James W. Gregory Introduction to Flight Testing

There are several different kinds of flight testing, driven by the objective of a particular program. These motivations include scientific research, development of new technologies or experimental capabilities, evaluation of operational performance, or airworthiness certification of new aircraft for commercial use. Other kinds of flight tests include production flight test (first flight of a new airframe of an already certified type, to verify compliance with design performance standards), systems flight test (new systems installed, new external stores on a fighter aircraft that must be tested for separation, new avionics systems), and post‐maintenance test flight. Here, we'll focus our attention on flight testing for scientific research, assessment of experimental technologies, developmental test and evaluation, operational test and evaluation, and airworthiness certification programs. Other perspectives on the different kinds of flight testing are provided by Kimberlin (2003), Ward et al. (2006), or Corda (2017).

1.2.1 Scientific Research

In many instances, the highest‐quality scientific research can only be done in actual flight. Even though wind tunnels are commonly available, results from these facilities are always limited in some way – facility effects such as streamwise pressure gradients in the test section, wall boundary layer effects, test section blockage, turbulence intensity level, constraints on model size, lack of Mach and/or Reynolds scaling, etc. are always present (see Tavoularis 2005 or Barlow et al. 1999 for a discussion of wind tunnels and their limitations). Similarly, computational fluid dynamics simulations are inherently limited in their ability to model viscous, unsteady separated flows, particularly when the model – such as a full aircraft – is large (see Cummings et al. 2015 for the limitations on computational aerodynamics). Grid resolution, turbulence modeling strategies, and time‐accurate solutions will always need validation of some kind. Thus, the ultimate proof of scientific principles associated with flight is to actually conduct experiments in flight.

The range of scientific experiments that can be studied via flight testing can be very broad and conducted by government labs, universities, and industry. University flight test efforts have included Purdue University's development of pressure‐sensitive paint (PSP) for in‐flight measurements of chordwise surface pressure distribution on an aircraft wing (Figure 1.7). The advantage of PSP is that there is minimal flow intrusiveness, compared to the traditional pressure belts mounted on top of the wing, which are banded and flexible tubes. Furthermore, it is much simpler to instrument the aircraft with PSP, since no tubing has to be run into the fuselage and connected to pressure transducers. In fact, the production Beechjet 400 shown in Figure 1.7 was returned to normal flight under its regular airworthiness certification immediately following flight testing (Lachendro 2000).

Another leading flight test program for scientific research is the University of Notre Dame's Airborne Aero‐optics flight research program (Jumper et al. 2015). Researchers at Notre Dame, led by Prof. Eric Jumper and Prof. Stanislav Gordeyev, study approaches for correcting optical aberrations to laser beams propagating through unsteady shear flows and turbulence. Their active correction schemes allow them to focus a laser beam emitted from one aircraft on the fuselage of a target aircraft such as the Dassault Falcon 10 shown in Figure 1.8. These concepts are used for applications ranging from optical air‐to‐air communications to directed energy for military applications.

The US government is also active with scientific research enabled by flight testing programs. One notable example is NASA's F‐18 high alpha research vehicle (HARV). The goal of the first phase of this program was to understand vortex formation, trajectory, and breakdown on the F‐18 operated at high angle of attack. The specially instrumented F‐18 had tufts (short pieces of yarn) taped to the top of the wing, smoke tracer particles released from orifices near the nose, dye flow visualization, and hundreds of pressure taps. These various techniques were used to study local flow separation and vortex trajectories. In‐flight measurements, shown in Figure 1.9, clearly documented the formation of vortices on the leading‐edge extension (LEX) of the F‐18 at high angle of attack, the trajectory of these vortices, and the specific location of vortex breakdown. The vortex breakdown phenomenon, when occurring in the vicinity of the aft tail, led to significant tail buffeting and issues with fatigue (see Fisher et al. 1990).

Figure 1.7 Inspection of pressure‐sensitive paint on Purdue University's Beechjet 400 following a flight test in 1999 (depicted left to right are Hirotaka Sakaue, Brian Stirm, and Jim Gregory).

Source: Photo courtesy of Nate Lachendro.

Figure 1.8 Notre Dame's Dassault Falcon 10.

Source: U.S. Air Force.

Figure 1.9 Smoke and tuft flow visualization on the NASA F‐18 High Alpha Research Vehicle at an angle of attack of 20°.

Source: NASA.

1.2.2 Experimental Flight Test

Now, we turn our attention from basic scientific and engineering studies to development and test of new vehicle concepts. NASA Armstrong Flight Research Center (formerly known as NASA Dryden) has led the way over the years with this type of flight research (for a good historical overview of NASA's many flight research programs, see Gorn 2001 or Hallion and Gorn 2003). This type of flight testing is all about pushing the boundaries of what is possible, through development and demonstration of new flight technologies. Beyond the Bell XS‐1 discussed earlier, there are numerous flight research programs that the US Government has conducted (Miller 2001; Jenkins et al. 2003). These cutting‐edge aircraft are generally classified as X‐planes, with the goal of proving out new technologies or advanced concepts (see Figure 1.10). The Bell XS‐1 was the first aircraft in this distinguished lineup, which includes over 60 aircraft (and counting!). Many of these X‐planes led to successful production flight vehicles after a period of focused flight testing (see Miller 2001; Jenkins et al. 2003; Corda 2017).

One interesting example is the X‐wing project, which had the goal of improving the forward flight speed of helicopters. This interesting vehicle, the Sikorsky S‐72 shown in Figure 1.11, is a hybrid between a fixed wing aircraft and a traditional rotorcraft. It could take off vertically like a traditional helicopter, but then its rigid rotors could be stopped mid‐flight as the aircraft transitioned from vertical flight to forward flight. Instead of articulating the rotor blades as a traditional helicopter does, the S‐72 used compressed air blown from the edges of the blades to achieve lift control (called circulation control – see Reader and Wilkerson 1977 for details). This innovative aircraft from the early 1980s has paved the way for high‐speed helicopters today, such as the Sikorsky S‐97 Raider or the Airbus RACER program.

Figure 1.10 Early X‐planes, including the Douglas X‐3 Stiletto (center) and (clockwise, from lower left) Bell X‐1A, Douglas D‐558‐1 Skystreak, Convair XF‐92A, Bell X‐5, Douglas D‐558‐2 Skyrocket, and the Northrop X‐4 Bantam.

Source: NASA.

Figure 1.11 Sikorsky S‐72 X‐wing testbed aircraft.

Source: NASA.

Vehicle flight testing programs are also pushing into the domain of unmanned aircraft systems (UAS), commonly known as drones. For example, The Ohio State University developed and flight tested the Avanti UAS, which is a 70‐lb jet capable of autonomous, unmanned, high‐speed flight (Figure 1.12). This flight vehicle featured dual‐redundant radio control links and a third independent satellite communications link, to provide robust beyond‐line‐of‐sight flight. Flight research with this vehicle assessed the robustness of the control links, along with adaptive control laws for real‐time in‐flight system identification (see Warwick 2017; McCrink and Gregory 2021; or Chapter 16 for details). In the midst of the flight testing program, the Ohio State team set official world records for speed (147 mph) and out‐and‐back distance (28 mi) of an autonomous unmanned aerial vehicle (UAV), as certified by the Fédération Aéronautique Internationale (FAI) and the National Aeronautic Association (NAA).

Figure 1.12 The Ohio State University's Avanti jet unmanned aircraft system.

Source: Photo courtesy of Kamilah King.

1.2.3 Developmental Test and Evaluation

Within the US military, a significant amount of time and energy are invested in development test and evaluation (DT&E) flight testing. This aspect of flight testing involves a careful assessment of how an aircraft flies, including evaluation of aircraft performance, stability, and handling qualities. DT&E also includes performance assessment of new weapons, new software, and new airframes. These tests are centered on assessment of compliance with performance standards and focus on identifying anomalies in new systems. Test pilots (see Figure 1.13) push the performance limits of the system and are often involved in test planning very early in the design cycle. For example, if a new weapon system is designed for an aircraft, the developmental test pilot will evaluate the separation characteristics, compatibility of the new weapon with the aircraft system across a wide range of flight conditions, and evaluation of flutter flight characteristics. This testing and evaluation are done through a gradual build‐up approach that minimizes (but does not eliminate) risk.

1.2.4 Operational Test and Evaluation

Operational test and evaluation (OT&E) involves assessment of an air vehicle's performance under representative operational conditions. This often includes operation on different runways under different conditions (e.g., rain, sleet, snow, etc.) or at high‐density altitude (high elevation, hot day). Operational testing also involves determination of crosswind limits on landing and taxiing operations. Aircraft manufacturers will also assess aircraft system robustness and reliability under a wide range of extreme weather conditions, including heat, cold, and icing.

Figure 1.13 Maj Rachael Winiecki, a developmental test pilot with the 461st Flight Test Squadron at Edwards Air Force Base, and the first F‐35 female test pilot. Also shown is Airman 1st Class Heather Rice, a crew chief with the 412th Aircraft Maintenance Squadron.

Source: U.S. Air Force.

1.2.5 Airworthiness Certification

Airworthiness certification is the process by which an aircraft is demonstrated to conform to approved design principles and that it is in a condition for safe operation. But what constitutes safe flight? This generally involves an insignificant hazard to people or property on the ground and minimal hazard to the occupants of the aircraft. Typically, a government's civil aviation authority, such as the Federal Aviation Administration (FAA) in the United States, grants an airworthiness certificate to an applicant submitting reports that document airworthiness for a new aircraft type. This process can be lengthy, involving flight testing to document aircraft performance and demonstrate compliance with safety standards.

In the United States, the regulatory authority for the FAA to certify the airworthiness of light aircraft is Title 14 of the Code of Federal Regulations (“Aeronautics and Space”), Chapter I (“Federal Aviation Administration, Department of Transportation”), Subchapter C (“Aircraft”), Part 23 (“Airworthiness Standards: Normal Category Airplanes”) – we'll refer to this as 14 CFR §23 or simply part 23 (U.S. Code of Federal Regulations 2021). Part 23 covers the certification standards for general aviation aircraft, which have a maximum takeoff weight of 19,000 lb or less and carry 19 or fewer passengers. Since the scope of this book focuses on light aircraft, Part 23 is most relevant for our purposes. The subpart that is most relevant for flight testing is Subpart B (14 CFR §23.2100 through §23.2165), which defines the requirements for flight testing of aircraft for airworthiness certification.

Aircraft certified under Part 23 are grouped into different certification and performance levels based on number of passengers that can be carried and flight speed (14 CFR §23.2005), which are summarized in Table 1.1. Each level indicates a higher hazard, and a correspondingly higher bar is set to mitigate the risks associated with those hazards. Aircraft at the higher certification levels and higher performance levels will have higher standards to meet for certification.

Part 23 details the standards of safe flight that must be met for an aircraft to be certified as airworthy by the FAA, organized into broad categories of performance metrics and flight characteristics. Performance metrics include defining limits on the aircraft weight and center of gravity position, the stall speed of the aircraft under various operating conditions, takeoff performance, climb performance, glide performance, and landing distance required. The flight characteristics for certification include demonstration that the airplane is controllable and maneuverable; that the airplane can be trimmed in flight; that it has static and dynamic longitudinal, lateral, and directional stability; that the aircraft has controllable stall characteristics in all maneuvers and that sufficient stall warning is provided; that spins are recoverable; that the airplane has controllable ground handling characteristics; and that vibration and buffeting do not interfere with control of the airplane or cause excessive fatigue. If certification is requested for flight into known icing conditions, then the aircraft performance and handling characteristics must be shown to the same level of safety even in icing conditions. This textbook provides an introduction to the underlying principles for some of these flight tests; more detailed information is available from Kimberlin (2003) or FAA Advisory Circulars (2003, 2011).

Table 1.1 Airworthiness certification levels defined by part 23.

Source: Based on FAA (2011).

VNO = maximum structural cruising speed, VMO = maximum operating limit speed, MMO = maximum operating Mach number, and KCAS represents the units for knots calibrated airspeed.

While the regulatory framework and overall safety criteria are defined in Part 23, the regulations are intentionally sparse on details on how to actually demonstrate compliance for certification. Instead, means of compliance (§23.2010) can be determined by the applicant, subject to approval by the FAA. Typically, the means of compliance is established by a consensus standard. A type certificate applicant for a new light aircraft could demonstrate compliance with a consensus‐based industry standard, which has been approved by the FAA. This compliance mechanism is a dynamic and flexible approach (compared to explicitly defining the compliance mechanisms in part 23), since consensus‐forming bodies can quickly respond to new technologies and develop consensus standards. One key example of such a body is ASTM International. The ASTM convenes a number of committees, which are populated by representatives from various industry groups, and also includes government (FAA) representatives. The key ASTM committee that covers certification standards for light aircraft is the F44 committee on General Aviation Aircraft and specifically the F44.20 subcommittee on Flight. At the time of writing this book, ASTM F44.20 had published standard specifications for flight test demonstration of aircraft weight and center of gravity, operating limitations, aircraft handling characteristics, performance, and low‐speed flight characteristics (ASTM 2017, 2018a, 2018b, 2019a, 2019b). Historical guidance from the FAA is also available for means of compliance with 14 CFR part 23 through nonregulatory advisory circulars (FAA 2003, 2011).

It's important to also be familiar with the historical approaches to airworthiness certification, since there are many aircraft flying today that were certified under older versions of the regulations. Predating certification of general aviation aircraft under part 23, certification was granted under the Civil Air Regulations (from the late 1930s until 1965). Kimberlin (2003, chapter 1) provides a good synopsis of these older regulations and how antique aircraft are still flying under airworthiness certificates granted under the older regulations.

For decades, certification of light general aviation aircraft followed regimented flight testing protocols that were explicitly defined in part 23. Over the years, the part grew more complex as additional safety measures and compliance protocols were codified. The resulting regulation was a rigid document that could not easily accommodate new technologies. For example, part 23 was strictly written to document how a type applicant must demonstrate the performance of internal combustion engines and the associated fuel system. This strict delineation of a compliance pathway was fine when all general aviation aircraft were powered by internal combustion engines running off Avgas. However, there are new propulsion system concepts emerging such as electric motors driven by fuel cells, batteries, or hybrid battery‐generator systems, but these could not be certified under the former regimented structure of part 23. Type certificate applicants would have had to demonstrate an equivalent level of safety and obtain waivers, but there was no established and agreed‐upon process for doing so. Thus, certification of new technologies such as electric propulsion would have been costly, with an uncertain outcome.

The current certification framework was developed in response to these challenges, leading to a complete rewrite of part 23 in 2016. With the rewrite of part 23, the FAA removed historical designations of various certification categories for airplanes. While these categories no longer exist for new aircraft certifications, any aircraft certified under the old part 23 will retain its category designation. These categories are normal, utility, acrobatic, and commuter. The commuter category is the designation for the largest general aviation aircraft, with a maximum takeoff weight of 19,000 lb, a passenger seating capacity of up to 19, and multiple engines. The normal, utility, and acrobatic categories all have a much lower weight limit of 12,500 lb and a seating capacity of up to 9. Normal category airplanes are approved for normal (routine) flying, stalls (but not “whip stalls”), and routine commercial maneuvers (less than 60° bank). Airplanes certified for utility category are approved for limited aerobatic maneuvers, which may include spins and commercial maneuvers at higher bank angles (up to 90°). Acrobatic category airplanes are approved for acrobatic maneuvers, which is basically any maneuver that a pilot can fly, and found to be safe in the flight testing program. For the normal, utility, and acrobatic categories, a given airplane could be certified for one, two, or all three, with varying operating limitations corresponding to each. Given that there are many aircraft routinely flying today that are well over 60 years old, one can anticipate that these legacy certification categories will persist for quite some time as historical and current aircraft continue flying.