These are the traits we'd like our unmanned air vehicles to have," said University of Southern California aerospace engineer Geoffrey Spedding, who along with his team measured the aerodynamic performance in the wing beats of a small species of bat and found that their flight is quite different from bird flight, particularly at very small scales.

"Bats with a body mass of 10 - 30 grams - or about the weight of one or two teaspoons of sugar - and tip-to-tip wing spans of 25 - 30 centimetres - about the length of a human hand - generate very different wakes. The telltale tracks in the airflow caused by the wing beat have a very different pattern for bats, and this difference can be traced to the peculiar upstroke. That, in turn, is likely caused by the collapsible membrane of the bat's wing, which needs to maintain some degree of tension," said Spedding.

"Instead of feathers projecting back from lightweight, fused arm and hand bones, bats have flexible, elastic membranes that stretch between specially extended, slender bones of the hand. The bones and wing membrane both change shape with every wing beat, flexing in response to the balance between forces applied by the muscles and competing forces due to the air motion around them.

"In contrast with bird wings, the bat wing membrane must be kept under tension, otherwise it will flap uselessly, like a flag. As a consequence, there are limits to how much the wing can be folded during flight," he said.

As such, aircraft designed with wings of flexible material could give better aeronautical performance during flight, he said.

"Bats have relied on very flexible wings for 50 million years to propel and lift themselves into the sky. We still have a lot more to learn about the aerodynamics of bat flight and how their wings allow them to manoeuvre through incredibly unsteady air flows and turbulent conditions," Spedding said.

The findings are presented in an article, "Bat Flight Generates Complex Aerodynamic Tracks" in the May 11, 2007 issue of the journal Science.

Source: ANI - Copyright 2007

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