Which rotating-aircraft concept did Leonardo da Vinci sketch around 1490?

The rotating-aircraft concept Leonardo da Vinci sketched around 1490 was the aerial screw.

Leonardo’s drawing shows a large helical sail mounted around a central shaft. He proposed that, if the screw were turned rapidly, its spiral surface would bite into the air and lift the machine. The idea is often described as an early conceptual ancestor of the helicopter, although it was not a helicopter in the modern engineering sense.

The aerial screw had no engine, and the available materials and power sources could not have produced enough lift for practical flight. Leonardo was reasoning from the behavior of screws and the resistance of air rather than working with experimentally validated rotor aerodynamics.

The design is especially notable because it separates the idea of vertical lift from the flapping wings used in many of Leonardo’s other flying-machine studies. Modern helicopters use powered rotors, aerodynamic blades, and control systems rather than a fabric-covered spiral.