Bat in flight
Courtesy BistrosavageBats - the only mammal to develop winged flight - seem to have picked some of their flying tips from insects, at least when flying slowly.

A research team at Lund University in Sweden has come out with a new study describing how the hefty-by-comparison “flying rodents” manage to retain lift while in a hover pattern. The team, led by theoretical ecologist, Anders Hedenström, also included Geoff Spedding, an aerospace and mechanical engineering professor from the University of Southern California. Florian Muijres, a Lund graduate student was lead author of the study that appeared in the February 29th issue of Science.

Of course, bats aren’t really flying rodents. They’re a unique Order (Chiroptera) that has populated the Earth since the Eocene Epoch some 50 million years ago, although a tooth from the Late Cretaceous is suspected to be that of an even earlier ancestor.

For a long time scientists weren’t certain if the beating of a bat’s wing could produce strong enough leading edge vortices (LEV) to keep an organism its size in the air. Insects, which are considerably smaller and lighter, use the technique to hover, as do hummingbirds. But current theories of flight seemed to be against it working for the nearly double-in-size bat.

Using digital particle image velocimetry (DPIV) to detect the movement of fog particles in the airflow around the flapping wings, the researchers videotaped three separate bats (Glossophaga sorcina) in a wind tunnel with a slight headwind as they hovered in front of a honey-water dispenser. A multi-screen video of it can be viewed here.

The study showed that during the downward flap, the tip of each of the bat’s wings produced a whirling clockwise-spinning vortex that helped keep the bat aloft.

“This leading edge vortex then swirls around the wing during the upward stroke. It stays attached to the wing, almost like it’s glued there,” Hedenström said.

The researchers estimated that the force created contributed about 40 percent of the lift necessary to keep the creature aloft. Insects produce something like 45-65 percent lift with the same technique.

But insect wings are thicker and can’t be controlled in the same way the wing of a bat can so they have to flap much faster. A bat’s wing is comprised of a thin membrane of skin stretched across specialized elongated fingers and thumb. By flexing its digits, the wing can be manipulated in the same way the flaps on an airplane wing can effect air movement.

"The high lift arises because the bats can actively change the shape (curvature) by their elongated fingers and by muscle fibers in their membranous wing. A bumblebee cannot do this; its wings are stiff. This is compensated for by the wing-beat frequency. Bats beat their wings up to 17 times per second while the bumblebee can approach 200 wing-beats per second."

The Hedenström team originally used DPIV to study bird flight in 2003. Last year, they completed a study of the aerodynamics of a bat’s wing using the same method.

SOURCES

American Association for the Advancement of Science story
Story in Science Daily
Biology News Net

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