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Magic number revealed for flying and swimming

18:00 15 October 03

NewScientist.com news service

A single magic number describes the locomotion of flying and swimming animals, from moths to dolphins. The finding could shed light on the flying skills of extinct animals and help defence researchers design tiny military spy drones that can fly around enemy-occupied buildings.

A simple number called the Strouhal number describes locomotion produced by the flapping of wings. It equals the frequency of flapping multiplied by its amplitude, divided by forward speed. Theory suggests that in most cases peak efficiency is reached when this number lies between 0.2 and 0.4.

Scientists already knew that fish to swish their tails in the most efficient way, cruising the oceans with a Strouhal number in the peak range (New Scientist print edition, 4 March 2000).

But Graham Taylor, Robert Nudds and Adrian Thomas of Oxford University, UK, wondered whether flying animals would also have the same narrow range. To find out, they rounded up data on wing movements and speed for 42 species of bats, insects and birds. It turned out that the Strouhal number for almost all these animals once again fell in the 0.2 to 0.4 range.

"Nature clearly has a rule that if you want to cruise efficiently, you need to operate at this Strouhal number," says Thomas. "It's astonishing that you get the same result for a moth and a whale - one's in air, the other's in water, and there's such a colossal difference in size."

Alien species

The rule is so general that it should allow biologists to estimate the cruising speeds of extinct animals simply from their anatomy. It may even apply to alien species. "If there are swimming or flying organisms on other planets, then we predict that it should apply to them too," the scientists say.

The result might also help defence researchers design insect-sized drones that beat their wings efficiently. Currently the US army uses bird-sized flapping machines to fly cameras into buildings for surveillance. But they would like to have much smaller robotic spies that people would mistake for insects and ignore.

The Oxford team can point them in the right direction. Their work shows that for efficient flight, a robotic spy with a 15-centimetre wingspan and a 10-centimetre wing amplitude should flap its wings 30 times a second.

One possible hurdle to making such small machines fly is stability in the air. So the US Air Force has funded Thomas's team to film hoverflies and find out their secret of staying upright.

Journal reference: Nature (vol 425, p 708)

Hazel Muir

New Scientist / Related Stories

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For more related stories search the print edition Archive

Weblinks

Animal flight group, Oxford University

Nature

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