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Aviation History
1944
1944 - 2006.PDF
> • 344 FLIGHT' SEPTEMBER 28TH, 1944 Air-cooled Engines Possible Developments and Some Problems Likely to be Encountered in Large Power Plants : 6,000 b.h.p. Within the Next Seven Years By B; G. MARKHAM, M.A., A.F.R.Ae.S., A.M.I.A.E. IN his recent lecture to the R.Ae.S., Mr. Markham first examined the probable requirements for engines of increased power in the next seven years and then, after a short reference to the possible limitations imposed by the airscrew and a short historical note on developments in the 1920s, an. outline was given of the probable increases in specific powers during the next seven years, and subse quently, specific problems involved in the design of the larger engines envisaged. It was believed that if engines of the 3,000 to 4,000 h.p. class were available at the present time, they would be accepted by designers of single-seater fighters. Engines capable ot giving these powers on present-day fuels and b.m.e.p.s could, it was thought, be developed to give 4,500 to 6,000 h.p. within the next seven years, and this would probably meet the fighter re quirements for this period. Bombers in the present-day heavy- bomber class would, in the near future, no doubt use engines of about 3,000 h.p., and these engines would be developed to give a substantial increase in power, although possibly not to the same extent as those used in fighter aircraft. For very h_avy aircraft of the order of 100 tons weight, which was about the maximum which could prudently be designed with the know ledge and experience at present avail able, good use could undoubtedly be made of engines of the 5,000/6,000 h.p. class. The time taken to design and produce prototype aircraft of this size meant that such engines were, not required immediately. For aircraft of this class there would be competition from the development of coupled engines. That is, the use of twice the number of engines, each of half the power, coupled together and driving concentric contra-rotating airscrews. Such an arrangement started off with the solid advantage of using a form of power plant which could almost cer tainly be developed more quickly than the single high- power engine, and it should gain something in reliability. Coupled engines could almost certainly be more easily buried in the wings, and they might have some small advantage in reduced drag for this reason, although they would probably have to pay a small weight penalty. For aircraft of 100 tons there was also the possibility of using two pairs of 5,000 to 6,000 h.p. engines, and this would entail the development ot contra-rotating airscrews capable of handling 10,000 to 12.000 h.p. It was probable that the bulk of these engines would outweigh the advan tages in the smaller area of disturbed flow over the wings, but the possibility was mentioned as it presents about the maximum h.p. per airscrew likely to be required. The sizes of airscrews required for the powers envisaged were as follows: — AIR-COOLED ENGINES HLP 3,000 6,000 10,000 Up to 20,000ft. 13ft. dia. 6-blade con- tra-prop. 16ft. dia. 8-blade con- tra-prop. 20ft. dia. 8-blade con- tra-prop. Up to 35,000ft. • 17ft. dia. 6-blade contra- prop. 20ft. dia. 8-blade contra- prop. Two 22ft. dia. 6-blade co-axial contra-props (i-Tractor, i-Pusher). None of these was in any way impracticable, although that suggested for the highest power was unconventional and indicated the difficulty of dealing with very high powers at great altitudes. For aircraft with speeds approaching 500 m.p.h. these suggestions would have to be modified to avoid compres sibility effects. For speeds much above this figure, it was unlikely that the highest-power reciprocating engine would be able successfully to compete with gas turbines or jet- propelled aircraft. Soon after the last war. Mr. Markham continued, -dBk attempt was made to develop a number of large enghiej^: of about 1,000 h.p., and the best known of these was pro bably the Napier Cub. It is interesting to note that none of them was really successful, and the first engines pro duced in large quantities to give powers approaching 1,000 h.p. were the Bristol Pegasus and the Rolls- Royce Kestrel which were developed from the Jupiter and the Eagle respec tively of similar swept volumes which, in 1922, gave about 380 h.p. It is apparent that in the post-1914 war period sufficient allowance was not made for the possibilities of meeting the growing power requirements of air craft by boosting up engines of exist ing sizes, but in the period between 1922 and 1934. specific powers in creased by about 150 per cent. About a third of this was due to increased piston speed, and the rest to better fuels and supercharging. The improved fuels giving higher knock ratings under both weak-mixture and rich-mixture conditions which, it is believed, will become available in the next few years, may be used either to allow higher boost pressures to be used at existing compression ratios, or to enable improved speci fic fuel consumption to be obtained by the adoption of higher compression ratios. Influence o! Compression Ratio Apart from the question of fuel consumption, a higher*^ compression ratio obtained by increasing the thermal effi ciency simplifies the cooling problem, and this may be of considerable advantage in certain types of engines. In addition, since the size ot cylinder for a given power out put must be greater with a high compression ratio, addi tional cooling area can be obtained. These two factors may allow an appreciable reduction in the internal cooling drag of the engine. On the other hand, the greater bulk of the high-compression engine may increase the form drag and more than offset any reduction in cooling drag. It is impossible to generalise on this because on certain aircraft the diameter of the engine nacelle is determined by the space required for the retractable undercarriage and not by the engine diameter. On other installations, how ever, it may be possible to submerge a small highly-boosted low-compression engine in the aircraft wing and so obtain a considerable reduction "in drag. For the purposes of the following comparisons, it is proposed to neglect any change in drag due to change in compression ratio. It is not proposed to go into all the factors affecting fuel consumption, but it is impossible to discuss the probable increase in specific powers without considering the probable trends in the compression ratios which will be used. Fig. 1 shows the variation in fuel weight plotted against MR. MARKHAM is head of the j Technical Department, Aero - Engine I Division of the Bristol Aircraft Co. Ltd., \ and th/s article Is a r6sum6 of his paper j entitled "Possible development in the \ design of air-cooled engines with par- \ titular reference to the problems to be \ faced in increase in size" recently : read by the author before the Royal \ Aeronautical Society.
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