Owls have the uncanny ability to sneak up on their prey while remaining almost completely silent. This is partially accomplished through their immense wingspan (Great Grey Owls such as the one shown in the picture above can have a wingspan of over 5ft); their large wings provide for large amounts of lift, which translates to them being able to glide for much longer before needing to flap their wings again. Dr. Justin Jaworski and his fellow mechanical engineering researchers at Lehigh University in Pennsylvania have been looking at how the material properties associated with various parts of owls’ wings affect their silent aerodynamic characteristics.
Dr. Jaworski’s team has highlighted three distinct features of the owl wing that help to mask its aeroacoustic characteristics in flight, as seen in the picture below. The first feature is that of the trailing edge, which is typically the dominant source of noise on airfoils of all sorts (including aircraft wings, where turbulent eddies or vortices interact with the hard trailing edge, thus creating noise). On an owl wing, the trailing edge is made up of a flexible fringe that possesses a combination of specific porous and elastic properties that attenuate or diminish the sound frequencies that would otherwise be produced off the wing’s trailing edge. In the lab, these porous and elastic characteristics could be manipulated in order to block specific sound frequency ranges and thus provide a stealth solution that could be customized for different applications. With the noise from the trailing edge silenced, the dominant source of noise becomes that associated with the other two regions of the wing.
The second wing feature/region is the mid-wing velvety down which is compliant, yet rough on the macro-scale (like a soft carpet). This section of the wing is the subject of the researchers’ current studies and thus is not very well understood yet, but they believe that somehow the “forest-like structure” of this section acts to absorb the sound that would typically be produced by the rough surface.
The last section of interest is that of the leading edge, which is made up of stiff feathers that add a serrated-like tip to the edge, producing geometry that is analogous to the tubercles that can be found on a whale’s fins. This serration divides up the flow over the wing into smaller, channeled flow streams which produce less wingtip vortex generation and thus less noise (in addition to decreasing parasitic drag).
The combined effect of all of these wing features translate to the owl’s mastery of stealth and could in the future lead to quieter airplane flight, wind turbines that produce less noise, and submarines with greater stealth characteristics. For more information click here.