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Gliding flight in birds
Gliding is a comparatively inexpensive flight mode. It comes at the cost of losing potential energy, so the bird will inevitably end up on the ground.
In the Lund wind tunnel it is possible to generate updrafts by tilting the air stream making it possible to have a bird gliding for prolonged periods of time. Conveniently lift and drag, essential in flight performance, become simple functions of weight and tilt angle.
We used the wind tunnel to examine gliding flight performance in the jackdaw Corvus monedula over a wide range of flight speeds. A linear relation between wingspan and fight speed was found, contrary to the expected behaviour for minimised 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 was studied in this wind tunnel. Similar results as found with 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 was extended with wake velocity measurements using stereo PIV, giving for example information about body drag.
In the near future more studies of gliding flight will be performed, leading to more accurate models for estimating flight performance in more species of flying animals.
The vortex wake behind a gliding swift in the Lund wind tunnel, measured using PIV.
Last modified 16 Oct 2012
Top view of a jackdaw gliding in the Lund wind tunnel.