Prediction of VRS Boundaries of Helicopters in 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). While VRS is a region of descent flight of unsteady flow through the rotor, a determination of the precise boundary surrounding this region has posed a challenge to the researchers over the years. Several criteria have been proposed in the literature for the determination of VRS boundaries. This paper considers prediction of VRS boundaries using three different criteria involving zero transport velocity of rotor tip vortices, bifurcation of equilibria and zero heave damping. Two different inflow models previously developed at ONERA and Georgia Tech are used in the VRS boundaries predictions. It is shown that, within the accuracy of the inflow models used, VRS boundary predictions are significantly influenced by the criteria used.