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Aviation History
1928
1928 - 0442.PDF
SUPPLEMENT TO FLIGHT 38 THE AIRCRAFT ENGINEER MAY 31, 1928 weight; but by keeping the minus limit zero, and retaining a small positive limit, consistent -with good rolling-mill practice, we are enabled to design and build steel structures from thin strip with safety and without serious increases of weight over estimates based on the nominal thickness. A uni-lateral (i.e. minus nothing plus something) system of tolerances makes a special appeal to the stress calculator, for he does not wish to make reference to a file of specifications to ascertain what thickness he must subtract from the standard sizes which he is accustomed to using. punched and the rivets clinched in a very short" time ; if, however, the assembly is carried out by hand tools (hammers sets, etc.), the process will be long and costly. Such mediaeval methods can find no place in the factory properly equipped for metal aeroplane construction. The method of assembly as shown in Fig. 3 has the great advantage of permitting rapid spar production. It cannot be said definitely at the moment that this method of wing assembly results in a saving of time in the building up of the wing as a whole. It is a question of the production of the greater number of rib pressings and the Something might be said from the workshop point of view in fayour of the bi-lateral system, but we will refer to this when we deal with workshop matters. The question of corrosion will be dealt with later. Regarding the Editor's second question, the writer thinks that this point would not have been raised had particulars regarding the frame been given more completely. It should have been stated that Fig. 1, page 14, represents one side of a tail portion of a fuselage, the total ultimate load on the tail skid of which was 1,600 lbs., that is, 800 lbs. per side. It was, however, stated later in the article (page 16) that Fig. 1 was a uni-planar structure. Presumably there would be two such plane frames each carrying 800 lbs., and they would be suitably braced together to make the complete fuselage. A few observations on the assembly into wings of box type spars, as illustrated in Figs. 5, 6 and 7, page 24, will indicate some important features which have a direct bearing on the question of stress development in these members. Fig. 1 shows a portion of a two-spar wing of a normal construction ; Fig. 2 shows the method of joining the ribs to the spars as used in the majority of " Bristol" metal aero- planes ; Fig. 3 shows an alternative method not so extensively used. The first method needs no description: a single pressed abutment is secured to both riveted edges, that is, each rib is secured to the two spars by four such posts. The second method may need a little explanation : the rib is held in place by springing the toggle post from the dotted position A to position B and engagement is retained with the spars by virtue of the resilience of portions of the booms. The movable posts rotate about Y, Y, in which position the projections C come into contact with the spar lips. Thus, the ribs are threaded on to the spars with the toggles in position A; the same post acts as a toggle relative to the spar webs in each case, usually the post forward of each spar is made from a single pressed abutment, which is riveted to the base of the rib boom channel. In Fig. 4 details of a toggle post are shown clearly. There are two objections to the first method, one being that the continuous and complete automatic assembly of flanges and webs becomes impracticable ; the second objection is that riveting of rib posts to rib flanges or booms must be carried out during wing assembly. The latter objection is not nearly so important as is generally supposed, for, given the correct appliances, the eight holes can be fitting of these into a rihfversus the punching of eight holes, inserting and clinching eight rivets, which latter, given the suitable equipment, can be carried out very expeditiously. The fact of the matter is that insufficient production experience has as yet been obtained with these methods to enable one to decide the matter finally. It is evident that these rib posts actually give considerable support to the spar webs. The following is a comparison^of two spars of section nearly identical with that shown in Fig. 1, page 23, the flange main radius being 0-45 in. instead of 0 • 5 in. To the sides of one spar, rib posts as in Fig. 2 were secured and the second spar had no such supports. The constants of the sections were I = 0 • 38 ; y = 1 • 98 ; A = 0-208; Z = 0-192. I, the thickness of the flanges and webs = 0-015 in. The method of loading was as shown in Fig. 2, page 23, and the ratio —r = 24 (see page 24).W The spar fitted with rib posts failed at a value of W = 435 lbs., the central deflection being 1-2 in. 4046
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