Page 227 - The Jet Engine

Performance

temperature is constant up to 65.617ft. The I.S.A. not in pounds of thrust the factors are different. For
standard pressure at sea level is 14.69 pounds per example, the correction for s.h.p. is:
square inch falling to 3.28 pounds per square inch at S.h.p. (corrected) =
the tropopause (refer to I.S.A. table fig. 21-10). 30 273 + 15
s.h.p. (observed) x x
P 0 273 + 0 T
= atmospheric pressure (in.Hg.)
where P 0
ENGINE THRUST ON THE TEST BENCH (observed)
= atmospheric temperature in deg.C.
T 0
8. The thrust of the turbo-jet engine on the test (observed)
bench differs somewhat from that during flight. 30 = I.S.A. standard sea level pressure
Modern test facilities are available to simulate (in.Hg.)
atmospheric conditions at high altitudes thus 273 + 15 = I.S.A. standard sea level
providing a means of assessing some of the temperature in deg.K.
performance capability of a turbo-jet engine in flight 273 + T 0 = Atmospheric temperature in
without the engine ever leaving the ground. This is deg.K.
important as the changes in ambient temperature In practice there is always a certain amount of jet
and pressure encountered at high altitudes consider- thrust in the total output of the turbo-propeller engine
ably influence the thrust of the engine. and this must be added to the s.h.p. The correction
for jet thrust is the same as that in para. 10.
9. Considering the formula derived in Part 20 for
engines operating under 'choked' nozzle conditions, 12. To distinguish between these two aspects of the
power output, it is usual to refer to them as s.h.p. and
Thrust = P ( − P0 )⋅ A + WV J thrust horse-power (t.h.p.). The total equivalent
g 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
it can be seen that the thrust can be further affected the net jet thrust. For estimation purposes it is taken
by a change in the mass flow rate of air through the that, under sea- level static conditions, one s.h.p. is
engine and by a change in jet velocity. An increase in
mass airflow may be obtained by using water equivalent to approximately 2.6 lb. of jet thrust.
Therefore :
injection (Part 17) and increases in jet velocity by
using afterburning (Part 16). jet thrust lb .
. p . h . e . t = . p . h . s +
10. As previously mentioned, changes in ambient 6 . 2
pressure and temperature considerably influence the 13. The ratio of jet thrust to shaft power is
thrust of the engine. This is because of the way they influenced by many factors. For instance, the higher
affect the air density and hence the mass of air the aircraft operating speed the larger may be the
entering the engine for a given engine rotational required proportion of total output in the form of jet
speed. To enable the performance of similar engines thrust. Alternatively, an extra turbine stage may be
to be compared when operating under different
climatic conditions, or at different altitudes, correction required if more than a certain proportion of the total
power is to be provided at the shaft. In general,
factors must be applied to the calculations to return turbo-propeller aircraft provide one pound of thrust
the observed values to those which would be found for every 3.5 h,p. to 5 h.p.
under I.S.A. conditions. For example, the thrust
correction for a turbo-jet engine is: Comparison between thrust and horse-power
Thrust (lb.) (corrected) = 14. Because the turbo-jet engine is rated in thrust
30 and the turbo-propeller engine in s.h.p., no direct
thrust (lb.) (observed) x
P 0 comparison between the two can be made without a
where P 0 = atmospheric pressure in inches of power conversion factor. However, since the turbo-
mercury (in. Hg.) (observed) propeller engine receives its thrust mainly from the
30 = I.S.A. standard sea level pressure propeller, a comparison can be made by converting
(in.Hg.) the horse-power developed by the engine to thrust or
the thrust developed by the turbo-jet engine to t.h.p.;
11. The observed performance of the turbo- that is, by converting work to force or force to work.
propeller engine is also corrected to I.S.A. For this purpose, it is necessary to take into account
conditions, but due to the rating being in s.h.p. and the speed of the aircraft.

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