Page 230 - The Jet Engine

Performance

temperature rise affects the compressor delivery air
temperature proportionately and, in consequence, to
maintain the required thrust, the engine must be
subjected to higher turbine entry temperatures. Since
the maximum permissible turbine entry temperature
is determined by the temperature limitations of the
turbine assembly, the choice of turbine materials and
the design of blades and stators to permit cooling are
very important.

22. With an increase in forward speed, the
increased mass airflow due to the 'ram ratio' effect
must be matched by the fuel flow (Part 10) and the
result is an increase in fuel consumption. Because
the net thrust tends to decrease with forward speed
the end result is an increase in specific fuel
consumption (s.f.c.), as shown by the curves for a
typical turbo-jet engine in fig, 21-4.

23. At high forward speeds at low altitudes the 'ram
ratio' effect causes very high stresses on the engine
and, to prevent overstressing, the fuel flow is auto-
matically reduced to limit the engine speed and
airflow. The method of fuel control is described in
Part 10.
24. The effect of forward speed on a typical turbo-
propeller engine is shown by the trend curves in fig.
21 -5. Although net jet thrust decreases, s.h.p.
increases due to the 'ram ratio1 effect of increased
mass flow and matching fuel flow. Because it is
standard practice to express the s.f.c. of a turbo-
propeller engine relative to s.h.p., an improved s.f.c.
is exhibited. However, this does not provide a true
comparison with the curves shown in fig. 21-4, for a
typical turbo-jet engine, as s.h.p, is absorbed by the
propeller and converted into thrust and, irrespective
of an increase in s.h.p., propeller efficiency and
therefore net thrust deteriorates at high subsonic
forward speeds. In consequence, the turbo-propeller
engine s.f.c, relative to net thrust would, in general
comparison with the turbo-jet engine, show an
improvement at low forward speeds but a rapid dete-
rioration at high speeds.
Effect of afterburning on engine thrust
25. At take-off conditions, the momentum drag of
the airflow through the engine is negligible, so that
the gross thrust can be considered to be equal to the Fig. 21-5 The effect of aircraft speed on
net thrust. If afterburning (Part 16) is selected, an s.h.p. and fuel consumption.
increase in take-off thrust in the order of 30 per cent
is possible with the pure jet engine and considerably 26. Under flight conditions, however, this advantage
more with the by-pass engine. This augmentation of is even greater, since the momentum drag is the
basic thrust is of greater advantage for certain same with or without afterburning and, due to the
specific operating requirements. ram effect, better utilization is made of every pound

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