A helicopter rotor in descent flight encounters its own wake, resulting in a doughnut-shaped ring around the rotor disk, known as the Vortex Ring State (VRS). Flight in VRS condition can be dangerous as it may cause uncommanded drop in descent rate, loss of control effectiveness, power settling, excessive thrust and torque fluctuations, and vibration. As simple momentum theory is no longer valid for a rotor in VRS, modeling of rotor inflow in VRS continues to challenge researchers, especially for flight simulation applications. In this dissertation, a simplified inflow model, called the ring vortex model, is developed for a helicopter rotor operating in descent condition. By creating a series of vortex rings near the rotor disk, the ring vortex model addresses the strong flow interaction between the rotor wake and the surrounding airflow in descent flight. In addition, the total mass flow parameter in the existing inflow models is augmented to create a steady state transition between the helicopter and the windmill branches. With the ring vortex model, rotor inflow can now be adequately predicted over a wide range of descent rates. Validations of the ring vortex model for helicopter rotors are conducted extensively in axial and inclined descent. Effects from blade taper, blade twist, and rotor thrust are also investigated with further application of the finite-state inflow model. The ring vortex model is applied to a single main-rotor helicopter. The main effort is to establish VRS boundary based on heave stability criterion. In addition, two important phenomena observed in the descent flight tests are addressed in the dynamic simulation, including uncommanded drop in descent rate and loss of collective control effectiveness. The ring vortex model is further applied to a side-by-side rotor configuration. Lateral thrust asymmetry on the side-by-side rotor configuration can be reproduced through uneven distribution of vortex rings at the two rotors. Two important issues are investigated, including the impact of vortex rings on lateral thrust deficit and on lateral AFCS limit.