Swept forward wings

For aircraft designed for cruise in the transonic range the use of swept forward, rather than swept back, wings offers some advantages. An optimum spanwise load distribution can be obtained with conventional taper towards the tip. The problem of boundary layer drift towards the tip, which encourages tip stall is also alleviated. Because the velocity component along the leading edge is now directed inboard, the boundary layer tends to thicken towards the root rather than the tip.

With this catalogue of virtues the reader may wonder why forward sweep has not been employed exclusively. The main problem lies with the struc­tural behaviour of the wing. When the wing is loaded the angle of attack increases unlike the swept back wing (Fig. 9.13). Because of this the tip lift is increased and the deflection worsens. The wing can then suffer progressively increasing twist. This condition is known as divergence and is encountered again in Chapter 14. The problem is made worse because an increase in load at the tip of a swept forward wing will produce a nose-up pitching moment thus increasing the angle of attack over the whole wing and again increasing the load.

After early attempts at using forward sweep (e. g. the Junkers 287 in 1942) the structural problems led to the virtual disappearance of the idea. Recently, however, advances in structural materials have led to renewed interest in the concept. Modern composite materials (Chapter 14) allow suitable flexural behaviour to be designed into the wing to prevent the occurrence of divergence. Another technique which can be employed is to automatically sense the twist as it occurs and to use a computer-driven system to deflect the ailerons down­ward simultaneously in order to cancel the twisting effect.

The X-29 aircraft (Fig. 9.20) is an example of an experimental forward – swept configuration. Radical new design features, such as forward sweep, are, however, expensive and commercially risky to introduce. It is likely to be many years, therefore, before the conventional swept back configuration is seriously challenged.