Most seem to agree that every airplane should have them, if only for the sexy appeal that they add to the overall style of an aircraft. In fact, as “cool factor” goes, they can transform even the most mundane of flightline designs into something that appears ready to set class speed records at Reno’s annual air races. Quite what it is aesthetically that universally seems to capture on-looker’s approval is open to conjecture, but winglets (or variations thereof) can, at a stroke, transfigure an airplane’s appearance. But, visual impressions aside, exactly what is it that these devices do for the performance of an aircraft?
Specially designed wingtips have been found to be useful in controlling induced drag and wingtip vortices. Given that the pressure of the air flowing over a wing is less than its pressure flowing under, the flow over the wing’s top surface tends to flow inward while that underneath tends to flow outward and curl upward over the wingtip. When the two airflows unite at the trailing edge of the wingtip, they crossflow into eddies and vortices that resist against the wing’s forward motion, resulting in induced drag. Winglets – those small vertical airfoil sections at the ends of the wings – have been found to reduce this drag and diffuse the energy of the wingtip’s vortex flow.
Interestingly, though the devices are associated to modern wing configurations, the concept for their use was actually pioneered by an Englishman, Frederick Lanchester, in 1897. Soon after, in an effort to achieve stability in some of their production models, the Wright Brothers were using a variation of the idea too. However, it wasn’t until the 1970's that NASA’s Richard Whitcomb began conducting wind tunnel experiments to deduce if the imposition of a vertical surface at the ends of the wings could reduce wingtip crossflow tendencies. In theory, Whitcomb figured, this vertical surface would also provide thrust much like a sail does on a sailboat tacking into the wind.
Whitcomb’s research found useful reductions in induced drag with associated improvements in lift-to-drag ratios. Each airplane, however, is different and, therefore, requires a variation on the winglet layout to achieve proportional benefits in performance. For this reason, any given winglet on an aircraft will have different upward angles (or cant, as seen from the side), vertical angles (or camber, as seen from the front), and inward angles (i.e. twist or toe). The camber and twist to the winglet create a lift force that has a forward component which reduces the total wing drag. Properly designed winglets can potentially add an effective wingspan increase that is double what would be achieved if the winglet height was merely added to the wing as a wingspan increase itself. They are of greatest benefit when the wingtip vortex is strong; therefore, low- aspect-ratio wings profit more than high-aspect-ratio wings from the use of winglets.
The advantages winglets provide are many: increased speed at altitude, improved density altitude performance, increased range and fuel savings, and improved climb. They are especially useful when re-certifying an aircraft for a higher takeoff weight. If the wing’s span is not lengthened, the process necessary to increase a given aircraft’s takeoff weight results in a span loading hike which, in turn, increases its induced drag. Elongating the wingspan is expensive and complicated; the addition of winglets avoids an induced drag increase without having to extend the wing’s length. (Not having to extend the wing’s length ensures it can still fit into the same size parking slots too.)
Not everything about winglets is golden: they can add weight behind an axial point of the wing whereat flutter could result. Furthermore, the camber and twist in a winglet is optimized for one speed which means that, at other speeds, they don’t provide the full benefit for which they were designed.
Winglets tend to be used as add-ons for existing wing designs. While it is generally considered better to add span to the wing to enhance performance, there can be no denying that they add a dash of panache to the look of any aircraft.