Читать книгу Introduction to Flight Testing - James W. Gregory - Страница 17

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 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 briefing, Williams would review the lessons learned from the previous flight and detail the objectives of the upcoming mission (Gorn 2001, pp. 195–196).

As Yeager flew at progressively higher flight speeds, he noticed significant changes in the trim condition of the aircraft. At certain Mach numbers, the trim condition would change nose‐up, and at other Mach numbers it would trend toward nose‐down, all accompanied by buffeting at various flight conditions. For example, on one flight at Mach 0.88 and 40,000 ft, Yeager was unable to put the aircraft into a light stall (even with the stick full aft), due to the lack of control authority. Then, on October 10, 1947, Yeager piloted another mission in a series of powered flights to ever‐higher Mach numbers to test the response of the aircraft in this untested regime. After accelerating up to an indicated Mach number of 0.94 at an altitude of 40,000 ft, Yeager found that he had lost virtually all pitch control! He moved the control stick full fore and aft, yet obtained very little pitch response. Fortunately, the XS‐1 was still stable at this flight condition, if not controllable. At this point, Yeager cut off the engines and came back for a landing on the expansive Rogers lakebed (Young 1997).

All of these various anomalies were due to compressibility effects, which were only poorly understood at the time. As the aircraft exceeded the critical Mach number, shock waves would form at various locations on the aircraft body. Furthermore, these shock waves could move substantially, with only a minor adjustment in freestream Mach number. Since there is a significant pressure gradient across a shock wave, this could result in dramatic changes in the forces and moments produced on control surfaces, and the strong pressure gradient across the shock would often lead to boundary layer separation. Thus, if a shock happened to be present at a hinge line for the elevator, the shock‐induced boundary layer separation would create a thick unsteady wake flow over the elevator, causing the dynamic pressure on this control surface to drop dramatically and the elevator to lose effectiveness. With some foresight, researchers at NACA and designers at Bell anticipated this eventuality and designed the XS‐1 to enable pitch control by moving the incidence angle of the entire horizontal tail (rather than inducing pitch changes using the elevator alone). So, as Yeager and Ridley discussed the phenomena occurring on October 10 and earlier, Ridley encouraged Yeager to adjust the horizontal tail angle of incidence to achieve pitch control, instead of using the elevator.

The plan for the next flight was to go for it – Yeager's intent was to fly supersonic. However, with the technical uncertainty associated with loss of elevator control and shock‐induced buffeting, the NACA engineering team admonished Yeager to not exceed Mach 0.96 unless he was completely certain that he could do so safely. Beyond the NACA team, however, Jack Ridley was the one whom Yeager trusted the most. Ridley thought Yeager would be just fine controlling pitch with the moving horizontal tail, actuating it in increments of a quarter or a third of a degree to achieve pitch control without using the elevator. Ridley explained, “It may not be much, and it may feel ragged to you up there, but it will keep you flying” (Young 1997, p. 56). Yeager trusted Ridley implicitly – much more so than the NACA team. He later recounted, “I trusted Jack with my life. He was the only person on earth who could have kept me from flying the X [S]‐1” (Young 1997, p. 56).

So, on the morning of October 14, 1947, Yeager set out with his team to fly faster than Mach 1. With Cardenas at the controls of the B‐29, the Superfortress carried the Bell XS‐1 up to altitude. On the way up, Bob Hoover and Dick Frost joined up in formation in their FP‐80s. Hoover positioned himself in the “high chase” position: 10 mi ahead of the B‐29 at an altitude of 40,000 ft, to give Yeager an aiming point as he climbed and accelerated in the XS‐1. Frost joined up slightly to the right of and behind the B‐29 in order to observe the rocket firing and drop of the XS‐1.

When everyone was ready, Cardenas put the B‐29 in a slight dive and started a countdown: “10‐9‐8‐7‐6‐5‐3‐2‐1” (yes, he skipped “4”!, as he often skipped a number on these flights) and pulled the release mechanism at 10:26 a.m. an altitude of 20,000 ft and an airspeed of 250 knots. This airspeed was slightly lower than planned, causing the XS‐1 to nearly stall. Yeager pitched the nose down to regain airspeed and then lit all four burners to rapidly accelerate upward. As he breezed past the high‐chase FP‐80, Hoover was able to snap the world‐famous photo of Yeager's flight (Figure 1.5) as the XS‐1 continued going faster and higher. Yeager then shut down two of the rocket chambers in order to keep the vehicle's acceleration in check. Accelerating through Mach 0.83, 0.88, and 0.92, he tested the aircraft's response to horizontal stabilizer control. With the small increments of a quarter or a third of a degree that Ridley recommended, Yeager was able to maintain effective control of the aircraft. Then, as Yeager recounted in his postflight report: “At 42,000' in approximately level flight, a third cylinder was turned on. Acceleration was rapid and speed increased to .98 Mach_i. The needle of the machmeter fluctuated at this reading momentarily, then passed off the scale. Assuming that the off‐scale reading remained linear, it is estimated that 1.05 Mach_i 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.