RF energy.  Electromagnetic energy with a frequency high  enough to radiate from any conductor through which it is  flowing. Rich blowout.  A condition in which the fire in a gas turbine  engine goes out because the fuel-air mixture ratio is too rich  to sustain combustion. Rich mixture.  A fuel-air mixture that contains less than 15  parts of air to 1 part of fuel, by weight. Riffle file.  A hand file with its teeth formed on a curved  surface that resembles a spoon. Rms.  Root mean square. A dimension that is the square  root of the average of an infinite number of varying values.  An rms dimension is used to indicate the allowable surface  roughness of a reciprocating engine cylinder wall. Rocker arm.  A pivoted arm on the cylinder head of a  reciprocating engine. The pushrod forces one end of the  rocker arm up, and as the other end moves down, it forces  the poppet valve off of its seat. Rocker box.  The enclosed part of a reciprocating engine  cylinder that houses the rocker arm and valve mechanism. Rocket engine.  A form of reaction engine whose fuel and  oxidizer contain all of the oxygen needed for the release of  heat energy. The released heat expands the gases which are  ejected at a high velocity from a nozzle at the rear of the  rocket. Because rocket engines carry their own oxygen, they  can operate in outer space where there is no atmosphere. Rotary radial engine.  A form of reciprocating engine used  in some early aircraft. The crankshaft is rigidly attached to  the airframe, and the propeller, crankcase, and cylinders all  revolve as a unit. Rotating combustion (RC) engine.  A form of internal  combustion engine in which a rounded, triangular-shaped  rotor with sliding seals at the apexes forms the combustion  space inside an hourglass-shaped chamber. Expanding gases  from the burning fuel-air mixture push the rotor around and  turn a geared drive shaft in its center. The RC engine was  conceived in Germany by Felix Wankel in 1955. RPM.  Revolutions per minute. A measure of rotational speed.  One rpm is one revolution made in one minute. Run in.  A time of controlled operation of a new or freshly  overhauled engine that allows the moving parts to wear  together. Runout.  A measure of the amount a shaft, flange, or disk is  bent or fails to run true. Runout is normally measured with  a dial indicator. Run up.  A procedure in which an aircraft engine is operated  on the ground to determine its condition and performance. SAE.  Society of Automotive Engineers. A professional  organization that has formulated standards for the automotive  and aviation industries. Safety gap.  A location in a magneto that allows a spark to  jump to ground from the secondary circuit before the voltage  rises high enough to damage the secondary insulation. Sand casting.  A method of molding metal parts in a mold  made of sand. A pattern that duplicates the part to be molded  is made of wood and is covered with a special casting sand  that contains a resin to bind it. The mold is separated along  a special parting line, and the pattern is removed. The mold  is put back together, and molten metal is poured into the  cavity. When the metal cools, the sand is broken away  from the molded part. Sand casting is less expensive than  permanent-mold casting. Saybolt Seconds Universal (SSU) viscosity.  A measurement  of viscosity (resistance to flow)of a lubricating oil. The  number of seconds needed for 60 milliliters of oil at a  specified temperature to flow through a calibrated orifice.  The viscosity number used for commercial aviation engine  lubricating oil relates closely to the SSU viscosity of the oil  at 210 ºF. Scavenge subsystem.  The subsystem in the lubrication  system of a gas turbine engine that collects oil after it has  lubricated the bearings and gears and returns it to the oil tank. Scimitar shape.  The shape of the blades of the propellers  mounted on UHB engines. The name is derived from the  shape of a curved Asian sword that has its edge on the convex  side. See UHB engine. Scramjet.  Supersonic combustion ramjet. A special type of  ramjet engine whose fuel can be ignited while the vehicle is  mobbing at a supersonic speed. Scuffing.  Severe damage to moving parts caused when one  metal part moves across another without sufficient lubricant  between them. Enough heat is generated by friction to cause  the high points of the surfaces to weld together; continued  movement tears, or scuffs, the metal.  G-24