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tested. This vibration produces an electrical signal which is displayed on the screen of a cathode-ray
oscilloscope.
The display produced by the vibrations passing through the material being tested is compared with
the display produced by a vibration of the same frequency passing through a similar material known to
have no faults. Any difference in the displays is an indication of an internal fault.
ultrasonics. The branch of science and technology that deals with mechanical vibrations having a
frequency higher than the human ear can normally hear. Ultrasonic energy has a frequency greater than
20,000 hertz.
ultrasonic soldering. A method of soldering aluminum with soft solder. Aluminum is difficult to solder
because, when it is heated, oxides form on its surface and prevent the solder wetting the metal.
The tip of a special soldering iron is vibrated at an ultrasonic frequency as it heats the metal and
melts the flux in the solder. This vibration breaks the oxide film away from the surface of the metal, and it
floats on top of the flux. The flux covers the hot metal and prevents new oxides forming. Soft solder will
stick to aluminum when there are no oxides on it.
ultraviolet-blocking dope. Dope that contains tiny aluminum flakes or some other pigment that blocks the
passage of ultraviolet rays from the sun. This coat of dope protects the organic fabrics and the clear dope
from deterioration by these rays.
ultraviolet lamp. A form of vapor lamp that produces electromagnetic radiation whose wavelength is
slightly shorter than that of visible light. Ultraviolet light is often called “black light.”
ultraviolet radiation. Radiation of electromagnetic energy whose wavelengths are between about 100 and
4,000 angstroms. See angstrom. The wavelengths of ultraviolet rays are just shorter than those of rays
visible to the human eye.
umbilical cord (rocket component). A cable used to carry power from a launch pad to a rocket or
spacecraft being readied for flight. The umbilical cord is disconnected before the vehicle leaves the pad. An
umbilical cord is also used to tether and to supply oxygen to an astronaut when operating outside a
spacecraft in space.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 604
unbalanced cell (nickel-cadmium battery condition). A cell in a nickel-cadmium battery that has become
discharged to a lower level than the other cells in the battery. An unbalanced cell will take more charging
current than the other cells, and this can lead to overheating and thermal runaway of the battery.
Unbalanced cells can be corrected by deep-cycling the battery.
unbalanced transmission line (radio transmission line). A form of two-conductor cable used to carry an
electromagnetic signal from a transmitter to its antenna. An unbalanced line does not have the same voltage
between each of the two conductors and ground. A coaxial cable is a form of unbalanced transmission line.
uncontrolled spin (aircraft flight maneuver). A spin in an airplane in which the controls are ineffective
in recovering to straight and level flight. See spin.
undamped oscillation. Oscillation that continues with an unchanging amplitude once it has started.
undercarriage (aircraft structure). The landing gear of an aircraft.
undercast (meteorology). A cloud layer of ten-tenths (1.0) coverage, as viewed from an observation point
above the layer.
undercompounded generator (electrical generator). A condition in an electrical generator which has
both a series and a shunt field in which the effect of the shunt field is greater than that of the series field.
The output voltage tends to drop as the electrical load increases.
undercurrent relay (electrical protection device). An electrical circuit protection device that opens a
control circuit when the current drops to a specified low value.
underpowered (aircraft condition). An aircraft is underpowered when there is not enough power
delivered to its propeller or rotor to give it the required performance.
undershoot (aircraft landing). A condition in an aircraft landing in which the aircraft touches the ground
before it reaches the end of the runway or the landing strip. A pilot can correct an undershot approach by
applying engine power as soon as it is evident the aircraft will not reach the spot of intended touchdown.
underslung rotor (helicopter rotor). A helicopter rotor with its center of gravity below the point at which
the rotor is attached to the mast.
underspeed condition (propeller governor condition). A propeller governor is in its underspeed
condition when the engine is operating at a speed lower than the governor calls for. The speeder spring
holds the pilot valve down and the flyweights in. With the pilot valve down, the governor directs oil in the
　
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