Bird wings inspire new approach to flight safety

"These flaps can both help the plane avoid stall and make it easier to regain control when stall does occur," said Aimy Wissa, assistant professor of mechanical and aerospace engineering and principal investigator of the study, published in the Proceedings of the National Academy of Sciences.

The flaps mimic a group of feathers, called covert feathers, that deploy when birds perform certain aerial maneuvers, such as landing or flying in a gust. Biologists have observed when and how these feathers deploy, but no studies have quantified the aerodynamic role of covert feathers during bird flight. Engineering studies have investigated covert-inspired flaps for improving engineered wing performance, but have mostly neglected that birds have multiple rows of covert feathers. The Princeton team has advanced the technology by demonstrating how sets of flaps work together and exploring the complex physics that governs the interaction.

Girguis Sedky, postdoctoral researcher and the paper's lead author, called the technique "an easy and cost-effective way to drastically improve flight performance without additional power requirements."

The covert flaps deploy or flip up in response to changes in airflow, requiring no external control mechanisms. They offer an inexpensive and lightweight method to increase flight performance without complex machinery. "They're essentially just flexible flaps that, when designed and placed properly, can greatly improve a plane's performance and stability," Wissa said.

A wing's teardrop form forces air to flow quickly over its top, creating a low-pressure area that pulls the plane up. At the same time, air pushes against the bottom of the wing, adding upward pressure. Designers call the combination of this pull and push "lift." Changes in flight conditions or a drop in an aircraft's speed can result in stall, rapidly reducing lift.

Wissa's team designed a series of experiments in the wind tunnel at Princeton's Forrestal Campus to understand how flaps mimicking the feathers would affect flight performance, especially near stall, which usually happens when the plane is at a steep angle, when covert feathers were observed to deploy. The tunnel allowed the team to examine the way different flap arrangements affected variables like air pressure around the wings, wind speed over the wing and vortices that impact performance.

The team attached the covert-inspired flaps to a 3D-printed model airplane wing and mounted it in the wind tunnel, a 30-foot-tall metal contraption that simulates and measures air flow. "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," Sedky said.

The wind tunnel is equipped with sensors that read the forces felt by the wing, as well as a laser and high-speed camera that measure precisely how air is moving around the wing.

The study uncovered the physics by which the flaps improved lift and identified two ways that the flaps control air moving around the wing. One of these control mechanisms had not been previously identified. The researchers uncovered the new mechanism, called shear layer interaction, when they were testing the effect of a single flap near the front of the wing. They found that the other mechanism is only effective when the flap is at the back of the wing.

The researchers tested configurations with a single flap and with multiple flaps ranging from two rows to five rows. They found that the five-row configuration improved lift by 45%, reduced drag by 30% and enhanced the overall wing stability.

"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," Wissa said. "Especially because we found that the more flaps you add to the front of the wing, the higher the performance benefit."

Following the results of the wind tunnel experiments, the team moved outside the lab and into the field to test the covert-inspired flaps on a scaled model aircraft. Princeton's Forrestal Campus was once an airport and still features an operational helipad. So, the researchers teamed up with Nathaniel Simon, graduate student in mechanical and aerospace engineering who researches drone flight, and demonstrated the technology in real-world conditions by equipping a radio-controlled (RC) airplane with covert-inspired flaps.

The researchers worked with members of the Somerset RC model aircraft club to select a model plane. The researchers then modified the airplane body to outfit it with an onboard flight computer, and Simon drew on his experience piloting drones to fly it. They programmed the flight computer to stall the aircraft autonomously and repeatedly. Simon said it was amazing to see the flaps deploy in-flight and to see that they helped delay and reduce stall intensity, just as they did in the wind tunnel. "It's cool to be able to collaborate in the shared space at the Forrestal campus, and to see how many areas of research this project touched," he said.

Sedky said that in addition to improving flight, their findings could be extended to other applications where modifying the surrounding fluid would benefit performance. "What we discovered about how coverts alter the airflow around the wing can be applied to other fluids and other bodies, making them applicable to cars, underwater vehicles, and even wind turbines," he said.

Wissa said that this study could open the door to collaborations with biologists to learn more about the role of covert feathers in bird flight, and that the results of this study will help form new hypotheses that can be tested on birds. "That's the power of bioinspired design," she 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."