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Aviation History
1914
1914 - 0048.PDF
(/jJGHT) designers, and for the moment one cannot conceive how crank shafts of (literally speaking) paper thickness can be expected to do the work required of them. The secret of success in many cases is the absence of a flywheel, which latter probably introduces more stress in the average crank-shaft than anything else. An extremely heavy flywheel rigidly compels the crank-shaft to rotate with an exceedingly small variation in angular velocity, the stresses in the crank-shaft due to the acceleration and deceleration of the pistons being in this case considerably greater. « From this it would seem that an engine designed to run well with out a flywheel ofanv sort, excepting the light propeller fitted, saves not only the weight of the flywheel, but indirectly a considerable amount of crank-shaft weight. With regard to the ratio of connecting-rod length to stroke, this is a point of design in which the aero engine builder cannot allow himself to follow his own technical reasonings. To him the angularity of the connecting-rod has to be the greatest he can use with success, as a short rod gives him a much lighter engine, and it is no uncommon sight to see a connecting rod the length of which is only some 1*6 or 17 times the length of the stroke. In a 4-cylindered vertical engine he of course cannot so readily ignore the importance of a long connecting rod, but this type of engine is seldom used for aeronautical purposes to-day. The largest diameter cylinder that can be used with success as an air-cooled engine appears to hover in the neighbourhood of 100mm., although in some of the rotary motors even larger than this is used with success. Air-cooled stationary cylindered engines, with a bore in excess of 110 mm., seem to have a desire to run hot on occasions, and to continue firing spasmodically after the ignition has been cut oft. Thus, when high powers are required, it is not unusual to see engines with cylinders amounting up to twenty in number. Trouble does not appear to arise in tuning up eight or even twelve cylinders, but one would imagine that to detect an occasionally missing cylinder out of 20, would necessitate a more than ordinary knowledge of that particular engine. If air-cooling is in the future used for very high powers, it is more than likely that long ftroke engines will be used in preference to engines with an abnormal num ber of cylinders. The question of bearings and lubrication of aeronautical engines is one of the most important in the advance of aviation to-day. Automobile engines running most of their life under light load, with their high quality white metal and highly efficient oil circulating systems, have given engineers an altogether too favourable impres sion of the reliability of such bearings. Under continuous loads of a high order, connecting rod big end bearings in particular show excessive wear on the inside of the crank, and a momentary stoppage of the oil system entails a complete overhaul, and the fitting of new white metal. A reliable lubricating system is excellent in its way, but to have to risk the ruination of a valuable engine by a small piece of dirt or an inefficient gauze is quite sufficient to warrant our looking out for a system that offers more chance of success. It is, indeed, surprising that car engine designers have not shown more enterprise in the matter of bearings, and have, one and all, concentrated on trying to improve s syitem which can never hope tf> reach perfection. A network of tiny oil passages, designed to supply even the remotest bearings in the engine with the correct amount of oil is not, one would think, finality in bearing design. Even the motor cycle is tome two or three years ahead of the most luxurious car engines in this respect, and although car lubrica tion systems were tried, other things besides economy taught that the best way to achieve success is not to try to induce the bearing to perform Us duty by fitting a better method of lubricating it, but by looking round for a bearing that does not require a lubricating system at all. It is interesting to note that one of the most successful road racing cars of this year has the crank-shaft of the engine running on a number of ball bearings. These last mentioned, however, are not ideal for an aero engine, as their weight for the load they will carry is certainly high, and they are not altogether free from possible breakdown or deranee- ment. A much improved bearing, and one which promises every success in the future, is the roller bearing. These bearings have, of course, been known in different forms for years, with apparently little success, chiefly because long rollers were used which had a habit of twisting out of parallel with the axis of the shaft, and materials of the quality we have at our disposal to-day could not be obtained. In these early roller bearings wear used to take place through the rollers binding and skidding on the shaft, and serious end thrust was obtained through the rollers Incoming tapered. The rollers of to-day which have a length equal to their diameter, seem to behave much better than any other form of bearing in use. The author's experience of roller bearings in the connecting rod big 48 JANUARY 10, 1914. ends of engines running up to 6,500 r.p.m., convinces him, after trying every other type, that the roller bearing has a great future. The keynote for aero engineers is undoubtedly simplicity in every thing possible. An engine may be designed to perform a certain duty, and made to do its duty by the addition of many extraneous fittings and appliances, but if another engine can be designed to perform the same duty as well, without the extra appliances, the Uist mentioned will inevitably be more successful and more reliable. Bolts and nuts are now looked upon as very useful and necessary details at one's disposal, whereas there can be no doubt that, although unfortunately often necessary, they are the bete noir of tne designer, as well as the manufacturer. , , -, The whole design of an engine may be altered by mechanics of different muscular strength assembling it, for what designer can tell the amount of initial stress that will be placed on a bolt or important part, and what amount of strength is left in the bolt or part to perform the duty it is intended to perform ? Two or three aero engines give evidence that the designer already realizes that the use of initially stressed parts is not the way to obtain the maximum amount of strength with the smallest amount of material, and the engine that uses the smallest number of bolts and nuts, and is designed so that all parts where possible are inter locking, and not stressed except by the work they have to do, stands- a better chance of success. As we cannot, to-day, have engines of absolute simplicity, the next best thing to do is to leave nothing to chance. Every possible or even conceivable risk must be removed, every part examined microscopically if necessary, and the mechanic impressed with the importance of properly securing every single bolt and nut on the engine. A thorough knowledge of the properties of every kind of material used in the manufacture of the aero engine is most essential, not particularly as regards the composition of the materials, as this is possibly better left to the specialist, but a knowledge of how each class of material behaves under stresses, its actual tensile strength (not that often stated by the manufacturers), its yield point, which is even more important, its percentage of elongation, and its resiliency. Steel of 30 or 40 tons per sq. in. ultimate tensile should never find a place in an aero engine, when steel of from 90 to 120 tons tensile, with 85 to 115 tons yield, and 15 to 9 per cent, elongation respec tively, can be obtained every day. The greatest care should be exercised in using case-hardened steel for stressed parts. After case- hardening the elongation is generally nothing at all, or at the most one half or one per cent., and as in aero engines these parts have to be so light they will not stand shocks or bending loads. The system of filling up the sump with oil and using the same over and over again is not, from the author's experience, highly satisfactory. It seems to take as long as from five to six hours on full throttle for the average water-cooled engine to attain its maxi mum heat all over, after this point the difference in temperature between the engine and the surrounding air is such that no increased heating takes place, no matter how long the engine is run within reason. * The quality of the oil meanwhile has become impoverished, and it is too much to ask the engine, once it ha§ attained its maximum heat, to continue running on oil that is nearly at boiling point, and that has lost its lubricating qualities. On the other hand an engine extravagant in oil, unless some method of draining the oil away is used, becomes carbonized in the cylinder and valves get gummed up necessitating constant overhaul. Valves and valve gear (with perhaps ignition) constitute the most unreliable parts of almost every aero engine, and there can be not the slightest doubt that an engine without valves will quickly supersede the present type as soon as such an engine can be found. Sleeve valves do not offer a solution to the problem, because they necessitate increase of weight over present types, whilst decrease of weight is the most important factor. The two-stroke cycle principle does not appear to be receiving the attention it deserves, and because it is already successful in that it has very few parts, will actually run, and is already a commercial proposition, one cannot help thinking it ought to be developed more, despite the enormous amount of natural prejudice against this type. ® ® ® ® AERONAUTICAL SOCIETY OF GREAT BRITAIN. Official Notices, Meeting.- The fifth meeting of the forty-ninth session will be held on Wednesday January 21st, at 8.30 p.m., when Sir Alfred Keogh, -\. j ' ' P,eside- Mr- Leonard Bairstow, A.R.C.Sc, will read a paper, to be followed by a discussion on " The Stability of Aeroplanes," illustrated by experiments with paper gliders. Members are reminded that, under the Rules, they may introduce visitors to General Meetings. Tickets for visitors, not introduced, may be obtained from the Secretary, 11, Adam Street, Adelphi, W.C BERTRAM G. COOPER, Secretary.
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