Stress diffusion adjacent to gaps in the inter-spar skin of a stressed-skin wing

Abstract

The diffusion of stress in the .neighbourhood of chordwise gaps in the wing surface is an important structural design problem. Such gaps occur at wing joints and at undercarriage and bomb-bay cut-outs, and can involve local stress concentrations which require to be estimated. This report gives, subject to certain simplifications (including representation of the stringers by an equivalent sheet, carrying direct end load only), a theoretical analysis of the problem, and derives formulae for the stress distribution. Approximate formulae are found for (i) the direct stress in the flanges and (ii) the shear stress in the skin at the flanges and at the chordwise gap. These approximate formulae, applicable with negligible error when chordwise gaps are not closer than about one and a half times the inter-spar distance, enable a rapid estimate to be made of the stress concentration. A numerical example to illustrate the application to design is given, and shows that the maximum additional skin shear stress can be as much as two to three times the maximum additional flange direct stress. Although various factors (for example, flexibility of riveted joints between the spar flanges and the skin, local buckling and plastic flow) are likely to reduce the stress concentration if present calculations predict it to be high, some reinforcement of the skin is likely to be necessary.