Rigid rotor. A rotor system permitting blades to feather, but not flap or hunt.

Rotational velocity. The component of relative wind produced by the rotation of the rotor blades.

Rotor. A complete system of rotating airfoils creating lift for a helicopter.

Rotor brake. A device used to stop the rotor blades during shutdown.

Rotor disk area. See disk area.

Rotor force. The force produced by the rotor, comprised of rotor lift and rotor drag.

Semirigid rotor. A rotor system in which the blades are fixed to the hub, but are free to flap and feather.

Shaft turbine. A turbine engine used to drive an output shaft, commonly used in helicopters.

Skid. A flight condition in which the rate of turn is too great for the angle of bank.

Skid shoes. Plates attached to the bottom of skid landing gear, protecting the skid.

Slip. A flight condition in which the rate of turn is too slow for the angle of bank.

Solidity ratio. The ratio of the total rotor blade area to total rotor disk area.

Span. The dimension of a rotor blade or airfoil from root to tip.

Split needles. A term used to describe the position of the two needles on the engine/rotor tachometer when the two needles are not superimposed.

Standard atmosphere. A hypothetical atmosphere based on averages in which the surface temperature is 59 °F (15 °C), the surface pressure is 29.92 "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 rpm or when the rotor is stopped.

Steady-state flight. The type of flight experienced when a helicopter 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.