Tuesday, February 5, 2013
The Way Of Active Winglets
It’s not cheap, but it adds performance and safety to a variety of airplanes
Nick Guida, chief engineer and CEO of Tamarack Aerospace Industries in Sandpoint, Idaho, has done exactly that. Guida has been working for two years on a system of Tamarack active winglets. The Tamarack Active Technology Load Alleviation System (inevitably acronymed ATLAS) consists of two major components—the Tamarack Active Control Surfaces (TACS) and the winglets themselves. Together, the system effectively sidesteps most of the problems associated with winglets.
Winglet technology has been around for years and has been employed on a wide variety of aircraft, from sailplanes and homebuilts to military transports and airliners. The concept of winglets is fairly simple, though the design and execution is considerably more complex. Winglets increase aspect ratio without a proportionate increase in span.
Aspect ratio of a wing is the relationship of span to average chord. A Lockheed U2 has an enormous wingspan and therefore a very high aspect ratio. A Lockheed F-104 (which uses the same basic fuselage) has an aspect ratio of 2.45, less than a tenth of the U2s. The result is glide characteristics for the U2 far in excess of anything short of a high- performance, competition sailplane. The F-104, in contrast, has very poor glide characteristics, so bad that standard procedure in the event of a total power failure is simply to eject.
Back in the 1970s, a NASA aerodynamicist, Richard Whitcomb, did initial research on winglets. He compared a wing with a winglet to one with a slightly longer wing. Both devices put an equal structural load on the wing. Whitcomb proved that winglets reduced tip drag by about 20% and offered double the improvement in the wing's lift-to-drag ratio.
Part of the reason was/is that winglets provide an artificial method of increasing the wing-aspect ratio without a proportional increase in span. They also can impart a slight forward thrust in the same manner a sailboat's sail can propel a boat forward into the wind. (Just think—if we could do the same with airplanes, we wouldn't need engines for cruise.)
In brief, winglets reduce the drag produced by the confluence of the airflow on the upper and lower surface of the wing where they meet the tip. This conjunction of air at the tip forms a vortex that trails behind the aircraft and creates drag.
Winglets won't work with some airfoils, and they're only marginally effective on others. In selected applications, however, they can generate significant additional lift or reduce drag, both of which result in the same benefits. Many airlines, flying aircraft never initially designed for winglets, have discovered that tipsails can improve speed and efficiency dramatically, sometimes resulting in as much as 5-7% percent reduced fuel burn. On the Boeing 737 NG and BBJ, for example, blended winglets reduce the tip vortices dramatically, and that means less drag.
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