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promethium. A radioactive, rare-earth chemical element used as a source of beta rays. Promethium’s
symbol is Pm, its atomic number is 61, and the mass number of its most stable isotope is 145.
prony brake. An instrument used to measure the amount of horsepower an engine or motor is delivering to
its output shaft. The engine is operated at a specified RPM, and a brake is applied to its output shaft. The
amount of torque applied to the brake is measured, and this, with the RPM, is converted into brake
horsepower.
Dynamometers have replaced prony brakes for measuring brake horsepower.
propeller. A rotating airfoil driven by an airplane engine to produce thrust to pull or push an airplane
through the air. 14 CFR Part 1: “A device for propelling an aircraft that has blades on an engine-driven
shaft and that, when rotated, produces by its action on the air, a thrust approximately perpendicular to its
plane of rotation. It includes control components normally supplied by its manufacturer, but does not
include main and auxiliary rotors or rotating airfoils of engines.”
propeller anti-icer. A system that spreads a mixture of alcohol and glycerin along the propeller blades
when the aircraft is flying in icing conditions. The alcohol and glycerin prevent water reaching the metal of
the propeller blade and freezing on it.
propeller blade tipping (wooden propeller component). Thin sheet metal tipping on the blades of a
wooden propeller. The metal reinforces the thin tips and prevents damage to the wood by water, sand, and
rocks thrown into the propeller.
propeller cones. Tapered metal cones placed over a splined propeller shaft to hold the propeller centered
on the shaft. A single-piece bronze cone is used as the rear cone and it slips over the shaft and contacts the
thrust bearing retainer nut that is screwed onto the propeller shaft. The front cone is a two-piece,
chrome-plated steel, matched set. It contains a groove into which a flange on the propeller retaining nut fits.
When the retaining nut is tightened, the propeller hub is squeezed between the two cones which
centers it on the shaft.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 462
propeller efficiency. The ratio of the thrust horsepower produced by the engine-propeller combination to
the brake horsepower delivered to the propeller shaft.
propeller end (aircraft engine nomenclature). The end of a reciprocating engine to which the propeller is
attached.
propeller shaft. The shaft on which the propeller is mounted. On a direct-drive engine, the propeller
mounts on the end of the engine crankshaft. On a geared engine, the propeller is mounted on a separate
shaft which is part of the reduction gear system.
Before and during World War II tapered shafts were used on most of the smaller engines and
splined shafts were used on all of the larger engines. Almost all modern engines except some of the very
largest turboprop engines are equipped with flanged propeller shafts.
propeller synchronization. The function of keeping the RPM of all the engines in a multiengine aircraft
exactly the same. One engine is used as the master engine, and the speed of each of the other engines,
called slave engines, is compared to it. The propeller governor on each slave engine is automatically
adjusted, to change the pitch of its propeller to maintain an RPM the same as that of the master engine.
proper fraction (mathematics). A common fraction in which the numerator is smaller than the
denominator.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 463
Propfan engine. The trade name registered by Hamilton Standard for an ultrahigh-bypass turbine engine.
See UHB engine, and UDF engine.
proportional control. A type of control in which the magnitude of the corrective action (the output) is
directly proportional to the magnitude of the error (the input signal). An autopilot controls an airplane by
proportional control. If a wing drops slightly (the error), the autopilot servo causes the proper aileron
movement to smoothly return the aircraft to level flight.
proposed departure time (air traffic control). The time a scheduled flight will depart the gate for
scheduled operators, or the actual runway-off time for nonscheduled operators. For expected departure
clearance time (EDCT) purposes, the air traffic control system command center (ATCSCC) adjusts the
proposed departure time for scheduled operators to reflect the runway-off time.
propulsive efficiency. A measure of the effectiveness with which an aircraft engine converts the fuel it
burns into useful thrust, and is the ratio of the thrust horsepower produced by a propeller to the torque
　
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