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
