Vortex ring state

The vortex ring state, also known as settling with power, is a dangerous condition that may arise in helicopter flight, when a vortex ring system engulfs the rotor causing severe loss of lift. The Federal Aviation Administration (FAA) sees these terms as synonymous, whereas Transport Canada sees them as two different phenomena. The vortex ring state, also known as settling with power, is a dangerous condition that may arise in helicopter flight, when a vortex ring system engulfs the rotor causing severe loss of lift. The Federal Aviation Administration (FAA) sees these terms as synonymous, whereas Transport Canada sees them as two different phenomena. This condition also occurs with tiltrotors, and was responsible for an accident involving a V-22 Osprey. Vortex ring state caused the loss of a heavily modified MH-60 helicopter during Operation Neptune Spear, the 2011 raid in which Osama bin Laden was killed. In forward flight, there is no upward flow (upflow) 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 (air pulled or 'induced' down through the rotor system) 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. 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. 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, so as to get out of the column of air created by the rotor downwash, 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 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 (FOCA) 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 (cross controls). Recovery is complete when the rotor disc reaches the upwind part of the vortex. 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 able to do this, when unladen. 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.