What spiral flying-machine concept did Leonardo da Vinci sketch around 1489?

Leonardo da Vinci sketched an aerial screw, a spiral flying-machine concept, around 1489.

The aerial screw drawing shows a large helical structure supported by a framework. Leonardo proposed that the spiral could climb if it were turned rapidly, an idea that resembles the basic lifting principle of a modern helicopter rotor. The surviving design is usually associated with his studies of flight and mechanical motion.

The machine was not a helicopter in the modern engineering sense. Leonardo did not provide a workable engine, and the materials and power available in the Renaissance could not have made the sketch fly as intended. Its fabric-covered spiral also differs from the rigid rotor blades used by helicopters.

The aerial screw is often called a helicopter predecessor, but it was a conceptual design rather than a proven aircraft. Leonardo’s notebooks contain several unrelated flight ideas, including flapping-wing machines and a pyramid-shaped parachute.