|Winglets on the tip of the wings of a Boeing 737-800 in the foreground, and wingtip fences on a Airbus A319, in the background.|
Wingtip devices are usually intended to improve the efficiency of fixed-wing aircraft. There are several types of wingtip devices, and though they function in different manners, the intended effect is always to reduce the aircraft's drag by altering the airflow 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 without materially increasing the 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 (e.g., available width at airport gates).
Wingtip devices increase the lift generated at the wingtip (by smoothing the airflow across the upper wing near the tip) and reduce the lift-induced drag caused by wingtip vortices, improving lift-to-drag ratio. This increases fuel efficiency in powered aircraft and increases cross-country speed in gliders, in both cases increasing range. U.S. Air Force studies indicate that a given improvement in fuel efficiency correlates directly with the causal increase in L/D ratio.
A winglet is a near-vertical extension of the wing tips. The upward angle (or cant) of the winglet, its inward or outward angle (or toe), as well as its size and shape are critical for correct performance and are unique in each application. The wingtip 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 wingtip vortex to an apparent thrust.
This small contribution can be worthwhile over the aircraft's lifetime, provided the benefit offsets the cost of installing and maintaining the winglets. 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. This possibility is greatest near airports, where slow approach and departure speeds create the strongest vortices, and the minimum spacing requirements between aircraft operations at airports is largely due to these vortices.
Winglets and wing fences also increase efficiency by reducing vortex interference with laminar airflow near the tips of the wing, by 'moving' the confluence of low-pressure (over wing) and high-pressure (under wing) air away from the surface of the wing. Wingtip vortices create turbulence, originating at the leading edge of the wingtip and propagating backwards and inboard. This turbulence 'delaminates' the airflow over a small triangular section of the outboard wing, which destroys lift in that area. The fence/winglet drives the area where the vortex forms upwards away from the wing surface, since the center of the resulting vortex is now at the tip of the winglet.
Aircraft such as the Airbus A340 and the Boeing 747-400 use winglets. Other designs such as some versions of the Boeing 777 and the Boeing 747-8 omit them in favor of raked wingtips. Large winglets such as those seen on Boeing 737 aircraft equipped with blended winglets are most useful during short-distance flights, where increased climb performance offsets increased drag. Raked wingtips are now preferred over small winglets for long-distance flights, where increased fuel economy during the cruise phase is more important.
The initial concept dates back to 1897, when English engineer Frederick W. Lanchester patented wing end-plates as a method for controlling wingtip vortices. In 1905, the Wright brothers used a concept they called "blinkers" on the canard of their Flyer III and later, on their Wright Model A aircraft. In 1910 they installed "side curtains" and modified blinkers on their production Wright Model B aircraft, to improve its stability.
Dr. Sighard Hoerner was a pioneer in the field, having written a technical paper published in 1952 that called for drooped wingtips whose pointed rear tips focused the resulting wingtip vortex away from the upper wing surface. Drooped wingtips are often called "Hoerner tips" in his honor. Gliders and light aircraft have made use of Hoerner tips for many years.
Hoerner's concept was further developed by Richard T. Whitcomb, an engineer at NASA's Langley Research Center, in response to the sharp increase in the cost of fuel after the 1973 oil crisis. Whitcomb's designs were flight-tested in 1979-80 by a joint NASA/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 implemented by McDonnell Douglas on the derivative MD-11, which was rolled out in 1990. NASA's own most notable application of wingtip devices is on the Boeing 747 Shuttle Carrier Aircraft. Located on the 747's horizontal stabilizers, the devices increase the tailplane's effectiveness under the weight of the Space Shuttle orbiter.
Even before NASA did flight testing on winglets, Burt Rutan incorporated them in his innovative Rutan VariEze homebuilt aircraft design, which made its 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 reduced the resulting drag penalty by assigning double duty to the winglets; they also serve as vertical stabilizers and rudders in his canard, pusher configuration aircraft. 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 aircraft to circumnavigate the world without refueling in 1987. However, the aircraft's wingtips were damaged when they dragged along the runway during takeoff, breaking off about a foot of each wingtip, so the flight was made without benefit of winglets..
Learjet exhibited the prototype Learjet 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 that 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 with newer models including the Learjet 55, 31, 60, 45 and Learjet 40.
Gulfstream Aerospace 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 several other business jets to reduce take-off distance, enabling operation out of smaller secondary airports, and allowing higher cruise altitudes for overflying bad weather, both of which are 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 the Dassault Group, whose French designers resisted applying them on their Dassault Falcon line until recently, were forced to run a contrarian marketing campaign. Cessna 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 announced a new version of the 747 in October 1985, known as the 747-400, with an extended range and capacity. With that particular model, Boeing used a combination of winglets and increased span to carry the additional load. The winglets increased the 747-400's 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.
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 parasitic 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 retrofitted to production sailplanes, but within 10 years of their introduction, most high-performance gliders were equipped from the factory with winglets or other wingtip devices. 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 significant competitive advantage. Many non-competition pilots installed them for handling benefits such as 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 pilot's preference.
Some airlines capitalize on the visibility of winglets to passengers. AirTran Airways, American Airlines, Southwest Airlines, WestJet and Ryanair advertise their website addresses on the inboard side of their 737's winglets.
Winglets are employed on many aircraft types, such as:
A wingtip fence is a winglet variant, with surfaces extending both above and below 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.
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, Hawker 800 and the Falcon 2000 with winglets designed by Aviation Partners, a Seattle, Washington-based firm that develops and markets blended winglets. 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 (introduction to airline service is slated for March 2009), and LAN Express and Delta Air Lines following soon after.
Airbus tested similar blended winglets, designed by Winglet Technology, for the Airbus A320 family, but determined that their benefits did not warrant further development. In December 2008, Airbus announced that, in conjunction with Aviation Partners, they are restarting their winglet testing program for the A320, stating they are putting into practice the lessons learned from tests two years before. The stated aim of the new tests is to consider "an integrated Airbus programme".
In 2009 Airbus launched a new blended winglet design which the company called a "Sharklet", designed to enhance the payload-range performance of the A320 Family. Offered as a retro-fit option, Sharklets are expected to result in a reduced fuel burn of at least 3.5 percent over longer sectors, corresponding to an annual CO2 reduction of around 700 tonnes per aircraft. The A320 will be the first model fitted with Sharklets, which will be delivered in 2012.
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 efficiency and 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, the short-range Boeing 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 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 parasitic 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 wingtip designs. Glider winglets were originally retrofitted directly to planar wings, with only a small, nearly right-angle, transition area. Once the performance of the winglet itself was optimized, attention was 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.
Closed surfaces at the end of winglets are a possible way to eliminate the wake vortices induced at the tips of a wing. An example of a closed-surface winglet is the Spiroid winglet, a design currently under development by Aviation Partners. Initial testing using a Gulfstream II test aircraft has shown the winglet design to reduce fuel consumption in the cruise phase by over 10%.
Non-planar wingtips (without winglets) are or will be employed on:
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.
Wingtip devices are also used on rotating propeller, helicopter rotor, and wind turbine blades to reduce drag, reduce diameter, reduce noise and/or improve efficiency. By reducing aircraft blade tip vorticies interacting with the ground surface during taxiing, takeoff, and hover, these devices can reduce damage from dirt and small stones picked up in the vortices.
The main rotor of the AgustaWestland AW101 (formerly the EH101) has a special "winged tip"; pilots have found that this alters the downwash field and reduces brownout which limits visibility in dusty areas and leads to accidents.
Hartzell Propeller developed their "Q-tip" propeller used on the Piper PA-42 Cheyenne and several other fixed-wing aircraft types by bending the blade tips back at a 90-degree angle to get the same thrust from a reduced diameter propeller disk; the reduced propeller tip speed reduces noise according to the manufacturer. Modern scimitar propellers have increased sweepback at the tips, resembling a raked tip on an aircraft wing.
The McDonnell Douglas MD-11, one of the first airliners to benefit from winglet research
Embraer ERJ 145XR wing
An Ultraflight Lazair showing the 1978 design of its winglets
The Swiss flag on the winglet of an Airbus A340-300