VORTEX RING STATE —A tran- sient condition of downward flight (descending through air after just pre- viously being accelerated downward by the rotor) during which an appre- ciable portion of the main rotor sys- tem is being forced to operate at angles of attack above maximum. Blade stall starts near the hub and pro- gresses outward as the rate of descent increases. UNDERSLUNG —A rotor hub that rotates below the top of the mast, as on semirigid rotor systems. TRANSLATIONAL LIFT —The additional lift obtained when entering forward flight, due to the increased efficiency of the rotor system. TRANSVERSE-FLOW EFFECT —A condition of increased drag and decreased lift in the aft por- tion of the rotor disc caused by the air having a greater induced velocity and angle in the aft portion of the disc. 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. 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. 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 differ- ence between true north and magnetic north; indicated on charts by isogonic lines. TRUE ALTITUDE —The actual height of an object above mean sea level. TURBOSHAFT ENGINE —A tur- bine engine transmitting power through a shaft as would be found in a turbine helicopter. VERTICAL VIBRATION —A vibra- tion in which the movement is up and down, or vertical, as in an out-of-track condition. YAW —The movement of a rotorcraft about its vertical axis. TWIST GRIP —The power control on the end of the collective control. G-5