Page 145 - Aeronautical Engineer Data Book
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120 Aeronautical Engineer’s Data Book
100
90
Turboprop
Engine efficiency (%) 80
70
60 Turbofan
Turbojet
50
0.5 0.6 0.7 0.8 0.9
Mach No. (cruise)
Fig. 8.4 ‘Order of magnitude’ engine efficiencies
the main types and Figure 8.4 an indication of
engine efficiency at various flight speeds.
8.2.1 The simple turbojet
The simple turbojet derives all its thrust from
the exit velocity of the exhaust gas. It has no
separate propeller or ‘power’ turbine stage.
Performance parameters are outlined in Figure
8.5. Turbojets have poor fuel economy and high
exhaust noise. The fact that all the air passes
through the engine core (i.e. there is no bypass)
is responsible for the low propulsive efficiency,
except at very high aircraft speed. The
Concorde supersonic transport (SST) aircraft is
virtually the only commercial airliner that still
uses the turbojet. By making the convenient
assumption of neglecting Reynolds number,
the variables governing the performance of a
simple turbojet can be grouped as shown in
Table 8.1.
