Researchers at EPFL have shown that flexible membrane wings, inspired by bats, generate more lift than rigid ones, offering insights for more efficient drones and renewable energy technologies.
Karen Mulleners with her lab's flexible membrane wing. Image Credit: Alain Herzog
In 1934, French entomologist Antoine Magnan famously noted that, in theory, bumblebees should not be able to fly—at least not according to conventional aerodynamic principles. Their small wings seemingly lack the lift necessary to support their bodies. However, modern high-speed cameras have since revealed the secret behind insect flight: the leading-edge vortex. This phenomenon occurs when air flowing around the front edge of flapping wings forms a vortex, creating a low-pressure zone that enhances lift.
Unlike insects, bats rely on flexible membrane wings, which allow them to fly just as efficiently—if not more so. Some bats, in fact, expend up to 40 % less energy than moths of comparable size.
Researchers at EPFL’s Unsteady Flow Diagnostics Laboratory set out to explore the aerodynamic potential of flexible wings using an experimental platform equipped with a highly deformable membrane made from a silicone-based polymer. Their findings challenge traditional assumptions: rather than generating vortices, air flows smoothly over the curved membrane wings, producing more lift and making them even more efficient than rigid wings of the same size.
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. 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.
Alexander Gehrke, Researcher, Brown University
Alexander Gehrke was a Ph.D. Student at EPFL.
Design Insights for Drones or Energy Harvesters
To study the behavior of these flexible wings, the researchers mounted a deformable membrane onto a rigid frame with rotating edges. To visualize airflow, they submerged the setup 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. 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.
Karen Mulleners, Head, Unsteady Flow Diagnostics Lab, EPFL
The results provide valuable insights for both biologists and engineers.
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. By using a simplified bio-inspired experiment, we can learn about nature’s fliers and how to build more efficient aerial vehicles.
Alexander Gehrke, Researcher, Brown University
As drones become smaller, they encounter greater aerodynamic instability and sensitivity to turbulence. Traditional quadrotor designs struggle at tiny scales, but flexible flapping wings—like those found in nature—could offer a solution, improving hovering capabilities and payload efficiency.
Beyond aerial vehicles, these findings could enhance existing energy technologies such as wind turbines or advance emerging systems like tidal energy harvesters, which passively harness ocean currents. With the integration of advanced sensors, control systems, and artificial intelligence, flexible membrane wings could be precisely regulated to optimize performance for varying weather conditions or flight missions.
This research not only deepens our understanding of biological flight but also opens up the possibility for innovative engineering solutions in aviation and renewable energy.
Journal Reference:
Gehrke, A. and Mulleners, K., (2025) Highly deformable flapping membrane wings suppress the leading edge vortex in hover to perform better. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2410833121