March 26, 2005 A flight research project that put a 21st century twist on a century-old technology -- a high-tech derivative of the Wright brothers' wing-warping method of controlling an aircraft's turning ability -- can be summed up in two words: "It works!" That was the conclusion of project manager Larry Myers as flight tests in the Active Aeroelastic Wing (AAW) project at NASA's Dryden Flight Research Center at Edwards Air Force Base, Calif., neared their end. Jointly sponsored and managed by NASA, the U.S. Air Force Research Laboratory and Boeing's Phantom Works, the AAW project is evaluating active control of lighter-weight flexible wings for improved maneuverability of high-performance military aircraft.
AAW concluded its second phase of flights in early March, evaluating the ability of software in its flight control computer to drive its modified control system so that aerodynamically induced twisting of the wings would provide roll control at transonic and supersonic speeds. An earlier phase of test flights conducted in late 2002 and early 2003 provided baseline data on the flexibility of the modified wing prior to new control laws being developed and installed. In all, about 80 flights were flown in the project's two phases. A small number of follow-on flights to evaluate several other control laws will be flown before the project concludes flight tests later this month.
"We have demonstrated a number of subsonic and supersonic flight conditions where we have actually taken advantage of the aeroelasticity of the wing," Myers explained. "We've gotten excellent results, good agreement with a predicted results (and) roll rates are comparable to what we predicted in simulation. It looks like we've proven the AAW concept."
"AAW represents a new philosophy for designing highly efficient wings in terms of structural weight, aerodynamic efficiency and control effectiveness," said Air Force AAW program manager Pete Flick, with the AFRL Air Vehicles Directorate at Wright-Patterson AFB, Ohio.
AAW research flights demonstrated banking or rolling performance at transonic and supersonic speeds close to that of production F/A-18s due to wing aeroelastic effects alone, without using differential stabilator movements to assist in roll control and with smaller control surface movements.
"We defined 18 test points for this second phase of the flight test program," explained NASA Dryden AAW project test pilot Dana Purifoy. "The test points started at 5,000 feet altitude and a speed of .85 Mach and went out to 25,000 feet and 1.3 Mach, (or 1.3 times the speed of sound). There were small variations on some points, but in general, we gained good knowledge about what kind of roll rates we expected to see."
Flick said the benefits of AAW depend on the specific application.
"With AAW, the control surface deflections can be chosen to produce an aeroelastic shape that minimizes the load on the structure (resulting in reduced structural weight), minimizes the drag of the aircraft (improving fuel efficiency or range), or maximizes the maneuver rates of the aircraft (enhancing maneuverability)," he said.
Data obtained from flight tests at Dryden will provide benchmark design criteria as guidance for a wide variety of future aircraft design concepts, ranging from high-performance fighters to high altitude, long endurance UAV concepts, large transport aircraft and high-speed, long-range aircraft.
The test aircraft-an F/A-18A obtained from the U.S. Navy--was modified with additional actuators to differentially control the split leading edge flaps and thinner wing skins that allowed the outer wing panels to twist up to five degrees. The traditional wing control surfaces-trailing edge ailerons and leading edge flaps-were used to provide the aerodynamic force needed to twist or "warp" the wing.
Extensive instrumentation measured the twisting and bending of the wing during flight. Numerous strain gauges were installed on the both wings, along with a flight deflection measurement system incorporating an optical sensor package in a dorsal pod atop the fuselage and 16 infrared light emitting diode markers on the upper surface of the left wing.
The F/A-18's modified wings underwent six months of structural loads testing in Dryden's Flight Loads Laboratory in 2001. The AAW F/A-18 then underwent extensive systems tests and simulation before flight tests began.
Once the flight research is successfully completed, Flick said the inventors will turn toward spreading the AAW design philosophy to the technical community.
"Transitioning AAW will likely be a relatively long process since it represents a design philosophy," he said. "The application to future (aircraft) will depend on specific design requirements of those future systems. The benefits are greatest when a vehicle design is initiated with AAW in mind, and limited when applied to an existing vehicle."
