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from the compressor through the coils of a radiator-like condenser. Air from outside the aircraft blowing
across the condenser coils absorbs heat from the refrigerant vapor, causing it to change back into a liquid.
The liquid refrigerant now repeats its cycle through the evaporator and picks up more heat.
vapor degreasing. A method of removing grease and oil from a metal part by spraying it with hot vapors
of a solvent such as trichlorethylene.
vaporization. The act of changing of a liquid into a vapor.
vapor lock (reciprocating engine malfunction). A condition of fuel starvation that can occur in a
reciprocating engine fuel system.
If the fuel in the line between the tank and the carburetor is heated enough to cause it to vaporize,
a bubble of fuel vapor will form in the line. If the vapor pressure of this bubble is high enough, it will block
the fuel and prevent it from flowing to the engine. This is a vapor lock.
vapor pressure. The pressure of the vapor above a liquid needed to prevent the liquid evaporating, or
changing into a vapor. When an aircraft ascends to an altitude at which the air pressure is lower than the
vapor pressure of the fuel, vapor leaves the fuel. The fuel “boils.” Vapor pressure is measured in pounds
per square inch at a specified temperature.
vapor trail. A long trail of visible moisture left by an airplane as it flies at high altitude through very cold
air. Vapor trails are caused by the condensation of water vapor in the exhaust gases leaving the engines.
Vapor trails look like long, thin clouds, and they are often called contrails (condensation trails).
vapour. The British spelling of vapor.
VAPC (variable absolute pressure controller). A type of turbocharger controller that uses a cam actuated
by the engine throttle to maintain a constant upper-deck pressure for each position of the throttle valve. A
VAPC differs from an absolute pressure controller (APC) which maintains a constant upper-deck pressure
only for full-throttle operation. The bellows in the VAPC controls the position of the valve, but the throttle
controls the position of the valve seat.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 612
variable-angle stator vanes (gas turbine engine component). Compressor stator vanes whose angle is
controlled by fuel pressure from the fuel control unit. By varying the stator-vane angle, the air directed into
the first stages of the compressor meets the rotor blades at an angle that minimizes the possibility of
compressor stall.
variable capacitor (electrical component). A capacitor whose capacity can be changed. The capacity of a
variable capacitor can be changed by varying the area of the plates, the distance between the plates, or the
dielectric between the plates.
variable-displacement fluid pump. A type of fluid pump whose output can be changed to suit the
demands placed on it. A centrifugal pump moves an amount of fluid determined by the opposition at its
discharge port. The less opposition there is, the greater the amount of fluid the pump moves.
Variable-displacement piston pumps control their output by either changing the physical distance
the pistons move by using a variable wobble plate, or by changing the length of time the intake ports of the
cylinders are open.
variable-geometry aircraft. An aircraft whose wings can be moved in flight, to change the amount of
sweepback. When flying at a high speed, the wings are sharply swept back; but for takeoff or landing, the
wings are moved into a position where there is much less sweepback.
variable-geometry air inlet duct (turbojet airplane component). The type of air inlet duct used on a
supersonic airplane. The shape and area of the inlet duct can be varied in flight to provide the correct air
velocity and pressure at the inlet of the engine as the airspeed of the airplane changes.
variable inductor (electrical component). A coil whose inductance can be changed. The inductance of a
coil is determined by the number of turns of wire and by the type of material used as the core. Changing
either of these varies the inductance.
variable-pitch propeller. A propeller whose pitch can be changed in flight. A two-position propeller is one
that can be set to a low pitch angle for takeoff, and then in flight, the pitch angle can be increased for a
more economical cruise.
A constant-speed propeller is a variable-pitch propeller whose pitch is controlled by a governor to
keep the RPM of the engine constant as the air loads on the propeller change.
variable resistor (electrical component). A resistor whose resistance can be changed by varying the
length of the resistance element in the circuit. The most commonly used variable resistors are rheostats and
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