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Aviation History
1932
1932 - 0290.PDF
FLIGHT, MARCH 25, 1932 Vhe Industry A NEW ALL-METAL SHOCK ABSORBER STRUT •"• Jp-^HE case must be almost un- (^ II known where aircraft con- JL structors have obtained an improvement in design which halves both the weight and frontal area of a component. This is, nevertheless, true of the Shock Absorber Strut which has been developed during the last few years by Mr. G. H. Dowty. The struts are of all-metal construc tion, using steel spring suspension. The secret of the important reduction in weight and frontal area lies in two ingenious arrangements of the spring ing elements. In the first case the whole of the strut length is utilised for housing the springs, every inch of space is effectiveiy employed by coup ling, in parallel, two or three spring units placed vertically one above the other so that the sprung load is divided equally between the spring units. It will be seen that the greater the number of spring units employed the smaller will be the strut diameter. The number of units that can be used is, of course, limited by the pin centre length of strut and the spring travel required, but in no instance has it been found that the strut width ex ceeds one half that of other struts having a similar shock-absorbing capacity. It will be appreciated that the small diameter and the use of small fittings is largely responsible for weight reduction. In early shock-absorber struts, where steel springs were used, considerable loss in weight resulted from several reasons. The use of a single spring was not economical because of the large coil diameter and the low maxi mum stress which can be developed in a large diameter wire. This latter feature resulted in a heavy spring, but the weight was also seriously influ enced by the employment of a spring which, compared with compression rubber or pneumatic suspension, was too powerful. It is well known that the load de flection diagram of a steel spring is a 1200 1000 800 < 600 o •J 400 200 ^gf ^f R SPRINGS /7 , it j / / 10 20 30 40 50 DEFLECTION straight line, whereas rubber or pneu matic systems show a gradual building up of load, with a high deflection rate at the commencement of compression and a low rate at the end. It is evident, therefore, that a single steel spring does not compare favourably with rubber or pneumatic springs. The use of two or more springs, oper ating in series, one having a high deflection rate and the other a low deflection rate would be practically ideal and result in considerable weight saving. By carefully selecting the coil and wire diameter it is possible accu rately to reproduce the load deflection curve of compression rubbers or any other springing medium. The springing in these struts con sists of two separate arrangements. Two or more spring units are coupled together so that they operate in parallel and each of these units con sists of two or more springs, of vary ing capacities operating in series. The struts are manufactured with or with out oil dashpots, but in all cases the spring recoil is checked. To obtain an idea of the advantages, the charac teristics of several struts at present in use are tabulated below, and com parison with other types go to show that the claims advanced are justified. Static Load Pin Centre Maximum Strut Weight on Strut. (lb.) 800 1,700 2,000 3,700 Length. (in.) 27 65 42 56 Dia. (in. 1-625 1-875 1-875 2-25 Travel. (in.) 6-0 6-0 6-5 7-0 (lb.) 6 IS 13 23 Comparison of Load-Deflection Curve for Compression Rubber and Three Steel Spring of Varying Capacities, Coupled In Series. The photograph shows an illustra tion of a strut for 2,000 lb. static load and the section diagram the lay out of a strut employing three spring units, each unit consisting of two springs of different capacities operat ing in series. The effect of this latter refinement is given on the graph. In this instance three springs have been used and the close approximation to a rubber load deflection diagram will be noticed. Although these struts have only recently been placed on the market, seven aircraft manufacturers have stan dardised them, and arrangements for manufacture in several foreign coun tries have been made. Further particulars can be obtained from the manufacturers and sole licensees, The Aircraft Components Company, Grosvenor Place South, Cheltenham (Telephone: Cheltenham 3755). AUXILIARY ENGINES ALMOST all aerodrome authorities, " aircraft manufacturers and those connected with the aircraft-engine in dustry have occasion to use auxiliary engines for providing them with elec tric light or power. To these people the question must immediately arise whether it is better for them to utilise two-stroke or four-stroke engines. Petters, Ltd., of Yeovil, have for many years specialised on the two-stroke %*£ • The " Dowty " Strut in section and elevation. cycle type of engine, either as a paraffin engine or as a semi-Diesel and latterly also for their high speed atomic Diesel type. There is no doubt that the two-stroke heavy oil engine has many advantages. First of all there is the question of regularity, which is influenced by the fact that the four-stroke cycle engine has three out of four strokes negative, or, as it might be termed, unproductive, while the two-stroke has only every other stroke unproductive, since it requires only one revolution of the flywheel for a complete cycle of operations to take place. Another great advantage in the two-stroke type is the absence of valves and valve gear. It has always been admitted that the exhaust valve is the weakest link in the chain as regards a four-stroke engine, and this, in the case of high-speed engines, has invariably been a matter of the greatest concern to designers. The two-stroke cycle, therefore, enables the designer to produce an extremely simple engine, particularly when com pared with the four-stroke cycle type, and the experience of Petters, Ltd., has been that since abandoning the four-stroke cycle, the demand for spare parts has fallen to negligible propor tions. Yet another matter of im portance for engines used for this type of work is its ultimate life, and here the two-stroke type scores very con siderably. It has often been said that the scavenge of the cylinders in the two-stroke engine is not as perfect 266
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