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bezel. The rim which holds the glass cover in the case of an aircraft instrument.
BFO (beat-frequency oscillator). A variable radio-frequency electronic oscillator used with
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communications receivers when receiving unmodulated CW (continuous wave) signals.
The unmodulated CW has a frequency that is too high to hear, so it is mixed with the output of the
BFO. When the BFO is adjusted so its frequency is nearly the same as that of the CW signal being received,
the two signals beat together and produce an audio-frequency signal that can be heard. The frequency of
this AF is the difference between the frequency of the unmodulated CW and that produced by the BFO.
B-H curve (magnetic specifications). A curve showing the relationship between the flux density (B) in a
piece of magnetized material and the magnetizing force (H) needed to produce the flux density.
Each type of ferromagnetic material, material that contains iron and is magnetizable, has a
different B-H curve.
Each B-H curve consists of the virgin magnetization curve, the curve of B and H, when the
material is first magnetized, and the closed hysteresis loop that shows the effect of hysteresis, the
magnetism that remains in the material.
BHP (brake horsepower). The horsepower actually delivered to the output shaft of a rotating machine.
Brake horsepower gets its name from the fact that this power was originally measured with a prony brake.
Today, brake horsepower is measured with a dynamometer. BHP is actually the usable horsepower.
biannual. Occurring twice each year. Biannual must not be confused with biennial, which means occurring
every two years.
bias current (transistor operation). Current flowing in the emitter-base circuit of a transistor. The amount
of bias current can be adjusted by the choice of components to set the operating conditions of the transistor.
bias-cut surface tape (aircraft covering material). A fabric tape in which the threads run at an angle,
usually 45°, to the length of the tape. Bias-cut tape is often used as a surface tape when covering an aircraft,
because it can be stretched around a compound curve without wrinkling.
bias voltage (vacuum tube operation). The DC voltage placed on the grid of an electron tube which sets
the operating point about which an AC input signal is applied. The proper bias voltage allows the tube to
amplify the signal applied to its grid without distortion.
bicarbonate of soda. Common baking soda (NaHCO3).
bicycle landing gear. See bogie landing gear.
bidirectional antenna (radio antenna). A radio antenna that produces a maximum signal strength in two
directions.
bidirectional data transfer. A method of data transfer in which signals can travel in either direction.
bidirectional fibers. Fibers in a piece of composite material oriented in such a way that they can sustain
loads in two directions.
bifiliar resistor (electrical component). A special type of wire-wound resistor used in electrical circuits.
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A bifiliar resistor is wound of wire that is doubled back on itself to decrease the inductance in the resistor.
bifiliar transformer (electrical component). A type of electrical transformer in which the two windings,
the primary and the secondary, are wound side-by-side. Bifiliar winding increases the coefficient of
coupling between the windings.
bilge area. The lowest part of an aircraft structure in which water and contaminants collect, and corrosion
is likely to form. The area under the cabin floor is normally called the bilge.
bill of materials (aircraft drawing). A list of all of the materials used in the component or assembly
shown in an aircraft drawing.
billet (metal manufacturing). A bar of semifinished steel.
A billet is rolled from an ingot, and it generally has a square cross section.
bimetallic circuit breaker (electrical component). A circuit breaker in which one of the current-carrying
contacts is mounted on a bimetallic strip.
If more than the rated current flows through a bimetallic circuit breaker, the bimetallic strip will
heat up and warp. When it warps, it snaps the contacts apart and opens the circuit.
bimetallic hairspring. A flat, spiral-wound spring made of two strips of metal laid side-by-side and
welded together. The two metals have different coefficient of expansion, and as the temperature changes,
the spiral either tightens or loosens.
The pointer of an analog thermocouple-type temperature indicator is mounted on a bimetallic
hairspring to compensate its indication for changes in temperature of the reference junction.
　
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