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Aviation History
1928
1928 - 0807.PDF
AUGUST 30, 1928 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT the next operation : the insertion of distance pieces and rivets, The rib is then taken from the jig and the rivets clinched under a power press, the empty jig being sent along on the return track for a repetition of the operations. Metal con- struction of ribs by such methods is thus shown to be a very inexpensive matter. I am doubtful, however, as to how long these and the Caudron ribs would remain intact under a vibration test similar to that officially used in this country ; at the same time I do not suggest that these ribs give trouble under service conditions. On this point I have unfortunately no information. For small components, fins, rudders, etc., ribs pressed from sheet were seen, this is good practice for small ribs. In America, and on some German machines, this practice is extended to large ribs, but such a practice, cannot on large members result in economy in either cost or weight. The Hanriot products had timber ribs, on metal spars, a practice adopted in the early stages of metal wing development in this country ; although abandoned, this system even now has its advocates here. Turning now to wing spars, the majority of French uncovered exhibits had spars of rectangular tubular section, these were in some cases lightened in both flanges and webs by plain (unflanged) circular holes (Fig. 5). The fact that this construction should be entertained after English practice is well known, aa it must be, is really amazing. " The proof of the pudding is still in the eating," but one could not help wondering what intensity of stress would be developed round the edges of some of these holes in the narrow sides of the rectangles, particularly if the real position of points of contra- flexure and maximum stress did not coincide with those calculated. The holes were not continuous nor evenly spaced along the whole length of the spar, but occurred only at places presumably of minimum stress. The spars in the Fiat machine shown at the last Exhibition had holes in the sides, they were made from two pieces of approximate^* channel shape, the lap joints being carried along the upper and lower sides or flanges of the spars, thus giving these portions where maximum stress occurs some reinforcement by reason of the double thickness. If these solid drawn rectangular spars with unsupported lightening holes are light and have adequate strength, then all that is required is done. One set of wings having such spars was exhibited, stood on end on the Liore et Oliver stand and stretched from the floor almost to the roof of the Grand Palais. The lower portion had what appeared to be an internal reinforcement of timber, whether this is used on the actual aircraft to give additional strength to the lightened webs of the spars, or whether it was merely for handling purposes or to enable the wings to be stood on end. one cannot say. The Caudron spars were of a novel form. An idea of the construction may be gained from Fig. 6. Each spar consists of two channel-like members, made up of a flat-plate web (unlightened) to the longitudinal edges of which are bolted channels, having one side shorter than the other. The channels are decreased in width outwards along the span, thus becoming angles with a continuously decreasing hori- zontal side. Whether this refinement is worth making is questionable, the weight saving can be but small. Finally, these main built-up channels are secured together along their upper and lower edges by a large number of closely-spaced bolts and distance pieces. A third design, more in keeping with English standards, is the Nieuport spar, as used on the Nieuport-Delage. This is simply a box spar made from four pieces in the usual manner with four outwardly projecting flanges, the flat plate flange being laminated to give some uniformity of stress. In similar duralumin spars made in this country, the flange is usually corrugated, although, perhaps, to the extent of only of one lobe. This permits of some reduction in thickness of metal, and in no way interferes with lamination. Another spar shown was that usually used on Breguet products, a solid-drawn hollow girder section, lightness and taper at the tip being obtained by inserting two channels secured back to back, these being held in place so far as one could judge by set screws filed off flush with the external surface of the girder section. Another form of Breguet spar used is that of the single-centre web with lobed flanges formed from strip secured to the upper and lower edges of the web, a form of spar quite well known in this country. The Dyle et Bacalan Company had no machine on view, but showed several examples of construction, including a built-up box spar, this again being liberally lightened in the webs. Other points in spar construction may arise when we consider complete assembled units. Fuselages were in the main wire-braced. One noticeable example of a fabric-covered fuselage without wires was seen on the Caudron stand ; in fact apart from control cables and external wing bracing, there was not a wire on this machine. In the " strut and wire " constructions a, good deal of sectioned tubing was seen. The most-favoured method was to use square or circular longerons and square or circular tubes for bracing flattened at the ends as far as the ductility of the metal permits ; a typical joint is shown in Fig. 7, which is a sketch of a fuselage joint taken from the Caudron machine, the bulkhead member being omitted for the sake of clearness. A variation of this is shown in Fig. 8, while the front end of the fuselage on the Ateliers des Mureaux Stand had round tubes for bracings ; this follows a well-known English practice. The fuselage on the Liore stand had square longerons with strut and wire bracing, the struts being fitted into end sockets which were held to the longerons by bolts passing through these members, the, plan and aide view members being slightly offset to allow of this. We may say then that, apart from the Caudron fuselage, in which even the bulkhead members were tubes, there is no substan- tial departure from English practice, except in the material used. Strut and wire duralumin fuselages are not unknown in this country. Attention must be drawn to one important detail, that is the use on French machines of duralumin wiring plates. Whatever may be argued for and against duralumin spars, ribs, etc., there can be no two opinions as to which is the better material for wiring plates. It only remains to describe a few special types of fuselage construction, but before doing so we will mention a matter which has recently been under discussion : we refer to the alleged dangers of the '' rigid " welded fuselage as typified in the construction of the forward end of the Fokker machine. It has been said that, probably due to unequal cooling after welding, such fuselages may fall apart under their own internal stresses and that some wire bracing is essential for safety in this construction. Large numbers of Fokker machines have been used for a long time without recorded mishap due to failure of these parts : it is difficult in the light of this experi- ence to see why the construction is considered to be very unsafe, but caution and experience must be exercised in the building. A part of the care necessary pro bably consists in the assembly of the struts in proper sequence. A metal monocoque fuselage of pleasing appearance, although perhaps of no better appearance than the Short fuselages, is that of the Nieuport-Delage. I think the internal structure of the Nieuport is different from that in the English products, the hoops being of half-round section with out- wardly extending edges, these edges are bolted to the base of flanged channels, and the outwardly extending edges of these channels are riveted to the skin of the fuselage at the lap joints. The body appears to have a double curvature and would therefore be a more expensive proposition than the large flat-sided fuselage exhibited on the other side of the Nieuport stand. Fig. 9 shows a section of a corner of this fuselage. The Wibault fuselage, although metal covered, is built on quite different lines, and is simply an N-braced girder with metal covering. The longerons and struts are simple angles and are secured together by means of gusset plates. The corrugated duralumin sheets forming the skin are placed with the corrugations vertical so that there are in consequence a number of vertical rows of rivets ; internally at these lap joints, light stiffening angles are used and where they cross the main girder members their inwardly projecting sides are cut away. The skin gives the impression of fragility, one could only hazard a guess at the thickness, which appeared to be 748c
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