Retreating blade stall.  A stall that begins at or near the tip of a blade in a helicopter because of the high angles of attack required to compensate for dissymmetry of lift. 

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. 

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.  The condition of increased drag and decreased lift in the aft portion of the rotor disk caused by the air having a greater induced velocity and angle in the aft portion of the disk. 

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. 

