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Aviation History
1917
1917 - 0487.PDF
MAY 17, 1917. divergent. Hence on the present occasion little further attention need be devoted to standard engine design for car practice. Suffice it to observe that to date the non-technical opinion of the buying public, which opinion is not to be depreciated altogether, has exercised a not inconsiderable and, on occasion, detrimental influence on the designer and manufacturer. It will be observed, incidentally, that the element affects the proposition of aircraft engine design scarcely at all, especially under the conditions which are beginning to govern the industry towards the conclusion of the third year of war. The Analogy of the Racing Car Engine. By contrast, there is another type of engine specially built as distinct from standardised, and which is fitted to a few motor chassis only each year in relation to the total number produced, because it is evolved and employed for racing purposes sole] '. Admittedly, in the beginnings of the motor industry the racing car of one year became the standard vehicle the succeeding season. With the lapse of time, however, racing became' so highly specialised that if the individual competitor was to enjoy any prospect of success I during the last four or five years the racing engine had become a proposition utterly distinct from those standardised for service or ordinary civilian motor vehicle uses. This point is proved by a summary of the main characteristics required of a racing car engine, and which we find are to a considerable extent identical with those needed for an avia- tion engine. Thus :—(1) Weight is of importance. (2) Cost is unimportant. (3) The amount of labour and the time necessary for production are matters of relative indifference provided the maximum output of horse-power is obtained for a given size of engine. That demand has led manufac- turers' to employ overhead valves, which are also used in aviation service and which so far have been employed com- paratively little in standard car practice, partly on account of the principle not being so quiet in operation as the side- valve system. Every part of a racing car engine must be machined. The connecting-rods are milled to the minimum section, and so forth. (4) Silence is of no importance what- ever. (;) The racing car engine does all its work at prac- tically full power, but the evenness of its torque has to be extended over very much wider ranges of speed than is needed so far in the case of an aircraft engine. From 1,600 to 3,400 crankshaft r.p.m. is called for in the former case, whereas in the latter the normal speed is 2,100. The last- named figure chances to be no less than 1,300 r.p.m. slower than the capacity of Sunbeam racing car engines. There- fore it will be appreciated that the engine for racing car service is submitted to bigger stresses than the present- day aviation engine ; but that this period of high stress in the case of the vehicle variety is much shorter than obtains in that of the aviation type unless, indeed, the car is being run on a track. Even in that event 12 consecutive hours is considered a very long spell, whereas in aircraft service that period of uninterrupted powej: output is held to be all in the day's work. \(6) Under the heading of flexibility the engine for your racing car must be more akin to standard car re- quirements than to those of aircraft service. This charac- teristic, therefore, works out as a disadvantage to the racing car engine. When employed on dry roads with efficient gears and so forth the starting torque mounts up to a high figure, whereas in the aircraft engine at starting there is no load on the propeller. It increases, roughly, with the cube of the revolutions. (7) The racing car engine resembles the aviation type in that a very high mean effective pressure has to be obtained with both. In some racing car engines it has amounted to 135 lbs. to the square inch, taken from the brake horse-power developed at the flywheel. (8) As the problem is power for engine weight and volume, and not silence and low cost, great freedom is allowed the racing car engine designer as regards piston clearances, valve timing, com- pression, largeness of valve area, strength of valve springs, and so forth, the opportunities in this connection approximat- ing much more to aviation than to standard car engine prac- tice. (9) The high mean effective pressure necessary, coupled with the fact of nearly all the work being done at full power, calls for lubrication methods quite distinct from standard car practice, albeit as yet these have rarely ap- proximated to that of aircraft engine practice, though the problems of maintaining pressure in the oil circuit and of keeping the temperature of the lubricant normal are common to racing and aviation engine service. (10) Comparatively large- horse-power is needed in the case of all engines for racing cars, the average being anything from 80 to 225 horse-power, therefore much more on a plane with the demands for aircraft service than with those for the touring car, the town carriage, or the utility motor vehicle. Lastly, outside influence, traceable in the case of designing the private car engine and the commercial motor vehicle one is scarcely, if at all, to be detected in those of the racing car engine and of the aircraft variety. The racing car type has been developed with almost amazing rapidity through various stages along the lines ofv maximum power combined with low, as distinct from minimum, weight, and with the utmost reliability, notably with a view to enabling the machine to be run for long spells without loss of power. We might, perhaps, complete our survey of the inter^ relationship of these three branches of motor engineering enterprise by adding that in the aircraft engine we have to economise weight considerably over the degree that will suffice for racing car practice, while economy of fuel and oil consumption are also more important in the case of the former than the latter. Admittedly, in the racing car engine those two features constitute a special and important factor, but not one that has had to be studied yet on entirely different lines to standard car practice. Accordingly, perhaps we might conclude not unreasonably by stating in general terms that one. stage in the development of the aircraft engine is represented by racing car enterprise as well as, perhaps, by certain sporting motor-boat engine work. Endeavours in these directions provided us with the data from which were designed the first engines evolved on lines to be of such efficiency as the present-day aircraft variety. In face of our being now in the third year of war, and therefore for the most part somewhat out of practice in the matter of racing car engine design, whereas the leading firms in the industry in Europe have by now accumulated much experience of standardising aircraft engines, though of recent years none of them have ever standardised any for racing service, it may be said that the data on which aircraft engines are being designed to-day derives wholly from cumulative experience of aircraft engines, and has ceased to depend in any way on racing car experience. Indeed, on the coming of peace, doubtless it will be found that the position has been wholly reversed from that which obtained before the war. In the future not a little of racing car engine design may derive from aircraft engine practice. Desiderata in Aircraft Engine Design. To approach the problem from the correct point of view, we must recognise that the outstanding desiderata in designing aircraft engines to-day may be summarised thus :-=— (a) Light weight,'combined with low fuel and oil consumption, per horse-power, (b) Reliability. If we can but attain those characteristics with units of not less than, say, 200 horse-pOwer—better still, if we can exploit them in units each up to 600 horse-power—then we can afford more or less to neglect other desiderata as being of minor importance. Nevertheless, happily we can already go a far way towards realising what we might style the minor desiderata, which at this period of the war include :—(a) Simplification to the utmost in face of these engines being placed, for the most part, in the hands of a great number of men semi-skilled in even flying and maintaining them, (b) Fool- proof as much as possible in that some of the most daring Service fliers have not either the temperament or the under- standing to spare the engines of which they are put in charge. (c) Accessibility in face of the frequent attention needed by all aircraft engines and of the fortunes of war rendering it necessary on occasion to replace the most vital parts, (d) Standardisation because for the first time in the history of motor engineering we are making engines of high output in series in place of about a half-dozen examples at a time, (e) Suitability of exterior form that the power plant may be accommodated conveniently in the aircraft and occasion the minimum displacement. Influences on the Aircraft Constructor of the Particular Employment. Thus there are strict limits to the diametrical size of radial engines, whether of the rotary or of the stationary type, which it is profitable to employ for aircraft work ; while in regard to the vertical, or to the V-shaped engines, the nature of the particular service to which each individual engine is to be put likewise imposes certain limits. In certain cases strict limits must be set to overall length of the engine, particularly at a time of war in the air, when, at need, it is essential to lose the minimum time in altering the flight path of the machine from a diving attitude to a very steepfclimbing one. Again, some sorts of aircraft call for the minimum engine head re- sistance, but are less imperative as to overall length ; hence the six-cylinder type would be suitable for such service, whereas the V-type variety would not be. In other words, at this period it is impossible to lay down any arbitrary rules as to any one type of aircraft engine being suitable for the needs of all aircraft service. Those needs are 487
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