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Aviation History
1961
1961 - 0092.PDF
92 FLIGHT, 20 January 1961 Trends in Propulsion AN RAeS LECTURE ON ROLLS-ROYCE RESEARCH 0-2 0-6 08 10 BY-PASS RATIO Fig 7 (7e/t). By-pass optimization study; engine plus fuel weight for 1,500 n.m. Fig 2 (right). Increase in combustion temperatures of turbojet engines IN the introduction to the main lecture which was to be readbefore the Southampton branch of the Royal AeronauticalSociety yesterday, January 19, Mr Harry Pearson, chiefengineer (performance and research) of Rolls-Royce, said that it was trends within the next five to seven years and not the futureof aircraft propulsion which he intended to discuss. It had been found in America, said Mr Pearson, that conversionof existing engines to fan engines was a fairly economical method of increasing thrust, particularly for take-off. Also, take-off noiseannoyance acted as a catalyst in favour of the by-pass type. Almost overnight the Rolls-Royce by-pass engine was facedwith competition from fan engines with by-pass ratios of some- thing like 1.5. Rolls-Royce took leave to doubt the correctness ofthese conversions; they had always been of the opinion that at current operating temperature the optimum ratio was between0.6 and unity, take-off noise being improved at higher ratios. Specific fuel consumption diminishes as by-pass ratio increasesbut above unity the improvement is small and a higher ratio means a larger engine diameter. On installed six. alone 1.2 isthe maximum useful by-pass ratio, while above 0.6 powerplant weight increases rather rapidly, and for ranges between 1,500 n.m.and 4,650 n.m. (London - San Francisco) 0.8 is near the optimum. Comparing engines on a basis of noise levels, where a givencruise requirement has to be met and a given take-off is required, with a low by-pass engine take-off is achieved asa design requirement and the cruise conditions will be some- what throttled back. For high by-pass engines the cruise becomesthe design requirement, take-off being a relatively easier condition. The results of one study are shown in Fig. 1. Reduction of take-off noise by increasing by-pass ratio islargely fruitless because compressor and other noise will become a dominating source. At present operating temperatures andspeeds by-pass ratios above unity are not desirable. But as combustion temperatures are increased somewhat higherby-pass ratios tend to rectify the situation where jet velocities increase with detriment to both propulsive efficiency and noise. Other parameters besides by-pass ratio are changing withresearch and development. As compressor temperature rises or pressure ratio increases specific fuel consumption diminishes, butengine weight increases with additional compressor and turbine stages. If combustion temperature continues to rise at the presentrate higher pressure ratios are worth exploiting. Within seven years a ratio of 20 could be achieved in two-shaft engines. Supersonic aircraft act as a spur to an increase in combustiontemperatures and at higher by-pass ratios a worthwhile improve- ment in economy occurs; in seven years a cruising temperatureof about 1,35O°K could be achieved and take-off combustion temperature equivalent would be about 1,450 °K. By 1967 or so engines in the 10,000-20,0001b thrust class couldhave 18 per cent more take-off thrust than a current engine for the same airflow and 23 per cent more cruising thrust. Specificfuel consumption would be 3^ per cent higher. The engine would be about five decibels noisier so it seems desirable to operateat constant specific thrust and a by-pass ratio of about 1.5 to 1.6; noise and pod drag would then be unaltered, specific fuel con- sumption four per cent greater, and specific weight ten per centlighter. Development of noise reduction techniques—such as variable area devices within the engine—should make the nextgeneration of jets some 12 PNdb better than at present. For short-range jet aircraft simpler engines of higher fuel con-sumption but of reduced weight and cost are required. The future of turboprops remains in doubt; they are complexand military demand has not been great. Improvements in per- formance should come about by raising combustion temperatureand pressure ratio and from improvement in design and weight. Provided designers are not faced with designing smaller andsmaller engines combustion temperatures could reach cruising values of 1,350'K towards the end of the next decade giving aseven per cent improvement in range. These improvements applied to an engine of Tyne dimensions could give an increasedpower of 16 per cent coupled with a reduction in s.f.c. of eight per cent. Airflow could be increased by about 20 per cent givinga power increase of 39 per cent and a net gain in specific weight of 20 per cent, although the overall specific weight of engine pluspropeller would remain about the same. Supersonic flight presents the turbo-jet in its most favourablelight, overall efficiency at Mach 2 comparing with best power station practice.But if engine size is not to become prohibitive supersonic trans- port success turns on the ability to run engines at high combustiontemperatures. Apart from this and a bulky intake-control system the engine becomes simpler than current types. Reheat may becalled for if the transition height to supersonic flight is very great. Combustion temperatures 100-200 °C higher than those current(cruising temperatures of about 1,35O°K) will be necessary and blade material suitable for another 100°C must be forthcoming.This appears possible. The design of propulsion nozzles is not yet well understood; a variable nozzle of some type must be usedand an ejector nozzle has possibilities, but no satisfactory solution seems yet available. Airfield noise levels will depend on climbtechnique but although installed thrust is high, lift/drag ratio is low, and the height of a supersonic aircraft over residential com-munities will not be so high as a subsonic aircraft of similar power. Landing noise will also present problems but the variableair intakes that are needed (Fig 4) should help to solve this. No completely acceptable solution for VTOL transports hasemerged from propulsion studies. Separate jets allow power- plants to be chosen to operate in the most efficient manner buta lifting engine is an additional penalty. Low jet-velocity is mandatory for low noise level and more success may be achievedby designing round a propulsion-and-lifting duct system. For supersonic interceptor aircraft propulsion powerplants will prob-ably be capable of lifting the aircraft off vertically by powerplant rotation or jet deflection. With strike aircraft there is an advantagein the use of only one powerplant but consumption is impaired because the engine must be heavily throttled for cruise. The useof several special lift engines and a separate propulsion engine, on the other hand, has the disadvantage that a number of lift engineshave to be provided. The next four years should see important developments in this field. I O2 INCLUDING VARIATION IN POD DRAG -2O -10 O RELATIVE NOtSE-Dbl IO Fig 3 (left). Variation of direct operating cost and take-off noise with combus- tion flame temperature and design by-pass ratio Fig 4 (right). Variable wedge-intake control arrangement for a supersonic aircraft CRITICAL CONTROLTO PREVENT K INTAKE INSTABILITY
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