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Aviation History
1954
1954 - 1216.PDF
544 FLIGHT C B Two distinct stages in Nomad development are shown by these scale photographs of the E.12S engine (above) and the present E.145 Nomad (below). The layout of the original Nomad is shown above diagrammatically; redesign eliminated the centrifugal compressor, auxiliary turbine, and combustion chambers and forward airscrew, while other modifications increased the cylinder capacity, turbine output and compressor efficiency, and reduced heat-losses. The key to the diagram is as follows : A, one cylinder block (of two); B, piston-driven airscrew; C, delivery from centrifugal impeller; D, main gas turbine; E, auxiliary turbine; F, axial compressor; G, turbine- driven airscrew; H, projected heat-exchanger; J, auxiliary combustion chamber; K, tailpipe; L, flap valve. NAPIER NOMAD . . . The first project was a big 24-cylinder "H-type" diesel of some 75 litres capacity, with two crankshafts each driven by cylinder blocks 150 deg apart; had the two sets of cylinders been directly opposed there would have been no room for exhaust ducting. The work progressed to the stage at which single- and two- cylinder units were being run on test rigs, while the axial com pressor was tested by the surplus steam-turbine capacity of a Northampton power station—which, under M.o.S. authority, was also used in the development of some of our earliest gas turbines. Then, in 1946, Napiers decided against the 6,000 h.p. project; instead, the 75-litre unit was, figuratively speaking, cut in half The variable-ratio gear is diagram matically and photographically illus trated on the left and below. In the diagram, the boost-control piston pivots the planetary arms about fulcrums shown in solid black. to form a horizontally-opposed 3,000 h.p. engine for which a wider rang: of applications was envisaged. The resulting twelve-cylinder unit became the first Nomad; it is illustrated both photographically and diagrammatically above. The two main rotating parts of the engine, the crankshaft and the coinpressor/turbine assembly, were connected only by the gas flow, and each section drove its own airscrew. During 1948 the compressor and turbine assemblies were cold-tested, and it is worth noting that the axial compressor was very similar to that of the Naiad turboprop. By 1949 the Nomad prototype was running as a complete unit. The original working cycle required about 6J ata boost and, as this pressure could not be obtained with an axial compressor alone (at that time), a centrifugal compressor was added in series, driven from the crankshaft. The centrifugal blower was also essential during starting, owing to the inflexibility of early axial units; actually, the first axial was designed to give a pressure ratio of about 5.5 :1, and an intercooler was projected but not used. The thermodynamic cycle proposed was that heat would be added to the air until the combustion temperature in the diesel cylinder reached 2,500 deg C; the gas would then be cooled by doing work, and would finally be exhausted at atmospheric pressure by expansion across the turbine. A two-stroke cycle was chosen with compression ignition, and with a PV curve for the reciprocating component fairly typical of modern diesels. Com pared with a turboprop, the Nomad provided very much greater compression and expansion ratios, as well as greater peak pressures and temperatures: for example, the overall air /fuel ratio of a turboprop is normally of the order of 100:1, while the corresponding figure for the Nomad was about 40:1 overall (including scavenging) and 28:1 in the cylinder. Even the fresh scavenge air was by no means wasted, as this was in any case needed to reduce the gas temperature to that permitted at the turbine inlet. Regarding air compression, the initial compression was split roughly equally between the axial and the centrifugal units, the overall pressure ratio being about 11.3:1, resulting in a maximum boost of 95.5 lb/sq in absolute. This unprecedented supercharge in turn kept down the fixed-displacement compression in the cylinder to 8:1; nevertheless, the peak cylinder-pressure of over On the right is shown the variable-ratio gear in close- up, with an inset which emphasizes the avoidance of metal-to-metal contact.
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