The Secret Behind an Impossible Flight

The way a dragonfly hovers and zigzags in the air seems an impossible feat--at least, conventional physics has been at a loss to explain how these and many other insects fly. Until now, that is. Ending years of mystery surrounding the aerodynamics of insect flight, scientists report in tomorrow's issue of Nature that insects stay aloft thanks to previously undetected swirls of turbulent air that dance above their wings.

The primary force that keeps most self-propelled objects airborne is a lift from steady motion--a helicopter, for instance, gets lift from the constant rotation of its propellers. In contrast, most insects' wings flap almost willy-nilly, at rapidly changing speeds and directions. "Insect flight cannot be explained by conventional aerodynamic principles," says biologist R. McNeill Alexander of the University of Leeds, U.K.

Biologist Charles Ellington and colleagues at the University of Cambridge first tried to solve the perplexing problem by studying a hawkmoth--a rather large critter with a 10-centimeter wingspan--tethered in a wind tunnel that blew smoke toward it. As the moth beat its wings downward, high-speed photos captured smoke tendrils caught in swirling vortices that appeared near the wing base and swirled along the top toward the tip--"like rollers that rotate when a crate is pushed over them," Alexander says. This "leading-edge" vortex creates a low-pressure region that helps lift the wing.

To get an even better look at vortex formation in a larger and better controlled subject, Ellington's group built a Frankenmoth--a mechanical version of a hawkmoth about 10 times the size of the real thing. Photos of the Flapper, as they dubbed it, tethered in the wind tunnel confirmed the formation of these roller-like vortices. But the photos also showed, surprisingly, that air whipped out of the vortices toward the wing tip in widening helices--"this could not have been predicted by conventional aerodynamic analysis," Alexander says. This motion appears to stabilize the vortex longer into the wing's downward stroke. Ellington's team calculates that in the hawkmoth's case, the lift overcomes the wing's unstable motion so well that the insect can theoretically carry aloft half its weight. "In effect," says Alexander, "they have discovered how a typical insect flies."