Page 229 - The Jet Engine

Performance

engine, simplifying, can be expressed as:
W v ( J − V )
P ( − P0 ) A +
g
Fig. 21-2 provides a diagrammatic explanation.

Effect of forward speed
19. Since reference will be made to 'ram ratio' and
Mach number, these terms are defined as follows:
Ram ratio is the ratio of the total air pressure at
the engine compressor entry to the static air
pressure at the air intake entry.
Mach number is an additional means of
measuring speed and is defined as the ratio of
the speed of a body to the local speed of sound.
Mach 1.0 therefore represents a speed equal to
the local speed of sound.
20. From the thrust equation in para. 18, it is
apparent that if the jet velocity remains constant,
independent of aircraft speed, then as the aircraft
speed increases the thrust would decrease in direct
proportion. However, due to the 'ram ratio' effect from
the aircraft forward speed, extra air is taken into the
engine so that the mass airflow and also the jet
velocity increase with aircraft speed. The effect of
this tends to offset the extra intake momentum drag

Fig. 21-4 The effect of aircraft speed on
thrust and fuel consumption.
due to the forward speed so that the resultant loss of
net thrust is partially recovered as the aircraft speed
increases. A typical curve illustrating this point is
shown in fig. 21-3. Obviously, the 'ram ratio' effect, or
the return obtained in terms of pressure rise at entry
to the compressor in exchange for the unavoidable
intake drag, is of considerable importance to the
turbo-jet engine, especially at high speeds. Above
speeds of Mach 1.0, as a result of the formation of
shock waves at the air intake, this rate of pressure
rise will rapidly decrease unless a suitably designed
air intake is provided (Part 23); an efficient air intake
is necessary to obtain maximum benefit from the ram
ratio effect.
21. As aircraft speeds increase into the supersonic
Fig. 21-3 Thrust recovery with aircraft region, the ram air temperature rises rapidly
speed. consistent with the basic gas laws (Part 2). This

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