Soft and silent – Owls' wings provide an example for airplanes

No other bird flies as quietly as an owl. Their wings have special characteristics that make this possible. As a result, they are able to approach prey so quietly that they go unnoticed. Another reason evolution has favored their stealth is that they use their keen ears to hunt. Noisy flapping would make them unable to use their outstanding hearing to find their meals.
Scientists at RWTH Aachen University are trying to unlock the riddle of owl wings in order to make aircraft less noisy. It has long been known that owls are quiet flyers, but until now no one has found out why. The young researchers are using new technologies to investigate owl wings. The have analyzed three-dimensional models in their computer and designed a prototype for airplanes. They have also investigated the dynamic properties of owl wings in a wind tunnel and compared the vortices they created with those produced by their prototype.


One aim of the research project is to develop bionically designed airplane wings that would make planes quieter and safer. Computer cooling systems and air conditioning could be made quieter by the technology as well. ____________________________________________________________________



The barn owl Lulu is the star of the air show. She flies more quietly than any other bird. Over millions of years, these owls have evolved into superior airborne predators.



The biologist Thomas Bachmann is observing owls to find out why they can fly so quietly. He'll use that information to develop soundless airplane wings. Owls hunt at night, using their keen sight and hearing to find their next meal. They also reduce the movement of their wings in order not to scare their prey away.


Thomas Bachmann:"I think owls are totally fascinating. Their flying style is extremely delicate and their plumage is marvelous. And they're also intelligent, dignified animals."



Bachmann is working in cooperation with aerodynamics specialist Stephan Klän to take a closer look at owls' wings. In a wind tunnel, they measure the vortices created by the birds' wings. The smaller the amount of air moved, the quieter the flight.


Thomas Bachmann: "No matter which textbook you consult about owls' wings, they all say owls can fly so quietly because of their plumage. That's been known since 1934, when scientists first described the characteristic. Since then, there's been a great deal of research about why owls can fly so quietly, but they haven't really explained it yet."



So Bachmann is searching for what makes owls' wings so special. One possibility would be little hooks on the leading edge of the feathers, that cut the air. Or perhaps it’s the fringes on the trailing edge that dampen the sound. To find out more, the scientists use a laser to make a detailed image of the wing.


The experiment is only just getting off the ground, but they have already solved some riddles of the workings of an owl's wings.



This screen shows, for example, how the vortices are reduced. And this is a prototype wing they've produced, using their findings as an example.



The researchers modeled the wing on a computer with the help of a mathematic calculation program. It incorporates all the data about owl wings – bone structure, feathers and shape – and uses it to calculate optimal wing dimensions.



The decisive element of the innovation is a velvety surface designed to mimic the softness of an owl's wing. The scientists found that the velvety surface in particular produced a surprising effect.



Thomas Bachmann, RWTH Aachen: "On one hand, these images show wings with a smooth surface, while in the others, you can see the treated, velvety surface, performing a different speeds. What really stands out are vortices forming in blue, and they tend to get too big, particularly when airspeed is low and the bird or aircraft is carrying out tricky maneuvers. What we have discovered is that wings with a velvety surface make the vortices smaller and moves them forwards, which has the effect of stabilizing the air currents at low speeds."



Due to their velvety feathers, owls are not only more quiet when they fly slowly, they're also flying more safely. This first result is encouraging Thomas Bachman and his team to find out even more about owls' wings.


Thomas Bachmann: "In the future, we would also like to take a look at owls in free flight. We'll use different instruments to record the geometry of their wings during flight and optimize our models even further."


And not only planes may be made quieter from these insights. Computers and air conditioners could be hushed up as well.


Andreas Neuhaus