Rectifier.  A device that allows electrons to flow in one  direction while preventing their flow in the opposite direction.  Rectifiers are used to change AC into DC. Ramjet engine.  The simplest type of air-breathing reaction  engine. Air entering the front of the engine at a high velocity  has fuel sprayed into it and ignited. A barrier formed by the  incoming air forces the expanding gases to leave through  the nozzle at the rear. The energy added by the burning fuel  accelerates the air and produces a forward thrust. Ramjet  engines are used in some military unmanned aircraft that  are initially boosted to a speed high enough for the engine  to function. Reheat system.  The British name for an afterburner. See  afterburner. Reid vapor pressure.  The amount of pressure that must be  exerted on a liquid to keep it from vaporizing. Reid vapor  pressure is measured at 100 ºF. Ratiometer indicator.  An analog temperature measuring  instrument in which the pointer deflection is proportional to  the ratio between the current flowing in an internal reference  circuit and that flowing through the temperature-sensing  probe. Reach (spark plug specification).  The length of the threads  on the shell of a spark plug. Reaction engine.  A form of heat engine that produces thrust  by heating a mass of air inside the engine and discharging  it at a high velocity through a specially shaped nozzle. The  amount of thrust is determined by the mass of the air and the  amount it is accelerated. Reactive power.  Wattless power in an AC circuit. It is the  power consumed in the inductive and capacitive reactances.  Reactive power is expressed in volt-amps reactive (var) or  in kilovolt-amps reactive (kvar). Reamed fir.  The fit of a shaft in a hole in which the hole  is drilled undersize and cut with a reamer to the correct  diameter. Reamed holes have smooth walls and a consistent  diameter. Rebuilt engine.  A used engine that has been completely  disassembled, inspected, repaired as necessary, and  reassembled, tested, and approved in the same manner and to  the same tolerances and limits as a new engine, using either  new or used parts. However, all parts used must conform to  all production drawings, tolerances, and limits for new parts,  or be of approved oversize or undersize dimensions for a  new engine. According to 14 CFR part 91, section 91.421,  a rebuilt engine is considered to have no precious operating  history and may be issued a zero-time logbook. Only the  engine manufacturer can rebuild an engine and issue a zero- time record. Reciprocating engine.  A type of heat engine that changes  the reciprocating (back-and-forth) motion of pistons inside  the cylinders into rotary motion of a crank-shaft. 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. 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. G-23 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. 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. Residual voltage.  The voltage produced in a generator  armature when the armature is rotated in the residual  magnetism.  Residual magnetism.  The magnetism that remains in the  field frame of a generator when no current is flowing in the  field coils. Residual magnetic particle inspection.  A form of magnetic  particle inspection for small steel parts that have a high degree  of retentivity. The part is magnetized, removed, and inspected  away from the magnetizing machine. Reliability.  The ability of an aircraft engine to perform its  designed functions under widely varying operating conditions.