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Aviation History
1931
1931 - 0742.PDF
FLIGHT, JULY 17. 1981 MONOSPAR WING IN SKELETON: THIS PHOTO- GRAPH, TAKEN WHILE THE WING WAS BEING MOUNTED ON THE FUSELAGE. SHOWS ONE OF THE TANKS IN FRONT OF THE SPAR. (FLIGHT Photo.) ferent, l>eing one with a nearly stationary centre of pres-sure. How its maximum lift coefficient compares with that of the Fokker we do not know. If it is markedlylower, so that the stalling speed becomes appreciably higher, the comparison becomes a little unfair to theFokker. It is likely, however, that there will not be very much difference in this respect. the leading edge to the forward ends of the kingposts,this forming a tie which stabilises the structure against drag forces. A wire joining the rear ends of the kingpostsserves as a " return wire " and anti-drag bracing. The details of the monospar wing for the Fokker arevery simple. The main spar is a built-up beam, consisting of top and bottom booms and interconnecting braces. Thelatter are arranged to form a series of " N's," and are plain duralumin channel sections, the vertical braces beingperfectly plain, while the sloping braces have the channel sides flattened out at the ends, where they are riveted tothe spar booms. The spar booms are of built-up box section, with " Z "-shaped vertical strips and fiat cover-ing strips. Actually, the vertical strips are not quite plain flat " Z " sections, but have a corrugation running alongthem centrally for stiffness. And the outer covering strip is not quite flat either, but has a similar corrugation.Only the top boom is a closed box section, the inner cover plate being flat. The lower boom, which is in tension, isleft open on the inner side. Not only so, but, as one goes from root to tip, the inner edges of the " Z " strips arecut away, first slightly, and then more and more as the tip is approached. The horizontal cover plates are taperedoff, the spar tapering both in plan and elevation. The taper in elevation is, of course, obtained by bringing thespar booms closer together and shortening the braces. Thus, the material in the spar is used very economically,but little superfluous material being left anywhere. With single-spar construction, one would be inclinedto think that the ribs must be a little heavier than those of a normal two-spar wing. Actually, this is not the case,at any rate, in the monospar wing built for the Fokker. DETAILS OF THE SPAR : THE SPAR BOOMS (DURALUMIN) ARE BUILT UP BOXES, AND THE BRACES ARE PLAIN DURALUMIN CHANNELS. It may be recalled that, put very briefly, Mr. Stieger'smonospar type of construction is based upon the use of a single main spar made strong enough to take the bendingloads. Such a spar, however, is obviously weak in torsion, and would not unaided produce a stable or rigid wing.Torsional strength is provided by a " spiral " wiring system which may be imagined as being made up of aseries of kingposts and two sets of wires, the wires from the front ends of one kingpost to the rear end of adjacentkingposts crossing each other on the outer face of the spar, to which they are attached. It is not essential that theattachment between spar and kingpost centres should be a strong and rigid one. In fact, it is not necessary to thestability of the structure to have the kingposts attached to the spar at all. In the wing for the Fokker they arelightly secured to the spar, but this is merely to locate them, and to reduce the stresses in the kingposts byreducing their free length. It will be obvious that such a girder, consisting ofspar, kingposts, and torsion bracing, has little strength against drag loads. In fact, none other than that of thespar itself loaded horizontally. In the monospar wing for the Fokker, the strength in drag is provided by joining The wing section used is, as already stated, one having avery nearly stationary centre of pressure. The spar is, of course, located along the c.p. line, and so there is littlec.p. travel to take care of. The ribs are tubular girders, and, owing to the monospar construction, the nose andtrailing edge portions are cantilevers overhanging from the spar. With this plain load distribution, the whole of thetop rib boom is in compression, while the whole of the bottom boom is in tension. Consequently the top boom ismade of larger diameter tube, as can be seen in the photo- graphs. The result is a rib which is no heavier than anormal two-spar rib of the same chord. The torsion bracing is in the form of plain duralumintube kingposts and tierod bracing. The tubes are merely steadied at their centre, where they pass through thespar. The drag bracing is completed by a reinforced lead- ing edge of duralumin sheet, which has a channel sectionriveted to it. A certain amount of weight has been added by thenecessity for the wing to " pick up " on the same four points as those used for the standard Fokker wing. Forthis purpose diagonal box-section members are used. These can be seen in the photographs. 696
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