Best Asymmetrical Airfoil for small experimental wingsails

Discussion in 'Sailboats' started by misanthropicexplore, May 4, 2018.

  1. misanthropicexplore
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    misanthropicexplore Junior Member

    I'm working on a paper project for a wingsail for a dingy with something much like a Gibbons rig. (About 750 lbs displacement.) I figured I'd make the sail 6-10 wide and 12-18 ft tall.

    The best airfoil I know of for this sort of thing is an S1223, but it looks like a pain to build with a very whippy tail. It seems to me with a chord length of around 7' I don't really need special low reynolds airfoil for this, being the Reynolds number is >250k at a 5mph wind. So I think a Clark-Y might work., but the NACA 23112 looks very easy to build, like you could put twin spars in it very easily. Any thoughts?
     
  2. tspeer
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    tspeer Senior Member

    The section needs to be symmetrical if you want the boat to sail well on both tacks. Or is this to be a flip-tacker? Or an Amoeba?
    Amoeba_section.png

    Single element sections can have a high top speed, but they accelerate slowly. Most wingsails use two symmetrical sections in tandem that form a slotted flap when deflected. NACA 4-digit sections work well, although for the main element the maximum thickness can be further aft. The flap section should be fairly thin with the maximum thickness very far forward. The leading edge suction peak on a thin flap section can be ameliorated by narrowing the gap. The gap is controlled by the hinge location. The hinge line should be on the order of 90% of the main element chord, and the flap leading edge should just barely clear the main element trailing edge when tacking.
     

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  3. misanthropicexplore
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    misanthropicexplore Junior Member

    Ok, the Amoeba is super cool, but much more complex than I have the funds to make light enough to be worthwhile. I have an idea for a form of flip-tacker which should allow me to use an asymmetrical airfoil. My goal is a knock-about rig, and the appeal of the wingsail is operational simplicity rather high performance. I'm looking performance per square foot that is as good as a Bermuda, performance per lb about as good as junk rig, and ease of use about as good as a spiritsail.

    Another reasons for leaning towards the old designs is that I've heard that due to fabric stretch over ribs and surface finish, mean high performance airfoils cannot be realized in fabric/rib construction. Any thoughts on that?

    Also the 23112 family has a long history of use in variable incident foils, like helicopter blades and Mignet formula craft. The Clark Y has slightly higher performance on the top end, but I figure 50 years worth of variable incidence wing designers having the option of Clark-Y and using the 23000's instead must mean something, thought I can't see what it is by just looking at polars.
     
  4. tspeer
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    tspeer Senior Member

    You do know how the C-class flip-tackers ended up, don't you?
    [​IMG]
    [​IMG]

    If those are your goals, a flip-tacker is an unlikely candidate. It is operationally complex, and to have the performance per lb, it has to be built very light and fragile. Any wingsail must be "flown" 100% of the time. A cambered single element section has a nonzero moment at zero lift and negative lift at zero moment. That means it will not weathervane by itself, and if you let go of the sheet it will uncontrollably produce a substantial force to windward. That is the very thing that nearly capsized ETNZ in the 2013 America's Cup. This also applies when tacking, when you have little control over the wing. It will try to fly itself to a negative angle of attack as it goes over the top, unless you can maintain positive control over it or employ a flap that will trim the moment. Either approach requires more complication in the control system and operation.

    I think a two-element symmetrical wingsail is more likely to meet your requirements. A self-tacking wingsail (with a Hubbard style control system ala the AC45) is as easy to operate (once stepped) as a spritsail rig and has a higher maximum lift than a single element wing. Its minimum drag is close to that of a single element section. The flap can be set at zero degrees, making the section completely symmetrical and it will weathervane by itself. If the boat is free to swing on its mooring, the wingsail can be left up overnight, as was common practice with the AC45 and AC72. The principal quality of a spritsail rig is all the spars can be contained within the length of the boat, allowing it to be stepped or struck on the water. No wingsail has that kind of simplicity.

    It is best to design a section specifically for your requirements. Don Stackhouse has designed model glider sections that are designed to take the covering stretch into account. The wingsail on the AC72 17 had a D-tube structure that provided a smooth surface for the leading edge and back as far as laminar flow was expected, and the contour of the open cell part of the section was nearly flat. The lee side covering on the open cells actually bulged outward slightly under airloads and the contour was surprisingly smooth. Of course, it oil-canned inward significantly on the windward side, but it's not clear if that was a good thing or a bad thing with regard to the section shape.

    A front-loaded shape like the NACA 23112 is a good choice for low Reynolds numbers, at least compared to, say, a Wortmann section. But a wingsail made up of a NACA 0020 for the first 60% chord followed by a NACA 0008 as a 40% chord flap will probably outperform it substantially and have the same physical thickness for structural stiffness.
     
  5. misanthropicexplore
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    misanthropicexplore Junior Member

    I didn't want to explain my camber adjustment idea before now, because I'm not sure I can make it clear without drawing a picture, but I'll try. My plan for the wingsail is to use a top and bottom boom that can be rotated axially +/- 30 degrees. Each boom has 3 airfoils crossing each other at centerline, longwise. Looking at the boom from the end, the sectional profile would look like a flattened asterisk. A NACA 0012 sits at the 9 o'clock to 3 o'clock line. A "left hand" 23112 sits at the 10 o'clock to 4 o'clock line. A "right hand" 23112 sits at the the 8 o'clock to 2 o'clock line. The sail is like a sock on a pencil, fully enclosing the booms. When the booms are left 9/3, there is no lift. When the boom is at either 10/4 or 8/2 it generates lift in opposite directions.
     
  6. tspeer
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    tspeer Senior Member

    I get it. However, the shape in between your booms will not match the shape of the booms. You will need a lot of tension from top to bottom to get the sailcloth to not sag in between, even for a small boat. You might be able to use a semi-flexible skin that consisted of a sheet of rods connected together, so it could flex in the chordwise direction but be rigid in the spanwise direction, although I question whether even that could provide enough rigidity at a reasonable weight. A series of rotating ribs from top to bottom might help, but you still have the issue of controlling twist and the large moments that will be applied to the booms from spanwise tension. And it still won't be better than the two-element section.

    After reading your first post, though, I think you have a bigger issue. The largest source of drag is not profile drag, it's lift-induced drag. You are proposing a sail with a low aspect ratio. The induced drag associated with a sail that is so short for its area will be high. If the height of the mast is set by stability considerations, then you have a lot of area that is not very highly loaded, and you can reduce the chord to cut the drag. I think you'll find that properly sizing the rig is more powerful than any choice of section shape.

    I'm sorry to always sound so negative, but I don't believe you've really thought through all the issues. Asking what section shape to use is one of the last things you need to determine, not the first.
     
  7. misanthropicexplore
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    misanthropicexplore Junior Member

    No worries, the reason to ask here is so I learn the problems before I spend money on them. I wondered if 2 booms would be enough, and if it wouldn't need to be a sort of 2 sided junk rig, which in fact might get too heavy to be worth the effort. But something that puzzles me about the aspect ratio, shouldn't the airfoil always have higher lift than and less drag than a sail of the same size? Shouldn't the CL/cd of the airfoil at 0.8-1.3 beat the sail at 0.5-0.8 with the same area, aspect ratio and planform?
     
  8. CT249
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    CT249 Senior Member

    While I'm not an expert by any means, your question may already have been answered here by Tom and other true experts, such as Prof Mark Drela and Mikko Brummer. I hope they don't mind if I note what they have told us.

    In threads and posts such as the ones here Soft Wing Sail https://www.boatdesign.net/threads/soft-wing-sail.18422/page-4 they said things like; "Thin airfoils are capable of the highest CL and CL/CD values"; "a thin airfoil which always has the appropriate camber shape dialed in at any given operating point will in general be superior to a thick airfoil" and " the notion that because aircraft wings are very efficient and have thick sections, while sails have thin sections and generally lower lift/drag ratios, and therefore a thick sectioned sail will aerodynamically superior to a sail rig with a thin section simply because it is thick, is a mistaken idea. Airplanes have thick sections because they are structurally stronger and because they have to operate efficiently at low lift coefficients in cruise. This is generally not the case for most sailing craft, except for very high-speed craft like landyachts and iceboats."

    Another point that has been raised here and by designers is that an airfoil (ie wingsail) cannot generally be validly compared with a thin sail of the same area, aspect ratio and planform, since wingsails are normally heavier and therefore their span is normally shorter, all else being equal. For the given weight or heeling moment, a conventional thin and soft sail can be taller than a wingsail, and that lowers induced drag substantially. Obviously there are also classes in which wingsails work very well, but those are arguably the exceptions, and we can see good reasons why C Class cats and AC boats do better with wingsails. That doesn't meant that wingsails are better for other craft.

    Furthermore, we've got about a century of practical experience with wingsails in "conventional" craft (ie monos etc) and basically not one has proven to have superior performance to thin sails. Some people claim that the lack of success is due to lack of effort or conservatism, but IMHO a more detailed examination of the long experience with wingsails and wingmasts in monos shows that to be untrue. The reason is that (as Tom and Mark have noted) wingsails are NOT better per se, and also cause major issues with factors such as gust response, pitching moment, ability to reef etc.

    One interesting underlying factor is that many of the reasons that people give about why wingsails (and similar leading-edge devices) are not adopted more widely are completely wrong, yet they keep on popping up, apparently because people like to denigrate others as being overly conservative, victims of rigid rating rules, or in some other unfavourable light. The reality appears to be that wingsails, wingmasts, pocket luffs etc are uncommon for very good physical reasons, and that there are no good reasons to denigrate those who do not use them. I say that as someone who owns four wingmasts and about 10 pocket-luff sails, and know that they are not "better" than my conventional rigs. :)
     
  9. misanthropicexplore
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    misanthropicexplore Junior Member

    Well thank you both for your replies, tspeer and CT.

    I read through several of pages on tspeer's blog, as well as this paper, https://pdfs.semanticscholar.org/8858/a72d474d6fc3dd6a3f45cfe68798ec5a307a.pdf which made the interesting point that below 8 knots winds speed, a conventional Bermuda worked better because it was lighter, but above 8 knots the wing sail preformed better because it's extra weight made bought higher efficiency.
     
  10. CT249
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    CT249 Senior Member

    Well, I'm not a scientist in any way, but I do know the history of sailing and I can say there's good reasons for having reservations about that paper. For one, I don't think a single element sail has ever shown that sort of performance in reality. Secondly, the two or three trials in medium size cats (18 Squares, A Class) have not shown superior performance, even using sophisticated double-element wings. Thirdly, at least part of the explanation for the supposed advantage of the wingsail is simply incorrect. The paper states that the ability of soft sails to handle a wider range of Rn "alone may explain why they have persisted on modern designs even after the preponderance of evidence has demonstrated that wings are vastly superior." However, there is no such "preponderance of evidence (that) has demonstrated that wings are vastly superior" - they have been tried for eons and only worked in a narrow band of cases, so in fact the preponderance of evidence demonstrates quite the opposite.

    Furthermore, the paper states that "a cloth sail suffers from aeroelastic collapse when pointed high into the wind (the sail is said to be luffing). This causes a great deal of drag when sailing closehauled and effectively limits how high the boat can point into the wind." That appears to be wrong. There are thousands of full-batten high-tension cloth sails that don't suffer from aeroelastic collapse, unless I have the term wrong. High-tension full-batten sails as seen on skiffs, Moths, many windsurfers and cats (among others) don't luff (as the term is normally used) - however there are still limits to their pointing.

    I also note that the paper claims "Inspection of a conventional sailboat shows a large block and tackle with eight or more loops of line attached to the boom is required to trim the main sail. With a winch, an additional 8:1 mechanical advantage is required to hold the boom in. To control this effectively in an automatic manner, a very large and fast-acting actuator is required. These types of actuators quickly become very expensive and a typical one would cost more than the entire budget for the project. By contrast, the wing can be designed to pivot near the center of pressure of the wing itself."

    Again, it seems to indicate that the writer has a limited knowledge. There are soft sails that can pivot near the centre of pressure, as early Windsurfer sails do much of the time and as balestron rigs do - check out some cruisers and many RC yachts. As early as the '60s there were "cranked" masts that moved the pivot point. It's not a hard thing to do.

    Rather curiously, the writer uses Prof Mark Drela's programme to work out the efficiency of the rig and yet as far as I can see (and note I'm no aero expert) he contradicts Prof Drela's information (referred to above) that says that thin sails actually have a better maximum L/D ratio.

    When we get down to it, the writer still doesn't explain why, if wingsails are so much better, in a century of experiment they have only gone better on a fairly narrow range of boats. A lot of the time, the guys who write such stuff seem to have only sailed a very conventional rig, and to not know much about the developments in modern rig design.
     
  11. misanthropicexplore
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    misanthropicexplore Junior Member

    Yeah, I found some of the things he said to be odd, though I did enjoy his break down of how he designed the foil.

    Individuals can be stupid. Groups can have stupid beliefs. But as a rule, groups of people actually doing something don't hold on to stupid ideas. There is a sort of Darwinian selection that takes place, and practical ideas tend to stick, while impractical ones don't. I've never bought the idea that 100 years worth of sailors were using cloth sails instead of wing sails purely out of bias or tradition.

    Do you know if a Mignet formula wingsail has ever been tried? (Close coupled tandem wing, instead of wing/flap/elevator combo)
     

  12. misanthropicexplore
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    misanthropicexplore Junior Member

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