Winglets and other devices

At the time of writing, the most popular wing-tip device appears to be winglet, which may be seen on the Airbus A340 shown in Fig. 4.17.

As illustrated in Fig. 4.18, winglets take advantage of the strong side – wash that occurs at the wing tip. Due to the sidewash, the air flow meets the vertical winglet at an angle of attack, and thus a sideways force is generated. The winglet therefore has its own horseshoe vortex system, as shown in Fig. 4.18(a). At the wing-tip/winglet junction, the winglet vortex system partly cancels the wing-tip vortex, so that effectively, the main ‘tip’ vortex forms at

Fig. 4.17 Wing-tip winglets on the Airbus A340 reduce drag

the tip of the winglet. This vortex is above the plane of the main wing, and so its downwash effect is reduced. In fact, the winglet modifies the whole of the spanwise distribution of trailing vorticity in a way that reduces the downwash and induced drag. In addition, the sideforce on the winglet can have a forward thrust component, as shown in Fig. 4.18(b). This also contributes to the reduc­tion in drag.

An inward sidewash occurs on the upper surface of the wing, as air is drawn in towards the low pressure. Conversely, an outward sidewash occurs on the lower surface, where air flows away from the high pressure. Thus, winglets can be fitted both above and below the wing tip. Because of the requirements of ground clearance, however, they are often only fitted above the tip.

There was some initial scepticism concerning the claimed advantages of such devices, because it seems to be a little like picking oneself up by one’s boot­straps. However, theoretical study (Yates et al. 1986) shows that they do not contravene any of the laws of nature, and that significant reductions in trailing vortex drag are possible using such devices. These theoretical predictions are well supported by experimental evidence.

Devices such as winglets are described as non-planar, because the wing is not in a single plane. A full analysis of non-planar lifting surfaces is given in Yates et al. (1986). In general, theoretical analysis indicates that for a given span, there is a wide range of non-planar wing shapes that should give less

Outwash under Inwash over

trailing vortex drag than a simple elliptical planform wing. The monoplane with winglets, and the biplane are two examples. However, in many cases, including that of the biplane, unless there is some good reason for limiting the wing span, it is cheaper and easier to use a simple monoplane, and to reduce the drag by increasing the aspect ratio.

It should be noted that winglets will not significantly reduce the drag if added to a wing that has already been optimised for low drag. If winglets are to be used to full effect, the wing has to be designed to take account of their presence from the outset. They can also be used to reduce the drag of a wing which was not optimised for low drag.

Both sails and winglets modify the distribution of vorticity downstream of the wing, and generally inhibit the formation of a well defined vortex at the tip. This has been shown to be a useful side-effect for crop-spraying aircraft, as it prevents the spray from being lifted above the wing and blown off target by a cross-wind.

Although these devices modify the trailing vortex field in a way that has a beneficial influence on the trailing vortex drag, they do not destroy the trailing vorticity as is popularly believed. Spillman (1988) reports that in flight trials with wing-tip sails, the disturbance effects far downstream were, if anything, slightly increased.

Winglets and other devices can produce a low-drag wing, but they add to the cost and complexity of construction. They also modify the handling and stability characteristics. In one case tested, the cross-wind stability of the air­craft in landing was severely affected, and in another, interference with the flow over the ailerons produced a control reversal effect in some circumstances. Even though the influence on handling and stability may not be detrimental in all applications, the effects must be fully evaluated for certification purposes, and this can also be a costly process.

An ingenious use of winglets was made in the design of the Beech Starship (Fig. 4.10). Here, the winglet also served as the vertical fin, and is thus a necessary, rather than additional feature.

The modern use of composite construction makes it possible to design much more complex out-of-plane wing geometries, as on the Airbus A350XWB shown in Fig. 14.6.

In addition to the use of these fixed devices, drag reductions have also been obtained by using spanwise blowing (Tavella et al. 1985).