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which in addition to permitting a significant reduction
in the total number of parts, enables rotating
assemblies to be more effectively matched and to
work closer to optimum conditions, thus minimizing
the number of compressor and turbine stages for a
given duty. The use of higher strength light-weight
materials is also a contributory factor.
50. For a given mass flow less thrust is produced by
the by-pass engine due to the lower exit velocity.
Thus, to obtain the same thrust, the by-pass engine
must be scaled to pass a larger total mass airflow
than the pure turbo-jet engine. The weight of the
engine, however, is still less because of the reduced
size of the H.P. section of the engine. Therefore, in
addition to the reduced specific fuel consumption, an
improvement in the power-to-weight ratio is obtained.
Performance
227
Rolls-Royce RB168 Mk202/Mk203
Rolls-Royce RB39 Clyde
Encouraged by results obtained from the
Trent, Rolls-Royce decided to go ahead with
an engine designed from the start as a turboprop.
Named the Clyde it utilized the axial
compressor from the Metrovick F2 as first
stage and a scaled up supercharger impeller
from a Merlin as second stage. First running in
August 1945 at 2000 shp, later engines
produced up to 4200 shp.
22: Manufacture
Contents Page
Introduction 229
Manufacturing strategy 230
Forging 231
Casting 233
Fabrication 234
Welding 235
Tungsten inert gas (T.I.G.) welding
Electron beam welding (E.B.W.)
Electro-chemical
machining (E.C.M.) 237
Stem drilling
Capillary drilling
Electro-discharge
machining (E.D.M.) 238
Composite materials and
sandwich casings 240
Inspection 240
INTRODUCTION
1. During the design stages of the aircraft gas
turbine engine, close liaison is maintained between
design, manufacturing, development and product
support to ensure that the final design is a match
between the engineering specification and the manufacturing
process capability.
2. The functioning of this type of engine, with its
high power-to-weight ratio, demands the highest
possible performance from each component.
Consistent with this requirement, each component
must be manufactured at the lowest possible weight
and cost and also provide mechanical integrity
through a long service life. Consequently, the
methods used during manufacture are diverse and
are usually determined by the duties each
component has to fulfil.
3. No manufacturing technique or process that In
any way offers an advantage is ignored and most
available engineering methods and processes are
employed in the manufacture of these engines, In
some instances, the technique or process may
appear by some standards to be elaborate, time
consuming and expensive, but is only adopted after
confirmation that it does produce maximized
component lives comparable with rig test achievements.
4. Engine components are produced from a variety
of high tensile steel and high temperature nickel and
cobalt alloy forgings. A proportion of components are
cast using the investment casting process. Whilst
fabrications, which form an increasing content, are
produced from materials such as stainless steel,
titanium and nickel alloys using modern joining
229
techniques i.e., tungsten inert gas welding,
resistance welding, electron beam welding and high
temperature brazing in vacuum furnaces.
5. The methods of machining engine components
include grinding, turning, drilling, boring and
broaching whenever possible, with the more difficult
materials and configurations being machined by
electro-discharge, electro-chemical, laser hole
drilling and chemical size reduction.
6. Structural components i.e., cold spoiler, location
rings and by-pass ducts, benefit by considerable
weight saving when using composite materials.
7. In addition to the many manufacturing methods,
chemical and thermal processes are used on part
finished and finished components. These include
heat treatment, electro-plating, chromate sealing,
chemical treatments, anodizing to prevent corrosion,
chemical and mechanical cleaning, wet and dry
abrasive blasting, polishing, plasma spraying, electrolytic
etching and polishing to reveal metallurgical
defects. Also a variety of barrelling techniques for
removal o! burrs and surface improvement. Most
processes are concerned with surface changes,
some give resistance to corrosion whilst others can
be used to release unwanted stress.
8. The main structure of an aero gas turbine engine
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