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windings and the common point. A Y-connected circuit is sometimes called a wye-connected circuit or a
star-connected circuit.
yellow arc (aircraft instrument marking). A type of range marking for aircraft instruments that identifies
a caution area. A yellow arc on the dial of an airspeed indicator identifies a range of airspeeds that can be
safely flown in calm air, but the aircraft must not be flown at these speeds in rough air. There must be no
abrupt maneuvers made while flying in this speed range.
yield point (material strength). A measure of the amount of stress needed to permanently deform a
material. When a piece of material is stressed to a value within its elastic limit, it will deform, but will
return to its original size and shape when the stress is removed. When the material is stressed to its yield
point, it will not return to its original size and shape when the stress is removed; it will be permanently
deformed.
yield strength. The amount of stress needed to permanently deform a material.
yoke (airplane control). The control column in an airplane on which the control wheel is mounted.
Back-and-forth movement of the yoke moves the elevators, and rotation of the control wheel moves the
ailerons.
Young’s modulus. The ratio of the amount of tensile stress applied to a material to the amount of resulting
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 635
strain parallel to the stress.
ytterbium. A soft, bright, silvery, rare-earth, metallic chemical element. Ytterbium’s symbol is Yb, its
atomic number is 70, and its atomic weight is 173.04. Ytterbium is used as an X-ray source and in the study
of superconductivity.
yttrium. A silvery, rare-earth, metallic chemical element. Yttrium’s symbol is Y, its atomic number is 39,
and its atomic weight is 88.905. Yttrium is used in nuclear technology and as an alloy for other metals.
Z
Zahn cup (viscosity measuring instrument). A specially designed viscosimeter, or cup for measuring the
viscosity of a liquid. The Zahn cup is filled with the liquid whose viscosity is to be measured. A specially
calibrated hole in the bottom of the cup is opened, and the time needed for the liquid to flow from the cup is
measured. The number of seconds between the start of the flow and the time at which the first break in the
flow occurs relates to the viscosity of the liquid.
Z-axis. The vertical, or yaw, axis of an aircraft. The X-axis is the longitudinal, or roll axis, and the Y-axis
is the lateral, or pitch axis. See vertical axis.
zener diode (electronic component). A special type of semiconductor device that allows electrons to flow
through it in one direction, but normally opposes flow in the opposite direction.
If the voltage across the zener diode, in the direction that does not allow flow, is increased to a
specific value, the zener diode will break down, and electrons will flow through it. The voltage at which a
zener diode breaks down is called the zener voltage. Zener diodes are also called avalanche diodes.
zenith. The point in the orbit of a satellite or any body rotating around another body, at which the orbiting
body is the greatest distance from the object around which it is rotating. The zenith of an orbit is opposite
the nadir of the orbit.
zephyr (meteorology). A mild breeze, usually one blowing from the west.
Zeppelin. A large, rigid, lighter-than-air flying machine. The Zeppelin gets its name from Count Ferdinand
von Zeppelin who pioneered the design and manufacture of these large, cigar-shaped airships. Zeppelins
were used in World War I, and to a limited extent in commercial air travel, until the time the huge German
airship Hindenburg burned in 1939.
Zerk fitting (grease fitting). The registered trade name for a type of fitting used to put grease into a
bearing. A hole is drilled through the structure to the location where grease is needed, and a Zerk fitting is
installed in this hole.
The Zerk fitting has a ball-shaped top that is gripped by the nozzle of the grease gun when
pressure is applied. Grease is pumped through the fitting into the bearing until it flows out between the
surfaces of the bearing. When the grease gun is removed from the Zerk fitting, a check valve inside the
fitting prevents grease leaking out, and prevents dirt getting into the bearing through the fitting.
zero (mathematical quantity). A numerical quantity that has no value. Zero is used as a reference point on
a number line from which values on both sides of zero are measured.
zero (temperature). A reference used in the measurement of temperature. In the measurement of absolute
temperatures using both degrees Kelvin and degrees Rankine, zero is the point at which all molecular
　
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