Source: One of Leonardo Da Vinci's Inventions - The Ariel Screw (considered as an early helicopter)

In Leonardo's infancy a hawk had once hovered over his cradle. Recalling this incident, Leonardo saw it as prophetic. “An object offers as much resistance to the air as the air does to the object. You may see that the beating of its wings against the air supports a heavy eagle in the highest and rarest atmosphere, close to the sphere of elemental fire. Again you may see the air in motion over the sea, fill the swelling sails and drive heavily laden ships. From these instances, and the reasons given, a man with wings large enough and duly connected might learn to overcome the resistance of the air, and by conquering it, succeed in subjugating it and rising above it."