On Birds: Steering Flight by the Wing-Beat
How the amplitude and speed of up- and down-strokes tilt the bird's path
Headed "On birds," this page analyzes how the relative amplitude and speed of a bird's up- and down-strokes steer its flight, illustrated by a bird drawn with wings in a raised (n m) and lowered (o p) position in the upper right margin. Leonardo argues that raising the wings higher, or beating them faster, below the center of gravity lifts the flight path and tilts the head above the tail, while a faster upstroke makes the bird descend obliquely toward the earth. He grounds each claim by citing numbered propositions of the book (the eighth, ninth, fourteenth, and seventeenth) on the percussion and condensation of air.
On this page
Raising the wings higher than lowering lifts head above tail
A bird that in beating raises its wings above the center of gravity more than it lowers them below it will hold its head higher than its tail. The wing that moves more downward than upward receives greater percussion from the air below it than from the air above, so its straight motion becomes oblique from below upward.
Oblique flight from the raise-to-lower proportion (n m, o p)
The upper-margin bird is drawn with wings in two positions: n m the raised wings, o p the lowered wings. The proportion between raising and lowering is the same as the proportion from b a to b c, and because of this the bird's motion is oblique.
A faster downstroke bends level flight upward
If, with equal wing amplitude above and below the center of gravity, the bird beats faster downward than upward, it bends its level motion upward. Faster strokes condense the air beneath them more, and percussion rebounds more from a more resistant site.
A faster upstroke makes the bird descend toward earth
By the converse: if the wings, in equal motion above and below the center of gravity, are faster in raising than in lowering, they make the bird's motion oblique, declining toward the earth.