Ring Vortex Model for Descent Flight

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, excessive thrust and torque fluctuations, vibration, and loss of control effectiveness. As simple momentum theory is no longer valid for a rotor in VRS, modeling of rotor inflow in the VRS continues to daunt researchers, especially for flight simulation applications. A simplified inflow model, called the 'Ring Vortex Model (RVM)', is proposed for a rotor in descent condition. The inflow model utilizes the concept of vortex rings to create nonlinear effect on rotor induced velocity. Several important features of the RVM are illustrated in detail, including estimations of vortex rings' convection speed, strength and number, and steady state transition. With the RVM, it is feasible to predict rotor inflow over a whole range of descent rates. Validations of the RVM in axial and inclined descent conditions reveal good correlation with experimental data. The RVM is computationally efficient and can easily be integrated into current rotorcraft flight simulation models. It can also be utilized to study the influence from model parameters, including blade taper, blade twist, and disk loading.