Recent investigations show that “ shear lag ” has a small effect on the distribution of the bending stresses in the section ; but the effect of the stiffness variation, of the various parts of the structure, has a considerable bearing on this distribution.

Compression tests on panels of wing covering comprised 20 S.W.G. and 22 S.W.G. plating, with stringers of 18 S.W.G. to 22 S.W.G. rolled sections, show that, due to the bowing and waving of the light covering and stringers, the effective E is reduced to about 4.5 × 106 to 5 × 106 lbs. per sq. ins. for light alloy panels of these sizes. Compression tests on light alloy extruded struts show a value of E of 10 × 106 lbs. per sq. ins.