How maple fruits fall

A heavy body and big wing help seeds stay airborne

View a video of the maple seeds and the mini-vortexes they generate.

A falling maple fruit spins thanks to its heavy seed and asymmetrical wing, as shown here. Inset: The spinning generates a whirling vortex of air above the wing, providing lift, scientists report. Modified from D. Lentink

A heavy body and lone, stubby wing seem unlikely features for an object trying to fly—but they help the seeds of maple trees travel thousands of meters from a parent tree, researchers report in the June 12 Science.

The helicopter-like seeds—technically fruits called samaras—dangle from trees in pairs. As the seeds age, they separate into single units, each a heavy, round mass at the base of a stubby, asymmetrical wing. This design makes the maple fruits spin as they descend, which generates an upward sucking mini-tornado atop the leading edge of the wing. And the flying fruits have lift.

For maples, the main point of the travel is to get seeds away from the parent plant, but insects, bats and hummingbirds also use a similar swirling vortex to get their lift on, says David Lentink of Harvard University, who led the new work. The technique probably wouldn’t work at scales as large as an airplane, but the research could help in designing mini-parachutes that zoom in with a camera to survey the surface of a planet, Lentink says.

With colleagues at Wageningen University in the Netherlands and at Caltech in Pasadena, Calif., Lentink analyzed the samara’s spin in a smoke-filled wind tunnel.  The researchers also had a robot repeatedly drop oversized plastic models of the seed into a tank of mineral oil. The analyses suggest that stubbier wings are better because they can support a larger vortex, meaning more lift. The angle at which the leading edge of the seed hits the air is also critical. When this “angle of attack” for an airplane wing gets to be near or above 20 degrees, the plane loses lift and begins to stall, says Lentink. “This is the angle of attack that a maple seed really starts to work,” he says. “It’s astonishing.”

Credit: Movie courtesy of David Lentink

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