Flapping Flight in Nature

While flapping, birds systematically twist their wings to produce aerodynamic effects in ways similar to that of the ailerons on the wings of conventional airplanes. Specifi­cally, one wing is twisted downward (pronated) to reduce the angle of attack (AoA)

[51].

and corresponding lift, while the other wing is twisted upward (supinated) to increase lift. With different degrees of twisting between wings, a bird is able to roll [1]. For a bird to be able to deform and twist its wings, an adaptation in the skeletal and muscular systems is required. The key features that seem desirable are modifica­tion of camber and flexing of the wing planform between upstroke and downstroke, twisting, area expansion and contraction, and transverse bending. To perform these functions, birds have a bone structure in their wings similar to the one in a human arm or a bat wing, as shown in Figure 1.17. Despite these similarities birds have more demanding muscle and bone movement in their wings during flight than a human arm is capable of making. Figure 1.18 compares the cross-sectional shapes of a pigeon wing and a conventional transport airplane wing. To a noticeable extent, the pigeon wing exhibits a greater number of variations in camber and thickness along the span.