Retarder (finishing system component).  Dope thinner that  contains certain additives that slow its rate of evaporation  enough to prevent dope blushing. Retread.  The replacement of the tread rubber on an aircraft tire. Retreating blade.  The blade on a helicopter rotor whose  tip is moving in the direction opposite to that in which the  helicopter is moving. Retreating blade stall.  The stall of a helicopter rotor disc  that occurs near the tip of the retreating blade. A retreating  blade stall occurs when the flight airspeed is high and the  retreating blade airspeed is low. This results in a high angle  of attack, causing the stall. Return manifold.  The portion of a fluid power system  through which the fluid is returned to the reservoir. Reverse polarity welding.  DC-electric arc welding in which  the electrode is positive with respect to the work. Rib thread.  A series of circumferential grooves cut into the  tread of a tire. This tread pattern provides superior traction  and directional stability on hard-surfaced runways. Ribbon direction.  The direction in a piece of honeycomb  material that is parallel to the length of the strips of material  that make up the core. Rigid conduit.  Aluminum alloy tubing used to house electrical  wires in areas where they are subject to mechanical damage. Rigidity in space.  The characteristic of a gyroscope that  prevents its axis of rotation tilting as the earth rotates. This  characteristic is used for attitude gyro instruments. Rime ice.  A rough ice that forms on aircraft flying through  visible moisture, such as a cloud, when the temperature is  below freezing. Rime ice disturbs the smooth airflow as well  as adding weight. Rivet cutters.  Special cutting pliers that resemble diagonal  cutters except that the jaws are ground in such a way that  they cut the rivet shank, or stem, off square. Rivet set.  A tool used to drive aircraft solid rivets. It is a piece  of hardened steel with a recess the shape of the rivet head in  one end. The other end fits into the rivet gun. RMI.  Radio magnetic indicator. G-29 Scarf joint.  A joint in a wood structure in which the ends to  be joined are cut in a long taper, normally about 12:1, and  fastened together by gluing. A glued scarf joint makes a  strong splice because the joint is made along the side of the  wood fibers rather than along their ends. Satin-weave fabric.  Fabric in which the warp threads pass  under one fill thread and over several others. Satin-weave  fabrics are used when the lay-up must be made over complex  shapes. Sandwich material.  A type of composite structural material  in which a core material is bonded between face sheets of  metal or resin-impregnated fabric. Sailplane.  A high-performance glider. Saddle gusset.  A piece of plywood glued to an aircraft  structural member. The saddle gusset has a cutout to hold a  backing block or strip tightly against the skin to allow a nailing  strip to be used to apply pressure to a glued joint in the skin. Ruddervators.  The two movable surfaces on a V-tail  empennage. When these two surfaces are moved together  with the in-and-out movement of the control yoke, they act  as elevators, and when they are moved differentially with the  rudder pedals, they act as the rudder. RPM.  Revolutions per minute. Roving.  A lightly twisted roll or strand of fibers. Rotary actuator.  A fluid power actuator whose output is  rotational. A hydraulic motor is a rotary actuator. Rosette weld.  A method of securing one metal tube inside  another by welding. Small holes are drilled in the outer tube  and the inner tube is welded to it around the circumference  of the holes. Roots-type air compressor.  A positive-displacement air  pump that uses two intermeshing figure-8-shaped rotors to  move the air. Roll (aircraft maneuver).  Rotation of an aircraft about its  longitudinal axis. Rocking shaft.  A shaft used in the mechanism of a pressure  measuring instrument to change the direction of movement  by 90º and to amplify the amount of movement.