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Aviation History
1917
1917 - 0515.PDF
MAY 24, 1917. In~the manufacture of aircraft engines more and more use is being made of aluminium alloys of varied analyses, each to suit some particular condition of work. With a continuance of advance in knowledge of heat treatment, and so forth, there is no gainsaying that this material will be employed for aircraft engine construction to a greater and yet more great extent. Aluminium Alloy Pistons. Under this head it falls to be observed that for about two years the author has standardised aluminium alloy pistons with excellent results. Even when they have been made with a green sand core no trouble can be said to have been ex- perienced with them. A point to note, however, is that the greater clearance needed when the aluminium alloy piston is cold represents a disadvantage in comparison with cast-iron pistons, and which has not yet been overcome. Special Alloys in Place of Gunmetal for Oil Pump. Another special alloy is being used in place of gunmetal for the construction of the oil pump employed for the forced feed lubricating system. These details are of the gear-wheel type. As regards both strength and bearing qualities the alloy employed has proved to the full as satisfactory as the gunmetal used formerly. Problems Presented by New Materials. Before quitting the subject of materials in general, at this juncture it will be convenient to pass the general situation in rapid review. Manufacturers have been called on to make immense efforts in the matter of supplying a wide variety of materials for multi-cylinder aircraft engines. They have met and mastered right ably the usual sequence of difficulties that materialise Whenever man attempts to break fresh ground. In particular, the high-tensile steel stamping now being supplied, for instance, for a six-throw crankshaft for a 500 h.p. aircraft engine, is a splendid example of the steelmakers' craft to-day. There would have been no call for it had not the advent of war made it necessary on a sudden for us to standardise in these islands aircraft engines of high output. Of course, the necessity for using unpre- cedentedly high-class materials for these constructions has presented alike steel makers and alloy producers, as well as the engine maker's machine shops, with a series of fresh problems which have had to be overcome detail by detail before it has been possible to obtain that degree of success which is necessary ere any given product can be regarded as a practical proposition. Briefly, there has had to be an all-round improvement in method ; more scientific control has had to be exercised and procedure elaborated ; while, of course, the last link with rule of thumb method has been broken in our factories by the coming of the standardised aircraft engine or proportionately great power output per unit. Obviously, there has been a call for devoting the greatest attention to detail, since it will not suffice merely to employ more expensive workmanship and higher grade materials. In regard both to design and to procedure, each part must be accorded, besides, greater attention to detail than any that has sufficed for car engine design and production to date. In what direction this extra study is needed is discovered, of course, by the laborious process of experiment. After that knowledge has been attained, when it comes to standardising practice in the shops, very special attention has to be given to avoid points likely to start flaws. At this stage these are a prolific source of trouble in the production of aircraft engines. Each man has to be trained to give the correct proportionof attention to the various details of bis job; therefore a more highly-skilled class of labour is needed. Your individual worker must know precisely what function his particular task and the part which he is engaged in fashioning plays in the scheme of the complete and, necessarily, at present somewhat complicated aircraft engine. Though only a matter of lightening, such details as boring parts, with which the car engine builder is not concerned, have to receive more than ordinary intelligent and conscientious attention during manufacture. Connecting Rod Forms. Of course,_ both the radial and the V-type aircraft engine have introduced problems of design, the solution of which does not appear to be unanimous yet. In these varieties of engine design you have more than one piston attached to each crankpin. The diversity of opinion concerns the means by which this is done. Naturally the design of eight and of 12-cylinder standard engines for car service, such as our American friends are producing in large quantities, does not call for the same amount of care in this regard that is essential in the case of the more heavily loaded aircraft engine. The original form was that in which the end of the subsiduary connecting rod is mounted on the outside of the main connect- ing rod. This, however, makes relatively a very heavy big end not suitable for engines from which is demanded the high duty necessary for aircraft service, as the wear of the big end is controlled by its Weight and oil pressure. A lighter construction is achieved by the link-rod method, wherein the subsidiary member is attached to the main one by a pin placed as near the centre of the crankpin as possible. This arrangement has the additional advantage of rendering is considerably simpler to take up any wear in the big end bearings. For obvious reasons, in the former type that operation is not a very easy one. The latter type, however, introduces a further problem. Owing to the centres of the bottom pins of the subsidiary rods not being coincident with the centres of the main pins, the movement of them is along an elliptical path in contradistinction to that of the crankpin, which, of course, is circular. The ellipse has the effect of altering the stroke of the piston attached to the subsidiary rod. The extent of the variation depends on the angular position of this pin with the centre line of the main connecting rod, the distance from the main pin to the rod pin remaining constant. The Lesser of Two Evils. It will be readily appreciated that the length of the piston travel can be corrected by tilting the axis of the ellipse in such a manner as to give the precise stroke desired. Owing to its change of velocity, also to its greater stress on the main rod due to the reversal of that rod during the firing stroke, this would have the further effect of altering the acceleration of the piston attached to the link rod. In going into this question the author came to the conclusion that the alteration of the stroke was the lesser of two evils. . The design adopted in certain radial engines, in which gear-wheels are used to maintain the correct position of the link-rod pin, is a very clever method of overcoming these difficulties. Nevertheless, it makes for relatively heavy construction and tends to provide a further cause of mechanical failure. In the case of the three-row 18-cylinder 500 h.p. Sunbeam- Coatalen aircraft engine, the design of connecting rod arrange- ment in regard to the link pin details is such that the centre row of pistons that are attached to the main rod of each series have a travel of 160 mm., while each of the pistons of the two side rows of cylinders has a stroke of 168 mm. As it is essential to employ as light a connecting rod arrange- ment as possible, the question of loads on the bearings becomes of great importance. Despite the fact that one engine of great power has been designed with balance weights and has certainly given satisfactory results as standardised, and that, of course, these can be used in car practice, nevertheless such accessories arc practically precluded in the case of the latter-day aircraft engine on consideration of the weight per horse-power of the given engine. Lubrication. As has been indicated, lubrication is prominent among the matters that illustrate the difference between car and aircraft engine design and practice. While everything possible is done to reduce the bearing pressure in the aircraft engine, neverthe- less in practice it is found that the best design is one in which less bearing surface is allowed than obtains in the case of the car engine. This is rendered practicable in the aircraft engine by giving more careful consideration to the problem of lubri- cation. Thus a pressure in the oil circuit up to 100 lbs. to the square inch is being used. This ensures an excellent condition of bearings, while the design of the oilways in the bearings themselves is such as to prevent excess of lubricant getting to the pistons. Because an aircraft engine has to carry its supplies of lubricant aboard the machine, obviously it is only less desirable to ship an excessive amount of oil for any given flight than it is to carry too little. Moreover, the fact that the engine bearing pressure is much higher than in motor-car practice, and as much more heat is generated and has to be absorbed by the film of oil on which the lubricated surfaces are floated, renders it impossible to carry the oil in the base-chamber, as in ordinary motor-car engine practice. In aircraft engine design you aim at achieving the maximum economy of lubricant consumption in combination with maintaining effective pressure in the system by preventing the oil becoming too hot and thereby losing its viscosity. Therefore you design an aircraft engine with what is styled a dry base-chamber, placing the oil supply in a tank set somewhere else in the machine, probably in the slipstream of the propeller, so that it may be exposed to the maximum draught and its contents cooled as rapidly as possible, to be served again to the engine SIS
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