Page 231 - The Jet Engine

Performance

of air flowing through the engine. The following output to match the reduced mass airflow, so
example, using the static values given in Part 16, maintaining a constant engine speed.
illustrates why afterburning thrust improves under
flight conditions. The fall in air temperature increases the density
of the air, so that the mass of air entering the
27. Assuming an aircraft speed of 600 m.p.h. (880ft. compressor for a given engine speed is greater.
per sec.), then Momentum drag is: This causes the mass airflow to reduce at a
880 = 27 5 . ( approximat ely ) lower rate and so compensates to some extent
32 for the loss of thrust due to the fall in atmospheric
This means that every pound of air per second pressure. At altitudes above 36,089 feet and up
to 65,617 feet, however, the temperature
flowing through the engine and accelerated up to the
speed of the aircraft causes a drag of about 27.5 lb. remains constant, and the thrust or s.h.p. is
affected by pressure only.
28. Suppose each pound of air passed through the
engine gives a gross thrust of 77.5 lb. Then the net Graphs showing the typical effect of altitude on
thrust, s.h.p, and fuel consumption are illustrated in
thrust given by the engine per lb. of air per second is fig. 21-6 and fig. 21-7.
77.5 - 27.5 = 50 lb.
Effect of temperature
29. When afterburning is selected, assuming the 30
per cent increase in static thrust given in para. 25, 33. On a cold day the density of the air increases so
the gross thrust will be 1.3 x 77.5 - 100.75 lb. Thus, that the mass of air entering the compressor for a
given engine speed is greater, hence the thrust or
under flight condition of 600 m.p.h., the net thrust per
pound of air per second will be 100.75 - 27.5 = 73.25 s.h.p, is higher. The denser air does, however,
lb. Therefore, the ratio of net thrust due to increase the power required to drive the compressor
73 . 25 or compressors; thus the engine will require more
afterburning is = 1.465. In other words, a 30 fuel to maintain the same engine speed or will run at
50
per cent increase in thrust under static conditions a reduced engine speed if no increase in fuel is
available.
becomes a 46.5 per cent increase in thrust at 600
m.p.h. 34. On a hot day the density of the air decreases,
thus reducing the mass of air entering the
30. This larger increase in thrust is invaluable for
obtaining higher speeds and higher altitude perform- compressor and, consequently, the thrust of the
ances. The total and specific fuel consumptions are engine for a given r.p.m. Because less power will be
required to drive the compressor, the fuel control
high, but not unduly so for such an increase in
performance. system reduces the fuel flow to maintain a constant
engine rotational speed or turbine entry temperature,
31. The limit to the obtainable thrust is determined as appropriate; however, because of the decrease in
by the afterburning temperature and the remaining air density, the thrust will be lower. At a temperature
usable oxygen in the exhaust gas stream. Because of 45 deg.C., depending on the type of engine, a
no previous combustion heating takes place in the thrust loss of up to 20 per cent may be experienced.
duct of a by-pass engine, these engines with their This means that some sort of thrust augmentation,
large residual oxygen surplus are particularly suited such as water injection (Part 17), may be required.
to afterburning and static thrust increases of up to 70
per cent are obtainable. At high forward speeds 35. The fuel control system (Part 10) controls the
several times this amount is achieved. fuel flow so that the maximum fuel supply is held
practically constant at low air temperature conditions,
Effect of altitude whereupon the engine speed falls but, because of
32. With increasing altitude the ambient air the increased mass airflow as a result of the increase
pressure and temperature are reduced. This affects in air density, the thrust remains the same. For
the engine in two interrelated ways: example, the combined acceleration and speed
control fuel system (Part 10) schedules fuel flow to
The fall of pressure reduces the air density and maintain a constant engine r.p.m., hence thrust
hence the mass airflow into the engine for a increases as air temperature decreases until, at a
given engine speed. This causes the thrust or predetermined compressor delivery pressure, the
s.h.p. to fall. The fuel control system, as fuel flow is automatically controlled to maintain a
described in Part 10, adjusts the fuel pump constant compressor delivery pressure and,

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