Researchers Discover Mechanical Differences between Bat and Bird Flight

Bat in flight where the study was conducted
[Image © Science]

Scientists using fog machines and a wind tunnel have discovered how bats use a thin membrane stretched across each wing and wing mechanics different than birds to stay in the air, according to research in the 11 May issue of Science.

Using high speed computers to analyze the bats' wake, represented by the movement of fog particles around the wings, the researchers discovered that the thin membranes push air when the wing moves up and down.

While bats push air on both the upstroke and the downstroke, birds separate their wings' primary feathers, like the slats on a Venetian blind, so that the upstroke does not produce aerodynamic force.

While bats can stretch and collapse their wing membranes, they cannot make them aerodynamically inactive like birds feathers, the authors say.

"Bats and birds represent two independent evolutionary pathways solving the same problem—powered vertebrate flight," wrote lead author Anders Hedenström, professor of theoretical ecology at Lund University in Sweden.

A bat in flight. The arrows represent the air movement produced by the wing motion.
[Image © Science]

While previous studies have studied bird flight by imaging their wakes in wind tunnels, Hedenström's team was among the first to use the technology for bats.

Their analysis suggests that at slow speeds, bats flick their wing tips backward during the upstroke, generating additional loft.

During medium and high flight speeds, wake imaging suggests one wing portion creates positive lift while another creates negative lift, resulting in flight.

In addition to shedding light on the multiple ways that nature has solved the problem of powered vertebrate flight, these findings may be useful for designing new flying devices, the authors say.

Kathy Wren and Benjamin Somers

10 May 2007