RATO (rocket assisted takeoff). RATO is a method of assisted takeoff that can be used by a heavily loaded airplane when taking off from a high-altitude airport. Auxiliary boost rockets, attached to the airframe, are ignited at the critical time during takeoff to provide the needed additional thrust.
rattail file. A long, narrow, tapered, round file. Rattail files are used for filing a radius in the corner of a cutout.
rawhide mallet. A soft-face mallet whose head is made of a tightly wound roll of rawhide. Rawhide mallets are used to form soft sheet metal and to pound parts that must not be damaged by hitting them with a hard hammer.
rawinsonde observation (radar meteorology). A combined winds aloft and radiosonde observation. Winds are determined by tracking the radiosonde by radio direction finder or radar.
RC circuit (electrical circuit). An electrical circuit that contains both resistance and capacitance.

RC time constant. See capacitive time constant.
RDF (radio direction finding). A method of locating a radio transmitter by using radio receivers equipped with directional antennas. The station to be located is received by two or more RDF stations, and the bearings (the direction from the receivers to the transmitter) are plotted on a chart. The transmitting antenna is located at the point the bearings from all of the RDF stations cross.
reach (spark plug design feature). The length of the threads on a spark plug shell. The reach needed by a spark plug is determined by the thickness of the cylinder head into which the spark plug screws. The end of the spark plug should be flush with the inside of the cylinder head.
A long-reach 18-mm aircraft spark plug has a reach of 13/16-inch; a short-reach 18-mm spark plug has a reach of 1/2-inch.

reactance (X). Opposition to the flow of alternating current caused by capacitance and inductance in the circuit. Reactance opposes the flow of alternating current without using any power.
Reactance caused by capacitance is called capacitive reactance, its symbol is XC, and it is measured in ohms. Capacitive reactance causes the current in the circuit to lead the voltage.
Reactance caused by inductance is called inductive reactance, and its symbol is XL. Inductive reactance is also measured in ohms, and it causes the current in a circuit to lag behind the voltage.
reaction. A response to an action that occurs in a direction opposite to the action. The recoil produced when a gun is fired is an example of reaction. When the powder in the cartridge burns, the resulting gas forces the bullet out the barrel. The action forcing the bullet from the barrel causes an opposing reaction that pushes back on the gun with exactly the same amount of force.
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 by the amount it is accelerated. Reaction engines which include rocket engines are commonly called jet engines.