Controllable-Slope Soaring is a variation of slope soaring whereby the slope is controlled so as to both sustain and control an aircraft in free flight by modifying the wind in the vicinity of the aircraft. A controllable slope is any object which is used to move the air: a piece of cardboard, your hands or even your head! The controllable slope is usually manipulated by a person following the glider in flight (please see photo at right). Using Controllable-slope soaring, the free flying aircraft does not need an onboard system for actuating control surfaces or onboard propulsion system.