What spiral-powered aircraft concept did Leonardo da Vinci sketch?

Leonardo da Vinci sketched an aerial screw as a spiral-powered aircraft concept.

Leonardo drew the design around 1489, showing a large helical surface mounted on a framework. He reasoned that turning the screw could force air downward and lift the structure upward, an idea that resembles the operating principle of a modern helicopter rotor.

The aerial screw was not built and tested by Leonardo, and the materials and power sources available in the Renaissance could not have made it a practical aircraft. Its historical importance lies in the early recognition that rotary motion might produce vertical lift.

The design is often called a helicopter precursor, but it was not a helicopter in the modern engineering sense. Modern helicopters use powered rotors with carefully shaped blades and control systems. Leonardo’s sketch lacked the engine, rotor aerodynamics, and control technology needed for actual flight.