If you ever sit in an airplane row overlooking a wing, then you probably notice multiple flaps along its edges that adjust during takeoffs and landings. Much like bird feathers, these components are necessary for controlling the vehicle’s rotation, lift, and drag during flight. Unlike their avian counterparts, however, these mechanisms are generally placed in single rows along a wing, and require electronic components to control during travel.
Birds, meanwhile, have taken to the skies for millions of years equipped with exponentially more “flaps” in the form of covert feather groupings that passively adjust to airflow. By taking a cue from them, some engineers believe planes can be built to be safer and more energy efficient. Their results, published on October 28th in the Proceedings of the National Academy of Sciences, appear to support such feathery plane upgrades.
Researchers at Princeton University recently upgraded a small remote-controlled model airplane to include rows of flaps that mimic covert feathers—the feather groupings on birds that passively adjust during complex maneuvers such as navigating wind gusts and landing. In doing so, the team thinks similar biomimicry-based designs may one day help aircraft improve overall performance and avoid potentially dangerous stalling emergencies.
While previous “studies suggest [covert feathers] can enhance flight during maneuvers such as landing or flying through gusts,” the team wrote that “there is no existing consensus on their underlying physics or the implications of having multiple rows.” To fix this, they first installed between two-to-five rows of covert-style flaps onto 3D-printed, scale-model plane wings, then subjected their prototypes to wind tunnel tests in a 30-foot-tall installation. Inside the tunnel, a combination of sensors, as well as both laser and high-speed cameras, minutely measured airflow around the wings during various condition simulations. They also used a wing model built with standard, single-row flaps to serve as a control.
“The wind tunnel experiments give us really precise measurements for how air interacts with the wing and the flaps, and we can see what’s actually happening in terms of physics,” Girguis Sedkey, a postdoctoral researcher and study lead author, said in a university profile on Monday. After analyzing the data, Sedkey and their team pinpointed specific ways that flaps control airflow around a wing. One of these, “shear layer interaction,” had never before been documented in aeronautical testing.
“The discovery of this new mechanism unlocked a secret behind why birds have these feathers near the front of the wings and how we can use these flaps for aircraft,” added Aimy Wissa, a mechanical and aerospace engineering assistant professor and the study’s principal investigator. Wissa added that, out of all the models, the five-row design performed best, improving lift by 45-percent while reducing drag by 30-percent.
“[T]he more flaps you add to the front of the wing, the higher the performance benefit,” she explained.
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Following these initial experiments, Wissa’s team then transitioned to outdoor flight tests using a bird-sized R/C drone plane on loan from Princeton’s Somerset RC model aircraft club. Engineers first installed covert flap rows, then programmed an onboard flight computer to autonomously stall out. From there, researchers launched their model and watched it navigate its aerial challenges. Each time the computer initiated a stall, the plane’s covert flap rows passively deployed to mitigate stall intensity.
“That’s the power of bioinspired design,” Wissa said. “The ability to transfer things from biology to engineering to improve our mechanical systems, but also use our engineering tools to answer questions about biology.”
Wissa and her colleagues believe avian covert feathers may lend themselves to more applications than just airplanes. Given airflow fluid dynamics, they note the potential to explore similar adaptations to improve the efficacy and safety of cars, submersibles, and potentially even wind turbines.
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