AVIATION is about to go back to its roots. Nearly 100 years after the Wright brothers鈥� first heavier-than-air powered flight, the US Air Force is testing an experimental plane that uses 鈥渨ing warping鈥�, the steering and control technique that kept Orville Wright aloft in 1903. But this time round, it will be at supersonic speeds.
Unlike conventional aircraft wings, which use movable surfaces like flaps and ailerons for control, wing warping bends the entire wing. The USAF calls it 鈥渁ctive aeroelastic wing鈥� technology, and is investing $41 million in the project in the hope that it will lead to lighter, more manoeuvrable supersonic planes.
For the first flight test next month at NASA鈥檚 Dryden Flight Research Center in California, the USAF has modified the wings of an early version of an F-18 fighter. This aircraft was chosen because its wings are more flexible than those of most others.
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On the modified plane, the entire front and back of each wing has a single flap running from root to tip (see Graphic). This will exploit the natural flexibility of the wings to steer the plane. Instead of relying solely on ailerons and flaps, the wing itself acts as a control surface. 鈥淭he control surfaces are deflected like conventional flaps, but the way that they are used is different,鈥� explains Pete Flick, programme manager at the Air Force Research Laboratory at the Wright-Patterson Air Force Base in Ohio.
鈥淣o aircraft wing is completely rigid,鈥� says Flick. A lot of twisting occurs during flight, and this normally makes the aircraft鈥檚 performance worse. However, the USAF has discovered in simulations that its wing-twisting technique actually boosts manoeuvrability and reduces vibration.
By reducing aerodynamic drag and load on the wings, the new technique should allow planes to be built with thinner, lighter wings. According to Flick, design studies show twisty-winged fighter aircraft could be 10 to 20 per cent lighter.
The new approach mirrors the way Orville Wright steered the Wright Flyer. After months spent observing and analysing the flight of turkey vultures, the Wrights concluded that the birds twist their wing tips to steer and to maintain level flight. So the brothers gambled that wing warping might be the key to making a manned aircraft controllable. They were right.
By shifting his weight in a control 鈥渟addle鈥�, Orville was able to deform the left or right wing tip to control the rolling motion of the aircraft. With the new aeroelastic wing technology, slight deflections of the leading and trailing edge will change the profile of the whole wing.
The computer models developed by the USAF to predict the outcome of different kinds of deflections have come up with some surprising results. In particular, they show that the aeroelastic wing might actually bend less than a conventional wing.
Next month鈥檚 test flight will compare these predictions with the actual behaviour of the aircraft. The technology will be tested at subsonic and supersonic speeds, though not in the unpredictable range close to the speed of sound. 鈥淲e won鈥檛 have the system engaged when we go through the sound barrier,鈥� says Denis Bessette, project manager at NASA Dryden.
If the project proves successful we can expect to see wing warping used on other aircraft, perhaps even using smart materials such as shape memory alloys to generate more precise electrically or heat-actuated wing twist. Ultimately, aeroelasticity represents a change in aviation design philosophy and a return to its beginnings, says Flick. 鈥淎ircraft will eventually sense their environment and morph their shape to perform optimally in many different flight conditions.鈥�