Pitching Equilibrium, Wing Span and Tail Span in a Gliding Harris' Hawk, Parabuteo Unicinctus

Abstract

1. The centre of area of the wings of a Harris' hawk gliding freely in a wind tunnel moved forward 0.09 wing chord lengths when the hawk increased its wing span from 0.68 to 1.07 m. The movement of the centre of area probably produces a positive pitching moment that, if unopposed, would cause the bird's head to rise. The tail remained folded until wing span reached 87% of maximum and then began to spread. This behaviour is also typical of gliding birds in nature, which spread their tails when the wings are near maximum span. Tail spreading probably produces a negative pitching moment that compensates for the forward movement of the wings at maximum span. 2. As the tail spread, its centre of area moved backwards. This movement, together with the increase in tail area, can keep the centre of area of the combined wings and tail from moving forward, even at maximum wing span. 3. The tail can generate an estimated 10% of the hawk's total lift at maximum wing span and 5 % or less at shorter wing spans. 4. I moved the centre of area of the hawk's wings forward experimentally by clipping 76 mm from primary feathers 6–10 and 38 mm from primary feather 5. The effect of this operation on the hawk's behaviour indicated that the forward movement of the centre of area of the wings caused a positive pitching moment. The hawk pitched up more in flight. It held its wings at shorter than normal spans, which partially compensated for the effects of clipping by moving the centre of area of the wings backwards. It also spread its tail at shorter than normal spans, which would compensate for an increase in the pitching moment of the wings.