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Flexible secrets of how bats fly

11 May 07 08:53 GMT
By Richard Black
Environment correspondent, BBC News website

Using a wind tunnel, a computerised imaging system and the odd dash of honey, scientists have analysed the highly flexible flap of bats' wings.

Bats evolved flight independently from birds and their wings work differently.

The most striking aspect is that on the upstroke, the wings turn upside down to generate more lift.

Writing in the journal Science, the researchers suggest their findings could help in the development of machines which fly by flapping.

"This is completely different from birds," said study leader Anders Hedenstrom from Lund University in Sweden.

"There is nothing else that is similar to a bat in flight because their wings are so flexible," he told the BBC News website.

Fast snappers

The research took place in a wind tunnel at Lund University, where Professor Hedenstrom has spent more than a decade studying the flight patterns of birds.

The scientists selected Glossophaga soricina, a small species of bat that feeds on nectar.

Placed on a net in the wind tunnel near to feeding tubes bearing honey water, the bats would decide from time to time to fly across for a slurp or two of food.

As they flew, their wings displaced tiny water droplets, about one micron (one thousandth of a millimetre) in diameter, which had been injected into the apparatus.

"Then we create a thin light sheet with a pulsed laser," explained Professor Hedenstrom, "which illuminates areas of the tunnel, and we capture pairs of images."

The computer system scans the images for clusters of water droplets which it can then track from one snapshot to the next.

"And then repeating that again and again, we can get velocity information on droplets across the whole image," Professor Hedenstrom added.

Previous studies relied on filming bats in flight, but this team believes its method showed the mammals' flying secrets with much greater precision and detail.

On the beat

The patterns of the wing beats are complex, and change as the bats fly faster or slower.

But the key part is the wing inversion. With the wing moving upwards, you might expect power to drop off and the wing to push the animals downwards.

By turning the wing over, and then moving it backwards during the upstroke, power is maintained.

Birds have evolved a completely different way round the problem, separating and turning their feathers so they can pass through air more freely as the wing moves upwards.

Could this be useful to engineers trying to develop new modes of mechanical flight? Perhaps so, according to Geoffrey Spedding, an aerospace engineer from the University of Southern California (USC), who collaborated on the research.

"Bats are agile hunters, capable of plotting and executing complex manoeuvres through cluttered environments," he said.

"These are the traits we'd like our unmanned air vehicles to have because there are so many complex rural and urban environments in which we could use them."

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