Shape-memory polymers and other technologies may create an airplane for all missions.
- By Michael Milstein
- Air & Space magazine, March 2007
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But if morphing is so great, why is it only taking off now? It hasn’t been much of an option before, since the stress of high-speed flying demands tough airframes with little flexibility in the air. So designers built different airplanes for different purposes: fuel-efficient, long-winged reconnaissance craft and high-speed fighters. The trouble is, the multi-aircraft approach can be cumbersome on today’s battlefields.
“If you’re a unit pinned down in Iraq, you don’t want to wait an hour” for help, says Terry Weisshaar, manager of DARPA’s morphing aircraft structures program.
That urgency is merging with advanced new materials into shape-shifting schemes that could outmaneuver rigid-winged foes. A Lockheed Martin design starts out with long wings with plenty of lift and maneuverability. Then the wings fold up. The inner half nests into the fuselage, leaving the outer half as a shorter wing better suited for high speed. So far it exists only as a wind tunnel model, but it’s loaded with new features. A polymer expands when heated by an electrical current, which reshapes the leading edge of the wing. The wing has a seamless skin, so when its joints move, gaps do not open up and create drag. One option is a “shape-memory” polymer that relaxes to allow movement and then stiffens back into shape.
Lockheed’s main competitor in morphing research, NextGen Aeronautics in Torrance, California, devised latticework wings with stretchable skin that sweep back, bat-like. NextGen is now designing an unmanned model that can not only morph in flight, but also shift shape to make sharper turns and steeper climbs.
If it’s a trick to morph an aircraft, imagine piloting the thing. Flight controls usually depend on predictable responses: Bank right, and the airplane turns right. But it may not if the wings are shifting shape at the same time. “If you shape-change too quickly, you destabilize the whole aircraft,” says Ephrahim Garcia, a Cornell University mechanical and aerospace engineer.
So engineers must devise smart flight control systems that adapt seamlessly to changes in the airplane during flight, something birds do instinctively. Lind and his students have studied seagulls and bats for flying tips. Their model maneuvers by twisting its wings. After all, a bird has no flaps or ailerons. It changes course by changing shape, and that’s the most effective morphing of all.