Aircraft Winglets
Many of us who fly regularly have most probably seen a so-called winglet or wingtip device at the end of the wing of an airliner at least once. It is showing up more and more often on more and more types of aircraft, thus we felt it’s time to give an overview to our readers about these sometimes funny, sometimes cool and stylish looking aircraft parts.
Winglet on Virgin Atlantic A340-600 - c by Dan Valentine on Airliners.net
History, Reason and Benefits
The initial theoretical concept goes back to times before even the Wright Brothers first took to the skies in 1905, but it was picked up and developed by Richard T. Whitcomb of NASA after the 1973 oil crisis - in order to reduce fuel consumption. The first tests were carried out in 1979/80 in cooperation with the U.S. Air Force. At almost the same time, but independent of any U.S. military organization, a private jet producer, LearJet exhibited a prototype in 1977: the LearJet 28 that featured the first winglets on a jet and a production aircraft. Flight tests made with and without winglets showed that the winglets increased range by about 6.5 percent and also improved directional stability for the LearJet- these two factors are the major reasons behind using this facility at any fixed wing aircraft ever since.
A winglet is a (near) vertical extension of the wing tips. The upward angle of the winglet, its inward angle as well as its size and shape are critical for correct performance - this is why they can look quite different. Air rotating around the wing strikes the surface of the winglet that directs it in another direction - thus creating an extra force, basically converting otherwise wasted energy to thrust. This is a small contribution but can save a lot for an operator in an aircraft’s lifetime. Another potential benefit of winglets is that they reduce the strength of wingtip vortices, which trail behind the plane. When other aircraft pass through these vortices, the turbulent air can cause loss of control, possibly resulting in an accident.
Winglet Types
In general any wingtips that not end the wing simply horizontally are considered as some kind of a winglet. Even though in strictly technical terms Wingtip Fences are not real extensions of the wing, and Raked Wingtips do not have a vertical part, they are still widely considered as winglet variants.
WINGTIP FENCES are a special variant of winglets, that extend both upward and downward from the tip of the wing. Preferred by European plane-maker Airbus, it is featured on their full product range (except the A330/340 family and the future A350). The Airbus A300 was actually the first jet airliner to feature this kind of solution by default, but it was a very small version of the tool. Provided that most of the Airbus planes (including all A320 family jets) feature such wingtip fences, this may be the most seen and most produced winglet type. Even the new Airbus A380 double-decker features wingtip fences.
Airbus Winglets as seen from the outside
Airbus Winglets as seen from onboard
BLENDED WINGLETS (the real “Winglets”) are the most popular winglet type, leveraged by Airbus, Boeing, Embraer, Bombardier but also by Russian Tupolev and Iljushin. Blended winglets were first introduced on the McDonnel Douglas MD-11 aircraft in 1990 with launch customer Finnair (it also features a smaller winglet at the bottom side of the wing). In contrast to Airbus who applies the wingtip fences by default on most of their aircraft (and the winglets on the A330/340 family), 
blended winglets are considered by Boeing for example as an optional extra feature on their products, except for the Boeing 747-400. For some of the older Boeing jets (737 and 757) such blended winglets have been offered as an aftermarket retrofit, these are the newer, tall designs and do not connect to the tip of the wing with a sharp angle, but with a curve instead. These winglets are popular among airlines that fly these aircraft on medium/long haul routes as most of the real fuel savings materialize while cruising. Longer flights mean longer cruising, thus larger fuel savings. And they also server as marketing surface for airline logos or web addresses usually.
Just recently the Boeing 767-300ER has received 3.4 m high (!) winglets produced by Aviation Partners Inc. with American Airlines as the launch-customer with Air New Zealand and Hawaiian Airlines following with orders of 5 and 8 aircrafts respectively. 141 shipsets have been pre-sold already as the forecasted fuel savings range around 4%-6% for medium/long-range flights. Airbus earlier tested similar blended winglets designed by Winglet Technology for the A320 series, but determined that their benefits did not warrant further development and they stayed with the wingtip fences instead. Aviation Partners Boeing claims that winglets on 737s and 757s have saved a collective 1.2 billion gal. of fuel since they were introduced and 11.5 million tonnes of CO2 while reducing those types’ noise footprint by 6.5%. It has sold winglets to 140 airlines and 95% of all 737NGs are fitted with them. It is working on four winglet concepts for the 777 and hopes to finalize a design for that aircraft type by December, 2008.
Blended Winglets on Several Aircraft Types
RAKED WINGTIPS are the most recent winglet variants (they are probably better classified as special wings, though), where the tip of the wing has a higher degree of sweep than the rest of the wing. They are widely referred to as winglets, but they are better described as integrated wingtip extensions as they are (horizontal) additions to the existing wing, rather than the previously described (near) vertical solutions. The stated purpose of this additional feature is to improve fuel economy, climb performance and to shorten takeoff field length. It does this in much the same way as “traditional” winglets do. In testing by Boeing and NASA, raked wingtips have been shown to reduce drag by as much as 5.5%, as opposed to improvements of 3.5% to 4.5% from conventional winglets. Airliners to use raked wingtips: Boeing 747-8, Boeing 767-400ER, Boeing 777(-200LR; -300ER; and freighter versions) plus the new Boeing 787 Dreamliner and the Airbus A350. The 747-8, the 787 and the A350 will have special, new kind of wings, which do not have a separate winglet, but have raked, and blended wingtips integrated - without a sharp angle between the wing and the winglet.
Raked Wingtips on the new Boeing 787 and Airbus A350
As you can see, wingtips/winglets have developed and changed very much over the last 30 years, but are becoming the standard, which is not proven better by anything else than the wing designs of future aircraft by the largest airplane-makers that feature a built-in winglet at the tip of their new, revolutionary wings.
