What rotating device did Leonardo da Vinci study as a possible flying machine?

Leonardo da Vinci studied an aerial screw as a possible flying machine.

His famous sketch shows a helical surface mounted around a vertical frame. Leonardo reasoned that if the screw rotated rapidly, its shape might press against the air and lift the machine. The drawing appears in a notebook associated with his studies of flight in the late fifteenth century.

The aerial screw is often described as a forerunner of the helicopter, but it was not a working helicopter. Leonardo did not provide a practical engine, a lightweight structure, or a proven method for producing enough rotation. A helicopter uses powered rotors that generate lift through rotating blades, whereas Leonardo’s screw resembles a large airborne helical sail. The concept is historically important because it recognized vertical-lift flight as a distinct engineering problem.