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surrounds the winding and keeps magnetic lines of flux from radiating out from the transformer.
SHF (Superhigh Frequency). Electromagnetic energy with a frequency between 3 and 30 gigahertz
(3 · 109 and 30 · 109 hertz). The SHF band is used for weather and doppler radar.
shielded-arc welding. A form of arc welding in which the arc and the metal being welded are shielded, or
covered, with an inert gas or with melted flux that keeps air away from the molten metal in the weld.
shielded conductor (electrical conductor). Insulated electrical wire enclosed in a conducting braid,
usually made of tinned copper. The braid is grounded so any electromagnetic radiation picked up by the
braid is carried to ground. This prevents interference with the signal being carried in the conductor.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 520
shielded ignition cable (reciprocating engine ignition system component). A type of ignition cable used
to carry high voltage from the magneto distributor to the spark plug. Shielded ignition cable is used in
aircraft installations where the electromagnetic radiation from the high-voltage spark plug leads could
cause radio interference.
The spark plug leads are enclosed in a metal braid. One end of the braid is connected to a terminal
in a shielded spark plug, and the other end is connected to the magneto distributor. Any radio-frequency
energy that radiates from the wire when the spark is jumping the gap in the spark plug is picked up by the
braid and carried to ground, rather than radiating into space and causing radio interference.
shielded spark plug (reciprocating engine ignition system component). A spark plug entirely enclosed
in a steel housing. A shielded ignition lead terminates with an insulated connector that fits into an insulated
cavity inside the spark plug. The nut on the end of the ignition lead screws onto the spark plug barrel.
Electromagnetic energy released when the spark jumps the gap in the spark plug is picked up by
the metal in the spark plug and the shielded ignition lead, and is carried to ground preventing it from
causing radio interference.
shielded wire. Electrical wire enclosed in a braided metal jacket. Electromagnetic energy radiated from the
wire is trapped by the braid and carried to ground.
shielding. A metal braid that encloses wires which carry high-frequency alternating current or high-voltage
DC that has radio-frequency energy superimposed on it. The shielding intercepts any electromagnetic
radiation and carries it to the engine structure so it will not interfere with any installed electronic
equipment.
shim. A thin piece of material used to fill a space between two objects or to level an object. Thin metal
shims are used in some bevel gear systems to adjust the gear preload, and behind some bearing inserts to
adjust the clearance between the bearing and the shaft.
shimmy. Abnormal, and often violent, vibration of the nose wheel of an airplane. Airplane nose wheel
installations must be loose enough to allow them to be steered, but if they are too loose, they will shimmy.
Shimmying is prevented by a shimmy damper. See shimmy damper.
shimmy damper (aircraft landing gear component). A small hydraulic shock absorber installed between
the nose wheel fork and the nose wheel cylinder which is attached to the aircraft structure. The piston
inside the shimmy damper cylinder is free to move back and forth when it is moved slowly enough to allow
fluid to transfer from one side of the piston to the other through a restrictor.
Slow movement of the piston allows the nose wheel to pivot so the aircraft can be steered on the
ground. But, the restrictor slows the flow of fluid inside the cylinder and prevents the piston from moving
back and forth as fast as it would if the nose wheel were shimmying.
shock absorber (aircraft landing gear component). A device in an aircraft landing gear that absorbs the
shock which occurs when an aircraft touches down on landing. The most commonly used shock absorber is
the oleo (oil and air) shock strut. The initial landing impact is taken up by oil transferring from one
chamber inside the shock absorber to another through a metering orifice. The much smaller shocks that
occur when the aircraft is taxiing are taken up by a cushion of compressed air.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 521
shock mount. A type of vibration isolator used between a moving vehicle and a component sensitive to
vibration. Shock mounts allow the component a certain degree of free movement, but absorb the
high-frequency vibrations that are so destructive to delicate mechanisms.
shock stall (high-speed aerodynamics). A type of stall that affects some airplanes as they fly in the
　
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