EPFL Researchers Develop a High-Efficiency Wing for Drones and More — Inspired by Bats

Researchers at the EPFL School of Engineering's Unsteady Flow Diagnostics Laboratory have turned to nature for inspiration in making more efficient drones — designing wings inspired by those of the bat.

"We know that bats hover and that they have deformable membrane wings. How the wing deformation affects the hovering performance is an important question, but doing experiments on live animals is not trivial," first author Alexander Gehrke, former PhD student at EPFL and now a researcher at Brown University, explains in an interview with EPFL's Celia Luterbacher. "By using a simplified bio-inspired experiment, we can learn about nature’s fliers and how to build more efficient aerial vehicles."

"The main finding of this work is that the gain in lift we see comes not from a leading-edge vortex, but from the flow following the smooth curvature of the membrane wing," Gehrke continues. "Not only does the wing have to be curved, but it has to be curved by just the right amount, as a wing that is too flexible performs worse again."

The team's experiments saw the creation of a more flexible wing, made from a silicone-based polymer attached to a rigid frame with edges that rotate around their axes. Immersed in a water talk filled with polystyrene-based tracer particles, the researchers were able to visualize the flow of air in a way that would be impossible with an animal — finding that rather than creating a leading-edge vortex, as with an insect wing, the bat-like wing showed a smooth flow of air with increased lift and higher efficiency than an equivalent rigid wing.

"Our experiments allowed us to indirectly alter the front and back angles of the wing, so we could observe how they aligned with the flow," explains co-author Karen Mulleners, head of the Unsteady Flow Diagnostics Lab. "Due to the membrane's deformation, the flow wasn't forced to roll up into a vortex; rather, it followed the wing's curvature naturally without separating, creating more lift."

That finding could mean both improved efficiency for all kinds of aerial vehicles and a way to build even-smaller drones by swapping traditional rotors for bat-like flapping wings. The team also proposes using the findings to investigate potential efficiency gains in other systems, including wind and tidal energy harvesting devices.

The team's work has been published in the Proceedings of the National Academy of Science (PNAS) under closed-access terms; a preprint is available on Cornell's arXiv server.

Main article image of Karen Mulleners, PhD, courtesy of EPFL.