Balancing the Airborne Man; a Bird's Course in the Wind
The pilot balances like a boatman; the bird falls along a mean line between wind and wing
At the top a man stands within a flying apparatus: Leonardo notes he must stay free from the waist up to balance himself as in a boat, shifting the joint centre of gravity of man and machine as need requires. A lettered scheme then composes a bird's motion from two 'powers', a descent of 4 along its open wings and a wind of 2 striking from below, concluding that the bird falls along a mean line a e rather than following the wind to f or its own slope to g. A further figure begins the case of a descent of 4 against a wind of 8, and a last bird shows how beating one wing lower turns the bird to that side.
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The airborne man balances like a boatman
The man in flying machines must remain free from the waist upward to balance himself as in a boat, so that the centre of gravity of himself and of the instrument may shift where necessity demands, following the change of the centre of its resistance.
The descent as a mean line between wind and wing
With the bird in power 4 to descend along its open wings a d c and the wind b a in power 2 from below, Leonardo argues the bird will not go with the wind to f nor along its slope to g, but fall along the mean line a e.
Turning by beating one wing lower
When the bird wishes to turn right or left while beating its wings, it beats lower the wing toward which it wishes to turn, so that it twists its motion behind the impetus of the wing that moved.