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 Machi. 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 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: