Owl-inspired coating for noiseless aircraft

Scientists have developed a new coating which mimics the unique wing structure of owls to substantially reduce noise in aircraft, wind turbines and computers.
Early wind tunnel tests of the coating have shown a substantial reduction in noise without any noticeable effect on aerodynamics, researchers said.
Since wind turbines are heavily braked in order to minimise noise, the addition of this new surface would mean that they could be run at much higher speeds - producing more energy while making less noise.
For an average-sized wind farm, this could mean several additional megawatts worth of electricity.
"Many owls - primarily large owls like barn owls or great grey owls - can hunt by stealth, swooping down and capturing their prey undetected," said Professor Nigel Peake of University of Cambridge's Department of Applied Mathematics and Theoretical Physics, who led the research.

"While we've known this for centuries, what hasn't been known is how or why owls are able to fly in silence," said Peake.

Peake and his collaborators at Virginia Tech, Lehigh and Florida Atlantic universities used high resolution microscopy to examine owl feathers in fine detail.
They observed that the flight feathers on an owl's wing have a downy covering, which resembles a forest canopy when viewed from above.
In addition to this fluffy canopy, owl wings also have a flexible comb of evenly-spaced bristles along their leading edge, and a porous and elastic fringe on the trailing edge.
"No other bird has this sort of intricate wing structure," said Peake.

"Much of the noise caused by a wing - whether it's attached to a bird, a plane or a fan - originates at the trailing edge where the air passing over the wing surface is turbulent.
"The structure of an owl's wing serves to reduce noise by smoothing the passage of air as it passes over the wing - scattering the sound so their prey can't hear them coming," Peake added.
In order to replicate the structure, the researchers looked to design a covering that would 'scatter' the sound generated by a turbine blade in the same way.
Early experiments included covering a blade with material similar to that used for wedding veils, which despite its open structure, reduced the roughness of the underlying surface, lowering surface noise by as much as 30 decibel.

While the 'wedding veil' worked remarkably well, it is not suitable to apply to a wind turbine or aeroplane.
Using a similar design, the researchers then developed a prototype material made of 3D-printed plastic and tested it on a full-sized segment of a wind turbine blade.
In wind tunnel tests, the treatment reduced the noise generated by a wind turbine blade by 10 decibel, without any appreciable impact on aerodynamics.