On Birds: Simple, Compound and Decomposed Descent
Straight, curved and vertical flight paths, and the concave-convex wing
Headed "On birds," this page classifies the kinds of oblique descent, illustrated by a column of small marginal birds and wing studies at the right. Simple obliquity along the bird's length gives a rectilinear descent; adding the obliquity of the open wings' width makes the path curved, its concave side toward the lower wing (a spiral); and wind striking the lower wingtip decomposes that curve back into a straight oblique fall. Leonardo closes by noting the wing's cross-section, concave beneath from elbow to shoulder and convex elsewhere, with the air circling in the concave part and pressing and condensing in the convex.
On this page
Simple obliquity: a straight descent along the bird's length
If the oblique motion of the bird is made simply along the direction of its length, then such obliquity will be rectilinear.
Compound obliquity: a curved, spiral descent
But if to the obliquity of the bird's length is added the obliquity of the width of its open wings, the motion becomes curved, and this curvature has its concave side toward the lower wing.
Decomposed motion: wind on the lower wingtip straightens the curve
If the bird is struck by the wind on the tip of its lower wing, its motion, composed of curved obliquity, is decomposed and becomes simply oblique and rectilinear.
The wing's cross-section: concave below, convex beyond; air behavior
The wing is always concave on the underside in the part extending from elbow to shoulder, and convex in the rest. In the concave part the air circles about, while in the convex part it is pressed and condensed.