A simplified inflow model called the ‘ring vortex model’ is developed for a rotor in descent condition. The ring vortexmodelaccountsforthe flowinteractionbetweenrotorwakeandsurroundingairflowindescent flightbyusing a series of vortex rings along the rotor wake. Important features of the ring vortex model are illustrated in detail, including the convection speed, the vortex strength, and the number of vortex rings. An augmentation to total mass flow parameter in the existing inflow equation is proposed to create a steady-state transition between the helicopter and the windmill branches. With the ring vortex model, it is feasible to predict rotor inflow over a wide range of descent conditions. Numerical results show good correlation with experimental data for both axial and inclined descent. In addition, the ring vortex model is used to explain the influence of model parameters (such as blade twist and blade taper) on rotor-induced velocity during descent flight. Nomenclature CQ = rotor torque coefficient CT = rotor thrust coefficient CT = average rotor thrust coefficient kring = nondimensional factor used to compute induced velocity from a vortex ring k� = strength factor of a vortex ring Nb = number of blades Nring = number of vortex rings R = rotor radius T = rotor thrust Vh = rotor-induced velocity at hover Vi = rotor-induced velocity Vver;con = vertical convection speed of a vortex ring Vx = rotor horizontal speed (positive forward) Vz = rotor vertical speed (positive in climb) � = angle of descent � = strength of a vortex ring � T = amplitude of thrust fluctuation with respect to mean value of thrust � Vi = increment of rotor-induced velocity due to the presence of vortex rings � = normalized rotor vertical speed (positive in climb) � peak = normalized rotor vertical speed corresponding to the peak value of induced velocity � = nondimensional total inflow � m = nondimensional average induced velocity � = average total inflow � = advance ratio � z = nondimensional climb rate in the tip path plane � � = rotor advance ratio normalized by induced flow at