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Aviation History
1954
1954 - 1217.PDF
30 April 1954 545 2,000 lb/sq in was high for a commercial diesel engine, and demanded robust construction. Exhaust release—controlled by the uncovering of the cylinder ports by the piston—also took place at high pressure; for example, the full-throttle exhaust pressure was some 78 lb/sq in, indicating a drop in pressure of about 11 lb/sq in across the cylinder. No valve gear of any kind was used, but a simple ignition system was provided for starting purposes. It was considered that the only method of meeting the opposing demands of maximum take-off and cruising efficiencies was to have an auxiliary turbine for use only in the former condition. Thus, the main power turbine was designed for efficient operation at a single point while passing the full gas flow, i.e., its choking line was reached approximately at maximum continuous power. For take-off, an additional power turbine was brought into operation by opening a flap valve; at the same time, the gear ratio of the centrifugal impeller was changed from S to M (the converse of what might have been expected) and, to preserve proper matching through all parts of the engine, additional fuel was injected into the diesel exhaust—there was plenty of oxygen there—for com bustion in special cans which can be seen in the upper photo graph on p. 544. (It is worth stressing that, even with this additional combustion, the Nomad turbines—unlike gas turbine The cross-section (left) is made in the plane of the rear pair of cylinders, look ing forward. In the bottom corners of the crankcase can be seen the two shafts connecting the rear and front gear trains. Above is a diagram of the Nomad gas flow, emphasizing the smooth path which has been achieved. rotors—are not required to operate at the limit of either stress or temperature, so that very long life should be possible.) All the foregoing discussion refers to the original form of the Nomad, as illustrated and drawn on p. 544. Early Nomads were exhibited at Farnborough and, while undoubtedly impressive, were of such bulk and complexity as to receive very litde firm interest from operators concerned with immediate requirements. There were many who thought that the Nomad would be inflexible and difficult to control and, in fact, would not make a really "usable" power plant except for a restricted range of applica tions. In 1951, brief details of the first engine were made public, and that year's S.B.A.C. flying display and exhibition saw Nomads on the Napier stand and airborne in the nose of a Lincoln test- bed. Starting the engine was soon found to be quite straight forward, and no trouble resulted from the fact that the two co-axial airscrews absorbed different powers. Altogether, this first Nomad (the Nomad 1, at NNm.3 rating) taught Napiers a very great deal. By 1951 the company had been able to "take a second look" at the engine and, so great had been the advances made in the preceding five years, it was very soon decided to re-design the whole unit. On the Naiad turboprop, for example, the company had done a great deal of work on axial compressors, and had eventually obtained pressure ratios exceeding 10:1 from single "spools," with very good flexibility and with peak efficiencies at least in the late 80s. The company's engineers began to see their way clear to an immense simplification of the engine, by doing all compression in the axial unit. In addition, the adoption of variable- incidence inlet guide vanes increased flexibility and ease of, start ing. At this point, therefore, it is appropriate to include a basic description of the present E.145 Nomad, before going on to discuss the whys and wherefores of its layout. The mechanical details of the Nomad (from here on it is the E.145 engine which is referred to) are very well shown in the accompanying drawings. The two main parts of the engine are the reciprocating engine and the turbine-compressor set which is situated underneath. The reciprocating engine is a two-stroke diesel, with two horizontally-opposed banks of six valveless cylinders, each of 6in bore and 7|in stroke, giving a total swept volume of 41.1 litres. The crankcase is cast in RZ-5 magnesium-zirconium alloy, and is split on the vertical centre-line to permit insertion of the continuous six-throw, nitrided-steel crankshaft. The two cylinder- blocks are castings in L.51 aluminium alloy, and contain liquid- cooling passages. Above the blocks are the two sets of injection pumps, which feed the injectors mounted centrally in the head of each cylinder. The axial compressor is hung from four flexible links beneath the crankcase, and its forward-facing intake is arranged to take full advantage of ram air from an intake imme diately behind the airscrew spinner. There are 12 stages of blades, the maximum mass flow and pressure ratio being 13 lb/sec and 8 : 1 respectively. The compressor is driven by a three-stage turbine, which is attached to the rear gear casing of the engine by a tubular framework. Both the turbine and compressor are mounted on shafts with internally-toothed couplings at the ends to accommodate thermal expansion; these shafts enable a positive drive to be effected between the turbine and the compressor and rear gear train. The rear gear casing itself is, like the crankcase, a magnesium- zirconium alloy casting. In addition to housing the main portion of the gear drive between the turbine and the reciprocating engine it also contains the oil sump, die oil pressure and scavenge pumps i
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