The SR-71 Blackbird
The joint NASA-Air Force YF-12 research program lasted 10 years (1969-1979) and produced a wealth of data on materials, structures, loads, heating, aerodynamics, and performance for high-speed aircraft.
Initially, NASA researchers planned for the YF-12 program to focus on propulsion technology, especially inlet performance. Since the YF-12 featured a mixed-compression inlet, engineers planned to investigate drag, compressor face distortion, unstart margins, control parameters, air data requirements, and bleed system effects. But problems associated with high-temperature instrumentation delayed the propulsion investigation. This postponement gave NASA engineers time to develop a second initiative: a structures research program involving thermal stresses and aerodynamic loads. The overall effort relied on wind-tunnel data, analytical prediction, and flight research. Since supersonic aircraft undergo aerodynamic as well as thermal loads, the NASA team planned a series of experiments to measure both types of loads, combined and separately.
Strain gauges placed in several locations within the fuselage measured aerodynamic loads. At the same time, instruments on the left side of the aircraft recorded skin temperatures. The airplane enjoyed ideal qualities for thermal research. Previous research aircraft, such as the X-15, had experienced high temperatures but only for short periods of time. The YF-12, however, could sustain high-speed thermal loads for relatively long periods during cruise, enabling temperatures to stabilize. After collecting flight research data over most of the YF-12 performance envelope, researchers compared it to data collected during ground testing in the High Temperature Loads Laboratory (HTLL) at the FRC during 1972 and 1973. The process of comparison involved several steps.
Flight research data provided measurements of the combined effects of temperature and loads. Once this information had been gathered, technicians put the aircraft into the HTLL and heated the entire structure to the same temperatures it had experienced in flight. By measuring the strain outputs from temperature alone, NASA engineers could then separate the thermal effects from the flight data to obtain accurate measurement of aerodynamic loads. Results of the heating experiments showed that the predictions largely agreed with the laboratory results. Data obtained during flight, however, indicated temperatures as much as 20 degrees higher than anticipated because of the differences in the process of heat transfer. The rate of radiant heating is lower than that for aerodynamic heating in areas of higher structural mass. Moreover, the dry fuel tanks used in the ground tests also influenced the results. In flight, the aircraft’s fuel acted as a heat sink. Given the absence of fuel in the aircraft during ground-based heating tests, the fuel tank skin temperatures exceeded those obtained in flight. The simulation and flight measurements converged as the flight test aircraft depleted its fuel supply. Once these values converged, researchers established a correction for in-flight strain gauge measurements.
Remarkably, this research still supports one of the ten goals of NASA's Office of Aeronautics and Space Transportation Technology by providing design tools for the next generation of aircraft. In addition, with respect to the YF-12A alone, the thermal calibration on the ground corrected high-Mach-number loads data for adverse thermal effects, which frequently proved to be large and were always significant.
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