rel="nofollow" href="#fb3_img_img_5d787019-b0fe-5351-9ee2-7c06004f1ffa.png" alt="Photo depicts a more realistic view of the people behind flight testing - a team effort is required to promote safety and professionalism of flight. Depicted here is the team of NACA scientists and engineers who supported the XS-1 flight test program. "/>
Figure 1.2 A more realistic view of the people behind flight testing – a team effort is required to promote safety and professionalism of flight. Depicted here is the team of NACA scientists and engineers who supported the XS‐1 flight test program.
Source: NASA.
Flight testing is a critical endeavor in the overall design cycle of a new aircraft system. The main objective is to prove out the assumptions that are inherent to every design process and to discover any hidden anomalies in the performance of the aircraft system. Aircraft design typically proceeds by drawing upon historical data to estimate the performance of a new aircraft concept, but there is always uncertainty in those design estimates. The initial stages of design have very crude estimates made for a wide range of parameters and theories applied to the design. Over time, the design team reduces the uncertainty in the design by refining the analysis with improved design tools and higher‐fidelity (more expensive!) analysis, wind tunnel testing, and ground testing of functional systems and even the entire aircraft. But, then the moment of truth always comes, where it is time for first flight of the aircraft. It is at this point that the flight test team documents the true performance of the airplane. If differences arise between actual and predicted performance, minor tweaks to the design may be needed (e.g., the addition of vortex generators on the wings). Also, the insight gleaned from flight testing is documented and fed back into the design process for future aircraft.
This chapter will provide a brief overview of the flight testing endeavor through a historical anecdote that illustrates the key outcomes of flight testing, how flight testing is actually done, and the roles of all involved. Following this, we'll take a look at the various kinds of flight testing that are done, with a particular emphasis on airworthiness certification of an aircraft, which is the main objective of many flight testing programs. We'll then conclude this chapter with an overview of the rest of the book, including our objectives in writing this book and what we hope the reader will glean from this text.
1.1 Case Study: Supersonic Flight in the Bell XS‐1
A great way to learn about the essential elements of a successful flight test program is to look at a historical case study. We'll consider the push by the Army Air Forces (AAF) in 1947 to fly an aircraft faster than the speed of sound. Along the way, we'll pick up some insight into how flight testing is done and some of the values and principles of the flight test community.
At the time, many scientists and engineers did not think that supersonic flight could be achieved. They observed significant increases in drag as the flight speed increased. On top of that, there were significant loss‐of‐control incidents where pilots found that their aircraft could not be pulled out of a high‐speed dive. These highly publicized incidents led some to conclude that the so‐called “sound barrier” could 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
