Researchers have created flexible wings resembling those of bats, which enhance lift and flight efficiency. This advancement could pave the way for improved drones or new energy-harvesting devices.
Back in 1934, French insect expert Antoine Magnan claimed that bumblebees “should not be able to fly,” since their tiny wings presumably do not generate enough lift. However, modern high-speed cameras revealed the secret behind flying insects: the leading-edge vortex. This occurs when air flows around the front of flapping wings and forms a vortex, creating a low-pressure area that enhances lift.
In contrast, bats—equipped with flexible membrane wings—can fly as well as or even better than insects. Some bat species are known to use up to 40% less energy compared to similarly-sized moths. Researchers from the Unsteady Flow Diagnostics Laboratory at EPFL’s School of Engineering investigated the aerodynamic capabilities of more flexible wings by using a test platform with an elastic membrane made from a silicone-based material. Their research showed that, instead of forming a vortex, air flows smoothly over the curved wings, resulting in increased lift and greater efficiency than rigid wings of the same size.
“Our key discovery is that the enhanced lift comes not from a leading-edge vortex, but from the airflow seamlessly following the smooth curve of the membrane wing,” explains Alexander Gehrke, a former EPFL student now researching at Brown University. “The wing’s curve needs to be precisely right; if it’s too flexible, the performance declines.”
Gehrke is the leading author of a study detailing this research in the Proceedings of the National Academy of Sciences.
Insights for Drone and Energy Harvesting Design
The researchers attached the flexible membrane to a sturdy frame with rotational edges. To visualize the airflow around the wing, they submerged their setup in water mixed with polystyrene tracer particles.
“Our tests allowed us to indirectly modify the angles at the front and back of the wing, enabling us to see how they aligned with the airflow,” states Karen Mulleners, head of the Unsteady Flow Diagnostics Lab. “Due to the deformation of the membrane, the airflow followed the wing’s curvature naturally without forming a vortex, which increased lift.”
Gehrke notes that their findings are valuable for both biologists and engineers.
“We understand that bats can hover and that their membrane wings are flexible. Investigating how this wing deformation influences their hovering capability is crucial, but experimenting on live animals poses challenges. Through a simplified bio-inspired experiment, we can explore nature’s fliers and learn how to create more efficient aerial vehicles.”
He adds that as drones shrink in size, they become more susceptible to minor aerodynamic disturbances and unpredictable gusts compared to larger aircraft. Standard quadrotor drones struggle to function effectively at smaller scales. One potential solution could be mimicking the flapping motions of animals to develop enhanced drones capable of hovering and carrying payloads more effectively.
The team’s discoveries could also lead to improvements in existing energy technologies, such as wind turbines, or the development of new systems like tidal harvesters that harness energy from ocean currents passively. Progress in sensors and control techniques, along with artificial intelligence, might allow for precise manipulation of flexible membrane wings, adapting their performance to changing weather conditions and flight requirements.
