Nearly every child has played with the twirling seeds of maple trees, which spin to the ground like a helicopter when they are thrown in the air. The aerodynamic mechanisms allowing the spinning seeds to fall slowly and helping them to disperse over great distances has remained a mystery. Now, Dutch and American scientists from Wageningen University and the California Institute of Technology have unraveled the aerodynamic secret of swirling seeds which doubles the lift of not only these seeds. Also insects, hummingbirds and even bats use the principle to hover. Their study has been published as cover story in the June 12th issue of the scientific journal Science.
In the joined Dutch - American study David Lentink and Michael Dickinson show that swirling maple seeds generate a tornado-like vortex that sits atop the front leading edge as they spin slowly to the ground. This leading edge vortex lowers the air pressure over the upper surface of the maple seed, effectively sucking the wing upward to oppose gravity.
This vortex doubles the lift compared to the performance of non-swirling seeds. Surprisingly, this use of a leading edge vortex to elevate lift on spinning seeds is remarkably similar to the trick employed by insects, bats, and hummingbirds when they swing their wings back and forth to hover. Thus, the new study shows that plants and animals have converged on an identical aerodynamic solution for improving their flight performance a tornado-like vortex that boosts the lift of the wing.
Visualizing the vortex
To measure the flow of air created by swirling seeds, the scientist mad
e plastic models of the seeds and spun them through a large tank of mineral oil using a specially-designed robot. The robot they used was modified from a device at the California Institute of Technology called ‘Robofly’, that had previously helped to unravel the aerodynamic forces that keep insects aloft. The size of the model seed, the speed at which it spun through the tank, and the stickiness of the oil was chosen so that the flow created by the model was identical to that made by real 
maple seeds. Next, the scientists used light from a powerful laser to measure the motion of tiny glass beads in the oil as the model seed spun through the tank. The images revealed the presence of a tornado-like vortex lying near the front leading edge of the spinning seed. Force measurements showed that the swirling vortex created extra lift that would act to slow the descent of a seed as it spun to the ground. To make sure that the results from the robot seed models were accurate, the team built a wind tunnel at the University of Wageningen (Netherlands) to examine the flow created by real maple seeds as they spin. They used smoke to visualize the flow of air around spinning seeds, and confirmed that real seeds make a leading edge vortex which is similar in structure to that made by the flapping wings of insects, bats, and hummingbirds when they hover.
Parachutes and micro helicopters
Having found the trick that both animals and plants employ to generate exceptionally high lift, the scientist suggest that this finding might have technological implications as well. Swirling parachutes and vehicles have been designed by space agencies to slow down the descent of their planetary probes for exploring the atmosphere of planets such as Mars. Maple seeds might represent the most basic and simple design for a miniature helicopter, if the swirling wing could be powered by a micro motor. Such single-rotor helicopters have been built and flown successfully with wing spans of roughly a meter, but never at the scale of a maple seed. The finding that a leading edge vortex boosts the lift of swirling seeds could inspire the design of swirling parachutes and micro helicopters, and will engage anyone who has ever wondered as a child why maple seeds fly so well.
The research was financed by the Netherlands Organization for Scientific Research (NWO) and by the US National Science Foundation.
Short captions: (from top to bottom:)
- Maple seed swirling through the air [Cover photo Science]
- Tornado-like leading edge vortex measured on top of a maple seed model
- Tornado-like vortex visualized on top of a freely flying maple seed
- Maple seeds attached in pairs to a branch of their tree
(Elaborated caption in the appendix)