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Aviation History
1920
1920 - 0417.PDF
APRIL 15, 1920 RIGID AIRSHIP CONSTRUCTION: "-.'....,. A Suggestion By A. E. RANKIN, A.Memb.Inst. Naval Architects. Now that it is possible to once more turn our attention to airship construction for commercial work, one of the first problems needing careful consideration is that of cost of production. Our rigid airships of today involve a con- siderable sum to build, and therefore do not appear as a very tempting proposition in commercial circles. Financiers are not likely to indulge in airship ventures until this problem has been solved. At the present time, and likely for some years to come, demonstrated economy on the financial side will be a very prominent factor in enlisting support. The the girder will fail at the weakest point, the member in com-pression will fail first as the nearest rivet will have an excess of strength over the compressive member by 700-330 = 370 lbs.Therefore, a single-riveted connection is strong enough for the girder. The bearing surface of a 4 mm. rivet is sufficient forthe connections in question. Now let us compare the work involved in the two types ofgirders, assuming the length of girdere to be 15 ft. 6 ins. long, approximately. Type " A " will have 78 crosses, involving 936 rivets. F/G. 3 DIAGRAMMATIC r/OA/ THRO V TYPE. A' LIPS FLATTENED OUT IN tVAV Of K/r£TS P/ACRAMMATIC • SECTION THROUGH Cc*m?£ of GROSS Fie 4- Type. 8' PRCSSEO SUfflCICHTLY nT/D£ To TAKE. 4- % RlrET author, therefore, offers the following suggestion as one meansof reducing the cost of construction. To most readers of this article it will be well known that thehull of a rigid airship is composed of a series of braced girders. We know from the experience and investigation of severalwell-known authorities in recent years, on the strength of braced girders and columns, that we cannot depart generallyfrom the fundamental rules governing the spacing of bracings. Let us, therefore, see what we can do in the actual manufactureof these girders. It can easily be seen that the cost of braced girders variesas the number of bracings and the numbers of rivets in each bracing. The present recognised type of bracing is shown in Fig. i,i n which it will be seen that one complete cross has 12 rivets. The lips of the members are flattened out at the centre andat the ends to receive the rivets. Let us call this Type " A." Fig. 2 shows proposed type of bracing in which only five rivets are used. This we shall call Type " B." Fig. 3 shows part girder Type " A"Fig. 4 shows part girder Type " B." On examining the two types of girder,that the double-riveted connections Type ' stiffness to the main channels than Type " B," and it has beenthe custom to look upon the elementary pin-jointed girders in Type " A " as fixed-end girders, and in Type " B " as free-end girders. However, in practice, the disparity is not so great, and whenwe realise that the bracings on one side of a triangular girder overlap those on the adjacent side by one-third the pitch, theunsupported length " 1 " is very short, and consequently very stiff. Especially is this the case in double-braced girders, wherethe actual length " 1 " is very short indeed. If we take the un- supported length " 1" in Type " A " and compare it with thatin Type " B," we see that " B " requires to be a little stronger, such that 1 + c f_Y : 1 + c (- ) = 1. We should find \K/A \K> that the increased section would be approximate, as shownin Fig. 6, and the increase in weight, after deducting the saving in rivets, would be about one half of one per cent., of the total, lift of the ship. The single shear of a 4 mm. rivet is 700 lbs., and a bracing 01Type " B " would have a breaking strength of approximately 330 lbs. in compression and 2,000 lbs. in tension.When the girder is under stress, one member of each pair of bracings will be in compression and the other in tension it would appearA " afford more Type " B " will have 78 crosses involving 390 rivets. The huge saving in labour and cost is at once apparent,the saving on a finished girder being over 33 per cent, of the whole cost (labour only). The estimated saving on a ship of present-day proportionsis, approximately, ^20,000, and the number of rivets saved would be about 700,000. This enormous saving will have been obtained at the expenseof approximately o • 5 per cent, of the total lift, which amount, I feel sure, could be considered insignificant in commercialaviation at least for the next few years ; as it cannot be denied that, to foster a new industry, our greatest aims mustbe not only a reliable article, but quick production and cheapness also. If these aims can be obtained by such a smalj As sacrifice, then let us, by all means, make the sacrifice, untilsuch times as this industry is well established. , " ': One other feature I should like to mention before closing -this article, and that is in respect to kingposts and trusses in main transverse framing. In view of the cost, it would, '. i.apparently, be more preferable to increase the size of section „> in main transverse girders than te introduce kingposts, '.especially if we realise that,a complete kingpost and truss ••.„'•** alone cost one and a half times the price of the plain main •,transverse girder. The total saving in cost for one ship would be, approximately, ^3,000. There is plenty of scope for the metallurgist to produce acheaper alloy, and also for the fabric experts to introduce cheaper means of production. 417 E
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