Flight of the Maple Seed

Unlike acorns, maple seeds do fall far from the tree — sometimes a mile or more.

How do they do it?

Aerospace engineer David Lentink of Wagenigen University in the Netherlands used a smoke-filled vertical wind tunnel to photograph vortexes forming on maple seeds whirling in place (above).

More in Henry Fountain's June 16, 2009 New York Times Science section "Observatory" feature, which follows.

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Filming the Flight of a Winged Maple Seed

An acorn may not fall far from the tree, but the same cannot be said for a maple seed, with its distinctive wing shape. As it falls, the heavier nut end of the wing causes it to whirl in the air, slowing its descent and allowing the wind to carry the seed, sometimes as far as a mile or more.

Studies have shown that the seed’s whirling, called auto-rotation, gives it extra lift, but why this occurs has never been explained. It took an aerospace engineer, David Lentink of Wagenigen University in the Netherlands, to figure it out.

Dr. Lentink, with Michael H. Dickinson of the California Institute of Technology and colleagues, report in Science that the wings generate a leading edge vortex — a spinning horizontal tunnel of air along the wing — as they descend. This vortex is stable, Dr. Lentink said, because it has a low-pressure core that reduces the air pressure over the wing, causing the wing to be sucked up. “It really increases the lift,” he said.

Dr. Lentink suspected that the seed might generate such vortexes; many wings do, given the right conditions. To prove it, he and his colleagues first created a model, a rotating robotic wing in mineral oil. The model was dynamically scaled, meaning it matched the aerodynamics of a real seed in the air.

But Dr. Lentink realized a model was not enough. “Biologists worry about the small details,” he said. “I had to make sure that real seeds produce these vortices.”

Using a smoke-filled vertical wind tunnel and adjusting the wind speed precisely, he managed to photograph vortexes forming on real maple seeds, whirling in place. It was painstaking work.

Understanding how maple seeds create extra lift may prove useful in the design of tiny whirling craft, powered or not, that could be used to carry sensors, cameras or other devices through the air. “If you want to make miniature helicopters,” Dr. Lentink said, “then it definitely makes sense to use these vortices.”

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Here's a link to the abstract of the Science magazine report; the abstract follows.

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Leading-Edge Vortices Elevate Lift of Autorotating Plant Seeds

As they descend, the autorotating seeds of maples and some other trees generate unexpectedly high lift, but how they attain this elevated performance is unknown. To elucidate the mechanisms responsible, we measured the three-dimensional flow around dynamically scaled models of maple and hornbeam seeds. Our results indicate that these seeds attain high lift by generating a stable leading-edge vortex (LEV) as they descend. The compact LEV, which we verified on real specimens, allows maple seeds to remain in the air more effectively than do a variety of nonautorotating seeds. LEVs also explain the high lift generated by hovering insects, bats, and possibly birds, suggesting that the use of LEVs represents a convergent aerodynamic solution in the evolution of flight performance in both animals and plants.