The RPV Comes of Age as RDT&E Asset: The F-15 RPRV/SRV

NASA’s work with the RPV concept came of age when the agency applied RPV technology to support the Research, Development, Test, and Evaluation (RDT&E) of a new Air Force fighter, the McDonnell – Douglas (subsequently Boeing) F-15 Eagle. In 1969, the Air Force selected McDonnell-Douglas Aircraft Corporation to build the F-15, a Mach-2- capable air superiority fighter airplane designed using lessons learned during aerial combat over Vietnam. The prototype first flew in July 1972.

In the months leading up to that event, Maj. Gen. Benjamin Bellis, chief of the F-15 System Program Office at Wright-Patterson Air Force Base,

OH, requested NASA assistance in testing a three-eights-scale model F-15 RPRV to explore aerodynamic and control system characteristics of the F-15 configuration in spins and high-angle-of-attack flight. Such maneu­vers can be extremely hazardous. Rather than risk harm to a valuable test pilot and prototype, a ground pilot would develop stall/spin recov­ery techniques with the RPRV and pass lessons learned to test pilots fly­ing the actual airplanes.

In April 1972, NASA awarded McDonnell-Douglas a $762,000 con­tract to build three F-15 RPRV models. Other contractors provided electronic components and parachute-recovery equipment. NASA technicians installed avionics, hydraulics, and other subsystems. The F-15 RPRV was 23.5 feet long, was made primarily of fiberglass and wood, and weighed 2,500 pounds. It had no propulsion system and was designed for midair recovery using a helicopter. Each model cost a little over $250,000, compared with $6.8 million for a full-scale F-15 aircraft.[907] Every effort was made to use off-the-shelf components and equipment readily available at the Flight Research Center, including

hydraulic components, gyros, and telemetry systems from the lifting body research programs. A proportional uplink, then being used for instru­ment-landing system experiments, was acquired for the RPRV Ground Control Station (GCS). The ground cockpit itself was fashioned from a general-purpose simulator that had been used for stability-and – control studies. Data-processing computers were adapted for use in a programmable ground-based control system. A television cam­era provided forward visibility. The midair recovery system (MARS) parachute mechanism was taken from a Firebee drone.[908] The first F-15 RPRV arrived at the Flight Research Center in December 1972 but wasn’t flown until October 12, 1973. The model was carried to an altitude of about 45,000 feet beneath the wing of a modified B-52 Stratofortress known as the NB-52B. Following release from the launch pylon at a speed of 175 knots, ground pilot Einar Enevoldson guided the craft through a flawless 9-minute flight, during which he explored the vehi­cle’s basic handling qualities. At 15,000 feet altitude, a 12-foot spin – recovery parachute deployed to stabilize the descent. An 18-foot engage­ment chute and a 79-foot-diameter main chute then deployed so that the RPRV could be snagged in flight by a hook and cable beneath a helicopter, and set down gently on an inflated bag.[909] Enevoldson found the task of flying the RPRV very challenging, both physically and psychologically. The lack of physical cues left him feeling remote from the essential reassuring sensations of flight that provide a pilot with situational feed­back. Lacking sensory input, he found that his workload increased and that subjective time seemed to speed up. Afterward, he reenacted the mission in a simulator at 1.5 times actual time and found that the pace seemed the same as it had during the flight.

Researchers had monitored his heart rate during the flight to see if it would register the 70 to 80 beats per minute typical for a piloted test flight. They were surprised to see the readings indicate 130 to 140 beats per minute as the pilot’s stress level increased. Enevoldson found flying the F-15 RPRV less pleasant or satisfying than he normally did a difficult or demanding test mission.[910] "The results were gratifying,” he wrote in his postflight report, "and some satisfaction is gained from the

success of the technical and organizational achievement—but it wasn’t fun.”[911] In subsequent tests, Enevoldson and other research pilots explored the vehicle’s stability and control characteristics. Spin testing confirmed the RPRV’s capabilities for returning useful data, encouraging officials at the F-15 Joint Test Force to proceed with piloted spin trials in the preproduction prototypes at Edwards.[912] William H. "Bill” Dana piloted the fourth F-15 RPRV flight, on December 21, 1973. He collected about 100 seconds of data at angles of attack exceeding 30 degrees and 90 sec­onds of control-response data. Dana had a little more difficulty control­ling the RPRV in flight than he had in the simulator but otherwise felt everything went well. At Enevoldson’s suggestion, the simulator flights had been sped up to 1.4 times actual speed, and Dana later acknowl­edged that this had provided a more realistic experience.

During a postflight debriefing, Dana was asked how he liked flying the RPRV. He responded that it was quite different from sitting in the cockpit of an actual research vehicle, where he generally worried and
fretted until just before launch. Then he could settle down and just fly the airplane. With the RPRV, he said, he was calm and cool until launch and then felt keyed up through the recovery.[913] The first of several inci­dents involving the MARS parachute gear occurred during the ninth flight. The recovery helicopter failed to engage the chute, and the RPRV descended to the ground, where it was dragged upside down for about a quarter mile. Fortunately, damage was limited to the vertical tails, can­opy bulge, and nose boom. The RPRV was severely damaged at the end of the 14th flight, when the main parachute did not deploy because of failure of the MARS disconnect fitting.

Rather than repair the vehicle, it was replaced with the second F-15 RPRV. During the craft’s second flight, on January 16, 1975, research pilot Thomas C. McMurtry successfully completed a series of planned maneuvers and then deployed the recovery parachute. During MARS retrieval, with the RPRV about 3,000 feet above the ground, the towline separated. McMurtry quickly assumed control and executed an emer­gency landing on the Edwards Precision Impact Range Area (PIRA). As a result of this success and previous parachute-recovery difficulties, further use of MARS was discontinued. The RPRV was modified with landing skids, and all flights thereafter ended with horizontal touch­downs on the lakebed.[914] The F-15 RPRV project came to a halt December 17, 1975, following the 26th flight, but this did not spell the end of the vehicle’s career. In November 1977, flights resumed under the Spin Research Vehicle (SRV) project. Researchers were interested in evalu­ating the effect of nose shape on the spin susceptibility of modern high – performance fighters. Flight-testing with the F-15 model would augment previous wind tunnel experiments and analytical studies. Baseline work with the SRV consisted of an evaluation of the basic nose shape with and without two vortex strips installed. In November 1978, following nine baseline-data flights, the SRV was placed in inactive status pending the start of testing with various nose configurations for spin-mode determi­nation, forebody pressure-distribution studies, and nose-mounted spin – recovery parachute evaluation. Flights resumed in February 1981.[915]

When the SRV program ended in July 1981, the F-15 models had been carried aloft 72 times: 41 times for the RPRV flights and 31 times for the SRV. A total of 52 research missions were flown with the two aircraft: 26 free flights with each one. There had been only 2 ground aborts, 1 aborted planned-captive flight, and 15 air aborts prior to launch. Of 16 MARS recoveries, 13 were successful. Five landings occurred on the PIRA and 34 on the lakebed.[916] Flight data were correlated with wind tunnel and mathematical modeling results and presented in vari­ous technical papers. Tests of the subscale F-15 models clearly demon­strated the value of the RPRV concept for making bold, rapid advances in free-flight testing of experimental aircraft with minimal risk and max­imum return on investment. R. Dale Reed wrote that, "If information obtained from this program avoids the loss of just one full-scale F-15, then the program will have been a tremendous bargain.”[917]

Indeed it was: spin test results of the F-15 model identified a poten­tially dangerous "yaw-trip” problem with the full-scale F-15 if it had an offset airspeed boom. Such a configuration, the F-15 RPRV showed, might exhibit abrupt departure characteristics in turning flight as angle of attack increased. Subsequently, during early testing of F-15C aircraft equipped with fuselage-hugging conformal fuel tanks (like those subse­quently employed on the F-15E Strike Eagle) and an offset nose boom, Air Force test pilot John Hoffman experienced just such a departure. Review of the F-15 RPRV research results swiftly pinpointed the prob­lem and alleviated fears that the F-15 suffered from some inherent and major flaw that would force a costly and extensive redesign. This one "save” likely more than paid for the entire NASA F-15 RPRV effort.[918]