Bat wings boost hovering efficiency

Bumblebees shouldn’t be able to fly because their small wings shouldn’t be able to produce enough lift. It took modern high-speed camera technology to uncover the leading-edge vortex allowing airborne insects to fly. This phenomenon occurs when air flow around the leading edge of flapping wings rolls up into a vortex, creating a low-pressure region that boosts lift.

But bats – with their flexible membrane wings – can fly just as well as insects, if not more efficiently. Some bats expend as much as 40% less energy than moths of a similar size. Researchers in the Unsteady Flow Diagnostics Laboratory in EPFL’s School of Engineering studied the aerodynamic potential of more flexible wings using a highly deformable membrane made from a silicone-based polymer. Instead of creating a vortex, researchers found air flows smoothly over the curved wings, generating more lift and making them even more efficient than rigid wings of the same size.

“The gain in lift comes not from a leading-edge vortex, but from the flow following the smooth curvature of the membrane wing,” says former EPFL student Alexander Gehrke, now a researcher at Brown University. The wing must be curved by just the right amount, as a wing that’s too flexible performs worse.

The researchers mounted the flexible membrane onto a rigid frame with edges that rotate around their axes. To help visualize the flow around the wing, they immersed their device in water mixed with polystyrene tracer particles.

“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,” says the lab’s head Karen Mulleners. “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.”

Gehrke adds, “By using a simplified bio-inspired experiment, we can learn about nature’s fliers and how to build more efficient aerial vehicles.”

He explains that as drones get smaller, they are more strongly affected by small aerodynamic perturbations and unsteady gusts than larger vehicles. One solution could be to use the same flapping wing motions as animals to build improved versions of drones that can hover and carry a payload more efficiently.

Advances in sensors and control technology, potentially combined with artificial intelligence (AI), could enable the precise control required to regulate the deformation of flexible membrane wings and adapt the drones’ performance to varying weather conditions or flight missions.

École Polytechnique Fédérale de Lausanne (EPFL)
https://www.epfl.ch