The Adaptive Compliant Trailing Edge (ACTE) project, a joint effort between NASA and the U.S. Air Force Research Laboratory (AFRL), has taken another step forward with the completion of the first series of successful flight tests. Flight testing is being conducted at NASA's Armstrong Flight Research Center (AFRC) in Edwards, CA, to determine whether flexible trailing-edge wing flaps are a viable approach to improve aerodynamic efficiency and reduce noise generated during takeoffs and landings.

Using a variable geometry airfoil system called FlexFoil designed and built by FlexSys Inc., of Ann Arbor, MI, researchers replaced the primary trailing edge wing flaps on a Gulfstream III business jet with 18-ft-span FlexFoil aircraft control surfaces on each wing, including 2-ft-wide fairings at each end to eliminate noise-generating gaps in the airframe. Advanced lightweight materials will reduce wing structural weight and give engineers the ability to aerodynamically tailor the wings to promote improved fuel economy and more efficient operations, while reducing environmental impacts.

Noise, drag, and weight benefits are not part of the current phase of testing, according to Thomas Rigney, ACTE Project Manager at NASA AFRC, instead focusing exclusively on increasing the technology readiness level and establishing its airworthiness.

“That means it can handle the structural aero loads and that we can have the predicted aero data match the test aero data,” Rigney said in an interview with Aerospace Engineering. “We have to test this in a relevant environment first. That’s the biggest hurdle that we have in front of us right now. After that, the other hurdles will be smaller. For example, the noise testing would be done on a more optimized aero shape than the one we’re currently testing.”

Thus far, three flights have been conducted with testing scheduled to continue through April 2015. Systems-level analysis will then be completed by September 2015.

“The three flight tests that we’ve done so far, the researchers are all very happy with the results,” Rigney said. “We’re all pleased that there was nothing structurally or otherwise that has developed that we would consider to be a problem. It’s actually better than anticipated. One of the pilot’s comments on the first flight was that it handled like a GIII. That’s a good thing because you don’t want it to be an exciting flight with something new that happens. We want it to be a standard flight where nothing exciting happens. A boring flight is a good flight.”

Researchers are using a build-up approach for the testing, setting the flaps at a specific angle for each flight and progressively ramping up the angle.

“What we do is set it at 0 degrees, for example, and then do four flights each at different altitudes and speeds, and then we’ll land the aircraft each time and after the four flights are concluded then we’ll go to 2 degrees and then 5 degrees, incrementally going up higher and higher until the limit that we determine, whatever that might be for this aircraft,” Rigney said.

The speed at which the flaps can be actuated—up to 30 degrees per second—and the sound they produce when deformed have been a surprise to even Rigney.

“When we first actuated [the flaps], the NASA engineers were expecting lots of popping, cracking, and all kinds of sounds you expect when something rigid moves like that, but when it was actuated it was as quiet as could be. It’s amazingly fluid in its movement,” he said.

This is actually not the first time that NASA has tested morphing flaps, having flight-tested an F-111 Aardvark with a morphing trailing edge and morphing leading edge in 1987. Although deemed successful performance-wise, it was heavier, more complex, and less reliable, which prevented the technology from advancing to the next phases.

“That technology is a different way of morphing the flaps that uses hinges, joints, and standard type of technology that we had available,” Rigney said. “This is the first time that we’ve had this type of morphing flaps. It uses different materials and different type of structure altogether that is more advantageous than the last one. This technology has advantages in that it is lighter, simpler, and therefore more reliable and less subject to fatigue.”