Owl wings the focus for bio-inspired next-gen aircraft design

Iowa State researchers, left to right, Bharat Agrawal, Andrew Bodling and Anupam Sharma are studying how owl wings can inspire ultraquiet aircraft and wind turbines. Photo credit: Christopher Gannon/Iowa State University.

Engineers look to nature for clues for developing ultraquiet aircraft and wind turbines.

Anupam Sharma, an Iowa State assistant professor of aerospace engineering and Walter W. Wilson Faculty Fellow, used a preserved wing of a short-eared owl for design inspiration.

“The owl is almost completely silent in flight,” said Sharman, who started working in aeroacoustics during graduate school and a previous position at GE. “Owls are not only silent in gliding flight, but also in flapping flight, which is amazing.”

Sharma points to the three-part “owl hush kit” that’s responsible for silent flight as the system he hopes to replicate.

First, at the leading edge of the wing there are small, fine, comb-like structures. Second, all the feathers at the trailing edge of the wing end in a pliable and porous fringe. And third, there’s a downy coat on the flight feathers.

One of the main components of the study was to scan owl wing specimens in order to learn exactly how that hush kit manipulates air flow, turbulence and pressure to produce silent flight. Sharma was able to create digital models and run multi-day simulations that use more than 16,000 processors.

“We can get into details that there is no way you can study with experiments,” Sharma said.

Sharma is not the only one researching the owl wing — the downy coat of owl wings inspired collaborators at Virginia Tech to design model airfoils (the curved shape of an aircraft wing) with a regular series of small, thin “finlets” and canopies near the trailing edge of the blade and running parallel to the airflow. William Devenport, a Virginia Tech professor of aerospace and ocean engineering and director of the university’s Stability Wind Tunnel, led the experiments in Virginia.

Both research teams compared the performance of the owl-inspired airfoils to a standard, flat-surfaced airfoil.

Sharma said the computer simulations showed the owl-inspired airfoils substantially reduced the unsteady pressure on the back end of the blade surface. The sound radiated by the owl-inspired design was reduced by up to 5 decibels over a wide frequency range. This noise reduction was observed without sacrificing aerodynamic performance.

A 2016 paper by Sharma’s team and published in the International Journal of Aeroacoustics notes that tests of airfoils with a serrated leading edge inspired by owl wings found the serrations substantially reduced airfoil noise. The paper also established the physical mechanisms that caused the noise reduction.

Sharma stresses that we shouldn’t expect wind turbine blades or next-gen aircraft to look like owl wings as the approach is bio-inspired not bio-mimicry.

“Our designs won’t look like owl wings,” Sharma said. We’re studying the physical mechanisms behind the owl’s silent flight. Then we’re taking simplified geometries inspired by the owl wings and applying those to aircraft wings, rotor blades of jet engines and wind turbines.”

According study findings, the owl has potential to help engineers develop ultraquiet flight and wind energy, although applications might start at smaller scales and low speeds, such as drones or unmanned aerial vehicles.

“The results of this research could have an impact on the design of silent air vehicles with application in national defense, in commerce and in transportation,” Sharma wrote in a project summary.