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Aviation History
1924
1924 - 0023.PDF
JANUARY 10, 1924 ,vm WATER-COOLED AERO ENGINES UNDER above title Mr. A. J. Rowledge read a very interesting paper before the Institution of Automobile Engineers on January 9, 1924. The paper was one of considerable length, and space does not allow of giving it here in full. Some of the main points are, however, dealt with in the following summary : The lecturer pointed out that the aero-engine designer was not, in most cases, an aeroplane designer also, and that he often found it difficult to place the true relative value of the various points in his engine specification. With regard to the two main divisions into which aero engines fall, i.e., water-cooled and air-cooled, he thought it probable that both types would continue to be used, as neither had reached the limits of its possible development, and each had special characteristics making it especially suitable under certain conditions. For the water-cooled engine the lecturer stated that it could be claimed to have accomplished almost all the great performances in the air, and that it might be expected that it would be possible to build a faster machine with a water-cooled engine than with an air-cooled. With water cooling, Mr. Rowledge added, the temperature was under better control, and the problem of heating the mixture pipes and carburettor was also considerably simplified. Low weight per brake horse-power was an important point, and if the consideration could be taken far enough to take the weight of the whole power plant against the thrust horse power developed, the whole question of suitability would be fairly simple. The lecturer thought that the direct-injection heavy-oil engine was in a very early stage of development, but that if the system met with success it would solve many outstanding problems. Continuing, Mr. Rowledge said : " The question of economy in its various aspects, covering cost of production, running costs, and maintenance costs, requires more than superficial consideration, and has to be considered in relation to the complete aeroplane. It is of little use to have an engine that can be built for a low price per brake horse power and use a low-priced fuel if its performance is poor, and if it not only requires to be of much greater horse-power itself, but requires also a larger and more costlv aeroplane to perform the same duty." Taking the case of an aeroplane with a total weight per horse-power of 15 lbs. and whose engine weighed 3 lbs./h.p., the engine would have to develop 218 b.h.p. to carry 1,000 lbs. useful load for four hours. If the engine weight was increased to 5 lbs./h.p., thS engine would have to develop 374 h.p. and would need a larger machine. The dry weight of a water- cooled engine was, he said, smaller than the dry weight of an air-cooled engine of the same cylinder dimensions and arrange ment, but the added weight of water and radiators brought the weight up to a greater figure than that of an air-cooled engine. He claimed, however, that the water-cooled engine gave greater power for the same cylinder capacity. Variable Pitch Airscrews On the subject of variable pitch airscrews and two-speed gears Mr. Rowledge said : " Another subject that it is worth our while to investigate in considering the best type of aero engine, and one which will have a great effect on the type, is the provision of a variable pitch airscrew, or the alternative of a variable speed gear. Engine builders in the past have usually left the airscrew to the aeroplane designer, and pro bably each has been too busy on essential details to concentrate on this apparently difficult problem. It certainly seems unfortunate that the variable pitch airscrew has not made more rapid progress. The alternative, a two-speed gear, is at present being designed and constructed. A good two-speed gear does not appear to present a difficult problem, and it is probably easier to deal with than a variable pitch airscrew. If we regard the additional weight due to the two-speed gear as negligible, the result will be greatly improved take-off and climb, and better fuel efficiency when cruising, as the engine can be run at its most economical load. It is rapidly becoming the practice of civil aviation firms to run their engines as though they were on the low gear all the time. In this case the advantages will be better fuel economy, and a great improvement in reliability and engine life, owing to the lower revolution speed and higher brake mean effective pressure when cruising, but we cannot expect to improve the climbing speed as well. The weight per brake horse-power climbing can be reduced, say, 10 per cent., an improvement well worth while. Such a gear can also be used to maintain the speed of the airscrew when flying at high altitudes, although in this case, as the engine will have to run faster, it will be at some loss of reliability, and justifiable only as a military expedient." On the question of number of cylinders and bore/stroke ratio Mr. Rowledge arrived at the conclusion that a fairly long stroke, say 1 -4 times the bore, was desirable, while for engines up to 750 h.p. the 12-cylinder V-type was, on balance, preferred, although he considered that there was very little to choose between the 12-cylinder V-type and the triple-four type. Reference was made to the Report of the Safety and Economy Committee, in which it was suggested that the desirable airscrew speed should be based on a formula such as 20,000/ VbThTp., while in the regulations for the forthcoming French engine competitions it was stipulated that the airscrew speed must not exceed 32,000/v'b.h.pT; the following formula being used for obtaining the power at the airscrew :— Rower at airscrew = b.h.p. X (32,000/revs, -/b.h.p.) 01. This formula, he said, gave us something to work upon, but was unsatisfactory in assuming indefinite increase in propeller efficiency with decrease in revolutions, whereas it should give maximum marks for an engine driving the airscrew at "the most efficient speed, and deduct marks from engines in which the airscrew ran either faster or slower than this speed. The lecturer then added : " Difficulties of this sort always occur in drawing up competition rules, but it is evident that the compilers in this case were very anxious to encourage low speed. If a complete set of regulations for a competition for aero engines were drawn up by machine designers, it would be of very great value to engine designers when deciding their programme of future development without any competition ever taking place. " Taking as an example an engine of 500 b.h.p., having its airscrew mounted directly on the crankshaft, and running at 1,400 r.p.m., the power at the airscrew by the above formula is 501. If a reduction gear is interposed between the engine and the airscrew, and we take the revolutions per minute as 893 from the formula of the Safety and Economy Committee, then, using the same formula for power at the airscrew as in the first case, we arrive at a figure of 525 against 501 in the direct-drive case. These figures look curious, but evidently the framers of the competition rule were taking a high-speed direct-drive job as their basis." Cooling System Under this heading Mr. Rowledge stated that : " Water as a medium for transferring the heat from the cylinder-wall surfaces to the radiator surface possesses many advantages to offset the apparent simplicity of direct air cooling. The high specific heat of water means that it can absorb a large number of B.Th.Us. without its temperature rising unduly. It also prevents local hot spots in the combustion space, an important point assisting reliability and fuel economy. In addition, it enables the cylinders to be arranged in a suitable form for small head resistance due to the engine, and lends itself to body forms which give the pilot a good view, while the cylinders can be placed closer together, making the engine more compact, and, of course, lighter. The latest American racing machines are examples of how the head resistance due to cooling can be reduced to a minimum by the use of this system, a result that air cooling can never attain. The weight of the cooling system adds to the dry weight of the engine about 0 -6 lb. per brake horse-power for a long-range machine, including the weight of the radiator, radiator shutters, pipes, tanks and water. " The system has to get rid of from 20 to 24 B.Th.Us. per brake horse-power per minute. A certain amount of heat is used to warm the carburettors and mixture pipes, and this is not included in the figures to be dealt with by the radiator. The provision of an alternative source for the supply of the heat necessary for this purpose in an air-cooled engine is a matter of difficulty. " The water circulation is always maintained by a pump of the centrifugal type. The spindle of the water pump is in most engines grease-lubricated, but in the R.R. ' Eagle ' engine the lubrication of this part is satisfactorily effected by using the circulating water only for the purpose. The British standard rate of flow through the radiator is 15 gals, per 100 b.h.p. per minute, an ample quantity under all conditions. "It is probable that slight pressure loading of the system will be resorted to in the future to increase the efficiency by raising the maximum temperature that can be used, thus increasing the difference between the radiator and air temperatures.'' Crankshafts Stiffness was, perhaps, a more important attribute than strength in this important part. The complete stressing of a crank design was a very complicated piece of work taking 23
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