Table I. Commonly Used Gas Turbine Engine Symbols and Abbreviations
c. Engine speed notation. The rotational speed of the engine
is represented by the capital letter N. The first rotating mass, the
gas producer has the symbol N1. Any time a number is placed after the
letter N it denotes a specific system on the gas turbine engine.
1.17.
ENGINE MODEL DESIGNATIONS
Letter designators are used to differentiate the jet
propulsion engines from reciprocating engines. A letternumber
combination identifies each type of the various gas turbine engines.
The J series designates true turbojet engines; the T designates
turboprop or turboshaft engines; and the TF, turbofan engines.
One of these letters or letter combinations begins each engine
model number, each part of which has a special significance. For
example, in the engine model number T53L13, the T means turboshaft;
the 53 is simply the number assigned to this model by the Air Force
when the engine was accepted or used experimentally. The Air
Force/Army designation numbers are odd, while engines developed
originally for the Navy get even numbers. The letter L is added by
the manufacturer (in this case the Lycoming Division of
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