Page 232 - The Jet Engine

Performance

Fig. 21-6 The effects of altitude on thrust
and fuel consumption.
therefore, thrust. Fig. 21-8 illustrates this for a twin-
spool engine where the controlled engine r.p.m. is
high pressure compressor speed and the
compressor delivery pressure is expressed as P3. It
will also be apparent from this graph that the low
pressure compressor speed is always less than its Fig. 21-7 The effect of altitude on s.h.p. and
limiting maximum and that the difference in the two fuel consumption.
speeds is reduced by a decrease in ambient air
temperature. To prevent the L.P. compressor over- 36. The pressure ratio control fuel system (Part 10)
speeding, fuel flow is also controlled by an L.P. schedules fuel flow to maintain a constant engine
governor which, in this case, takes a passive role. pressure ratio and, therefore, thrust below a prede-

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