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square wave (electrical waveform). The waveform produced by pulsating direct current in which the
voltage alternates between zero and its maximum value. It changes almost instantaneously from zero to its
maximum value, remains at this value for a given period of time, and then returns instantly to zero. The
length of time the voltage remains at maximum is the same as the time it remains at zero.
Alternating current can also have a square wave if the voltage changes almost instantaneously
from its maximum positive value to its maximum negative value. The voltage at both the maximum
positive and the maximum negative values remains constant until it shifts to the opposite value. The voltage
remains positive for the same length of time it remains negative.
square-wave generator (electronic device). An electronic device that produces alternating current with a
square waveform. The frequency and the amplitude of the square-wave AC can be changed to any value
desired.
squaring shear (sheet metal shop tool). A large shear used to make straight cuts across a sheet of metal.
Squaring shears used for light-gage steel and thin aluminum alloy sheets are operated by a foot pedal, and
shears used to cut heavy plate steel are operated by energy stored in a large flywheel by a small electric
motor.
squat switch (aircraft safety switch). A switch mounted on the shock strut of an aircraft landing gear. The
squat switch is in a circuit whose actuation depends upon the configuration of the landing gear. For
example, the squat switch prevents the landing gear retraction handle being moved any time weight is on
the landing gear. A squat switch can also be installed in the cabin pressurization system to prevent the cabin
being pressurized if the airplane is on the ground and weight is on the landing gear.
“Squawk” (air traffic control). A term used by ATC to request a pilot to activate specific modes, codes,
or functions on the aircraft transponder. For example, “squawk three/alpha, two one zero five, low.”
“Squawk ident” (air traffic control). A phrase used by ATC to request a pilot to activate the ident feature
on the aircraft transponder. Activation of the ident feature allows the ground controller to immediately
identify the aircraft that is squawking.
squealer tip (compressor blade tip). See profile tip.
squeegee. A rubber scraper blade mounted on a handle in such a way that the blade forms a T with the
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 551
handle. Squeegees are used to move a liquid over a surface. Some of the common uses for squeegees are
removing water from a window after the window has been washed, spreading paint through the open fibers
in a silk screen, and removing water from a photographic print after it has been washed.
squeeze bottle. A bottle made of a soft plastic material. The contents of the bottle can be forced out
through a small opening in the cap by squeezing the sides of the bottle.
squeeze riveter. A power riveting tool used to install solid aluminum alloy rivets in a sheet metal structure.
The squeeze riveter can be made in the form of a C-clamp or an alligator clamp. One jaw of either type
clamp is a fixed part of the riveter frame, and the other jaw is moved against the fixed jaw by the piston in
an air cylinder.
The fixed jaw is fitted with a rivet set that fits the manufactured head of the rivet, and the movable
jaw is flat. Shims are placed under the rivet set so the jaws come close enough together to form a shop head
of the correct thickness. Squeeze riveting is fast, and rivets are more uniform than those driven by a rivet
hammer and bucking bar.
squelch circuit (radio receiver circuit). A circuit in a radio receiver that keeps the volume down when no
signal is being received. As soon as a signal is received, the squelch circuit allows it to come through loud
enough to be comfortably heard. Squelch circuits are used in communications radios that receive long
periods of silence between transmissions.
squib (fire extinguishing system component). A small explosive charge that drives a cutter through the
seal in a high-rate discharge (HRD) bottle of fire extinguishing agent. The container of fire extinguishing
agent is sealed with a thin metal diaphragm. When the operator presses the AGENT RELEASE switch in
the cockpit, the squib is ignited. As the squib burns, it produces enough gas pressure to drive a cutter
through the metal diaphragm and release the agent into the fire extinguishing system.
squirrel-cage induction motor. A type of AC motor whose rotor has current induced into it from
stationary field coils. The rotor is made of a stack of soft iron laminations, mounted on a steel rotor shaft.
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航空术语词典Dictionary of Aeronautical Terms 下(67)
