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Wing feathers enable near-silent flight: owl

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Comb-like edge of owl feather / Kersti Nebel.. / LicenseCC-by-sa - Attribution Share Alike

Specialized feathers of the owl enable near-silent flight by altering air turbulence and absorbing noise.

Owls are known as silent predators of the night, capable of flying just inches from their prey without being detected. The quietness of their flight is owed to their specialized feathers. When air rushes over an ordinary wing, it typically creates a “gushing” noise as large areas of air turbulence build up. But the owl has a few ways to alter this turbulence and reduce its noise.

First, the leading edge of the owl’s wing has feathers covered in small structures (hooks and bows) that break up the flowing air into smaller, micro-turbulences. These smaller areas of turbulence then roll along the owl’s wing toward the trailing edge, which is comprised of a flexible fringe. This fringe breaks up the air further as it flows off the trailing edge, resulting in a large reduction in aerodynamic noise. Then, any remaining noise that would be detectable by the owl’s prey is absorbed by velvety down feathers on the owl’s wings and legs. These soft feathers absorb high frequency sounds that most prey, as well as humans, are sensitive to.

All together, these features enable owls to remain undetected when they fly. However, it’s believed that the wing’s serrated leading edge is most effective at reducing noise when the wing is at a steep angle—which would happen when the owl is close to its prey and coming in for a strike.

See and hear the difference between a flying owl and other birds in this video, and check out this KQED Deep Look video to learn more and see owl feathers up close.  

Video content © 2015 KQED INC. All Rights Reserved.

This summary was contributed by Ashley Meyers.
"Barn owl wings differ from those of other birds and aircraft... The distal wing of barn owls resembles a slightly cambered plate...over which the airflow normally tends to separate, especially at low-speed flight... Serrations at the leading edge prevent a separation and increase the lift by generating a turbulent boundary layer over the airfoil upper surface... The turbulent boundary layer delays leading- and trailing-edge flow separation to higher angles of attack...similarly as shown in experiments by Soderman (1972) at higher Reynold’s numbers.
As mentioned above, Neuhaus et al. (1973) showed that serrations influence the noise generation only at steep angles of attack. One explanation might be that in cruise flight conditions the stagnation point at the leading edge results in a relative low air flow through the serrations due to the low air stream velocity. At sharp angles of attack, however, serrations comb through the air like a plough through a field. By their bending and orientation, serrations induce tiny vortices running over the dorsal wing surface as shown by Ito (2009). This phenomenon increases the lift and reduces the noise of barn owl wings during flapping flight and striking, which is extremely useful for the owl.” (Bachmann and Wagner 2011: 200)

"Serrated feathers on the front edges of owl wings funnel air smoothly over the wings, reducing the noise of rushing air. Each wing has a scarflike, fringed back edge that prevents the abrupt air-pressure changes--and noise--produced by rigid wings." (Courtesy of the Biomimicry Guild)
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Liang G-Q; Wang J-C; Chen Y; Zhou C-H; Liang J; Ren L-Q. 2010. The study of owl's silent flight and noise reduction on fan vane with bionic structure. Advances in Natural Science. 3(2): 192-198.
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Bachmann T; Wagner H. 2011. The three-dimensional shape of serrations at barn owl wings: towards a typical natural serration as a role model for biomimetic applications. Journal of Anatomy. 219: 192-202.
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Bachmann T; Klän S; Baumgartner W; Klaas M; Schröder W; Wagner H. 2011. Morphometric characterisation of wing feathers of the barn owl Tyto alba pratincola and the pigeon Columba livia. Frontiers in Zoology [Internet],
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Geoffrey Lilley. 1998. A study of the silent flight of the owl. Aeroacoustics Conferences. 1-6.
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American Physical Society's Division of Fluid Dynamics. 24 Nov 2013. The Secrets of Owls' Near Noiseless Wings. Science Daily. 1.
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I am impressed to discover that about owls. Awesome!
about 1 year ago
This is perfect strategy.
over 6 years ago
Thanks to Duarte Miguel Prazeres for finding and uploading this photo of a barn owl.
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