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Aviation History
1921
1921 - 0522.PDF
Dr. Sablatnig's reason for adopting a braced wing may be partly to save weight, but is more probably dictated by the desire to provide wings which fold fiat against the sides of the fuselage, an arrangement which would not be possible with a one-piece cantilever wing. Having decided to have folding wings, the problem was to provide the simplest form of bracing so that the wing could be dismantled in the shortest possible time. The form which the bracing has taken will be seen from the illustrations : On each side there is one pair of struts, running from the lower longerons of the fuselage to the wing spars at points slightly nearer to the wing tips than to the root. There is no " incidence " bracing between these struts, but an external drag wire runs to the nose of the fuselage from the point of attachment of the rear strut to the rear wing spar. It would appear to be somewhat doubtful whether this bracing is capable of giving sufficient rigidity, and one would rather expect that the wing might be liable to a considerable amount of deflection, and especially torsion, during flight. However, we are not aware STRUT SECTION AUGUST 4, 1921 of such struts, as he does not appear to have taken out a patent in Germany. From the illustrations it will be seen that the front strut of the Sablatnig is steadied in the centre by a tube running up to the front spar of the centre-section. It would therefore appear that, even with the special strut construction employed, a strut of this length cannot be relied upon to take compressive loads of great magnitude, such as might occur when there is a down-load on the front spar in a steep dive. The problem of a good terminal attachment for these struts is also one of some importance, owing to the change in direction of the load. Fitting ordinary eyebolts into the wood spar would scarcely be satisfactory, as such a bolt might, and probably would, work loose in the wood and give rise to " play." A similar argument applies to the strut fitting itself, and it is therefore not without interest to examine how Dr. Sablatnig has attacked the problem. The spar fitting by means of which the lift struts are secured, is shown in one of the drawings. First of all a sheet-steel SECTION A-B. j THE SAB. P. 3. : Some wing details. 1, Diagram showing method of building-up a wing strut, and section of the latter. 2, The unusual fitting on the ends of the wing struts. 3, Diagram of spar construction. 4, End fitting on centre-section spar. 5, Swivel lug attachment of lift struts to main spars. that in practice this has been found to be so. The manner of building up the wing struts is of more than passing interest, as it appears to be identical with the method invented and patented in this country by Mr. Fairey during the War. As the illustration will indicate, the struts are made of two halves, the inner faces of which are rounded off and the ends forced together, thus putting an initial tension on the outer fibres of the wood. Mr. Fairey has found that such a strut is very much stronger in compression than one made in the usual way, although it would probably be a matter of some difficulty to give satisfactory proof of the why and wherefore. Certainly the outer fibres will not begin to become loaded in compression until a fair load is on the strut, and thus the zero load point occurs at an actual load of so many pounds. On the other hand, the inner fibres are put into compression in making the strut, and might thus be expected to fail earlier than if the two halves had not been stressed originally. However, the fact remains that struts made in this manner have been found to give very good results. Probably Mr. Fairey himself did not originally realise the full advantage stirrup fits over the lower face of the spar, to which it is secured by horizontal bolts. In order to prevent the tighten- ing up of these bolts from bending the relatively thin sheet steel and thus crushing the wood, it appears that metal bushes are housed in the three-ply sides and spruce fillers of the spar at this point, as indicated in the section of the spar and fitting. This stirrup plate forms a base plate for the swivelling fork-end, which has a circular base working in a cup formed in the second sheet-steel plate. The latter is held to the stirrup plate first by rivets, and later the whole is held together by four long bolts passing up the sides of the spar and through a flanged plate on top of the spar. Inci- dentally this latter plate serves as the attachment, when the wing is folded, of the rear spar to the tubular pyramid on the fuselage. As regards the fitting for the lower ends of the lift tubes, this will be dealt with under fuselage construction. The fitting on the ends of the lift struts themselves is an elaborate affair, necessitated by the fact that the struts may be called upon to work in tension as well as in compression. Perhaps 522
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