James W. Gregory Introduction to Flight Testing

(2) Offer aerospace engineering students the context for connecting engineering theory with practice through guided flights in an aircraft. This provides the student with a visceral, empirical way of connecting their theoretical knowledge of flight with practical knowledge. The goal is for the student to develop a tacit understanding of flight beyond the explicit knowledge gleaned in traditional classrooms.

(3) Introduce the concepts and practice of digital data acquisition and signal processing, which is the underpinning of complex industrial and governmental flight test programs. These concepts are typically not taught in the undergraduate aerospace curriculum, but are important for knowing how to acquire and analyze flight test data using advanced, micro‐scale sensors and digital data acquisition systems.

(4) Provide an overview of many of the foundational flight test topics encountered in performance flight testing. Individual chapters address each topic in turn, starting with the theoretical basis for that aspect of aircraft performance and moving on to flight test methods for acquiring and analyzing data for each performance metric.

This text is partitioned into two main segments – the first half of the book (Chapters 1–6) deals with preliminary content and fundamental principles, while the second half (Chapters 7–16) covers a series of flight test topics in detail. The flight tests covered here focus predominantly on the performance and stability characteristics of an aircraft. We predominantly focus on light general aviation aircraft and UAVs, since these are accessible to most students, and optimal learning takes place when a student can experience flight testing firsthand. The material is designed to be accessible such that a student can go with a qualified pilot in nearly any general aviation aircraft and acquire meaningful flight test data. Dedicated flight test instrumentation, modifications to the aircraft, or expensive hardware is not required. Thus, many of the flight test methods presented here may be simplified relative to what is done in industry.

Figure 1.14 Ohio State University students Greg Rhodes and Jennifer Haines following turn performance flight testing in a Piper PA‐28R at the Ohio State University Airport.

Source: Courtesy of Greg Rhodes and Jennifer Haines.

This textbook should not be regarded as a definitive or even advisory source on how to conduct flight testing. Instead, this book should be considered a general introduction to the ideas, scientific principles, theoretical foundations, and some of the best practices associated with flight testing. We provide a mix of aircraft performance theory with flight testing methods. Our goal is to invite the student or practitioner into understanding the physical fundamentals underlying flight testing – this will enable the reader to more fully appreciate why flight testing is done the way it is, to spot errors or problems in theory or procedures, and to know how to adapt established practices to unanticipated circumstances or new vehicle concepts. So, our aim is to provide a general overview and introduction to flight testing: a general idea of the nature of the field and a sound theoretical basis for what is done. We hope that this book will be a good first step as preparation for entry into the flight testing domain, where more detailed methods can be picked up on the way.

Official publications, standards, and advisory documents from the relevant civil aviation authority must be regarded as the definitive source for guidance on how to safely conduct flight testing and how to provide sufficient information to comply with the certification requirements. In the United States, this documentation is primarily found in 14 CFR 23, FAA Advisory Circular 23‐8C, and any consensus standards accepted by the FAA (such as standards produced by ASTM International's F44 committee on General Aviation Aircraft). Other helpful sources of procedural and practical information are found in Hamlin (1946), Smith (1981), Stoliker et al. (1996), Stinton (1998), Kimberlin (2003), Ward et al. (2006, 2007), McCormick (2011), Mondt (2014), Corda (2017), and the publicly available flight test guides from the governmental flight test organizations (Herrington et al. 1966; USAF TPS 1986; USN TPS 1977, 1997; Gallagher et al. 1992; Stoliker 1995; Olson 2003). More advanced details on system identification for aircraft are available from Klein and Morelli (2006), Tischler and Remple (2012), or Jategaonkar (2015).

Flight testing is a fascinating, exhilarating field of aerospace engineering. It's incredibly rewarding to connect theory with practice, and we hope that the thoughts we provide here will draw students into a deeper understanding of flight through the intertwined approaches of theory and flying in flight test. And we hope to inspire the next generation of flight test professionals (Figure 1.14) to pursue this fascinating line of work. Hang on for a wild ride!

Nomenclature

MMO

maximum operating Mach number

VMO

maximum operating limit speed

VNO

maximum structural cruising speed

Acronyms and Abbreviations

AAF

Army Air Forces

CFR

Code of Federal Regulations

CG

center of gravity

DT&E

developmental test and evaluation

FAA

Federal Aviation Administration

HARV

high alpha research vehicle

LEX

leading‐edge extension

NACA

National Advisory Committee for Aeronautics

NASA

National Aeronautics and Space Administration

OT&E

operational test and evaluation

PSP

pressure‐sensitive paint

UAV

unmanned aerial vehicle

References

ASTM Committee F44 on General Aviation Aircraft, Subcommittee F44.20 on Flight. (2017). Standard Specification for Weights and Centers of Gravity of Aircraft. F3082/F3082M‐17, approved 15 October 2017, West Conshohocken, PA: ASTM International. doi: https://doi.org/10.1520/F3082_F3082M-17.

ASTM Committee F44 on General Aviation Aircraft, Subcommittee F44.20 on Flight. (2018a). Standard Specification for Performance of Aircraft. F3179/F3179M‐18, approved 1 May 2018, West Conshohocken, PA: ASTM International. doi: https://doi.org/10.1520/F3179_F3179M-18.

ASTM Committee F44 on General Aviation Aircraft, Subcommittee F44.20 on Flight. (2018b). Standard Specification for Aircraft Handling Characteristics. F3173/F3173M‐18, approved 1 December 2018, West Conshohocken, PA: ASTM International. doi: https://doi.org/10.1520/F3173_F3173M-18.

ASTM Committee F44 on General Aviation Aircraft, Subcommittee F44.20 on Flight. (2019a). Standard Specification for Establishing Operating Limitations and Information for Aeroplanes. F3174/F3174M‐19, approved 1 May 2019, West Conshohocken, PA: ASTM International. doi: https://doi.org/10.1520/F3174_F3174M-19.

ASTM Committee F44 on General Aviation Aircraft, Subcommittee F44.20 on Flight. (2019b). Standard Specification for Low‐Speed Flight Characteristics of Aircraft. F3180/F3180M‐19, approved 1 May 2019, West Conshohocken, PA: ASTM International. doi: https://doi.org/10.1520/F3180_F3180M-19.

Barlow, J., B., Rae, W. H., and Pope, A., 1999, Low‐Speed Wind Tunnel Testing, 3, New York: Wiley.

Corda, S. (2017). Introduction to Aerospace Engineering with a Flight Test Perspective. Chichester, West Sussex, UK: Wiley.

Cummings, R.M., Mason, W.H., Morton, S.A., and McDaniel, D.R. (2015). Applied Computational Aerodynamics. Cambridge, UK: Cambridge University Press.

Federal Aviation Administration (2003). Small Airplane Certification Compliance Program, Advisory Circular 23‐15A. Washington, DC: U.S. Department of Transportation.

Federal Aviation Administration (2011). Flight Test Guide for Certification of Part 23 Airplanes, Advisory Circular 23‐8C. Washington, DC: U.S. Department of Transportation.

Fisher, D.F., Del Frate, J.H., and Richwine, D.M. (1990). In‐flight flow visualization characteristics of the NASA F‐18 high alpha research vehicle at high angles of attack. NASA Technical Memorandum 4193 http://hdl.handle.net/2060/19910010742.

Gallagher, G.L., Higgins, L.B., Khinoo, L.A., and Pierce, P.W. (1992). Fixed Wing Performance. USNTPS‐FTM‐No. 108,. Patuxent River, MD: Naval Air Warfare Center.

Gorn, M.H. (2001). Expanding the Envelope: Flight Research at NACA and NASA. Lexington, KY: University Press of Kentucky.

Hallion, R.P. (1972). Supersonic Flight; The Story of the Bell X‐1 and Douglas D‐558. New York: Macmillan.

Hallion, R.P. and Gorn, M.H. (2003). On the Frontier: Experimental Flight at NASA Dryden. Washington, DC: Smithsonian Books.

Hamlin, B. (1946). Flight Testing Conventional and Jet‐Propelled Airplanes. New York: Macmillan Company.

Herrington, R. M., Shoemacher, P. E., Bartlett, E. P., and Dunlap, E. W. (1966). Flight Test Engineering Handbook, USAF Technical Report 6273, Edwards AFB, CA: US Air Force Flight Test Center. Defense Technical Information Center Accession Number AD0636392, https://apps.dtic.mil/docs/citations/AD0636392.

Jategaonkar, R.V. (2015). Chapter 2. In: Flight Vehicle System Identification: A Time‐Domain Methodology, 2e. Reston, VA: American Institute of Aeronautics and Astronautics.

Jenkins, D. R., Landis, T., and Miller, J. (2003). American X‐Vehicles: An Inventory – X‐1 to X‐50. Monographs in Aerospace History No. 31, NASA SP‐2003‐4531.

Jumper, E.J., Gordeyev, S., Davalieri, D. et al. (2015). Airborne Aero‐Optics Laboratory – Transonic (AAOL‐T). AIAA 2015‐0657,. Kissimmee, FL: American Institute of Aeronautics and Astronautics, 53rd Aerospace Sciences Meeting.

Kimberlin, R.D. (2003). Flight Testing of Fixed‐Wing Aircraft. Reston, VA: American Institute of Aeronautics and Astronautics.

Klein, V. and Morelli, E.A. (2006). Chapter 9. In: Aircraft System Identification: Theory and Practice. Reston, VA: American Institute of Aeronautics and Astronautics.

Lachendro, N. (2000). Flight testing of pressure sensitive paint using a phase based laser scanning system. MS thesis, West Lafayette, IN: School of Aeronautics and Astronautics, Purdue University.

McCormick, B.W. (2011). Introduction to Flight Testing and Applied Aerodynamics. Reston, VA: American Institute of Aeronautics and Astronautics.

McCrink, M. H. and Gregory, J. W. (2021). Design and development of a high‐speed UAS for beyond visual line‐of‐sight operations. Journal of Intelligent & Robotic Systems 101: 31. https://doi.org/10.1007/s10846-020-01300-2.

Miller, J. (2001). The X‐planes – X‐1 to X‐45, 3e. Hinckley, UK: Midland Publishing.

Mondt, M.J. (2014). The Tao of Flight Test: Principles to Live By. Boone, IA: J. I. Lord.

Olson, W.M. (2003). Aircraft Performance Flight Testing, AFFTC‐TIH‐99‐01. Edwards AFB, CA: Air Force Flight Test Center.

Peebles, C. (2014). Probing the Sky: Selected NACA Research Airplanes and their Contributions to Flight. Washington, DC: National Aeronautics and Space Administration.

Reader, K. R. and Wilkerson, J. B. (1977). Circulation Control Applied to a High Speed Helicopter Rotor. Report 77‐0024, David W. Taylor Naval Ship Research and Development Center, Bethesda, MD: DTIC accession number ADA146674.

Smith, H.C. (1981). Introduction to Aircraft Flight Test Engineering. Basin, WY: Aviation Maintenance Publishers.

Stinton, D. (1998). Flying Qualities and Flight Testing of the Airplane. Reston, VA: American Institute of Aeronautics and Astronautics.

Stoliker, F.N. (1995). Introduction to Flight Test Engineering, AGARD Flight Test Techniques Series, AGARD‐AG‐300, vol. 14. Neuilly‐sur‐Seine, France: Advisory Group for Aerospace Research and Development.

Stoliker, F., Hoey, B., and Armstrong, J. (1996). Flight Testing at Edwards: Flight Test Engineers' Stories 1946–1975. Lancaster, CA: Flight Test Historical Foundation.

Tavoularis, S. (2005). Measurement in Fluid Mechanics. Cambridge, UK: Cambridge University Press.

Tischler, M.B. and Remple, R.K. (2012). Aircraft and Rotorcraft System Identification, 2e. Reston, VA: American Institute of Aeronautics and Astronautics.

U.S. Code of Federal Regulations. (2021). Airworthiness Standards: Normal Category Airplanes. Title 14, Chapter I, Subchapter C, Part 23 (14 CFR §23). http://www.ecfr.gov (accessed 01 January 2021).

U.S. Air Force Test Pilot School (1986). Performance Phase Textbook, vol. 1, USAF‐TPS‐CUR‐86‐01. Edwards AFB, CA: US Air Force Test Pilot School.

U.S. Naval Test Pilot School. (1977). Fixed Wing Performance: Theory and Flight Test Techniques. USNTPS‐FTM‐No. 104, Patuxent River, MD: Naval Air Test Center.

U.S. Naval Test Pilot School. (1997). Fixed Wing Stability and Control: Theory and Flight Test Techniques. USNTPS‐FTM‐No. 103, Patuxent River, MD: Naval Air Warfare Center.

Ward, D., Strganac, T., and Niewoehner, R. (2006). Introduction to Flight Test Engineering, 3e, vol. 1. Dubuque, IA: Kendall/Hunt.

Ward, D., Strganac, T., and Niewoehner, R. (2007). Introduction to Flight Test Engineering, vol. 2. Dubuque, IA: Kendall/Hunt.

Warwick, G. (2017). Ohio State pushes speed envelope in UAS Research. Aviation Week and Space Technology 18 (2017): 44.

Wolfe, T. (1979). The Right Stuff. New York: Farrar, Straus and Giroux.

Yeager, C. and Janos, L. (1985). Yeager: An Autobiography. Toronto: Bantam Books.

Young, J.O. (1997). Meeting the Challenge of Supersonic Flight. Edwards AFB, CA: Air Force Flight Test Center.

2

The Flight Environment: Standard Atmosphere

In this chapter, we will discuss the properties of the environment for flight testing – Earth's atmosphere. It is critical to understand the nature of the atmosphere, since aircraft performance depends significantly on the properties of air. For example, the lift produced by the aircraft is proportional to the air density, and the amount of power produced by an internal combustion engine also varies with density. For these two reasons, aircraft performance decreases as density decreases. We will see in this chapter that density decreases with altitude, so key aircraft performance metrics such as takeoff distance, rate of climb, acceleration, etc. all degrade with altitude. Since aircraft performance depends significantly on the local properties of air, we need some way to factor out altitude effects. We also need to be able to predict the performance of an aircraft as a function of altitude, once its baseline performance is known. Thus, we need an agreed‐upon standard definition of the properties of the atmosphere – this is the standard atmosphere. Definition of the standard atmosphere allows us to evaluate and compare aircraft performance in a consistent manner, no matter what the altitude is.

The important atmospheric parameters are the atmospheric temperature, pressure, density, and viscosity, which depend on the distance from the earth surface, geographic location, and time. In order to describe the atmosphere in a universal way, a standard atmosphere model has been developed, where the atmospheric parameters are determined as the univariate functions of altitude from sea level. Temperature exhibits strong variations with time of year, geographic location, and altitude. And, on a daily basis, temperature depends on current weather conditions in a stochastic manner. It is impossible to develop a first‐principles model that will capture all of these parameters that influence the temperature profile; thus, the standard temperature profile is determined from an average of a large ensemble of atmospheric measurements. The variation of pressure with altitude, however, is rigorously described by some basic physical principles – we will derive these here. In fact, pressure is so intricately and reliably linked to altitude that aircraft altimeters measure pressure and convert the measurement to an indicated altitude through the definition of the standard atmosphere. Density is related to the estimated value of temperature and the derived value of pressure via the ideal gas law. Finally, we will provide a relationship that determines the viscosity of air as a function of temperature. Based on these developments, we will define a standard atmosphere that can be expressed in tabular form, or equations coded for computational analysis. This chapter will start with a physical description of the atmosphere and then present a detailed development of the standard atmosphere. Most of the development of the standard atmosphere presented in this chapter will rely on SI units, since this was the unit system used to define the standard atmosphere and the boundaries of atmospheric regions. The input and output of the standard atmosphere can be easily converted from SI units to English units as needed.

2.1 Earth's Atmosphere

Earth's atmosphere is an envelope of air surrounding the planet Earth, where dry air consists of 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.031% carbon dioxide, and small amounts of other gases (NOAA et al. 1976). In addition, air contains a small amount of water vapor (about 1%). The entire atmosphere has an air mass of about 5.15 × 1018 kg (1.13 × 1019 lb), and three quarters of the total air mass are contained within a layer of about 11 km (∼36,000 ft) from the Earth's surface.

There is a general stratification of Earth's atmosphere, which leads to the definition of distinct regions of the atmosphere: the troposphere (0–11 km), stratosphere (11–50 km), mesosphere (50–85 km), and thermosphere (85–600 km). The atmosphere becomes thinner as the altitude increases, and there is no clear boundary between the atmosphere and outer space. However, the Kármán line has been defined at 100 km and is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km. Aircraft propelled by internal combustion engines and propellers are generally limited to operating in the troposphere, while jet‐propelled aircraft routinely operate in the stratosphere.

Figure 2.1 illustrates the bottom three layers of Earth's atmosphere, which is where all atmospheric flight vehicles conduct flight. The delineation between the various regions of the atmosphere is based on historical measurements of temperature profiles, which lead to distinct regions with different temperature lapse rates. In the troposphere (the layer of the atmosphere nearest the surface), the air temperature generally decreases linearly with the altitude. This temperature reduction with altitude is due to the increasing distance from Earth and a concomitant reduction in heating from Earth's surface. Weather phenomena are directly dependent on this temperature reduction with altitude, causing most storms and other weather phenomena to develop and reside within the troposphere. The dividing boundary between the troposphere and the next layer (the stratosphere) is called the tropopause, at 11 km. Within the lower portion of the stratosphere (11–20 km), the air temperature remains constant; it then increases with altitude in the upper stratosphere (20–50 km), due to absorption of the sun's ultraviolet radiation by ozone in this region of the atmosphere.

Figure 2.1 The layers of Earth's atmosphere.

In contrast with the temperature–altitude profile, the variation of pressure with altitude is highly repeatable and deterministic. Air pressure continually decreases with altitude from Earth's surface all the way to the edge of the atmosphere. The primary reason for this is the action of Earth's gravitational acceleration on air, causing a given mass of air to exert a force on the air below it. Air at a given altitude must support the weight of all of the air mass above it, and it balances this force by pressure. As altitude increases, there is less air mass above that altitude, so there is less force (weight) acting on the air at that point and the pressure decreases. Thus, pressure decreases as altitude increases. We will discuss this physical mechanism in greater detail in Section 2.2, when we derive an expression for the variation of pressure with altitude.

2.2 Standard Atmosphere Model

A standardized model of the atmosphere allows scientists, engineers, and pilots in the flight testing community to have a commonly agreed‐upon definition of the properties of the atmosphere. The definition of the standard atmosphere includes the variation of gravitational acceleration, temperature, pressure, density, and viscosity as a function of altitude. There is actually more than one definition of the standard atmosphere: the U.S. Standard Atmosphere (NOAA et al. 1976) and the International Civil Aviation Organization (ICAO) Standard Atmosphere (ICAO 1993). Thankfully, the two definitions are identical at lower altitudes where aircraft fly – the only differences are in the upper stratosphere and beyond. Our discussion here will generally follow the development of the U.S. Standard Atmosphere (NOAA et al. 1976).[1]

2.2.1 Hydrostatics

The development of the standard atmosphere directly results from the hydrostatic equation, which is derived here based on a control volume analysis. Figure 2.2 illustrates an arbitrary control volume, measuring dx × dy × dhG, and the forces acting upon it (here, hG is the geometric altitude, or height above mean sea level (MSL)). The forces due to pressure acting on all of the side walls balance one another out in this static equilibrium condition, and we will consider only the forces acting in the vertical direction. The force acting upward on the bottom surface of the control volume is the pressure, p, times the cross‐sectional area dx dy. Similarly, on the top surface, we have a force of (p + dp)dx dy acting downward. (Here, the differential pressure dp accounts for pressure changes in the vertical direction.) Finally, we have the weight of the air inside the control volume acting downward, W = mg, where g is the local gravitational acceleration and the mass of the air inside the control volume can be found from the product of density and the volume,

(2.1)

Figure 2.2 Forces acting on a hydrostatic control volume.

Summing all the forces in the vertical direction and setting equal to zero (from Newton's second law applied to a stationary control volume), we obtain

(2.2)

Canceling terms leads to

(2.3)

which is the hydrostatic equation as a function of geometric altitude. This expression mathematically expresses the physical explanation that we presented earlier for the variation of pressure with altitude. As altitude increases (positive dhG), the minus sign indicates that the pressure decreases (negative dp). The ρg term is an expression of the weight of the air inside the control volume, which is the reason for the pressure difference.

2.2.2 Gravitational Acceleration and Altitude Definitions

As we proceed with the development of the standard atmosphere, we must consider how gravitational acceleration varies with altitude. From Newton's law of universal gravitation, we know that gravitational acceleration varies inversely with the square of the distance to the center of the earth. Thus, we have