The Flight of Birds that have Wings and a Tail: Variable Geometry Expands the Envelope of Flight Performance

✍ Scribed by Adrian L.R. Thomas

• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 214 KB
• Volume: 183
• Category: Article
• ISSN: 0022-5193
• DOI: 10.1006/jtbi.1996.0217

✦ Synopsis

The geometry of a bird's lifting surfaces (wings and tail) varies during flight. The wing span, wing area, tail spread, and tail angle of attack can all change substantially during a single wingbeat, there is systematic variation in geometry with flight speed, and the geometry adopted during manoeuvres or accelerating flight can differ substantially from that in steady flight.

Here I use a simple aerodynamic model to examine the effect of the tail and of variable wing geometry on the power required for flight. The tail can reduce the power required for flight at low speeds, reducing the wing span can reduce the power required for high speed flight. The tail can also be used to add lift in addition to that of the wings and this will improve performance during turning or accelerating flight.

To minimise power in flight the wings and tail should be spread widely at low speeds with the tail at a high angle of attack. As speed increases the angle of attack of the tail should be gradually reduced and then completely furled. At high speeds reducing the wing area and wing span reduces power required for flight.

Whereas previous flight models give a simple U shaped curve relating speed and power, when the influence of the tail is included the shape of the power curve becomes dependent on the morphology of the bird. For birds with a large forked tail the power curve can be W shaped with two separate local minima: one at low speeds with the tail spread, one at a higher speed with the tail furled.