A Flettner rotor is a smooth cylinder with disc end plates which is spun along its long axis and, as air passes at right angles across it, the Magnus effect causes an aerodynamic force to be generated in the direction perpendicular to both the long axis and the direction of airflow. The rotor sail is named after the German aviation engineer and inventor Anton Flettner, who started developing the rotor sail in the 1920s.
In a rotor ship, the rotors stand vertically and lift is generated at right angles to the wind, to drive the ship forwards.
In a rotor airplane, the rotor extends sideways in place of a wing and upwards lift is generated.