Engineers have discovered a method to assist airplanes in avoiding stalls by utilizing lightweight plastic flaps that react to fluctuations in pressure. These flaps are inspired by a type of feathers essential for birds’ most adventurous flying techniques.
Drawing inspiration from bird feathers, researchers at Princeton have determined that integrating rows of flaps onto the wings of remote-controlled aircraft can enhance their flight capabilities and help prevent stalling, a condition that can threaten a plane’s ability to remain airborne.
“These flaps not only aid in preventing stalls but also facilitate regaining control if a stall occurs,” explained Aimy Wissa, an assistant professor of mechanical and aerospace engineering and the main researcher behind the study published in the Proceedings of the National Academy of Sciences.
The flaps imitate a category of feathers known as covert feathers, which extend when birds execute specific aerial movements, such as landing or adjusting to gusts of wind. While biologists have studied the deployment of these feathers, no research has quantitatively assessed their aerodynamic significance during bird flight. Previous engineering studies have looked into layers of covert-inspired flaps to enhance wing performance but often overlooked that birds possess multiple rows of these feathers. The Princeton team has advanced this field by demonstrating how collections of flaps can work in coordination and investigating the complex physics that underlies their interaction.
According to Girguis Sedky, postdoctoral researcher and lead author of the paper, this method is “a simple and cost-effective means to significantly enhance flight performance without the need for extra power.”
The covert flaps respond to changes in airflow by deploying or flipping up, eliminating the need for external control mechanisms. This approach provides an affordable and lightweight solution to boost flight capability without intricate machinery. “Essentially, they’re just flexible flaps that, when properly designed and positioned, can greatly enhance an aircraft’s performance and stability,” Wissa stated.
A wing’s teardrop shape causes air to flow swiftly over its upper surface, creating a low-pressure zone that lifts the airplane. Concurrently, air pushes against the wing’s bottom, contributing to an upward force. This combination of pulling and pushing is characterized as “lift.” However, fluctuations in flight conditions or a decrease in the aircraft’s speed may lead to stall, which rapidly lessens lift.
Wissa’s team devised a series of experiments within Princeton’s wind tunnel to explore how these flaps emulating feathers affect flight performance, particularly as stall approaches, which typically occurs at steep angles where covert feathers have been recorded to deploy. The tunnel allowed the team to analyze different flap arrangements’ impacts on factors like air pressure around the wings, wind speed over the wing, and vortices that influence performance.
The team attached the covert-inspired flaps to a 3D-printed model of an airplane wing, securing it in the wind tunnel—a 30-foot metal structure that simulates and measures airflow. “The wind tunnel tests provide highly accurate data on how air interacts with the wing and flaps, allowing us to observe the physics at play,” Sedky remarked.
Equipped with sensors to capture the forces experienced by the wing, as well as a laser and high-speed camera for precise air movement measurements, the wind tunnel provided ample data for the researchers.
The research unveiled the physics behind how the flaps enhance lift and identified two mechanisms by which the flaps influence the airflow around the wing. One of these mechanisms had not been previously recognized. The new mechanism, termed shear layer interaction, emerged while testing the effects of a single flap near the front of the wing, while the other mechanism was effective only when the flap was positioned at the back of the wing.
The team conducted tests with configurations that included one flap and variations with two to five rows of flaps. Their findings showed that the five-row arrangement enhanced lift by 45%, decreased drag by 30%, and improved overall wing stability.
“This discovery about the new mechanism clarifies why birds have these feathers near the front of their wings and how we can leverage these flaps in aircraft design,” Wissa noted. “Particularly since we found that adding more flaps to the front of the wing yields greater performance benefits.”
After analyzing wind tunnel results, the team transitioned their efforts outside the laboratory to test the covert-inspired flaps on a scaled aircraft model. Given that Princeton’s Forrestal Campus was once an airport and still hosts an active helipad, the researchers collaborated with Nathaniel Simon, a graduate student specializing in drone flight, to validate the technology under real-world settings by equipping a radio-controlled (RC) airplane with these flaps.
They chose a model plane in collaboration with members of the Somerset RC model aircraft club and modified the aircraft to include an onboard flight computer, with Simon piloting the craft utilizing his drone flying expertise. The flight computer was programmed to autonomously initiate stalling of the aircraft repeatedly. Simon expressed amazement at witnessing the flaps actively deploying during flight, effectively delaying and minimizing stall intensity as observed in the wind tunnel tests. “Collaborating within the shared resources at the Forrestal campus was fantastic, and it was exciting to see the diverse research areas this project has touched,” he stated.
Sedky noted that beyond enhancing flight, their discoveries might be adaptable to other fields where altering surrounding fluid can enhance performance. “What we uncovered regarding covert feathers’ influence on airflow around the wing can be applied to different fluids and bodies, making it useful in automotive engineering, underwater vehicles, and even wind turbines,” he observed.
According to Wissa, this research paves the way for collaborations with biologists to delve deeper into the functions of covert feathers in bird flight, and the findings will aid in developing new hypotheses for testing in avian studies. “That’s the essence of bioinspired design,” she concluded. “The ability to translate biological insights into engineering solutions to refine our mechanical systems, while also using our engineering tools to explore biological questions.”
