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horsepower of the shaft turning the propeller.
The more nearly the speed of the aircraft is to the speed of the exhaust jet or propeller wake, the
less kinetic energy is lost in the jet or wake, and the higher the propulsive efficiency.
propwash. A commonly used term for the air blown back from an airplane by the propeller.
In the early days of aviation, one of the ways of hazing a fledgling mechanic was to send him
across the airport “to get a bucket of propwash.”
protactinium. A rare, radioactive chemical element in the actinide group. Protactinium’s symbol is Pa, and
its atomic number is 91. The most common isotope of protactinium, Pa 231, has a half-life of more than
32,000 years.
proton. The positively charged particles in the nucleus of an atom.
prototype device. A working model of a device usually made by hand. The prototype of an aircraft is built
to prove the design or design concept. After all of the design “bugs” are worked out of the prototype,
production tooling is set up, and production models are built. The production models are quite similar to,
but not exactly like the prototype.
protractor. An instrument used to measure angles.
protruding head rivet (aircraft structural component). An aircraft rivet whose head protrudes, or sticks
up, above the surface of the metal it is used to join. The universal head rivet (MS20470) is the most
commonly used protruding head rivet in modern aircraft construction.
PRT (power recovery turbine). A turbine driven by the exhaust gases from several cylinders of a
reciprocating engine. Energy extracted from the exhaust gases by the turbine is coupled, through a fluid
clutch, to the engine crankshaft.
Prussian blue dye. A type of industrial dye used to measure the amount of contact between mating parts.
One of the surfaces to be mated is given a light coat of Prussian blue dye, and the parts are assembled.
During the assembly, one part is moved slightly with respect to the other. The parts are then disassembled,
and the amount of dye transferred from one part to the other gives an indication of the amount of physical
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Page 464
contact between the two parts.
psychrometer (meteorological instrument). An instrument used to measure the amount of water vapor in
the air. Two thermometers are used to measure the air temperature. One thermometer has a cloth wick
around its sensitive end, and this wick is dipped in water and air is blown across it.
The difference in temperature shown on the wet-bulb thermometer and the dry-bulb thermometer
is an indication of the amount of cooling done by the evaporating water. The amount of cooling is inversely
proportional to the amount of water vapor in the air.
P-type semiconductor material. A semiconductor material, either silicon or germanium, that has been
doped with a few parts per million of an impurity atom having three electrons in its valence shell.
Boron, aluminum, gallium, and indium are elements used to dope silicon and germanium to make
P-type material.
public aircraft. 14 CFR Part 1: “Aircraft used only in the service of a government, or a political
subdivision. It does not include any government-owned aircraft engaged in carrying persons or property for
commercial purposes.”
pulley (aircraft control system component). A grooved wheel around which a steel control cable passes.
Pulleys are used to change the direction of movement of a control cable.
pull test (aircraft fabric test). A test to measure the strength of the fabric used to cover an aircraft. A strip
of fabric exactly one inch wide is cut from the aircraft and all of the dope removed from it. This strip is put
into a tensile tester and pulled until it breaks. The number of pounds of pull required to break the strip is the
strength of the fabric in pounds per inch.
pulsating direct current. A flow of electrons that varies in its rate of flow, but does not change its
direction of flow. A rectifier changes alternating current into pulsating direct current.
pulse (electrical energy). A sudden change in voltage in an electrical circuit, lasting only a short period of
time. A pulse can be either an increase or decrease in the voltage, and can be used as a bit of information
transmitted in the circuit.
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Page 465
pulse (radar technology). A brief burst of electromagnetic radiation emitted by a radar. A pulse has a very
short time duration.
pulse amplifier. A wide-band electrical amplifier used to increase the voltage of a square wave of
alternating current or pulsating direct current. An effective pulse amplifier does not change the waveform
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