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The total and specific fuel consumptions are
high, but not unduly so for such an increase in
performance.
31. The limit to the obtainable thrust is determined
by the afterburning temperature and the remaining
usable oxygen in the exhaust gas stream. Because
no previous combustion heating takes place in the
duct of a by-pass engine, these engines with their
large residual oxygen surplus are particularly suited
to afterburning and static thrust increases of up to 70
per cent are obtainable. At high forward speeds
several times this amount is achieved.
Effect of altitude
32. With increasing altitude the ambient air
pressure and temperature are reduced. This affects
the engine in two interrelated ways:
The fall of pressure reduces the air density and
hence the mass airflow into the engine for a
given engine speed. This causes the thrust or
s.h.p. to fall. The fuel control system, as
described in Part 10, adjusts the fuel pump
output to match the reduced mass airflow, so
maintaining a constant engine speed.
The fall in air temperature increases the density
of the air, so that the mass of air entering the
compressor for a given engine speed is greater.
This causes the mass airflow to reduce at a
lower rate and so compensates to some extent
for the loss of thrust due to the fall in atmospheric
pressure. At altitudes above 36,089 feet and up
to 65,617 feet, however, the temperature
remains constant, and the thrust or s.h.p. is
affected by pressure only.
Graphs showing the typical effect of altitude on
thrust, s.h.p, and fuel consumption are illustrated in
fig. 21-6 and fig. 21-7.
Effect of temperature
33. On a cold day the density of the air increases so
that the mass of air entering the compressor for a
given engine speed is greater, hence the thrust or
s.h.p, is higher. The denser air does, however,
increase the power required to drive the compressor
or compressors; thus the engine will require more
fuel to maintain the same engine speed or will run at
a reduced engine speed if no increase in fuel is
available.
34. On a hot day the density of the air decreases,
thus reducing the mass of air entering the
compressor and, consequently, the thrust of the
engine for a given r.p.m. Because less power will be
required to drive the compressor, the fuel control
system reduces the fuel flow to maintain a constant
engine rotational speed or turbine entry temperature,
as appropriate; however, because of the decrease in
air density, the thrust will be lower. At a temperature
of 45 deg.C., depending on the type of engine, a
thrust loss of up to 20 per cent may be experienced.
This means that some sort of thrust augmentation,
such as water injection (Part 17), may be required.
35. The fuel control system (Part 10) controls the
fuel flow so that the maximum fuel supply is held
practically constant at low air temperature conditions,
whereupon the engine speed falls but, because of
the increased mass airflow as a result of the increase
in air density, the thrust remains the same. For
example, the combined acceleration and speed
control fuel system (Part 10) schedules fuel flow to
maintain a constant engine r.p.m., hence thrust
increases as air temperature decreases until, at a
predetermined compressor delivery pressure, the
fuel flow is automatically controlled to maintain a
constant compressor delivery pressure and,
Performance
221
27.5 (approximately)
32
880 =
50
73.25
therefore, thrust. Fig. 21-8 illustrates this for a twinspool
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
limiting maximum and that the difference in the two
speeds is reduced by a decrease in ambient air
temperature. To prevent the L.P. compressor overspeeding,
fuel flow is also controlled by an L.P.
governor which, in this case, takes a passive role.
36. The pressure ratio control fuel system (Part 10)
schedules fuel flow to maintain a constant engine
pressure ratio and, therefore, thrust below a prede-
Performance
222
Fig. 21-6 The effects of altitude on thrust
and fuel consumption.
Fig. 21-7 The effect of altitude on s.h.p. and
fuel consumption.
termined ambient air temperature. Above this
temperature the fuel flow is automatically controlled
to prevent turbine entry temperature limitations from
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