NASA sees folding wings as a key aeronautical technology for the aircraft of tomorrow, and to make it practical, the space agency is looking to a cutting edge, lightweight memory alloy. Developed for the Spanwise Adaptive Wing (SAW) project, the new alloy allows an aircraft's wing and control surfaces to change their shape in flight without heavy hydraulic systems.
Aircraft wings have come a long way from the ones made out of spruce wood and duck for the 1903 Wright flyer, but they've also become very complicated. They're also limited in how efficient they can be, because they depend on shifting and tilting rigid control surfaces to work.
If a wing was a bit more "rubbery," it could reshape itself into a variety of forms to meet different flight conditions while maintaining a smooth, aerodynamic shape to minimize turbulence. It's not a new idea. The Wright flyer's flight controls worked by warping the wings using simple pulleys, and for decades aerospace engineers have explored the concept, including in a modified North American XB-70 Valkyrie in the 1960s.
The problem is that the hydraulic mechanisms used to fold wings are heavy, bulky, and energy-consuming to the point where their disadvantages offset any of their advantages. As part of a joint effort, NASA's Armstrong Flight Research Center, Glenn Research Center, Langley Research Center, Boeing Research & Technology, and Area-I Inc are studying how to replace the usual hydraulics and motors with actuators that are 80 percent lighter and run by shape memory alloy as a way of creating wings that work better at subsonic, transonic, and supersonic speeds.
Recently, NASA carried out a series of flight tests over Rogers Dry Lake at Edwards Air Force Base in California using the remotely-controlled Prototype Technology-Evaluation Research Aircraft (PTERA) fitted with wings with tips that can fold between zero and 70 degrees up and down while in flight. The carbon composite UAV is kitted out with extensive telemetry and sensors, and was flown in a racetrack pattern with the wings in the zero position, then in the 70 percent up and down positions on subsequent flights.
The drone was able to do this using an actuator consisting of a tube made of thermal shape memory alloy. A memory alloy is an alloy that is triggered by some outside stimulus, usually heat, and springs back into its original state. This means that you could, for example, take a strip of alloy, crumple it up, and then watch it uncrumple itself when heated.
In the PTERA's wings, heating the tube causes it to twist, causing the tips of the wings to bend up or down.
"The performance of this new alloy that we developed between NASA and Boeing really showed outstanding performance," says Jim Mabe, Technical Fellow with Boeing Research and Technology. "From the time we started initial testing here at Boeing, up to the flight tests, the material behaved consistently stable, and showed a superior performance to previous materials."
According to NASA, the ability to fold their wings may one day allow for lighter, simpler aircraft with longer and more slender wings, stabilizers, and rudders that make them not only more fuel-efficient, but better able to taxi at airports and to morph their wings to exploit different air conditions instead of suffering from them. In addition, it would make supersonic flight easier.
"There's a lot of benefit in folding the wing tips downward to sort of 'ride the wave' in supersonic flight, including reduced drag. This may result in more efficient supersonic flight," says SAW Principal Investigator Matt Moholt. "Through this effort, we may be able to enable this element to the next generation of supersonic flight, to not only reduce drag but also increase performance, as you transition from subsonic to supersonic speeds. This is made possible using shape memory alloy."
NASA will continue with SAW flights later this year, concentrating on folding wings between 70 degrees up to 70 degrees down in a single flight. This will be followed by scaling up the technology for installation on an F-18.
"We put the SAW technology through a real flight environment, and these flights not only proved that we can fly with this technology, but they validated how we went about integrating it," says Moholt. "We will use the data from these flights to continue to improve upon the actuation system, including speed and smoothness of actually folding the wings, and we'll apply them as we get ready to fly again in 2018."
The video below shows one of the PTERA flights.
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