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Aviation History
1914
1914 - 0019.PDF
JANUARY 3, 1914. [/OGED AERO ENGINES.* By GRANVILLE THERE is probably no form Of prime mover in existence that is more highly stressed, or that has a more strenuous life, than the Aero •engine, and there is undoubtedly no engine that has greater claims on n liability. These facts at once suggest that the aero engine requires greater accuracy of de ign, wider knowledge of the properties of the materials availab e for the construction of such engines, and the highest possible workmanship that can be obtained. It has been said that the modern aeroplane, although not by any means perfect, has already reached a stage of efficiency such that •extreme lightness of the power plant is no longer necessary to the evoluti m of the successful flying machine. This ha-, appeared in engineering periodicals on several occasions, and it is interesting to note that this form of argument invariably comes from those who have had little, if any, praciical experience in aeroplane design, construction, or piloting. The aeroplane manufacturer's cry for the extremely light engine is probably greater to-day than it has ever been in the history of .aviation. The demands of the authorities who purchase aeroplanes are such lhat probably so much as 90 per cent, of the factors which -determine the most successful machine, are governed directly or indirectly by the weight efficiency and fuel efficiency of the engine ; by the former is meant, of course, the number of pounds of weight for every horse-power developed. (That the engine shall be extremely reliable is of course taken for granted.) Amongst the essential features of all successful aeroplanes are the following:— 1st. That it shall climb very quickly. 2nd. That it shall have a good gliding angle. 3rd. That it shall have a combination of fast and slow flying speeds. 4th. That it shall be a machine safe to handle in all winds, both with and without the engine in operation. 5th. That it shall be able to remain in the air for long periods. Investigating these in the order just cited, we find that the first -essential feature, i.e., "that it shall climb quickly," depends almost eniirely on the weight efficiency of the engine. For our purpose, and because the design of the aeroplane does not enter into •our subject, I propose to assume the aeroplanes themselves to be of •equal efficiency in flight, and working on this assumption it is readily seen that the rate of climb varies directly as the power developed, and indirectly as weight to be lifted. That the aero plane shall be very efficient in this particular can easily be under stood when one remembers that its capabilities of evading destruction from projectiles depend to a great extent on how quickly it can get out of range of such projectiles. It must also be efficient in climbing in order to successfully rise from a small field sui rounded by tall trees, which may be necessitated by a forced landing during a cross country flight over a populous district. The second essential, i.e., " that it shall have a good gliding angle," or in other words, that from any given height it shall be able to glide for a great distance, is also governed indirectly by the weight of the machine, and consequently by the weight of the power plant, because a machine with a heavy power plant must be designed with a larger lifting surface, and must be stronger in proportion. Wiih the same lifting surface and head resi-tance, the angle of descent of the heavy engined machine wi 1 be steeper than that of the light machine, as higher speed is necessary in order to support the increased weight. The th rd essential feature, i.e., "that it shall have a combination of fast and slow flying speeds " is one of paramount importance, and one that aeroplane constructors are paying probably the greatest amount of attention to. Just as ihe fast battleship is infinitely superior to the slow one, so will the fast aeroplane in time of war be able to frustrate and limit the usefulness of the slow machine. To have a machine that will only fly at a high rate of speed, however, is almost useless, and at the same time extremely dangerous. For purpo es of alighting in bad weather on rough and unknown ground, a machine that will not fly at a lower speed than about 70 mile.- per hour is anything but desirable, such a machine being of course much more liable to be broken up when alighting. The capabilities of a machine to fly slowly as well as fast, depend almost entirely on the adoption of an extremely light and p iwerful engine. If the machine is designed for very high speed, slow speed is only possible by the machine, and consequently the power plant, being -very light. One cannot wonder therefore at the aeroplane con structor crying for a light engine. The fourth essential, i.e., "that it shall be a machine safe to * Paper read before the Scottish Aeronautical Society, on Wednesday, December loth 1911, at the Engineers and Shipbuilders Institute, Glasgow. E. BRADSHAW. handle in all winds both with and without the engine in operation," depends again to a great extent on the weight of the power plant. Aeroplanes have been built that will carry as much as from 15 to 20 lbs. per sq. ft of supporting surface, but constructors nowadays are agreed that the lightly loaded machine is the safer to handle, and the average loading on the planes is to-day generally in the neigh bourhood of 4 or 5 lbs. per sq. ft. A heavily loaded machine depends to a great extent on high speed of flight in order to maintain it in the air. Should the speed fall (unconsciously to the pilot) through loss of engine power, or from any other cause, tht control becomes sluggish, and will not answer quickly, the aeroplane, unless the nose is put down very quickly to increase the speed, flounders about like a log in the sea, and gener ally ends in a sideslip and one of those terrible nose dives that have deprived us of so many of our best pilots. The life of the pilot of the heavily loaded machine is more dependent upon the good behaviour of the engine than is the life of the pilot of the lightly loaded machine, and the latter could probably go on flying in search of a good alighting ground with two or three of the cylinders not firing at all. The fifth and last essential, i.e., "that it shall be able to remain in the air for long periods," depends chiefly upon the oil and petrol consumption of the engine, and without efficiency in this respect the extremely light power plant is practically useless, as flights of only a few minutes' duration are not likely to be of much use in serious warfare. All the essentials just enumerated, and particularly the last, depend, of course, on the engine being absolutely free from any breakdown, which point has not been dealt with, as it is not a debatable one. We are all without doubt of one mind on this matter. The foregone, to the writer's mind, proves that the light engine is not only desirable but essential. Competition demands it, the man who has to fly the machine demands it, and the nation demands it. There can be no doubt that the nation will be foremost in aviation which can produce the lightest, the most economical, and the most reliable aero engine. We occasionally read in periodicals devoted entirely to auto- mobilism, that if some of our car engine designers, who are so successful on the track and on the road, were to turn their attention to aero engines for a little time, and slightly modify the designs of their most successful racing engines, we should immediately have the perfect aero engine, and the aeroplane constructor's quest would be at an end. Indeed, there seems to be an impression in automobile circles that the aero engine designer is a freak inventor with most weird and unmechanical ideas of making the cylinders run round the crank shaft, or of building engines of peculiar shapes, more with the idea of giving vent to their ingenuity than with a determination to evolve a successful engine. As a matter of fact there is hardly a successful aero engine designer in existence who has not tried at some time or other the adoption of car practice to aeronautical purposes, and found in a remarkably short space of time that in designing aero engines he must first dismiss from his mind all thought of automobile practice. And really, when one comes to look at it in the light of the five essential features of a successful aeroplane, already dtalt with, the aero engine must be built on lines almost directly opposite to those of the present-day high-speed car engine. Just at the moment there appear to be three schools of thought in aero engine de ign. The first builds an engine of very large capacity, and arranges induction and carburation so that little brake hor>epower is developed for the cylinder size. By this means it is found that the -weight can be cut down to a greater extent than the horse-power, but at th» same time bearing surfaces are as large as if the engine were double the power, and consequently the life of the engine is very great. This type of engine must always be the most reliable, as it is like any oiher engine running on two-thirds throttle, and is at the same time easily kept cooL It is generally in the form of a rotary cy'inder, or radial stationary cylindered engine, with air- cooling, but it has its disadvantages. In the first place it uses as much oil as an engine developing the full power of the cylinder capacity, and if it is a rotary cylindered engine it often uses more. Secondly, it is like an engine running on about two-thirds throttle, and the petrol consumption is conse quently high per brake horse-power developed. It is also difficult to see just at the present how such engines will work in when high powers are required, such as say 300 to 400 horse-power. The size in this case will be considerable, and may lead to difficulties in fitting. 19
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