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retard breaker points. When the crankshaft turns far enough for the retard points to open, the pulsating DC
in the primary winding induces a high voltage in the secondary winding of the left magneto. This high
voltage causes the spark plugs connected to the left magneto to produce a “shower of sparks.”
show-type finish (aircraft finish). A type of finish put on some aircraft that are built for show. A
show-type finish is usually made up of many coats of dope or lacquer, with much sanding and rubbing of
the surface between coats.
shrink fit. An interference fit between pieces of an assembly. The hole into which a part is to fit is enlarged
by heating it, and the piece to fit into the hole is shrunk by chilling it. The two pieces are assembled while
their temperatures are different. Then, when they reach the same temperature, the fit is so tight no relative
movement can take place between the parts.
shrinking blocks (sheet metalworking tool). A pair of wooden blocks used to clamp a sheet of metal to
keep it from buckling when it is hammered with a mallet to shrink it.
shrink-wrapping. A method used to protect products from damage while they are being displayed for sale.
The material to be shrink-wrapped is covered with a thin film of transparent thermoplastic material. Heat is
applied, and the film shrinks to form a smooth protective cover.
shroud (air-cooled engine component). A sheet-metal cover placed over an air-cooled engine to pick up
and force cooling air through the cylinder fins to remove heat from the engine.
shrouded-tip turbine blades (gas turbine engine component). A type of turbine blade with a T-shaped
tip. The tips of the blades touch each other to form a ring around the turbine wheel to support the blades.
shunt (electrical instrument component). A precision resistor installed in parallel with a voltage
measuring instrument to measure the amount of current flowing in a circuit. Load current flows through the
shunt and produces a voltage drop proportional to the amount of current flowing through it.
A millivoltmeter whose dial is calibrated in amps is used to read the voltage drop across the shunt.
The standard shunt used with aircraft ammeters produces a voltage drop of 50 millivolts when the shunt’s
rated current flows through it.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 524
shunt circuit (electrical circuit). A parallel circuit, or a circuit in which there are more than one path for
electrons to flow.
shunt ohmmeter. A type of ohmmeter used for measuring very low values of resistance. Voltage across
the meter is adjusted until the meter pointer deflects full scale. Then, the unknown resistor is connected in
parallel (in shunt) with the meter. The resistance of the unknown resistor is indicated by the decrease in the
current flowing through the meter.
shunt winding. Field coils in an electric motor or generator that are connected in parallel with the
armature.
shunt-wound generator. An electrical generator whose field windings are connected in shunt (parallel)
with the armature winding. Most aircraft generators are shunt-wound, because their output voltage can be
controlled by varying the amount of current flowing through the shunt-connected field winding.
shunt-wound motor. A commutator-type electric motor in which the field windings are connected in shunt
(parallel) with the armature. Shunt-wound motors do not have a large amount of starting torque, but they
operate at a relatively constant speed, and they do not need to have a mechanical load connected to them to
prevent them from overspeeding.
Shunt-wound motors are used for blowers and other low-torque applications.
shut-off valve (fluid power system component). A valve used in a fluid power system to shut off the flow
of fluid.
shuttle valve (fluid power system component). A type of automatic selector valve installed in a critical
portion of a fluid power system. Fluid for normal operation of the system flows through the shuttle valve
into the actuating cylinder. But if the normal system pressure fails, emergency fluid pressure can be
directed into the actuator through the shuttle valve.
Shuttle valves are used in aircraft landing gear and brake systems to allow compressed air to be
used to extend the landing gear or apply the brakes if the normal hydraulic system should fail.
SI. The universal abbreviation for the International System of Units. See International System of Units.
SID (Standard Instrument Departure) charts. Aeronautical charts designed to expedite clearance
delivery and facilitate transition between takeoff and enroute operations.
sidebands (radio transmission). The bands of frequencies on either side of the carrier frequency in
　
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