at this reading momentarily, then passed off the scale. Assuming that the off‐scale reading remained linear, it is estimated that 1.05 Machi was attained at this time. Approximately 30% of fuel and lox remained when this speed was reached and the motor was turned off” (Young 1997, p. 75).
Figure 1.5 Yeager accelerates in the Bell XS‐1 on his way to breaking the “sound barrier” on October 14 1947.
Source: NASA.
Yeager had done it! As mentioned in his postflight report, his Machmeter indications were a bit unusual. In fact, during the flight he radioed: “Ridley! Make another note. There's something wrong with this Machmeter. It's gone screwey!” (Young 1997, p. 73). That radio transmission heralded the dawn of a new era in aviation to supersonic speeds and well beyond. After maintaining supersonic flight for about 15 seconds, he shut down the rocket motors, performed a 1‐g stall, and descended for a landing on Rogers dry lakebed.
Postflight analysis of the data, including corrections of the Machmeter reading for installation error, revealed that Yeager had reached a maximum flight Mach number of 1.06. A reproduction of this data is shown in Figure 1.6, where the initial jump in total and static pressures heralded the formation of a shock wave in front of the probe tip, causing a loss of total pressure. This is the characteristic “Mach jump” experienced by every Machmeter as the aircraft accelerates to supersonic speeds.
There are a number of interesting and revealing features of this story that can tell us something about flight testing. First, we see that this endeavor was anything but an individual effort. There was a large team with many players involved – pilots, engineers, managers, analysts, range safety officers, and so on. In this particular case, the flight test program was a collaboration between two separate organizations – the AAF was leading the program execution, and were supported by NACA's technical experts. Even though there was tension between these two groups, they were able to rise above those difficulties to work together in an effective manner to achieve the test objectives.
The source of the tension was inherently due to different test objectives – the AAF crew was tasked with breaking the sound barrier as quickly and safely as possible, while the NACA team was focused on developing a scientific understanding of transonic and supersonic flight, requiring a slower and more methodical approach. Flight test programs sometimes have such competing objectives in mind, which requires deft coordination and program management in order to ensure safety of flight and accomplishment of the test objectives. There is always a tension between programmatic needs, budget, and safety.
Figure 1.6 Plot of the total and static pressure for the first supersonic flight of the XS‐1 on October 14 1947.
Source: Data from NASA.
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
