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allows the rivet set to move back and forth, but prevents it being driven out of the gun.
beep button (helicopter control). A switch on the collective control of a turbine engine-powered
helicopter. The beep button is used to adjust the steady-state RPM of the engine.
beeswax. A natural wax applied to rib lacing cord used to attach the fabric to a fabric-covered aircraft.
Beeswax protects the cord from moisture and keeps knots in the cord from slipping.
before bottom center (reciprocating engine piston position). The position of the piston in the cylinder of
a reciprocating engine as the piston is moving downward, toward the bottom of its stroke, with the
crankshaft turning in the direction of normal rotation.
The position is normally expressed in degrees BBC, which is the number of degrees the crankshaft
must rotate before the piston reaches its bottom dead center position.
before top center (reciprocating engine piston position). The position of the piston in the cylinder of a
reciprocating engine as the piston is moving upward, toward the top of its stroke, with the crankshaft
turning in the direction of normal rotation.
The position is normally expressed in degrees BTC, which is the number of degrees the crankshaft
must rotate before the piston reaches its top dead center position.
bel. A dimensionless number used to express the ratio of two powers or intensities. The number of bels is
the logarithm of the ratio of the two values. A decibel (dB), one tenth of a bel, is more commonly used than
the bel.
bell gear. The large gear in a planetary gear system. The bell gear may be either stationary or driven, and it
surrounds, and meshes with, the planetary gears as they rotate around the sun gear.
bell jar. A heavy glass jar with a dome-shaped top and an open bottom. Bell jars are used as vacuum
chambers in science laboratories.
The bottom of the bell jar is ground perfectly flat, so it forms an airtight seal with a flat surface
plate, and a vacuum pump connected to the inside of the bell jar pumps all the air out of it.
bell mouth. The shape of a smooth convergent duct. The bell-mouth shape allows the maximum amount of
air to be drawn into the duct with a minimum of loss.
bell-mouth inlet duct (gas turbine engine component). A form of convergent inlet air duct used to direct
air into the inlet of a gas turbine engine. The area of a convergent duct gets smaller as the air flows into the
engine.
A bell-mouth inlet duct is extremely efficient, and is used where there is little ram pressure
available to force the air into the engine. Bell-mouth ducts are used in engine test cells and on engines
installed in helicopters.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 70
bell crank. A double lever in an aircraft control system used to change the direction of motion. Bell cranks
are often used in the aileron system of an airplane to change spanwise movement of a push-pull rod into
fore-and-aft movement of the control rod that raises or lowers the aileron.
bellows (pressure measuring device). Circular corrugated metal capsules that are either evacuated or
filled with an inert gas under a specific pressure.
Bellows are used to measure pressure. As the pressure acting on the outside of the bellows
changes, the bellows expand or contract. The change in the thickness of the bellows is measured, and this
change relates to the change in pressure acting on the bellows.
Bellville washer. A cupped, spring steel washer used to produce a large amount of compressive force, but
with a limited amount of movement.
below minimums (air traffic control). Weather conditions below the minimums prescribed by regulations
for the particular action involved. These are conditions such as landing minimums, takeoff minimums.
belt frame (aircraft structural component). A frame that goes around the inside of the fuselage of an
aircraft. Belt frames are normally made in the form of a channel or hat section.
bench check. A functional test of a piece of equipment in which the equipment is set up on a test bench
and operated to find out whether or not it functions as it should.
A bench check can tell whether or not a piece of equipment is working satisfactorily by itself, but
it tells little about the way it will perform when it is interconnected with other equipment.
bench plate. A flat cast-iron plate built into a bench used for working sheet metal. Holes in the bench plate
support stakes used to form sheet metal.
bend allowance (sheet metal layout). The amount of material actually needed to make a bend in a piece of
sheet metal. When a flat layout of a piece of sheet metal is being made, an allowance must be made for the
bend. The amount of material in the bend is determined by both the radius of the bend and the number of
　
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