Getting the load distribution right

The loading distribution for a swept wing of constant section and geometric incidence along the span (Fig. 9.14) shows that, as well as an increase in local load towards the tips, there is a decrease in the centre section. The region of high load means low pressures on the top of the wing surface. This in turn means that the local velocity, and hence Mach number, will also be high in this region. Thus the tip region will be the first to encounter the transonic drag rise and stall, while the rest of the wing, particularly the centre section, is com­paratively lightly loaded.

If this state of affairs is not corrected, the wing will not be particularly efficient. It may be thought that this would not create too much difficulty because the Mach number could be pushed up slightly and the reduction in performance near the tip tolerated. However increase of Mach number in the tip region would lead to unacceptable shock-induced flow separation resulting in buffet and even stall.

We saw in Chapter 2 that the load variation across the span could be altered by modifying the wing planform. This is also true with swept wings. Unfortunately, in order to remove the tip load peaks and boost the load in the centre section an inverse taper would be required. This is clearly not a good idea from the structural point of view. The alternative solution of using twist is

the one nearly always adopted. Figure 9.15 shows a rare example of an aircraft with inversely tapered wings.

Typically the wing of a transonic transport will have 5% washout (reduced geometric incidence towards the tip) over the span and employ the more struc­turally acceptable conventional taper.

Unfortunately the use of twist to correct the load distribution can only pro­duce the desired result at a particular design angle of attack. As the speed reduces so the load distribution will tend to revert to the previous undesirable form and leading and trailing-edge flaps must be used to correct further.