Prediction of vortex ring state boundary of a helicopter in descending flight by simulation

A helicopter rotor in descending flight may encounter 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 descending 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, viz., zero transport velocity of rotor tip vortices, bifurcation of equilibria, and zero heave damping. Two different inflow models previously developed at ONERA and the Georgia Institute of Technology are used in the VRS boundaries predictions. It is shown that, within the accuracy of the inflow models used, the VRS boundary predictions are significantly influenced by the criterion used.