EVER since the Wright brothers came up with the idea of wing warping to steer their plane, aircraft designers have dreamed of altering the contours of their creations on the fly. The shape of an aircraft鈥檚 tail and the length of its wings, for example, determine its flying characteristics, so being able to alter them during flight could make existing designs more versatile, and turn specialist aircraft into workhorses that can carry out all kinds of tasks.
Now a new design approach based on smart materials and actuators could turn shape-shifting from a dream into the hottest aeronautical must-have since the jet engine. The result would be planes with wings that grow, bend and shrink on command, morphing into the best shape for the task in hand.
This kind of shape-shifting has been tried before. The US, Europe and the USSR all developed aircraft in the 60s and 70s that could swing their wings forward or back as required, including the MiG-29, the F-14 and the Tornado. But the severe limitations of this design soon became apparent. Mounting the wing on a single pivot means the joint has to be strong, heavy and mechanically complex. The extra weight slashes fuel economy, and the complexity increases costs. 鈥淪wing wings are a really inefficient way to do it鈥�, says Terry Weisshaar of the Pentagon鈥檚 Defense Advanced Research Projects Agency in Washington DC 鈥� the same hotbed of ideas that created the internet in the 1960s.
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As head of DARPA鈥檚 鈥渕orphing aircraft structures鈥� project, Weisshaar has handed out tens of millions of dollars to three companies 鈥� Lockheed Martin, Raytheon and NextGen Aeronautics 鈥� to see which can come up with the best way for a plane to change its wing shape safely mid-flight. 鈥淭he aim is to give a single military airplane the ability to perform drastically different missions,鈥� says Weisshaar. The shape of a plane that needs to 鈥渓oiter鈥� for a long time, on reconnaissance say, and the shape of one that needs to fly fast to intercept a target, are very different. Loitering requires long wings with a large surface area, whereas high speeds call for narrow, smaller wings. So DARPA aims to create wings that can change their surface area by more than 150 per cent.
So how will they do it? DARPA has three ideas on the drawing board. Lockheed Martin, based in Fort Worth, Texas, is exploring an origami-like folding wing (see Diagram), while NextGen Aeronautics, based in California, envisages a 鈥渂atwing鈥� comprising a series of 鈥渟liding skins鈥� that unlatch, fan out to create new wing shapes, and then latch together again. Raytheon, based in Waltham, Massachusetts, is working on a wing that telescopes out of the fuselage 鈥� initially for cruise missiles.
To reshape wings during flight requires arrays of tiny but powerful devices called compact hybrid actuators. These motors use a piezoelectric material 鈥� one which deforms slightly when an electric field is applied to it. Apply the field repeatedly and the motor can be made to ratchet along a toothed track, pushing or pulling a section of wing as it goes. Small piezoelectric motors could sit inside a wing and deliver force where it is needed.
But shape-shifting will create another headache: any gaps that appear as you open up a joint in a wing will cause drag. And if an aircraft is travelling at supersonic speeds, the stresses this drag creates could rip a wing apart. So DARPA is looking at a new generation of 鈥渟hape memory鈥� materials that might stretch and move with the wings, sealing joints as they open and close.
Shape memory metals have been around since the 1930s, but it is new shape memory polymers that are catching DARPA鈥檚 eye, since they are more versatile and flexible. An SMP鈥檚 chemical structure allows it to be stretched and pulled into various shapes, yet it will automatically resume its original shape when heated above a certain threshold. For example, an aerofoil-shaped sheet of SMP could be bent into a flat sheet, but when it is heated above its transition temperature, it would snap back into an aerofoil shape.
The plan is to cover the joints of a folding or telescoping wing with SMP, so as the wing changes shape, the polymer stretches with the movement. When the shape shift is complete, tiny heaters attached to the SMP would warm the material above its transition temperature and the SMP would reseal the joints around the new wing configuration. Research is focusing on making these polymers strong and yet stretchy enough to cope with repeated use at high air speeds.
Engineers also plans to revisit an idea the Wright brothers borrowed from birds. If you can morph the wings quickly enough, shortening one wing slightly and tilting the other, it should be possible to steer the craft much like a bird steers. This would save weight and reduce drag, since you would not need ailerons, rudders or tail fins.
Boeing, NASA and the US Air Force are also interested in borrowing from birds. Together they are developing what鈥檚 called an 鈥渁ctive aeroelastic鈥� wing. By turning an aircraft鈥檚 entire wing into a deformable, control surface you can dispense with ailerons and flaps 鈥� when you want to steer or roll you simply twist the whole wing. Boeing expects the technique to produce a 20 per cent saving in fuel. Clearly Orville and Wilbur had it right all along.
