The twirling seeds of maple trees spin like miniature helicopters as they fall to the ground. Because the seeds descend slowly as they swirl, they can be carried aloft by the wind and dispersed over great distances. Just how the seeds manage to fall so slowly, however, has mystified scientists. In research published in the June 12 issue of the journal Science, researchers from Wageningen University in the Netherlands and the California Institute of Technology describe the aerodynamic secret of the enchanting, swirling seeds.
The research, led by David Lentink at Wageningen and Michael H. Dickinson, a professor of bioengineering at Caltech, revealed that, by swirling, maple seeds generate a tornado-like vortex atop the leading edge of the seeds as they spin slowly to the ground. That leading-edge vortex lowers the air pressure over the upper surface of the maple seed, effectively sucking the wing upward to oppose gravity, giving it a boost. The vortex doubles the lift generated by the seeds, compared to seeds that don’t whirl.
This use of a leading-edge vortex to increase lift is remarkably similar to the trick employed by insects, bats and hummingbirds when they sweep their wings back and forth to hover.
To measure the flow of air created by swirling seeds, the scientists built plastic models of the seeds with radii of about five inches, or 5 to 10 times larger than a maple seed. The seeds were spun through a large tank of mineral oil using a specially designed robot, modified from a device at Caltech called “Robofly.” Previously, Robofly helped to determine the aerodynamic forces that keep insects aloft. The size of the model seed, the speed at which it spun through the tank, and the viscosity of the oil were chosen so that the characteristics of the fluid flow generated by the model were identical to those produced by maple seeds flowing through air.
The scientists used a powerful laser to create a sheet of light that illuminated tiny glass beads added to the oil, and a camera to capture images of the beads’ motion as the model seed spun through the tank. The images revealed a tornado-like vortex lying near the leading edge of the spinning seed. Force measurements attached to the model 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 verify their results, the team built a wind tunnel at Wageningen University to examine the flow created by real maple seeds spinning freely. Smoke was used to show the flow of air around the spinning seeds. The studies, which included 32 specimens, confirmed that real seeds do, indeed, produce a vortex that generates exceptionally high lift, and that the vortex is aerodynamically similar in structure to the vortex made by the flapping wings of insects, bats and hummingbirds when they hover.
The research might have implications for the design of swirling parachutes, which have been designed by space agencies to slow the descent of future planetary probes exploring the atmospheres of planets such as Mars, and of micro-helicopters.
“Maple seeds could represent the most basic and simple design for a miniature helicopter, if the swirling wing could be powered by a micromotor,” said Lentink. 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, he said.
“There is enormous interest in the development of micro air vehicles, which, because of their size, must function using the same physical principles employed by small, natural flying devices such as insects and maple seeds,” Dickinson said. “This is still an open challenge for future aerospace engineers, and our aerodynamic study of maple seeds could help design the first successful powered ‘maple’ helicopters,” he added.
To read the Science article: http://www.sciencemag.org/cgi/content/abstract/324/5933/1438
For a video of the vortex: http://mr.caltech.edu/assets/619-mapleseed.mp4
Other links: http://waynesword.palomar.edu/plfeb99.htm, http://theseedsite.co.uk/sdwind.html,