Since at least the myth of Icarus, wings seen in nature have inspired humanity’s dreams of flight. In the more than a century since the Wright brothers flew the first airplane, Flyer 1, people have begun making these fantasies a reality, flying high enough above the Earth to make it all the way to the moon.
However, in many ways, nature still has much to teach aircraft designers. For instance, while airplanes now often break the sound barrier, their stiff wings limit their performance. In contrast, the wings of bats and birds can alter their shapes, helping them fly more efficiently and granting them the versatility to cruise, maneuver, and dive as needed.
Now, aerospace engineers from a number of different institutions are developing morphing wings, including ones that can bend and flex much like the wings of bats and birds and are covered by skins of overlapping pieces that resemble scales and feathers. “I would say there are at least 10 groups worldwide working on different approaches to morphing,” says Daniel Inman, chair of aerospace engineering at the University of Michigan at Ann Arbor and one of the many engineers investigating morphing wings. This biological mimicry extends into the very airframes of these wings—they are made of modules, a bit like how animals are made of cells. This new approach could lead to a wide range of new aircraft and other morphing robots.
Nowadays, conventional aircraft rely on hinged flaps such as ailerons, rudders, slats, and tabs on their wings to help them control the way they roll, pitch, and yaw as they fly. While these new morphing wings rely on cutting edge materials science and computer-aided control systems, the way they work has not changed for decades.
The world’s very first airplane, the Wright brothers’ Flyer 1, did not have hinged flaps like modern aircraft. Instead, it used wires and pulleys that bent and twisted the aircraft’s pliant wood-and-canvas wings. Essentially, Flyer 1 had primitive morphing wings.
“They were trying to create as lightweight a structure as they could, and given that it was flexible, they utilized that flexibility. That was an amazing innovation to see,” says Kenneth Cheung, a research scientist at NASA Ames Research Center at Moffett Field, California, and another engineer developing morphing wings.
Though the new wings draw inspiration from the Wright brothers’, they are far more sophisticated, promising to improve the efficiency, agility, and stealth of future aircraft. What’s more, they could be far easier—and cheaper—to produce.
The Wright brothers’ warping wings went out of style once airplanes got faster. Wood-and-canvas wings fare poorly at high speeds, so engineers substituted stiffer materials, which required wings with hinged flaps.
Still, the kind of flexible wings seen in nature have long tantalized scientists for the benefits they can provide. Birds and bats can greatly vary the shape of their wings, helping them rapidly switch between a variety of tasks in complex environments. For instance, falcons can save energy loitering aloft until they see prey, at which point they can adopt a strike configuration to swoop downwards, adjusting their wings at the last second to help them deftly capture desperately jinking prey.
Although stiff aircraft wings are each designed for optimal flight at one speed and one angle of attack—the angle between the wing and the direction of the airflow—they do not behave as well at other speeds or angles of attack, Inman explained. For example, commercial airliners have wings that sweep moderately toward the rear, while the wings of many jet fighters sweep much farther back to enable supersonic speeds.
The hinged flaps needed to control flight also create gaps at the edge of wings. This can lead to turbulent airflow, and “turbulence tends to increases drag on the wing, reducing fuel efficiency,” Inman says. “A morphing or shape-changing wing has the potential for being optimal across all flight conditions, thus reducing fuel consumption,” Inman says.
In addition, the absence of tilted surfaces and sharp edges on morphing aircraft could reduce their visibility and radar signature, thus enhancing their stealth properties.
For decades, scientists have sought to develop aircraft with varying levels of ability to change the shape of their wings. For instance, the Grumman F-14 Tomcat, first flown in 1970, could alter the angle at which its wings swept back. At lower speeds, the wings swing forward to help provide lift, and as velocity increases, the wings swing backward to allow a high-speed cruise that consumes less fuel.
Most previous attempts to develop morphing wings that can flex relied on adding mechanical control structures within the wings. These add weight, complexity and cost, usually offsetting any benefits that morphing could provide.
The main advance underlying the morphing aircraft from Cheung and his colleagues involves not adding new control mechanisms into the wings, as has often been tried with previous morphing aircraft. Instead, “we’re turning the wing into the control mechanism,” says physicist Neil Gershenfeld, director of the Center for Bits and Atoms at MIT and Cheung’s former advisor.
The new morphing wings came about when Spirit AeroSystems, a major supplier to aerospace titans such as Boeing and Airbus, approached Cheung, Gershenfeld, and their colleagues and “asked us if we could 3D-print an entire aircraft,” Cheung says. Instead of 3D printing, Gershenfeld, Cheung, and their colleagues had long explored building objects from Lego-like parts. “Just as electronic hardware in essence is built from a few kinds of components, so too were we trying to think about how physical and even mechanical systems could be constructed using the building-block method,” Cheung says. Just as birds and bats are made of cells, “so, too, is this a cellular approach,” he adds.
The interior of each new wing that Cheung, Gershenfeld, and their colleagues developed consists of a system of tiny, strong, lightweight modules made of black carbon-fiber composites. “We take sheets of solid material and use water-jets to cut shapes out of it that can snap together into little diamond or square shapes that are hollow in the middle,” Cheung says. “Imagine each of these shapes as Legos stacked together as 3D structures, but with excess material taken out of each Lego. Put together, it looks like foam.”
Some of the modules in these structures are rigid, while others are relatively flexible. For instance, the 12-sided dodecahedral modules are easier to deform than eight-sided octohedral ones, and modules consisting of thinner struts are naturally easier to squash than ones made of thicker struts, Cheung says.
The rigidity or flexibility of each module influences how a wing flexes when force is applied to it. Through a tube reaching across the wing to each wingtip, a small motor located in the center of an aircraft can apply twisting pressure to change the shape of each wing uniformly along its length.
To accommodate that flexing, the exteriors of these new wings are covered in orange artificial “skins” made of overlapping strips of flexible resin that resemble fish scales or bird feathers. These strips are attached to the wings via steel retaining pins and slide across each other as the wings change shape, providing a smooth outer surface.
In wind-tunnel tests, a radio-controlled aircraft equipped with these new morphing wings matched the aerodynamic properties of a comparable standard aircraft, but at just roughly one-tenth the weight. “The test pilot said it handled just like a good aircraft — it flew straight and level when he wanted, turned when he wanted, went where he wanted it to go,” Cheung says. “He even did some acrobatics with it.”
Ultimately the goal is to not just build morphing wings, “but to progress to a whole aircraft whose shape you can deform,” Gershenfeld says. Cheung adds that a virtue of building an entire airplane using their approach is potentially much greater simplicity.
“A Boeing 747 has about 6 million parts,” Cheung says. “About 3 million of those are rivets, and let’s say they’re all the same rivet, although they are not. Of the remaining 3 million parts, on the order of 1 million are unique parts. In contrast, we’ve estimated that, to build an aircraft with our method that serves the same function, you’d probably need several times more parts, maybe 30 million, but instead of 1 million unique parts, we’re talking about fewer than 1,000 unique parts as a rough estimate. Having that much fewer unique parts is a huge benefit since they are easier to mass manufacture and replace or repair.”
A number of major companies have expressed interest in this new work. “Airbus is actively involved, as is Moog in Buffalo, which works on control systems for all major aircraft companies from cockpit to tail,” Cheung says.
Several other groups around the world are developing their own style of morphing aircraft. For example, Inman and his colleagues have received roughly $6 million in funding from the Air Force Office of Scientific Research to spend five years investigating bird-inspired morphing vehicles.
“We have three bird biologists in our group, and we’re looking at many different kinds of birds to understand their aerodynamics and integrate what we learn into small aircraft and unmanned aerial vehicles, from radio-controlled planes up to the Predator drone,” Inman says.
Instead of using simple tugs to control the wings, Inman and his colleagues are investigating piezoelectric materials that convert electricity into physical movement as well as shape memory alloys that change shape when supplied with electricity. “We also have funding from some automotive companies to look at morphing as a way to increase fuel economy of road vehicles,” Inman says.
All morphing aircraft research currently underway will take a long time before it helps build passenger jets, due to the highly risk-averse nature of the passenger aircraft industry. “The best-case scenario that anyone in commercial aviation will ever give you is that it will take 30 years before you see something new in an airport,” Cheung says.
Instead, in the near-term, he says they are focusing on creating new unmanned morphing aircraft. If they succeed, morphing-wing drones may be able to stay aloft efficiently for long times to deliver medicine or wireless phone or internet access to far-off areas, Gershenfeld says.
In the long run, Gershenfeld says that such research extends well beyond aviation, possibly leading to wind turbines driven by giant morphing rotors that are better at collecting wind power than conventional machines. It could also inspire to robotic arms, legs, and bodies whose shapes can bend continuously across their entire length, instead of at fixed joints. “You could one day have boats or submarines that swim more like fish, as well as planes that fly more like birds,” Cheung says.
In a way, any future menagerie of animal-inspired morphing robots will owe much of its success to the flexing wings of the Wright brothers. “All aviation is following in the footsteps of the Wright brothers,” Cheung says. “They were the first to show that flexing wings worked.”