Nasa is investigating wings that fold down in flight to reduce drag and increase stability: (paid subscription required)

Folding the tips of a wing in flight can increase stability and reduce drag, NASA flight tests have shown. Now researchers plan additional flights to test control laws that actively adjust wing fold in flight to minimize drag. They are also proposing a project to test wing folding in supersonic flight.

The Spanwise Adaptive Wing (SAW) project, a rapid feasibility assessment under NASA’s Convergent Aeronautics Solutions (CAS) program, showed folding the outer sections of the wing in flight improved directional stability and control. In a new aircraft design, this would allow tail size and drag to be reduced.

One of the interesting bit of tech here is the actuator for wing folding.

As opposed to the rather large and heavy hydraulic actuators used by the Valkyrie, they are using memory metals and heating:

The tests involved NASA’s subscale unmanned prototype-technology evaluation and research aircraft (PTERA), essentially an 11%-scale Boeing 737, with the outer 15 in. on either side of its 176-in.-span wing hinged to fold up or down by up to 75 deg. The sections were folded in flight using shape memory alloy (SMA) actuators built into the hinge lines.

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SAW is built around torsion actuators made of an alloy that, when heated electrically, “remembers” and returns to its original twisted shaped, and in doing so moves the wingtip. The PTERA uses an actuator with a single SMA tube that produces 500 in.-lb. of torque. NASA Glenn has ground-tested a 5,000 in.-lb. actuator with nested SMA tubes. This was used to fold the outer wingbox of the F/A-18 wing.

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NASA Glenn has developed the nickel-titanium-hafnium shape-memory alloy and is working to scale up the tubes to sizes never before produced. “Glenn is working with the material supplier, pouring melts and breaking records,” says Moholt. “They are working to make sure it scales, with the right crystalline structure.”

The raw SMA stock is provided to Boeing, which gun-drills the tubes and assembles them into an actuator. The 20,000 in.-lb. SAW actuator has 12 0.5-in.-dia. tubes, each with a gear at the end driving a ring gear that moves the wing. Boeing is also “training” the SMA actuators, a process that requires thousands of thermal cycles.

The SAW project ends in September. The team is proposing a follow-on project that would demonstrate SMA wing folding on a supersonic aircraft. Folding the wingtips down in supersonic flight generates compression lift from shockwaves under the wing and can dramatically reduce induced drag, says Moholt. This was used in the North American XB-70 bomber. Folding the tips down also increases lateral stability and control in supersonic flight, allowing a smaller tail.

Also, at supersonic speeds, the drooped wingtips capture the shock-wave from the bottom of the aircraft, and can increase lift.

It’s a neat piece of kit.