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Aviation History
1925
1925 - 0118.PDF
FEBRUARY 26, 1925 LIGHT AEROPLANE ENGINE DEVELOPMENT THE paper under above title read by Lieut.-Col. L. F. K. Fell, before a joint meeting of the Institution of Automobile Engineers and Royal Aeronautical Society, on February 19, proved a very interesting one, and produced a valuable dis- cussion. Colonel Fell commenced by saying that it had frequently been stated and written that in order to popularise light aircraft, the first essential was the production of a reliable engine capable of being easily maintained and having a long life, at the same time selling at a low figure. The first part of the paper was then devoted to an attempt to show that this ideal is difficult of attainment, and the second part dealt with a consideration of the various types of engine and their claim to adoption. Difficulties The lecturer stated that the public, and even aircraft designers, had been misled as to the type of engine that was really required by statements made in the non-technical and semi-technical press to the effect that it was possible to fly an aeroplane satisfactorily with a motor-cycle engine. At this stage, he said, it was desired to state quite definitely that this was impossible, as figures which would be given later clearly indicated. The method of rating on capacity had caused misapprehension, and the statement had been made that a complete motor-car with an engine of 1,100 c.c. capacity could be purchased at the same price as a light aeroplane engine of similar capacity. It was, however, the normal power that an engine could maintain indefinitely which had to be paid for, and if viewed in this light the light aircraft engine as we knew it today was not particularly expensive. A table was then given showing the horse-power taken from the engines in the last Lympne competition, the average figure for the engines considered being 34 b.h.p. This was about three times the normal for a motor-car engine of similar size, and compared favourably with the best Brook- lands efforts of engines of similar capacity. Considering that during the competitions certain engines were running at powers fully 50 per cent, in excess of their normal makers' rating, it was not surprising that unreliability was experienced Requirements of Light 'Plane Engines The lecturer divided the requirements of satisfactory light aircraft engines into two heads : (A) Reliability and (B) Light weight in working order. (A) could only be obtained as a result of careful design and laborious and expensive testing on the bench, while (B) was actually more difficult to attain in light aircraft engines than in large ones, for the following reasons :—Though it was true that a slightly higher horse- power per urit of cylinder volume was permissible with the small engine certain parts, such as cylinders, had to be made thicker and heavier than was dictated by stressing, in order to obtain the necessary rigidity. The use of higher horse- power per unit of volumne was limited by propeller speed unless gearing was introduced, which was undesirable, as it put up the cost and introduced vibration unless the engine was multi-cylinder or fitted with a flywheel. Colonel Fell gave a table of brake horse-power per cubic inch taken from a number of engines, ranging from the Napier " Lion " to the Blackburne and Cherub engines at Lympne, and these indicated that in the case of the Blactburne, when run at 4,000 r.p.m., the brake horse-power per cubic inch was nearly 1.8 times that taken from the Napier "Lion " Series II. The power/weight ratio of small engines was worse than that of large engines, and the lecturer stated that not one of the light aircraft engines approached 2 Ibs./h.p., and were mostly around 3 lbs./h.p. The small engine was at a dis- advantage, in that accessories formed a large proportion of the weight, and could not very well be reduced in weight. On the question of carburettors, the lecturer thought alti- tude control was a necessity, and to get good results the finest workmanship was required. Dual ignition was essential, and merely to provide a magneto firing two plugs were insuffi- cient. For correct lubrication at high speeds without over- oiling a positive dry base system was indispensable. The design and manufacture of big-ends for a radial engine was probably the most serious problem to be solved in this type of engine, and was the limiting factor in its development. Finally, the light aircraft engine was a high efficiency engine, comparing as far as motor vehicles was concerned, with the rack racing engine only. The most thorough design, the finest materials, and the highest class of workmanship only could, therefore,', be employed. Choice of a Type, and Design Generally On the method of rating, Colonel Fell then repeated his opinion that rating by capacity was definitely undesirable. The choice of cylinder capacity should be left to the designer, and should only be settled by him after selection of the type of engine he wished to build. It was, he said, safe to say that had the 1,100 c.c. rating not been a necessary condition lor the engines to fulfil in the Lympne competition, two- cylinder engines would have been produced with larger bore cylinders running at lower speeds, and having a very much greater reliability at a negligible increase of weight. " In the lecturer's opinion the normal ground level horse-power and propeller speed capable of fulfilling the duties of the various classes of light aeroplanes should first be determined and laid down, and the engine builder thereafter be given a free hand to produce an engine of the best power/weight ratio he could at the horse-powers and speeds given, with no other restriction than that his engine must be capable of passing the Air Ministry's standard test of reliability. On the subject of air-cooling versus water-cooling the lecturer was rather in favour of the latter, and showed outline diagrams of a three-cylinder radial air-cooled, a flat twin air-cooled, and a four-cylinder in line water-cooled engine, in order to demonstrate the relative space taken up by the three The water-cooled was intended to be of the type known in the Air Ministry as an inverted engine, i.e.. mounted with its cylinders hanging down from the crank- case. It had been proved, Col. Fell stated, that the air- cooled engine was not necessarily lighter or cheaper than the water-cooled, and that the water-cooled could be produced to give considerably less head resistance than the air-cooled. In support of his contention the lecturer said that owing to the cylinder block construction the water- cooled gave greater rigidity and consequently less vibration. It was a simpler matter to make a multi-cylinder water-cooled, and higher speeds were therefore more easily obtained. These advantages were, he thought, sufficiently great to warrant serious consideration of the water-cooled, and, furthermore, there were in use on the track quite a large number of water-cooled engines capable of performances equal to the requirements for light aircraft ; these engine- would only require to be developed in a lighter form to make them suitable. The application of supercharging to light aircraft engines was suggested by the lecturer, but only in the form in which it is now used on the track, i.e., for the purpose of obtaining an output greater than normal for a given capacity. Figures were quoted relating to the increase in power obtained in the 1,500 c.c. Sunbeam racing cars by using the super charger. The power went up from 5(1 b.h.p. to 72 b.h.jv, or the b.h.p /W0 c.c. from 3.33 to 4-66. Turning to the question of types of engines suitable for light aeroplanes Col. Fell saw no particular merits in the Vee-tvrin type. Concerning the 180° flat twin, he said that from the point of view of simplicity of design and production this was perhaps the best type. Unless very strong mounting was used, the torque fluctuation was apt to result in damage to the machine structure Also there seemed to be a limit Outline diagrams showing space occupied by flat-twinand 3-cylinder radial air-cooled, and 4-cylinder in-line inverted water-cooled engines. 18
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