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Aviation History
1954
1954 - 1222.PDF
30 April 1954 551 gap being initially ionized by a high-voltage, low-energy spark, after which follows the low-voltage discharge widi really high energy. In conjunction with K.L.G. plugs, one of which is mounted beneath each injector, this system provides trouble-free starts, and appears to be completely unaffected by any condition of die plug points. The installation of the Nomad cannot yet be fully discussed, for it is not yet completely developed. At present, two engines are being made up into power-plants for installation in an Avro Shackleton MR.l flying test-bed, replacing die outboard Griffons. The engine is slung from four vibration-damping mountings car ried by a pair of semi-cantilevered bearers which lie along die upper flanges of die crankcase, above the injection pumps. The cowling fits tightly, widi small streamlined blisters covering the cylinder heads. Immediately behind and below die spinner is die main air intake. The coolant radiators can conveniently be placed in die wing adjacent to the nacelle, and it may be noted diat radiator size is governed solely by die heat-release rate required by the diesel component at take-off and not by die power of the complete engine (for the turbine needs no cooling), which in practice is equivalent to a reduction of some 12 per cent. Many readers will no doubt wish to be able to assess die advantages possessed by die Nomad in comparison widi die Wright Turbo-Compound engine. The latter is, as we said in our "Aero Engines" number on April 9th, "a compounded R-3350 18-cylinder radial engine, with diree small, single-stage, blow-down power-recovery turbines fitted on radial shafts spaced around die engine at 120 deg intervals. These turbines are spun by the exhaust gas whenever the engine is running and they are con nected, via fluid couplings, to the crankshaft, to which diey feed up to 600 h.p." Thus, the basic unit of die Turbo-Compound is a highly-developed petrol engine operating, as do most high-power piston aero engines, up to the limits permitted by the onset of detonation. It is, therefore, essential that any turbine fitted in die exhaust system should impose the absolute minimum back-pressure on die cylinders, in order to avoid accentuating the already acute detonation problem and possible difficulty caused by such con siderations as overheating of exhaust valves and reduction in volumetric efficiency. The Wright turbines are, therefore, of die blow-down type, in which die drive is effected by die impulsive velocities in the exhaust ducts. These velocities are continuously changing throughout each exhaust period and, moreover, are affected by r.p.m., boost and altitude. It is an inescapable con sequence that the turbine efficiency is lower than that attainable with a relatively steady-pressure gas supply and is also far more variable with engine operating conditions. In die Nomad, detonation problems do not exist, and the back pressure imposed on die cylinders can be so chosen as to give the best overall operating conditions. As a result, not only is die turbine efficiency significandy higher than diat of any blow-down unit, but it is also maintained over a very wide range of operating conditions. Thus, while die best specific fuel consumption of die Turbo-Compound is about 0.38 lb/hr/b.h.p., and is achieved only at the difikult-to-maintain optimum condition of speed, power and altitude, die Nomad can get down to about 0.327; and —as the performance curves show—it maintains figures below 0.35 over almost the entire practical range of conditions. These performance curves do not indicate turbine/compressor r.p.m., which vary between 19,800 in the "operational necessity at altitude" case, through the following: full power at sea level, 2,050 crankshaft r.p.m. and 18,200 turbine r.p.m.; recommended cruise, 1,750/14,960 r.p.m. at sea level and 1,750/19,600 r.p.m. at altitude (die latter showing the variation in gear ratio required of the variable-speed gear, 14,960 r.p.m. maintaining 43 Ib/sq in boost in the cylinder at sea level) and a flight idling turbine r.p.m. of 6,700, equivalent to 4 lb boost. At present, as the data table shows, the weight/power ratio is about 1.171b/h.p. Naturally, this is a poorer figure than can be achieved by a modern turboprop—but, quite apart from con siderations of fuel consumption, the turboprop is still invariably a full-throtde engine, and the Nomad begins to look more com petitive at altitudes above 20,000ft. At present, development is in hand aimed at still further increasing the output of die basic Nomad, for very little increase in weight. With water injection into the intake manifold, powers of the order of 3,580 eJi.p. are available, representing 87.2 h.p. per litre and corresponding to a specific weight of 1 lb/h.p. Still further increases in power result from the combustion of a small quantity of additional fuel in the exhaust manifold between die cylinders and the turbine, there being sufficient excess oxygen available to make this well worth while. With combined water injection and "reheat" the take-off power can be increased to no less dian 4,095 e.h.p., corresponding to values of 99.6 h.p. per litre and 0.93 lb/h.p. Performance of this order, coupled widi die Nomad's other advantages—such as its ability to burn not only diesel fuel, for which it was designed, but also wide-cut petrol, kerosine, and other fuels, and also its comparative insensitivity to temperature changes (it loses less than two per cent take-off power for each 10 deg C rise in ambient temperature) would seem to assure a wide acceptance of the engine in several fields. The next five years may, in fact, see this engine making reconnaissance patrols of unprecedented duration and, in commercial use, permitting freight and passenger fares far below the present I.A.T.A. standards. In fact, it is certain diat no other engine can at present match the economy of the Nomad—nor, in all probability, will any engine, until such time as an atomic power unit is developed. W.T.G. [An important lecture on the Nomad, by Mr. E. E. Chatterton, chief engineer of the Napier piston-engine division, is summarized overleaf.] These performance curves refer to the Nomad 2 (£145) at the NNm.6 development stage. The left-hand graph shows shaft horse-power and s.f.c.in I.C.A.N. atmosphere at 300 knots T.A.S.; the right-hand figure shows equivalent horse-power under the same conditions. iJBOO ifiOO 1,200 ipoo- 800 2OO0 -RECOMM&'OeD CRUISING 17SO cpm LOW OUTPUT CONDITION "T"—' 15QO \ rjxn. SJL. 5,000 tt,000 1SJM0 , X20P00 ALTITUDE, (ft) 2SO00 50PO0 10,000 HOOD 20,000 ALTITUDE (ft) 3,4°°| OPERATIONAL NECESSITY )50r.pnv 2 0-4Q- I 10.000 \$fxa zo.ooo ALTITUDE frt) 25,000 50p00 30,000 | 2^-35 •152 Orpm^ "T^i ,2,050 rpm. -3="H-1900r*nV| ,^0^. S.L. 5,000 40,000 <5,000 20.000 ALTITUDE (ft) 25,000 50,000
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