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Aviation History
1934
1934 - 0288.PDF
FLIGHT, MARCH 22, 1934 SOME DEVELOPMENTS IN AIRCRAFT CONSTRUCTION By H. J. POLLARD, Wh.Ex., A.F.R.Ae.S. A summary of the lecture delivered before the Royal Aeronautical Society on Thursday, March 15 R. POLLARD confined his lecture for the most part to. the consideration of the problems of stressed skin construction. He fixed the mini- mum practical thickness of the covering material at 0.02 in., a condition which he considered ruled out the use of steel and made that of light alloy imperative, and taking the simple bending theory as being true, deter- mined that the maximum stresses in the skin of a covered fuselage caused by the vertical component of the tail skid loading to be from 2 to 6 tons/sq. in. He considered that the use of a corrugated skin could be ruled out owing to the drag involved and, therefore, that internal reinforce- ment by means of hoops and stringers of angle, channel or other suitable section riveted to the skin was necessary. He lucidly explained the forms of buckling which were to be expected in a structure of this nature- when subjected to compressive and/or shear loads. He favoured Professor von Karman's formula (Ref. 2) and (Rej. 3) to obtain the effective width of sheet between the reinforcements: = C x t Where 2w = effective width of sheet. p = crippling stress at failure. t = thickness of the material. E — Young's Modulus of the material. C = constant. He pointed out that this formula referred only to rein- forced flat sheets or those having small curvature (Ref. 4), Dealing then with the form of buckling to be expected owing to shear, he showed that this formed diagonally and differed appreciably from that due to compression, and that as it was normal practice tb leave the stringers and hoops unconnected, the sheet had to carry the whole of the shear load and the reinforcing had to resist all the components of force from the pull of the sheet. The formula p = kE (tlb)2 was used to show the crippling stress in panels under shear or compression. Where p = crippling stress. E = Young's Modulus. t = thickness of the sheet. b = width of sheet. k = a constant. From this formula it was shown that it was possible to place the various structural materials in their order of merit, but it was pointed out that while the value of E for steel was about three times that of aluminium, yet the latter can be approximately three times thicker than the former for the same weight of panel, and, as the buckling load increases as the cube of the thickness, it did not at the moment seem probable that the desirable properties of steel outweighed the disadvantage of weight. Attention was drawn to R. & M. 1553, " Summary of the present state of knowledge regarding sheet construc- tion," by H. L. Cox, a publication which rendered it easy for anyone to become conversant with the problems of thin sheet construction. Reference was also made to the work of Mr. S. C. Redshaw, of the Imperial College Civil Engineering staff, and his work " Elastic instability of a thin curved panel subjected to an axial thrust," published in R. & M. 1565. Mr. Pollard considered that Mr. Red- shaw's results proved that it was fundamentally sound to estimate the strength of a large component, as for example a monocoque fuselage, from a test on a small element representing the construction at the most highly stressed part. He considered that the theoretical crippling stresses were practically independent of the length of the specimen and that a test specimen having the correct curvature and breadth, and being of a convenient length, might easily be constructed and tested by applying loads, moments and shears of the values which would be found at that parti- cular section of the whole component ; a great practical advantage over constructing and testing a whole fuselage. While not wishing to give the impression that the problem of the strength and stiffness of reinforced smooth sheet construction had been solved, Mr. Pollard stated that investigations of the subject were being actively pur- sued at the Bristol works on behalf of the Directorate of Scientific Research. Part of this work was also an inves- tigation of the all-important matter of holes in the cover- ing, but this had not proceeded far enough for the estab- lishment of formula. Turning then to the consideration of wing spars and wing covering, the investigations of Professor Wagner (Ref. 7, 8, 9, 10 and Dr. Mathar (Ref. 11, 12) were dis- cussed, particularly with reference to the use of thin sheet web girders and the methods whereby undue buckling of the web could be avoided. He pointed out also that the web performed another duty in addition to resisting shear loads, in that it assisted the sheet covering in stabilising the spar booms. The important effect of a rigid wing covering for monoplanes in increasing the torsional stiffness of the wings was also discussed, and Batho's formula (Ref. 14) was taken, as being the best for estimating the loads of any members of a wing under torsion, while the stresses in the spar webs could be estimated by the method adopted by Wagner. Mr. Pollard discussed at some length s. 1 . Surface in compression. Elastic buckles. Permanent buckle at one stringer. 9SS
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