What spiral rotor did Leonardo da Vinci sketch as a proposed aircraft?

Leonardo da Vinci’s proposed vertical-lift aircraft was called the aerial screw. His famous 15th-century sketch shows a helical sail intended to rise by “screwing” into the air, an idea often compared with a helicopter rotor.

The drawing appears in the Codex Atlanticus, compiled from Leonardo’s papers. It depicts a large framework covered with reeds, linen and starch, with a rotating helical surface. Leonardo reasoned that if the screw could rotate rapidly enough, it might compress air and lift the structure.

The machine was never built or tested successfully, and its proposed materials and human-powered mechanism could not provide adequate lift. Nevertheless, the aerial screw is regularly cited as an early conceptual ancestor of the helicopter, while modern helicopters use powered rotors that generate lift through aerodynamic blades.