April 09, 2009 | 12:59:57 PM
Categories: Aeronautics, Animals
Courtesy of Wired Blog Network
A unifying theory of winged locomotion could explain the magical mid-air maneuvers of birds and insects, and guide the design of flying robots.
Using high-speed video, biologists modeled how hummingbirds and hawkmoths use asymmetrical flapping to make slow, mid-air turns. The model predicted how five other flyers turned at full speed, hinting at a universal turning technique for flying creatures.
"It's basically an exponential damping system," said Ty Hedrick, a University of North Carolina animal aerodynamics expert. "The strength of braking increases in proportion to speed."
Though scientists understand the principles underlying many flight-enhancing physiologies, from birds' hollow bones to dragonflies' flexible wings, the biomechanics of turning was in many ways a mystery.
Researchers were unsure whether different species used fundamentally different mechanisms, or variations on a basic theme. Hedrick's findings, published Thursday in Science, describe a common solution shaped by evolutionary pressures in the 150 million years since dinosaurs took to the air.
Though the dynamics probably can't work at large scales — building-sized robotic birds won't ever be as agile as a swallow — they could be harnessed in small drones used by explorers or the military. Compared to the average hummingbird or fruit fly, such craft are now clumsy and unstable.
"The results will inform all future research into maneuvering flight in animals and biomimetic flying robots," wrote University of Montana, Missoula biomechanicist Bret Tobalske in an accompanying commentary.
Hedrick's team used 1,000 frame-per-second video cameras to watch hawkmoths and hummingbirds hovering before a feeder. As each turned away, one wing flapped faster on its down-stroke, while the other flapped faster on its up-stroke.
The asymmetry causes flyers to lose speed as soon as they start to turn. The effect is strongest when velocity is highest.
"The moment they start turning their wings and stop symmetrically flapping, their bodies act like a brake," said Hedrick.
Measurements of the motion provided a model that, adjusted for size differences, predicted the mid-air turn motions of four insect species, a cockatoo, a hummingbird and a bat.
In animals with proportionally similar bodies, rates of wing flapping — not body size — controlled turning ability. Agile hummingbirds and fruit flies flap their wings the same number of times to complete a turn.
"To understand the importance of this result, consider the array of solutions that flying animals have at their disposal to modulate aerodynamic forces," wrote Tobalske. "The fact that the flapping counter-torque model is robust over a wide range of body size indicates that it represents a universal model," he wrote.
The effect probably helps flyers regain equilibrium when hit by gusts of wind, providing a natural stabilizer that engages before their brains can react to a perturbance, said Hedrick.
The study's other co-authors, Darpa-funded University of Delaware mechanical engineers Xin-Yan Deng and Bo Cheng, will use the findings to refine their insect-inspired unmanned aerial vehicles.
As for Hedrick, he next plans to study mechanisms used in more complicated aerial maneuvers, perhaps equipping swallows and other small birds with sensor-filled backbacks.
"Animals are doing things so smoothly and gracefully that we don't even realize that these are very hard tasks," Hedrick said. "In a robot, we have trouble replicating that behavior."
Citations: "Wingbeat Time and the Scaling of Passive Rotational Damping in Flapping Flight." By Tyson L. Hedrick, Bo Cheng, Xinyan Deng. Science, Vol. 324, April 10, 2009.
"Symmetry in Turns." By Bret W. Tobalske. Science, Vol. 324, April 10, 2009.
Images: 1. Flickr/peasap
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