Introduction to Flight Testing. James W. Gregory
not be broken. We now know, however, that this barrier only amounted to a lack of insight into the physics of shock–boundary layer interaction, shock‐induced separation, and the transonic drag rise, along with a lack of high‐thrust propulsion sources to power through the high drag. Scientific advancements in theoretical analysis, experimental testing, and flight testing, along with engineering advancements in propulsion and airframe design, ultimately opened the door to supersonic flight.
In a program kept out of public sight, the U.S. Army Air Forces, the National Advisory Committee for Aeronautics (NACA, the predecessor to NASA), and the Bell aircraft company collaborated on a program to develop the Bell XS‐1 with the specific intent of “breaking the sound barrier” to supersonic flight. (Note that the “S” in XS‐1 stands for “supersonic”; this letter was dropped early in the flight testing program, leaving us with the commonly known X‐1 notation.) The XS‐1 (see Figure 1.3) was a fixed‐wing aircraft with a gross weight of 12,250 lb, measured 30‐ft 11‐in. long, had a straight (unswept) wing with an aspect ratio of 6.0 and a span of 28 ft, and an all‐moving horizontal tail (a detail that we'll soon see was important!). The XS‐1 was powered by a four‐chamber liquid‐fueled rocket engine producing 6000 lb of thrust. The overarching narrative of the program is well documented in numerous historical and popular sources (e.g., see Young 1997; Gorn 2001; Peebles 2014; Hallion 1972; Hallion and Gorn 2003; or Wolfe 1979), but we'll pick up the story in the latter stages of the flight test program at Muroc Army Airfield, positioned on the expansive Rogers Dry Lake bed that is today the home of Edwards Air Force Base and NASA Armstrong Flight Research Center.
Figure 1.3 Three‐view drawing of the Bell XS‐1.
Source: NASA, X‐1/XS‐1 3‐View line art. Available at http://www.dfrc.nasa.gov/Gallery/Graphics/X‐1/index.html.
The XS‐1 had an aggressive flight test schedule, with not too many check‐out flights before going for the performance goal of supersonic flight. The extent of the test program was actually a matter of contentious debate between the AAF, the NACA, and Bell. In the end, Bell dropped out of the mix for contractual and financial reasons, and the NACA and AAF proceeded to collaborate on the flight test program. But the continued collaboration was not without tension. The AAF leaders and pilots continually pushed for an aggressive flight test program, making significant steps with each flight. The NACA, on the other hand, advocated for a much slower, methodical pace where substantial data would be recorded with each flight and carefully analyzed before proceeding on to the next boundary. In the end, the AAF vision predominantly prevailed, although there was a reasonable suite of instrumentation on board the aircraft. The XS‐1 was outfitted with a six‐channel telemeter, where NACA downlinked data on airspeed, altitude, elevator position, normal acceleration, stabilizer position, aileron position, and elevator stick force, along with strain gauges to record airloads and vibrations (Gorn 2001, p. 195). On the ground, the NACA crew had five trucks to support the data acquisition system – one to supply power, one for telemetry data, and three for radar. The radar system was manually directed through an optical sight, but if visual of the aircraft was lost, the radar system could be switched to automatic direction finding (Gorn 2001, pp. 187–188).
To lead the flying of the aircraft toward the perceived “sound barrier,” the AAF needed a pilot with precision flying capabilities, someone who was unflappable under pressure, and someone with scientific understanding of the principles involved. The Army turned to Captain Charles E. “Chuck” Yeager – a young, 24‐year‐old P‐51 ace from World War II – for the honor and responsibility of being primary pilot. According to Colonel Albert Boyd who selected him, Yeager had impeccable instinctive piloting skills and could work through the nuance of the aircraft's response to figure out exactly how it was performing (Young 1997, p. 41). Not only could he fly with amazing skill, but the engineering team on the ground loved him for his postflight debriefs. Yeager was able to return from a flight and relate in uncanny detail exactly how the aircraft responded to his precise control inputs, all in a vernacular that the engineering staff could immediately appreciate (Peebles 2014, p. 29). But it wasn't just Yeager doing all of the work – he had a team around him. Backing him up and flying an FP‐80 chase plane was First Lieutenant Robert A. “Bob” Hoover, who was also well known as an exceptional pilot. Captain Jackie L. “Jack” Ridley, an AAF test pilot and engineer with an MS degree from Caltech, was the engineer in charge of the project. Others involved included Major Robert L. “Bob” Cardenas, pilot of the B‐29 Superfortress carrier aircraft and officer in charge, and Lieutenant Edward L. “Ed” Swindell, flight engineer for the B‐29. Backing up these AAF officers was Richard “Dick” Frost, a Bell engineer and test pilot who already had flight experience in the XS‐1 and got Yeager up to speed on the intricacies of the aircraft. This cast of characters is depicted in Figure 1.4.
Beyond this core group of military professionals was a team of NACA scientists and engineers led by Walt Williams (see Figure 1.2). This team was focused predominantly on understanding the flight physics in this exploratory program, providing deep technical insight and support to the Air Force crew. Yet, this objective was inherently at odds with the AAF's stated desire to push to supersonic flight as quickly and safely as possible. This tension was aptly described by Williams: “We were enthusiastic, there is little question. The Air Force group – Yeager, Ridley – were very, very enthusiastic. We were just beginning to know each other, just beginning to work together. There had to be a balance between complete enthusiasm and the hard, cold facts. We knew that if this program should fail, the whole research airplane program would be set back. So, our problem became one of maintaining the necessary balance between enthusiasm and eagerness to get the job completed with a scientific approach that would assure success of the program. That was accomplished” (Gorn 2001, pp. 194–195).
Figure 1.4 The Air Materiel Command XS‐1 flight test team, composed of (from left to right): Ed Swindell (B‐29 Flight Engineer), Bob Hoover (XS‐1 Backup and Chase Pilot), Bob Cardenas (Officer‐in‐charge and B‐29 Pilot), Chuck Yeager (XS‐1 Pilot), Dick Frost (Bell Engineer), and Jack Ridley (Project Engineer).
Source: U.S. Air Force.
In the run‐up to the first supersonic flight, the team carefully pushed forward. On Yeager's first powered flight on August 29, 1947, he accelerated up to Mach 0.85, exceeding the planned test point of Mach 0.8. This negated NACA's need to acquire telemetered data in the Mach 0.8–0.85 range, leading to further tension between Yeager and Williams. In Yeager's words, “They [the NACA engineers and technicians] were there as advisers, with high‐speed wind tunnel experience, and were performing the data reduction collected on the X[S]‐1 flights, so they tried to dictate the speed in our flight plans. Ridley, Frost, and I always wanted to go faster than they did. They would recommend a Mach number, then the three of us would sit down and decide whether or not we wanted to stick with their recommendation. They were so conservative that it would've taken me six months to get to the [sound] barrier” (Young 1997, p. 51 – quoted from Yeager and Janos (1985), p. 122).
Yeager was admonished by Colonel Boyd to cooperate more carefully with the NACA technical specialists and to follow the test plan. This led to careful preflight briefings that, while Yeager considered to be tedious, were essential to flight safety and accomplishment of the test objectives. At each