Published online 28 February 2008 | Nature | doi:10.1038/news.2008.633

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Technique shows how bats and insects share the secret of slow flight.

F.T. Muijres, Lund University Flying in a fog: laser light illuminates air flow. Watch the video.

Bats have a clever aerodynamic trick to make flying easier, researchers have found: the sharp edge at the front of their wings cuts through the air in such a way as to create a vortex on top of the wing, producing up to 40% of the lift needed to stay aloft.

“It explains how these animals are able to fly at very slow speed,” says Anders Hedenström from Lund University in Sweden, who led the research — published in Science1 — that showed the effect with a live bat.

The phenomenon of a 'leading-edge vortex' is known to help insects to fly; this discovery helped to work out how the bumble bee manages to stay airborne. But it hasn’t been definitively seen before in a non-insect with live animals.

Hedenström filmed a bat feeding on nectar placed in a wind tunnel. The wind tunnel was filled with fog and a laser light was pulsed onto the fast-flapping bat as it hovered in front of the nectar. The light illuminated individual fog particles, which were filmed, allowing the researchers to get a clear picture of what the air on and around the bat’s wings was doing.

Using this method, Hedenström's team have previously illuminated the swirling vortices in the wake of a flying bat, showing how these complex air patterns are different from those of a flying bird, and are likely to have helped with a bat's manoeuvrability. They saw that the bat was clearly getting a lot of lift with its flap too, but couldn't work out exactly how (see Bats fly like a bee).

This time they moved the laser to above the bat and shone it directly on the leading edge of the wing, hoping to highlight the technique used to achieve lift. It worked.

Controlled flappers

Hedenström and his team showed that the down-stroke of a bat's wing moves forward as well as down, and is tilted at a sharp angle — just like that of an insect in flight. This produces a powerful lifting vortex. The swirling air closely follows the surface of the wing during the down-stroke: that's a good thing, because if the vortex moved away from the wing, a slow-flying bat would stall, dropping out of the sky. To keep the vortex close to the wing takes some delicate wing movement. “The bats control their wing curvature in a very subtle way,” says Hedenström.

John Videler, from Leiden University and Groningen University in the Netherlands, had previously suggested that swifts use the same vortices to fly. He has seen this in model studies, but hasn't been able to confirm it with live birds. He admits to being a little jealous that Hedenström is first to film the effect in a live animal, but is pleased that it has now been proven. “It’s a very strong effect,” says Videler. "The leading-edge vortex is much more advantageous than the conventional way of lift gain," which simply involves flapping faster. “Slow flight is the most difficult,” he says.

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Hedenström is now going to extend his work to birds and other bats. He expects to see similar results, except, perhaps in big animals that can’t hover.

The phenomenon will be of use for engineers trying to make small autonomous flapping vehicles, says Hedenström. But to do this, the subtle changes in the wing shape that the bat uses to keep hold of the vortex will need to be understood better. “This shows us how we need to control the skin membrane of a controlled flapper,” he says.