Axial inflow factor

The axial inflow factor is the fraction a that relates the mean axial velocity at the propeller disk to the free stream in actuator-disk momentum theory: the disk velocity is V_A(1 + a) and the slipstream far downstream reaches V_A(1 + 2a). It expresses the induced acceleration the working disk imparts to the inflow, and the ideal (Froude) efficiency of a lightly loaded disk follows as eta_i = 1 / (1 + a). The factor rises with thrust loading coefficient C_T, since a heavier-loaded disk accelerates its inflow more, which is why high disk loading caps achievable propulsive efficiency and motivates large, slow-turning wheels.