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  • Bored With Winglets?
    Posted by Graham Warwick 8:16 PM on Aug 05, 2011

    Winglets are everywhere these days, on almost every business jet and Boeings up to through the 767. Most Airbuses have wingtip fences, but the A320NEO and A350XWB will join the A330/A340 in having more conventional upward-turbing winglets.

    The winglets we see today are evolutions of the drag-reducing wingtip devices developed in the 1970s by NASA aerodynamicist Richard Whitcomb, but only the McDonnell Douglas MD-11 was built with the original two-surface design. Most have a single upward-canting winglet on each tip.

    Now Airbus and Qinetiq have been granted patents for downward-canted wingtip devices that aim to overcome some of the disadvantages of winglets.

    Winglets reduce the drag produced by wingtip vortices, but they also produce lift and a sideforce that increases the bending moment at the wing root and pushes up weight. The new designs claim to provide the drag reduction without the increase in bending moment and weight penalty.

    blog post photo
    Graphic: Airbus, via USPTO

    Airbus's downward-turned winglet (above - US patent 7,988,100) has a region that is canted more than 180deg from the vertical so that the lift it generates has a downward component to offset the bending moment on the wing. According to the patent, the preferred cant is less than 210deg from vertical.

    blog post photo
    Graphic: Qinetiq, via USPTO

    Qinetiq's scythe-shaped wingtip device (above - US patent 7,971,832) is also designed to reduce wing-root bending moment, by using a combination of sweep and reducing chord to avoid loading up the tip. In addition, the device is curved downwards so the lift vector is progressively rotated outboard to reduce the bending moment on the wing.

    Another new Airbus patent (below - 7,988,099) describes a different way to reduce the disadvantages by providing a means to control the lift generated by the winglet. This would allow winglet lift to be reduced during maneuvers as part of an active load alleviation system. In the cruise, the winglet would function as normal.
    blog post photo
    Graphic: Airbus, via USPTO

    The Airbus patent describes several ways to vary lift, including leading- or trailing-edge flaps. Other ways to reduce winglet lift include a leading-edge trip device to cause flow separation on the upper surface; and a passage through the winglet that would open to allow air to flow from the high-pressure lower surface to the upper surface.

    Tags: awt, aerodynamics

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