Shown below is the X-31 Enhanced Fighter Maneuverability demonstrator, which in 1995 concluded a multiyear flight test program that accomplished what was termed a "landmark contribution" to military aeronautics. Built by Rockwell International and the German Deutsche Aerospace, the X-31 made hundreds of flights from 1990 to 1995 demonstrating the potential of thrust vectoring (maneuvering by directing the exhaust flow) and an advanced flight control system for close-in air combat at very high angles of attack (the angle between an airplane's wing and the air through which it is flying).
The tests were conducted at Dryden Flight Research Center by an International Test Organization (ITO), a component of the NATO Cooperative Research and Development Program, under the management of the U.S. Advanced Research Projects Agency (ARPA). In addition to ARPA and NASA, ITO included representation from the German Ministry of Defense, the U.S. Air Force and Navy, and the co-manufacturers.
The X-31 tests attained all program goals and compiled an impressive array of technically significant "firsts." It demonstrated exceptional agility at extremely high angles of attack, well beyond the normal aerodynamic stall limit; the combat value of its advanced technologies by winning 117 of 128 simulated combat engagements against two of the nation's highest performing military aircraft; and the effectiveness of thrust vectoring for stability and control at supersonic speed, including a successful simulation of controlled flight without a vertical tail.
The X-31 program is representative of the type of flight research conducted by NASA to explore new technologies and new flight regimes. NASA conducts such programs independently or in cooperation with U.S. industry and the Department of Defense, sometimes in cooperation with international development teams.
Another example is the research work performed by NASA's SR-71 (lower right), a one-time reconnaissance plane converted to duty as a NASA research platform. In 1994-95, the SR-71 was used on three major test programs: the Optical Air Data Experiment, involving tests of an advanced laser-based angle of attack/airspeed/sideslip measurement system; a Handling Qualities Experiment, in which the SR71 served as a stand-in for the reference model High Speed Civil Transport (HSCT); and a sonic boom test program designed to measure the effects of boom pressure waves under various meteorological conditions at various altitudes. The SR-71 flights were conducted for Langley Research Center's High Speed (HSR) program (see page 34) with the goal of building a database to help engineers predict the shape and intensity level of sonic booms generated by future high speed aircraft, such as the HSCT. The SR-71 was employed as the boom generator, while an instrumented F-16XL aircraft probed the boom's shock wave from a position several hundred feet from the SR-71. Data was also collected by two other aircraft at lower altitudes and by sensors on the ground. Data on the characteristics and intensity of the booms may help designers shape future supersonic aircraft to soften the boom effect.
While one F-16XL was engaged in the boom tests, another‹the F-16XL Supersonic Laminar Flow Control (SLFC) aircraft (right)‹was participating in a different aspect of Langley's HSR program. The SLFC aircraft is fitted with a "suction glove" on its left wing. The glove, made primarily of titanium, has millions of microscopic laser- drilled holes. The objective is to pull turbulent air through the holes by means of a suction system, thereby creating a laminar (smooth) flow over the wing. Laminar flow lowers air drag and permits faster speed, longer range or reduced fuel consumption. The 1995 flights represented a continuation of a program in flight test status since 1992, but the new series involved evaluation of a new laminar flow control system, one that more closely resembles the type of apparatus that would be installed on a HSCT.
A veteran research aircraft, NASA's F-18 HARV (High Alpha Research Vehicle) embarked on a new phase of an investigation that has been in flight test since 1987. High alpha refers to the high angle of attack, in which the nose is angled up as much as 70 degrees while the aircraft flies horizontally. This is called a post-stall maneuver, because the aircraft would stall and spin without the control assistance supplied by its thrust vectoring system. After several series of successful tests with the thrust vectoring system alone, the HARV was fitted with a nose-mounted strake system. The new series of flights is evaluating how the pilot-actuated strakes improve control at high alpha when coupled with the thrust vectoring system already on the HARV.