Engineers look to learn from stealthy owls

Civil engineers are hoping that research into how owls fly so quietly could lead to new ways of making wind turbines quieter.

Most birds make a noise when they flap their wings but not owls. The aspiration is to adapt the same principles to civil engineering applications.

Now research published in the Institution of Civil Engineers (ICE) journal Bioinspired, Biomimetic and Nanobiomaterials shows that owls are also capable of supressing the vibrations that occur when they flap their wings, despite repeated flapping requiring tremendous force and resulting, for most birds, in noise.

Scientists already knew that many owls are able to glide inches from their prey without being detected due to special feathers that absorb aerodynamic sound – the type of noise a low-flying high speed aircraft generates. But the new study1 shows that the silent flying ability of the owl is even greater than previously thought.

The study, led by Professor Jinkui Chu from Dalian University of Technology in China, showed this is due to the owl’s feather structures, which during flight are able to extract the mechanical energy from the vibrations and convert it into heat. This enables the owl to remain stable and quiet.

It compared the behaviour of long eared owl, golden eagle and pigeon primary feathers during flight, using laser displacement sensors and high speed cameras. All three birds, while differing in size, have a similar flapping style in flight.

Amplitude–time analysis results indicated that loss factors of long-eared owl’s complete primary feathers and barbs are higher than those of the golden eagle and pigeon feathers. Thus, vibration amplitudes of the long-eared owl feathers and their barbs decay much faster, showing a superior vibration-damping characteristic. Consequently, mechanical noise caused by the relative motion of the owl primary feather structures during flight may be effectively suppressed.

Professor Jinkui Chu said: “Many owls have a unique and fascinating ability to fly so silently that they are out of their prey’s hearing range, due to their feather structure. This behaviour has long been of interest to engineers, as we seek to apply the owl’s noise-reduction mechanisms to other purposes and situations that benefit society.

“Now however, we know the owls’ silent flight ability is even more superior than we thought, you could say of all birds it is the ‘king of acoustic stealth’. It not only manages to suppress aerodynamic noise when gliding, but also mechanical noise caused by vibration during flying.

“This is remarkable, considering the sudden jumping, bending and twisting the wings are subjected to when flapping and the noise that creates for other birds. In the scientific world, the process used to eliminate this mechanical noise is called ‘damping’ – which means the extraction of mechanical energy from a vibrating system usually by converting it into heat and allowing it to remain steady.

“Our research showed the long eared owl has superior ‘damping’ skill – meaning it can remain mind-blowingly stable and eliminate mechanical noise caused by the movement of its feathers – quite a feat of engineering.

“This study will hopefully provide further insight into the owls’ silent flight mechanism and help engineers develop ideas for special materials or structures such as on-shore wind turbines, where similar noise elimination can be applied.”