The Nature of Vortex Ring State

A framework for understanding vortex ring state (VRS) is outlined and a simple flight model for vertical descent is developed. At low forward speed, when descent rate is sufficient to reverse the rotor wake trajectory, the rotor blade tip vortices begin to organize in stable, recirculating patterns beneath the rotor disk. With increased descent rate, the pattern transitions into a ringlike structure that passes through the rotor plane and settles above the rotor disk where stable flight conditions can once again exist. In VRS, the organized wake elements act as an equivalent single vortex ring that dominates the descending rotor’s inflow field. The self-induced velocity of the vortex ring is the propulsive mechanism that allows the vortex ring to lead or follow the descending rotor. Simultaneously, the ring’s self-induced velocity field increases the rotor’s inflow in spite of the descent rate. Inflow always reduces the blade’s effective angle of attack, leading to thrust loss with the absence of blade stall. An established vortex ring can only propel itself in a single axial direction. A curved flight trajectory therefore precludes VRS. This principle is the governing mechanism that allows a maneuvering rotorcraft to suppress VRS.