Owls' Silent Flight May Inspire Quiet Aircraft Tech

John Roach
for National Geographic News
December 17, 2004
A few years ago, the silent brush of a barn owl's wing sent Trish Nixon reeling from her porch in the still of the night. She never heard the owl, just saw its "ghostly white form float past."

Nixon is a raptor specialist with The Peregrine Fund in Boise, Idaho. She often speaks about the silent flight of owls, but the porch incident spoke to her louder than words. "The owl lifted from the ground, and I didn't hear a sound, which is why I totally lost my cool when a wing brushed against me," she said.

The silent flight of owls has long fascinated ornithologists. No other birds fly with such stealth. Now flight engineers are looking at the unique design of owl feathers in hopes of making aircraft as quiet as possible.

Geoffrey Lilley, a professor emeritus of aeronautics and astronautics at the University of Southampton in England, is a pioneer in the study of the owl's silent flight and its potential applications in the aeronautics industry.

He has been working on the issue with the Quiet Aircraft Technology project within the Vehicle Systems Program at NASA's Langley Research Center in Hampton, Virginia.

Silent Owls

Flight, for most birds, including day-active owls, is a noisy affair. Air rushing over the birds' feathers produces turbulence. This turbulence creates a swishing sound that is audible to most prey animals.

But nocturnal owls are birds of a different, often quieter feather. Three main reasons explain why, Lilley said.

First, the leading edge, or primary feathers, on the owls' wings are serrated like a comb. Second, the trailing feathers on the back end of the wing are tattered like the fringe of a scarf. And third, the rest of the owls' wings and legs are covered in velvety down feathers.

The serrated feathers on the leading edge of owl wings have more to do with keeping the raptors stable than quiet, Lilley noted.

The fringe on owls' trailing feathers, however, allows for "a very large noise reduction at the speed owls fly," he said.

Research scientist David Lockard is a Langley Research Center-based colleague of Lilley's. Lockard said owls' tattered fringe feathers help to break up the sound waves that are generated as air flows over the top of their wings and forms downstream wakes.

"If you put serrations on the trailing edge or change the angle at which [the air] flows, you can reduce the amount of noise generated there," he said.

The noise reduction achieved from the tattered fringe makes owls the quietest flying birds, Lilley said.

He added that the velvety down feathers found elsewhere on owls' wings and legs absorb the remaining sound frequencies above 2,000 hertz, making owls completely silent to their prey.

"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," Lilley said.

Nixon, the raptor specialist, readily agrees that unique feather design gives owls the power of stealth flight. She also notes that the silence allows owls to more efficiently use their ears, which are positioned asymmetrically in their skulls: One ear is higher and more forward than the other, and each is shaped slightly different.

Human ears, by contrast, are shaped and positioned symmetrically on each side of our skulls.

But owls, because of their ears' asymmetry, hear sounds slightly differently through each ear. This allows the raptors to more accurately determine where a sound is coming from.

"This is often what really helps [owls to] pinpoint their prey," Nixon said, noting that the ability is sometimes more helpful to owls than night vision.

Quiet Aircraft

In aircraft design, noise reduction takes a backseat to fuel efficiency, Lockard said. The NASA research scientist notes, however, that the lessons learned from the study of owl feathers have the potential to increase where and how often planes can take off and land.

"Major airports like Chicago [O'Hare] and London Heathrow have requirements on the amount of noise per day an air carrier can generate. If you can reduce that, you can have more flights come in a day, which is important … for these airlines," he said.

Incorporating owl-feather-inspired technology to dampen sound in aircraft requires that engineers clear a critical hurdle: coming up with a design that does not create extra drag when aircraft have reached cruising speeds and altitude.

Ideas under consideration include a retractable, brushlike fringe to mimic an owl's trailing feathers and a velvety coating on aircraft landing gear, Lilley said.

"The owl has been doing this silent flight for something like 20 million years. We are just now coming along and hope we might make use of some these ideas to get an airplane quiet within 20 years and even more quiet within 20 to 50 years," Lilley said.

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