horsepower (shp), which is used to drive the output shaft of the
engine. The gas then exits the engine through the exhaust section to
the atmosphere. Army helicopters use a divergent duct to eliminate
the remaining thrust. The various kinds of exhaust ducting are
discussed in detail with the engine using that particular ducting.
Figure 1.2. Typical FreePower Turboshaft Engine.
The turbojet is the engine in most common use today in high
speed, highaltitude aircraft, not in Army aircraft. With this
engine, air is drawn in by a compressor which raises internal
pressures many times over atmospheric pressure. The compressed air
be ignited and burned. Burning the fuelair mixture expands the gas,
which is accelerated out the rear as a highvelocity jetstream. In
the turbine section of the engine, the hot expanded gas rotates a
turbine wheel which furnishes power to keep the compressor going.
The gas turbine engine operates on the principle of intake,
compression, power, and exhaust, but unlike the reciprocating engine,
these events are continuous. Approximately twothirds of the total
energy developed within the combustion chamber is absorbed by the