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the first transistor that turns it off. When the first transistor turns off, it turns the second one on.
A free-running multivibrator has no stable condition, and because of this, it is called an astable (no
stable condition) oscillator.
free turbine (gas turbine engine component). A turbine or stage of turbines in a gas turbine engine that is
not used to drive the compressor in the gas-generator section of the engine.
A free turbine may be used to drive the reduction gears for the propeller in a turboprop engine, or
the transmission and a rotor of a helicopter. Free turbines are also used in industrial turbine engines to drive
pumps or generators. Free turbines are sometimes referred to as free power turbines.
free wheeling unit (helicopter component). A component part of the transmission or power train of a
helicopter which automatically disconnects the main rotor from the engine when the engine stops or slows
below the equivalent RPM of the rotor.
freezing. The process in which a liquid is changed into a solid by the removal of heat energy. When
enough heat energy is removed from liquid water, it freezes, or turns into solid water (ice).
freezing level (meteorology). The level in the atmosphere at which the temperature is 0°C (32°F).
freezing point. The temperature at which a liquid changes into a solid. In petroleum specifications,
freezing point is the temperature at which solids, such as wax crystals, separate from a hydrocarbon fuel as
it is cooled.
French fell seam (machine-sewed fabric seam). A type of machine-sewed seam in which the edges of the
two pieces of fabric are folded together, and a double row of stitches passes through four thicknesses of the
material.
Freon. The registered trade name for a fluorinated hydrocarbon compound used as a refrigerant and a fire
extinguishing agent. There are a number of Freon compounds identified with numbers. Each compound
having different characteristics.
Freon-12 has been the most commonly used refrigerant in aircraft air conditioning systems, but
because of environmental considerations it is being phased out and replaced by R-134a which is more
environmentally friendly.
frequency. The number of complete cycles of a recurring event that take place in one unit of time.
frequency converter (electronic circuit). A circuit which changes the frequency of alternating current.
Alternating current with a frequency of 1,000 hertz, for example, can be changed to 200 hertz by
combining it with an AC signal having a frequency of 1,200 hertz.
When alternating currents having these two frequencies are combined, or mixed, two more
alternating currents are produced: one having a frequency which is the sum of the two, 2,200 hertz, and the
other a frequency which is the difference between the two — in this case, 200 hertz, the desired frequency.
frequency divider (electronic circuit). An electronic circuit whose output has a frequency that is an exact
divisor of the frequency at the input. If a signal of 100 kilohertz is placed on the input of a frequency
divider that divides by two, the frequency at the output will be 50 kilohertz.
frequency doubler (electronic circuit). An electronic circuit whose output is resonant at the second
harmonic of the input signal. The frequency of the second harmonic of an alternating current is exactly two
times the frequency of the input AC. The output frequency of the frequency doubler is two times that of the
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 265
input.
frequency meter (electrical measuring instrument). An electrical instrument that measures the frequency
of an alternating current and shows this frequency directly on its dial.
One commonly used type of frequency meter is a vibrating reed-type instrument. A series of metal
reeds of different lengths is excited by the AC, and the reed that vibrates with the greatest amplitude is
resonant at the frequency of the AC. The frequency at which each reed is resonant is marked beside each
reed.
frequency modulation (radio communications). One method of putting an audio frequency signal on a
radio frequency carrier. Rather than the AF signal changing the amplitude of the carrier, as is done in
amplitude modulation (AM), the AF signal changes the frequency of the carrier by an amount determined
by the amplitude of the audio signal.
The amplitude of the frequency-modulated carrier is clipped so static caused by electrical
interference causes a minimum of distortion.
frequency multiplier (electronic circuit). An electronic circuit whose output has a frequency that is a
multiple of the frequency of the input signal. Frequency doublers and triplers are commonly used frequency
　
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