VERTICAL VIBRATION— A vibration in which the movement is up and down, or vertical, as in an out-of-track condition.

VORTEX RING STATE— A transient condition of downward flight (descending through air after just previously being accelerated downward by the rotor) during which an appreciable portion of the main rotor system is being forced to operate at angles of attack above maximum. Blade stall starts near the hub and progresses outward as the rate of descent increases.

WEIGHT— One of the four main forces acting on a rotorcraft. Equivalent to the actual weight of the rotorcraft. It acts downward toward the center of the earth.

YAW— The movement of a rotorcraft about its vertical axis.