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.
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