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16. Noise absorbing ’lining’ material converts
acoustic energy into heat. The absorbent linings (fig.
19-6) normally consist of a porous skin supported by
a honeycomb backing, to provide the required
separation between the facesheet and the solid
engine duct. The acoustic properties of the skin and
the liner depth are carefully matched to the character
of the noise, for optimum suppression. The disadvantage
of liners is the slight increase in weight and
skin friction and hence a slight increase in fuel
consumption. They do however, provide a very
powerful suppression technique.
CONSTRUCTION AND MATERIALS
17. The corrugated or lobe-type noise suppressor
forms the exhaust propelling nozzle and is usually a
separate assembly bolted to the jet pipe. Provision is
usually made to adjust the nozzle area so that it can
be accurately calibrated. Guide vanes are fitted to
the lobe-type suppressor to prevent excessive losses
by guiding the exhaust gas smoothly through the
lobes to atmosphere. The suppressor is a fabricated
welded structure and is manufactured from heatresistant
alloys.
18. Various noise absorbing lining materials are
used on jet engines. They fall mainly within two
categories, lightweight composite materials that are
used in the lower temperature regions and fibrousmetallic
materials that are used in the higher
temperature regions. The noise absorbing material
consists of a perforate metal or composite facing
skin, supported by a honeycomb structure on a solid
backing skin which is bonded to the parent metal of
the duct or casing. For details of manufacture of
these materials refer to Part 22.
Noise suppression
205
Rolls-Royce Conway
Rolls-Royce RM60
Produced in response to an Admiralty contract
for a coastal-craft engine with good cruising
economy, the RM60, although based on
aeroengine philosophy, was designed from
the first as a marine gas turbine. Two RM60s
went to sea in 1953 in the former steam
gunboat HMS Grey Goose, the world’s first
warship to be powered solely by gas turbines.
20: Thrust distribution
Contents Page
Introduction 207
Distribution of the thrust
forces 207
Method of calculating the
thrust forces 209
Calculating the thrust of
the engine 209
Compressor casing
Diffuser duct
Combustion chambers
Turbine assembly
Exhaust unit and jet pipe
Propelling nozzle
Engine
Inclined combustion chambers
Afterburning 212
INTRODUCTION
1. Although the principles of jet propulsion (see Part
1) will be familiar to the reader, the distribution of the
thrust forces within the engine may appear
somewhat obscure- These forces are in effect gas
loads resulting from the pressure and momentum
changes of the gas stream reacting on the engine
structure and on the rotating components. They are
in some locations forward propelling forces and in
others opposing or rearward forces. The amount that
the sum of the forward forces exceeds the sum of the
rearward forces is normally known as the rated thrust
of the engine.
DISTRIBUTION OF THE THRUST FORCES
2. The diagram in fig. 20-1 is of a typical singlespool
axial flow turbo-jet engine and illustrates where
the main forward and rearward forces act. The origin
of these forces is explained by following the engine
working cycle shown in Part 2.
207
3. At the start of the cycle, air is induced into the
engine and is compressed. The rearward accelerations
through the compressor stages and the
resultant pressure rise produces a large reactive
force in a forward direction. On the next stage of its
journey the air passes through the diffuser where it
exerts a small reactive force, also in a forward
direction,
4. From the diffuser the air passes into the
combustion chambers (Part 4) where it is heated,
and in the consequent expansion and acceleration of
the gas large forward forces are exerted on the
chamber walls.
5. When the expanding gases leave the combustion
chambers and flow through the nozzle guide vanes
they are accelerated and deflected on to the blades
of the turbine (Part 5). Due to the acceleration and
deflection, together with the subsequent straightening
of the gas flow as it enters the jet pipe, considerable
’drag’ results; thus the vanes and blades are
subjected to large rearward forces, the magnitude of
which may be seen on the diagram. As the gas flow
passes through the exhaust system (Part 6), small
forward forces may act on the inner cone or bullet,
　
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