F-15 RPRV landing on lakebed. Public domain image of NASA aircraft.

This photograph shows NASA's 3/8th-scale remotely piloted research vehicle landing on Rogers Dry Lakebed at Edwards Air Force Base, California, in 1975. In April of 1971, Assistant Secretary of the Air Force for Research and Development Grant Hanson sent a memorandum noting the comparatively small amount of research being conducted on stalls (losses of lift) and spins despite the yearly losses that they caused (especially of fighter aircraft). In the spring and summer of that year, NASA's Flight Research Center (redesignated in 1976 the Dryden Flight Research Center, Edwards, California) studied the feasibility of conducting flight research with a sub-scale fighter-type Remotely Piloted Research Vehicle (RPRV) in the stall-spin regime. In November, NASA Headquarters approved flight research for a 3/8-scale F-15 RPRV. It would measure aerodynamic derivatives of the aircraft throughout its angle-of-attack range and compare them with those from wind tunnels and full-scale flight. (Angle of attack refers to the angle of the wings or fuselage with respect to the prevailing wind.) The McDonnell Douglas Aircraft Co., builder of the full-size F-15, designed and constructed three 3/8-scale mostly fiberglass, unpowered F-15 RPRV's for a little more than $250,000 apiece (compared with $6.8 million for a full-size F-15). The FRC set up a dedicated RPRV control facility in a room on the first floor next to the hangar for the RPRV and set up a much more sophisticated control system than was used for an earlier RPRV--the Hyper III. The control facility featured a digital uplink capability, a ground computer, a television monitor, and a telemetry system. Launched from a B-52, the first F-15 RPRV flew its initial flight on October 12, 1973. The initial flights were recovered in mid-air by helicopters, but later flights employed horizontal landings by the remote research pilot, who "flew" the aircraft from the RPRV control facility. Chosen because of the risks involved in spin testing a full-scale fighter aircraft, the remotely piloted research technique enabled the pilot to interact with the vehicle much as he did in normal flight. Flying remotely, however, called for some special techniques to make up for the cues available to a pilot in the airplane but not to a remote pilot. It also allowed the flight envelope to be expanded more rapidly than conventional flight research methods permitted for piloted vehicles. During its first 26 flights, through the end of 1975, flight research over an angle-of-attack range of minus 20 degrees to plus 53 degrees with the 3/8-scale vehicle in the basic F-15 configuration allowed FRC engineers to test the mathematical model of the aircraft in an angle-of-attack range not previously examined in flight research. The basic airplane configuration proved to be resistant to departure from straight and level flight, hence to spins; however, the vehicle could be flown into a spin using a technique developed in the simulator. Data obtained during the first 26 flights gave researchers a better understanding of the spin characteristics of the full-scale fighter. Researchers later obtained spin data with the vehicle in other configurations at angles of attack as large as minus 70 degrees and plus 88 degrees. There were 35 flights of the 3/8-scale F-15s by the end of 1978 and 52 flights by mid-July of 1981. These included some in which the vehicle--redesignated the Spin Research Vehicle after it was modified from the basic F-15 configuration--evaluated the effects of an elongated nose and a wind-tunnel-designed nose strake (among other modifications) on the airplane's stall/spin characteristics. Results of flight research with these modifications indicated that the addition of the nose strake increased the vehicle's resistance to departure from the intended flight path, especially entrance into a spin. Large differential tail deflections, a tail chute, and a nose chute all proved effective as spin recovery techniques, although it was essential to release the nose chute once it had deflated in order to prevent an inadvertent reentry into a spin. Overall, remote piloting with the 3/8-scale F-15 provided high-quality data about spin characteristics. The SRV was about 23 and one-half feet long and had a 16-foot wing span.
NASA Identifier: NIX-ECN-4891