What rotating-aircraft concept did Leonardo da Vinci draw around 1489?

Leonardo da Vinci drew an aerial screw around 1489 as a concept for a machine that might rise by rotating a helical surface through the air.

The sketch, commonly known as the aerial screw or helical air screw, shows a large spiral structure supported by a light frame. Leonardo proposed that the screw could climb if its linen-covered surface were turned rapidly, comparing the action to a screw moving through a solid material.

The machine was not a functioning helicopter and was never built or tested by Leonardo. Its proposed power source is also unclear, since the human muscle available in the fifteenth century could not have driven such a large rotor effectively. Modern helicopters use powered rotating blades that produce lift through airfoils, a different aerodynamic principle.

The aerial screw nevertheless became one of Leonardo's best-known flight studies. It is often described as a helicopter precursor, not as the first helicopter itself. The drawing illustrates Leonardo's interest in translating familiar mechanical ideas into machines for aerial movement.