In aeronautics and marine hydrodynamics, the advance ratio at which a propeller is operating is the ratio between the distance the propeller moves forward through the fluid during one revolution, and the diameter of the propeller. When a propeller-driven vehicle is moving at high speed relative to the fluid the advance ratio of its propeller(s) is a high number; and when it is moving at low speed the advance ratio is a low number. The advance ratio of a propeller is analogous to the angle of attack of an airfoil
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Advance ratio
The advance ratio J is a non-dimensional term given by:
where
Va is the speed of advance per unit of time, typically the true airspeed of the aircraft or the water speed of the vessel n is the propeller's rotational speed in revolutions per unit of time D is the propeller's diameter
The advance ratio is a means of describing the incoming angle of the fluid relative to the propeller blade.