HOW BLENDED WINGLETS REDUCE DRAG?

So what is it?

Boeing offers blended winglets as standard equipment on the BBJ and as optional equipment on the 737-700, -800, and -900 Extended Range (ER). Blended winglets also are available as a retrofit installation from Aviation Partners Boeing for the 737-300/-500/-700/-800/-900, 757-200/-300, and 767-300ER (both passenger and freighter variants) commercial airplanes. More than 2,850 Boeing airplanes have been equipped with blended winglets.

The carbon-fiber composite winglets allow an airplane to save on fuel and thereby reduce emissions. The fuel burn improvement with blended winglets at the airplane's design range is 4 to 5 percent. For a 767 airplane, saving half a million U.S. gallons of jet fuel a year per airplane translates into an annual reduction of more than 4,790 tonnes of CO2 for each airplane. The addition of winglets can also be used to increase the payload/range capability of the airplane instead of reducing the fuel consumption. Airplanes with blended winglets also show a significant reduction in takeoff and landing drag.

This article provides background about the development of blended winglets, describes the principle behind their operation, and outlines the types of performance improvements operators can expect from them.

The motivation behind all wingtip devices is to reduce induced drag. Induced drag is the part of the airplane drag due to global effects of generating lift. In general, wings will produce air motion, called circulation, as a result of generating lift. This motion is characterized by downward flow between the wingtips and upward flow outboard of the wingtips (see fig. 2). As a result, the wing flies in a downdraft of its own making. The lift vector is thereby tilted slightly backward . It is this backward component of lift that is felt as induced drag.

The magnitude of the induced drag is determined by the spanwise lift distribution and the resulting distribution of vortices . The vortex cores that form are often referred to as "wingtip vortices," but as is shown, the entire wing span feeds the cores. Any significant reduction in induced drag requires a change in this global flow field to reduce the total kinetic energy. This can be accomplished by increasing the horizontal span of the lifting system or by introducing a nonplanar element that has a similar effect. (More information about the aerodynamic principles of blended winglets can be found in AERO 17, January 2002.)

Blended winglets are upward-swept extensions to airplane wings. They feature a large radius and a smooth chord variation in the transition section. This feature sacrifices some of the potential induced drag reduction in return for less viscous drag and less need for tailoring the sections locally.

Although winglets installed by retrofit can require significant changes to the wing structure, they are a viable solution when gate limitations make it impractical to add to wingspan with a device such as a raked wingtip.

BLENDED WINGLET PERFORMANCE IMPROVEMENTS

The drag reduction provided by blended winglets improves fuel efficiency and thereby reduces emissions. Depending on the airplane, its cargo, the airline's routes, and other factors, blended winglets can:

1) Lower operating costs by reducing block fuel burn by 4 to 5 percent on missions near the airplane's design range.

2) Increase the payload/range capability of the airplane instead of reducing the fuel consumption.

3) Reduce engine maintenance costs.

4) Improve takeoff performance and obstacle clearance, allowing airlines to derate engine thrust.

5) Increase optimum cruise altitude capability

Source: Boeing Company.