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Aviation History
1921
1921 - 0646.PDF
44© lbs.), and we are informed that the weight of the wings of the monoplane is 1,110 lbs. Judged on a basis of weight per sq. ft., this is a heavy wing (2.52 lbs./sq. it.), but when the high lift coefficient is" taken into account, the matter of wing weight assumes a different aspect. We have no data relating to the L^ of the wing of the D.H. 29^ but assuming that a landing speed of 55 m.p.h. has been allowed, and that the total weight of the machine fully loaded h 7,000 lbs. (which is somewhere near the figure for which she has been stressed), the \,c max. works out at just over 1. This does not appear unattainable in a thick high-lift section. As the Lr max. of an ordinary section is generally about 0.6, the equivalent weight of the monoplane cantilever wing is virtually 2.52 x 0.6 = 1.51 lbs./sq. ft. This can scarcely be said to be a prohibitive figure in view of the various undoubted advantages of the cantilever wing. At the same time, we are not going SEPTEMBER 29, 19,21 their upper and lower surfaces are warped, or rather shaped, to lie snugly against the rib flanges without leaving any space to be packed up, as was the case in the German Fokkers. The ribs vary somewhat in construction according to location, but generally speaking they are spruce flanges with light webs and distance pieces ail spruce and three-ply. One of our sketches shows the general rib construction. The compression struts are indicated in the same sketch, but at certain points they take another form. This is where the internal drag bracing is attached. With regard to the latter it is of interest to note that it is in duplicate, not in the usual way, but in two sets, of which one is near the upper surface and one near the lower surface of the wing section. Coupled with the fact that the compression struts are practic- ally the full depth of the wing spars, this arrangement should help materially in giving rigidity to the wing. The SOME DETAILS OF THE UNDERCARRIAGE OF THE D.H. 29 MONOPLANE : rl, General arrange- ment of the undercarriage. 2, Details of the swivel attachment of the front chassis strut to lower longeron. 3, Ball-and-socket attachment of rear chassis strut. 4, Details of the rubber shock-absorbers and oleo gear. Note how the arrangement has been simplified as compared with that of the D.H. 18. to assert that even better results might not be obtained by a combination of thick high-lift aerofoils, more lightly built and having a certain amount of external bracing. In our opinion it appears quite probable that the future machine will have thick wings so as to be able to do with a minimum of outside bracing ; for instance, one set of struts for a biplane so large that ordinarily two sets of struts would be employed. That, however, is another story. At present we may be presumed to have indicated sufficiently that conipared with the ordinary high-speed low-lift wing the cantilever is not nearly so bad aa many appear to think. Coming now to the actual wing construction, the spars of the D.H. 29 are built up of spruce flanges with ply-wood sides, forming a box. As will be seen from the accompanying sketches, the flanges themselves are laminated, and consist of three strips with their vertical surfaces glued together. The spars have a pronounced taper from root to tip, and '.' incidence " bracing (the wing is so deep that one feels justified in so terming it) is in the form of spruce struts framing a flat Z, which, with the duplicate drag bracing, completes the wing structure. The greatest difficulty with a cantilever wing is not so much to provide against direct bending stresses as against twisting with travel of the c.p. In the de Havilland monoplane it would appear that this difficulty haS been entirely overcome, with the result that the wing is very rigid, and does not show any signs of dither. Another alleged difficulty with cantilever wings appears to have been refuted in the D.H. 29. It has generally been thought that with thick-section high-lift wings it was a necessity to have a rigid covering such as ply-wood or sheet metal, as such wings have been thought to be very sensitive to quite minute changes in curvature, such as may be caused by a slight sagging of the covering. Although the D.H. 29 is covered with fabric in the ordinary way, there does not 646
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