Introduction to Flight Testing

Aerospace Series

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James W. Gregory

The Ohio State University

Tianshu Liu

Western Michigan University

This edition first published 2021

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Library of Congress Cataloging‐in‐Publication Data

Names: Gregory, James W., author. | Liu, T. (Tianshu), author.

Title: Introduction to flight testing / James W. Gregory, The Ohio State University; Tianshu Liu, Western Michigan University.

Description: First edition. | Hoboken, NJ : Wiley, 2021. | Includes bibliographical references and index.

Identifiers: LCCN 2020048350 (print) | LCCN 2020048351 (ebook) | ISBN 9781118949825 (hardback) | ISBN 9781118949795 (adobe pdf) | ISBN 9781118949801 (epub)

Subjects: LCSH: Airplanes–Flight testing–Textbooks.

Classification: LCC TL671.7 .G74 2021 (print) | LCC TL671.7 (ebook) | DDC 629.134/53–dc23

LC record available at https://lccn.loc.gov/2020048350

LC ebook record available at https://lccn.loc.gov/2020048351

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Dedicated to

Deb, Alina, and Maggie – J. W. G.

Ruomei and Ranya – T. L.

About the Authors

James W. Gregory is professor and chair of the Department of Mechanical and Aerospace Engineering at The Ohio State University (OSU). He received his Bachelor of Aerospace Engineering from Georgia Tech in 1999 and his PhD in Aeronautics and Astronautics from Purdue University in 2005. He has been a faculty member at OSU since 2008 and served as Director of OSU's Aerospace Research Center from 2017 to 2020. In 2017, he led a team of research staff and students to set FAI/NAA‐sanctioned world records for speed and distance for an autonomous drone. He teaches classes at OSU on Flight Test Engineering and Introduction to Aerospace Engineering. Prof. Gregory also recorded a series of video lectures on the Science of Flight, produced by the Great Courses. He is an instrument‐rated commercial pilot and holds a remote pilot certificate.

Tianshu Liu is a professor in Department of Mechanical and Aerospace Engineering at Western Michigan University (WMU). He received a PhD in Aeronautics and Astronautics from Purdue University in 1996. He was a research scientist at NASA Langley Research Center from 1999 to 2004. His research focuses on experimental aerodynamics and fluid mechanics, particularly on global measurement techniques for various physical quantities such as pressure, temperature, heat flux, skin friction, velocity, aeroelastic deformation, and aerodynamic force. He teaches classes in aerodynamics and flight testing at WMU.

Series Preface

The field of aerospace is multidisciplinary and wide‐ranging, covering a large variety of platforms, disciplines, and domains, not merely in engineering but in many related supporting activities. These combine to enable the aerospace industry to produce innovative and technologically advanced vehicles. The wealth of knowledge and experience that has been gained by expert practitioners in the various aerospace fields needs to be passed onto others working in the industry and also researchers, teachers, and the student body in universities.

The Aerospace Series aims to be a practical, topical, and relevant series of books aimed at people working in the aerospace industry, including engineering professionals and operators, engineers in academia, and allied professions such as commercial and legal executives. The range of topics is intended to be wide‐ranging, covering design and development, manufacture, operation and support of aircraft, as well as topics such as infrastructure operations and current advances in research and technology.

Flight testing is a vital part of the certification and validation phase of all new aircraft and is performed to determine or verify the performance and handling qualities. Although the flight characteristics are predicted in the design and development stages of new aircraft programs, the real‐world capabilities are not known until the aircraft is flown and tested. Most aircraft flight testing programs are focused on meeting airworthiness certification requirements and demonstrate all aspects of the flight vehicle's performance and handling characteristics to ensure flight safety.

This book, Introduction to Flight Testing, is aimed at advanced‐level undergraduate students, graduate students, and practicing engineers who are looking for an introduction to the field of flight testing. With a focus on light aircraft and UAVs, the book covers the engineering fundamentals of flight, including the flight environment, aircraft performance and stability and control, combined with the piloting, sensors, and digital data acquisition and analysis required to perform flight tests. This book is a very welcome addition to the Wiley Aerospace Series.

October 2020

Preface

The goal of this book is to provide an accessible introduction to the fascinating and intriguing world of aircraft flight testing. This unique discipline directly straddles the domains of engineering and piloting, requiring knowledge of both the theory and practice of flight. Our target audience is advanced‐level undergraduate students, beginning graduate students, and practicing engineers who are looking for an introduction to the field of flight testing. Flight testing professionals (engineers, pilots, managers, etc.) may also find this to be a helpful resource if they wish to solidify their understanding of the fundamentals beyond what is provided in most other flight testing resources. We have attempted to write this book in an engaging, conversational style that invites the reader into understanding the fundamental principles.

Both authors teach a senior‐year technical elective course at our home universities on the topic of flight test engineering. Within this context, we have found that students best learn the material when they actively engage with flight testing practice. Experiencing flight in an aircraft is the best way to develop a tacit understanding of the principles of flight, to augment and deepen the intellectual knowledge of engineering practice that students receive in the classroom. In working with our senior‐year engineering students, we have developed the following learning objectives for our courses, as well as for this book. Our aim is that readers of this book will:

• Have an appreciation for the purpose, scope, and magnitude of historical and modern flight test programs in the commercial and military aircraft sectors.

• Understand the theoretical foundations of the flight environment, aircraft performance, and stability and control as it applies to flight testing.

• Be familiar with aircraft cockpit instrumentation, supplemental sensors for flight testing, and digital data acquisition techniques.

• Be able to plan a flight test to evaluate the performance or handling qualities of a general aviation or unmanned aircraft.

• Have the ability to coordinate with an experienced pilot to successfully conduct the flight test.

• Have the knowledge and background needed to perform postflight analysis and data reduction.

• Be able to professionally and succinctly communicate the findings of a flight test program via oral and written communication.

• Have a general familiarity with piloting, aviation weather, and flight planning.

Thus, this book is best approached in conjunction with flight in an actual aircraft. No specially instrumented aircraft are needed in order to do most of the flight tests in this book. No exceptional training is needed in order to fly the basic maneuvers described in this book. In fact, all of the procedures described herein are routine maneuvers that are encountered often in piloting practice. An interested reader can simply head to their nearest airport and work with a qualified, professional pilot (e.g., a certified flight instructor) to conduct the flights described here. In our educational contexts, we collaborate with flight instructors in the aviation programs at The Ohio State University and Western Michigan University to conduct flight tests for our students, but any flight instructor would be capable of performing these basic maneuvers.

We must be sure to emphasize that flying an aircraft involves elevated risk compared to other routine activities in daily life. It is critical for the pilot in command to always maintain positive control of the aircraft and to maintain flight within the performance envelope of the aircraft. All operating limitations of the Pilot's Operating Handbook, as well as all regulatory limitations and best practices for safety of flight, must be observed. Flights should be conducted with a minimum crew of two, where the pilot is solely focused on safe operation of the aircraft. Since precise flying is important for acquiring quality data, the pilot should be experienced – a pilot with a commercial license is likely a safe minimum standard for piloting credentials. The second crew member – the flight test engineer – should be dedicated to acquiring flight data and not have any responsibilities related to ensuring safe operation of flight. The flight test engineer is essentially a passenger for these flights, and all piloting authority and responsibility for the flight rest with the pilot in command. Chapter 6 of this book describes the principles of flight test safety and risk management, which form an essential foundation for the flight test profession. Fly safely!

While there are several other resources on flight testing already available, we saw a specific need for this textbook. Some of the existing resources are targeted toward flight testing professionals and may not be as accessible to the general student. Other resources have become dated, with the relatively recent rewrite of airworthiness certification standards for normal category airplanes (Title 14 of the U.S. Code of Federal Regulations, Part 23). In writing this book, we have sought to provide a modern and accessible resource for flight test educators and students, with several unique features that we hope will set it apart as a helpful and leading resource. Our primary audience is engineering students, with the goal of drawing connections between engineering practice and flight testing experience. We have provided guidelines on how to conduct each flight test, which will guide the reader in the flight test planning process. We have also included unique chapters on digital data acquisition and analysis techniques, uncertainty analysis, and unmanned aircraft flight testing. These are all modern topics that are not covered in the flight testing literature, but are now critical topics. And, with the proliferation of smartphones (repurposed as digital data acquisition devices in manned flight testing) and drones, the modern principles of flight testing are more accessible than ever.

The focus of our book is predominantly on light aircraft (small general aviation airplanes) and small unmanned aircraft. We have homed in on this subset of aviation since these aircraft are generally accessible to the public. While our focus is on light aircraft, the principles described here are equally applicable to all regimes of flight testing. This book provides an introduction, while other resources can be consulted for more advanced topics. The discussion here has been tailored to academic classroom instruction to convey the main principles of flight testing, rather than as a “field manual” for definitive best practices in all situations for flight testing. Having said that, we have made a reasonable effort to align the guidance provided here with accepted best practices. Also, we have decided to omit discussions of spin flight testing and flutter flight testing. These are significant and important topics in flight testing practice, but these are hazardous flight tests. We wish to encourage the reader to engage only with the safer dimensions of flight testing as an entry point.

Throughout this book, we'll predominantly use English units. This is primarily because aviation practice in North America has mostly converged on English units. For example, most air traffic control organizations around the world assign altitude in units of feet and airspeed in units of knots. While SI units are generally preferable in science and engineering environments, we'll generally work with the aviation standard. We view this choice as an educational opportunity for the reader to become acquainted with and proficient in multiple unit systems. There is clear pedagogical value in learning how to quickly convert between and track various units – we hope that students and professional engineers alike will become comfortable with all units and how to convert between them. Appendix B includes a range of unit conversion factors and discussion on best practices for handling units in aviation and engineering practice.

Furthermore, we have avoided embedding implied units into equations. This practice can be convenient for some cases when input and output units for a formula are well established and clearly documented – this can facilitate situations where rapid computations are needed without encumbering the analysis with unit conversions. However, in many cases this practice leads to confusion or ambiguity since the input and output units are seldom clearly documented or agreed upon. Another disadvantage of embedded units is that constants must be embedded in the formula, which have no basis on the physics. This can be confusing to a student who is exposed to theory for the first time. Finally, embedded units force the reader into one specific unit system. Our approach with equations that are unit‐agnostic will allow the reader to use either English or SI consistent units as desired. Thus, the assumption throughout this text (unless otherwise specified) is that equations are based on standard, consistent units.

To the reader – thank you for picking up this book. We are passionate about flight testing and are eager to share our deep interest in this domain with you. We hope that this book will be rewarding, enriching, and fascinating.

September 6, 2020

Acknowledgements

The authors are grateful for a wide array of colleagues, collaborators, and students who have shaped our thinking and provided input and feedback on this book.

JG would like to thank Profs. Stacy Weislogel and Gerald M. Gregorek, who as pioneers of educational flight testing in the 1970s were inspirations for this work. Prof. Hubert C. “Skip” Smith was also generous with ideas and resources along the way. The flight education department and colleagues in Aviation at The Ohio State University have been extremely helpful in providing tactical support over the years – D. Gelter, D. Hammon, B. Mann, S. Morgan, S. Pruchnicki, C. Roby, B. Strzempkowski, and S. Young. Special thanks go to Profs. Jeffrey Bons and Cliff Whitfield, who helped review significant portions of a near‐final version of the manuscript. (Any remaining errors or inaccuracies are solely the responsibility of the authors). Portions of this book were written in 2014–15 while JG was on sabbatical at the Technion in Israel; the support of Ohio State University and the Fulbright foundation is gratefully acknowledged.