Wing Feathers Enable Near-Silent Flight

Biological Strategy

Owl

AskNature Team

Image: Luc Viatour / Wikimedia commons / GFDL - Gnu Free Document License

Manage Turbulence

A turbulent force occurs when air or water creates a chaotic or irregular motion. The source can be such things as wind, waves, and eddies caused by obstructions to air or water flow (such as that created by a rock in a stream). Because the force is irregular, it acts in unpredictable ways on multiple parts of a living system at any given time, decreasing the living system’s efficiency. Strategies used to manage turbulence include dampening the amount of turbulence, having flexibility to handle sudden changes, and making quick adjustments. An example is the mucus on aquatic organisms, such as barracuda sharks, that can reduce turbulent friction of seawater by 66%. In doing so, it decreases drag and increases the sharks’ swimming efficiency.

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Move in/Through Gases

Living systems must move through gases (which are less dense than liquids and solids) such as those in the earth’s atmosphere. The greatest challenge of moving in gases is that because the living system is heavier than the gas, it must overcome the force of gravity. Moving efficiently in this light medium presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag and increase lift so that they can stay aloft and take advantage of variable currents. Additionally, they must overcome gravity when moving from a liquid or solid into the air. The fairyfly, the smallest known insect, is a tiny wasp that must move through the air. To the wasp, air feels like a heavy liquid and to move through it, it uses special feathery oars rather than wings.

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Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

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

Introduction

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.

The Strategy

First, the leading edge of the owl’s wing has feathers covered in small structures that project out from the wing. One hypothesis is that these serrations break up the flowing air into smaller flows that are more stable along the wing. Furthermore, this change in airflow patterns also appears to reduce the noise of the flowing air. The wing’s serrated leading edge appears to be 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.

Detail view of the stiff curved serrations on the leading edge of an owl wing feather

A close view of the leading edge of an owl's outer wing feather shows the stiff serrations that help create a smooth flow of air above the wing. Photo © Radd Icenoggle, used with permission.

detail of feathers on an owl wing showing the leading and trailing edges

While stiff serrations break up air flow on the leading edge of owl wing feathers, softer serrations on the trailing edge (upper left) help to tame turbulence in the feathers' wake. Photo © Radd Icenoggle, used with permission.

These smaller airflows then roll along the owl’s wing toward the trailing edge, which is comprised of a fine 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 feather features enable owls to remain unheard when they fly.

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The Potential

Aspects of owl feather aerodynamics have already been employed in wind turbines and the Shinkansen train aerofoil. They could similarly improve efficiency and minimize noise for the wings of airplanes and the blades of helicopters and drones. Fan blades for home and industrial use could also make use of these strategies.

Owl feather strategies could also be used to create different aerodynamic effects for fabric-based wings, such as kites, parachutes, gliders, banners and more.

Related Innovations

Innovation: Industry

Wind Turbine Tech Inspired by Owl Feathers and Maple Seeds

Biome Renewables

Biome Renewables adjusts the aeroacoustics of existing turbine blades to decrease turbulence and increase efficiency.

Innovation: Industry

Low-Noise Retrofit for Wind Turbine Blades Inspired by Owl Wings

Siemens Gamesa

DinoTail Next Generation from Siemens Gamesa has fine combs and serrated edges that help to reduce turbulence and noise while increasing power in wind turbines.

Innovation: Industry

Low-Noise Fan Inspired by Owl Wings

ZIEHL-ABEGG

FE2owlet from Ziehl-Abegg is an efficient, low-noise fan that has winglets and serrated edges to reduce noise and save energy.

Innovation: Academia

Turbulent-Reducing Aerofoil Inspired by Owl Feathers

City University London

An aerofoil from City University of London has finlets that stabilize flow and reduce turbulence.

Last Updated September 21, 2019

References

“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)

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