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when forward current is flowing through it.
forward wing. 14 CFR Part 1: “A forward lifting surface of a canard configuration or tandem-wing
configuration airplane. The surface may be a fixed, movable, or variable geometry surface, with or without
control surfaces.”
fouled spark plug (malfunctioning ignition system component). A spark plug that is unable to fire
because of contamination in its firing-end cavity. Oil, soot, silicon deposits, or lead deposits can build up in
the firing end until they provide a bridge that allows the voltage to leak off before it can build up high
enough to jump across the gap between the electrodes.
four-stroke cycle. A constant-volume cycle of energy transformation that has separate strokes for intake,
compression, power, and exhaust. See Otto cycle.
four-stroke-cycle reciprocating engine. A form of reciprocating engine which has an intake and an
exhaust valve in each cylinder. Four strokes of the piston (two in and two out) are needed to change
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chemical energy in the fuel into mechanical energy at the rotating crankshaft.
During the intake stroke, the piston moves inward, with the intake valve open and the exhaust
valve closed. A mixture of fuel and air is pulled into the cylinder through the carburetor.
On the compression stroke, the piston moves outward with both valves closed. This compresses
the fuel-air mixture. Near the end of the compression stroke, the spark plug fires and ignites the compressed
fuel-air mixture.
The burning fuel heats and expands the air inside the cylinder, and the expanding air forces the
piston inward on the power stroke. Near the end of the power stroke, the exhaust valve opens.
During the exhaust stroke, the piston moves outward with the exhaust valve open. As the piston
moves outward, it forces the burned gases from the cylinder, and the cylinder is prepared for another cycle
of operation.
Fowler flap (airplane secondary flight control). A type of wing flap which moves out of the trailing edge
of a wing on tracks. Fowler flaps increase both the wing area and camber which increases the lift and drag
produced by the wing. They allow the wing to produce its lift at a slower airspeed so less runway is
required for takeoff and landing.
FPD (freezing point depressant). A chemical compound added to a liquid to lower its freezing
temperature. FPDs are used in the fluid sprayed on an aircraft to deice it before flight.
fractional distillation. The procedure used for separating the various components from a physical mixture
of liquids. Crude oil is a mixture of many different types of hydrocarbon fuels which can be separated by
carefully raising its temperature. The first products to be released, those having the lowest boiling points,
are some of the gaseous fuels; then gasoline, kerosine, diesel fuel, heavy fuel oils, lubricating oils, and
finally, tar and asphalt.
fractus (meteorology). Clouds in the form of ragged, irregular shreds, appearing as if they were torn.
Fractus clouds of the cumulus family are called fractocumulus, and those in the stratus family are called
fractostratus.
francium. An extremely unstable, radioactive chemical element. Francium’s symbol is Fr, and its atomic
number is 87. The most stable of francium’s isotopes is Fr 223, which has a half-life of 21 minutes.
frangible. Breakable, or easily broken.
free balloon. A lighter-than-air device that is free to rise in the air. A free balloon is used in weather
observations to find the height of the base of the lower layer of clouds. A rubber balloon is filled with
helium until it has a specified diameter, and it is released into the air.
The rate of rise of this balloon is known, and the time between its release and its disappearance
into the clouds is measured. This time determines the height of the base of the clouds.
free electrons. Electrons that are not bound to any particular atom. They are free to move about from one
atom to another in a material. A semiconductor material having a large number of free electrons is called an
N-type material.
free fit (threaded fastener). A loose fit between moving parts, used when accuracy of movement is not
important. A nut that turns easily over the threads of a screw or bolt is said to have a free fit.
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free-running multivibrator (electronic oscillator). A form of oscillator that produces a square-wave
output. It uses two transistors arranged in a circuit in such a way that when one transistor begins to conduct
(when it turns on), it turns the other transistor off. When the second transistor turns off, it feeds a signal to
　
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