The result is an unsteady turbulent flow over a large area of the disk which causes loss of rotor efficiency even though power is still supplied from the engine. As blade airspeed decreases nearer the disk center, downward velocity is less. The descent is so rapid that induced flow at the inner portion of the blades is upward rather than downward. The upflow caused by the descent has overcome the downflow produced by blade rotation. This figure shows the relationship of horizontal speed versus vertical speed for a typical helicopter in a descent.
Straight lines emanating from the upper left corner are lines of constant descent angle.
Superimposed on this grid are flow state regions for the typical helicopter. For steeper approaches, vortex ring state can be avoided by using a speed either faster or slower than the area of severe turbulence and thrust variation. At very shallow angles of descent, the vortex ring wake is shed behind the helicopter. At steep angles, the vortex ring wake is below the helicopter at slow rates of descent and above the helicopter at high rates of descent. Power settling is an unstable condition.