The impacts of variable rotor speed on closed loop stability of rotorcraft are discussed. In previous work a model following controller was designed to provide high bandwidth control of a utility helicopter while providing improved performance using variable rotor speed. However, reduction of rotor speed can result in rotor body coupling and even instability. A rotor state feedback control law can be designed independently and easily integrated into the model following control architecture. Simulations were performed to verify the effectiveness of RSF control to stabilize the rotorcraft dynamics or improve the command tracking performance in the presence of reduced rotor RPM.