In forward flight, there is no upward flow of air in the hub area. As forward airspeed decreases and vertical descent rates increase, an upflow begins because there are no airfoil surfaces in the mast and blade grip area. As volume of upflow increases, the induced flow of the inner blade sections is overcome and the blades begin to stall near the hub. As the inner blade sections stall, a second set of vortices, similar to the rotor tip vortices, form in the center of the rotor system, and combined with the outer set of vortices, result in severe loss of lift. The failure of a helicopter pilot to recognize and react to the condition can lead to high descent rates and ground impact.
Occurrence
A helicopter normally encounters this condition when attempting to hover out of ground effect above the hovering ceiling for the aircraft, hovering out of ground effect without maintaining precise altitude control, and while making downwind or steep, powered approaches when the airspeed drops to nearly zero.
Detection and correction
The signs of settling with power are a vibration in the main rotor system followed by an increasing sink rate and possibly a decrease of cyclic authority. In single rotor helicopters, the vortex ring state can be corrected by moving the cyclic control in any direction, which controls the pitch angle of the rotor blade, slightly pitching nose down, and establishing forward flight. In tandem-rotor helicopters, recovery is accomplished through lateral cyclic or pedal input. The aircraft will fly out of the vortex ring into "clean air", and will be able to regain lift. Another correction, the Vuichard Recovery Technique, was developed by Claude Vuichard, a Federal Office for Civil Aviation inspector in Switzerland. This technique uses the assistance of tail rotor thrust: apply cyclic in the direction of tail rotor thrust, and increase the collective to climb power, coordinated with the power pedal to maintain heading. Recovery is complete when the rotor disc reaches the upwind part of the vortex.
Powering out of vortex ring state
It is possible to power out of vortex ring state, but this requires having about twice the power it takes to hover. Only one full-scale helicopter, the Sikorsky S-64 Skycrane, is documented as being able to do this, when unladen.
Pilot or operator reaction
Helicopter pilots are most commonly taught to avoid settling with power by monitoring their rates of descent at lower airspeeds. When encountering settling with power, pilots are taught to apply forward cyclic to fly out of the condition or lowering collective pitch. While transitioning to forward or lateral flight will alleviate the condition by itself, lowering the collective to reduce the power demand decreases the size of the vortices and reduces the amount of time required to be free of the condition. However, since the condition often occurs near the ground, lowering the collective may not be an option; a loss of altitude will occur proportional to the rate of descent developed before beginning the recovery. In some cases, vortex ring state is encountered and allowed to advance to the point that the pilot may lose cyclic authority due to the disrupted airflow. In these cases, the pilot's only recourse may be to enter an autorotation to break the rotor system free of its vortex ring state.
In a tandem rotor helicopter, forward cyclic will not arrest the rate of descent caused by settling with power. In such a helicopter, which utilizes differential collective pitch in order to gain airspeed, lateral cyclic inputs must be made accompanied by pedal inputs in order to slide horizontally out of the vortex ring state's disturbed air.
Multirotors are subject to normal rotorcraft aerodynamics, including vortex ring state. Frame design, size and power affect the likelihood of entering the state and recovering from it. Multirotors that do not have altitude hold are also more likely to succumb to operator error, where the pilot drops the craft too fast resulting in the upwash at the rotor hubs that can lead to vortex ring state. Those that are equipped with that feature, on the other hand, tend to control their descent automatically and can usually escape the dangerous condition.
