CASE
9
The Evolution of Remotely Piloted Research Vehicles
For over a half century, NASA researchers have worked to make remotely piloted research vehicles to complement piloted aircraft, in the forms of furnishing cheap “quick look” design validations, undertaking testing too hazardous for piloted aircraft, and furnishing new research capabilities such as high-altitude solar-powered environmental monitoring. The RPRV has evolved to sophisticated fly-by-wire inherently unstable vehicles with composite structures and integrated propulsion.
Case-9 Cover Image: One of two small APV-3 aircraft flown in the joint Ames-Dryen Networked UAV Teaming Experiment flares for landing on a roadway on a remote area of Edwards Air Force Base. NASA.
Since the mid-1990s, researchers at the National Aeronautics and Space Administration (NASA) have increasingly relied on unmanned aerial vehicles (UAVs) to fill roles traditionally defined by piloted aircraft. Instead of strapping themselves into the cockpit and taking off into the unknown, test pilots more often fly remotely piloted research vehicles (RPRVs) from the safety of a ground-based control station. Such craft are ideally suited to serve as aerodynamic and systems testbeds, airborne science platforms, and launch aircraft, or to explore unorthodox flight modes. NASA scientists began exploring the RPRV concept at Dryden Flight Research Center, Edwards, CA, in the 1960s. Since then, NASA RPRV development has contributed significantly to such technological innovations as autopilot systems, data links, and inertial navigation systems, among others. By the beginning of the 21st century, use of the once-novel RPRV concept had become standard practice.
There is no substitute—wind tunnel and computer modeling notwithstanding—for actual flight data. The RPRV provides real-world results while providing the ground pilot with precisely the same responsibilities and tasks as if he were sitting in a cockpit onboard a research airplane. As in piloted flight-testing, the remote pilot is responsible for performing data maneuvers, evaluating vehicle and systems performance, and reacting to emergency situations.
A ground pilot may, in fact, be considered the most versatile element of an RPRV system. Since experimental vehicles are designed to venture into unexplored engineering territory, the remote pilot may be called upon to repeat or abort a test point, or execute additional tasks not included in the original flight plan. Not all unmanned research vehicles require a pilot in the loop, but having one adds flexibility and provides an additional level of safety when performing hazardous maneuvers.[1]
Reducing the High Cost of Flight Research
Research aircraft are designed to explore advanced technologies and new fight regimes. Consequently, they are often relatively expensive to build and operate, and inherently risky to fly. Flight research from the earliest days of aviation well into the mid-20th century resulted in a staggering loss of life and valuable, often one-of-a-kind, aircraft.
This was tragically illustrated during experimental testing of advanced aircraft concepts, early jet-powered aircraft, and supersonic rocket planes of the 1940s and 1950s at Muroc Army Air Field in the Mojave Desert. Between 1943 and 1959, more than two-dozen research airplanes and prototypes were lost in accidents, more than half of them fatal. Among these were several of Northrop’s flying wing designs, including the N9M-1, XP-56, and both YB-49 prototypes. Early variants of Lockheed P-80 and F-104 jet fighters were lost, along with the two Martin XB-51 bomber prototypes. A rocket-powered Bell X-1 and its second-generation stablemates, the X-1A and X-1D, were lost to explosions—all fortunately nonfatal—and Capt. Milburn Apt died in the Bell X-2 after becoming the first human to fly more than three times the speed of sound.
By the 1960s, researchers began to recognize the value of using remotely piloted vehicles (RPVs) to mitigate the risks associated with flight-testing. During World War I and World War II, remotely controlled aircraft had been developed as weapons. In the postwar era, drones served as targets for missile tests and for such tasks as flying through clouds of radioactive fallout from nuclear explosions to collect particulate samples without endangering aircrews. By the 1950s, cruise-missile prototypes, such as the Regulus and X-10, were taking off and landing under radio control. Several of these vehicles crashed, but without a crew on board, there was no risk of losing a valuable test pilot.[2] Over the years, advances in electronics greatly increased the reliability of control systems, rendering development of RPRVs more practical. Early efforts focused on guidance and navigation, stabilization, and remote control. Eventually, designers worked to improve technologies to support these capabilities through the integration of improved avionics, microprocessors, and computers. The RPRV concept was attractive to researchers because it built confidence in new technology through demonstration under actual flight conditions, at relatively low cost, in quick response to demand, and at no risk to the pilot.
Taking the pilot out of the airplane provided additional savings in terms of development and fabrication. The cost and complexity of robotic and remotely piloted vehicles are generally less than those of comparable aircraft that require an onboard crew, because there is no need for life-support systems, escape and survival equipment, or hygiene facilities. Hazardous testing can be accomplished with a vehicle that may be considered expendable or semiexpendable.
Quick response to customer requirements and reduced program costs resulted from the elimination of redundant systems (usually added for crew safety) and man-rating tests, and through the use of less complex structures and systems. Subscale test vehicles generally cost less than full-size airplanes while providing usable aerodynamic and systems data. The use of programmable ground-based control systems provides additional flexibility and eliminates downtime resulting from the need for extensive aircraft modifications.[3]
Modeling the Future: Radio-Controlled Lifting Bodies
Robert Dale Reed, an engineer at NASA’s Flight Research Center (later renamed NASA Dryden Flight Research Center) at Edwards Air Force Base and an avid radio-controlled (R/C) model airplane hobbyist, was one of the first to recognize the RPRV potential. Previous drone aircraft had been used for reconnaissance or strike missions, flying a restricted number of maneuvers with the help of an autopilot or radio signals from a ground station. The RPRV, on the other hand, offered a versatile platform for operating in what Reed called “unexplored engineering territory.”[4] In 1962, when astronauts returned from space in capsules that splashed down in the ocean, NASA and Air Force engineers were discussing a revolutionary concept for spacecraft reentry vehicles. Wingless lifting bodies—half-cone-shaped vehicles capable of controlled flight using the craft’s fuselage shape to produce stability and lift—could be controlled from atmospheric entry to gliding touchdown on a conventional runway. Skeptics believed such craft would require deployable wings and possibly even pop-out jet engines.
Reed believed the basic lifting body concept was sound and set out to convince his peers. His first modest efforts at flight demonstration were confined to hand-launching small paper models in the hallways of the Flight Research Center. His next step involved construction, from balsa wood, of a 24-inch-long free-flight model.
The vehicle’s shape was a half-cone design with twin vertical-stabilizer fins with rudders and a bump representing a cockpit canopy. Elevons provided longitudinal trim and turning control. Spring-wired tricycle wheels served as landing gear. Reed adjusted the craft’s center of gravity until he was satisfied and began a series of hand-launched flight tests. He began at ground level and finally moved to the top of the NASA Administration building, gradually expanding the performance envelope. Reed found the model had a steep gliding angle but remained upright and landed on its gear.
He soon embarked on a path that presaged eventual testing of a full-scale, piloted vehicle. He attached a thread to the upper part of the nose gear and ran to tow the lifting body aloft, as one would launch a kite. Reed then turned to one of his favorite hobbies: radio-controlled, gas-powered model airplanes. He had previously used R/C models to tow free flight model gliders with great success. By attaching the towline to the top of the R/C model’s fuselage, just at the trailing edge of the wing, he ensured minimum effect on the tow plane from the motions of the lifting body model behind it.
Reed conducted his flight tests at Sterk’s Ranch in nearby Lancaster while his wife, Donna, documented the demonstrations with an 8-millimeter motion picture camera. When the R/C tow plane reached a sufficient altitude for extended gliding flight, a vacuum timer released the lifting body model from the towline. The lifting body demonstrated stable flight and landing characteristics, inspiring Reed and other researchers to pursue development of a full-scale, piloted lifting body, dubbed the M2-F1.[5] Reed’s R/C model experiments provided a low-cost demonstration capability for a revolutionary concept. Success with the model built confidence in proposals for a full-scale lifting body. Essentially, the model was scaled up to a length of 20 feet, with a span of 14.167 feet. A tubular steel framework provided internal support for the cockpit and landing gear. The outer shell was comprised of mahogany ribs and spars covered with plywood and doped cloth skin. As with the small model, the full-scale M2-F1 was towed into the air—first behind a Pontiac convertible and later behind a C-47 transport for extended glide flights. Just as the models paved the way for full-scale, piloted testing, the M2-F1 served as a pathfinder for a series of air-launched heavyweight lifting body vehicles—flown between 1966 and 1975—that provided data eventually used in development of the Space Shuttle and other aerospace vehicles.[6]
Radio-controlled mother ship and models of Hyper III and M2-F2 on lakebed with research staff. Left to right: Richard C. Eldredge, Dale Reed, James O. Newman, and Bob McDonald. NASA.
By 1969, Reed had teamed with Dick Eldredge, one of the original engineers from the M2-F1 project, for a series of studies involving modeling spacecraft-landing techniques. Still seeking alternatives to splashdown, the pair experimented with deployable wings and paraglider concepts. Reed discussed his ideas with Max Faget, director of engineering at the Manned Spacecraft Center (now NASA Johnson Space Center) in Houston, TX. Faget, who had played a major role in designing the Mercury, Gemini, and Apollo spacecraft, had proposed a Gemini-derived vehicle capable of carrying 12 astronauts. Known as the “Big G,” it was to be flown to a landing beneath a gliding parachute canopy.
Reed proposed a single-pilot test vehicle to demonstrate paraglider-landing techniques similar to those used with his models. The Parawing demonstrator would be launched from a helicopter and glide to a landing beneath a Rogallo wing, as used in typical hang glider designs. Spacecraft-type viewports would provide visibility for realistic simulation of Big G design characteristics.[7] Faget offered to lend a borrowed Navy SH-3A helicopter—one being used to support the Apollo program—to the Flight Research Center and provide enough money for several Rogallo parafoils. Hugh Jackson was selected as project pilot, but for safety reasons, Reed suggested that the test vehicle initially be flown by radio control with a dummy on board.
Eldredge designed the Parawing vehicle, incorporating a generic ogival lifting body shape with an aluminum internal support structure, Gemini-style viewing ports, a pilot’s seat mounted on surplus shock struts from Apollo crew couches, and landing skids. A general-aviation autopilot servo was used to actuate the parachute control lines. A side stick controller was installed to control the servo. On planned piloted flights, it would be hand-actuated, but in the test configuration, model airplane servos were used to move the side stick. For realism, engineers placed an anthropomorphic dummy in the pilot’s seat and tied the dummy’s hands in its lap to prevent interference with the controls. The dummy and airframe were instrumented to record accelerations, decelerations, and shock loads as the parachute opened.
The Parawing test vehicle was then mounted on the side of the helicopter using a pneumatic hook release borrowed from the M2-F2 lifting body launch adapter. Donald Mallick and Bruce Peterson flew the SH-3A to an altitude of approximately 10,000 feet and released the Parawing test vehicle above Rosamond Dry Lake. Using his R/C model controls, Reed guided the craft to a safe landing. He and Eldredge conducted 30 successful radio-controlled test flights between February and October 1969. Shortly before the first scheduled piloted tests were to take place, however, officials at the Manned Spacecraft Center canceled the project. The next planned piloted spacecraft, the Space Shuttle orbiter, would be designed to land on a runway like a conventional airplane does. There was no need to pursue a paraglider system.[8] This, however, did not spell the end of Reed’s paraglider research. A few decades later, he would again find himself involved with paraglider recovery systems for the Spacecraft Autoland Project and the X-38 Crew Return Vehicle technology demonstration.
Hyper III: The First True RPRV
In support of the lifting body program, Dale Reed had built a small fleet of models, including variations on the M2-F2 and FDL-7 concepts. The M2-F2 was a half cone with twin stabilizer fins like the M2-F1 but with the cockpit bulge moved forward from midfuselage to the nose. The full-scale heavyweight M2-F2 suffered some stability problems and eventually crashed, although it was later rebuilt as the M2-F3 with an additional vertical stabilizer. The FDL-7 had a sleek shape (somewhat resembling a flatiron) with four stabilizer fins, two horizontal, and two that were canted outward. Engineers at the Air Force Flight Dynamics Laboratory at Wright-Patterson Air Force Base, OH, designed it with hypersonic-flight characteristics in mind. Variants included wingless versions as well as those equipped with fixed or pop-out wings for extended gliding.[9] Reed launched his creations from a twin-engine R/C model plane he dubbed “Mother,” since it served as a mother ship for his lifting body models. With a 10.5-foot wingspan, Mother was capable of lofting models of various sizes to useful altitudes for extended glide flights. By the end of 1968, Reed’s mother ship had successfully made 120 drops from an altitude of around 1,000 feet.
The Hyper III, with its ground cockpit visible at upper left, was a full-scale lifting body remotely
piloted research vehicle. NASA.
One day, Reed asked research pilot Milton O. Thompson if he thought he would be able to control a research airplane from the ground using an attitude-indicator instrument as a reference. Thompson thought this was possible and agreed to try it using Reed’s mother ship. Within a month, at a cost of $500, Mother was modified, and Thompson had successfully demonstrated the ability to fly the craft from the ground using the instrument reference.[10] Next, Reed wanted to explore the possibility of flying a full-scale research airplane from a ground cockpit. Because of his interest in lifting bodies, he selected a simplified variant of the FDL-7 configuration based on research accomplished at NASA Langley Research Center. Known as Hyper III—because the shape would have a lift-to-drag (L/D) ratio of 3.0 at hypersonic speeds—the test vehicle had a 32-foot-long fuselage with a narrow delta planform and trapezoidal cross-section, stabilizer fins, and fixed straight wings spanning 18.5 feet to simulate pop-out airfoils that could be used to improve the low-speed glide ratio of a reentry vehicle. The Hyper III RPRV weighed about 1,000 pounds.[11]
Reed recruited numerous volunteers for his low-budget, low-priority project. Dick Fischer, a designer of R/C models as well as full-scale homebuilt aircraft, joined the team as operations engineer and designed the vehicle’s structure. With previous control-system engineering experience on the X-15, Bill “Pete” Peterson designed a control system for the Hyper III. Reed also recruited aircraft inspector Ed Browne, painter Billy Schuler, crew chief Herman Dorr, and mechanics Willard Dives, Bill Mersereau, and Herb Scott.
The craft was built in the Flight Research Center’s fabrication shops. Frank McDonald and Howard Curtis assembled the fuselage, consisting of a Dacron-covered, steel-tube frame with a molded fiberglass nose assembly. LaVern Kelly constructed the stabilizer fins from sheet aluminum. Daniel Garrabrant borrowed and assembled aluminum wings from an HP-11 sailplane kit. The vehicle was built at a cost of just $6,500 and without interfering with the Center’s other, higher-priority projects.[12] The team managed to scrounge and recycle a variety of items for the vehicle’s control system. These included a Kraft uplink from a model airplane radio-control system and miniature hydraulic pumps from the Air Force’s Precision Recovery Including Maneuvering Entry (PRIME) lifting body program. Peterson designed the Hyper III control system to work from either of two Kraft receivers, mounted on the top and bottom of the vehicle, depending on signal strength. If either malfunctioned or suffered interference, an electronic circuit switched control signals to the operating receiver to actuate the elevons. Keith Anderson modified the PRIME hydraulic actuator system for use on the Hyper III.
The team also developed an emergency-recovery parachute system in case control of the vehicle was lost. Dave Gold, of Northrop, who had helped design the Apollo spacecraft parachute system, and John Rifenberry, of the Flight Research Center life-support shop, designed a system that included a drogue chute and three main parachutes that would safely lower the vehicle to the ground onto its landing skids. Pyrotechnics expert Chester Bergener assumed responsibility for the drogue’s firing system.[13] To test the recovery system, technicians mounted the Hyper III on a flatbed truck and fired the drogue-extraction system while racing across the dry lakebed, but weak radio signals kept the three main chutes from deploying. To test the clustered main parachutes, the team dropped a weight equivalent to the vehicle from a helicopter.
Tom McAlister assembled a ground cockpit with instruments identical to those in a fixed-base flight simulator. An attitude indicator displayed roll, pitch, heading, and sideslip. Other instruments showed airspeed, altitude, angle of attack, and control-surface position. Don Yount and Chuck Bailey installed a 12-channel downlink telemetry system to record data and drive the cockpit instruments. The ground cockpit station was designed to be transported to the landing area on a two-wheeled trailer.[14] On December 12, 1969, Bruce Peterson piloted the SH-3A helicopter that towed the Hyper III to an altitude of 10,000 feet above the lakebed. Hanging at the end of a 400-foot cable, the nose of the Hyper III had a disturbing tendency to drift to one side or another. Reed realized later that he should have added a small drag chute to stabilize the craft’s heading prior to launch. Peterson started and stopped forward flight several times until the Hyper III stabilized in a forward climb attitude, downwind with a northerly heading.
As soon as Peterson released the hook, Thompson took control of the lifting body. He flew the vehicle north for 3 miles, then reversed course and steered toward the landing site, covering another 3 miles. During each straight course, Thompson performed pitch doublets and oscillations in order to collect aerodynamic data. Since the Hyper III was not equipped with an onboard video camera, Thompson was forced to fly on instruments alone. Gary Layton, in the Flight Research Center control room, watched the radar data showing the vehicle’s position and relayed information to Thompson via radio.
Dick Fischer stood beside Thompson to take control of the Hyper III just before the landing flare, using the model airplane radio-control box. Several miles away, the Hyper III was invisible in the hazy sky as it descended toward the lakebed. Thompson called out altitude readings as Fischer strained to see the vehicle. Suddenly, he spotted the lifting body, when it was on final approach just 1,000 feet above the ground. Thompson relinquished control, and Fischer commanded a slight left roll to confirm he had established radio contact. He then leveled the aircraft and executed a landing flare, bringing the Hyper III down softly on its skids.
Thompson found the experience of flying the RPRV exciting and challenging. After the 3-minute flight, he was as physically and emotionally drained as he had been after piloting first flights in piloted research aircraft. Worries that lack of motion and visual cues might hurt his piloting performance proved unfounded. It seemed as natural to control the Hyper III on gauges as it did any other airplane or simulator, responding solely to instrument readings. Twice during the flight, he used his experience to compensate for departures from predicted aerodynamic characteristics when the lift-to-drag ratio proved lower than expected, thus demonstrating the value of having a research pilot at the controls.[15]
The Next, More Ambitious Step: The Piper PA-30
Encouraged by the results of the Hyper III experiment, Reed and his team decided to convert a full-scale production airplane into a RPRV. They selected the Flight Research Center’s modified Piper PA-30 Twin Comanche, a light, twin-engine propeller plane that was equipped with both conventional and fly-by-wire control systems. Technicians installed uplink/downlink telemetry equipment to transmit radio commands and data. A television camera, mounted above the cockpit windscreen, transmitted images to the ground pilot to provide a visual reference—a significant improvement over the Hyper III cockpit. To provide the pilot with physical cues, as well, the team developed a harness with small electronic motors connected to straps surrounding the pilot’s torso. During maneuvers such as sideslips and stalls, the straps exerted forces to simulate lateral accelerations in accordance with data telemetered from the RPRV, thus providing the pilot with a more natural “feel.”[16] The original control system of pulleys and cables was left intact, but a few minor modifications were incorporated. The right-hand, or safety pilot’s, controls were connected directly to the flight control surfaces via conventional control cables and to the nose gear steering system via pushrods. The left-hand control wheel and rudder pedals were completely independent of the control cables, instead operating the control surfaces via hydraulic actuators through an electronic stability-augmentation system. Bungees were installed to give the left-hand controls an artificial “feel.” A friction control was added to provide free movement of the throttles while still providing friction control on the propellers when the remote throttle was in operation.
When flown in RPRV configuration, the left-hand cockpit controls were disabled, and signals from a remote control receiver fed directly into the control system electronics. Control of the airplane from the ground cockpit was functionally identical to control from the pilot’s seat. A safety trip channel was added to disengage the control system whenever the airborne remote control system failed to receive intelligible commands. In such a situation, the safety pilot would immediately take control.[17] Flight trials began in October 1971, with research pilot Einar Enevoldson flying the PA-30 from the ground while Thomas C. McMurtry rode on board as safety pilot, ready to take control if problems developed. Following a series of incremental buildup flights, Enevoldson eventually flew the airplane unassisted from takeoff to landing, demonstrating precise instrument landing system approaches, stall recovery, and other maneuvers.[18] By February 1973, the project was nearly complete. The research team had successfully developed and demonstrated basic RPRV hardware and operating techniques quickly and at relatively low cost. These achievements were critical to follow-on programs that would rely on the use of remotely piloted vehicles to reduce the cost of flight research while maintaining or expanding data return.[19]
Extending the Vision: The Evolution of Mini-Sniffer
The Mini-Sniffer program was initiated in 1975 to develop a small, unpiloted, propeller-driven aircraft with which to conduct research on turbulence, natural particulates, and manmade pollutants in the upper atmosphere. Unencumbered and flying at speeds of around 45 mph, the craft was designed to reach a maximum altitude of 90,000 feet. The Mini-Sniffer was capable of carrying a 25-pound instrument package to 70,000 feet and cruising there for about 1 hour within a 200-mile range.
The Aircraft Propulsion Division of NASA’s Office of Aeronautics and Space Technology sponsored the project and a team at the Flight Research Center, led by R. Dale Reed, was charged with designing and testing the airplane. Researchers at Johnson Space Center developed a hydrazine-fueled engine for use at high altitudes, where oxygen is scarce. To avoid delays while waiting for the revolutionary new engine, Reed’s team built two Mini-Sniffer aircraft powered by conventional gasoline engines. These were used for validating the airplane’s structure, aerodynamics, handling qualities, guidance and control systems, and operational techniques.[20] As Reed worked on the airframe design, he built small, hand-launched balsa wood gliders for qualitative evaluation of different configurations. He decided from the outset that the Mini-Sniffer should have a pusher engine to leave the nose-mounted payload free to collect air samples without disruption or contamination from the engine. Climb performance was given priority over cruise performance.
Eventually, Reed’s team constructed three configurations. The first two—using the same airframe—were powered by a single two-stroke, gasoline-fueled go-cart engine driving a 22-inch-diameter propeller. The third was powered by a hydrazine-fueled engine developed by James W. Akkerman, a propulsion engineer at Johnson Space Center. Thirty-three flights were completed with the three airplanes, each of which provided experimental research results. Thanks to the use of a six-degree-of-freedom simulator, none of the Mini-Sniffer flights had to be devoted to training. Simulation also proved useful for designing the control system and, when compared with flight results, proved an accurate representation of the vehicle’s flight characteristics.
The Mini-Sniffer I featured an 18-foot-span, aft-mounted wing, and a nose-mounted canard. Initially, it was flown via a model airplane radio-control box. Dual-redundant batteries supplied power, and fail-safe units were provi