Propeller is a device which transmits power by converting it into thrust for propulsion of a vehicle such as an aircraft, ship, or submarine, by rotating two or more twisted blades about a central shaft, in a manner analogous to rotating a screw through a solid. The blades of a propeller act as rotating wings, and produce force through application of both Bernoulli's principle and Newton's third law, generating a difference in pressure between the forward and rear surfaces of the airfoil-shaped blades. Propellers of all types are referred to as screws, those on aircraft are usually referred to as airscrews. Changes to a propeller's efficiency are produced by a number of factors, notably adjustments to the helix angle, the angle between the resultant relative velocity and the blade rotation direction, and to blade pitch. Constant-speed propellers automatically adjust the blade pitch angle to alter resistance torque in response to sensed changes in rotational speed.