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. 

Run up. A procedure in which an aircraft engine is operated on the ground to determine its condition and performance. 

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. 

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. 

