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Gliding flight in birds

Gliding flight is a comparatively inexpensive flight mode. Instead of the bird actively adding energy by flapping its wings, energy for the forward motion is instead harvested from the potential energy. This means, however, that the bird will lose altitude as it flies forward and, if not resuming flapping or gaining altitude by soaring at some point, will end up on the ground.

In the Lund wind tunnel it is possible to generate updrafts by tilting the whole structure and thus making it possible to have a bird gliding stationary in the lab frame of reference for prolonged periods of time. This arrangement allows us to study the kinematics and aerodynamics of the animal to a level of detail that would otherwise be impossible. Furthermore, lift and drag which are both essential parameters in flight performance, conveniently become simple functions of weight and tilt angle.

Using this approach we performed experiments on the gliding flight performance in the jackdaw Corvus monedula over a wide range of flight speeds. A linear relationship between wingspan and flight speed was found, contrary to the behaviour expected for minimising drag. Best gliding performance was found at 8.5 m/s with a lift to drag ratio of 12.6.

Also the gliding flight in swifts has also been studied in the wind tunnel. Similar results as found for the Jackdaw were obtained, i.e. a linear variation of the span and a maximum lift to drag ratio of 12.5 at an airspeed of 9.5 m/s. The experiment on the swift was extended with wake velocity measurements using stereo PIV, which resulted in the first wake visualisation of a gliding bird and the first measurement of body drag on a live animal in free flight.

We are currently performing more studies of gliding flight that will hopefully lead to more accurate models for estimating flight performance in animals.

The wake behind a swift in gliding flight
The vortex wake behind a gliding swift in the Lund wind tunnel, measured using PIV.
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A jackdaw in gliding flight in the wind tunnel
Top view of a jackdaw gliding in the Lund wind tunnel.