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the electromagnetic field in the relay pulls the contacts open. Field current is reduced and the generator
output voltage drops. As soon as the voltage is low enough, the relay contacts close and field current again
flows. The contacts vibrate open and closed several hundred times a minute to hold the generator voltage
within the desired range.
vibratory torque control (reciprocating engine component). A patented coupling between the propeller
shaft and the crankshaft of the Continental Tiara series aircraft engines. A short quill shaft absorbs torsional
vibrations under certain operating conditions, and a centrifugal lock bypasses the quill shaft under
conditions in which a solid shaft will produce less vibration. See quill shaft.
video amplifier (electronic component). An electronic amplifier capable of amplifying alternating current
over a wide band of frequencies. Video amplifiers normally amplify AC over a frequency band of from
about 15 hertz to five megahertz (5,000,000 hertz).
video map. An electronically displayed map on the radar display that may depict data such as airports,
heliports, runway center line extensions, hospital emergency landing areas, navaids and fixes, reporting
points, airway/route center lines, boundaries, hand-off points, special use tracks, obstructions, prominent
geographical features, map alignment indicators, range accuracy marks, and minimum vectoring altitudes.
VIFF (vectoring in forward flight). The ability of the pilot of an aircraft equipped with a vectored-thrust
engine to change the direction of the exhaust gas flow while the aircraft is in forward flight. VIFF allows
the pilot to accomplish flight maneuvers that are impossible with a conventional engine installation.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 619
VI (viscosity index) improver. An additive used to produce a multiviscosity lubricating oil. The polymer
additive expands as temperature increases and contracts as the temperature decreases. VI improvers cause
the viscosity to increase as the oil becomes hot and decrease when it becomes cold.
virga (meteorology). Water or ice particles falling from a cloud, usually in wisps or streaks, and
evaporating before reaching the ground.
viscosity. The resistance of a fluid to flow. The viscosity of a liquid is a measure of its internal friction, or
its “stiffness.”
viscosity index. See VI.
viscosity index improver. See VI improver.
viscosimeter. An instrument used to measure the viscosity of a fluid. A cup-type viscosimeter is a specially
shaped cup with an accurately sized hole in its bottom.
The cup is filled with the fluid whose viscosity is to be measured, and the hole is opened so fluid
can flow from it. The number of seconds is measured between the beginning of the flow and the time the
first break in the flow occurs. This time relates to the viscosity of the fluid.
viscous damping. A method of damping vibration or oscillation by converting some of the vibrational
energy into heat. Viscous damping is done by allowing the vibrating object to force a fluid, either a liquid
or gas, through a small orifice. The amount of damping is determined by the viscosity of the fluid and the
size of the orifice.
An aircraft nose wheel shimmy damper operates on the principle of viscous damping. See shimmy
damper.
vise. A bench-mounted clamp used to hold material while it is being worked on. One jaw of the vise is
fixed, and the other is mounted on a screw that allows it to be moved away from or close to the fixed jaw.
Vise-grip pliers. A patented type of pliers in which the jaws can be locked together by the over-center
compound action of a toggle. A screw in one handle of the pliers allows the basic size of the jaw opening to
be adjusted, and when the handles are squeezed together, the jaws lock tightly on the object between them.
Vise-grip pliers are made with many different types and shapes of jaws for many different uses.
visibility. The ability, as determined by atmospheric conditions and expressed in units of distance, to see
and identify prominent unlighted objects by day, and prominent lighted objects by night. Visibility is
reported in statute miles, hundreds of feet, or meters.
visible light. The very small range of electromagnetic radiation that produces sensations the human eye and
brain can accept. Vibrations making up visible light have wavelengths of between approximately 4,000 and
7,700 angstroms. One angstrom is equivalent to one billionth of a centimeter.
The different wavelengths of electromagnetic energy give light its color. Visible light with the
shortest wavelength has the color of violet, and that with the longest wavelength has the color of red.
　
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