Feathers, Wing-Bones, and How a Wing Loads Its Supports
A bird seen from below and from the front, with the statics of rigid and flexible wings
Studying a bird from below and from the front, Leonardo observes that feathers farthest from their attachment are the most flexible, so the wing tips always ride higher than their roots and the wing bones, the heaviest part, lead the motion up and down. He asks where along the bird's breadth the wing presses the air most, then lays down a statics principle: a rigid body gives equal weight to supports equally distant from its centre of gravity. Two lettered schemes of a wing (b d e c a) show a four-pound body loading two supports with two pounds each, while a flexible body loads the support nearest its centre of gravity more heavily.
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Flexible feathers and the leading wing-bones
Feathers farthest from their attachment are the most flexible, so the tips of the wing feathers always stand higher than their roots. The wing bones stay lower than any part of the wing when the wing is lowered and higher when it is raised, because the heaviest part always guides the motion.
Where the wing presses the air most
Leonardo asks in what part of the underside of the bird's breadth the wing presses the air more than in any part of the wing's length, opening the question of where the wing's action on the air is greatest.
Rigid versus flexible bodies on their supports
A four-pound body suspended from supports a b loads each with two pounds; on the middle support c alone it would carry the whole weight. But a flexible body of varying thickness loads the support nearest its centre of gravity more than those over the lighter parts.