Powered Flight: Flapping

An alternative method to gliding and soaring that is used by many biological flyers to produce lift and thrust is flapping wing flight. The similarities between the aerody­namics of a flapping wing and that of a rotary wing, although limited, illustrate a few key ideas. For example, the rotors of a helicopter rotate about the central shaft con­tinuously, while the relative flow around the rotors produces lift. Likewise, a flapping wing rotates, swings in an arc around its shoulder joint, and reverses direction every half-stroke. Helicopters and biological flyers also use similar techniques to acceler­ate from hovering to forward flight. Helicopters tilt the rotational plane of rotors from horizontal to forward. The steeper the tilt of the rotor, the faster the helicopter accelerates. Biological flyers also tilt their flapping stroke plane: down and forward on the downstroke, and up and backward on the upstroke. To fly faster, biological flyers make the stroke more vertical by increasing the up-and-down amplitude of the movements. When biological flyers decrease their speed, they tend to flap their wings more horizontally, similar to the way helicopters change the angle of the rotors.

Birds, bats, and insects apply a variety of different flapping patterns in hovering and in forward flight to generate lift and thrust. Larger birds have relatively simple wingtip paths. For example, an oval tip path is often associated with albatrosses (see Fig. 1.23). Smaller flyers exhibit more complicated flapping patterns. Figure 1.23 illustrates the highly curved tip paths of a locust and a fruit fly, the figure-eight

pattern of a pigeon (see Fig. 1.23b), and the more complicated paths of June Beetles and blowflies.