Advance ratio

The propeller advance ratio or coefficient is a dimensionless number used in aeronautics and marine hydrodynamics to describe the relationship between the speed at which a vehicle (like an airplane or a boat) is moving forward and the speed at which its propeller is turning.

The advance ratio is a useful non-dimensional quantity in helicopter and propeller theory, since propellers and rotors will experience the same angle of attack on every blade airfoil section at the same advance ratio regardless of actual forward speed.

Helicopter rotors pitch the retreating blade to a higher angle of attack to maintain lift as the relative velocity decreases.

, used in wind turbine aerodynamics:[6] In operation, propellers and rotors are generally spinning, but could be immersed in a stationary fluid.

Wind turbines use the reciprocal to prevent infinite values since they start stationary in a moving fluid.