The Nature of Vortex Ring State

At low forward speed, when a helicopter's descent rate approaches the downward speed of its wake, the blade tip vortices organize in recirculating patterns beneath the rotor. A stable, ring-like wake structure emerges. Vortex ring state (VRS) is detected when the toroidal wake formation enters a buildup phase where organized vorticity accumulates more rapidly than it dissipates. The highly organized VRS wake dominates the descending rotor's inflow field, reducing blade angle of attack and creating thrust loss. The main event, observed in piloted rotorcraft during VRS, occurs when the ring makes a single transition through the rotor plane from below. Steady speed wind tunnel tests do not appropriately simulate VRS because thrust loss does not lead to higher descent rates in a wind tunnel. A vortex ring can only propel itself in a single direction. Thus, prior to transition, a curved flight trajectory, or maneuver, will tend to preclude or suppress VRS.