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potentiometers. Rheostats have two terminals, one at the end of the resistance element and one at the wiper,
or movable contact. Potentiometers have three terminals, one at either end of the resistance element and
one at the wiper.
variable restrictor (fluid flow control device). An adjustable device in a fluid power system. A variable
restrictor is usually some form of variable orifice that controls the amount of fluid allowed to flow in a
given time when it is acted on by a specific pressure.
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Variac. The registered trade name for a toroidal-wound autotransformer. A portion of the winding is
connected to the power line, and it serves as the primary of the transformer. A carbon brush riding on a
bare portion of the winding is moved by the control knob. The position of this brush determines the number
of turns of wire used as the secondary winding.
A Variac can vary the secondary voltage from zero to approximately 117% of the primary voltage.
variation (magnetic compass error). The compass error caused by the difference in the physical locations
of the magnetic north pole, the location to which the magnetic compass points, and the geographic north
pole, the reference from which navigational charts are drawn.
Variation is shown on a chart by isogonic lines, along which the variation is a constant number of
degrees. For land navigation, variation is called declination.
varistor (electronic component). A special type of semiconductor device whose resistance varies as the
voltage across it changes. A varistor is sometimes called a voltage-dependent resistor (VDR).
varmeter (electrical measuring instrument). An electrical instrument used to measure the reactive power
in an alternating current circuit. Reactive power is the power caused by the portion of the current not in
phase with the voltage. Reactive power is measured in volt-amps reactive (VARs) or kilovolt-amps reactive
(KVARs).
varnish (finishing material). A material used to produce an attractive and protective coating on wood or
metal. Varnish is made of a resin dissolved in a solvent, and thinned until it is the proper viscosity to spray
or brush. The varnish is spread evenly over the surface to be coated, and when the solvents evaporate, a
tough film is left.
Enamel is made by mixing a finely ground colored pigment into a varnish base.
varnish (reciprocating engine damage). A hard, glass-like deposit that forms on the cylinder walls of a
reciprocating engine when the lubricating oil bakes onto the surface. Varnish often forms on the cylinder
walls of an engine that is not operated properly when it is first run after it has been overhauled.
Varnish on the cylinder walls prevents the piston rings seating as they should.
varsol (cleaning fluid). A petroleum product similar to naphtha. Varsol is used in aircraft maintenance
shops as a degreaser.
V-belt. An endless belt used to drive a pump or generator from a source of power. The cross section of a
V-belt has the shape of a trapezoid, and it fits into V-shaped pulleys, with the top of the V-belt even with
the top of the pulleys.
V-blocks (wood). Blocks of wood with a V-shaped cut in one of their edges. Two V-blocks are used to
hold a round object when clamping it in a vise.
V-blocks (metal). A fixture that allows a shaft to be centered and rotated to measure any out-of-round
condition.
vector (aircraft navigation). Aircraft approaching a busy terminal area are directed from the ground by a
process known as vectoring. The air traffic controller on the ground has radar contact with the aircraft, and
by giving the pilot a vector, or a heading to fly, the aircraft can be positioned to avoid other traffic, to fly
around bad weather, or be directed to a position from which a landing can be made.
vector (mathematics). A quantity which has both direction and magnitude.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 614
vectored-thrust engine. A turbojet or turbofan engine whose exhaust nozzle is mounted on a swivel in
such a way that it may be rotated in flight to produce forward, vertically upward, or rearward thrust.
vector sum (mathematics). A single vector with the same magnitude and direction as a number of vectors
would have if they acted on a single point, one at a time. The vector representing the vector sum is called
the resultant vector, or just the resultant.
veering (meteorology). Shifting of the wind in a clockwise direction with respect to either space or time.
Veering, which is the opposite of backing, is commonly used by meteorologists to refer to an anticyclonic
shift, clockwise in the northern hemisphere, and counterclockwise in the southern hemisphere.
　
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