39.
An electric spark results when an electric potential between two conducting surfaces reaches the point at which the accumulation of electrons has acquired sufficient energy to bridge the gap between the two surfaces and complete the circuit. At this point, electrons break through the dielectric medium between the conducting surfaces and, moving from negative (the tool electrode) to positive (the workpiece), strike the latter surface with great energy; fig, 22-13 illustrates a typical spark erosion circuit.
40.
When the sparks strike the workpiece, the heat is so intense that the metal to be removed is instan-taneously vaporized with explosive results. Away
Manufacture
from the actual centre of the explosion, the metal is torn into fragments which may themselves be melted by the intense heat. The dielectric medium, usually paraffin oil. pumped into the gap between the tool electrode and the workpiece, has the tendency to quench the explosion and to sweep away metallic vapour and molten particles.
41.
The amount of work that can be effected in the
system is a function of the energy of the individual
sparks and the frequency at which they occur.
42.
The shape of the tool electrode is a mirror image
of the passage to be machined in the workpiece and,
to maintain a constant work gap, the electrode is fed Fig. 22-13 Electro-discharge machining
into the workpiece as erosion is effected. circuit.
COMPOSITE MATERIALS AND SANDWICH CASINGS
43.
High power to weight ratio and low component costs are very important considerations in the design of any aircraft gas turbine engine, but when the function of such an engine is to support a vertical take-off aircraft during transition, or as an auxiliary power unit, then the power to weight ratio becomes extremely critical.
44.
In such engines, the advantage of composite materials allows the designer to produce structures in which directional strengths can be varied by directional lay-up of fibres according to the applied loads.
45.
Composite materials have and will continue to replace casings which, in previous engines, would have been produced in steels or titanium. By-pass duct assemblies comprising of three casings are currently being produced up to 4ft-7in. in diameter and 2ft-0in in length using pre-cured composite materials for the casing fabric. Flanges and mounting bosses are added during the manufacturing process, which are then drilled for both location and machined
Manufacture
for peripheral feature attachment on C.N.C. machining centres, which at one component load, completely machine all required features. Examples of composite material applications are illustrated in fig. 22-14.
46. Conventional cast and fabricated casings and cowlings are also being replaced by casings of sandwich construction which provide strength allied with lightness and also act as a noise suppression medium. Sandwich construction casings comprise a honeycomb structure of aluminium or stainless steel interposed between layers of dissimilar material. The materials employed depend upon the environment in which they are used.
INSPECTION
47. During the process of manufacture, component parts need to be inspected to ensure defect free engines are produced. Using automated machinery and automated inspection, dimensional accuracy is maintained by using multi-directional applied probes that record sizes and position of features. The C.N.C. inspection machine can inspect families of components at pre-determined allotted intervals
Manufacture
Fig. 22-15 Advanced integrated manufacturing system (A.I.M.S.).
without further operator intervention. In the chip integrity is achieved by use of ultrasonic, radiologi-machining (i.e., turning, boring, milling etc.) and cal, magnetic particle and penetrant inspection metal forming processes C.N.C. machine tools techniques, as well as electrolytic and acid etching to enable consistency of manufacture which can be sta-ensure all material properties are maintained to both tistically inspected i.e., one in ten. Component laboratory and quality acceptance standards.
Rolls-Royce Turbomeca RTM 332 Turboshaft
Rolls-Royce RB 93 Soar
Developed as a lightweight expendable engine for winged missiles, the Soar first ran in 1952. At that time it had the best thrust to weight ratio of any gas turbine in the world, producing 1810 lb thrust from only 275 lb total weight. The Soar was flight tested in a Gloster Meteor with one engine on each wingtip. It was also built under licence in the USA as the J81 for the XQ4 supersonic drone.
23: Power plant installation
Contents Page
Introduction 243 Power plant location 243 Air intakes 245 Engine and jet pipe mountings 248 Accessories 249 Cowlings 249
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