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Aviation History
1927
1927 - 0053.PDF
JANUARY 27, 1927 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT THE PROBLEM OF THE SAFETY FUEL TANK. By F. L. M. BOOTHBY, Captain R.N. (Ret.). Aeroplane accidents are regrettably frequent, and much too often we read that a machine bursts into flames after a crash and destroys the occupants, who may be otherwise uninjured. An ordinary metal tank is in common use. When partially full we have a layer of petrol vapour lying on top of the liquid fuel, and partially mixed with air, which enters through the necessary vent. Should any spark occur on a crash there is the mixture ready to ignite. A match may be dropped into liquid petrol without ill effect, but a match dropped into the mixture indicated will have evil results. The ideal fuel tank should not permit of this explosive mixture being formed. A metal fuel tank bursts very easily on striking the ground ; it deforms, and deformation opens the seams or cracks the metal. The perfect petrol tank should be capable of any deformation without affecting its strength. In the ordinary petrol tank air or exhaust gas are admitted into contact with the petrol. Both contain water. The water condenses and the well-known trouble results. The fuel tank we require should not allow of any moisture getting into the tank. The ordinary fuel tank is connected to the engine by an inelastic pipe : should the position of the tank relative to the engine increase, even to a small extent, the pipe will be broken and fuel escape. Should an electric light lead be run alongside the petrol pipe the disaster will then be complete, as in R.38. Let us consider how we can overcome these difficulties, and start from crude principles. When Kut was being re-provisioned from the air it was the practice to pack such substances as sugar, rice, etc., in a sack. This sack would be enclosed in a larger one. When dropped the inner sack sometimes burst, but the contents were retained by the outer. ("onsider a flexible but inelastic ban in the form of a sphere and quite full of liquid. On being dropped the whole of the energy developed will be absorbed in bursting the walls of the Dag. Suppose the same bag to be half full of liquid and half full of air. It will no longer remain in the shape of a sphere and may be constrained by elastic suspension, etc., to take any desired form. On being dropped in this condition the energy generated must compress the air in the tank and shape the tank into such a form that the air pressure is sufficient to burst it. Much work is absorbed in compressing the air and in stretching the elastic suspension, and there is often insuffi- cient energy left to burst the container. If the container is surrounded by another and larger bag. a double process has to be gone through. The inner container must be burst, and then the remaining energy must be sufficient to compress the air in the outer and burst that— which in practice it generally does not accomplish. What we can do is to take a tank made of balloon fabric— three-ply is generally sufficient, but any number of plies to give the required strength can be used. This is flexibly suspended inside a larger tank, which in turn is flexibly suspended in the aeroplane's fuselage. A single lead of flexible petrol tubing is led into the inner tank, passing through a gland on both the outer and inner tank, and so arranged that on a heavy pull being given the piping can slip through the gland. The piping is extended from the top to the bottom of the tank, so that there is a foot or two to draw on should the engine start to move away from the neighbourhood of the tank. Between the inner and outer tank we take a lead from the exhaust of the engine. The exhaust pressure acting through the walls of the inner tank forces the petrol out through the pipe. Further, it fireproofs the tank, as it drives all air away from the neighbourhood of the fuel and surrounds it with a layer of inert gas. Any incendiary bullet can be fired through the tank without setting it alight. So far, so good. Now come the difficulties. Rubber-proofed balloon fabric will not hold petrol, so the fabric must be " doped." There is no difficulty in finding a dope that will resist petrol. Unfortunately aeroplanes now use " cocktail fuels " with a large percentage of benzol. No known dope will stand that. Gold-beater's skin will do so, and it is possible to get a glue that will do the same. The gold-beater's skin and glue do not take kindly to a rubber fabric, so we have to stick them to cotton. This makes a perfectly good petrol and benzol-proof lining. Another difficulty then arises. This lining is petrol-proof but not waterproof. We can take precautions that no water gets into the tank with the fuel, but water from the exhaust gases or from the air might come in contact with the outside of the lining. Fortunately a cellulose dope will render this waterproof. Our tank then takes this final form : an outer cover of balloon fabric, then a layer of inert gas under pressure from the exhaust, then an inner tank of balloon fabric with a lining of petrol- and benzol-proof fabric. A single pipe, both for filling and supply, passes into the inner tank and reaches to the bottom, allowing the tank to move relative to the engine without breaking the pipe, as already described. Such a tank is reasonably crash- and fire-proof, but it must not be cut by sharp metal edges, such as are found on some machines. Generally the cross-bracing wires are sufficient to prevent this, should the tank tend to be thrown towards the engine. Tanks of this kind can be fitted inside the wings, or under them ; in the latter case they would be made of streamline form and kept in shape by pressure from the exhaust or from the slip stream of the propeller, as in non-rigid airships. The weight of such a tank is about the same as a metal one of the same capacity. MORANE-SAULNIER PERFORMANCE FIGURES. In the table of data relating to the machines exhibited at the Paris Aero Show, which was published in the December 30, 1926. issue of THE AIRCRAFT ENGINEER, no performance figures were given for the Morane-Saulnier monoplanes. As these were school machines this is not, perhaps, a serious matter, but as the figures have now become available it is thought that it may be of interest to give them here so that those wishing to do so may add them in their appropriate place in the table referred to. Three types were shown, the 129, the ]32. and the 35. Following are the missing figures relating to these three machines. The type 129 has a top speed of 200 km. h. (124 m.p.h.). a climb to 4.000 m. in 30-9 mins., and a ceiling of G.300 m. (20.650 ft.). Its '" high-speed figure " is 15 -8 and its '" altitude figure "' 7-7. The type 132 has a top speed of 172 km./h. (107 m.p.h.). climbs to 4.000 m. (13.100 ft.) in 41-5 mins., and its ceiling is 5.300 m. (17,380 ft.). The " high-speed figure " is 15-5 and the " altitude figure " 7-9. In the case of the type 35 the top speed is 131 km./h. (81 -4 m.p.h.). the climb to 4.000 m. occupies 53 • 17 mins.. while the ceiling is 4.500 m. (14.760 ft.). The high-speed and altitude figures are 9 • 2 and 6 • 7 respectively. THE " EVERLING QUANTITIES." In connection with the article entitled '' Comparative Quantities in Aircraft Statistics " published in the November 25, 1926, issue of the AIRCRAFT ENGINEER, which was a translation of an article by Prof. E. Everling that appeared originally in the German journal Zeitschrift fur Flugtechnik und Motorluftschiffahrt. we have received a letter from Mr. J. Yanier, Technical Translator. U.S. Air Corps, McCook Field, Dayton. Ohio. U.S.A., in which the writer points out that, although the German word " zahl " literally does mean " figure." the expression is used in the article in the same sense as the German word " Beiwert," i.e., coefficient or factor. Thus, Schtieilflugzahl would be translated high-speed factor ; Weitflugzahl. distance or range factor ; Hoehflugzahl, height or altitude factor ; Bauzahl. load factor ; Flugzahl, criterion for climb and ceiling. As it is rather important that the right translations should be found for these quantities if they are to pass into the English language, we have very great pleasure in giving hero Mr. Vanier's suggested translations, and would ask the opinion of our readers on the subject. 46g
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