Disadvantages of swept wings

On a swept wing, only the normal component of velocity changes, and thus pressure changes are produced only by this component. A swept wing flying at a speed V, therefore, behaves like a straight wing flying at a lower speed; roughly V x cosine of sweep angle. Thus, we can see that wing sweep reduces the amount of lift produced for a given flight speed, wing area and angle of attack. Correspondingly, to give the same amount of lift as an unswept wing, the swept wing will need to be larger, and consequently heavier.

Although the lift is dependent on the normal component of velocity, both components contribute to the drag. The swept wing, therefore, tends to have a poorer ratio of lift to drag than an equivalent straight wing.

Swept wings and straight wings are influenced differently by the downwash effect of the trailing vorticity. We can explain this by use of the Lanchester – Prandtl vortex line model. Referring to Fig. 2.19 we see that an inboard line of trailing vorticity starting at A, will have more effect at C, than an equal strength outboard line starting at B. The nearest part of the outboard line is simply further away than that of the inboard line. On an unswept wing, both lines would have an equal and opposite effect. By considering the direction of rotation, we see that the inboard line tends to produce an upwash at C, while the outboard line produces a downwash.

On an unswept untapered wing, the upwash effect of inboard vortex lines is more than cancelled by the downwash produced by the large number of lines concentrated near the tip. On a swept untapered wing, the stronger influence of the inboard lines has the effect that the downwash decreases towards the tips. This is compounded by the fact that on a swept-wing configuration, the bound vorticity on one wing produces a downwash effect on the other. The mutual interference effect will again tend to produce a greater downwash at the centre than at the tips.

On a tapered swept wing, the trailing vorticity is less concentrated towards the tips, so the outboard downwash is further reduced. In fact, there can even be an upwash at the tips.

The upwash or reduction in downwash at the tips produces problems, since, when the wing approaches the stalling angle, the tips tend to stall first, giving the undesirable effects described earlier. On a swept-wing aircraft, the effect of tip stall is particularly serious. As the tips lose lift, the centre of lift will move

forwards, causing the aircraft to pitch nose-up, thereby increasing the stall in a runaway manner. The problem of tip stall was encountered on many early swept-wing aircraft. One solution is to sweep the wing forward, as described in Chapter 9.

Figure 2.20 shows what happens on a simple swept wing at high angles of attack. A separated conical vortex starts to form. On highly swept wings this vortex more or less follows the line of the leading edge, but on moderately swept wings, it bends away inboard. The tips produce very little lift, and any control surfaces near the tips become ineffective. There may be little or no loss in overall lift, however, since the separated vortices produce a contribution to lift, as we described in the previous chapter.

The problems encountered on early swept-wing aircraft were primarily those of loss of stability and control. In later chapters, we show how improvements in wing design, and advances in control systems have largely overcome such difficulties.

Swept wings only show an advantage for aircraft designed to fly close to or above the speed of sound. For low speed aircraft, they have positive dis­advantages, as outlined above, and it would be a mistake to introduce wing sweep for purely aesthetic reasons. A small amount of sweep is sometimes used on low speed aircraft purely to enable the wing spars to enter or attach to the fuselage at a structurally convenient position.

Wing sweep is also used as a means of providing stability in tailless designs, as we shall show later.