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. 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. 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. Teetering hinge.  A hinge that permits the rotor blades of a  semirigid rotor system to flap as a unit. Rotor brake.  A device used to stop the rotor blades during  shutdown. Rotor disk area.  See disk area. Thrust.  The force developed by the rotor blades acting  parallel to the relative wind and opposing the forces of drag  and weight. Rotor force.  The force produced by the rotor, comprised of  rotor lift and rotor drag. Tip-path plane.  The imaginary circular plane outlined by  the rotor blade tips as they make a cycle of rotation. Semirigid rotor.  A rotor system in which the blades are fixed  to the hub, but are free to flap and feather. 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. Shaft turbine.  A turbine engine used to drive an output shaft,  commonly used in helicopters. 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. 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. Translational lift.  The additional lift obtained when entering  forward flight, due to the increased efficiency of the rotor  system. Slip.  A flight condition in which the rate of turn is too slow  for the angle of bank. 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. 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. 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. 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. 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. Static stop.  A device used to limit the blade flap, or rotor  flap, at low rpm or when the rotor is stopped. G-5 Steady-state flight.  The type of flight experienced when a  helicopter is in straight-and-level, unaccelerated flight, and  all forces are in balance.