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used voltmeters are actually current measuring instruments in series with a high resistance which limits the
amount of current that can flow through the meter. The deflection of the meter pointer is proportional to the
voltage across the meter and resistance combination.
voltmeter multiplier. A precision resistor in series with a voltmeter mechanism used to extend the range of
the basic meter or to allow a single meter to measure several ranges of voltage.
voltmeter sensitivity (electrical measuring instrument specification). The amount of current needed to
move the pointer of a voltmeter across its full scale. Voltmeter sensitivity, which is expressed in terms of
ohms per volt, is found by dividing the total resistance of the meter by its full-scale voltage.
volt-ohm-milliammeter. See VOM.
volume (physical measurement). The amount of space in a container. Volume is measured in cubic units,
and it is found by multiplying the area of the base of the container by its height. All measurements must be
in the same units.
volume control (electrical circuit). A circuit or control in an audio amplifier that can vary the output
volume.
volumetric efficiency (reciprocating engine specification). The ratio of the volume of the charge of fuel
and air inside the cylinder of a reciprocating engine to the total physical volume of the cylinder. The
fuel-air charge inside the cylinder is reduced to a standard pressure and temperature to measure the
volumetric efficiency.
The volumetric efficiency of a normally aspirated engine is always less than 100%, but a
supercharged engine can have a volumetric efficiency greater than 100%.
VOM (electrical measuring instrument). A combination voltmeter, milliammeter, and ohmmeter, all
using a single indicating instrument. One current-measuring instrument and a number of switches, resistors,
and rectifiers are used to measure a wide range of voltages, both AC and DC, current, normally in
milliamperes and microamperes, and resistance in ohms. A VOM may have either an analog or digital
display.
von Ohain, Dr. Hans Pabst. The designer and developer of the first turbojet engine to actually power an
airplane. Von Ohain’s HeS3b engine was built in Germany by the Heinkel Company, and it flew in a
Heinkel HE-178 airplane on August 27, 1939.
VOR (very-high-frequency omnirange navigation equipment). A type of electronic navigation
equipment in which the instrument in the cockpit identifies the radial, or line from the VOR station
measured in degrees clockwise from magnetic north, along which the aircraft is located. VOR is a
phase-comparison system in which two signals transmitted simultaneously from a ground station are in
phase only when they are received at a location directly magnetic north of the station. At a location
magnetic east of the station, they are 90° out of phase. At magnetic south, they are 180° out of phase, and
west of the station, they are 270° out of phase.
The pilot selects on the omni bearing selector (OBS) the radial on which he wants to fly, and the
needle of the course deviation indicator (CDI) centers when the aircraft is on the radial. If the aircraft
moves off the radial, the needle moves away from center to show the pilot which direction to turn to return
the aircraft to the radial. The indictor for the VOR is often also used as the localizer indicator.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 622
VORTAC (electronic navigation system). An electronic navigation system used by both military and
civilian aircraft. The name VORTAC is the combination of the names of two types of navigation equipment:
VOR, very-high-frequency omnirange navigation equipment, and TACAN, a military pulse-type tactical
navigation system. VOR and TACAN equipment are colocated at a VORTAC station, and signals from
both systems are transmitted simultaneously.
Military aircraft use the TACAN system for determining their direction and distance from the
VORTAC station. Civilian aircraft determine their direction from the station by the VOR and their distance
from the station by the distance-measuring portion of TACAN. This distance is shown in the aircraft by the
distance measuring equipment (DME).
vortex. A whirling motion in a fluid. The coriolis force caused by the rotation of the earth causes water to
flow out of a drain in a vortex. Air that spills over the wing tips of an airplane spins in the form of vortices.
A tornado is an extremely strong vortex of air.
vortex dissipator (turbine powered aircraft component). A high-velocity stream of compressor bleed air
blown from a nozzle into an area where vortices are likely to form. Vortex dissipators destroy the vortices
that would otherwise suck debris from the ground into engines mounted in pods below the wings.
　
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