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VORTEX VISION: Researchers visualized the swirls of air around flapping bat wings by shining laser light into a foggy wind tunnel.
© SCIENCE

Holy upside-down wing stroke, Batman! Researchers have pinpointed the forces generated by the flapping of bat wings, including an odd upside-down wing stroke that seems to help the furry fliers hover languidly. The results not only highlight differences between bat and bird flight, but may help designers of small flying robots—not to mention caped crusaders.

To get a grip on bat wings, a Swedish-led team blew foggy wind across two small bats as they hovered in midair, lapping from a source of nectar [see image below]. Laser pulses shined into the vapor exposed the size and speed of vortices swirling around the bats' wings, which in turn revealed the forces they produced [see image at right].

At slow wind speeds, the bats whipped their wings back and forth, tracing a horizontal figure 8 with their wingtips. As prior studies had found, the animals rotated their shoulders 180 degrees on the backswing so that the undersides of their membranous wings were turned face up.

The accompanying vortices confirmed that this seemingly awkward motion in fact produces lift, says theoretical biologist Anders Hedenström of Sweden's Lund University, first author of a report published online today in Science. "It actually generates a useful force also on the backswing, which is a very good thing when it hovers," he notes.

At higher wind speeds, the bats tilted their wings up and flapped more vertically [click to watch a brief video of a bat flapping its wings]. The whipped up vortices confirmed yet another prediction—that at the top of the bat's stroke, the outer part of the wing impedes lift by creating a swirl of air that pushes it down. The wing's inner region, however, still generates lift by sweeping air downward.

Bats seem to be adapted for slow speed and high maneuverability in the same way as hummingbirds, Hedenström says, whereas most flying birds are optimized for high speed. As birds swing their wings upward, the feathers separate like window blinds to let air through, which prevents the lift-reducing currents that the bats experienced, the researchers say.

The two creatures also leave different wakes: A bat's stretchy wings churn up two separate vortices—one behind each wing—but a pair of relatively rigid bird wings produces one vortex for the whole bird.

Hedenström is currently talking to researchers who build small flying machines to see if the wind tunnel results can help out. As he says, the study "gives detailed information about how a small autonomous flying system works."