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.
Propfan engine. The trade name registered by Hamilton Standard for an ultrahigh-bypass turbine engine. See UHB engine, and UDF engine.