Owl Wings Taught Fans and Wind Turbines to Hush

On a still summer evening at the edge of a meadow, a pale owl drifts out of the dusk, sweeps low over the long grass and is gone again. If you were standing nearby, you might have seen it, but you almost certainly did not hear it. No whoosh, no flutter, no beat of wings. A pigeon taking off from a pavement sounds like someone shaking out a rug; a large owl crossing a field sounds like nothing at all.

That silence is how the owl makes its living. Large owls such as the snowy owl, the great horned owl and the great grey owl are silent hunters, which gives them an edge over hawks and eagles, birds that rely on speed instead. The owl can hear the meadow, and the meadow cannot hear the owl. Engineers, who spend a great deal of time and money trying to make moving blades less noisy, have been studying that hush for years.

How an owl's wing stays quiet

The secret is not one trick but three, built into the feathers. Along the leading edge of the wing, the part nearest the owl's head, sits a row of stiff, evenly spaced fibres like the teeth of a comb. According to Audubon magazine, these serrations break up the turbulent air that would otherwise make a swooshing sound. The upper surface of the wing is covered in soft down with a texture much like velvet, and the trailing edge, at the back of the wing, ends in a soft, flexible fringe.

Each part does its share. Justin Jaworski, an engineer at Lehigh University who studies owl flight, describes the velvety down as a buffer layer that stifles sound, and says the flexible feathers at the back provide significant noise reduction. Krista Le Piane, a researcher at the University of California, Riverside, measured the wing feathers of 70 owl species in the Natural History Museum's bird collection at Tring, near London, and found a neat pattern: the wider the comb, the less sound the wing makes as it flaps.

What people often get wrong

It is natural to assume that a wing's noise comes from the front, where it pushes into the air. Jaworski's point is the opposite. The back edge, where the air leaves the wing, is the main source of noise for any blade passing through the air: not only an owl's wing, but an aircraft wing and a wind turbine blade too. That is why he calls the owl's porous, flexible trailing edge arguably the most important feature of the whole wing. A hard, sharp back edge is noisy; a soft, fringed one is far quieter.

From feathers to fan blades

Engineers have taken the hint. Researchers working to cut aircraft noise around airports studied the owl's quiet flight, and a 2011 study in the Journal of Aviation/Aerospace Education and Research fitted owl-style features to a model aeroplane and found no significant difference in how far it glided. Jaworski and colleagues at Virginia Tech have worked on adding a porous, flexible extension to the back edge of wind turbine blades, along with a fuzzy upper surface, in the hope of quieter wind farms.

In Germany, the fan maker Ziehl-Abegg built the idea into an everyday machine. Its FE2owlet axial fan has blades whose contours and structure are borrowed from an owl's wing, and the company sells it as a quiet fan for vehicles, machinery and industrial plants. So the next time a cooling fan hums more softly than you expected, it may owe a little of its manners to a bird that has been flying quietly through the night for a very long time.