Which Leonardo da Vinci design is often called the first conceptual helicopter?

Leonardo da Vinci’s aerial screw is often called the first conceptual helicopter.

Leonardo drew the design around 1489, during his studies of flight and rotating motion. The machine showed a large helical linen sail wrapped around a central shaft. Leonardo reasoned that turning the shaft could make the screw rise by pushing against the air, much as a screw advances through wood.

The aerial screw was a paper design, not a demonstrated flying machine. Its proposed materials and human-powered mechanism could not provide enough lift for practical flight. It is therefore best described as an early helicopter concept rather than a working helicopter. Leonardo’s drawing is important because it shows him applying mechanical principles to vertical flight centuries before powered helicopters became possible. The design is sometimes confused with his ornithopter, which used flapping wings instead of a rotating screw.