Winglets

Background

As early as the 1800s, it was widely recognized that the aerodynamic efficiency and drag of aircraft wing shapes were dependent on not only profile drag (largely a two-dimensional effect) but also the induced drag or drag due to lift. The induced drag is produced by the generation of three-dimensional airflow characteristics near the tips of aircraft wings. As the flow encounters the wingtip shape, it rolls up over the tip side edge resulting in the well-known trailing vortices displayed by lifting wings. The energy expended in this phenomenon is directly responsible for the induced drag, which can be extremely large for certain aircraft wing configurations—particularly under high-lift, low-speed flight conditions. Induced drag also represents a significant decrement in aircraft efficiency for subsonic transports operating at high subsonic speeds and accounts for as much as 50 percent of total drag.

Early studies by Frederick W. Lanchester and others in England indicated that vertical surfaces located at the wingtips could significantly reduce the three-dimensional effects and thereby reduce induced drag. On the basis of theoretical studies, experimental investigations of vertical endplates were undertaken and the results showed significant reductions in drag at high-lift conditions (Lanchester patented the endplate concept in 1897). Unfortunately, near cruise conditions, these configurations exhibited large areas of local flow separation; this resulted in large viscous drag increments in profile drag, which essentially negated the benefits to induced drag. Thus, although the potential drag-reducing effects of endplates on induced drag had been identified, the net effect of simple endplates was regarded as inconsequential because of the increase in profile drag.

Following his intensive studies that led to the development and application of supercritical airfoils, Langley’s Richard T. Whitcomb continued his quest to reduce cruise drag for high subsonic speeds. Inspired by an article in Science Magazine on the flight characteristics of soaring birds and their use of tip feathers to control flight characteristics, Whitcomb zeroed in on the wingtip flow phenomena associated with induced drag. Although many scientists had proposed wingtip configurations that mimicked birds (numerous wingtip feathers) for the reduction of induced drag, Whitcomb’s guiding principle was to analyze the detailed aerodynamics involved in the flow at the wingtip and develop a more practical wingtip configuration.

Aerodynamic flow mechanisms of winglet concept.

Whitcomb’s analysis of flow phenomena at the tip showed that the airflow about the wingtip of the typical aircraft in flight is characterized by flow that is directed inward above the wingtip and flow that is directed outward below the wingtip. Whitcomb hypothesized that a vertical, properly cambered and angled surface above or below the tip could utilize this crossflow tendency to reduce the strength of the trailing vortex and, thereby, reduce the induced drag. The drag reduction mechanism is achieved by a forward vectoring of the side force generated by the winglets. The aerodynamic effect of the winglet is very similar to that of a sailboat tacking upwind. The resulting reduction in aircraft drag for a properly designed winglet configuration can be extremely significant in terms of fuel consumption and range.

In essence, Whitcomb and his team provided the fundamental knowledge and design approach required for an extremely attractive option to improve the aerodynamic efficiency of civil and military aircraft. Throughout these development efforts, he emphasized to the technical community that the design of winglets requires considerable care and attention to airfoil aerodynamic characteristics. To emphasize this point, Whitcomb called the wingtip surfaces winglets to stress the fact that the design process required efforts similar in sophistication to those required for wing design.

Langley Research and Development Activities

Whitcomb’s initial studies of winglets followed his time-proven experimental approach in the Langley 8-Foot Transonic Pressure Tunnel in the early 1970s. Leading a team of researchers that included Stuart G. Flechner and Peter F. Jacobs, he launched an extensive research program that included theoretical calculations, physical flow considerations, and extensive exploratory wind-tunnel experiments. These studies recognized that the acceptability of the winglet concept would depend on the impact of the winglets on structural weight and the high-lift off-design performance of the wing, as well as the magnitude of drag reduction at design conditions. Many of his technical peers pointed out to Whitcomb that the improvement in induced drag could also be achieved by simply increasing the span of the wing. However, an increase in span also increases the bending moments in the wing structures; thus, the required increase in the wing structure to accommodate these additional loads produces an increment in aircraft weight that is larger than for winglets for the same aerodynamic improvement. In fact, in most applications the reduction in drag at the cruise condition for winglets is about twice that for a simple tip extension with the same increase in wing-root bending moment. An additional, very significant advantage of using winglets rather than wing extensions is that the configuration has a smaller wingspan. Therefore, winglets provide special benefits for configurations whose wings are structurally constrained or span constrained by airport ramp dimensions or runway length.

Early experiments on the winglet concept began in the 8-Foot Transonic Pressure Tunnel in 1974. Whitcomb’s team initially evaluated and developed winglet configurations that consisted of both upper and lower wingtip surfaces, with each surface designed for the local flow conditions. The upper winglet, which was the primary component of the winglet, was placed rearward on the wingtip to minimize adverse interference effects at the intersection of the wing and winglet. The experimental results showed that the leading edge of the root of the winglet should not be significantly ahead of the upper-surface crest of the wingtip section, and that the greatest winglet effectiveness was achieved with the trailing edge of the winglet near the trailing edge of the wing. In addition, the winglet airfoil should be shaped so that the desired inward force is obtained for the aircraft design flight condition, particularly for high subsonic supercritical conditions. In achieving this side-force requirement, designers can make use of Whitcomb’s supercritical airfoil technology for the cross-sectional shape of the winglet airfoil. The winglets must also be designed to avoid flow separation both on the winglet surface and the winglet-wing juncture at low-speed, high-lift conditions as well as cruise conditions.

Semispan model of KC-135 with winglets in Langley 8-Foot Transonic Pressure Tunnel.

DC-10 model during winglet studies in 8-Foot Transonic Pressure Tunnel in 1974.

The winglet concept was evaluated and tested extensively in the 8-Foot Transonic Pressure Tunnel from 1974 to 1976. In July 1976, Whitcomb published a general design approach that summarized the aerodynamic technology involved in winglet design. At that time, the tunnel tests indicated that, for typical subsonic transport aircraft configurations, the induced drag could be reduced by about 20 percent and the aircraft lift-drag ratio could be increased by about 9 percent. The improvement in lift-drag ratio was more than twice as great as that achieved by a wingtip extension producing the same wing-root bending moment. Because the lower winglet could adversely impact the ground-handling equipment for low-wing aircraft configurations, the lower winglets were subsequently eliminated from some low-wing applications, such as the Boeing 747-400.

The impressive results of the winglet studies were quickly disseminated to the U.S. civil and military communities. Flechner presented a summary of Langley wind-tunnel results obtained for winglet technology applied to four configurations (KC-135, Lockheed L-1011, McDonnell Douglas DC-10, and a generic high-aspect-ratio model) to an extremely large audience at a meeting on transport technologies at Langley in early 1978. Widespread interest and application studies for large commercial transports, business jets, and personal-owner aircraft rapidly grew on a nationwide basis.

As part of the NASA ACEE Program, Boeing, Douglas, and Lockheed studied the impact of winglets on near-term derivative aircraft. Boeing’s initial wind-tunnel and design evaluations for the Boeing 747 configuration in May 1977 indicated that the winglet would not provide adequate economic return to the airlines for the cost of fabrication. Despite this early negative assessment, Boeing later adapted winglets to the 747-400, as is discussed in a subsequent section. Lockheed’s studies indicated that extending the wingtips of the L-1011, together with the use of active controls to relieve loads, was a more favorable approach than the use of winglets. Douglas, however, was impressed with the potential benefits of winglets to structurally or span-constrained configurations, and the company proceeded to modify a DC-10 for flight tests.

Flight Evaluations

As NASA searched for a potential aircraft for in-flight evaluations and demonstrations of winglet technology, Langley carefully examined the impact for candidate configurations. The early jet transports featured an elliptical-type span loading with relatively high loads on the outer wing panels. The KC-135 exemplified this family of aircraft, as did its civilian derivative, the Boeing 707. The application of winglets was known to be more effective for configurations with highly loaded wingtips. In contrast, second-generation subsonic transports such as the DC-10 and L-1011 used nonelliptic loading to avoid pitch-up characteristics at postbuffet conditions; thereby the wing bending moments and structural requirements were reduced. Whitcomb and his staff emphasized the importance of wingtip loading as a beneficial factor in winglet performance, and preferred the KC-135 as a test vehicle because of its advantageous wingtip loading. In addition, the Air Force had expressed a keen interest in a potential winglet retrofit to the KC-135 fleet for improved fuel efficiency following a series of industry studies. The mutual NASA and Air Force interests quickly resulted in proposals for a flight program.

KC-135 winglet flight tests at Dryden Flight Research Center.

The aerodynamic winglet design for the KC-135 was completed by Whitcomb and his staff at Langley, and the structural design and fabrication were accomplished by Boeing, Wichita Division. Joint NASA-Air Force flight tests of the modified KC-135 military tanker aircraft were conducted at the Dryden Flight Research Center in 1979 and 1980. The results of the KC-135 flight tests verified the wind-tunnel results obtained in the Langley 8-Foot Transonic Pressure Tunnel. A 7-percent gain in lift-drag ratio and a 20-percent reduction in drag due to lift were achieved at the cruise condition. Despite these impressive benefits, Air Force priorities and limited budget options resulted in a decision to retrofit the KC-135 fleet with new engines, rather than winglets, as a more efficient fleet modification.

Modified DC-10 during Douglas winglet flight tests in 1981.

Full-span KC-135 model testing in Langley 8-Foot Transonic Pressure Tunnel preceding flight tests in 1977.

Under sponsorship of the NASA ACEE Program, Douglas proceeded with its flight evaluation of winglets on a modified DC-10 Series 10 aircraft in 1981 following a series of tunnel tests including performance tests of a 4.7-percent semispan model in the 8-Foot Transonic Pressure Tunnel. The 16-month flight test program involved leasing a test aircraft from Continental Airlines in April 1981, conducting 61 total flights with and without winglets, and returning the aircraft in November. Douglas conducted the configuration aerodynamic and structural designs and fabrication (two winglet spans were flown). The flights were made at the Douglas Long Beach facility at Edwards Air Force Base and the Douglas facility at Yuma, Arizona.

During the DC-10 tests, a buffet problem was experienced in low-speed flight due to aerodynamic flow separation at the winglet-wingtip intersection; leading-edge shape modifications on the upper and lower winglets were found to eliminate the problem. Douglas also found that the greatest drag reduction was obtained with both upper and lower winglets used and that the use of aileron droop improved the drag reduction.

From flight test results the estimation was that the application of a reduced-span winglet and aileron droop to a DC-10 Series 10 aircraft would yield a 3-percent reduction in fuel burned at the range for capacity loads. Although not retrofitted to the DC-10 because of unacceptable recertification costs, the improved performance provided by the winglets favorably impressed the Douglas organization, and winglets were later designed and implemented for the McDonnell Douglas MD-11 transport.

Applications

Arguably, the most aggressive initial U.S. applications of winglet technology came from within the general aviation and business jet community. The first aircraft to fly with winglets was the propeller-driven Vari-Eze light homebuilt aircraft, designed in 1974 by Burt Rutan, designer and builder of the Voyager aircraft, which made the first nonstop, nonrefueled around-the-world flight in December 1986. Rutan incorporated control surfaces on the winglets for rudder control. He designed the Vari-Eze to prove that the canard (wing stabilizer near the nose of the aircraft) configuration could be more efficient and safer than conventional designs. He demonstrated that efficiency by setting a world record in the under-500 kg (1100 lb) class in 1975. Advanced design features which helped the Vari-Eze achieve this world record performance included low-weight composite materials, a lightweight engine, innovative aerodynamic configuration, high-aspect-ratio wings, and winglets. Other propeller aircraft that utilize winglets include the Beech 1900D regional transport and Beech King Air 350 corporate turboprop.

In 1977, Learjet displayed an exciting new test-bed aircraft designated the Learjet Model 28 at the National Business Aircraft Association convention. The Model 28 had been involved in high priority developmental testing of a new wing for a major new Learjet project to be known as the Model 55. 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, and referred to the new wing as the Longhorn, which coupled the new NASA winglet technology with a wing that had higher aspect ratio. Although the Model 28 was intended to be a prototype experimental aircraft, the performance of the new aircraft was extremely impressive and 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 Model 28/29, first production jet aircraft to utilize winglets.

Record-setting Gulfstream V with supercritical airfoil sections for its winglet design.

Following the highly successful application of winglets, one of the Learjet prototype aircraft used in the winglet development program (aircraft 25-064) was acquired by the Langley Research Center for flight tests of new NASA research concepts in October 1984. Joseph Stickle was instrumental in the acquisition of the aircraft. Flying as NASA 566, the aircraft was used extensively by Langley researchers Cynthia C. Lee, Bruce J. Holmes, Clifford J. Obara, and Bruce D. Fisher in flight tests of natural laminar flow airfoils (1984 to1989) and studies of lightning phenomena (1990 to 1993).

Learjet’s application of winglets to production aircraft continued through the Model 28 to subsequent current applications including the Model 55, Model 31, Model 60, and Model 45.

Gulfstream had also been aggressively studying applications of winglets in the late 1970s (contemporary with the Lear activities) and incorporated winglets in its line of business jet transports including the Gulfstream III, Gulfstream IV, and Gulfstream V. The performance of the Gulfstream V has been spectacular. Its operational range of 6,500 nmi at a cruise Mach number of 0.80, and cruise speed capability up to Mach 0.89, permits routine nonstop business travel for routes such as New York–Tokyo. The Gulfstream V also holds over 70 world and national flight records.

Boeing 737-800 with winglets.

Boeing B747-400, first large U.S. commercial transport to incorporate winglets.

In October 1985, Boeing announced a new version of the 747, known as the 747-400, with extended range and capacity. With this particular model, Boeing introduced winglets for enhanced performance. The winglets increase the 747-400 range by about 3 percent. In addition to the 747 application, Boeing offers its winglet designs as customer options in its 737 business jet and its 737-800 aircraft. Building on the foundation acquired in its Energy Efficient Transport (EET) studies for the DC-10, McDonnell Douglas included the winglet concept in its design for the MD-11, which entered service in December 1990.

As frequently happens with the introduction of new technology, widespread interest quickly grew in the potential application of winglets as a retrofit to existing aircraft. Within the U.S. industry, a number of small consulting organizations have attempted to meet this interest with retrofit capabilities. For example, winglets have been retrofitted to the Boeing 727 configuration. The retrofit process involves a number of Federal Aviation Administration (FAA) regulatory constraints that must be complied with to permit such modifications. For example, the 727 modification was accomplished by removing part of the outboard wing before the winglet was added so as to maintain the same root-bending moments for the wing.

Worldwide applications of winglets have extended far beyond transport aircraft and business jets. For example, personal-owner aircraft have been modified and several competition gliders (15-m class) have been modified or designed with winglets. In this somewhat surprising application (the performance of high-aspect-ratio gliders would not be expected to benefit greatly from winglets), the winglets considerably improved the roll response of the aircraft. Other interesting applications have included the use of winglets to control the flow fields and wake turbulence behind an aircraft for aerial applications. By modifying the trailing wake and moving the tip vortex up from the wingtip to the winglet tip, Langley researchers were able to beneficially affect the dispersal characteristics of particles and sprays for aircraft used in agricultural or forestry missions.

Perhaps no single NASA concept has seen such widespread use on an international level as Whitcomb’s winglets. The multitude of applications is clearly visible across the spectrum of civil aircraft, even to the most casual air traveler.

McDonnell Douglas built on development experience gained
in NASA ACEE Program to design winglets for the MD-11.