber, 8, 213, 214, 215, 216, 325, 329, 331, 332, 334, 374, 421, 427, 434, 435, 436, 438, 439, 440, 455, 457, 531, 532, 534, 535, 541 combustion science, 295 command module (CM), 99, 390, 481, 485 composite material, 1-6 conservation of energy, 137-140 contrails (condensation trails), 140-142 control yoke, 59, 60 convection cell, 100, 101

647

Subject Index

Curtiss, Glenn H., 142-144 d’Arlandes, François Laurent, 189, 314, 386 d’Arlandes, Marquis, 183, 189, 190, 191 da Vinci, Leonardo, 145-150 DC plane family, 150-154 decalage, 124, 126 dethermalizer, 379, 380 differential calculus, 402, 403 differential equation, 154-158 dirigibles, 158-163 Discovery (shuttle), 255, 459, 474 dissipative forces, 137, 138, 139, 140 Doolittle, Jimmy, 163-165 Doppler effect, 80, 82, 361, 460, 466 Doppler radar, 61, 64 Doppler shift, 466 downdraft, 91, 95, 571, 574, 575 drag chute, 410 Earhart, Amelia, 167-171 electrical engineering, 61, 65, 69, 85 electromagnetism, 137, 264, 267 English Channel, 178-185 environmental studies, 268 ergonomics, 59, 61 error chain, 53, 54, 86 experimental aircraft, 307-313 Federal Aviation Administration (FAA), 173-178 Federal Aviation Regulations (FARs), 13, 65, 86, 89, 223, 311, 521, 545 fire-control system, 118, 119 fixed-wing aircraft, 105, 280, 281, 284, 285, 393, 441, 444, 506, 507, 537 flaps, 37-39 flight altitude, 192-194 flight balloons, 194-200 flight control systems, 200-203 flight control training, 39 flight instrumentation, 203-208 flight propulsion, 212-217 flight recorder (black box), 217-220 flight roll and pitch, 220-222 flight schools, 222-228 flight simulators, 228-231 flight testing, 231-236 flight training, 43, 65, 88, 89, 175, 208, 211, 223, 224, 225, 226, 227, 228, 231, 254, 342, 429, 430, 458, 515, 516, 520, 521, 522, 523, 542

648

Principles of Aeronautics

fluid dynamics, 236-240 fluid mechanics, 18, 247, 416, 441, 512 flying wing (all-wing or nurflügel), 240-242 forces of flight, 242-247 Fossett, Steve, 247-248 Fourier analysis, 121 free fall, 263, 265 Gagarin, Yuri, 249-250 geosynchronous orbit, 12, 212, 387, 435 GI Bill (1944), 223 Glenn, John, 253-255 glider planes, 256-260 Goddard, Robert H., 260-263 gondola, 127, 129, 159, 194, 195, 196, 197, 199, 289, 348, 349, 350, 352, 451 greenhouse gas emissions, 109, 112, 273 greenhouse gases (GHGs), 268-273 ground proximity warning system (GPWS), 62, 86, 90, 399 Guggenheim Aeronautical Laboratory at the California Institute of Technology (GALCIT), 335, 387 guidance systems, 61-65 gust loud alleviation system (GLAS), 493 gyrocompass, 62, 63 heavier-than-air (HTA) craft, 275-280 heavier-than-air (HTA) vehicle, 275 helicopters (choppers), 280-287 high-altitude flight, 287-290 high-speed flight, 291-295 Hindenburg (Luftschiff Zeppelin 129 (LZ-129)), 295-300 homebuilt aircraft, 307-313 hot-air balloons, 313-317 Hughes Aircraft Corporation, 317 Hughes, Howard R., 317-318 Hugoniot elastic limit, 460, 461 human-powered flight, 318-323 hydrodynamics, 19, 121, 122, 237, 238, 563 hyperbaric atmosphere, 287 hypersonic aircraft, 323-327 ideal gas law, 192, 193, 329, 331 inclinometer, 145, 149, 203, 206 induction icing, 48, 49 inertial navigation system, 62, 64 instrument landing system (ILS), 62, 116, 176, 569, 574 integral calculus, 402, 403 International Space Station (ISS), 11, 326, 339, 341, 390, 451, 474, 475, 476, 482, 487 interplane struts, 124

Principles of Aeronautics

Jeffries, John, 182, 183, 197, 350 jet aircraft, 31, 67, 140, 185, 187, 188, 287, 330, 331, 397, 460, 462, 464, 470, 542 jet engines, 329-334 Jet Propulsion Laboratory (JPL), 335-339 jet stream, 91, 414, 415, 572, 574, 575 Johnson Space Center (Lyndon B. Johnson Space Center), 339-343 kerosene, 109, 110, 113, 195, 204, 331, 349, 427, 428, 435 killer-stick, 129 kinetic energy, 92, 123, 137, 138, 139, 265, 266, 326, 419, 432, 457, 462, 531, 532, 533, 534, 578 Kollsman window, 192, 193, 203, 205 landing gear, 345-348 landing module (LM), 481 landing procedures, 208-212 law of conservation of energy, 137, 419, 436 law of conservation of momentum, 436, 440 Leibniz, Gottfried Wilhelm, 402, 403 Lighter-than-air (LTA) craft, 348-354 Lilienthal, Otto, 354-355 Lindbergh, Charles A., 355-358 LOX/LH2 engine, 419, 436 Luftwaffe, the, 250-253 lunar excursion module (LEM), 481 Mach angle, 362, 364 Mach bands, 359, 361 Mach cone, 362, 363, 364 Mach number, 361-364 Mach, Ernst, 359-361 magnetometric, 167, 170 magnetron, 80, 83 mass flow rate, 332, 420, 422, 425, 432, 434, 440, 533 materials science, 364-369 mean sea level (MSL), 192 mechanical engineering, 1, 3, 6, 26, 30, 37, 44, 48, 51, 53, 61, 65, 69, 76, 80, 85, 92, 98, 102, 106, 109, 114, 118, 121, 124, 127, 132, 137, 142, 145, 150, 154, 158, 163, 167, 178, 185, 189, 200, 203, 208, 212, 217, 220, 222, 228, 231, 236, 240, 242, 247, 249, 250, 253, 256, 260, 275, 280, 287, 291, 295, 300, 307, 317, 318, 323, 329, 335, 339, 345, 348, 354, 355, 361, 364, 369, 374, 378, 379, 382, 384, 387, 395, 398, 402, 407, 413, 414, 419, 425, 429, 431, 436, 441, 445, 451, 455, 459, 464, 469, 475, 481, 488, 491, 494, 499, 503, 507, 510, 512, 514, 519, 526, 528, 529, 530, 534, 537, 545, 548, 553, 559, 560, 564, 567, 569, 574, 576, 580, 584, 590, 593, 597, 603

Subject Index

Messerschmitt Aircraft, 369-373 meteorology, 100, 287, 430, 569 microburst, 86, 90, 91, 575 military aircraft, 374-378 military and commercial aviation, 30 minimal aircraft, 545, 547 Mitchell, Billy, 378-379 model airplanes, 379-382 modulators/demodulators, 475, 478 monomeric unit, 365 monoplanes, 382-384 Montgolfier brothers, 384-386 Montgolfier, Jacques-Étienne, 183, 189, 196, 275, 300, 313, 350, 384, 385 Montgolfier, Joseph-Michel, 127, 182, 189, 196, 384, 385 movable-wing aircraft, 507, 537 multiengine rating, 223, 226, 521 National Advisory Committee for Aeronautics (NACA), 395-398 National Aeronautics and Space Administration (NASA), 387-394 National Transportation Safety Board (NTSB), 398-402 negative stagger, 124, 126 Newton, Sir Isaac, 402-406 Newtonian fluid, 237, 560, 561, 563 nuclear physics, 137 open-loop wind tunnel, 576 ornithopter, 92, 109, 256, 257, 275 ornithoptic propulsion, 92, 94, 256, 275, 300, 319 paper airplanes, 407-409 parachute valve, 313, 315, 316 parachutes, 409-412 passenger plane, 35, 44, 45, 111, 150, 151, 152, 400, 497, 537 phraseology, 39, 40, 41, 43, 91 physiology, 92, 102, 114, 123, 124, 132, 142, 145, 217, 222, 231, 236, 249, 253, 268, 280, 287, 300, 307, 318, 335, 339, 354, 360, 378, 398, 407, 414, 416, 445, 451, 475, 481, 514, 519, 560, 564, 584, 597 Pilâtre de Rozier, Jean-François, 174, 183, 184, 189, 190, 191, 196, 197, 301, 314, 350, 386 pilot training, 12, 39, 59, 98, 208, 220, 223, 228, 249, 254, 256, 259, 263, 304, 313, 374, 400, 507, 510, 511, 512, 521, 522, 523, 545, 568, 569 piston aircraft, 31 pitot tube, 203, 205 plane rudders, 413-414

649

Subject Index

Poiseuille flow, 560, 562 positive stagger, 124, 126 Post, Wiley, 414-416 potential energy, 92, 122, 137, 138, 139, 265, 266, 419, 531 pressure, 416-418 propulsion technologies, 419-423 quarterline chord, 580 radial engine, 291, 292, 382, 383 ramjet engine, 425-429 resistojet, 6, 10, 421 Reynolds number, 93, 94, 238, 242, 244, 245 Rickenbacker, Eddie, 429-431 rocket, 436-441 rocket propulsion, 431-436 rotary-wing aircraft, 280, 281, 441, 507 rotorcraft (rotary-wing aircraft), 441-445 Rozier balloon, 197, 200, 348, 350, 353 Russian Space Program, 445-451 Rutan, Burt, 451-453 safety issues, 85-92 scramjet (supersonic combustion ramjet), 455-458 seaplane, 228, 292, 312, 318, 507, 509, 510, 512, 521, 601 Shepard, Alan, 458-459 shock front, 364, 460, 461, 462, 463, 466, 467 shock wave, 459-464 short takeoff and landing (STOL), 377, 441, 443, 545, 601 sidestick, 59 Sikorsky, Igor, 464-466 sonic boom, 9, 29, 30, 290, 294, 460, 463, 466, 468, 469, 497, 498 sound barrier, 466-469 space exploration, 31, 34, 35, 387, 388, 390, 391, 392, 432, 445, 447, 448, 449, 481, 487 space shuttle, 469-475 spacecraft engineering, 475-481 spaceflight (space exploration or space travel), 481-488 stabilizers, 488-491 stealth bomber (B-2 or Spirit bomber or Flying Wing), 491-494 structural icing, 48, 49 supersonic aircraft, 494-498 supersonic jetliners, 498-499 supersonic jets, 499-501 supersonic transport (SST), 27, 73, 290, 415, 468, 494, 497, 498, 499, 500, 530, 581 swashplate, 281, 283, 285, 465

650

Principles of Aeronautics

tail designs, 503-507 tail-dragger, 345, 347 tail-skid, 510, 511 takeoff procedures, 507-510 taxiing procedures, 510-512 temperature, 512-514 Tereshkova, Valentina, 514-519 test pilot, 16, 98, 99, 230, 250, 255, 294, 309, 311, 396, 397, 449, 456, 458, 515, 518, 519, 586, 589, 597, 603, 604 Thaden, Louise, 586 thermodynamics, 48, 123, 329, 512 throttleable rocket engine, 323, 325 total mechanical energy, 137, 139 Transport Canada Civil Aviation (TCCA), 310 tricycle landing gear arrangement, 345, 348, 510, 540 triplanes, 526-528 tropopause, 100, 575 Tsiolkovsky, Konstantin, 528-529 Tupolev, Andrei Nikolayevich, 529-530 turbofans, 530-534 turbojets, 530-534 turboprops (propjets or jetprops), 534-536 ultralight aircraft, 545-548 Unidentified Aerial Phenomena (UAP), 548-553 unidentified flying object (UFO), 548, 551 Uninhabited Aerial Vehicles (UAVs), 553-557 updraft, 91, 184, 574, 575 US Army Air Forces (USAAF), 586, 603 V-12 engine, 291, 292 variometer, 259, 313, 316 velocity gradient, 560, 561 Venturi principle, 48 Verne, Jules, 559-560 vertical takeoff and landing (VTOL), 280, 281, 441, 444, 599 viscosity, 560-564 viscous stress, 560, 561 von Kármán, Theodore, 335, 568 von Richthofen, Manfred (Red Baron), 564-565 wake turbulence (vortex hazard and Kármán’s vortex street), 567-569 warp clock, 1, 2, 3 weather conditions, 569-574 Weightless Environment Training Facility (WETF), 342 wind shear, 574-576 wind tunnels, 576-580 wing designs, 580-584

Subject Index

Principles of Aeronautics

wing-flapping propulsion, 319 winglet, 387, 392, 580, 583 wing-tip vortex, 140, 141 women in aviation, 584-590 Wright brothers, 590-593 Wright Flyer (Flyer 1, Aerostat, and the Flying Machine), 593-596

Wright, Wilbur and Orville, 31, 37, 109, 143, 174, 510, 590 X-Planes (X-1 to X-45), 597-602 yawing moment, 22, 576, 577 Yeager, Chuck, 603-606

651

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