. Principles and design of aëroplanes . 61 The frames are at present generallyconstructed of timber or stout bamboo,but steel tube is used in some cases. In considering the strength of theframe we have three main parts: (1) Longitudinal girder, sometimestriangular and sometimes rectangular. (2) The fore girder, rectangular insection. (3) The aft girder, rectangular insection. The latter is generally smaller thanthe fore girder. To find the forces act-ing in the parts of the frame we calcu-late on the same principles as in bridgeconstruction. Taking the longitudinalgirder, we have the following
. Principles and design of aëroplanes . 61 The frames are at present generallyconstructed of timber or stout bamboo,but steel tube is used in some cases. In considering the strength of theframe we have three main parts: (1) Longitudinal girder, sometimestriangular and sometimes rectangular. (2) The fore girder, rectangular insection. (3) The aft girder, rectangular insection. The latter is generally smaller thanthe fore girder. To find the forces act-ing in the parts of the frame we calcu-late on the same principles as in bridgeconstruction. Taking the longitudinalgirder, we have the following arrange-ment: 62. 63 We can for finding strength neglectthe weight of the longitudinal girder. The greatest bending moment will beeither ^(Lx-w^Xte-da)}- {w2x [x-(dl+da)][ or -[ (L2-w4) X (x - dx) J- - -J w3 X [x- (di-f- d2)] }■ Use feet and pounds for measure-ments, and the result will be in foot-pounds turning or bending moment. The resisting moment of the framewill be as follows (rectangular frame) : 2 X pull or push in one bar (lbs.) Xdepth of frame (feet) = bendingmoment. Using the greatest bending moment,we have then: n, i. i( pull in bottom bars ) _( or, push in top bars ) greatest bending moment2 X depth of frame If the frame is triangular, 64
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