Page 257 - The Jet Engine

Power plant installation

11. At higher supersonic speeds, the pitot type of air
intake is unsuitable due to the severity of the
shockwave that forms and progressively reduces the
intake efficiency as speed increases. A more suitable
type of intake for these higher speeds is known as
the external/internal compression intake (fig. 23-8).
This type of intake produces a series of mild shock
waves without excessively reducing the intake
efficiency.
12. As aircraft speed increases still further, so also
does the intake compression ratio and, at high Mach
numbers, it is necessary to have an air intake that has
a variable throat area and spill valves to accommodate

Fig. 23-9 Variable throat area intake.
and control the changing volumes of air (fig. 23-9).
The airflow velocities encountered in the higher speed
range of the aircraft are much higher than the engine
can efficiently use; therefore, the air velocity must be
decreased between the intake and the engine air inlet.
The angle of the variable throat area intake automati-
cally varies with aircraft speed and positions the shock
wave to decrease the air velocity at the engine inlet
and maintain maximum pressure recovery within the
inlet duct. However, continued development enables
this to be achieved by careful design of the intake and
ducting. This, coupled with auxiliary air doors to permit
extra air to be taken in under certain engine operating
conditions, allows the airflow to be controlled without
the use of variable geometry intakes. The fuselage
Fig. 23-8 External/internal compression intakes shown in fig. 23-10 are of the variable throat
intake. area type.

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