Secrets Of Owls' Noiseless Wings Could Improve Aircraft Stealth Technology

Noise-reduction mechanism of silent flight in birds of prey has far-reaching implications

By Ben Wolford on November 24, 2013 11:17 AM EST

Shutterstock
Shutterstock

Scientists have long known that owls can swoop down on prey in complete silence, with no loud flapping to announce their attack. But exactly how the birds manage to be so stealthy remained shrouded. New research from a team led by Justin Jaworski, of Lehigh University, suggests that down feathers on the top of the owls' wings could have more to do with the noiseless flight than previously thought. And Jaworski says continued efforts to understand the dynamics of this "soft carpet," as they describe it, could give mechanical engineers fresh ideas for reducing noise.

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"If the noise-reduction mechanism of the owl down can be established, there may be far-reaching implications," Jaworski said in a statement. He said it could revolutionize the way engineers think about the design of sound-absorbing liners, wind turbines, and the sound vibrations stirred up by the wings of aircraft and even submarines.

Copying owl feathers is not a new concept, however. Scientists at NASA, the University of Southampton in England, and other institutes have been scrutinizing the aerodynamics of owls' wings for years. "If the owl didn't have velvety feathers, the owl would be heard by the prey as the owl approached, and [the prey] would have time to scurry away," Geoffrey Lilley, of the University of Southampton, told National Geographic in 2004. He says the down feathers absorb whatever noise the owls' other wing features don't.

Jaworski's research builds on that knowledge by predicting the sound that would result from wind passing over those down feathers. Additionally, his team predicted "how the aerodynamic noise level varies with fiber composition," Jaworski said. Understanding the fiber composition is the key to stealing nature's schematic. The diagram below shows the three mechanisms that give owls their unique stealth. Stiff serrated feathers at the leading edge of the wing stabilize the appendage. And the trailing-edge fringe helps enormously to disrupt the waves of sound that come crashing over the wing. Imagine dragging a massive comb behind a boat's wake. And finally, there are the velvety down feathers, like a felt blanket over the whole production.

The three unique wing features believed to make owls fly silently are shown in this graphic.

"Friction noise between single feathers is also reduced [by] their velvety surface," Thomas Bachmann from the Technical University Darmstadt in Germany, who also studies owl mechanics, told BBC Nature last year. He predicted that it may be two more decades before engineers can present an aircraft wing that adapts these findings to their designs. "Until then, we will conduct many more experiments on owl wings."

Jaworski's research incorporated photographic studies from Virginia Tech that revealed "a surprising 'forest-like' geometry" in the down feathers. "Preliminary experiments performed at Virginia Tech show that a simple mesh covering, which replicates the top layer of the 'forest' structure, is effective in eliminating some sound generated by rough surfaces," the researchers announced.

The owl scientists will present their findings at the annual conference of the American Physical Society's Division of Fluid Dynamics in Pittsburgh this week. It's worth noting that another presentation at that conference, called "Urinal Dynamics," will likely be highly attended. Researchers from Brigham Young University will demonstrate how to prevent splashback from a "human male urine stream impacting rigid and free surfaces." Ah, science.

Top photo courtesy of Shutterstock

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