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sideslipped

Wingtip device

Wingtip devices are usually intended to improve the efficiency of fixed-wing aircraft. There are several types of devices, and though they function in different manners, the intended effect is to reduce the aircraft's drag by altering the flow near the wingtips. Wingtip devices can also improve aircraft handling characteristics, and enhance safety for following aircraft. Such devices increase the effective aspect ratio of a wing, with less added wingspan. An extension of span would lower lift-induced drag, but would increase parasitic drag, and would require boosting the strength and weight of the wing. At some point there is no net benefit from further increased span. There may also be operational considerations that limit the allowable wingspan.

The wingtip devices increase the lift generated at the wingtip, and reduce the lift-induced drag caused by wingtip vortices, improving lift-to-drag ratio. This increases fuel efficiency in powered aircraft, and cross-country speed in gliders, in both cases increasing range. US Air Force studies indicate that a given improvement in fuel efficiency correlates directly with the causal increase in L/D ratio.

Winglet

A winglet is a near vertical extension of the wing tips. The upward angle (or cant) of the winglet, its inward angle (or toe), as well as its size and shape are critical for correct performance, and unique in each application. The vortex which rotates around from below the wing strikes the cambered surface of the winglet, generating a force that angles inward and slightly forward, analogous to a sailboat sailing close hauled. The winglet converts some of the otherwise wasted energy in the wing tip vortex to an apparent thrust.

This small contribution can be very worthwhile, provided the benefit offsets the cost of installing and maintaining the winglets during the 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.

The Airbus A340, and the Boeing 747-400 use winglets. Other designs such as Boeing 777 and the 747-8 omit them in favor of raked wingtips, because the gain available is very small and would make the aircraft too large for a standard airport gate. Large winglets are mainly useful for short distance flights, where the benefit in climb performance is more important. Raked wingtips are now preferred over small winglets for long distance flights where cruise performance is more important.

History and Applications

NASA Development

The initial concept came from Frederick W. Lanchester, an Englishman, in 1897, when he patented wing end-plates to control wingtip vortices. The Wright Brothers beginning in 1905 used a concept they called "blinkers" on the canard of their Flyer III powered aircraft. In 1910 they installed "side curtains" and modified blinkers on their production Model Baircraft all to achieve stability. The concept was further developed by Richard T. Whitcomb, an engineer at NASA's Langley Research Center, after the cost of jet fuel skyrocketed in the 1973 oil crisis. Witcomb's designs were flight tested in 1979-80 by a joint NASA / U.S. Air Force team using a KC-135 Stratotanker based at the Dryden Flight Research Center. A Lockheed L-1011 and McDonnell Douglas DC-10 were also used for testing, and the latter design was directly used on the derivative MD-11 rolled out in 1990.

NASA's most notable application of wingtip devices is on the Boeing 747 Shuttle Carrier Aircraft. Located on the 747's horizontal stabilizers, the devices allow for stability under the weight of the Space Shuttle Orbiter.

Composite aircraft

Even before NASA did flight testing on winglets, Burt Rutan incorporated them in his innovative Rutan VariEze homebuilt aircraft design, making the first flight with winglets on May 21, 1975. The VariEze pioneered glass reinforced plastic composite construction in homebuilt aircraft, which simplified fabrication of the winglets. He avoided any drag penalty by using winglets to do double duty, serving as the vertical stabilizers in his rear wing, canard, pusher configuration. They were also used similarly on the derivative Rutan Long-EZ, and reappeared on his Beechcraft Starship business aircraft design that first flew in 1986.

Conventional winglets were fitted to Rutan's Rutan Voyager, the first fixed-wing aircraft to circumnavigate the world without refueling in 1987. However, the aircraft's wingtips were damaged as they dragged along the runway during takeoff and Dick Rutan, who was piloting, sideslipped the plane to cleanly shear off the winglets so that they would not pose a risk to the fuel tanks.

Business aircraft

Learjet exhibited the prototype Learjet Model 28 at the 1977 National Business Aviation Association convention. The Model 28 prototype employed the first winglets ever used on a jet and a production aircraft, either civilian or military. Learjet developed the winglet design without NASA assistance. Although the Model 28 was intended to be a prototype experimental aircraft, performance was so impressive it resulted in a production commitment from Learjet. Flight tests made with and without winglets showed that the winglets increased range by about 6.5 percent and also improved directional stability. Learjet’s application of winglets to production aircraft continued to newer models including the Learjet 55, 31, 60, 45, and Learjet 40.

Gulfstream also explored winglets in the late 1970s and incorporated winglets in the Gulfstream III, IV, and V. The performance of the Gulfstream V has been exemplary. Its operational range of 6,500 nmi permits routine nonstop business travel for routes such as New York–Tokyo. The Gulfstream V also holds over 70 world and national flight records.

Winglets are also applied to many other business jets to reduce take-off distance, enabling operation out of smaller secondary airports, or allowing higher cruise altitudes for overflying bad weather, valuable operational benefits for corporate travel. In addition to factory installed winglets on new aircraft, aftermarket vendors developed retrofit kits for popular jets and turboprops, to improve both aerodynamics and appearance. Winglets became so popular on this class of aircraft, that Dassault, whose French designers resisted applying them on their Falcon Jet line until recently, were forced to run a contrarian marketing campaign. Cessna Aircraft Company recently announced they were partnering with Winglet Technology, LLC of Wichita, Kansas to test a new wingtip device called Elliptical Winglets, which are designed to increase range and increase payload on hot and high departures.

Boeing airliners

Boeing announced a new version of the 747, in October 1985, known as the 747-400, with extended range and capacity. With this particular model, Boeing used a combination of winglets and increased span to carry the additional load. The winglets increase the 747-400 range by 3.5 percent over the 747-400D, which is otherwise aerodynamically identical but has no winglets. Winglets are preferred for Boeing derivative designs based on existing platforms, because they allow maximum re-use of existing components. Newer designs are favoring increased span, other wingtip devices, or a combination of both, whenever possible.

Gliders

In 1987, mechanical engineer Peter Masak called on Mark D. Maughmer, an associate professor of aerospace engineering, about designing winglets to improve performance on his 15-meter wingspan racing sailplane. Others had attempted to apply Whitcomb's winglets to gliders before, and they did improve climb performance, but this did not offset the parasite drag penalty in high speed cruise. Masak was convinced it was possible to overcome this hurdle.

By trial and error, they ultimately developed successful winglet designs for gliding competitions, using a new PSU–90–125 airfoil designed by Maughmer specifically for the winglet application. At the 1991 World Gliding Championships in Uvalde, Texas, the trophy for the highest speed went to a winglet equipped 15-meter class limited wingspan glider, exceeding the highest speed in the unlimited span Open Class, an exceptional result. Masak went on to win the 1993 U.S. 15 Meter Nationals gliding competition using winglets on his prototype Scimitar sailplane.

The Masak winglets were originally retrofit to production sailplanes, but within 10 years of their introduction, most high-performance gliders were equipped from the factory with winglets, or some other wingtip device. It took over a decade for winglets to first appear on a production airliner, the original application that was the focus of the NASA development. Yet, once the advantages of winglets were proven in competition, adoption was swift with gliders. The point difference between the winner and the runner-up in soaring competition is often less than one percent, so even a small improvement in efficiency is a large competitive advantage.

Many non-competition pilots installed them for the handling benefits, including increased roll rate and roll authority, and reduced tendency for wing tip stall. The benefits are notable, because sailplane winglets must be removable to allow the glider to be stored in a trailer, so they are usually installed only at the pilots' preference.

Advertising

Some airlines capitalize on the visibility of winglets to passengers. AirTran, American Airlines, Southwest Airlines, WestJet and Ryanair advertise their website addresses on the inboard side of their 737's winglets.

Notable examples

Winglets are employed on many aircraft types, some notable examples are:

Wingtip fence

A wingtip fence is a winglet variant with surfaces extending both upward and downward from the wingtip. Both surfaces are shorter than or equivalent to a winglet possessing similar aerodynamic benefits.

Wingtip fences are the preferred wingtip device of Airbus, employed on all their airliners except for the A330 and A340 families. The Airbus A350 will also make use of winglets rather than wingtip fences.

Blended winglets

A blended winglet is intended to reduce interference drag at the wing/winglet junction. A sharp interior angle in this region can interact with the boundary layer flow causing a drag inducing vortex, negating some of the benefit of the winglet. The blended winglet is used on business jets and sailplanes, where individual buyer preference is an important marketing aspect.

Blended winglets have been offered as an aftermarket retrofit for Boeing 737, 757 and Raytheon Hawker 800 with winglets series aircraft by Aviation Partners Inc., and the 737 version is now standard on the Boeing Business Jet derivative. Many operators have retrofitted their fleets with these for the fuel cost savings. Aviation Partners has also developed winglets for the 767-300ER with American Airlines being the first customer. Airbus tested similar blended winglets designed by Winglet Technology for the A320 series, but determined that their benefits did not warrant further development.

Raked wingtip

Raked wingtips are a feature on some Boeing airliners, where the tip of the wing has a higher degree of sweep than the rest of the wing. 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 that winglets do, by increasing the effective aspect ratio of the wing and interrupting harmful wingtip vortices. This decreases the amount of lift-induced drag experienced by the aircraft. 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. An increase in wingspan is generally more effective than a winglet of the same length, but may present difficulties in ground handling. For this reason, Boeing's short-range 787-3 design currently calls for winglets, instead of the raked wingtips featured on all other 787 variants.

Raked wingtips are or are planned to be employed on:

Non-planar wingtip

Non-planar wingtips are normally angled upwards in a polyhedral wing configuration, increasing the local dihedral near the wing tip. These provide the wake control benefit of winglets, with less parasite drag penalty if designed carefully. The non-planar wing tip is often swept back like a raked wingtip, and may also be combined with a winglet. A winglet is also a special case of a non-planar wingtip.

Aircraft designers employed mostly planar wing designs with simple dihedral after World War II, prior to the introduction of winglets. With the wide acceptance of winglets in new sailplane designs of the 1990s, designers sought to further optimize the aerodynamic performance of their wing tip designs. Glider winglets were originally retrofit directly to planar wings, with only a small, nearly right-angle transition area. Once the performance of the winglet itself was optimized, attention turned to the transition between the wing and winglet.

A common application was tapering the transition area from the wing tip chord to the winglet chord, and raking the transition area back to place the winglet in the optimal position. If the tapered portion was canted upward, the winglet height could also be reduced. Eventually designers employed multiple non-planar sections, each canting up at a greater angle, dispensing with the winglets entirely.

The Spiroid winglet, a winglet that loops back onto the wing to attempt to eliminate winglet tip induced vortices, is under development.

Non-planar wingtips (without winglets) are or will be employed on:

Actuating wingtip devices

There has been research into actuating wingtip devices, including a filed patent application, though no aircraft currently uses this feature as described. The XB-70 Valkyrie's wingtips were capable of drooping downward in flight, to facilitate Mach 3 flight using waveriding.

Gallery

References

See also

External links

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