gas stream passing across the turbines is the only link between these
two systems. The freeturbine engine can operate over wide power
ranges with a constant outputshaft speed.
In operation, the gas producer (N1) system automatically
varies its speed, thereby controlling the intensity of the gas stream
in relation to the load applied to the power (N2) shaft. This is
accomplished by a fuel metering system that senses engine
requirements. The free turbine design has revolutionized the methods
of application of shaft turbine engines. Why a shaft turbine? Why
is a perfectly good jet engine used to drive a propeller? Because in
the speed range that Army aircraft operate, the propeller or
helicopter rotor is more efficient. With a turbojet engine, power
(thrust) produced is roughly the difference between the velocity of
the air entering the engine and the velocity of the air exiting from
the engine. Efficiency of the engine (power producer versus fuel
consumed) increases with speed until it is 100 percent efficient when
the forward
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