Flight of bats may help develop robot planes

Bats use very different wing motions in flight than birds, according to a detailed study of how the mammals fly. The results could provide fresh ideas for engineers who are designing new, more manoeuvrable robotic planes.

Bats and birds represent two completely separate evolutionary solutions to the problem of flight. Archaeopteryx, the dinosaur ancestor of modern birds, developed its method around 150m years ago, but bats are relative newcomers to the skies. Earliest flying bat fossils date to around 50m years ago in the Eocene period.

The differences in the nature of a bat or bird wing are obvious, but now researchers have shown that these differences mean the two groups use very different aerodynamic strategies to get off the ground.

They studied long-tongued nectar bats (Glossophaga soricina) on the wing in a wind tunnel. To see how the bat's flapping changed the air-flow around their wings, the researchers added smoke to the airflow and took images of the bats at different points during each wing beat.

The main difference between flight in bats and birds stems from the fact that birds have feathers. On each upstroke, birds are able to separate their primary feathers like the slates of a Venetian blind, so that the upstroke does not undo the lift generated by the previous downstroke. Bats cannot do this because their flight membrane is continuous.

Anders Hedenström, of Lund University in Sweden, Geoffrey Spedding, from the University of Southern California in Los Angeles, and their colleagues, showed that bats are able to gain extra lift on each upstroke at low speeds by flicking their wings backward. Each bat wing also generates its own vortex in the air which are interlinked by vortex structures shed by the bat's body.

Another finding that differed from bird flight was that during each upstroke, different parts of the bat wing produce negative and positive lift. The findings are published today in the journal Science.

Prof Spedding, an aerospace engineer, said: "Bats are agile hunters, capable of executing complex manoeuvres through cluttered environments. These are the traits we would like our unmanned air vehicles to have, because there are so many complex rural and urban environments in which we could use them."

Engineers could copy the bats' aerodynamic tricks in their efforts to develop small, highly manoeuvrable robotic air vehicles for surveillance, he said.