Bats and Bugs Share Aerodynamic Trick for Staying Aloft

Pallas' long-tongued bat flying in front of a feeder in the wind tunnel.

[Image courtesy of F.T. Muijres, Lund University]

Researchers have known for awhile that little whirlpools of air stirred up by insects' wing motions can help keep these small organisms aloft as they fly slowly or hover, two activities essential for food foraging. But how a weightier organism—a bat—manages to stay aloft during slow flight has remained unclear.

A new report shows that bats have a similar mechanism as insects when flying slowly, according to a research group comprised of biologists and aerospace engineers. Their findings, which are published in the 29 February issue of Science, indicate that downward strokes of bats' wings produce small cyclones of air that pull the animal upward.

"Staying aloft when hovering and flying slowly is demanding," wrote the researchers in their Science report. The ability is inexplicable by standard theories of flight—which predict that slow-flying vertebrates would not be able create enough lift to stay aloft during hovering—and has led to proposals of alternative aerodynamic mechanisms.

A whirling cyclone known as a leading-edge vortex (LEV) is one such alternative mechanism to explain lift during slow flight. Insects use this aerodynamic trick, and hummingbirds have also shown LEV when hovering at nectar feeders. Researchers wondered whether the little whirlpool of air would be enough to maintain flight in a heavier organism such as a bat, which at around 10 grams weighs about twice as much as a hummingbird.

To see if bats are capable of generating and using LEV, lead author Florian Muijres and his colleagues in Sweden and the United States studied the airflow around the wings of the tropical Pallas' long-tongued bat (Glossophaga sorcina). Three individual bats were studied as they flew freely in front of a cone-shaped feeder containing honey water. The feeder and bats were placed in a wind tunnel with a gentle head wind. View video of the bats' flight.

Airflow measurements around the bats' wings were acquired using digital particle image velocimetry (DPIV), which calculates fog particles in the wind tunnel that are dispersed due to the flapping of bats' wings. The measurements showed a patch of negative, or clockwise, spin around the wing.

"The vorticity patch at the leading edge of the wing was present at all measured span-wise locations but was stronger near the wingtip," the researchers wrote in their Science report. The whirling air pattern above the wing contributed to more than 40% of bats' ability to stay aloft during slow-moving, hovering flight. For comparison, this force contributes to 45-65% lift in insects and 15% lift in hovering hummingbirds.

Muijres, a graduate student at Lund University and an aerospace engineer specializing in the aerodynamics of flapping wings, and the rest of the research team propose that "the sharp leading edge of the bat wing probably facilities the generation of the LEV" and that wing shape and angle may also contribute to LEV control.

The research team is led by Anders Hedenström, a Lund University theoretical ecology professor, and includes Geoff Spedding, a University of Southern California aerospace and mechanical engineering professor. The group published another Science report last year showing how a thin membrane on bats' wings contributes to their flight control.

Molly McElroy

28 February 2008