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 stiff wings. Those panels manipulate the airflow and thus the aerodynamic 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: http://www.sciencenews.org/sn_arc97/11_22_97/bob1.htm). Even on a modest scale, such reshaping of aircraft contours could greatly enhance vehicle control and performance.
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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.
Many aircraft already change their shapes in striking ways. Both the B1B 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.
Last spring, researchers from NASA, the Air Force Research Laboratory (AFRL), and Boeing Phantom Works, the advanced research arm of the aircraft manufacturer, conducted preliminary flight tests to measure forces and other wing parameters under various flight conditions.
The plane still has conventional control panels, such as ailerons that are typically raised or lowered to turn, or roll, the aircraft, and flaps, which are extended or retracted to vary the amount of lift. But these structures have been relegated to a supporting role, serving as the levers used to initiate specific wing reshapings that are wrought by onrushing air currents.
The so-called active aeroelastic wing has a major advantage: It can be up to 15 percent lighter than a conventional wing because it’s thinner and more flexible.
What’s more, using the entire wing as a control surface generates much more torque than using conventional flaps does. “We believe we could roll the aircraft better than a conventional F/A-18, under high-speed conditions,” says Peter M. Flick, AFRL’s program manager on the project.
At a lab in Germany, researchers have begun a related project in the realm of civilian aviation. The goal is to exploit in-flight twisting of the wings of Airbus transports to distribute aerodynamic forces more evenly along those wings. Shifting force from the wingtips toward the fuselage might reduce an aircraft’s weight by eliminating some of the heavy structural reinforcement from the wings’ outer reaches, says Hans Peter Monner of the German Aerospace Center (DLR) in Braunschweig. The challenge, he adds, is to make the twistable configuration lighter than the structural weight that would be avoided.
Not everyone is taking a full-wing approach to wing warping. Wing edges that can mold themselves into a variety of graceful curves and other deformations can also provide flight control while reducing drag.
In a recent joint effort by the Defense Advanced Research Projects Agency (DARPA), AFRL, and NASA, engineers devised a spine-like structure that snakes along a wing’s trailing edge. For the vertebra of that spine, researchers in the so-called Smart Wing Program used lightweight aluminum wedges. Each wedge could be independently extended or tilted using actuators. A stretchy silicone skin covered the spinal assembly.
In wind tunnel tests, the wedges on a model of the wing adjusted to form more than 70 different wing contours. What’s more, for the same amount of deflection that a conventional aileron provides, the deformable wing edge produced greater torque for turning the aircraft, the researchers report in an upcoming special issue of the Journal of Intelligent Materials Systems and Structures.
While such wing transformations look promising for high-speed, highly maneuverable combat jets, their value in large civil transports is doubtful, a recent study shows.
The DLR’s Monner and his colleagues have tested morphing structures, including a deformable trailing edge, intended to optimize wing lift-to-drag ratios and therefore fuel efficiencies for certain Airbus vehicles, such as the 300-to-400-passenger Airbus A340.
The performance of the flexible trailing edge was similar to that of a conventional set of flaps with an extra segment, the team reports. That’s because a layer of turbulent air some 2 centimeters thick hugs the wing surfaces of big, subsonic jets, making airflows less sensitive to underlying shapes, such as the sharp edges of conventional control surfaces, Monner explains.
Although wings have received the most attention from shape-change researchers, “there are advantages to making changes in the propulsion system as well,” says St. Louis–based Edward V. White of Boeing Phantom Works. What’s more, the advantages seem to apply to both military and civil aviation.
In a recently completed DARPA-funded study, White’s team demonstrated dynamic reconfiguring of engine-inlet nozzles of F-15 Eagle fighter aircraft that could boost the aircraft’s range by up to 20 percent.
Now, the Boeing engineers are looking into subtle shape shifting of teeth, called chevrons, which intrude slightly into the exhaust nozzles of passenger jets to reduce engine noise on take-off. Unfortunately, the chevrons also reduce fuel efficiency. By making chevrons that would reshape themselves to withdraw from the flow after take off, aircraft makers could recoup a small but significant amount of fuel.
“It’s something that would benefit just about all commercial aircraft,” White says.
Break the mold
Much more radical morphing is just beginning to come off some aeronautical engineers’ drawing boards. As a starting point, several companies are exploring major wing transformations.
Consider Skunk Works in Palmdale, Calif., the aeronautics research and development arm of Lockheed Martin. Engineers there have recently proposed an aircraft that would perform wing calisthenics. To transition between a large, fully extended wing suitable for cruising to a smaller, more combat-tailored wing positioned above the fuselage, the aircraft would lift and fold its wings while simultaneously drawing them together.
NextGen Aeronautics, a small company in Torrance, Calif., is investigating a morphing wing that would convert between the extremes of a swept-back “bat wing” for combat and a narrow, planklike wing for cruising.
Raytheon Missile Systems in Tucson, is exploring a telescoping wing for a cruise missile.
These participants in a year-old DARPA program called Morphing Aircraft Structures have pledged to create, by 2005, functional, scale-model wings that can vary in area or length by 50 percent. That’s a huge change, considering that the control surfaces of a conventional, fixed-wing aircraft modify wing areas by no more than about 5 percent, says DARPA’s Terry A. Weisshaar, who heads the program.
Each approach presents its own challenges. When an aircraft folds its wings on the fly, rapid, large shifts occur in its center of gravity and another balance point known as the aerodynamic center. Such shifts, absent among conventional fixed-wing aircraft, could make the plane spin or become otherwise unmanageable, says Daniel J. Inman of Virginia Polytechnic Institute and State University in Blacksburg.
“You could think of it like going out in a small fishing boat and trying to keep control of it while people jump around in front,” he says. Working with Lockheed Martin engineers, Inman and his colleagues have used computers to simulate the wing transition and have come up with sequences of control-surface adjustments that they predict would maintain stability.
“Stability through the transition is an issue,” says engineer Charles Chase of Lockheed Martin. However, he adds, wind tunnel tests—as well as computer simulations—indicate this transition can be managed. Going beyond DARPA’s requirement of producing a demonstration wing, the Lockheed Martin team is constructing a radio-controlled, jet-powered, unmanned aerial vehicle with which to demonstrate the folding-wing capability, says Chase, a program manager of the Skunk Works’ advanced development program. The researchers expect to start test flights of the vehicle, which will have a 9-foot wingspan, in the next few months.
As for the bat-wing structure proposed by NextGen, it would have to be “like an umbrella that you fold up and unfold back out,” says Jayanth N. Kudva, an aerospace engineer who led the Smart Wing program when he was at Northrop Grumman in El Segundo, Calif. Kudva founded NextGen last January. Making the bat-wing structure even more challenging is the requirement that the covering of this novel wing must remain taut and smooth at all times, Kudva says, unlike an umbrella’s fabric, which becomes taut only when the apparatus is fully open.
Compared with the bat wing structure, Raytheon’s telescoping-wing design may seem relatively straightforward. However, because cruise-missile wings are extremely small, there is little space for actuators and wing portions that slide into others. Moreover, of all the wing types being modified in the DARPA program, the cruise missile wings sustain the greatest forces, says Raytheon aerospace engineer Patrick O’Hagan.
Although the Wright brothers launched morphing research a century ago, that engineering approach has caught on only in the past few years. With many aeronautical designers now bent on applying all the know-how and technological progress of the last century to the task, a new phase of aviation may be taking off.
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