Raising Wings Is Easier; Widening the Tail; Man Might Fly
Convex wings, spread for lightness, and the weight of man borne on great wings
Leonardo argues that the simple raising of a bird's wing is easier than lowering it, both because the sinking weight lifts the wing by itself and because the wing—convex above, concave below—lets the air escape more easily on the upstroke than on the down, where the enclosed air condenses. He describes how birds widen the tail in rebounding so the condensed air resists beneath and the impetus is spent only on the wing's edge, and notes that spreading the wings and tail gains lightness. A marginal conclusion famously extends the principle to man, reasoning that a man's weight might sustain itself in the air by means of great wings. Birds in several attitudes of flight are sketched down the right margin.
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Why raising the wing is easier than lowering it
The simple motion of a bird's wings is easier to raise than to lower, for two reasons. First, the weight sinking down somewhat lifts the wings by itself. Second, the wing being convex above and concave below, on the upstroke the air escapes easily from the wing's percussion, whereas on the downstroke the air enclosed in the concavity sooner condenses than escapes.
Widening the tail in the rebound
Birds widen the tail in their rebounding motion so that the air condenses beneath it and resists with greater width, letting the impetus be spent only on the edge or front of the wing. Otherwise the acquired impetus would be lost partly toward the earth and partly in the rebound. Leonardo adds that the bird gains the more lightness the more it dilates and spreads its wings and tail.
A man might sustain himself on great wings
A margin note restates that a heavy body shows itself lighter the more it extends in width. From this Leonardo concludes that the weight of a man could, by means of a great width of wings, sustain itself amid the air—applying the principle of spread and lightness directly to human flight.