What rotating flight machine did Leonardo da Vinci call an “aerial screw”?

Leonardo da Vinci’s aerial screw was a rotating flight-machine concept from about 1489.

The design shows a large helical rotor made from a linen-covered reed or wire framework. Leonardo reasoned that the screw could rise if its surface pushed against the air, an idea often compared with the operating principle of a helicopter rotor.

The machine was never built and flown by Leonardo. Its proposed scale was very large, and the materials and human power available in the fifteenth century were not sufficient to make the concept practical.

The aerial screw is often called a helicopter precursor, but it was not a helicopter in the modern engineering sense. It had one large rotating spiral rather than powered, controllable rotor systems, and Leonardo did not provide a complete modern control system.