Langley Contributions to the X-31
In 1984, Rockwell proposed a cooperative program to Langley to assess and develop a Rockwell advanced design known as the Super Normal Attitude Kinetic Enhancement( SNAKE) configuration.
Joseph R. Chambers and Joseph L. Johnson, Jr. determined that the proposal was in concert with many Langley research interests in high- angle-ofattack technology, and the cooperative program on the SNAKE configuration was begun. Langley researcher Mark A. Croom was assigned the role of lead engineer, and he began a decade of personal participation in the X- 31 evolution and flight -test programs. The initial SNAKE configuration bore a superficial resemblance to the earlier HIMAT design ( canard and wing-mounted twin vertical tails ) ; however, the new configuration was designed analytically with computers and a minimum amount of wind-tunnel tests.
Unfortunately, Croom's aerodynamic tests of the initial SNAKE configuration in theLangley 30- by 60 - Foot ( Full - Scale ) Tunnel indicated unacceptable stability and controlcharacteristics. The configuration was unstable in pitch , roll, and yaw for all angles of attack. Based on their extensive experience with stability and control characteristics of advanced fighters , Croom and Johnson provided the Rockwell team with several recommendations to cure the problems exhibited by the SNAKE configuration.
The configuration modifications resulted in satisfactory characteristics, and the aerodynamic deficiencies of the initial SNAKE design had been eliminated. Rockwell was grateful for the guidance and innovation contributed by Langley in the evolution of the SNAKE configuration. In the early 1980's, an awareness of the benefits of thrust vectoring for dramatically improved control at high angles of attack surfaced. In addition to studies of advanced engine concepts with vectoring nozzles, interest arose over the use of simple thrust vectoring paddles in the engine exhaust to deflect the thrust for control augmentation.
As discussed in Langley Contributions to the F-14 , the Navy, with Langley's assistance, had taken the lead in this area with flight tests on an F-14 modified with single -axis yaw- vectoring paddles. In addition, during a cooperative program with Rockwell led by Langley researcher Bobby L. Berrier, Langley provided design data for multiaxis thrust vectoring paddle configurations using the Jet Exit Test Facility in 1985. Based on these fundamental research studies, Rockwell incorporated multiaxis thrust- vectoring paddles into the SNAKE configuration. Free - flight tests of the modified SNAKE model in theFull - Scale Tunnel by Croom's team in 1985 provided an impressive display of the effectiveness of thrust vectoring at extreme angles of attack.
In West Germany, Dr. Wolfgang Herbst of Messerschmitt-Bolkow-Blohm (MBB) aggressively touted the advantages of post-stall technology ( PST) for increased effectiveness during close-in air combat. Herbst's conclusions were based on windtunnel tests of a German advanced canard fighter configuration known as the TKF-90 and piloted simulator studies during which the application of simulated thrust vectoring resulted in rapid directional turns at high angles of attack had increased the turn rate by over 30 percent.
Technical discussions between the Rockwell SNAKE Program managers and Herbst were initiated in 1983 , and planning for a mutual program on PST ensued. Discussions with the Defense Advanced Research Projects Agency (DARPA )were very positive . When funding for collaborative international activities became available from the U.S. ( the Nunn-Quayle research and development initiative in 1986 ) and West German governments, the technical expertise of Rockwell and MBB were joined under DARPA sponsorship in the X- 31 Program .
In view of Langley's extensive experience in high-angle-of-attack technology, unique test facilities, and contributions to the Rockwell SNAKE Program , DARPA requested in 1986 that Langley become a participant in the X- 31 development program .
The Rockwell and MBB X- 31 design team merged their configuration candidates into acanard fighter powered by a single General Electric F404 engine with a single vertical tail . The initial design included an F- 16 canopy for cost- saving purposes. Extensive tests of the initial X- 31 configuration were carried out at Langley during 1987. These tests included static wind- tunnel tests and configuration component evaluations in the Langley 14- by 22- Foot High - Speed Tunnel, rotary - balance tests in the Langley 20 - Foot Vertical Spin Tunnel to determine aerodynamic characteristics during spins, and dynamic force tests in the Langley Full - Scale Tunnel .
Unfortunately, in 1988 the X- 31 configuration was revised , and an F- 18 canopy was incorporated. This change was regarded as significant, and a major portion of the previous wind- tunnel tests had to be repeated for the revised configuration. Rotary-balance tests of the revised configuration were conducted in 1988, and spin tests and static and dynamic tests were completed in 1989 for the updated configuration. In 1989, a 0.19- scale model of the X- 31 underwent extensive aerodynamic and free-flight tests in the Langley Full-Scale Tunnel.
Results from these ground -based studies indicated that the X - 31 might have marginal nose- down control at high angles of attack and that the configuration might exhibit severe , unstable lateral oscillations ( wing rock ) that would result in a violent, disorienting roll departure and an unrecoverable inverted stall condition. Fortunately, the results also indicated that a simple control law concept could prevent the aircraft from entering a spin . The awareness that such phenomena might exist for the full - scale aircraft enabled the X- 31 design team to configure the flight control system for maximum effectiveness .An exhaustive test , which included 498 paddle and nozzle configurations of the multi axis thrust -vectoring system, was conducted by Langley researcher Francis J. Capone in the Jet Exit Test Facility during 1988.
These data were used to select the final paddle and nozzle multiaxis thrust- vectoring configuration. These data were also critical to the design of the X- 31 flight control system , since vectored thrust imposes large forces and moments in addition to the normal aerodynamic parameters .A 0.27 - scale drop model was used by Langley to evaluate the post- stall and out- of control recovery characteristics of the configuration. The model, which weighed about 540 lb and included extensive instrumentation , was flown without an engine to assess the capabilities and characteristics of the basic airframe. The objective was to demonstrate that the X - 31 would be agile and have satisfactory characteristics without the additional augmentation provided by thrust vectoring. The drop -model test identifies characteristics and large amplitude flight motions that cannot be assessed in conventional wind or spin tunnels. In the X- 31 Program , the technique proved to be invaluable as an early indicator of the highly unconventional behavior of the configuration . In particular, the violent roll departure indicated by tests of the free- flight model was encountered during the drop -model tests . Several control schemes were evaluated to eliminate this problem . In addition , the drop -model test technique provided solutions to barrier problems during the full - scale flight - test program .
X-31 Flight Demonstration Program
The first flight of the first X-31 aircraft occurred at Palmdale, California, on October 11 ,1990, and the second aircraft made its first flight on January 19, 1991. During the initial phase of flight- test operations at the Rockwell facility at Palmdale, the two aircraft wereflown on 108 test missions . On the test missions, the aircraft achieved thrust vectoring inflight and expanded the post- stall envelope to 40-deg angle of attack. Operations were then moved to Dryden in February 1992, at the request of DARPA .At Dryden, the International Test Organization ( ITO) expanded the flight envelope ofthe aircraft, including military utility evaluations that compared the X- 31 to similarly equipped aircraft for maneuverability in simulated combat. The ITO, managed by DARPA, included NASA, the U.S. Navy, the U.S. Air Force, Rockwell Aerospace, the Federal Republic of Germany, and Deutsche Aerospace ( formerly Messerschmitt-Bolkow-Blohm). The first NASA flight under the ITO took place in April 1992.
As the X-31 full - scale aircraft flight tests began at Dryden, the Langley staff maintained a close support role for consultation and ground testing capability. Two problems surfaced during the X- 31 flight- test program , and both were considered significant enough to curtail flight tests until solutions were found. The first problem was encountered in the flight - test program when it became apparent that the pitch control effectiveness of the aircraft at post- stall conditions ( particularly at aft center of gravity conditions ) was marginal. Pilots reported that their ability to obtain positive, crisp, nose- down aircraft response was unsatisfactory and that increased control effectiveness was required if the X- 31 was to be considered tactically responsive at high angles of attack . As part of the X- 31 Team , Langley was requested to conduct windtunnel tests to explore options to provide the increased control at high angles of attack. Mark Croom and his team quickly responded and evaluated 16 configuration modifications to improve nose -down recovery capability in the Full- Scale Tunnel.
Mark Croom and his team quickly responded and evaluated 16 configuration modifica tions to improve nose -down recovery capability in the Full- Scale Tunnel. Results of the Investigation recommended that a pair of 6- by 65- in . strakes be mounted along the fuselage afterbody to promote nose- down recovery. The Langley recommendations, which were given within a week of the test request, provided a timely solution to the problem .The aft-fuselage strakes were incorporated in the X-31 , and the pilots reported that the nose-down control was significantly improved. The second problem that occurred in the X- 31 full- scale flight test was caused by large out of trim asymmetric yawing moments at high angles of attack . Shortly into the high angle- of-attack, elevated - g phase of the envelope expansion, a departure from controlled flight occurred as the pilot was performing a maneuver at 60- deg angle of attack . Data analysis by the X- 31 team indicated that aa large asymmetric yawing moment, in excess of the available control power, had triggered the departure. In response to an urgent request for solutions, Croom and the Langley team conducted tests in the Langley Full Scale Tunnel to design nose strakes that would minimize the problem . Once again, Langley responded rapidly with a strake configuration that permitted the flights to continue. The X- 31 Program logged an X-plane record of 524 flights in 52 months with 14 pilots from NASA, the U.S. Navy, the U.S. Marine Corps, the U.S. Air Force, the German Air Force, Rockwell International, and Deutsche Aerospace .Evaluation of the X- 31 as an enhanced fighter maneuverability demonstrator by the ITO concluded in early 1995.