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velocity. A vector quantity expressing both the speed an object is moving and the direction in which it is
moving. “One hundred miles per hour” is a measure of speed, because it tells only the rate at which an
object is moving. But “one hundred miles per hour, northward” is a measure of velocity. It tells both the
rate and the direction the vehicle is moving.
velocity turbine. A turbine designed in such a way that it is driven by forces produced by the velocity,
rather than the pressure, of the gases flowing through the vanes. Velocity turbines are used on
turbocompound engines as the power recovery turbines (PRTs).
vendor. A person or business that sells something.
veneer (wood product). A thin sheet of wood “peeled” from a log with a knife held against the log as it is
rotated in the cutter. Veneer is used for making plywood. Several sheets of veneer are glued together, with
the grain of each sheet placed at 45° or 90° to the grain of the sheets next to it.
V-engine (reciprocating engine). A form of reciprocating engine in which the cylinders are arranged in
two banks, separated by an angle of between 45° and 90°. Pistons in two cylinders, one in each bank, are
connected to each of the crankshaft throws.
vent. An orifice, or hole, in a container that maintains the air pressure inside the container the same as the
pressure on the outside.
ventilate. To allow fresh air to circulate through a room or compartment. A compartment is ventilated to
remove fumes and make it more comfortable for the occupants.
ventral fin (aircraft structural component). A fixed vertical surface on an airplane that extends below
the aft end of the fuselage. Ventral fins are used to increase the directional stability of an airplane.
venturi. A specially shaped restriction in a tube designed to speed up the flow of fluid passing through it.
According to Bernoulli’s principle, any time a fluid is speeded up without losing or gaining any energy
from the outside, the pressure of the fluid decreases.
Venturis are used in carburetors and in many types of fluid control devices to produce a pressure
drop proportional to the speed of the fluid passing through them. See Bernoulli’s principle.
verdigris. A green or a greenish-blue deposit that forms on the surface of copper when it is acted on by
certain acids or other contaminants.
vernier. A device used to allow very fine adjustments or very small measurements to be made. A vernier
coupling, for example, has two sets of splines, or teeth. One set has 35 teeth and the other set has 36 teeth.
When the coupling is moved ahead one tooth on the 35-tooth side, it moves ahead 10.285 degrees (360 ÷
35 = 10.285). When it is moved back one tooth on the 36-tooth side, it moves back 10 degrees (360 ÷ 36 =
10).
By moving the coupling ahead one tooth on the 35-tooth side and back one tooth on the 36-tooth
side, the shafts joined by the coupling have been moved by an amount of only 0.285 degree.
vernier caliper. A precision measuring instrument used to make inside or outside measurements with an
accuracy in the range of 1/1000 inch, or 1/50 millimeter. The movable jaw of the vernier caliper moves a
vernier scale next to the main scale. The distance between the end marks on the vernier scale is divided into
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 615
one more space than the same distance on the main scale.
To read the vernier caliper, read the inches, tenths, and hundredths of an inch from the main scale
opposite the zero on the vernier scale. Only one line on the vernier scale lines up with a line on the main
scale. Find this line, and it shows the number of thousandths of an inch to be added to the number read
from the main scale.
In the illustration, the zero on the vernier scale is past the 3.650 mark. The 20 line on the vernier
scale lines up with a line on the main scale; so, add 3.650 + .020 = 3.670 inch.
vernier coupling. A timing coupling used with base-mounted magnetos. The vernier coupling allows the
timing to be adjusted in increments of considerably less than one degree.
vernier micrometer caliper. A precision measuring device calibrated in units of one ten-thousandth of an
inch. A vernier scale consisting of 11 lines (10 spaces), is marked on the barrel of the caliper. Only one of
these marks lines up with a mark on the caliper thimble. To find the distance the caliper spindle has moved
out from its zero position, add the number of ten-thousandths of an inch shown on the vernier scale to the
number of thousandths of an inch read on the sleeve and the thimble.
In the illustration, the thimble has moved out more than 0.222 inch, as shown by the lines on the
　
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