What spiral-powered aircraft concept did Leonardo da Vinci call an aerial screw?

Leonardo da Vinci’s aerial screw was a spiral-powered aircraft concept often described as an early helicopter idea.

In a manuscript drawing usually dated to around 1489, Leonardo showed a large helical surface wrapped around a central mast. He reasoned that if the spiral were rapidly rotated, its surface might screw upward through the air, much as a screw moves through a solid material. The proposed machine was roughly 5 metres in diameter and used a reed frame, linen, and wire in the surviving reconstruction descriptions.

The device could not operate as designed because its crew had no sufficiently powerful and lightweight engine. Modern helicopters also use rotating blades to generate lift, but their rotor blades are shaped airfoils and are powered continuously by an engine. Leonardo’s aerial screw therefore resembles a helicopter in broad concept, not in engineering details. It was a speculative machine rather than a completed aircraft or a demonstrated invention.