X-36 Tailless Fighter Agility Demonstration

In 1989, engineers from NASA Ames Research Center and the Phantom Works, a division of McDonnell-Douglas—and later Boeing, following a merger of the two companies—began development of an agile, tailless aircraft configuration. Based on results of extensive wind tunnel test­ing and computational fluid dynamics (CFD) analysis, designers pro­posed building the X-36—a subscale, remotely piloted demonstrator—to validate a variety of advanced technologies. The X-36 project team con­sisted of personnel from the Phantom Works, Ames, and Dryden. NASA and Boeing were full partners in the project, which was jointly funded under a roughly fifty-fifty cost-sharing arrangement. Combined program cost for development, fabrication, and flight-testing of two aircraft was approximately $21 million. The program was managed at Ames, while Dryden provided flight-test experience, facilities, infrastructure, and range support during flight-testing.

The X-36 was a 28-percent-scale representation of a generic advanced tailless, agile, stealthy fighter aircraft configuration. It was about 18 feet long and 3 feet high, with a wingspan of just over 10 feet. A single

Williams International F112 turbofan engine provided about 700 pounds of thrust. Fully fueled, the X-36 weighed about 1,250 pounds.

The vehicle’s small size helped reduce program costs but increased risk because designers sacrificed aircraft system redundancy for lower weight and complexity. The subscale vehicle was equipped with only a single-string flight control system rather than a multiply redundant sys­tem more typical in larger piloted aircraft. Canards on the forward fuse­lage, split ailerons on the trailing edges of the wings, and an advanced thrust-vectoring nozzle provided directional control as well as speed brake and aerobraking functions. Because the X-36 was aerodynamically unsta­ble in both pitch and yaw, an advanced single-channel digital fly-by-wire control system was required to stabilize the aircraft in flight.[1013] Risks were mitigated by using a pilot-in-the-loop approach, to eliminate the need for expensive and complex autonomous flight control systems and the risks associated with such systems’ inability to correct for unknown or unfore­seen phenomena once in flight. Situational-awareness data were provided to the pilot’s ground station through a video camera mounted in the vehi­cle’s nose, a standard fighter-type head-up display, and a moving-map representation of the vehicle’s position.

Boeing project pilot Laurence A. Walker was a strong advocate for the advantages of a full-sized ground cockpit. When an engineer designs a control station for a subscale RPRV, the natural tendency might be to reduce the cockpit control and display suite, but Walker demonstrated that the best practice is just the opposite. In any ground-based cock­pit, the pilot will have fewer natural sensory cues such as peripheral vision, sound, and motion. Re-creating motion cues was impractical, but audio, visual, and HUD cues were re-created in order to improve situational awareness comparable to that of a full-sized aircraft.[1014] The X-36 Ground Control Station included a full-size stick, rudder pedals and their respective feel systems, throttle, and a full complement of mod­ern fighter-style switches. Two 20-inch monitors provided visual displays to the pilot. The forward-looking monitor provided downlinked video from a canopy-mounted camera, as well as HUD overlay with embedded flight-test features. The second monitor displayed a horizontal situation indicator, engine and fuel information, control surface deflection indi­cators, yaw rate, and a host of warnings, cautions, and advisories. An audio alarm alerted the pilot to any new warnings or cautions. A redun­dant monitor shared by the test director and GCS engineer served as a backup, should either of the pilot’s monitors fail.[1015] To improve the pilot’s ability to accurately set engine power and to further improve situational awareness, the X-36 was equipped with a microphone in what would have been the cockpit area of a conventional aircraft. Downlinked audio from this microphone proved to be a highly valuable cue and alerted the team, more than once, to problems such as screech at high-power settings and engine stalls before they became serious.

The X-36 had a very high roll rate and a mild spiral divergence. Because of its size, it was also highly susceptible to gusty wind condi­tions. As a result, the pilot had to spend a great deal of time watching the HUD, the sole source of attitude cues. Without kinesthetic cues to signal a deviation, anything taking the pilot’s focus away from the HUD (such as shuffling test cards on a kneeboard) was a dangerous distrac­tion. To resolve the problem, the X-36 team designed a tray to hold test cards at the lower edge of the HUD monitor for easy viewing.[1016] Walker

piloted the maiden flight May 17, 1997. The X-36 flight-test envelope was limited to 160 knots to avoid structural failure in the event of a flight con­trol malfunction. If a mishap occurred, an onboard parachute was pro­vided to allow safe recovery of the X-36 following an emergency flight termination. Fortunately, the initial flight was a great success with no obvious discrepancies.

The second flight, however, presented a significant problem as the video and downlink signals became weak and intermittent while the X-36 was about 10 miles from the GCS at 12,000 feet altitude. As pro-^^^B^9 grammed to do, the X-36 went into lost-link autonomous operation, giv­ing the test team time to initiate recovery procedures to regain control.

The engineers were concerned, as each intermittent glimpse of the data showed the vehicle in a steeper angle of bank, well beyond what had yet been flown. Eventually, Walker regained control and made an unevent­ful landing. The problem was later traced to a temperature sensitivity problem in a low-noise amplifier.[1017] Phase I of the X-36 program pro­vided a considerable amount of data on real-time stability margin and parameter identification maneuvers. Automated maneuvers, uplinked to the aircraft, greatly facilitated envelope expansion, and handling qual­ities were found to be remarkably good.

Phase II testing expanded the flight envelope and demonstrated new software. New control laws and better derivatives improved stabil­ity margins and resulted in improved flying qualities. The final Phase II flight took place November 12, 1997. During a 25-week period, 31 safe and successful research missions had been made, accumulating a total of 15 hours and 38 minutes of flight time and using 4 versions of flight control software.[1018] In a follow-on effort, the Air Force Research Laboratory (ARFL) contracted Boeing to fly AFRL’s Reconfigurable Control for Tailless Fighter Aircraft (RESTORE) software as a dem­onstration of the adaptability of a neural-net algorithm to compensate for in-flight damage or malfunction of aerodynamic control surfaces.

Two RESTORE research flights were flown in December 1998, with the adaptive neural-net software running in conjunction with the orig­inal proven control laws. Several in-flight simulated failures of con­trol surfaces were introduced as issues for the reconfigurable control algorithm to address. Each time, the software correctly compensated

for the failure and allowed the aircraft to be safely flown in spite of the degraded condition.[1019] The X-36 team found that having a trained test pilot operate the vehicle was essential because the high degree of air­craft agility required familiarity with fighter maneuvers, as well as with the cockpit cues and displays required for such testing. A test pilot in the loop also gave the team a high degree of flexibility to address prob­lems or emergencies in real time that might otherwise be impossible with an entirely autonomous system. Design of the ground cockpit was also critical, because the lack of normal pilot cues necessitated devel­opment of innovative methods to help replace the missing inputs. The pilot also felt that it was vital for flight control systems for the subscale vehicle to accurately represent those of a full-scale aircraft.

Some X-36 team members found it aggravating that, in the minds of some upper-level managers, the test vehicle was considered expend­able because it was didn’t carry a live crewmember. Lack of redundancy in certain systems created some accepted risk, but process and safety awareness were key ingredients to successful execution of the flight – test program. Accepted risk as it extended to the aircraft and onboard systems did not extend to processes that included qualification testing of hardware and software.[1020] The X-36 demonstrator program was aimed at validating technologies proposed by McDonnell-Douglas (and later Boeing) for early concepts of a Joint Strike Fighter (JSF) design, as well as unmanned combat air vehicle (UCAV) proposals. Results were immediately applicable to the company’s X-45 UCAV demonstrator project.[1021]