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On bended wing: flexible flying machines

A view of the F-18 testbed used in the Active Aeroelastic Wing project at NASA's Dryden Flight Research Center. Note the difference in direction of the two leading-edge wing flaps.
A view of the F-18 testbed used in the Active Aeroelastic Wing project at NASA's Dryden Flight Research Center. Note the difference in direction of the two leading-edge wing flaps. NASA / DFRC / AFRC

Most air travelers today take comfort in the knowledge that when flying from place to place, the wings on their aircraft always keep their shape.

But some aerospace researchers are working to do just the opposite; build a plane with malleable wings that bend and twist during flight. Unlike their fixed-wing cousins, airplanes with flexible wings could lead more efficient and maneuverable aircraft for both military and commercial aviation.

"The question is: Can you have an airplane that can fly fast efficiently and fly slowly efficiently," said George Lesieutre, an aerospace engineer and Pennsylvania State University professor who has been researching morphing wing technology. "You'd like one airplane to fulfill that entire mission."

Lesieutre and his Penn State colleagues have developed a mechanized internal truss that would pull on the outer skin of aircraft wing, reshaping it to fit changing flying conditions and speeds.

Meanwhile, researchers at NASA's Dryden Flight Research Facility (DFRC) at Edwards Air Force Base, California have already flown a plane with wings that bend, a modernized take on an aviation technique proven for more than 100 years.

"Basically, what we're trying to do is make the wings lighter," said Lawrence Myers, project manager of flight research for the Dryden portion of the Active Aeroelastic Wing project. "The biggest thing we're going here is actively controlling the plane's structure."

Wings that bend
The concept of reshaping an aircraft surface area is about as old as airplanes themselves. Orville and Wilbur Wright, for example, pulled on cables to warp the wings of their airplane in order to steer it during their historic 1903 flight.

Today, airplanes use motorized flaps and ailerons attached to fixed wings to control and maneuver an aircraft.

Dryden researchers, working in tandem with the Air Force Research Laboratory, have modified the wings of an F-18 jet to carry a wing-length back flap and a segmented leading-edge flap, both connected to a fixed center that includes lighter, more flexible panel material. Together, the already test-flown system allows the flexible F-18 wing to twist and bend under aerodynamic pressure to control aircraft roll.

Since the active aeroelastic wings in the Dryden project are lighter than non-modified versions, they could boost the range or payload capacities of future aircraft, Myers said, adding that a second phase of test flights should begin in August.

Lesieutre's research, on the other hand, seeks to develop wing technology for an all-purpose military aircraft.

His project is part of the broader Morphing Aircraft Structures project underway by the Defense Advanced Research Projects Agency (DARPA), which aims to successfully demonstrate a flying vehicle that is not only "aerodynamically efficient," but also possesses a capability for "radical shape change," according to the project's description.

The hope is to develop a single airplane that could combine the slow overflight capabilities of a reconnaissance plane with the high-speed talents of an attack jet.

"You'd like a vehicle that could fly from point A to point B fast, and once you get there fly around slowly to look a situation," Lesieutre said. "But this business of flying someplace very quickly then simply hanging around requires a dramatic change in the size of a wing."

The fastest jets use small, sometimes swept back, wings while slower reconnaissance planes use long, narrow wings.

Aerospace engineer George Lesieutre, of Pennsylvania State University, with a tabletop model of the compliant cellular truss structure designed to change an aircraft's wing shape in flight.Greg Grieco

Lesieutre and his team developed a model truss system of tiny actuators, struts and tendons to form the skeleton of a wing. While in flight, the interlocked, mechanized truss would pull on an attached wing skin of nickel-titanium or other memory alloy, shaping the outer surface from within.

To fly fast, the truss could fold into a small wing shape, then expand both wing cross-section and area to accommodate slower flight. Since memory alloys can return to their original shape, when the tendons pulling at them are released they'd just snap back into their default position, Lesieutre said.

"In a way, we're looking at an old idea with new materials," he added. "Manufacturing methods for actuators and other devices have really improved and people are still coming up with better ways all the time."

Lesieutre's truss system is still in the modeling stage, and he envisions it could be placed inside a wing with a segmented, overlapping skin akin to a fish, and push and pull on the surface as needed. It would have to start off with small planes first, growing up through unmanned aircraft stages to an eventual full-size jet, he added.

Borrowing from biology
Over the last few years, much of Lesieutre's research has taken a page from avian biology to develop what he calls micro air vehicles.

"At that scale, we wonder should we be looking at flapping our wings to fly," he said. "We look at birds that soar, like some hunting birds like hawks or falcons, which soar until they see a mouse or something to attack, then tuck in their wings and dive."

The key, Myers said, is a bird's control of the feathers that line the tips of its wings. By changing the orientation of those tip feathers, birds are able to roll and turn in flight, he added.

Even the Wright brothers looked at large soaring birds for hints at warping their wing shapes.

"This really is an old problem," Lesieutre said of the warped wing approach to aviation. "It actually predates the Wright brothers, but people continue to look at it and come at it from fresh angles."