The Silent Flight of Owls and its Possible Significance for Aviation

01/12/2020

Why can't you hear the sound of an owl beating its wings in flight? According to researchers from Aachen and London, small feather structures at the leading edge of the wing improve airflow and reduce noise. The findings could be of interest to the aviation industry.

  Professor Wagner with a barn owl Copyright: Peter Winandy RWTH zoologist Hermann Wagner has been doing research on owls for 35 years. Together with a team of researchers from Aachen and London, he has now published a study on the ability of owls to fly silently.

When the owl pounces on its prey at night, its wings do not make a sound. Now researchers in Aachen and London have discovered another piece of the puzzle of how the hunter manages to fly silently in the dark. The ability to fly silently is due a special physical characteristic of the feathers on the leading edge of the owl’s wings, which helps to reduce noise. This insight could point the way to the development of quieter aircraft, drones, or fans.

The structures of the feathers look like a comb with many small barbs, or filaments, positioned next to each other, as Professor Hermann Wagner from RWTH's Institute of Zoology explains. When the bird is flying, the air is directed inwards by this leading-edge comb, which improves airflow and reduces friction at the wing tip, turbulence, as well as other noisy effects. This is another reason why owls, unlike pigeons, geese or buzzards, do not make any noise when flying. This special effect of deflecting the air has now been proven for the first time in a recent joint study.

Professor Christoph Bruecker from City University London emphasizes that the microstructures of owl feathers may now inspire innovation in aviation. Bruecker received his doctorate and venia legendi from RWTH and is now Chair in Aeronautical Engineering at City. According to the aerodynamicist, the swept wings of aircraft create turbulence, which contributes to noise development at the wing. These effects could be reduced in future by adopting a comb-like leading edge device in future laminar wing design, inspired by the owl wing’s array of barbs. This has the potential to reduce fuel consumption and aircraft noise.

3D-Printed Feather Structure Model

As Wagner reports, owls hunt by ear at night. The Aachen zoologist has been conducting owl research for 35 years. To hear the noises of prey such as mice, it is essential for owls not to make any noise even when flying. The evolutionary solution to this can be found on the wings and wing feathers of owls: these have a unique surface with very soft hairs, flexible fringes on the back of the wings, and the comb of barbs, or finlets, on the leading edge of the wing.

Although researchers have assumed that these evolutionary adjustments contribute to noise suppression, they did not know exactly how the system worked. The function of the micro-structured finlets on the front edge of the wing are now more clearly understood – from the point of view of the zoologist, this is a step forward in basic research: "Our findings give us a new perspective on things, new insights, and are therefore important.”

For the study, the Aachen researchers scanned a barb comb from the small wing structures with a micro-CT scan and then 3D printed it, with tenfold magnification. According to Wagner, it was the first time that this structure was used as a basis for quantitative investigations.

Bruecker and his team used it to create an abstract generic model for flow investigations. They arrived at an astonishing result: the leading edge comb of barbs has a flow-turning effect, deflecting the air inwards. The inward deflection helps to suppress turbulence and thus to make the flight more silent and more effective. It was previously unknown that the barbs deflected the air in this way, as Wagner reports.