THE FLIGHT OF BIRDS THAT HAVE WINGS AND A TAIL - VARIABLE GEOMETRY EXPANDS THE ENVELOPE OF FLIGHT PERFORMANCE

The geometry of a bird's lifting surfaces (wings and tail) varies duri ng flight. The wing span, wing area, tail spread, and tail angle of at tack can all change substantially during a single wingbeat, there is s ystematic variation in geometry with fight speed, and the geometry ado pted 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 po wer required for flight. The tail can reduce the power required for fl ight at low speeds, reducing the wing span can reduce the power requir ed for high speed flight. The tail can also be used to add lift in add ition to that of the wings and this will improve performance during tu rning or accelerating flight. To minimise power in flight the wings an d tail should be spread widely at low speeds with the tail at a high a ngle of attack. As speed increases the angle of attack of the tail sho uld be gradually reduced and then completely furled. At high speeds re ducing the wing area and wing span reduces power required for flight. Whereas previous flight models give a simple U shaped curve relating s peed and power, when the influence of the tail is included the shape o f 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 t wo separate local minima: one at low speeds with the tail spread, one at a higher speed with the tail furled. (C) 1996 Academic Press Limite d