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Aviation History
1926
1926 - 0788.PDF
SUPPLEMENT TO FLIGHT 88 OCTOBER 28, 1926 THE AIRCRAFT ENGINEER II 10 9 8 7 ft ^j ii i : ao FAC T .OA D JA L I ACT l : ; JI M MM r i i i i I M I V\ / CEI1 MM X> >ITRE I T "i T Mil \ of PR •S5UF i I ii /- i \ • lEi i i 25 30 35 40 ~n~TT \ 1JL11 1 1 M MM Rg.16. V, MM 1 1 TV MIL ; M M til l M M - .Mir 45 50 Fig. 16.—Effect of Centre of Pressure Travel on Load Factor of Wings. Front and rear spars are assumed to be placed at 0 • 12 and 0-67 of the chord, measured from the leading edge. With the c.p. forward at 0-25 chord, the front spar has a factor of 7, and with the c.p. back at 0 -50 chord the rear spar has a factor of 5. As the c.p. moves back, the proportion of load carried by the two spars changes, as indicated below, the load factors following the changes. The maximum factor is 8-82, which is reached when the c.p. is at 0 -336 chord. C.P. Load on— Load Factor. Position. Front Spar. Rear Spar. Front Spar. Rear Spar. 0 25 0 30 0-35 0 40 0-45 0-50 0-336 0-764 0-673 0-583 0-492 0-401 0-309 0-607 0-236 0-327 0-417 0 • 508 0-59!) 0-691 0-393 7-00 7-93 9-16 10-86 13-32 17-28 8-82 14 • 62 10-57 8-28 6-80 5-76 5-00 8-82 happier in that a method of calculation is now available which is reasonably accurate for all practical purposes. Past experimental evidence conflicting with calculations requires re-investigation, when much will be found, no doubt, to be of low evidential value. The proposed scheme of factors of reliability and safety are too valuable to be neglected. The absolute values suggested need not be taken too seriously; for example, the factor of safety chosen was probably largely due to the fact that two was the smallest whole number larger than one. No such scheme ought to result in heavier aeroplanes, rather in lighter structure for the same reliability, probability of failure. Pending this work, investigations as to influences of change of geometry and scale on structure weight are vested with an underlying unreality which is disconcerting to those desiring to make real progress. We can, however, only examine the problem for what it is worth under conditions imposed today. In a previous article it was pointed out that the general expression for structure weight percentage does not cover the case of the wings since some part of the forces on them are relieved by gravitational and inertia forces. We may consider the weight of the wings with a fair degree of accuracy to be made up of parts following the structure law (Ws) and parts whose weight varies as W. Using the symbol (Wp)s for the former and Wp for the weight of the planes, this is expressed as W,= (Wt)lTKW. (Wp), = W,_ w " "w K is a constant depending on the design of the wings. Now, since the forces on them are relieved by the weight of the wings. w ~r w where C is a constant. The load factor (F) can also be introduced. w C F W / K w CFW5 urgent need of experimental investigation. In January, 1922, the position was summed up by the Load Factor Sub-Com- mittee as follows :— " In the past it has been the custom to calculate the stresses in an aeroplane under certain assumptions, in regard to the aerodvnamic loading, which admittedly did not cover extreme conditions. The validity of the methods employed in calcu- lation could be checked against the results of static tests made on a portion, or on the whole, of the structure, but no evidence for the accuracy of the assumptions could be adduced, beyond the general experience that machines stressed in this way, and designed with the factors of safety then customary, had proved strong enough in flight. This position is clearly not very satisfactory, in that everything which contributes to the strength or weakness of an aeroplane is taken indiscriminately into consideration. " For a logical system of stressing it is necessary to know (1) the aerodynamic loading on the various parts of the machine ; (2) the strength of thess parts ; and (3) an appropriate value of the factor of safety." The position in October, 1926, is practically the same as it was in January, 1922, since nothing appears to have been done towards obtaining the necessary experimental evidence. The position with regard to centre of pressure coefficient is 7026 which can be written W, C F W"2" (1 -KJ-j-K. In Fig. 17 this expression has been plotted for some selected W (W )values of —- and ——s, as follows :— W= 8,600 —f = 0 160W F= 5-5 from which C = 0-000299 and K= 0-032 and in Fig. 18 W = 8,600 w = 0128 C = w 0-000373 = 0160W K = 0. In the second case the whole wing is assumed to follow the structure law, while the other constants more nearly represent practical construction. The curves are important as to their form rather than for their actual numerical value. They
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