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gases; therefore, to prevent any distortion, vent
holes are provided to obtain a pressure balance.
16. The mixer unit used in low by-pass ratio
engines consists of a number of chutes through
which the bypass air flows into the exhaust gases. A
bonded honeycomb structure is used for the
integrated nozzle assembly of high by-pass ratio
engines to give lightweight strength to this large
component.
17. Due to the wide variations of temperature to
which the exhaust system is subjected, it must be
mounted and have its sections joined together in
such a manner as to allow for expansion and
contraction without distortion or damage.
Exhaust system
63
Fig. 6-6 An insulating blanket.
Rolls-Royce Gnome
De Havilland H2 Ghost
The Ghost was designed as a larger and more
powerful version of the Goblin. After running
for the first time on 2 September 1945 the
engine was cleared for flight in the outer
nacelles of an Avro Lancastrian at 4000 lb
thrust. The Ghost later went into production at
5000 lb thrust to power the de Havilland
Comet 1 airliner and Venom fighter.
7: Accessory drives
Contents Page
Introduction 65
Gearboxes and drives 65
Internal gearbox
Radial driveshaft
Direct drive
Gear train drive
Intermediate gearbox
External gearbox
Auxiliary gearbox
Construction and materials 69
Gears
Gearbox sealing
Materials
INTRODUCTION
1. Accessory units provide the power for aircraft
hydraulic, pneumatic and electrical systems in
addition to providing various pumps and control
systems for efficient engine operation. The high level
of dependence upon these units requires an
extremely reliable drive system.
2. The drive for the accessory units is typically
taken from a rotating engine shaft, via an internal
gearbox, to an external gearbox which provides a
mount for the accessories and distributes the
appropriate geared drive to each accessory unit. A
starter may also be fitted to provide an input torque
to the engine. An accessory drive system on a high
by-pass engine takes between 400 and 500
horsepower from the engine.
GEARBOXES AND DRIVES
Internal gearbox
3. The location of the internal gearbox within the
core of an engine is dictated by the difficulties of
bringing a driveshaft radially outwards and the space
available within the engine core.
4. Thermal fatigue and a reduction in engine
performance, due to the radial driveshaft disturbing
the gasflow, create greater problems within the
turbine area than the compressor area. For any
given engine, which incorporates an axial-flow
compressor, the turbine area is smaller than that
containing the compressor and therefore makes it
physically easier to mount the gearbox within the
compressor section. Centrifugal compressor engines
can have limited available space, so the internal
gearbox may be located within a static nose cone or,
in the case of a turbo-propeller engine, behind the
propeller reduction gear as shown in fig. 7-1.
5. On multi-shaft engines, the choice of which
compressor shaft is used to drive the internal
gearbox is primarily dependent upon the ease of
65
Accessory drives
66
Fig. 7-1 Mechanical arrangement of accessory drives.
engine starting. This is achieved by rotating the
compressor shaft, usually via an input torque from
the external gearbox (Part 11). In practice the high
pressure system is invariably rotated in order to
generate an airflow through the engine and the high
pressure compressor shaft is therefore coupled to
the internal gearbox.
6. To minimize unwanted movement between the
compressor shaft bevel gear and radial driveshaft
bevel gear, caused by axial movement of the
compressor shaft, the drive is taken by one of three
basic methods (fig. 7-2). The least number of
components is used when the compressor shaft
bevel gear is mounted as close to the compressor
shaft location bearing as possible, but a small
amount of movement has to be accommodated
within the meshing of the bevel gears. Alternatively,
the compressor shaft bevel gear may be mounted on
a stub shaft which has its own location bearing. The
stub shaft is splined onto the compressor shaft which
allows axial movement without affecting the bevel
gear mesh. A more complex system utilizes an idler
gear which meshes with the compressor shaft via
straight spur gears, accommodating the axial
movement, and drives the radial driveshaft via a
bevel gear arrangement. The latter method was
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