In the case of a helicopter, the object is the rotor blade (sail) and the fluid is air. Lift is produced when an air mass is deflected and it always acts perpendicular to the resulting relative wind. The airfoil must have a positive AOA to generate positive lift. At zero AOA, no lift is generated. A quadcopter can move on 6 different axes. Drag: The backward retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. Drag opposes thrust and acts backwards parallel to the relative wind. Weight: the combined load of the aircraft itself, crew, fuel and cargo or baggage. The weight pulls the plane down due to the force of gravity. It opposes lift and acts vertically downward through the center of gravity (CG) of the aircraft. Lift: opposes the downward force of weight, is produced by the dynamic effect of air acting on the airfoil, and acts perpendicular to the flight path passing through the center of lift. Thrust: generated by the rotation of the main rotor system. In a helicopter, thrust can be forward, backward, sideways or vertically. The resulting lift and thrust determine the direction of movement of the helicopter. "The quadrotors operate in a symmetrical design with the rotors located in each of the four corners. The rotors are fixed in their pitch and two of the rotors move clockwise.