How a Bird's Wing Sustains and Drives It in Still Air
The downward half prevents descent, the backward half drives the bird forward
Two notes on the mechanics of the wingbeat, illustrated by small pen sketches of a bird in flight and of a bird seen from the front with wings spread. Leonardo argues that in windless flight the wing does two jobs at once: its inner half beats downward to keep the bird from falling, while its outer half, toward the tip, sweeps backward to push the bird ahead. A second note describes how the wingtips draw together as the wing rises, spread apart through the first half of the downstroke, and close again toward the bottom of the beat.
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The two halves of the wing: lift and thrust
In flight without the help of wind the bird sends half of the wing downward and the other half, toward the tip, backward. The downward-moving part prevents the bird's descent, while the part that goes backward drives the bird forward.
Wingtips converge and spread across the beat
When the bird raises its wings it brings the tips close together. As it lowers them, up to the midpoint of the movement, it makes the tips more distant; and from the midpoint downward it draws them close again.