STANDARD ATMOSPHERE—A hypothetical atmosphere based on averages in which the surface temperature is 59°F (15°C), the surface pressure is 29.92 in. Hg (1013.2 Mb) at sea level, and the temperature lapse rate is approximately 3.5°F (2°C) per 1,000 feet.
STATIC STOP—A device used to limit the blade flap, or rotor flap, at low r.p.m. or when the rotor is stopped.
STEADY-STATE FLIGHT—A condition when a rotorcraft is in straight-and-level, unaccelerated flight, and all forces are in balance.
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.