Nature's Best Fliers Share a Shortcut for Stability

By Joel Achenbach
Washington Post Staff Writer
Monday, April 13, 2009

A hawk moth can fly fast, slow, up, down, sideways, even backward. It can hover. It can easily position itself at an open flower swaying in the breeze. Its entire life is an aerial show. Human engineers would love to be able to make a tiny flying device, a little robot insect, if you will, with even a fraction of the flying prowess of a hawk moth, a hummingbird, a bat or a fruit fly.

"Flying animals are ridiculously maneuverable and stable in flight compared to our own small craft," says Tyson Hedrick, a biologist at the University of North Carolina.

Until now, no one fully understood how these creatures -- which evolved independently from one another -- manage to fly so well. They flap their wings, sure enough. But how do they manage such feats of airmanship?

The simple question of how creatures fly is gradually succumbing to the probing of science. A study, led by Hedrick and published last week in the journal Science, has deconstructed one basic flying maneuver -- a turning motion -- and discovered that multiple creatures seem to employ the same principle. Indeed, it may be a universal principle of animal flight, independently derived by countless species over millions of years.

The scientists call it "flapping counter-torque." You can call it FCT.

There's a long scientific equation that explains the various vectors and variables of FCT, but publishing it here would be a typographical nightmare. (As an example, the Greek letter omega stands for wing stroke amplitude; alpha is the span-wise rotation angle of the wing; Cf is the mean aerodynamic resultant force coefficient; and so on.)

A mystery of flight is how animals keep from getting discombobulated and ankles-over-elbows as they swoosh around and interact with their environment. For example, scientists wanted to know how flying animals regain stability -- or "arrest the yaw" -- after initiating a turn.

The answer turned out to be surprisingly simple. First, the animal initiates the turn with asymmetrical flapping.

Then, rather than stopping the turn with a reverse asymmetrical flapping, the animal switches to regular symmetrical flapping. At that point, natural aerodynamics kick in.

"There's a simple rule for slowing back down: It's going back to being symmetric. You don't have to try anything special," said Hedrick. "What we have here is a shortcut rule for stability."

Hedrick and his colleagues universalized the principle when they noticed similarity between the flapping motions of hummingbirds and fruit flies. They studied nine types of creatures, including a hummingbird, a cockatoo, a bat and four species of insects, and all used FCT.

"The model also shows how animals may simultaneously specialize in both maneuverability and stability," Hedrick and his colleagues wrote in the Science paper.

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