X-56 Multi-Use Technology Testbed [MUTT]

The latest in a long series of experimental research aircraft, or X-planes, the X-56A was built by Lockheed Martin’s Advanced Development Projects division in Palmdale, California, under a contract from the U.S. Air Force Research Laboratory (AFRL), Wright-Patterson Air Force Base, Ohio. Powered by twin 85-pound-thrust JetCat P-400 micro jet engines, the airplane has a semi-flying-wing configuration with winglets at the tips. Lockheed Martin constructed two airframe center-bodies along with several sets of wings because plans called for tests involving both stiff and flexible airfoils.

The Multi-Use Technology Testbed [MUTT] is a small unmanned aircraft developed by the US Air Force Research Laboratory to test technologies needed for new kinds of lightweight, flexible aircraft. The 7.5-foot-long aircraft has a 28-foot wingspan and was powered by two 52-pound thrust JetCat P200-SX turbine engines. It was built in California under contract to Lockheed Martin Corp., which conducted the flight experiments for the Air Force Research Laboratory (AFRL).

Under a five-year project supported by the Fixed Wing Project of NASA's Fundamental Aeronautics Program, NASA is developing a flexible-wing flight control system and advanced sensors to measure wing shape and detect airflow separation. The project plans to use these to fly the X-56A with a slender, flexible wing with a much higher aspect ratio than currently used on commercial airliners. Research results from the low-speed sub-scale X-56A will benefit future designs for both subsonic and supersonic aircraft.

Dryden initially planned to conduct the flights for AFRL during summer 2012, and then take ownership of the X-56A MUTT for follow-on research after the Air Force tests were finished in early autumn 2012. The maiden flight of the first airframe – nicknamed Fido – in June 2013, by Lockheed Martin and AFRL initiated testing to explore technologies for active flutter suppression and gust-load alleviation for new kinds of lightweight, flexible aircraft. The second airframe – dubbed Buckeye – arrived at Armstrong on June 2, 2014. Three low-speed taxi tests were conducted in January 2015 and a medium-speed taxi test was accomplished in March. Buckeye’s initial flights will allow researchers to checkout aircraft systems, evaluate handling qualities, characterize and expand the airplane’s performance envelope, and verify preflight predictions regarding aircraft behavior. The results will inform planning for the next phase of testing.

The initial flight tests of the X-56A system, performed by Lockheed and AFRL in the latter half of 2013 and extending into early 2014, collected flight data on highly flexible structures and flutter suppression control technology. Initially flown with a conventional stiff wing, the aircraft is subsequently being used to evaluate active flutter suppression with the flexible wings in early 2014. After these flights are completed, the X-56A will be transferred to NASA Dryden to be used for research into lightweight structures and advanced control technologies for future efficient, environmentally friendly transport aircraft.

The modular X-56A system includes two center bodies, a set of stiff wings, three sets of flexible wings, a ground control station, and a transportation trailer. The X-56A has easily removed wings and is convertible to other wing configurations, such as a joined-wing planform or a wing-tail configuration. The aircraft is equipped with a ballistic parachute recovery system, which is intended to recover the fuselage and the majority of the aircraft systems in the event of an inflight wing failure.

Flexible wings and fuselages can result in significant reductions in the structural weight of aircraft. But unlike the short, stiff wings found on most aircraft today, long, thin wings like those on the X-56A are susceptible to uncontrollable vibrations, called flutter, that result from the force of air flowing over them. Thin wings can also be stressed by bending forces from wind gusts and atmospheric turbulence.

To maintain the long-term health of the structure and ride quality in a more flexible airplane, there is a need to actively alleviate gust loads on the airplane and suppress flutter, so gust load alleviation and active flutter suppression are two of the key technologies that NASA is working to advance. MUTT is designed to address this problem by enabling engineers to practice suppressing flutter by adjusting software programs in the aircraft’s flight control computer. With MUTT, researchers also expect to learn how better to ease gust loads, which will make flexible airplanes safer when they experience in-flight turbulence.

Although the X-56A MUTT is a low-speed, subsonic, sub-scale research aircraft, aircraft that fly faster than the speed of sound also can benefit from this research. The knowledge gained about flutter and gust suppression will be used in designing the proposed supersonic X-54, an aircraft that will demonstrate sonic boom-quieting technologies that could someday alleviate the noise concerns currently preventing supersonic commercial flight over land in the United States.

Researchers at NASA’s Armstrong Flight Research Center, Edwards, California, successfully conducted the agency’s first flight of the X-56A Multi-Utility Technology Testbed (MUTT) on 09 April 2015. The 20-minute flight marked the beginning of a research effort designed to yield significant advances in aeroservoelastic technology using a low-cost, modular, remotely piloted aerial vehicle.

Aeroservoelasticity involves the interaction of an airplane’s automatic flight controls with the response of non-rigid structures to aerodynamic forces. The X-56A is being flown in support of NASA’s Advanced Air Transport Technology (AATT) project’s Higher Aspect Ratio Wing subproject, Performance Adaptive Aeroelastic Wing element.

This was the first of eight planned Stiff Wing Controller Development (SWCD) envelope clearance flights. Successful efforts by the test team saw the X-56A attain an altitude of 4,000 feet above sea level and cleared for flight at up to 70 knots calibrated airspeed. Gary S. Martin, AATT associate project manager for integrated testing declared, “The flight went nearly exactly as rehearsed in the simulator.”