Interference corrections for asymmetrically loaded wings in closed rectangular wind tunnels

Abstract

The problem of tunnel interference on a complete lifting wing fitted with ailerons is considered in relation to aerodynamic measurements on a six-component balance. Asymmetric loading introduces corrections to the incidence of the wing, the drag and the rolling, pitching and yawing moments. The basic theory of wall interference in closed rectangular tunnels is outlined in sections 3 to 5. In section 6, the tunnel-induced upwash is expressed in terms of the loading on the wing and four quantities dependent on the shape of tunnel. These quantities are evaluated for a duplex tunnel in Tables 4 to 7 and may be computed for a general rectangular shape with the aid of Tables 1 to 3. Section 7 describes how the evaluation of tunnel interference is conveniently linked with Multhopp's lifting-surface theory to determine corrections to incidence, pitching moment and rolling moment. A worked example in the case of antisymmetrical loading is given in Appendix II, which concludes with an approximate procedure, suggested as a possible substitute for the lifting-surface method. The corrections to drag and yawing moment are discussed in detail in section 8. All the corrections are summarized in section 9 and expressed as products of experimental aerodynamic coefficients and theoretically determined quantities, which are evaluated in Table 8 for an arrowhead wing (Fig. 4) with various ailerons in a duplex tunnel. The corrections to incidence due to symmetrical loading are equivalent to corrections to lift of the opposite signs these vary from - 11 to - 5½ per cent depending on the type of loading. The corresponding corrections to rolling moment due to antisymmetrical loading are about - 2 per cent. Corrections to drag are very roughly + 20 per cent. When the spanwise loading is asymmetrical, there arises an induced yawing moment, which may require an interference correction of the order + 25 per cent.