Banking with the Wingtip Rudder Instead of a Whole Wing
The safer way to curve flight, and how to avoid rolling onto the side or over
Folio 44v explains why a bird prefers to steer with the small 'rudder' at the leading edge of its wing rather than by folding a whole wing. For a simple turn one wingtip rudder suffices; for a compound, banking turn the bird draws in one wing, but this risks rolling onto its side or flipping upside-down unless it keeps that wing's underside toward the ground and then gathers the other, extended wing back in. Leonardo concludes that disturbing the balanced resistance of the two spread wings, at whose midpoint the centre of gravity sits, is the more dangerous course, so steering with a wingtip rudder is the safer method. Marginal sketches show birds banking in the wind.
On this page
Steering with the wingtip rudder for a simple turn
The bird uses only one of the rudders placed at the fronts of its wings when it wants to curve its straight motion in the site of equality. For a compound, oblique curve it instead draws in one wing somewhat, making a curved motion declining toward the side where the wing draws in and showing it the convex motion.
The banking turn and its danger of rolling over
This device is dangerous, of turning onto the side and leaving the wingtip stretched toward the sky. To guard against it, the bird must extend the drawn-in wing, always showing the wing's underside to the earth, for if it showed the upper side the bird would turn upside-down; at the same moment it gathers in the lower, extended wing until it returns to the site of equality.
Balanced wing-resistance and the central centre of gravity
Curving the straight motion by folding a wing disturbs the equal resistance that the two wings, equally spread over the air, make between their extremes, in the middle of which sits the centre of gravity. Having shown this danger, Leonardo proves it is safer to bend one of the two wingtip rudders than to bend one of the two wings.
Marginal figures of banking birds
The right margin holds small pen figures of a bird banking in the wind with wings tilted, and another maneuvering below it. They illustrate the risky roll described in the text and the corrective adjustment of the wings.