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at various locations through the engine; for
instance, using the symbols shown in fig. 21-1 the
overall compressor pressure ratio is . These
symbols vary slightly for different types of engine; for
instance, with high by-pass ratio engines, and also
when afterburning (Part 16) is incorporated,
additional symbols are used.
7. To enable the performance of similar engines to
be compared, it is necessary to standardize in some
conventional form the variations of air temperature
and pressure that occur with altitude and climatic
conditions. There are in use several different
definitions of standard atmospheres, the one in most
common use being the International Standard
Atmosphere (I.S.A.). This is based on a temperature
lapse rate of approximately 1.98 K. degrees per
1,000ft,, resulting in a fall from 288.15 deg.K. (15
deg.C) at sea level to 216.65 deg.K (-56.5 deg.C.) at
36,089 ft. (the tropopause). Above this altitude the
Performance
216
Fig. 21-1 Temperature and pressure notation of a typical turbo-jet engine.
1
3
P
P
temperature is constant up to 65.617ft. The I.S.A.
standard pressure at sea level is 14.69 pounds per
square inch falling to 3.28 pounds per square inch at
the tropopause (refer to I.S.A. table fig. 21-10).
ENGINE THRUST ON THE TEST BENCH
8. The thrust of the turbo-jet engine on the test
bench differs somewhat from that during flight.
Modern test facilities are available to simulate
atmospheric conditions at high altitudes thus
providing a means of assessing some of the
performance capability of a turbo-jet engine in flight
without the engine ever leaving the ground. This is
important as the changes in ambient temperature
and pressure encountered at high altitudes considerably
influence the thrust of the engine.
9. Considering the formula derived in Part 20 for
engines operating under ’choked’ nozzle conditions,
it can be seen that the thrust can be further affected
by a change in the mass flow rate of air through the
engine and by a change in jet velocity. An increase in
mass airflow may be obtained by using water
injection (Part 17) and increases in jet velocity by
using afterburning (Part 16).
10. As previously mentioned, changes in ambient
pressure and temperature considerably influence the
thrust of the engine. This is because of the way they
affect the air density and hence the mass of air
entering the engine for a given engine rotational
speed. To enable the performance of similar engines
to be compared when operating under different
climatic conditions, or at different altitudes, correction
factors must be applied to the calculations to return
the observed values to those which would be found
under I.S.A. conditions. For example, the thrust
correction for a turbo-jet engine is:
Thrust (lb.) (corrected) =
thrust (lb.) (observed) x
where P0 = atmospheric pressure in inches of
mercury (in. Hg.) (observed)
30 = I.S.A. standard sea level pressure
(in.Hg.)
11. The observed performance of the turbopropeller
engine is also corrected to I.S.A.
conditions, but due to the rating being in s.h.p. and
not in pounds of thrust the factors are different. For
example, the correction for s.h.p. is:
S.h.p. (corrected) =
s.h.p. (observed)
where P0 = atmospheric pressure (in.Hg.)
(observed)
T0 = atmospheric temperature in deg.C.
(observed)
30 = I.S.A. standard sea level pressure
(in.Hg.)
273 + 15 = I.S.A. standard sea level
temperature in deg.K.
273 + T0 = Atmospheric temperature in
deg.K.
In practice there is always a certain amount of jet
thrust in the total output of the turbo-propeller engine
and this must be added to the s.h.p. The correction
for jet thrust is the same as that in para. 10.
12. To distinguish between these two aspects of the
power output, it is usual to refer to them as s.h.p. and
thrust horse-power (t.h.p.). The total equivalent
horse-power is denoted by t.e.h.p. (sometimes
e.h.p.) and is the s.h.p. plus the s.h.p. equivalent to
the net jet thrust. For estimation purposes it is taken
that, under sea- level static conditions, one s.h.p. is
equivalent to approximately 2.6 lb. of jet thrust.
Therefore :
13. The ratio of jet thrust to shaft power is
influenced by many factors. For instance, the higher
the aircraft operating speed the larger may be the
required proportion of total output in the form of jet
thrust. Alternatively, an extra turbine stage may be
required if more than a certain proportion of the total
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