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Aviation History
1928
1928 - 1101.PDF
NOVBMBEK 29, 1928 85 THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHT A portion of a Junkers monoplane wing is shown in Fig. 3, and considering the method of securing the bracings, it is obvious that the tubular spars must be subjected to greatly varying torsional forces. The tendency of the spars to twist is, of course, resisted by the skin. Nevertheless, it would be most instructive to know if in actual fact the wing tips of G.31 do remain sensibly free from angular movement. The writer has always regarded the Junkers type of wing con- struction favourably, but this feature of securing the bracings to the sides of the longitudinal tubes has always appeared to be scarcely an "engineering job," apart from the obvious difficulty of inserting and holding up a rivet in a hole in a portion of a l£-in. or 2-in. tube, perhaps 10 ft. away from the open end while a head is formed on the outside. The statement that a finished piece of work is or isn't an " engineering job " is constantly being levelled at the con- struction of particular parts of aeroplanes : this perhaps occurs most frequently in connection with the fabric covering of a lifting surface. No matter how excellent the structure itself may be in general principle of lay-out and detail design : as soon as it is covered with fabric it ceases to be an " engi- neering job " ! Can an " engineering job " be defined ? Assuming that the specification is on a sound basis it is clear that the design of a structure should involve considera- tions of:— (a) Compliance with the specification regarding strength and weight. (b) Durability both as regards freedom from corrosion and security against slackening of joints. (c) Complete immunity from fatigue failures over any period of time. (d) Ability to withstand severe man-handling. (e) Ease of repair or replacement of parts if damaged. (/) Quickness of assembly. (g) Low initial cost and upkeep. (A) Ease of inspection. As to whether a structure is or is not an " engineering job " depends on the number of the above conditions which it satisfies. It is frequently difficult to give an opinion on first consi- deration as to whether a structure is an " engineering job " or not, and one's early opinion is often modified as experience is gained in any particular case. As regards the Junkers wing construction, providing the designer gets the performance he requires and the machine is safe to fly under all conditions, then the fact that the longi- tudinal spars are subjected to a large number of offset loads is a matter of no consequence, but the question that one asks oneself is how much weight would be saved if all these secondary couples were avoided, and the answer appears to be that if sockets were used the weight, and certainly the cost would be increased, for the saving in weight on the longi- tudinals would probably not make up for the weight of the sockets. It might well be that the weight of the Junkers wings could be substantial^ reduced by using metal strip. It was interesting to note that in the centre section, where the loads are greatest, sockets round the tubes were used as a means of securing the bracings. We have seen that the wing loading of this particular machine is 17-6, while the figure for passenger machines of English design is only 10 or a little more. Allowing for the larger KL max. of the G.31, the Imperial Airways machines have one great advantage over the Luft Hansa land aircraft, which will no doubt appeal to the air travelling public, since the majority of passengers would probably prefer to take four hours on a journey and land at 50 m.p.h. rather than three hours and land at 75 m.p.h., particularly while the possibility of forced landing still exists. Returning to the Dornier exhibit, there is not much here that can be dealt with in detail; as stated at the beginning of these notes, there were a large number of tested structural specimens on the stands, but since no particulars of lengths, sections, dimensions or failing loads were avail- able, these specimens were more imposing than instructive. One saw confirmation in one test, however, of the necessity for having considerable strength reserve in the joints of tension members of bracings. When an open girder is tested to destruction, in the event of a tie member failing, the damage usually is. not confined to the tie, the booms being invariably very badly damaged, since they have to take the whole load in shear. On the other hand, however, if a strut fails in compression, the damage can usually be made good, and after repair the test can be continued. The reason why a compression failure is not so serious is because the load is momentarily relieved when the bracing " gives" and even after the commencement of buckling the member may withstand three-quarters of "the original load; at any rate, it is not usual for the whole of the shear load to be thrown on to the booms after buckling of a bracing com- pression member; moreover, there is often visual evidence Fig. 4. of the likelihood of a strut failure because of the bowing of the strut. The load in a tension member may be estimated with accuracy and sufficient metal be provided for carrying it; the trouble comes at the " rigid " riveted joint where the stress distribution may be very indeterminate and localised high stresses may start a failure which may spread across the whole joint with catastrophic results. There is also usually little previous warning that a tie failure either at the joint or in the member itself outside the joint is impending. One such girder spar having a fractured tie was on view, and the booms were damaged beyond repair, while close by a similar spar having a buckled compression member was shown ; in this case the place of failure was not at first appar- ent, and repair was obviously an easy matter. In later developments of Dornier spars it was noted that great pre- cautions were taken that the joints of tension members were right. A specimen of a large girder spar about 3 ft. deep was also on view. This was built on standard structural engineering lines, the material was £ in. thick or thereabouts in places and rivets that appeared to be of about f in. diameter were used. A sketch of this is shown in Fig. 4. The Dornier wing construction approximates most nearly to English standards inasmuch as there are two main spars and in some special cases a subsidiary spar. These wings are, of course, braced externally, the struts being set at a very small angle to the horizontal, thus relieving the bending moment in the wings at the expense of added end load. On one type of Rohrbach machine, external struts were used, but it is understood that the water resistance of the lower ends of these struts handicapped the machine in " getting off." Dornier gets over this difficulty by terminat- ing these external struts on two lower wings of small span ; these wings act as stabilisers in the water, and also must help the machine in the take-off. The relative merit of pure cantilever and " semi-cantilever " monoplane wing structures is a subject which obtrudes itself at this stage, but the discussion of this important matter must" be deferred to a later article. 1020c D2
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