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Aviation History
1965
1965 - 0036.PDF
22 AEROENGINES FUCHT International. 7 lanuory 19a for Advanced Subsonic Transports BY L. G. DAWSON, BSC* THERE is no doubt that a large amount of future passengerand cargo business will be carried subsonically. From thepassenger's point of view, the most important feature of future air travel is lower fares in relation to his or her wages. For many years the volume of fuselage which can be used efficiently on the supersonic transport will be limited. This will reduce the number of passengers which can be carried compared with the subsonic aircraft. At the moment, the chance of reducing fares seems to be greater with a subsonic civil transport. Air freight is of increasing importance. Safe handling and speed of delivery may accelerate the business more rapidly than the direct cost would suggest is likely. The predictions of subsonic passenger and freight business have encouraged us to study transport engines especially designed for this field. Estimated traffic growth is illustrated by Figs 1 and 2. With a background of four-and-a-half years of commercial operation of the Conway, we feel the service problems of the long-range transport engine are well understood. For several years, we have carried out feasibility studies of various cycles, by-pass ratios and new mechanical constructions leading to a new design of long-range transport engine. In assessing the technical factors which are important in long- range engines we are considering fuel consumption, engine cost and spare parts cost, and weight. Naturally the engine must be designed for long time between overhauls. In considering fuel consumption, the tacit assumption is usually made that ever increasing by-pass ratio is a good thing, but this is not necessarily true, and it might be as well to review the fundamentals of the subject. The fundamental qualities in the engine are related by the following equation:— Forward speed (ft/sec) Specific fuel consumption = 4,000 x thermal efficiency x propulsive efficiency To improve fuel consumption we can only improve the thermal and propulsive efficiency. The propulsive efficiency normally considered is the Froude efficiency. However, this is not the whole story; we should consider the effective propulsive efficiency which takes account of pod drag. This may be written:— Effective propulsive efficiency / Pod drag= Froude Efficiency. ^1- -^ Froude efficiency may be defined as:— 2+ ThrustIntake Momentum Drag For purposes of the argument this is a more useful form than the normal:— % ~~ \fo~T~Vj The intake momentum drag is:— lb/sec air consumed x forward speedg We write the effective propulsive efficiency as: -M1 2 2 + r thrustintake momentum drag pod drag intake momentum drag thrust intake momentum drag Chief engineer (projects), Rolls-Royce Ltd. This is illustrated in (Fig 3) overleaf by the graph for various values of pod drag against intake momentum drag. It will be seen that pod drag is of great importance. Various typical Rolls-Royce installations are shown for comparison. Consideration of installation drag leads one to aim for a value of thrust over intake momentum drag between 0.8 and 0.9. For aircraft which cruise at between Mach 0.8 and 0.86 in the strato- sphere, the corresponding value of the thrust/lb of air is about 20. Thermal efficiency may be improved by increasing cycle pressure ratio and, to some extent, turbine inlet temperature. Consider, as an example, a cycle pressure ratio of 20. If we plot, for a selected flight condition, specific fuel consumption against by-pass ratio for a constant thrust/lb of air, the curve shown in Fig 4 is obtained. Because the forward speed is fixed and the thrust/lb of air is chosen, the propulsive efficiency is constant. The change in specific fuel consumption is entirely associated with a change in thermal efficiency. Up to a turbine inlet temperature of 1,200° K and a by-pass ratio of two-and-a-quarter the thermal efficiency improves fairly rapidly. Beyond a turbine inlet temperature of l,200°K the improvement in thermal efficiency and specific fuel consumption is small. At constant thrust/lb of air, increasing the turbine inlet temperature and by-pass ratio reduces the size of the gas producer, and may reduce the weight of the engine, but the complication required at higher by-pass ratios is increasing, and the value chosen is a compromise between cost and weight. Fig / Estimates of passenger traffic growth until 1975 250 1961 ftQUS ROMSE 1965 \ SB!1882 \ y. ICA01960 I UNITED RESEARCH 1960 1SS5 T9K8 19*5 1970 19?S Fig 2 Estimates of freight traffic growth until 1975 res mis 800 KUU8X 196C 1978 197$
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