Wings of change: shape-shifting aircraft may ply future skyways - Cover Story

Science News, Dec 6, 2003 by Peter Weiss

Like a bird, the world's very first airplane had flexible wings. The lightweight wood, cloth, and wire flyer, built by Wilbur Wright and Orville Wright and first flown on Dec. 17, 1903, was steered and stabilized by pulleys and cables that twist the wingtips. Some aviation historians say that this bird-inspired control mechanism was the pivotal innovation that enabled the Wright brothers to achieve heavier-than-air flight whereas others pursuing that same goal had failed.

Although the Wright brothers control strategy worked, it vanished quickly from aviation. Stiff wings became the standard because they could withstand greater forces associated with increased flying speeds and vehicle weights. To control the sturdier aircraft, designers added movable panels to the ends of those stiffwings. Those panels manipulate the airflow and thus the aero-dynamic forces that pilots use to make an airplane take off, turn, or change altitude.

Now, at the centennial of powered human flight, the original technique for controlling aircraft is in the midst of a revival. Indeed, aeronautical engineers have recently completed the first test flights of an experimental, supersonic fighter jet in which subtle twisting of the wings may steer the aircraft.

Going beyond wings that merely flex, scientists and engineers have also been developing aircraft surfaces capable of molding themselves from one shape into another, much as arm muscles bulge and flatten. These possibilities arise largely from the use of so-called smart materials, a broad range of substances that can shorten, elongate, flex, and otherwise respond mechanically to electricity, heat, light, or magnetic fields (SN: 11/22/97, p. 328). Even on a modest scale, such reshaping of aircraft contours could greatly enhance vehicle control and performance.

Looking yet further down the air lanes, far more drastic and complicated transformations--for instance, wings that can telescope, curl, or fold--may be on the way, yielding extraordinarily versatile airplanes and missiles that change their shapes according to the missions they are expected to perform. If research programs that are just starting eventually reveal that such large-scale morphing is feasible, the first of those aircraft may streak across the skies 20 to 30 years from now.

MORPHER'S HELPERS Many aircraft already change their shapes in striking ways. Both the BIB bomber and F-14 fighter plane pivot their wings from outstretched to swept-back positions. The recently retired supersonic commercial transport, the Concorde, tilted its nose downward for subsonic flight. Even when an ordinary commercial jet deploys its wing flaps, it could be said to be changing shape, or morphing.

However, all those familiar changes follow the same paradigm: Chunks of aircraft get pushed or pulled by an actuator, be it a motor, a piston, or other device. Bit players taking part in the action include linkages, structural reinforcements, and hydraulic lines. All told, the capability to change shape in even limited ways is expensive in terms of added aircraft weight and complexity.

New morphing strategies strive for more dramatic and versatile shape changes by means of simpler mechanisms and with little or no added weight. Taking advantage of today's high-tech materials, some rely on substances that can be bent or stretched into a new shape until reheating snaps them back into the old one. Other strategies employ fluids that thicken when subjected to a magnetic field or materials that expand or contract in response to electricity or light.

Another approach uses so-called compliant structures. Typically injection molded or machined from a single piece of material, these frameworks of metal or plastic can serve as the interior structures of, say, wing edges or other malleable components. These frameworks distribute forces in such a way that they can simultaneously flex like woven basketry in some places while resisting deformation elsewhere.

TWIST AND SOAR Whether or not an aircraft changes its shape, the same aerodynamic conditions govern its flight. The contours of a wing are designed to make air pressure over the wing lower than the pressure beneath it, creating the net lifting force that enables the aircraft to take off and fly.

The airflow across wings and fuselage also exerts a force, known as drag, that resists the forward motion of the aircraft. Drag results mostly from friction between the moving wing surface and the air.

To enhance lift and minimize drag, aeronautical engineers design airplanes to have smooth, continuous contours, free of drag-inducing irregularities. But the movable panels typically deployed for steering and control create drag-inducing obstacles.

To avoid that drawback, aircraft builders are now reconsidering what the Wright brothers called wing warping.

Next summer, researchers expect to demonstrate wing warping on an aircraft that's otherwise about as different from the Wright Flyer as modern technology allows. It's a modified F/A-18A fighter capable of supersonic flight. The jet has been retrofitted with exceptionally thin wings that were part of the original design for the 1980s-era aircraft but were rejected as a safety hazard because they twisted too much.