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Jim and Steve,
Winglets work by 'flying' in the tip vortex; they are placed on the wing with a slight 'toe out' such that their lift vector is inward and slightly forward. They fly with a posivitve AOA at this toe out angle because their local relative wind is the top (inward rotating) part of the tip vortex. The forward component of their lift vector shows up as a reduction of induced drag. Winglets are effective only on aircraft that cruise at or near the stall speed, where the vortex is large. This is true on most bizjets and transport type jets; while their true airspeed is very high, their indicated airspeed is nearly down to the stall. Most of these types of aircraft cruise in this little 'corner' of the envelope, just above the stall speed and just below the limiting Mach numer. A little slower and you stall, a little faster and you get into bad Mach effects (buffeting or Mach tuck.) It is commonly called the coffin corner. Winglets are effective on these planes not because they are flying fast, but rather because (in indicated airspeed) they are flying slow (close to the stall). The low IAS means they fly at a high AOA (nearly stalled) and create strong vortices for the winglets to fly in. Winglets are typically of no or even negative value on light aircraft because our cruise speed is typically about twice the stall speed. This means we cruise with a much lower AOA, and therefore create much weaker wingtip vorticies. Without the strong vortex, there is little rotating flow for the winglet to fly in, and it just adds parasite drag. The voyager had winglets (until Dick scraped them off) because that aircraft was designed to fly at the airspeed of maximum range, which was near enough to the stall speed to make the winglets effective. That the aircraft made it around the world without them attests to the fact that the induced drag reduction is incremental (a few percent.) Regards, Gene |
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![]() wrote in message ups.com... Jim and Steve, Winglets are effective only on aircraft that cruise at or near the stall speed, where the vortex is large. This is true on most bizjets and transport type jets; while their true airspeed is very high, their indicated airspeed is nearly down to the stall. Thanks. Of course; one part I forgot. -- Jim in NC |
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