DISADVANTAGES OF WINGLETS

It has been shown in Chapter 5 that the most effective method of reducing vortex drag is by increasing the aspect ratio, i. e. increasing the wing span for a given total area. It follows that whatever the gain from using winglets, a similar improvement could be achieved by an increase in aspect ratio. This could be done by fitting a simple wing extension. Such a span extension would, of course, increase the bending loads on the mainplane and would add weight, so the best solution is again decided by economics rather than aerodynamics. Nonetheless, whereas winglets require considerable research and, usually, wind tunnel testing to ensure they are of the most favourable shape and set at the best angle, to lengthen the wing is comparatively simple. Moreover, stretching a wing in this way is guaranteed to reduce vortex drag at all airspeeds. A longer wing is more prone to flutter problems and slower in roll than a short wing, but adding winglets to a short wing also increases the danger of flutter and the additional mass at the tip creates more rolling inertia.

6.18 TIP SAILS

At about the same time as the Whitcomb winglets were being developed, J. J. Spillman was working on tip sails of the kind shown in Figure 6.9. These were inspired by the wing tip feathers of some large soaring birds, which are spread, finger-like, to form a series of separate wing extensions with slots between. Essentially, these are intended to work in the same way as the Whitcomb winglets, but there may be three, four or five tips ails, arranged radially and ‘en echelon’ round the tip. Each sail is adjusted to extract lift from the flow in its neighbourhood and, as with the winglet, some of this force is directed forwards, the rest

adds bending load to the wing. The results are comparable and the same economic considerations apply. As before, an increase in aspect ratio has the same effect.

6.19 NASA TIP SAILS

Even more reminiscent of the bird wing, the NASA tip modification suggested in Figure

6.10 is intended to spread the tip vortex and reduce its strength, and this, too, reduces the vortex drag. Additional loads, as usual, must be borne by the mainplane structure and the slender tip ‘feathers’ are prone to flutter.