In 1934, a French entomologist declared that bumblebees theoretically shouldn’t be able to fly — their wings were simply too small to generate enough lift. It took decades and high-speed cameras to solve that puzzle, revealing the leading-edge vortex: a spinning air mass that insects exploit to stay airborne.
But bats tell a different story. With flexible membrane wings, some species expend up to 40% less energy in flight than moths of comparable size — suggesting a separate aerodynamic advantage that science has yet to fully explain.
At EPFL’s Unsteady Flow Diagnostics Laboratory, researchers built a bio-inspired experimental platform to find out exactly what makes those soft, deformable wings so special.
A 90-Year-Old Puzzle About Flight
Antoine Magnan’s 1934 claim about bumblebees wasn’t simply wrong — it was incomplete. His calculations assumed wings worked like fixed airplane surfaces, generating lift through steady airflow. What he missed was the chaotic, spinning aerodynamics that insects actually exploit.
High-speed cameras eventually revealed the answer: the leading-edge vortex. When an insect flaps its wings, air curls around the front edge and rolls into a spinning mass, creating a low-pressure zone that pulls the wing upward. Turbulent, energy-intensive — but it works.
Bats, though, appear to bypass it entirely. Their membrane wings are soft and highly deformable, bending and reshaping with every stroke. Some bat species expend up to 40% less energy than moths of a similar size — a gap significant enough to point toward a different aerodynamic mechanism, one that researchers had not yet identified.
Building a Bat Wing in the Lab
Working with live bats introduces obvious complications. Animals move unpredictably, and isolating a single aerodynamic variable is nearly impossible in a living system. The EPFL team took a different approach.
They constructed an experimental platform using a highly deformable membrane made from a silicone-based polymer, mounted onto a rotating rigid frame. The setup allowed independent adjustment of the front and back angles of the wing, giving researchers precise control over how the membrane responded to airflow. Changing one variable at a time — something no live animal permits.
To make the invisible visible, the device was submerged in water mixed with polystyrene tracer particles. As the wing moved through the water, those particles traced the surrounding fluid motion, exposing flow patterns that would otherwise go undetected. Lab head Karen Mulleners described the advantage directly: by altering the wing’s angles indirectly, the team could observe how the membrane aligned with the flow in real time, without the noise and unpredictability of a living animal.
Smooth Flow, Not Vortex: The Key Finding
What the tracer particles revealed was striking in its clarity. Unlike insect wings, the flexible membrane did not generate a leading-edge vortex during hovering. Instead, air followed the wing’s curved surface smoothly, without separating from it — and that smooth attachment generated more lift than a rigid wing of the same size would produce.
Lead author Alexander Gehrke, now a researcher at Brown University, called it a fundamentally different aerodynamic strategy. “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,” he said.
The finding came with an important caveat: flexibility alone is not enough, and more is not always better. If the membrane deforms too much, performance actually drops. A wing that’s too stiff reverts toward rigid-wing behavior; one that’s too loose loses the smooth-flow advantage entirely. The optimal range is narrow, and locating it was central to the team’s results. The study was published in the Proceedings of the National Academy of Sciences.
What This Means for Drones and Energy Tech
The practical implications reach in two directions — aircraft design and energy systems.
On the drone side, the challenge is scale. As aerial vehicles shrink, standard quadrotor designs become increasingly vulnerable to small aerodynamic disturbances. A gust that a larger aircraft barely registers can destabilize a miniature drone entirely. Flapping membrane wings, which naturally adapt their shape to changing airflow, could offer a more resilient alternative where conventional rotors fall short.
Bat-inspired designs could allow tiny aerial vehicles to hover and carry payloads more efficiently than current rotor-based systems permit. Gehrke noted that the findings offer insights for engineers building improved flyers that operate where quadrotors stop working reliably.
Energy harvesting is the second frontier. The same principles that make flexible membranes efficient in flight could apply to systems extracting energy from moving fluids — improving wind turbines or advancing tidal harvesters that passively capture energy from ocean currents.
Looking further ahead, advances in sensors and control technology — potentially combined with artificial intelligence — could enable real-time regulation of membrane deformation. A drone or harvester equipped with such a system might adapt its wing shape continuously, responding to shifting wind conditions or changing mission requirements. That level of dynamic control remains a future goal, but the aerodynamic foundation for it is now considerably better understood.