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#1
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"steve.t" wrote in message oups.com... Is this why there are fences on wings? [I am assuming that the over sized chord shaped flat metal blade on a wing is a fence]. That is to keep the air flow straighter than it would otherwise flow? And speaking of winglets, would a winglet provide sufficient efficiency increase for piston singles to make it worth the cost of the modification (field approval or STC)? Later, Steve.T PP ASEL/Instrument Without getting into a bunch of searching for proof, what I recall reading is that winglets can be thought of as more wing, to keep the vortices from coming off the wing tip. Vortices still come off the winglet. Some of the energy is reclaimed by making the vortices from the winglet flow over the airfoil, adding lift. Angle and size are important, as being slightly wrong can soon destroy any gains. They are most efficient at speeds above what piston singles can attain. Fences and other trick wing tips (drooped, angled) are better at low speeds. You are better off adding more wingspan to increase the aspect ratio, at our size. Airliners can't simply add more wing length, and still fit in the gates and hangars, and keep the spars strong enough, so they put on the winglets. I'm sure that some of this is not quite right, but I think the general concepts are correct. -- Jim in NC |
#2
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Jim:
I've noticed and have flown C-15x with the exagerated wing tip (folded down) and I've not really noticed any improvement at low speed (which is what I think it is for, better handling at low speed for approach). What I am interested in is increased lift, and better performance (in terms of fuel usage) which is what the winglets do for the B-747 (something like 30% improvement in fuel economy! if memory serves me correctly). So if we could obtain the equivalent above 6000' MSL without wrecking the approach speed performance, I was wondering if it would be worth it. Particularly when looking at US$3/gal for 100LL. In my case, with speed mods for wheel pants, wing tips, rudder and flap gap seals, I get 1 Gal/hr better than the POH calls for with a fully run out engine (I've just had it overhauled and don't have current figures). What that means is, at 70% power, POH calls for 9.3 GPH at 6000' MSL (STD day). I get 8-8.3 GPH and 4-7 Knots better speed and a lower actual stall speed. This was rather expensive to do (all the STCs cost more than I would get back even flying 100+ hours/yr -- thankfully the plane already had them when I bought it). And I do not do aggresive leaning (I only lean until I start to get an engine drop of any RPM and then enrich 1.5 turns). But if a single mod could take advantage of the vortice energy to recover 30% at cruise, that might make it worth doing. Particularly if it added to climb rates (which I also see with all the STCs I currently have). Regards, Steve.T PP ASEL/Instrument |
#3
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"steve.t" wrote in message oups.com... Jim: I've noticed and have flown C-15x with the exagerated wing tip (folded down) and I've not really noticed any improvement at low speed (which is what I think it is for, better handling at low speed for approach). What I am interested in is increased lift, and better performance (in terms of fuel usage) which is what the winglets do for the B-747 (something like 30% improvement in fuel economy! if memory serves me correctly). Oh, no, not 30%! If that were true then every plane would have them. http://www.boeing.com/commercial/737...ets/wing2.html |
#4
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"steve.t" wrote Jim: I've noticed and have flown C-15x with the exagerated wing tip (folded down) and I've not really noticed any improvement at low speed (which is what I think it is for, better handling at low speed for approach). The fact that all small planes do not have that type of wingtip, is a testamony to what little benefit they must provide. If they were noticeable, by simple observation, everyone would be using them. I have no doubt that they have a measureable gain in slow speed performance, when measured in a wind tunnel, but enough for you to notice on the plane?....Gimmic?... They do look cool. g What I am interested in is increased lift, and better performance (in terms of fuel usage) which is what the winglets do for the B-747 (something like 30% improvement in fuel economy! if memory serves me correctly). 30% seems a little, or a lot high, from my memory. Longer wingspan, high aspect ratio wings are almost always more efficient. Look at sailplanes. That is the primary way winglets improve efficiency. Also, remember my comment about them being better at higher speeds than where we operate piston singles, or even piston twins. So if we could obtain the equivalent above 6000' MSL without wrecking the approach speed performance, I was wondering if it would be worth it. Particularly when looking at US$3/gal for 100LL. Unlikely. See above. But if a single mod could take advantage of the vortice energy to recover 30% at cruise, that might make it worth doing. Particularly if it added to climb rates (which I also see with all the STCs I currently have). Climb rates are at slower speeds, so even less gain than cruise. I am far from an expert on all things aerodynamic; I just read all I can. My take on winglets for small planes is like the argument I made on the drooped tips, earlier. If they were significant for use in small planes, everyone would be using them. Are all of the major manufactures of light planes using them? Nope. Wrong tree. (you're barking up) ;-) -- Jim in NC |
#5
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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 |
#6
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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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