On the flight of birds: wingtips, wind and turning
Flexible wingtips, wind striking beneath the wings, and the tail turning the bird's course
Continuing the study of birds, Leonardo argues that flight is of little value unless the wingtips are flexible, citing the "fifth of the Elements" that lateral power prevents heavy things from descending, as when a man braces himself between two walls or a chimney-sweep climbs; the twisting feather-tips press and bend against the air. He then shows how wind striking beneath the wings makes a descent more oblique the stronger the wind, and how lowering one side of the tail curves the flight so that the inner wing, nearer the centre of the turn like the hub of a wheel, moves more slowly than the outer. A marginal note restarts the proof of why a bird turns by bending its tail. Small feather or wing sketches sit at the upper right.
On this page
Flexible wingtips give flight its power
Flight is of little worth if the tips of the wings are not bendable. By the fifth of the Elements, lateral power prevents heavy things from descending, as a man braces himself with feet and back between two walls or a chimney-sweep climbs; the bird does likewise with the twisting tips of its feathers pressing and bending against the air.
Wind beneath the wings makes descent oblique
When the bird's direct descent is struck by wind under the wings, the descent becomes more oblique the greater the wind's power. This is proved by the ninth of this book: of equal heavy bodies descending through the air, the one struck by more impetuous wind descends more obliquely.
Lowering the tail turns the flight like a wheel
If a bird flying with wings of equal height lowers one side of its tail, its straight flight becomes curved, concave toward the lowered side, and the wing on that side moves more slowly. By the seventh of this book, the part of a revolving wheel nearest the centre moves least, so wingtip a at the centre moves least while the opposite wing at d has the greater motion g b.