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Aviation History
1949
1949 - 0384.PDF
250 FLIGHT MARCH 3RD, 1949 NTROSPECTION Radiographic Inspection of Gas- Turbine Castings : Armstrong Siddeley Technique Described WHEN A./Cdre. Banks gave the first Louis Bleriotlecture in Paris last May, he made some interestingobservations on the cost of developing prototype engines. He instanced a large, liquid-cooled engine of about 3,500 h.p. which, weighing 3,700 lb, would cost about £15 per lb, or £55,000, and cited this against the cost of a turboprop engine of approximately the same maximum power. The turboprop will, of course, weigh much less, but will absorb more design hours and cost more per lb to manufacture in prototype form. The total cost will, therefore, be of roughly the same order as that of the equivalent piston engine, assuming the turboprop to weigh 2,900 lb, "and to cost £20 per lb, or £58,000. These figures concern prototype engines, but they do bear some relation to the costs of manufacturing produc- tion engines, albeit accurate determination of the ratio might be difficult. What is certain, however, is that although the gas turbine, whether of turbojet or turboprop form, is essentially a simple unit physically, there are a great number of critical parts which are individually expen- sive, and of these the most numerous and most critical are blades. Quite what proportion of the total cost of a gas turbine is vested in blade cost is not common knowledge, but that it is over 50 per cent may be taken as certain for present units—although as time goes on and production techniques become more advanced the proportion will un- doubtedly get much less. Blade Types There are, of course, many forms of blades; in general, the usual British practice is to use forged blades for rotor stages and cast (lost-wax) blades for the stator stages of the turbine. For axial compressors, the blading both of the rotor and stator stages is usually machined from extruded bar stock. Blade material has now become more or less standardized, although an enormous amount of research has been put into this subject and, in fact, is still going on. Nevertheless, in most British axial engines, the compressor rotor and stator stages are of light alloy and the turbine rotor and stator stages are manufactured^ in one of the specialized high-temperature alloys, such as those of the Nimonic range. As part of their gas-turbine research programme Ann- strong Siddeley Motors, Ltd., are conducting investigations into the use of cast blades in a variety of materials. Aside The Siemens set in operation. On the lead viewing table is a typical set-out of guide-vane units on a negative envelope, ready for exposure. from this research, the company cast all their own pro- duction compressor inlet guide-vanes and turbine-nozzle vanes. A modified version of the lost-wax process is em- ployed and permits casting to limits within o.oo2in on thickness for all blade sizes. In so far as ferritic steels and nickel-chrome alloys of high physical strength are con- cerned, an interesting experiment is in the use of such metals for the initial stages of the compressor blading. At the high mass flows at which air is consumed by a gas turbine, the abrasive effects on the compressor blading of particles of matter which might be sucked in can be extremely severe. With light-alloy blading, the effect of this abrasion is frequently to destroy the protective anodic treatment so that the blade material is rendered vulnerable to corrosive attack, with the ensuing liability that a blade may frac- ture and -cause chaotic destruction of the unit, not only /by being torn up in passage through the compressor but, additionally, by destroy- ing the dynamic balance of the rotating mass. By making compressor blades of the first stages from relatively impervious srial these risks are virtually elimin- ed, for in passing through these stages •y intrusive solid which may penetrate the intake grill is normally minced down so small as to be harmless in its effect on the remaining stages of the compressor. Naturally enough, high-quality alloys are not cheap and as, quite apart from the cost of the material, the processing and machining operations are themselves critical, the rise in cost of the finished A general view of the X-ray laboratory showing, on the left, the large Siemens set and, on the right, the smaller Philips apparatus. D 2
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