Vibration was
also an issue on the PTERA flights. This caused a shift in a
sensor reading that was caught by comparison with a string-potentiometer
position sensor. “We need a more robust sensor suite,” says Benafan.
Future work would also look at the power required by the actuators, which
has not been addressed under SAW.
Researchers are
also looking at a locking mechanism that would enable the actuator to
move the wing, lock it in position, then relax. Another concept is
back-to-back actuators driving a ratchet mechanism (like a socket
wrench), alternating between one driving the wing fold while the other is
cooling down.
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.
Additionally, Moholt says,
the center of pressure shifts aft when an aircraft goes supersonic. The
Concorde compensated for this shift by pumping fuel from forward to rear
trim tanks to maintain a center-of-gravity location that minimized drag.
“By folding the wings down, we can keep the center-of-pressure position
and avoid having to pump fuel around,” he says. The team is proposing
building a 2,000-lb. unmanned aircraft to demonstrate
supersonic SMA wing folding, but has yet to identify funding.
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