Leonardo da Vinci’s rotating spiral flying-machine concept was the aerial screw. He sketched it around 1489, describing a device with a helical surface intended to screw upward through the air.
The design is often called a forerunner of the modern helicopter, although it could not have flown as drawn. Leonardo proposed a large wooden frame covered with linen and stiffened with reeds. A crew would turn a central mechanism to rotate the spiral.
The aerial screw was based on the idea that a screw could move through air in the same way that a ship’s screw moves through water. Its main weakness was the lack of a lightweight, powerful engine. Human muscle could not generate enough lift for the proposed structure.
It is easy to confuse the aerial screw with Leonardo’s ornithopter, which used flapping wings. The aerial screw used rotary motion instead, making it a distinct concept in Leonardo’s studies of flight.