Bats Have an Aerodynamic Edge Over Birds, Study Shows

Ben Harder
for National Geographic News
May 10, 2007
The flexible, flapping wings of bats work in a completely different way than those of birds and may pave the way toward versatile new types of aircraft, a new study reveals.

The sophisticated analysis looked at bats in a wind tunnel to uncover the key differences between how mammals and birds stay aloft.

Bats turn out to have a high degree of control over the ever-changing shape of their wings, said study leader Anders Hedenström, an ecologist at Sweden's Lund University.

The animals can therefore generate lift as their wings move both up and down—a big advantage when hovering.

The bats' trick resembles the way each rotor on a helicopter generates lift whether it's moving forward or backward, Hedenström pointed out.

By contrast, he said, birds' wings are optimally designed to generate lift on their downstroke, and they can "feather" their wings—spreading their feathers like the slats of Venetian blinds—to reduce drag on the upstroke.

Hedenström's findings, which appear in tomorrow's issue of the journal Science, agree with results that Brown University's Sharon Swartz and Kenny Breuer published in January.

"Aerodynamic forces generated by bat wings during flight are far more complicated than those of birds," said Swartz, an evolutionary biologist.

Birds' wings operate almost as if they were airplane wings on hinges. By comparison, "bat wings are more flexible," she said. "The materials—the skin and bone—are more stretchy. The bones actually bend when the bat is flying.

"At slower speeds, flexible wings seem to have advantages."

Lifting the Fog

To study the mechanics of bat flight, both research teams placed animals in a foggy wind tunnel and used lasers and high-speed video cameras to monitor how the bats' motion disturbed tiny water droplets that were hanging in the air.

By analyzing the air currents and vortices spilling off each wing, the scientists could determine how the bats generate lift, Swartz said.

The findings suggest that flying machines with batlike flapping wings would have advantages over birdlike devices and over conventional aircraft, especially in search-and-rescue operations and covert surveillance. (Related: "Seagulls May Inspire New Airplane Wings, Scientists Say" [October 24, 2006].)

Flapping flight "is more versatile," Hedenström, of Lund University, said. "You can fly at the slowest speeds and make tight turns, and fly into confined spaces."

Such machines could also be "hidden in plain sight," added Brown University's Swartz, whose research is funded by the U.S. Air Force.

"If you see something flying around in the sky that's not flapping, you know instantly that it's not natural," she said. "If it's flapping, you might not pay it any attention."

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