Finding Nature’s Most Efficient Flight Mechanism

Caltech's Robofly and Bride of Robofly, inspired by spinning maple seeds, have found that multiple evolutionary paths across both plants and animals all appear to lead to a universal ideal

Movie courtesy of David Lentink

With the help of a two-foot-wide robotic fly, a vat of oil, and some tricks with smoke and lasers, an aerospace engineer has learned that Mother Nature figured out long ago the most efficient way to fly. Well, at least if you’re really small.

It involves creating a vortex, like a miniature sideways tornado, over the leading edge of your wing, which reduces pressure above the wing’s surface. The low pressure virtually sucks the wing up, doubling the lift it would normally achieve.

Maple tree seeds do this, as David Lentink, a professor at Wageningen University in the Netherlands and a researcher at the California institute of Technology, announced last week in the journal Science. Insects, hummingbirds and bats also do the trick, by sweeping their wings back and forth. Maple seeds create the vortices as they swirl to the ground, allowing for a much slower fall and a landing much farther from the tree.

The fact that tree seeds developed leading-edge vortices, on an utterly different evolutionary path than the flying fauna that would eat them, suggests it’s one of the most efficient flight mechanisms around.

“It’s astonishing. What it shows is that aerodynamics is so important in shaping the wings in evolution, that very, very different organisms have all evolved the same mechanism to boost their lift,” Lentink said, adding that tree seeds evolved the trait without brains or muscles to help implement it. “It makes generating a leading-edge vortex a no-brainer, and that makes it really interesting.”

To find out what helps the seeds achieve such efficient lift, Lentink needed to see them in action. Thankfully, researchers at Michael Dickinson’s animal physiology laboratory at Caltech are experienced at measuring insect aerodynamics, using Robofly and the Bride of Robofly.

Robofly, a robotic fruit fly with a 23-inch wingspan that flaps 5 times per second, is immersed in two tons of mineral oil, which simulates the drag a real fruit fly would feel as it flapped its wings in the air. Its bride is a similar beast that can flap and simulate flying forward.

Lentink wanted to test the rotational movement of a maple seed twirling toward the ground, so he built a 5-inch maple seed — about 5 times the size of a real one — and hooked it up to Robofly’s motors.

Numerous miniature glass beads were added to the oil, and Lentink shone a powerful laser on them to film the swirl created by the “falling” seed.

“The seed is spinning, going through the laser light sheet, and it hits a nice cross-section of the wing and the flow around it. You see the flow because of the glass spheres moving around the wing,” he said.

He also tested 32 separate actual maple seeds in a vertical wind tunnel, to verify his findings with the real thing.

“It is surprising in many ways that these insects and maple leaves generate such lift,” he said. “It will be interesting to understand why different motions evolved. It all depends on what they evolutionarily inherited, like muscles that can flap wings but not spin them continuously. For maple, it makes a lot of sense biologically that they should fly very well with their continuous spinning wings; maple seeds are propelled by gravity and turbulent gusts of wind. There is lots of stuff we don’t know about this yet.”

So should we redesign our helicopters? Lentink said you probably wouldn’t want a human-sized maple-copter — bigger wings and faster speeds would lead to very unstable leading-edge vortices that would only last seconds. Plus, few passengers could stomach twirling 360 degrees throughout their flight, as maple seeds do.

But wee maple-like helicopters could do it, and they could be used for everything from surveillance to traffic monitoring to slowing down probes in the atmospheres of other planets. The Department of Defense has even tried developing miniature maple cameras.

“I think the future will be miniature helicopters with maple seed-like wings,” Lentink said. “If you want to design very effective, efficient micro- or nano-scale air vehicles, it is better to aspire to the vortex created by the maple seed than the vortex created by an insect.”