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Aviation History
1940
1940 - 0419.PDF
FEBRUARY 8, 1940 riveted tanks, handholes tend to lessen the effect of pantingdue to vibration, by breaking up a large surface. The gradual change of section obtained by welding-in theadaptor is more favourable to fatigue resistance than the riveted reinforcing ring and the sudden increase in sectiondue to the ring. There appears to be a tendency amongst designers to crowd out the inside of even small tanks with baffles, the function of which is usually given as a combination anti- surge device and stiffener. Fuel surging, in the case of small and medium-sized tanks, is an overrated factor. In the case of the old De Havilland Comet, which was sensi- tive enough fore and aft, the main petrol tanks are placed on and forward of the centre of gravity, have capacities 135 is given a cadmium coating and assembled with an insula- ting paste, preferably having a zinc or barium chromate base. It is not advisable to locate steel studs in a pocket on the tank bottom, where water could be trapped. External steel or aluminium alloy studs may be fitted in " Elektron " adaptors with insulating paste, but it is always advisable, where possible, to use fittings screwing into, or bolting on to, " Elektron "adaptors, made in " Elektron," or otherwise in an aluminium magnesium alloy, such as MG7 or Birmabright. Identification markings should be either stencilled or transferred on the paint film or, if of metal, fixed by the normal method, not to the sheet direct, but to a welded-in adaptor. All tanks should be finally treated with the standard cold FIG. 11 FIG.9 of 130 and no gallons respectively, and contain no baffles or anti-surge device whatever. No trouble was experi- enced at any time due to fuel surging. Baffles are a com- mon source of service trouble in any type of tank. Failure often occurs by cracking on the bend of a flange when it is directly riveted to the skin, or it may be due to work- hardening round a rivet head at attachment. Apart from this, they are often awkward to fit and they always increase cost and weight. ^- ; • Fewer Baffles v, ~1 ^£ If baffles are normally required for stiffness in aluminium tanks, then their number can be reduced in "Elektron" constructions made with a thicker shell. When baffles are "needed in the plane of suspension straps in aluminium -tanks, the best corresponding structure in "Elektron" is a welded-in extruded H-section as shown in Fig. 10, which gives a very stiff structure. When baffles are necessary, however, they can be attached as shown in Fig. 11 to the web of a welded-in T-section. It is better to use bolts for this purpose, as there is the possibility of damaging the sheet when closing a rivet in an inaccessible position. The minimum thickness of sheet for "Elektron" tank Shells may be taken as 18 G, and this can be used for .roost small- and medium-sized tanks having an initial test pressure of the order of 51b. per sq. in. Even for capacities as great as 400 gallons, 18 G sheet can prove perfectly •satisfactory, providing that the shell is suitably stiffened "by extrusions, or if the shape affords sufficient rigidity. 16 G sheet should be used when extra stiffness is required,: but it is rarely necessary to exceed this, except for profile •or load-carrying tanks. Internal baffles are usually made from 20 G sheet, and diffuser and hot well units can be '* made from 20 G or even 22 G, depending upon the nature of the welds involved. 22 G sheet can take a good weld, unless the seam is complicated or inaccessible. Although the calomel electrode potential differences be- tween magnesium and practically all other constructional ^metals and alloys is greater than .25 volt, this factor does not appear to have serious effects except under abnormal conditions. For internal fittings of oil tanks, the presence of steel studs or bolts in " Elektron " will cause no trouble. In petrol tanks-the practice may be allowed if the steel acid chromate dip inside and out, and externally shouldreceive two or three coats or an approved lacquer. Resistance to Vibration The aircraft designer will naturally want to know how tanks made of '' Elektron'' stand up to vibration in actual use. Some indication is given by official tests carried out last summer on an oil tank of 30.25 gallons total capacity and 26 gallons oil capacity. The tank was manufactured by Essex Aero, Ltd., and weighed, com- plete with filler cap, sump, dip-stick and filter, 12.4 lb. The tank shell was of DTD 118, and the extrusion stiffen- ing rings were DTD 142. The tank, it might be men- tioned, was of the same dimensions as the Alclad tank it was designed to replace, and the weight was 9.1 lb. less, a not inconsiderable saving for 26 gallons capacity. Three sets of test were carried out. First four runs of 25 hours each with the tank filled f, f, f and | respectively. The 100 hours were completed successfully at an ampli- tude of 0.005m. and a frequency of 1,850 per minute. Another four runs of 25 hours, but with the amplitude A cast-in stiffening corner web and a machined flange plateof an " Elektron '' tank
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