Residual voltage . The voltage produced in a generator armature when the armature is rotated in the residual magnetism.

Resistor spark plug . A shielded spark plug with a resistor between the ignition lead terminal and the center electrode. The resistor stops the flow of secondary current when its voltage drops to a specified value. The resistor prevents capacitive afterfiring.

Retarded sparks . The timing of the firing of the spark plugs used to start a reciprocating engine. The sparks for starting occur later in terms of crankshaft rotation than those used for normal operation. Retarding the sparks prevent the engine from kicking back when it is being started.

Retentivity . The ability of a magnetizable material to retain the alignment of the magnetic domains after the magnetizing force has been removed. Hard steel normally has a high retentivity, while soft iron and electrical steel both have very low retentivity.

Reverse-flow combustor . A type of combustor in which the air from the compressor enters the combustor outer case and reverses its direction as it flows into the inner liner. It again reverses its direction as it flows into the inner liner. It again reverses its direction before it flows through the turbine. Reverse-flow combustors are used where engine length is critical.

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 air/fuel mixture ratio is too rich to sustain combustion.

Rich mixture . A air/fuel 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 air/fuel 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 disc is bent or fails to run true. Runout is normally measured with a dial indicator.