Forward flight of birds revisited. Part 2.

Abstract

Thrust generation by flapping is accompanied by alternating pitching moment. On the down-stroke, it pitches the bird down when the wings are above its centre of gravity and up when they are below, on the up-stroke, the directions reverse. Because the thrust depends not only on the flapping characteristics but also on the angle of attack of the bird’s body, interaction between the flapping and body motions may incite a resonance that is similar to the one that causes the swinging of a swing.