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Aviation History
1917
1917 - 0488.PDF
[/JJGHT) almost'as various as are the demands for special varieties of steel and of alloys. Moreover, they are likely to multiply with the lapse of time. Yet between the widest varieties we perceive the essential characteristics of demand to be uniform. This is a great gain alike to the designer and to the manu- facturer on the one hand, and to the Services on the other. It means that, when the right scheme of design is evolved, the least possible disturbance is caused in the given factory, though it be concerned with producing power plant of various size, weight, and horse-power output. Therefore the maxi- mum production can be attained, while the problems of management and repair are correspondingly reduced to the minimum. Interchangeability can be exploited to the maximum, and, once the mechanics and pilots have mastered the principles of whatever system of construction is in question, it is found that those principles are applicable to. all varieties of the given system of construction. When we come to systems of construction, again there can be no laying down of hard-and-fast rules, for the sufficient reason that the suitability or otherwise of systems are pre- determined by the demands of the aircraft constructor and the aircraft user, which demands are ever varying. Therefore what system may be the most suitable thing possible to attain satisfaction of the demand of to-day is not necessarily the principle on which to work for satisfying the demand of next year, or of five years hence. Thus, while we may make bold to criticise the suitability of this system or that to satisfy the insistent demand of the hour, we must have a care not to be drawn into the making of sweeping assertions about the practicability of any system of construction for the necessarily nebulous needs of the future. Probably it is little realised that if the aircraft engine designer had not to think of the means at present available to tho manufacturer, nor of the time factor, nor of those to whose tender mercies the standardised product will be submitted, he would produce very different designs to fulfil any given purpose from those he evolves to-day. Aircraft engine design resembles motor-car engine production in this particular, that it is all the time -a, question of compromise. The most successful designer is he who exercises the soundest judgment in weighing a hundred and one factors of the hour and who gives the shrewdest estimate of the relative value of each. Having thus striven to give a notion of what one might style the psychology of aircraft engine designing at this or any period, let us take some of the governing factors of to-day somewhat more in detaiL It is not deemed desirable, there- fore it is not proposed, to give in a paper of this sort particulars of any engines such as are being or are about to be used by the Services in this war. Instead, it is held to be preferable to review with mote or less detail the points that have reaeived most attention in the development of the design of the latter- day aircraft engine. The Three Main Schemes. In regard to the general arrangement of aircraft engines, there are several main types, each of which involves advant- ages as well as disadvantages. The business of the designer is to effect the best compromise possible to fulfil the particular class of service that is had in mind in scheming the individual engine. Of course, multi-cylinders are common to all types of aircraft engines. But the arrangement of the cylinder groupings and settings differs entirely as between one type and another. Doubtless the most generally favoured form is the V type with either 7 z or eight cylinders per unit, these being set in two rows on a common crankcase whereby one crank- shaft suffices because one crankpin serves for each pair of opposed cylinders. Undoubtedly next in order of importance is the radial type, in which the cylinders are set in one or more planes with axes radiating from the centre line of the crank- shaft. The two sub-divisions of the radial type of engine are the rotating and the fixed variety. What we may style the straight-line engine constitutes the third main type. In this four, more generally six, and, in a few examples, even eight cylinders, and 12, are placed in a line and are set vertically on a crankcase, the pistons and connecting rods acting on a crankshaft with one crankpin per cylinder in the orthodox fashion of motor-car engine practice. ' Inasmuch as each of these three types has advantages peculiar to itself, it follows that each is the most suitable so far available for some particular form of aircraft. For instance, the cross-section or wind resistance area per horse- power is least in the straight-line engine and most in the rotating radial type. This includes the loss of power necessary to rotate the engine. The fore and aft length of the engine, however, which is of great importance in some aircraft, is jeast per horse-power in the case of the rotating radial type MAY 17, 1917. and greatest per horse-power in the straight-line engine. Moreover, when the straight-line engine is water-cooled, as is generally the case, the rotating radial type gains a further advantage on the score of decreased weight per horse-power. Against this, however, the economy cf fuel and oil con- sumption which can be obtained with the straight-line water- cooled engine is appreciably greater than is possible with the rotating air-cooled type as designed to-day. Somewhere between the two contrasted types of engines as regards the problems of wind resistance and overall length is what is styled the V-type of motor, wherein weight per horse-power is lighter than in the straight-line engine, owing, of course, to the proportionately muchsmaller crankshaft size in relation to the number of cylinders employed. But if we consider the case of the air-cooled V-type engine, under the score of weight per horse-power, of course, it has to yield place to the rotating radial type. Umisual Types. ' Yet another type which I have produced and stan- dardised during the past year with highly satisfactory results is a development of the V-form of engine in which more than two rows of cylinders are placed on a common crankcase. The particular engine had in mind employs three rows, each of six cylinders, on a common crankcase, each crankpin being connected to three pistons by articulated rods. In this 18- cylinder unit the centre lines of the cylinder make, in relation each to the other, an angle of 40 degrees. This allows of a very good firing diagram. This type of engine is one that is considered very promising for units of very large power. As regards weight per horse-power, it has advantages over both the V and the straight-line types of engines. A further development of this design, in which the rows of cylinders are increased, brings us to the con- sideration of the fixed radial engine, which, in my mind, i<i one that has been sadly neglected. I feel that we shall hear a deal more about it in the near future. Several forms of these engines have been designed and made, but it may be said, in broad terms, that the success of them does not yet appear to be as great as we should be led to anticipate from consideration of the possibilities of this particular form of design. The question of head resistance might be raised in regard to this engine, in that, when many cylinders are used, the diameter of the projected area of the power plant is increased. Suitability of Fuselage Section. In the case of most single-engined aeroplanes or seaplanes a fuselage of circular cross-section is admirable. It can be made large enough to accommodate the fixed radial type engine without increasing unduly the head resistance of the machine. This is not so, however, in the case of multi-engined aircraft, in which the power plant units are placed away from the body of the machine. In these cases increase of head resistance above the minimum necessary for each power unit is the greatest disadvantage ; therefdre its avoidance is of vital importance. Hence for multi-engined aircraft the straight- line type of engine is the more suitable, particularly as the power per unit at present demanded by the builders of these machines is well within the compass of types that have been produced on the principle wherein the cylinders employed are set in a single row vertically on the crankcase. In the circumstances in which we meet in mid-campaign it is not possible to discuss definitely the size of engine which is most likely to be adopted as standard in the near future. A particular effect of the war on the evolution of aviation is the rapidity of the advance which has been and which continues to be made in the design and production both of aircraft and of engines for them. Compared with the average of enterprise in normal times, the amount of experiment that has been carried out in these directions during the last year or two is amazing, and the practical results obtained are correspondingly important. In the Sunbeam factory experi- mental work is held to be of vital importance, in that the discovery of anything that gives advantage over any feature of previous practice is essential for the improvement of the product standardised. Doubtless this accounts for the rapidity with which changes are made in detail of design, also for the fact that the whole question of design is vastly more in a state of flux than the lay mind imagines. Further, the experience gained by our aviators since the beginning of the war, together with the demand for the engineer to meet their ever-growing needs, have called for continuous evolution in the design of aircraft, all of which has inspired corresponding enterprise in regard to engine con- struction and production. To be concluded.) 488
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