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140 CHAPTER FIVE

Jet Engine Power and Efficiency

The engine power developed by a turbojet, and thus the amount of air
drawn into the engine, is dependent on the amount of fuel injected
into the burner and not dependent on the speed of the airplane. If
more fuel is added, more force is put on the turbine, causing more air
to be brought in by the compressor. It is a characteristic of jet engines
that the engine power and the thrust are approximately only depen-
dent on the throttle setting. Thus an engine can develop full engine
power and thrust sitting on the runway or at cruise speed.
Now here comes the interesting part. The propulsive power is the
thrust times the speed of the airplane. So although the engine may be
developing full power and thrust as it starts to roll for takeoff, it is
producing almost no propulsive power. This is shown in Figure 5.3.
Since the wasted power is the difference between the engine power
and the propulsive power, almost all of the power is wasted! We have
said that the propulsive efficiency is the propulsive power divided by
the engine power. This is almost zero at takeoff and increases
Although a jet engine may be with speed. One sometimes hears that jet efficiency increases
developing full power and thrust with speed. This is the source of that increase in efficiency. As
as it starts to roll for takeoff, it will be seen in the chapter on airplane performance, the fact
is producing almost no that a jet’s propulsive power increases with speed while that
propulsive power. of the piston-driven engine is roughly constant affects how
these planes make climbs and turns.
Turbojets have another advantage over piston-driven airplanes.
Because of the design of the diffuser, the amount of air they take in
does not depend strongly on altitude. Therefore, they are able to fly
high where the parasitic power is greatly reduced while still
developing full power. This enhances the efficiency of jet airplanes.

The Turbofan

Earlier, in our discussion of efficiency, we noted that in order to opti-
mize the efficiency of a jet engine one wants to accelerate a large
amount of gas but to as low a velocity as necessary to produce the
needed thrust. The nature of turbojets limits the amount of air that