More about the tip flow

Problems with the tip region are not confined to the difficulties with boundary layer and local load distribution discussed above. The problem is compounded by an effective loss of sweep in this region. This can be seen by plotting isobars on the wing surface. Isobars are familiar to most people because they are in general use on weather maps. They are obtained by drawing lines through points on the wing surface having the same pressure thus providing a sort of ‘contour map’ of the distribution. Figure 9.16(a) shows how the isobars become less swept in the tip region which reduces the effectiveness of the geometric sweep angle.

This effect may be offset by using a thinner section in this region. Fortun­ately, from the structural point of view, the outboard sections of the wing are

the easiest to deal with in this way, since the bending moment is lower. The loading in the tip region can also be improved and made less ‘peaky’ by the use of local changes of camber and twist, but these modifications can also only be ‘tuned’ to a single design angle of attack. A further method, which will work throughout the range of angle of attack, is to modify the planform (Fig. 9.17). The particular form of taper shown in Fig. 9.17(a), while it produces the right result from the point of view of the ‘pure’ aerodynamic requirement, has clear drawbacks when the need to fit control surfaces or high lift devices is taken into account. In this case a straight trailing edge is an obvious advantage, and the planform of Fig. 9.17(b) results. Even so it may still not be completely possible to obtain quite the desired planform. Structural design must be considered and it may become necessary to demand a straight leading as well as trailing edge in order to fit a leading-edge slat over a sufficient span.