More boundary layer problems on swept wings

When air flows over a swept wing, the chordwise component of velocity changes in much the same way as the flow speed over an unswept wing, but the spanwise component remains more or less constant. This means that the local flow angle varies across the chord, and streamlines are consequently curved. Also, since the flow velocity varies with depth through the boundary layer, the amount of curvature will vary through the layer. This and other complex distorting effects hasten the transition to turbulence and increase the level of turbulence after transition. A further problem is that at the leading edge, the air is not brought to rest along a stagnation line as on a straight wing. On a swept wing, it is only the normal component that slows down to zero when the flow

meets the leading edge. The spanwise velocity component is little changed and with a strong spanwise component of velocity, there can be a turbulent boundary layer right at the leading edge.

The above features mean that on swept wings there is often little or no lami­nar boundary layer flow, and this creates a penalty in terms of the amount of surface friction drag generated. One way of improving the situation is to suck the boundary layer away through slots or small holes in the surface, as described previously. A significant benefit can be obtained even if only a small portion near the leading edge is made porous, since this can enable a region of laminar layer to become established. The reduction in drag thus obtained might be enough to offset the cost and complexity of such an approach. The technology for producing very small holes economically now exists, and there is renewed research interest in such boundary layer suction systems.