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Aviation History
1938
1938 - 2417.PDF
AUGUST 25. 1938 55 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT 174c REINFORCED SKIN An Adaptation and Elaboration of an Original American Method of Approximating Stressed-skin Strength By R. RODGER RECENTLY I had brought to my notice, through the favour of an American friend, a suggested method of designing reinforced duralumin skin structures to withstand compressive loads. The suggestion is, I understand, placed to the credit of the Curtiss Corporation, and although it is quite obviously only an approximation, the method is so delightfully simple and easy to apply that I feci sure it will be of interest and value to readers of The Aircraft Engineer. In the following notes I have elaborated the basic Curtiss data and adapted them to suit British standards without, however, departing from the general scheme. Finally, I have applied the data so obtained and compared the theoretical allowable unit stress indicated with that actually realised in a series of tests on an elliptical-section duralumin monocoque fuselage. The variable factors to be considered in reinforced skin construction are :— (i) The thickness of the skin, (ii) The curvature of the skin. (iii) The slcnderness ratio of the longitudinal stringers, (iv) The circumferential pitch of the longitudinal stringers. (v) The cross-sectional area of the longitudinal stringers, (vi) The end fixation ol the longitudinal stringers, (vii) The buckling strength of the longitudinal stringers, (viii) The size and spacing of the rivets. (ix) The stress distribution in the skin between the longitudinal stringers, (x) The modulus of elasticity of the material. Variable (iii) is a function of the hoop, or frame, spacing and the effective radius of gyration of the stringer section. Usually the radius of gyration of the stringer section with reference to the neutral axis normal to the tangent to the skin at the stringer is ineffective owing to the restraint offered by the skin itself. In other words the stringers usually tend to buckle radially rather than circumferentially. Variable (vi) is a function of the frame to stringer fixation, and of the support, i.e., gusset effect, if any, afforded by the skin. Variable (vii) is a function of stringer section form, or geometry, and the physical characteristics of the material. Variable (viii) is closely related to variable (vii) inasmuch as the rivet pitch will determine the ability of that part of the skin adjacent to the stringer to carry stress developed in the stringer. For example, a wide rivet pitch in com bination with a light skin and comparatively heavy stringer will allow the skin to fail as a column between rivets before the stringer has realised its failing stress. Variable (ix) is a function of the circumferential pitch and gauge of the stringer. Variable (x) is a constant for any given material. As previously stated, the Curtiss method is only approxi mate because it fails entirely to account for several of the above variables, whilst for others the test data are meagre and restricted in scope. Briefly, the Curtiss method is to consider as a standard the strength properties of flat sheets reinforced with longitudinal channel stiffeners as per Fig. 1, the slenderness, or Ijk, ratio of the stiffener being kept constant at a value of 70. Imp. 18 19 20 21 22 23 24 25 26 REINFORCE! 10 6,000 4,500 4,000 3,500 3,100 2,900 2,800 2,700 2,600 15 8,900 6,700 5,900 5,300 4,700 4,400 4,200 4,000 3,900 20 11,800 8,900 7,800 7,000 6,300 5,800 5,500 5,300 5,200 TABLE 1. l'LAT SHEET IN COMPRESSION. % Reinforcement. 25 14,700 11,100 9,800 8,800 7,900 7,300 6,900 6,700 6,500 30 17,600 13,300 11,700 10,500 9,500 8,700 8,300 8,000 7.700 35 20,500 15.500 13,700 12,300 11,100 10,200 9,700 9,300 9,000 40 23,400 17,700 15,600 14,000 12,600 11,600 11,100 10,700 10,300 45 26,500 20,300 17,900 15,800 14,000 13,300 12,800 12,000 11,900 50 29,600 22,900 20,200 17,000 16,500 15,000 14,400 13,900 13,400 NOTE.—Stress is in Ib./sq. in. lor Dural Sheet with constant stiffener slenderness ratio of 70. The stress is calculated on combined area of skin ami reinforcement. 1 have obtained the results shown in Table i by inter polating from the original curves which are very nearly linear through the origin. It will be apparent that the values quoted for compressive stress could be improved by the incorporation of a stiffener of more efficient cross- section, e.g., by lipping the free edges of the channel or the use of extruded bulb angles. The term " percentage reinforcement " is defined as the ratio of stiffener area to area of effective skin and stiffeners. The compressive strength of reinforced skin structures in light alloy appears to increase approximately linearly with this percentage reinforcement up to a value of about 50 per cent., when the stress becomes nearly constant with 1 •oj5"; Y L_ •75" I _J X in Fig. 1, standard channel stiffener of slenderness ratio 70. Fig. 2 (right) shows arrangement of elliptical-section duralumin monocoque fuselage test specimen.
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