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approved include spins, lazy eights, chandelles, and steep turns in which the bank angle exceeds sixty
degrees.
utility finish (type of aircraft finish). The finish of an aircraft that gives the necessary tautness and fill to
the fabric and the necessary protection to the metal, but does not have the glossy appearance of a show-type
finish.
U-tube manometer. A pressure measuring instrument made of a glass tube bent into the shape of the letter
U. A liquid such as water or mercury is poured into the tube until it is filled to a given level. Either a
positive or a negative pressure applied to one end of the tube will cause the liquid to rise in one side of the
tube and drop in the other side. The difference in the level of the liquid in the two sides of the tube is
proportional to the difference in the pressure at the two ends of the tube.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 608
V
vacuum. A space in which nothing exists, a completely empty space. For practical purposes, a vacuum is
considered to be a negative pressure, or a pressure less than that of the surrounding atmosphere.
vacuum bottle. A double-wall container used to store liquified gases such as liquid oxygen or liquid
nitrogen. A vacuum bottle is made of thin stainless steel sheet, formed into a double-walled container. All
air is pumped out of the space between the two walls, leaving a vacuum.
The walls are too thin to transfer much heat into the bottle by conduction, and almost no heat
passes across the vacuum. A vacuum bottle is a form of Dewar flask. See Dewar flask.
vacuum capacitor (electrical component). A capacitor that uses two concentric tubes as the plates, or
conductors, and a vacuum between the tubes as the dielectric. A vacuum capacitor has a higher breakdown
voltage than a similar capacitor that uses air as its dielectric.
vacuum distillation. A method of distilling a liquid by boiling it in a vacuum. The material being distilled
is placed in a container, and as much air as possible is pumped from it. Because of the low pressure, the
liquid boils at a lower temperature which prevents the material being damaged by heat.
vacuum forming. A method of forming parts from thermoplastic resin by using the weight of the
atmosphere to apply pressure to the softened material. A sheet of thermoplastic material is placed above a
female die and heated to soften it. When it is at the proper temperature, a low pressure, or vacuum, is
produced inside the die, and atmospheric pressure forces the softened material into the die.
vacuum metallizing. A method of depositing an extremely thin film of metal on a surface. A metallic
vapor is formed by evaporating the metal in a high vacuum, or extremely low absolute pressure. This vapor
is attracted to the surface to be metallized where it forms a microscopically thin coating.
vacuum pump. An air pump used to remove air from a container. There are two types of vacuum pumps:
those that produce a very low absolute pressure (a high vacuum) with almost no flow of air, and those that
produce less vacuum (a higher absolute pressure) but move a much larger volume of air.
vacuum tube (electronic component). An electron control valve that has most of the air removed from the
glass or metal envelope in which the electrodes are mounted. After as much air is removed from the
envelope as is practical, a “getter” is ignited inside the tube to combine with any oxygen left inside the
envelope. See getter.
vacuum-tube voltmeter. See VTVM.
valence electrons. Electrons that spin around the nucleus of an atom in its outer shell. It is the number of
valence electrons in an atom that determines its chemical and electrical characteristics, and it is these
electrons that can be moved from one atom to another to produce electron, or current, flow.
All chemical elements have between one and eight valence electrons. Elements with between one
and three valence electrons are good electrical conductors. Elements with five to eight valence electrons are
electrical insulators, and elements with four valence electrons are called semiconductors.
valence shell. The outer shell of electrons spinning around the nucleus of an atom.
valve (electronic device). The British name for a vacuum tube.
valve (mechanical component). A device that regulates or controls the flow of a fluid, either a liquid or a
gas. Fluid power systems use both flow control and pressure control valves. Aircraft pressurization systems
control the cabin pressure with outflow valves, and the fuel-air charge enters the cylinder of a reciprocating
engine through the intake valve, and the burned gases leave through the exhaust valve.
valve clearance (reciprocating engine specification). The clearance between the end of the valve stem
　
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