Unloaded rotor.  The state of a rotor when rotor force has  been removed, or when the rotor is operating under a low or  negative G condition. Useful load.  The difference between the gross weight and  the basic empty weight. It includes the flight crew, usable  fuel, drainable oil, if applicable, and payload. Variation.  The angular difference between true north and  magnetic north; indicated on charts by isogonic lines. 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 helicopter.  Equivalent to the actual weight of the helicopter. It acts  downward toward the center of the earth. Yaw.  The movement of a helicopter about its vertical axis.  G-6