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Aviation History
1919
1919 - 0277.PDF
spar weight, obtained by multiplying this " corresponding value on A curve " by d* value for front spar (i.e., .85 for this table), and dividing by an empirical constant .6325. This empirical constant comes in because I have assumed that with front spar at . 14 and the rear spar at .5, the weight of the front spar shall be taken as unity, and its breadth = . 5 of its depth. Columns xi. to xv give similarly obtained values for rear spar, the only difference being, of course, that the spar " figure of merit " is different of each line and is taken from Column v. Column xvi. is the sum of values in columns x. and xv., and is value for weight of both front and rear spars. I am afraid the methods are not explained particularly clearly, and they are not as accurate as might be desired, but further space cannot be afforded to them. Plate III. Here are given curves for weights of front spar, rear spar, and sum of front and rear spars, on a base of posi tion of rear spar. The values plotted are those found by the methods just outlined. It is interesting to note some of the points which these curves bring out. Front spar is throughout lighter than rear, it is nearly as heavy when it is at .06 and when rear spar is at from .66 to .67. Within the limits of the curves front spar is lightest when it is at .12 and rear spar at . 42 ; obviously front spar gets lighter as rear spar moves forwards, and would be zero with rear spar at .28. Rear spar is naturally lightest when front spar is at furthest back position, .16, and when rear spar itself is at about .64. It is interesting to note that rear spar weights increase on either side of some minimum position (from .6 to .64, dependent on front spar position), also that for far forward positions of rear spar this spar is lightest for most forward positions of, front spar, whilst for far back positions of rear spar this spar is lightest for farthest back position of front spar. The latter result is obvious, so is the former of course when we consider that when the rear spar is forward of .6 it is ahead of chosen centre of pressure, so that the nearer the front spar is to it the greater is the down load on the front spar. The most interesting point, however, is the total weight value. The lightest condition is given as with front spar at .08 and rear at .42, much more forward positions than in any current practice ; from the slope of the curves, moreover, it appears that the total spar weight would be still less for a still more forward position of rear spar. But the curves are very flat and bunched together for all cases when rear spar is ahead of .66 and front spar behind .08. Normal practice is about . 12 for front spar and .62 for rear, and from these curves we see that the total weight for this disposition is only about 6 per cent, greater than the minimum. We cannot have too far forward a position for the rear spar in practice, or the rib weight will go up, whilst since narrow ailerons are more efficient than wide ones (as we shall come to later on), if the rear spar be far forward we must add to the wing a secondary rear spar, behind the main one, for aileron attachment. It is noteworthy that German practice has inclined to a very much farther forward position of main spars than we usually employ, and to the addition of a secondary rear spar for ailerons. It would be interesting to investigate further for the economic position of main spars, including effect on rib weight and weight of secondary rear spar for aileron. Another matter worth investigating would be the effect on aerodynamic efficiency and strength obtained by a far forward rear spar and ribs more or less flexible behind the rear spar ; it is conceivable that the automatic variation of camber due to variation of load on the flexible rear portion \j STANDARD CRCT35 5ECTION FOR WING SPAR'S. lv =-oa5l?d?<-IOstjcl2- OfaAiM-t PiSai/' IT - •Q48fesb*<-OQ225dLl/. CURVES OF AgEAOFSECTlON A. OF VERTICAL MOMENT OF INERTIA, ly. fe OF TRWSV^rgSE MOMENT QF .NEIgTtA . \T , O^ *> SA^E OF P»TtO 1? -^ cL . S^K WCfSHT TABLE FOK HCHT 5WW AT -O, TC»B 5Wt *' So. 'SA, 58>g, toto V/O C.PTAKCN A^RT-ZftfDRjr'RONTSPKrt ^ASAT"^90 rt>K WEAK Jl Hi • iWrOIP»r*»TTvln» HTW SP*R TAKEN *& *7ST>WTFOF? 1WNT PLATE! 277
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