Introduction to Flight Testing. James W. Gregory
Another hallmark of successful flight testing is the careful probing of the edges of the flight envelope. Notice how the team approached the uncertain conditions associated with loss of control and buffeting. They gingerly pushed the Mach limits higher and higher, with the hope that any loss‐of‐control situation could be quickly recovered from. Despite the accelerated nature of the test program, the team took the time to carefully analyze the data and debrief after each flight. This was essential for gleaning insight from each test condition and informing the next step in the flight test program. They took an incremental buildup approach – starting from low‐risk flights with known characteristics and carefully advancing to higher‐risk flights, where the flight characteristics were unknown and potentially hazardous.
Also note how the aircraft was instrumented beyond what a normal production aircraft would have been. In fact, the record‐setting XS‐1 (the first airframe built) was only lightly instrumented compared to its sister ship, the second airframe off the production line, which was targeted for a much more detailed exploration of supersonic flight by the NACA team. This instrumentation is critical for understanding exactly what is happening during flight and preserving a record for postflight analysis. The analytical work was done by a large team of engineers, technicians, and, in that day, human “computers” who did many of the detailed computations of the data (see Figure 1.2).
After some initial renegade flying by Yeager, the flight test team settled into a rhythm of carefully planned and executed flights. Before each flight they carefully planned the objectives and specific maneuvers to fly on the next mission. The injunction was that the flight must proceed exactly as planned, with specific plans for various contingencies and anomalies. This culture of flight testing is absolutely essential for the safety and professionalism of the process. One common phrase captures this mentality of flight testing: “plan the flight, and fly the plan.”
This initial foray into exploring the flight testing program of the XS‐1 illustrates many of the hallmarks of flight test programs. We'll next discuss some of the different kinds of flight testing being done today. Clearly, not every flight test program is as ambitious or adventurous as the XS‐1 program, but a common objective is to answer the remaining unknown questions that are always present in an aircraft development program, even after rigorous design work backed up by wind tunnel testing and computational studies.
1.2 Types of 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.