The Formation of Regions of Separated Flow on Wing Surfaces Part I Low - Speed Tests on a Two - Dimensional Unswept Wing

Tests of a two-dimensional straight wing with a 10 per cent thick RAE 101 section have been made in a low-speed wind tunnel to check the validity of a criterion suggested by Owen and Klanfer for the type of bubble which will be formed when a laminar boundary layer separates from the surface of an aerofoil. The results confirm this hypothesis and show that if the boundary-layer Reynolds number based on displacement thickness at separation, calculated from an observed pressure distribution, is greater than 450 a short bubble is formed, and for (R 6 1)s less than 400 a long bubbl : is formed. For values of (R 6 1)s within the range 400 to 450 it is uncertain which type of bubble will occur. A method is given, based on these results, for predicting the type of bubble formed on a two-dimensional unswept wing of arbitary section shape for a given incidence and Reynolds number. A brief discussion of the physical structure of bubbles is given, and the more important problems yet to be solved are indicated. A hypothesis is put forward to explain the phenomenon of the 'leading-edge stall ' of moderately thin aerofoil sections, and some remarks are added on the scale effect on the maximum lift attained by aerofoils which experience this type of stall. 1. Introduction.-Aerofoil sections may be divided into three categories depending on the type of stall: (a) Trailing-edge stall, with the separation point of the turbulent boundary layer moving forward from the trailing edge as the incidence increases (b) Leading-edge stall, caused by an abrupt separation of the flow near the leading edge without subsequent reattachment. (c) Thin-aerofoil stall with laminar separation near the leading edge and turbulent re-attachment at a point which moves progressively rearward with increasing incidence.