What Leonardo da Vinci machine is often called an early helicopter design?

Leonardo da Vinci’s aerial screw is often called an early helicopter design.

Leonardo drew the machine in a manuscript now known as the Codex Atlanticus, probably around 1489. Its large helical sail was intended to screw upward through the air when turned by people operating a platform beneath it. The concept resembles a helicopter rotor in its use of a rotating lifting surface.

The aerial screw could not have flown as drawn because the available materials and human power were insufficient for sustained lift. It was therefore a theoretical machine rather than a working aircraft. Modern reconstructions generally treat it as an important historical step in thinking about vertical flight.

The term “helicopter” belongs to a much later period; Leonardo’s own design was called an aerial screw by modern historians.