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Aviation History
1931
1931 - 0516.PDF
40 SUPPLEMENT TOFLIGHT MAY 29, 1931 THE AIRCRAFT ENGINEER the covering is corrugated, this, of course, occurs auto- matically. The question of badly-formed rivet heads is frequently raised by inspectors, who are countered by " knowing " foremen claiming that, since rivets are only used in TECHNICAL LITERATURE SUMMARIES OF AERONAUTICAL RESEARCH COMMITTEE REPORTS These Reports are published by His Majesty's Stationery Office, London, and may be purchased directly from H.M. Stationery Office at the following addresses : Adastral House, Kingsway, W.C.2 ; 120, George Street, Edinburgh ; York Street, Manchester ; 1, St. Andrew's Crescent, Cardiff ; 15, Donegal! Square, West, Belfast; or through any bookseller. shear, the head shape is immaterial. Whilst this is to some extent true, it does not go the whole way. A half-formed head (Fig. 14) indicates that inefficient or insufficient hammering has been applied to it, with the result that the shank of the rivet is not properly ex- panded and pressed home into its hole. A single rivet in a long row badly formed may not be of importance, but one should suspect the joint if there are several thus. FIG 14- Hand riveting is expensive and often unsatisfactory. There is more possibility of damage to the surrounding plate, and the heads are frequently poor. Pneumatic riveting is satisfactory when carefully carried out. The tool is of the kind used in shipyards for light caulking. But the best workmanship is obtained from a single pressure machine, such as was illustrated in FLIGHT (THE AIBCHAFT ENGINEBH), October 25, 1928, page 936. This implies bench work, and is suitable for spars, fuselage fozmers and hull frames. It csannot be used in attaching skin plating to internal structure, although there are cases where large panels of plating may be put together before erection. Aluminium-alloy rivets should not be used in the annealed condition. Not only will they never develop their full strength, but they are liable to induce corro- sion in the surrounding plate. The usual procedure is to anodically coat them first and to follow this by full heat troament to 480° C. + 10, and quenching. If used within an hour, they are soft and pliable, and will age-harden to their full strength later. It is argued that in hull and seaplane work the rivet heads are the corrosion danger points, particularly as the hammered-up ends cannot have an anodic surface. A method of overcoming this fault, which is employed by Messrs. Saunders Roe, Ltd., is to hammer up on the inside of the hull, at least up to the waterline. This is only possible on hulls with an extremely accessible internal structure, and there are some points where it cannot be done. The principle may be applied in rivet- ing together panels of shell plating before erection. Whilst the obstruction to the air flow caused by the rivets may be comparatively small on large machines, it becomes of importance on the fuselages and main planes of small fast craft. In such aeroplanes the skin is usually too thin to allow of countersinking, and a satisfactory solution has yet to be found. Where the skin is supported by a robust member, this may be countersunk and the skin forced down into the hollow, causing in effect a countersink on the outer surface. But it is only a partial solution. Elsewhere the heads may be made flat instead of domed, a process difficult to carry out without cracking the edge of the rivet head. The aircraft industry awaits a really neat method of achieving a smooth surface to a thin riveted shell. FUBTHER EXPEBIMENTS ON THE BEHAVIOUR OF SLNGLB CRYSTALS OP ZINC SUBJECTED TO ALTERNATING TOESIONAL STRESSES. By H. J. Gough, M.B.E., D.Sc, and H. L. Cox, B.A. Work performed for the Department of Scientific and Industrial Research. R. & M. No. 1322 (M. 68). (20 pages and 9 diagrams.) August, 1929. Price Is. 6d. net. These experiments orm part of a lengthy investigation into the theoryof fatigue and relate mainly to experiments on single crystals of various metals. From the results of a previous experiment' on a single crystal ofline, it was concluded that the formation of twins in zinc occurred on planes of the 1012 type and that the particular operative twinning pjane (of the«lx available) was determined chiefly by the direction of slip on the originalt basal plane and possibly, to some extent, by the relative magnitudes of thenormal stresses on the possible twinning planes. In this previous experiment the orientation of the crystal was such that slip on the original basal planeoccurred in one direction only and one pair of complementary twins only was observed. From the results it was predicted that if a test were made on a crystal ofBUitable relative orientation of the crystallographic and straining axes such that all three slip dirertions became operative then the opeivtiri' twinningplanes should change with the slip direction. The present experiment was planned in order to test this prediction. Again, in the previous experiment,of the two possible pairs of complementary twinning planes associated with any one slip direction, it appeared probable that the choice of theoperative pair was influenced by considerations of normal stress on the twinning plane. The present experiment would, it was hoped, throw furtherlight on this aspect of twinning. The slip plane of zinc is the basal plane (0001); the slip direction is the mosthighly stressed primitive direction ; deformation by slip is controlled by the criterion of maximum resolved shear stress. The twinning planes of zinc are the six planes of the general type 1012. With the specimens employed in the present tests, due to the relativeorientation of crystallographic and straining axes, and to the type of applied stressing, three slip directions become operative in turn. The results showdefinitely: (a) One pair only of complementary twinning planes appear in the area associated with each operative slip direction, thus reducing thetotal of six possible twinning planes to three pairs ; (6) a change in slip direc- tion is accompanied by a change in the identity of the pair of operative twin-ning planes; (c) in any operative slip direction, the twinning planes con- taining that slip direction do not appear: (d) normal stress alone does notdetermine the choice of operative twinning plane. If conditions (a), (Mi (c) and (d) are fulfilled, only one sequence of operative twinning planes canresult, offering two alternative phases of the sequence. Both phases have been observed on different specimens. The important conclusion is thus reached that the occurrence of twins, uswell as slip bands, is controlled by the simple criterion of maximum resoiveu shear stress on the slip plane. • " Koy. Soc. Proc," A. vol. 123, pp. 143-167 (1929) and li. * "•No. 1183. t Associated with the initial structure of the unstressed crystal. AIRSCREWS FOR HIGH SPEED AEBOPLANES. i>y n- Glauert, M.A. Communicated by the Director of Scientific Research, Air Ministry. R. & M. No. 1342 (Ae. 474)- (18 pages and 7 diagrams.) June, 1930. Price Is. net. Since little is known of the characteristics of an airscrew at the his'a r*'^of advance which occur with modern racing aeroplanes, it 19 desiv.Mi examine theoretically the most suitable type of airscrew lor m°Ul:-ii-future high speed aeroplanes, to determine the efficiency ol these au *\1*" and to consider the possibility of improving the low static thrust which IMI»'<-accompanies a high pitch-diameter ratio. j Adopting a few simplifying assumptions, the characteristics of a «* * ^high pitch airscrews have been calculated, and these results nave »f. tto establish a relationship between the pitch-diameter ratio, torque ''>-" vd lidit f i erating near the state of maximum ci.jVJ ^ fficy 01 pand solidity of an airscrew operating near the state of m jJ Another approximate formula has been derived for the efficiency 0 • • (escrew, and these formula; have been used to determine the mosi• ••• type of airscrew for a high-speed aeroplane. A simple formula nas <-derived for the static thrust of a high pitch airscrew. The analysis suggests that the tip speed of the airscrew may ^'''ijLencythe moderate value of 900 f.p.s. and. using a suitable gear ratio, ai>> laIlfof over 80 per cent, may then be anticipated. For a modern raciut.' • f j ,ndthe most suitable airscrew is one with two blades of diam/.wr*l, of the it is necessary to use a gear ratio of the order of 0 • 6, but as the si^ ^aeroplane increases the need of a gear ratio will disappear »u " j^ of suitable airscrew will be one of smaller diameter with a larger u rf tMblades. The relatively poor static thrust is an inevitable con^eq" u- , tMhigh speed of the aeroplane, being directly proportional to t.h';/; rr(,,i«ctiveengine and inversely proportional to the speed of the aer°P'*™''«improve- of the diameter of the airscrew or of the gear ratio ; the only nope t»ment is to use a variable pitch airscrew.
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