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Aviation History
1917
1917 - 0177.PDF
FEBRUARY 23, 1917. set the stamping out of the lattice bars, " like hot cakes," to use a colloquialism, the rolling of the rails, the bending of the rails to shape over suitable formers and finally the riveting of the lattices to the rails, thus completing the fuselage. Then comes the question of strength for weight. Obviously if this is very much inferior to the ratio obtained in the ordinary girder body no amount of economy in production will make up for this deficiency. As regards the material to be employed, steel is the first to suggest itself, but if weight is to be kept down it would probably be found that sections would be obtained, which were so thin as to render the pre- vention of buckling under compressive strains difficult. Turning again to our enemies for information on what they have done, we find that aluminium alloys are the" materials used in airship construction, and as there are now several very good alloys on the market in this country it may be of interest to examine what results might be expected from their use. In this connection it may be well to mention at the outset what the N.P.L. says regarding the permanence of one alloy: " The peculiar behaviour for some considerable time after heat treatment, in undergoing a gradual hardening process, has naturally given rise to some misgivings as to the permanence and stability of the alloy in the resulting hardened condition. It was considered possible that the hardening process was the first stage in a series of changes which might after the lapse of considerable time lead to the disintegration or weakening of the material. On this point it can only be said that the evidence so far obtained is entirely negative. Specimens of the alloy over two years old have been re-tested and found to give results exactly 6imilar to those obtained on the new material, and there is at present no reason to suppose that any process of deterioration occurs in this material." As regards such properties as strength and ductility, these are * possessed to the required degree, or can be provided, in several commercial alloys. Without going into lengthy calculations of the strengths and weights of the wood girder fuselages, a fair standard of comparison may be obtained by taking the strength of an ash longeron of the dimen- sions generally employed, and examining what size and section aluminium alloy longeron is required to give the same strength. We suggest taking for a basis an ash longerom ij ins. x ij ins., as being a fair average dimension for the position of maximum stress. If the strength is calculated of such an ash longeron, treating it as a strut of length 20 ins., a reasonable figure for the distance between the struts of a fuselage near the point of maximum load, the following formula for the crippling load of a strut of any length can be employed : i/u A = area of cross section in inches ; / = length of strut in inches ; E = modulus of elasticity in lbs./sq. in.: and k = least radius of gyration of strut cross section. For an ash longeron ij ins. x ij ins., A =• 1.25 x 1-25 = 1.56 sq. in.f c for ash is taken as 10,000 lbs./sq. in.I is taken as 20 ins., a fair average distance between fuselage struts.E for ash is about 1,600,000. k for a square = 0.289 &&gt; where d = side of square, = 0.289 x X-25 = .36.We can now substitute in the formula, which becomes : 10,000 x 1.56 P = j 10,000 x^o8 3142 x 16060 ^ 3.142 x 1,606,000 x .362 2,051,200 = 5<400 lbs. approximately. Taking this figure as the crippling load of an ash longeron ij ins. x ij ins., the next step is to decide what size aluminium alloy longeron will be required to give a crippling strength of 5,400 lbs. Now, with the lattice girder type of construction it is possible to so space the lattices as to reduce very materially the length of longerons between two consecutive struts, to leave the " free " length of longeron so short in fact, that buckling is improbable, say 6 ins. fc for aluminium alloy = 35 tons = 78,400 Ibs./sq. in. E for aluminium alloy is taken as 10,400,000. k for equal angle section, which is a reasonably good section commercially as well as mechanically, = 0.204 d, where d = one side. Take the sides of longeron as 1 in., then k = .204. If an equal angle section with 1 in. sides is chosen the area of this section, or in other words the thickness of the angle, has to be found, which is done by substituting in the same formula previously employed. Here we have P = 5,400 lbs. and we can therefore write : 78,400 x A 5.400 78,400 x 6 3.I4'! x 10,400,000 x .204" where P *= crippling load of strut of any length I ;fc =intensity of stress at the yield point in compression; from which A = 0.12 sq. ins. approximately. This corresponds to a standard wire gauge of about 16, which should work out quite well as regards weight. Having now estimated the dimensions necessary to give the same strength as wood construction, there remains to find out how the weights of the two forms of construction compare. In his book " Aeroplane Design," Mr. Barnwell gives the following empirical formula for weight of aeroplane bodies : W = .057 P b d, where / = length of body in feet, b - mean breadth, and d = mean depth. For purposes of comparison the following dimensions will be assumed : length = 15.5 ft., mean breadth = 1.75 ft., and mean depth = 1.7 ft. According to Mr. Barnwell's formula the weight of a wire braced body with wood longerons and struts will then be = .057 x 15.5'- x 1.75 x 1.7 = about 41 lbs. (To be concluded.) The Exhibition of Zeppelin Relics. ADDITIONS during the past few days to the Exhibition ofZeppelin relics now being held in the Temple Gardens, Victoria Embankment, have made it still more representativeof all the Zeppelins brought down in this country. The new relics include a propeller blade and a cane device,apparently used to prevent the airship being damaged when leaving or entering the shed, from the twoZeppelins which were destroyed off the N.E. coast last November. The exhibition is to remain open for anotherfortnight, and those who have not done so should make a point of paying it a visit, as the proceeds will be dividedamong military charities. The charge for admission is sixpence. 177
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