A Note on Subsonic Linearised Theory for Symmetrical Cranked Wings at Zero Incidence

Extract

Research is in progress to assess, and if possible improve, the accuracy of a widely-used method for calculating the pressure distribution on swept wings in subsonic attached (inviscid) flow. Ref. 1 does not make specific recommendations on how to deal with wings having “cranked” planforms (i.e. wings with discontinuous changes in the sweep angle of the leading or trailing edges, other than at the central “kink” fundamental to the swept wing concept); but it implies (section 8.3) that the method of ref. 2 could be used for this purpose. A re-examination of this problem, in connection with the programme of work of the Transonic Aerodynamics Committee of the Royal Aeronautical Society, has revealed errors, which could be seriously misleading, in that part of ref. 2 dealing with the thickness effect. The object of the present note is to present the corrected formulae for the basic streamwise and span-wise components of perturbation velocity due to the thickness, at and near the crank, in the simplified example consisting of two semi-infinite wing panels of different sweep angles joined together; to examine the possible singularities in the flow field in the neighbourhood of the crank; and finally to discuss qualitatively the implications of these formulae on the magnitude of the “crank effect”, including the influence of free-stream Mach number.