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The turboprop engine and turboshaft engines, shown in figures 1.4 and 1.5, are of the same basic type as the turbojet. Instead of ejecting high-velocity exhaust gases to obtain thrust, as in the turbojet, a turbine rotor converts the energy of the expanding gases to rotational shaft power. A propeller or helicopter transmission can be connected to the engine through reduction gearing. This energy may be extracted by the same turbine rotor that drives the compressor, or it may be a free-power turbine which is independent of the compressor turbine and only linked to it by the expanding gases.

Axial-Flow Turboprop Engine
Figure 1.4. Axial-Flow Turboprop Engine.

Centrifugal-Flow Turbojet Engine
Figure 1.5. Centrifugal-Flow Turbojet Engine.

The free-power turbine is the type used in Army aircraft to harness the energy of the gases and convert this energy to rotational shaft power. This feature of having a free-power turbine enables the power output shaft to turn at a constant speed while the power producing capability of the engine can be varied to accommodate the increased loads applied to the power output shaft. Turbine engines may be further divided into three general groups, centrifugal-flow, axial-flow, and axial-centrifugal-flow, depending upon the type of compressor. Figure 1.4 shows an axial-flow turboprop engine, figure 1.5 shows a centrifugal-flow turbojet engine, and figure 1.5a shows an axial-centrifugal-flow compressor.

Axial-Centrifugal-Flow Compressor
Figure 1.5a. Axial-Centrifugal-Flow Compressor.


David L. Heiserman, Editor

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Revised: June 06, 2015