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is approached, the efficiency of this type of air intake
begins to fall because of the formation of a shock
wave at the intake lip.
8. The pitot-type intake can be used for engines that
are mounted in pods or in the wings, although the latter
sometimes require a departure from the circular crosssection
because of the wing thickness (fig. 23-5).
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Fig. 23-5 Wing leading edge intakes.
Fig. 23-4 Pitot-type intake.
9. Single engined aircraft sometimes use a pilottype
intake; however, because this generally involves
the use of a long duct ahead of the compressor, a
divided type of intake on each side of the fuselage is
often used (fig. 23-6).
10. The disadvantage of the divided type of air
intake is that when the aircraft yaws, a loss of ram
pressure occurs on one side of the intake, as shown
in fig. 23-7, causing an uneven distribution of airflow
into the compressor.
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Fig. 23-6 Single engined aircraft with fuselage intakes.
Fig. 23-7 Loss of ram pressure in divided intakes.
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
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 automatically
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
intakes shown in fig. 23-10 are of the variable throat
area type.
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247
Fig. 23-8 External/internal compression
intake.
Fig. 23-9 Variable throat area intake.
ENGINE AND JET PIPE MOUNTINGS
13. The engine is mounted in the aircraft in a
manner that allows the thrust forces developed by
the engine to be transmitted to the aircraft main
structure, in addition to supporting the engine weight
and carrying any flight loads. Because of the wide
variations in the temperature of the engine casings,
the engine is mounted so that the casings can
expand freely in both a longitudinal and a radial
direction. Types of engine mountings, however, vary
to suit the particular installation requirement. Turbojet
engines are usually either side mounted or
underslung as illustrated in fig. 23-11. Turbopropeller
engines are mounted forward on a tubular
framework as illustrated in fig. 23-13.
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248
Fig. 23-10 Fuselage intakes.
Fig. 23-11 Typical turbo-jet engine mountings.
14. The jet pipe is normally attached to the rear of
the engine and supported by the engine mountings.
In some installations, particularly where long jet
pipes are employed, an additional mounting is
provided, usually in the form of small rollers attached
to each side of the jet pipe. The rollers locate in
airframe-mounted channels and support the weight
of the jet pipe, whilst still allowing it to freely expand
in a longitudinal direction.
ACCESSORIES
15. An aircraft power plant installation generally
includes a number of accessories that are electrically
operated, mechanically driven or driven by high
pressure air.
16. Electrically operated accessories such as
engine control actuators, amplifiers, air control
valves and solenoids, are supplied with power from
the aircraft electrical system or an engine driven
dedicated electrical generator.
17. Mechanically driven units, such as generators,
constant speed drive units, hydraulic pumps, low and
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