Owl’s Flight Abilities Provide Clues to Mitigating Conventional Aircraft Noise

Using the wings of an owl as a model, researchers at the University of Cambridge, England, have  attempted to explore the dynamics behind its silent flight so that the same information could be used in aircraft design.

"Many owl species have developed specialized plumage to effectively eliminate the aerodynamic noise from their wings, which allows them to hunt and capture their prey using their ears alone," said Justin Jaworski with the department of applied mathematics and theoretical physics at the University of Cambridge. "No one knows exactly how owls achieve this acoustic stealth, and the reasons for this feat are largely speculative based on comparisons of owl feathers and physiology to other not-so-quiet birds such as pigeons."

According to the report, all wings, natural or engineered produce certain turbulent eddies as they slice through the air. As the  eddies slash against the trailing edge of the wing, they are amplified and scattered as sound. Conventional aircraft, which have hard trailing edges, are particularly noisy in this regard.

Three physical attributes of the owl that contribute to the silent flight capability involves, a comb of stiff feathers along the leading edge of the wing; a soft downy material on top of the wing; and a flexible fringe at the trailing edge of the wing.

But it is still a mystery whether it is a combination of the three attributes or just one single one that cause the noise reduction.

In an attempt to uncover this mystery, the researchers developed a theoretical basis for the owl's ability to mitigate sound from the trailing edge of its wing, which is typically an airfoil's dominant noise source.

Through previous findings it was known that wing noise is much less dependent on air speed and that there is a large reduction of high frequency noise across a range where human ears are most sensitive.

With the help of a mathematical model the researchers were able to demonstrate that elastic and porous properties of a trailing edge could be tuned so that aerodynamic noise would depend on the flight speed.

 "This implied that the dominant noise source for conventional wings could be eliminated," said Nigel Peake also of the University of Cambridge. "The noise signature from the wing could then be dictated by otherwise minor noise mechanisms such as the roughness of the wing surface."

Their findings were presented at the American Physical Society's (APS) Division of Fluid Dynamics meeting, held Nov. 18 - 20, in San Diego, Calif.