TURNING on a DIME
by: Ulrike K. Muller &
S CIENTISTS MAY THINK THAT INSECTS are the masters of unconventional lift but it seems that birds have caught on to the same trick, using it to outsmart their insect prey. swifts—agile aerial hunters that catch insects on the wing—produce unconventional lift: They use their wings to generate a so-called leading-edge vortex. Biologists first caught on to this vortex in 1996 when trying to explain how insects fly. Since then, this vortex has been observed again and again in flying insects . The new study reveals that a bird’s wing also can generate this type of vortex.
A leading-edge vortex forms on the top of a wing when the angle between the wing and the oncoming air flow is large. The flow then separates from the wing at the leading edge and rolls up into a vortex. To form a leading-edge vortex at lower angles of attack, some wings have a sharp rather than blunt leading edge. To exploit this vortex, the flying animal needs to keep the vortex close to its wing. Insects and swifts have found different solutions to this problem. To stabilize the vortex, flying insects beat their wings rapidly , whereas gliding swifts sweep their wings back-ward. The leading-edge vortex spirals out toward the tip of the wing, adopting the shape of a tornado. Like a tornado, the air pressure in the core of the vortex is low, sucking the wing upward and sometimes forward (during flapping).
Swifts have scythe-shaped wings that consist of a long curved hand-wing, which is attached to the body by a short arm-wing. The hand-wing is composed of primary feathers, which form a sharp and swept-back leading edge. Both features help to generate and stabilize a leading-edge vortex. Videler et al. cast a model of a single swift wing in fast gliding posture and recorded the flow fields around the wing in a water tunnel using digital particle image velocimetry. (Flow patterns in water are the same as in air as long as the same Reynolds number is used.) They observed that a vortex forms on top of the wing close behind the wing’s leading edge. This leading-edge vortex is robust against changes in flow speed and angle of attack observations that agree well with those of other biologists studying the leading-edge vortices of insects. However, surprisingly, the swift wing produces such a vortex at angles of attack as small as 50, compared with 250 to 450 typical for insects. The achievements of aerospace engineers have inspired biologists to study the aero-dynamics of flying animals. Engineers first discovered the extraordinary amount of lift that leading-edge vortices produce when they solved the problem of how to land supersonic fighter jets and passenger aircraft like the Concorde. Swept-back wings not only make supersonic flight possible, but also generate stable leading- edge vortices at high angles of attack. The resulting extra lift enables delta-wing aircraft to land safely despite their small wings, which are much smaller than those of conventional aircraft.
The swept wing of a swift generates a stable leading-edge vortex. Yet the exact role of this vortex in the swift’s flight performance can only be inferred from observations of their flight. Swifts in flight turn on a dime while catching insects, a spectacular acrobatic display. Anybody observing swifts circling in a yard will notice that the birds hold their wings swept back during fast flight and swiftly change the wing sweep to execute tight turns (see the figure above). Aerospace engineers converged on the same solution for their military aircraft, which have to perform optimally both during supersonic and subsonic flight. Pilots of fighter jets such as the F-14 Tomcat and the Tornado can choose between different wing sweeps for maximal dogfight and cruise performance.
The gliding flight of storks inspired the first airplane designs of Otto Lilienthal in the late 19th century. The benevolent flight characteristics of these slow and stately gliders invested airplane pioneers with the confidence to take to the skies. Swifts are radically different gliders from storks: They are nimble and fast. These attributes require the ability not only to generate large aerodynamic forces from unsteady lift mechanisms, but also to exercise exquisite control over these forces. The next challenge for Videler and his team is to elucidate how swifts use their variable wing sweep to gain direct control over leading-edge vortices in order to increase their flight performance. In the future, the swift’s flight control might inspire a new generation of engineers to develop morphing microrobotic vehicles that can fly with the agility, efficiency, and short take-off and landing capabilities of insects and birds.
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The authors are in the Department of Experimental Zoology,
Wageningen University, the Netherlands.
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