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Aviation History
1926
1926 - 0529.PDF
JULY 29. 1926 67 THE AIRCRAFT ENGINEER SUPPLFMENT TO FLIGHT It is unlikely that parts of such thickness will be employed that the strength falls helow 20 tons after the final heat treatment of the article. During the heat treatment of Duralumin certain changes of dimension occur. The heat treatment and ageing operations bring about definite structural modifications- in the alloy. It is usual for alterations in the structure of a metal to produce alterations in its density. In those metals in which the effect of heat treatment are completed suddenly the change of dimen- sions due to variation in density is usually obvious and easy to cope with, because at the completion of the quenching operation the change of density is also completed. This change of density is, of course, used very considerably in connection with the hardening of steel tools and dies, and in the heat treatment of case-hardened steel parts. In Duralu- min the structural changes that occur take place during both the quenching operation and the ageing period, and as a result they are more gradual. For this reason it is obvious that ihe final accurate machining of Duralumin parts to shape should not be carried out until the metal has aged completely. On the completion of ageing the density of the metal will have become stable, and no further change in dimensions is, there- fore, likely to ensue. Where the parts are of considerable dimensions, and particularly when they are also of a complicated shape, the influence of mass, as indicated above, once again comes into play. Naturally, if some portions of the article are fully hardened, whilst others are not fully hardened, the dimensional changes which occur in the metal will be different in the two portions. As a result the article as a whole is likely to warp somewhat during the ageing operation, because of the strains set up as a result of the varying dimensional changes produced in different portions of the article. F.ven if warping does occur, the forging is likely to be left in a state of strain. This strain may demonstrate its presence by causing a further distortion of the article when it is machined. The machining operation disturbs the equilibrium of the stresses. In all probability it removes the fully hardened and the partially hardened material to an unequal extent. Consequent upon the alteration of the equilibrium of the stresses, the strains in the different parts begin to operate and to bring about some warping or change of shape. The changes of dimensions that are referred to are necessarily of quite a small order, but in a large article they may accumulate to be sufficiently great to be noticeable, and to cause a certain amount of difficulty in securing accurate fitting after the final machining of the article. It is very desirable, therefore, that in all cases complete ageing shall be allowed so that as far as possible the material has reached its stable condition before any of it is removed in the machine shop. (To be continued.) toa S u o a,c/a •g CO £ o PM•o ere •aa> vo, C/3 a a>'3 £w S as a AIRSCREW TIP SPEEDS By R. K. PIEKSOX The effect of the tip speed of an airscrew upon its efficiency has been given little serious consideration until within the last few months. Owing to the tendency of designers of aircraft engines to increase power by increasing the revolutions and by dispensing with gearing, owing to its various dis- advantages, i.e., increase in engino weight, and efficiency loss, the need for reliable full-scale tests upon airscrews with high tip speeds is becoming acute. Very little experimental work has been carried out. and the only work published appears to be R &. It 884, giving the results of tests upon a model airscrew running at tip speeds up to the velocity of sound in air. Mention is made in this report of full-scale tests upon a DH.9a aeroplane, fitted with a Napier " Lion " engine with special gearing, but the results obtained are of very limited value. From recent full-scale tests of airscrews with high tip speeds, it would appear highly desirable to keep tip speeds down to 850 ft./sec, both on the score of efficiency and noise. 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