What rotating-aircraft concept did Leonardo da Vinci sketch with a helical sail?

Leonardo da Vinci sketched an aerial screw, a rotating-aircraft concept with a helical sail intended to rise through the air.

His drawing, usually dated to around 1489, shows a large spiral structure supported by a frame. Leonardo reasoned that if the helical surface were turned rapidly, it might screw itself upward through the air. The concept resembles a very early helicopter idea, although it was not a working helicopter.

The design was probably inspired by the screw principle used in devices such as the Archimedean screw. Leonardo’s proposal lacked a practical engine and a clearly workable lightweight structure, so it remained a paper study. The drawing is nevertheless important in the history of vertical flight.

An aerial screw is not the same as an ornithopter, whose wings flap, or a modern propeller, which produces thrust by rotating blades. Those distinctions explain why the helical sketch is usually discussed separately.