Birds flying in flocks: bounding flight in a crosswind
How a flank wind lets a flock glide in bounds without wingbeats; incident and reflected motion; impetus
A flock of birds drawn across the top of the sheet, labelled a b c d i K l m and e f g h with the 'incident' and 'reflected' paths and the compass directions, illustrates how a wind striking the flank lets birds make a long journey in bounds without beating their wings. Leonardo describes the incident motion made below the wind with the wings drawn in, and the reflected motion made above the wind with the wings open, the wind penetrating beneath the bird and lifting it like a wedge back to its starting height. A short closing note on the nature of impetus states that an impetus created along any line has the potential to end along any other line.
On this page
Bounding flight of a flock in a flank wind (a b c d i K l m / e f g h)
When birds flying in flocks make long journeys and the wind strikes them on the flank, they gain great favor, flying in bounds without effort of the wings. The incident motion is made under the wind with the wings somewhat drawn in and toward the intended course, while the reflected motion, made above the wind with the wings open, is always turned against the wind.
The wind lifts the bird like a wedge
The wind penetrating beneath the bird lifts it toward the sky in the likeness of a wedge driven under a heavy body placed above it. Raised to its due height, equal to the start of the incident motion, the bird turns its front toward its first path and begins the incident motion again.
The nature of impetus
An impetus created along any line is in potential to end along any other line. This closing maxim generalizes the changing directions traced by the gliding flock.