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Aviation History
1950
1950 - 0835.PDF
FLIGHT, 4 May 1950 543 problem and the changes introduced are shown in Fig. -2. If leaded fuel is used greater care is necessary in keeping flame-tube material at as low a temperature as possible to prevent lead attack. The lower cut fuels of A.N.A.58 variety also present further difficulties, due to aromatic content and carbon formation. We have encountered these problems on other engines and energetic action is under way to overcome them. We do not propose to release the Dart for any fuel'other than kerosene. (2) Nozzle Guide-vanes and Nozzles.—Operating tem- perature of the nozzle guide-vane is the limiting condition of the engine so far as power and specific consumption are concerned. The effect of flame temperatures on specific consump- tion is shown in the graph (Fig. 3). An increase of, say, 90 deg F (50 deg C) flame temperature is a prize well worth striving for. Our Fig. I. Resuhs of carbon-deposit accumulation on original type ^ of burner after '150 hours' running (left) and an anti-carbon type (below) after 500 hours. present limit is 2,034 deg F absolute (1,130 deg K) for take-off, and 1,890 deg F absolute (1,050 deg K) for maxi- mum cruise. These figures were established both from running on the Dart engine and using our Derwent ex- perience. Futhermorc, we have established our limiting temperature by running a complete model test at 2,105 deg F absolute (1,170 deg K max.) and 2,034 deg F absolute (1,130 deg K) for cruise. The nozzle guide-vanes were scrap after this test. At 2,034 deg F (I.I3° deS K max.) we have vanes which have completed 900 hours' running. Further development has shown that making nozzle guide vanes hollow and keeping as nearly as possible to constant metal section reduces the incidence of cracking and bending due to temperature shock. The next desirable step is to introduce air cooling, but this involves a rather complex problem. Increase in specific consumption and decrease in power due to the use of compressor air for cooling is indicated in the graph (Fig. 4). With the introduction of n.g.v. cooling -some increase in life can be assured at a price. If an acceptable compromise can be established, and this can only be done by operating experience, it jnay be possible to re- duce the metal temperature and still further increase flame tempera- ture. This would thereby restore specific consumption and improve n.g.v. life at a slight cost in b.h.p. The cooled n.g.v. is a project we are actively pursuing since its appli- Fig. 3 (Heft). Effect of engine working temperature on specific fuel consumption. F'g- 4 (right). Increase in specific con- sumption and decrease in power, caused by use of compressor air for cooling. cation to the turboprop re-acts in the manner described above, but on the pure jet it opens the possibility of increasing the rating as a war-time emergency or de-rating the material specification should the supply 61 strategic materials become difficult The method of intro- ducing cooling air through the hollow nozzle guide vanes is illustrated (Fig. 5). If air cooling drops the vane temperature by 180 deg F (82 deg C), which is then restored by increasing jet-pipe temperature, the effect is shown on Fig. 3. As regards materials for n.g.v., Vitallium has given the best results but supply in Britain is difficult, while Hastalloy C is showing promise and is almost equal to Vitallium. A good third is H.R. Crown Max, but this suffers from cracking due to temperature shock. I believe that concurrently with the introduction of cooling we shall be able to use a fabricated vane. This would considerably ease production as well as being cheaper initially and on overhaul. Experience to date shows that after 800-900 hours' run- ning about 5 per cent of the vanes have to be scrapped, mainly due to leading-edge cracking; these are all H.R. Crown vanes and not hollow. Nozzles are now very reliable, manufactured in Nimonic 75 with external air cooling and we do not anticipate any major expense on overhaul. (3) Turbine Blades.—Our present design of turbine blade has given excellent results as regards reliability, failures having been confined to accidental overheating and exter- nal damage. You will notice we use the shrouded type of blade—I mention this because while in England we have been busy putting shrouds on, 1 believe that in the U.S.A. you have been equally busy taking them off. Vibration characteristics of the blade have been put out- side the running range of the engine by shrouding; this in itself is a good reason for adopt- ing them, but the main reason is one of performance. The latest design of blade is shown in Fig. 6, where it will be noted the root has been extended. The reason is to keep the Nimonic 80A material for the hot part of the turbine and reduce the disc \ Fig.- 2. Sectional elevations of original type of burner (above) and redesigned type to overcome carbon-formation problems. 1,600 TURBINE 1.8OO 2.OOO 2.2OO ENTRY TEMPERATURE-°F, ABS. 1O 2 O 3O - INCREASE OF HIGH PRESS. COOLING AIR -V. ENGINE INTAKE a
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