SYMMETRICAL AIRFOIL— An airfoil having the same shape on the top and bottom.

TAIL ROTOR— A rotor turning in a plane perpendicular to that of the main rotor and parallel to the longitudinal axis of the fuselage. It is used to control the torque of the main rotor and to provide movement about the yaw axis of the helicopter.

TEETERING HINGE— A hinge that permits the rotor blades of a semirigid rotor system to flap as a unit.

THRUST— The force developed by the rotor blades acting parallel to the relative wind and opposing the forces of drag and weight.

TIP-PATH PLANE— The imaginary circular plane outlined by the rotor blade tips as they make a cycle of rotation.

TORQUE— In helicopters with a single, main rotor system, the tendency of the helicopter to turn in the opposite direction of the main rotor rotation.

TRAILING EDGE— The rearmost edge of an airfoil.

TRANSLATING TENDENCY— The tendency of the single-rotor helicopter to move laterally during hovering flight. Also called tail rotor drift.

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 portion of the rotor disc caused by the air having a greater induced velocity and angle in the aft portion of the disc.

TRUE ALTITUDE— The actual height of an object above mean sea level.

TURBOSHAFT ENGINE— A turbine engine transmitting power through a shaft as would be found in a turbine helicopter.

TWIST GRIP— The power control on the end of the collective control.

UNDERSLUNG— A rotor hub that rotates below the top of the mast, as on semirigid rotor systems.

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.