Because the blades are rotating about a central axis, the speed of each airfoil is lowest at the point where it connects to the hub-and-grip assembly. This fundamental physical reality means that the innermost portion of each blade has an inherent vulnerability to
stalling. In forward flight with
translational lift, there is no upward flow (
upflow) of air in the hub area. As forward airspeed decreases and vertical descent rates increase, an upflow begins simply because there are no airfoil surfaces in the area of the hub, mast and blade-grip assembly. Then, as the volume of upflow increases in the central region (i.e. between the hub and the innermost edges of the airfoils), the induced flow (air pulled or "induced" downwards through the rotor system) of the inner blade sections is overcome. This causes the innermost portions of the blades to begin to
stall. As the inner blade sections stall, a second set of vortices, similar to the rotor-tip vortices, begins to form in and around the center of the rotor system. This, combined with the outer set of vortices, results 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 catastrophic ground impact.
Detection and correction The signs of VRS 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 is traditionally corrected by slightly lowering the
collective to regain cyclic authority and using the
cyclic control to apply lateral motion, often pitching the nose down to establish forward flight. In tandem-rotor helicopters, recovery is accomplished through lateral cyclic or pedal input or both. The aircraft will fly out of the vortex ring into "clean air", and will be able to regain lift.
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 ==