Vortex Ring State (VRS)

The phenomenon of vortex ring state (VRS) has been previously mentioned in Section 2.13.6. Under descending flight conditions at high rates or at steep descending flight path angles, helicopter rotors can begin to operate in this adverse flight condition. For VRS to occur, the upward component of velocity normal to the rotor disk plane must be a substantial fraction of the average induced velocity downward through the rotor disk. In the VRS the wake vorticity produced by the blades cannot convect away from the rotor. Instead, the wake accumulates near the rotor plane, clumping or bundling together forming a violent, unsteady flow condition that is analogous to flight in a “vortex ring” – see Fig. 2.21. An early effort to analyze the VRS problem for a helicopter rotor was made by de Bothezat (1919).

Entry into the VRS manifests as rotor thrust fluctuations and also an increase in the average rotor-shaft torque (power required), the latter which is necessary to overcome the higher induced aerodynamic losses associated with rotor operations inside its own blade wake. Most helicopters do not have a lot of excess power available at low airspeeds, so the extra power required to overcome these additional induced losses can be of sufficient magnitude to negate the decreased rotor power requirements associated with giving up altitude (potential energy). Therefore, when in the VRS, the application of high rotor torque (power) may be required to maintain equilibrium flight, even though the aircraft is rapidly descending. This scenario is often referred to by pilots as “power settling” or “settling with power” and can be a safety of flight issue – see Vames et al. (2000). These latter terms, however, are not accurate descriptors of VRS conditions because such “settling” issues can

maw «nder operational flight conditions when VRS is clearly not present, such as when a helicopter transitions from hover into forward flight while in ground effect (see Section 5.8) or climbs out of ground effect at high gross weights or high density altitudes.