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Aviation History
1928
1928 - 0133.PDF
\ v; FEBRUABY 23, 1928 17 THE AIRCRAFT ENGINEER BvTtLMUMKT *• PLIGHT 130,0(10 120.000 110.000 100,000 90.000 60.000 a. 70.000 £ 60.000 Si z i~ 50000 +0,000 30.000 20.000 10.000 'S ^. [ ; ; : : ; : : : : \ >=; \ \ A B> -^. c \ \ THE \ \ \ \ ORE! FRON \ flCAL STRUT CURVES PORTUBEJ 1 STEELS HAVING YIELD POINTS A....65 TONS PER SQ. INCH " B...4O C. AV N IB On heshs Prom buiir up sh-uf has been Found rtiar* rhe res when plotted .lie slightly abc curv«A S > Ma s i^ ulfs ve "^•^ =^ DE «*^ —s : ; \ n TTT T j TTT T : \ \ 0 IC 20 30 40 SO 60 70 80 90 100 110 120 130 140 150 Fig. 11. comparison, therefore, may need revising as experience with these higher tensile tubes is obtained. Advances are, however, to be expected in the design and methods of manufacture of components made from steel strip. It is not suggested that the whole weight of 800 lbs. could be taken locally on the strip longeron section, but the same remark applies to the solid drawn tube. Provision for resting on trestles, lifting, etc., is easily made, and a fitting and method of attachment suitable for this is shown in Fig. 12. The above comparison is presented in as simple a way as possible. At the same time, the overall dimensions and externally-applied loads are such as might apply to a portion of the structure of an aeroplane of 4,500 lbs. gross weight or thereabout. If the investigation is pursued further, it will still be found to favour the strip construction, particularly in the matter of fittings for the attachment of equipment, control surfaces, cable guides, etc. ; the numerous " free edges " obviously lend themselves to this purpose. One such type of fixing is shown in Pig. 13. This is a tail plane spar attachment. Space does not permit of further illustration or description of fittings, but in general, a simple bent or flat plate is all that is necessary ; there is a sharp contrast between this and the machined fittings or clips with bolts that are common to tubular construction. While the writer believes that aircraft frames as described have only been built by the Bristol Aeroplane Co., yet descrip- tions and drawings of the various component sections have appeared from time to time; for instance, particulars of bracings made from two similar semi-circular channels joined together along their edges were advocated for aircraft more than 30 years ago ; similarly, drawings of longerons made from two parts shaped approximately as illustrated above have been published fairly recently, but such longerons have been shown discontinuous along their lengths, and it may be that this lack of continuity has been the reason for the abandonment of the method. Only one aspect of this construction has been dealt with : it may be possible in the future to describe further developments along these lines. allowance must be made for tube sockets, pins, fork ends, rivets, etc., exclusive of longeron fittings, the percentage increase in weight of the wired over the strip frame is found to be 18 per cent. There is also the weight of fittings to consider. The gussets would be 24 G., with suitably-shaped lightening holes. These would certainly be lighter than some forms of joint used in tubular construction, but as recently several very light, if costly joints for solid drawn tube work have been designed, it may be assumed that the weight of fittings is equal in each type of structure. The above is a fairly complete weight comparison of two methods of steel construction. A welded frame would show up very badly indeed beside these two cases if M.S. tube, the strut curve of which is shown on the chart, was the material used. Molybdenum or manganese steels would show up better, but it has been admitted that where tubes have been used, notably in America, finished structures are on the heavy side. This is probably due to the fact that it is not considered safe to join tubes by welding where the wall thickness is less than 22 G. It should be noted that if the material of the gussets is distributed over all the corrugated members, the thickness of the material would only be raised one-and-a-half thousandth of an inch. This fact should give the welding enthusiast food for thought. To further this comparison, it should be stated that the sections shown in Figs. 8 to 10 are practical propositions, although it would be wrong to give the impression that, without some experience on the part of the producer, such sections could be readily made. The question of the assembly of these members will be dealt with in a later article. A more favourable case could have been made out for the tubular Structure if a larger diameter and thinner gauge of T. 5 had been taken, but comparison with a tube outside the practical commercial range is useless. Tubes are now being offered to the aircraft industry of quality superior to T. 5, and these are said to be quite suitable for structural work : the above SEAPLANE STABILITY CALCULATIONS. By WILLIAM MTJNRO In the design of seaplanes it is quite as necessary to deter- mine by calculation the statical stability of the machine on the water, as in the case of ocean-going vessels, and the calcu- lations involved are solved along very similar lines. The statical stability is defined as the tendency the sea- plane has to return to the upright when inclined from that position, say by wind or waves. This stability is measured by a comparison of the " meta- oentric height" calculated for any given machine with the metacentric height of similar craft known to be successful, and is very largely a matter of experience and tabulated data It is proposed to outline the method adopted. Fig. 1. Shows the machine inclined at a small angle, and indicates the two equal forces acting. (i) Wt. acting down vertically through the C.G. (ii) Buoyancy acting vertically up through the new centre of buoyancy ; that is, the C.B. with machine tilted. When the machine is tilted, the total displacement remains the same, but the shape of the underwater surfaces changes, so that the centre of buoyance—which is the centre of gravity of the underwater volume—also changes from B to Hv The point M where the vertical through Bx cuts the centre line of machine, is termed the transverse metacentre. If, now, a line is drawn GZ perpendicular to the vertical through Bj, then the equal forces (i) and (ii) act at a distance GZ from each other, and the moment tending to right the seaplane is WxGZ. As the point M is generally assumed to remain constant for small angles of heel—to about 8°—we can substitute for GZ and say that the righting moment, or moment of statical stability is W X GM sin 6.
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