Aviation Maintenance Terms beginning with R
Radial bearing load
The load on a bearing perpendicular to the shaft on which the bearing is mounted. Centrifugal loads are radial loads.
Radial engine (static radial)
A form of reciprocating engine in which the cylinders radiate out from a small central crankcase. The pistons in the cylinders drive a central crankshaft which in turn drives the propeller.
Radial-inflow turbine
A turbine, similar in appearance to a centrifugal compressor rotor. Radial-inflow turbines are used to drive the compressor in reciprocating engine turbochargers and some of the smaller APU turbine engines. Hot gases flow into the turbine from its outside rim, then radially inward through the vanes and out of the turbine at its center.
Radiation
See electromagnetic radiation.
Ram air
Air whose pressure has been increased by the forward motion of the aircraft. Ram air pressure is the same as pitot pressure.
Ram drag
The loss of thrust produced by a turbojet or turbofan engine caused by the increase of velocity of air entering the engine. Ram drag is the difference between gross thrust and net thrust.
Ram pressure
Pressure produced when a moving fluid is stopped.
Ram-recovery speed
The speed of an aircraft at which the ram effect caused by the forward movement increases the air pressure at the compressor inlet so that it is the same as that of the ambient air.
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.
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.
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.
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°F.
Reliability
The ability of an aircraft engine to perform its designed functions under widely varying operating conditions.
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.
Residual magnetism
The magnetism that remains in the field frame of a generator when no current is flowing in the field coils.
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.
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.
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.
Restrict the list to specific publications by clicking on the menubar at the top of the screen.