The centre section

Any real swept wing with subsonic leading edges will not behave in quite the same way as it would at subsonic speed because of the fact that there must be a limit to its span. For simplicity we will first examine the simple case of a swept wing without a fuselage separating the two halves (Fig. 8.11).

In this case the flow can only be influenced at a finite distance upstream and for very thin wings at small angles of incidence the appropriate zone of influence will be approximately defined by the Mach waves at the apex of the wing (Fig. 8.11(a)). If the disturbance to the flow is bigger, because of increased wing thickness or angle of attack, then a shock wave forms at the apex (Fig. 8.11(b)) and, because of its higher propagation speed, the zone of influence of the wing will be extended slightly in the upstream direction.

We now see that for a real swept wing we will still generate wave drag due to this bow shock wave. However we have gained one important advantage and that is due to the fact that the wing now works in much the same way at both sub – and supersonic speeds over most of the span. This means that the problem of choosing a wing section which is a suitable compromise between high and low speed has been made very much easier than before.

If we now introduce a fuselage the overall picture does not look very differ­ent, although some clever aerodynamic design at the wing fuselage junction may well prove very worthwhile – but more of that later.

The tip region

Unlike the unswept wing which we discussed earlier in this chapter, the tip region lies within the Mach cone of all the upstream wing sections. This region thus again behaves in a manner very similar to its subsonic counterpart (Chapter 2) and a trailing vortex sheet will be generated along the trailing edge of the wing and this will roll up into two large vortices which stream down­stream of the wing in a position close to the tips.