YF-12 Flight Test: NASA’s Major Supersonic Cruise Study Effort

The XB-70 test program had focused on SST research, as it was the only large aircraft capable of high Mach cruise. In the 1970s, flight data col­lection could focus on a smaller aircraft but one that had already dem­onstrated routine flight at Mach 3. Lockheed’s Mach 3 Blackbird was no longer as secret as it had been in its CIA A-12 initial stages, and the USAF

was operating a fleet of acknowledged Mach 3 aircraft, although details of its missions, top speeds, and altitudes remained military secrets. NASA had requested Blackbirds as early as 1968 for flight research, but the Agency was rejected as being too open for the CIA’s liking. Later, as the NASA flight-test engineers Bill Schweikhard and Gene Matranga, both XB-70 test program veterans, assisted the USAF in SR-71 flight-test data analysis, the atmosphere changed, and the USAF was more willing to provide the aircraft but without compromising the secrecy of the details of the SR-71. The YF-12s were in storage, as the USAF had decided not to buy any further aircraft. Because the YF-12 had a different fuselage and earlier model J58s than the SR-71, a joint test program was pro­posed, with the USAF providing aircraft and crew support and NASA paying operational costs. Phase I of the program would concentrate on USAF desires to evaluate operational tactics against a high Mach target (such as the new Soviet MiG-25 Foxbat). NASA would instrument the aircraft and collect basic research data, as well as conduct Phase II of the test program with applied research that would benefit from a Mach 3, 80,000-foot altitude supersonic cruise platform.[1101]

Подпись: 10Between 1969 and 1979, flight-test crews flew 298 flights with 2 YF-12 Blackbird aircraft. The first of these was a modified YF-12A inter­ceptor. The second, which replaced another YF-12A lost from a fire dur­ing an Air Force test mission, was a nonstandard SR-71A test aircraft given a fictitious "YF-12C” designation and serial number to mask its spy plane origins. YF-12 supersonic cruise-related test results included isolation of thermal effects on aircraft loads from aerodynamic effects. The instrumented aircraft collected loads and temperature data in flight. It was then heated in purpose-built form-fit ovens on the ground—the High Temperature Loads Laboratory (HTTL)—so the thermal strains and loads could be differentiated from the aerodynamic.[1102] For a high – temperature aircraft of the future, separation of these stresses could be crucial in the event that underestimation could lead to skin failures, as experienced by XB-70 AV-1. One byproduct of this research was to correct the factoid still quoted into the 21st century that the airplane expanded in length by 30 inches because of heat. The actual figure was closer to 12 inches, with the difference appearing in structural stresses.

YF-12 Flight Test: NASA's Major Supersonic Cruise Study Effort Подпись: 10

Lockheed’s masterful Blackbird relied not only on lightweight tita­nium and a high fuel fraction for its long range but also a finely tuned propulsion system. At Mach 3, over 50 percent of the nacelle net thrust came from the inlet pressure rise, with the engine thrust being only on the order of 20 percent; the remainder came from the accelerated flow

exiting the nozzle (given the small percentage of thrust from the engine, Lockheed designer Kelly Johnson used to good-naturedly joke that Pratt & Whitney’s superb J58 functioned merely as an air pump for the nacelle at Mach 3 and above). It is not necessarily self-evident why the nozzle should produce such a large percentage of thrust, while the engine’s con­tribution seems so little. The nozzle produces so much thrust because it accelerates the combined flow from the engine and inlet bypass air as it passes through the constricted nozzle throat at the rear of the nacelle. Engine designers concentrate only on the engine, regarding the nacelle inlet and exhaust as details that the airframe manufacturer provides. The low percentage numbers for the jet engine are because it produces less absolute net thrust the faster and higher it goes. Therefore, at the same time the engine thrust goes down (as the plane climbs to high altitude and accelerates to high Mach numbers), the percentage of net thrust from nonengine sources increases drastically (mainly because of inlet pressure buildup). Thus, static sea level thrust is the highest thrust an engine can produce. Integration of the propulsion system (i. e., match­ing the nacelle with the engine for optimum net thrust) is critical for efficient and economical supersonic cruise, as opposed to accelerating briefly through the speed of sound, which can be achieved by using (as the early Century series did) a "brute force” afterburner to boost engine power over airframe drag.

Подпись: 10Air had to be bypassed around the engine to position shock waves properly for the pressure recovery. This bypass led to the added bene­fit of cooling the engine and to the system being referred to as a turbo ramjet. The NASA YF-12 inlets were instrumented, and much testing was devoted to investigation of the inlet/shock wave/engine interac­tion. Inlet unstarts in the YF-12 were even more noticeable and criti­cal than they were in the B-70, as the nacelles were close to mid-span on the wing and the instantaneous loss of the inlet thrust led to violent yaws induced in the less massive aircraft. The Blackbirds had auto­matic inlet controls, unlike XB-70 AV-1, but they were analog control devices and were often not up to the task; operational crewmembers spent much time in the simulator practicing emergency manual con­trol of the inlets. The NASA test sorties revealed that the inlet affected the flight performance of the aircraft during restart recovery. The excess spillage drag airflows from the unstarted inlets induced uncommanded rolling moments. This could result in a "falling leaf” effect at extreme altitudes, as the inlet control systems attempted to reposition the shock

Подпись:Подпись: Full-scale engine

YF-12 Flight Test: NASA's Major Supersonic Cruise Study Effort Подпись: 10
Подпись: Test module
Подпись: Flight. Dryden
Подпись: Airplane
Подпись: 10’ x 10' wind tunnel.
Подпись: Full-scale inlet model

YF-12 Flight Test: NASA's Major Supersonic Cruise Study Effort8×7 9*7 and 11-foot

wind tunnels. Ames

Propulsion Systems laboratory <PSl>

Altitude Test Facility, lewis

Comparison of inlet configurations and facilities.

YF-12 NASA propulsion research assets. NASA.

wave properly by spike positioning and door movements.[1103] This was an illustration of the strong interaction for a supersonic cruiser between aerodynamics and propulsion.

To further investigate this interaction, much research was dedi­cated to the inlet system of the YF-12.[1104] A salvaged full-scale inlet was tested in a supersonic wind tunnel and a one-third-scale inlet. Using this approach, flight-test data could be compared with wind tunnel data to validate the tunnel data and adjust them as required.

The digital computer era was appearing, and NASA led the way in applying it to aeronautics. In addition to the digital fly-by-wire F-8 flight

research, the YF-12 was also employing the digital computer. Originally, the Central Airborne Performance Analyzer (CAPA) general performance digital computer was used to monitor the behavior of the YF-12 Air Inlet Control System (AICS). It behaved well in the harsh airborne environ­ment and provided excellent data. Based upon this and the progress in digital flight control systems, NASA partnered with Lockheed in 1975 to incorporate a Cooperative Airframe/Propulsion Control System com­puter on the YF-12C (the modified SR-71) that would perform the flight control system and propulsion control functions.[1105] This was delivered in 1978, only shortly before the end of the YF-12 flight-test era. The sys­tem requirements dictated that pilot interface be transparent between the standard analog aircraft and the digital aircraft and that the aircraft system performance be duplicated digitally. The development included the use of a digital model of the flight controls and propulsion system for software development. Only 13 flights were flown in the 4 months remaining before program shutdown, but the flights were spectacu­larly successful. After initial developmental "teething problems” early in the program, the aircraft autopilot behavior was 10 times more pre­cise than it was in the analog system, inlet unstarts were rare, and the aircraft exhibited a 5-7-percent increase in range.[1106] Air Force test pilots flew the YF-12C on three occasions and were instrumental in persuad­ing the USAF Logistics Command to install a similar digital system on the entire SR-71 fleet. The triple-redundant operational system—called Digital Automatic Flight and Inlet Control System (DAFICS)—was tested and deployed between 1980 and 1985 and exhibited similar benefits.

Подпись: 10For the record, the author himself was a USAF SR-71 flight-test engineer and navigator/back-seater for the developmental test flights of DAFICs on the SR-71 and during the approximately 1-year test program experienced some 85 inlet unstarts! Several NASA research papers speculated on the effect of inlet unstarts on passenger, using anecdotal flight-test data from XB-70 and YF-12 flights. The author agreed with the comments in 1968 of XB-70 test pilot Fulton (who also flew the YF-12 for NASA) that he thought paying passen­gers in an SST would put up with an unstart exactly once. The author

also has several minutes of supersonic glider time because of dual inlet unstarts followed by dual engine flameouts, accompanied by an unrelated engine mechanical problem inhibiting engine restart. During a dual inlet unstart at 85,000 feet and subsequent emergency single engine descent to 30,000 feet, he experienced the "falling leaf” mode of flight, as the inlets cycled trying to recapture the shock waves within the inlet while the flight Mach number also oscillated. The problems during the test program indicated the sensitivity of the integration of propulsion with the airframe for a supersonic cruise aircraft. Once the in-flight "debugging” of the digital system had been accomplished, however, operational crews never experienced unstarts, except in the event of mechanical malfunction. One byproduct of the digital system was that the inlet setting software could be varied to account for differ­ences in individual airframe inlets because of manufacturing tolerances. This even allowed inlet optimization by tail number.

Подпись: 10Other YF-12 research projects were more connected with taking advantage of its high speed and high altitude as platforms for basic research experiments. One measured the increase in drag caused by an aft-facing "step” placed within the Mach 3 boundary layer. As well, researchers measured the thickness and flow characteristics of this turbulent region. The coldwall experiment was the most famous (or infamous).[1107] It was a thermodynamics heat transfer exper­iment that took an externally mounted, insulated, cryogenically cooled cylinder to Mach 3 cruise and then exposed it to the high – temperature boundary layer by explosively stripping the insulation. Basic handling qualities investigations with the cylinder resulted in loss of the carrier YF-12A folding ventral fin. It was replaced with a newer material, producing a bonus materials experiment. When the experiment was finally cleared for deployment, it resulted in sending debris into the left inlet of the YF-12 carrier and unstarts in both inlets, not to mention multiple unstarts of the YF-12C chase aircraft. Both aircraft were grounded for over 6 weeks for inspections and repairs. Fortunately, the next 2 deployments with fewer explosives were more routine. An implicit lesson learned was that at Mach 3, the seemingly routine flight-test techniques may require careful review to ensure that they really are routine.