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American Association for the Advancement of Science

How whirlybird seeds catch air

"Helicopter" seeds from a maple tree.
[Image Science/AAAS]

Plants and flying animals have evolved the same aerodynamic trick for fighting gravity while flying, scientists have discovered.

This research, which appears in the 12 June 2009 issue of the journal Science, might inspire the design of new spinning vehicles, such as parachutes or tiny, robotic helicopters.

Now that spring has arrived to the northern hemisphere, trees like the maple are releasing fleets of "helicopter" or "whirlybird" seeds that spin as they travel to the ground. If a breeze catches the seeds during their fall, it can carry the seeds long distances, improving the chances that they will find an unoccupied patch of ground to grow in.

Seeds on a maple tree.
[Image Science/AAAS]

David Lentink of Wageningen University in the Netherlands and colleagues have now discovered that helicopter seeds buy time in the air thanks to a spiral of air that develops above the seed's papery wing, generating lift for the seeds.

This spiral, called a "leading edge vortex" also serves the same purpose for hovering insects, bats and possibly birds, as recent studies have shown.

Lentink's team first built a model seed controlled by a robotic arm in a tank of mineral oil and measured the vortex that developed in the oil as the seed dropped.

Then, they filmed real maple seeds spinning in a smoke-filled wind tunnel. The films showed that the vortices were similar to those made by the flapping wings of insects, bats and hummingbirds.