Aerodynamic Characteristics of 2D Sail sections

Discussion in 'Hydrodynamics and Aerodynamics' started by Remmlinger, Feb 16, 2021.

  1. Remmlinger
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    Remmlinger engineer

    I need the 2D characteristics for my VPP. I reviewed the literature and used XFOIL to compute the required polar curves. The paper is attached. May be someone is interested in my results and has helpful comments.
     

    Attached Files:

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  2. revintage
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    revintage Senior Member

    Thanks, will print out!
     
  3. patzefran
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    patzefran patzefran

    Impressive work ! You should look at
    Aerodynamics of Teardrop Wingmasts http://www.tspeer.com/Wingmasts/teardropPaper.htm
    Tom Speer did some numerical computations on 2D rotating wingmast + sail .
     
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  4. Remmlinger
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    Remmlinger engineer

    Everybody who works in this field knows Tom's webpage. So yes, a rotating wingmast has great potential, therefore I mentioned it in my paper. In a first step I wanted to be able to predict conventional rigs. Once I have my VPP runnig, I will surely investigate rotating masts. It will be interesting, whether the speed increase is 1% or 10% and if its worth the increased complexity.
     
  5. patzefran
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    patzefran patzefran

    I made a VPP code for my cats in the past (I am a retired engineer). I used experimental available data for sails aerodynamic.
    Speed is more related to hydrodynamic drag than Aero ! Thirty years ago I also made a 2D aero code using singularities method + boundary layer for foresail + wingmast with sail combination.
    Now XFOIL is available and more powerful.
     
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  6. CT249
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    CT249 Senior Member

    If I may make a couple of comments, from the viewpoint of someone who does not claim to have any expertise in aerodynamics;

    1- The paper claims that there is no advantage to camber over 15% is of no benefit in headsails, but sailmakers like North Sails have found that higher cambers do work well in some highly developed classes (ie J/35s, to use one example I know off by heart) higher up the luff. I can't easily find figures for mid depth, but I'm fairly sure that cambers over 15% do work in real life in some classes in many situations.

    2- Is it "a well known fact" that there is significant disturbance of flow over the mast? Mikko Brummer's CFD studies reported here don't seem to show any such thing. Nor, arguably, does real life. Wingmasts have been tried in NZ R Class, Australian Gwens, 12 Footers, British Merlins, Finns, supermaxis, Int Canoes, windsurfers and many other classes where the claims of wingmast superiority have not been borne out in practise. I've raced against a sailmaker who has won world titles and Olympic medals in wingmast classes. Both of us were surprised that in real life, his wingmasted dinghy was often at a significant disadvantage to a Laser. The Laser seems to show fairly turbulent flow behind the mast but it doesn't seem to dramatically impair the boat, to use one example. So when real life differs to theory and tests, one may wonder whether the tests are truly accurate.

    3- Significantly, in classes with wingmasts reality seems to show that the advantage is far less significant than some theory would claim. For example, in the Taipan 4.9 cats, which have a fairly deep wingmast, some top sailors de-rotate in strong wings. If rotation was such a huge advantage then why would boats that de-rotate go faster?

    Similarly, in a class like Tasars (short-chord wingmast) the speed gained by rotating the wingmast properly is actually comparatively small; for example it may be about the same as moving the jib lead 12cm too far outboard. That's not a huge amount, in practise, and far less than some theories claim.

    4- Theory seems unable to model gust response, rig weight and other issues with wingmasts.
     
  7. patzefran
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    patzefran patzefran

    Your comments is essentially a requisitory against wingmasts. Except Taipan 4.9 , your arguments are based on dinghys . If wingmasts are ineffective, why don't we see any beach catamaran with fixed masts, why racing maxi trimarans (including foilers) uses wing masts, why Vendée globe 60 ft open racing monohulls (including foil assisted) uses rotating wingmasts, not to speak of last generation of Americas cup monohull foilers. Your argument about Taipan's is poor, many class of racing beach cats (including A cats) use under rotation to depower the sail in high wind conditions, this is an advantage of rotating masts or wing wingmasts which allows to depower the rig using the best angle of rotation to spill the wind in the top of the sail.
    About Theory claims. Theory don't claim anything but numerical results using mathematical models and hypothesis in some defined conditions, these results can be compared to experimental measurement in the same conditions. Anyone can do the same numerical computation or measurements to check, this is the only claim !
     
  8. CT249
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    CT249 Senior Member

    No, I'm not against wingmasts, nor did I claim that they were ineffective. I own four of them so I'm not against them; I'm merely interested in the complexities of how much they affect performance, and why they often do not increase performance as much as many theories predict.

    The original poster said that it would be interesting to see what the speed increase was and whether it was worth the (alleged) extra complication and therefore I was trying to introduce evidence that shows that the real-life speed advantage may be less than claimed by many studies, which shows that the studies may be innaccurate or may not include some of the relevant factors.

    It's not "poor" to use Taipans as an example; I wanted to specifically isolate one class for discussion and the Taipan seemed like a good one because it has an unusually large-section wing for a one design class. The point is that the fact that many cats de-rotate seems to confirm that the advantage given by the improved flow over a wingmast is less, in some situations, than the advantage given by depowering. That seems to be significant because it demonstrates that the tests and theory that concentrate on the improved aerodynamics do not cover many of the factors relevant to the question that the OP was analysing, which is the speed increase that wingmasts give.

    Yes, many of my examples are to do with dinghies. So what? Why should they be ignored? The fact that wingmast dinghies rarely appear to perform as theory says is surely something that should be considered when the question is about how much wingmasts increase speed. Why ignore so many practical examples of wingmast use?

    Why do cats, AC boats etc use wingmasts? Because they work well in many situations. The fact that other high performance classes, such as Moths, Skiffs and windsurfers, do not use wingmasts shows that they do not work well in all situations and exploring the reasons for that is surely relevant when trying to ascertain the speed increase and whether it is worth the (alleged) extra complexity.
     
  9. Remmlinger
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    Remmlinger engineer

    It all depends on the profile of the sail. I used the NACA a= 0.1 mean line. In this case an increase of the camber beyond 15% only increases the drag. If a circular arc is used instead, larger lift coefficients can be achieved and even an 18% camber will still give a slightly higher CL. The price to pay is a significantly higher drag. I have attached a comparison with the XFOIL predictions.

    I was citing the RANS based CFD-simulations by Paton and Morvan and I wanted to make clear that their results are well accepted and in good agreement with the experiments of Wilkinson. The XFOIL-predictions are in line with these findings.
     

    Attached Files:

  10. patzefran
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    patzefran patzefran

    About Dinghy and windsurf,
    About Moth, Skiffs and windsurf (including laser ) their sails are sheated around the mast, and often the whole rig is able to rotate (laser or windsurf, moth !),
    so their leading edge are streamlined and rotate to the apparent wing like wingmast, this explain they may as aerodynamically clean . However on larger boat, one ask what is more complex, sheat the sail around the mast , or rotating spar !
     
  11. CT249
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    CT249 Senior Member

    Skiffs don't have pocket luff sails. Like other classes (Bembridge Redwing, R Class, Merlin, Int Canoe, etc) they could use pocket luffs and wingmasts but don't, because they found they were slower than "conventional" sails. As Tom Speer has said here, I think, part of the reason is that in many situations the best way to reduce drag is to make a very light spar that allows you to have a taller rig, rather than a wingmast which tends to be heavier. Secondly, in the real world, gust response is critical and in high-speed boats, handling is also critical.

    My understanding is that a pocket luff, as used in Moths, Windsurfers and Lasers, is not anywhere near as efficient as a wingmast because of the thick rounded leading edge and the rather abrupt transition from the round spar into the much flatter sail. But Moths, like As, Redwings and other boats, have tried wingmasts and wingsails but abandoned them because in the real world, the speed lost through pitching, gust response and probably stall at low apparent windspeeds and high apparent wind angles, means they don't really work in such classes.

    The classes where wingmasts tend to work seem to be those in which the dynamics of the rest of the craft mean that they can use very flat sails on stable platforms so the limited gust response of a wingmast isn't such a problem (as in cats) or where the platform is fairly easily handled and doesn't have a particularly large or tall downwind sail area (Finns, NS14s and Tasars) and therefore the handling issues are not as critical, and increasing downwind power is vital.

    This isn't an attack on wingmasts, it's an attempt to discuss whether the theoretical advantages are seen in the real world.
     
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  12. patzefran
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    patzefran patzefran

    I understand you point of view. But as well as your first post, your last sentence may be understood as the theoretical (aerodynamics) advantages of wingmast (Lift, Lift/drag ratio) are not real and not seen in the real world !
    The platform ability to carry a wingmast is another issue than pure aerodynamic, about hull drag, righting moment, dynamic stability, weight, inertia moment ........
     
  13. Remmlinger
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    Remmlinger engineer

    CT249's first comment inspired me to look into optimal sail shapes on reaching courses. I added a chapter about parabolas with large camber and high lift coefficients. The revised paper can be downloaded at http://www.remmlinger.com/2D aerodynamics.pdf
     
  14. gonzo
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    gonzo Senior Member

    I think that a rig and sailplan that doesn't account for it being on a boat is unrealistic. Response to the actual operating conditions should be taken into account in the study of a mast. Wind is gusty. A rig that responds to a gust by accelerating more than heeling will be preferable; regardless of how it performs in a study with steady air flow. Also, mast move and whip around as the boat rolls, heaves and pitches. The change of sail shape and turbulence generated by the movement and bending of the mast. has a huge influence on the efficiency of the rig. Perhaps the theories are wrong because the wing masts performance tests do not represent a boat rig.
     

  15. patzefran
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    patzefran patzefran

    Agree with the first sentence, not the second. You cannot say a theory is wrong, only users of theories are wrong, good theories are consistent with their hypothesis. To assess performance of a rig on any sailboat, you most deal with unsteady aerodynamics, aeroelasticity, boat response to sea state......etc. It is another complex coupled modeling problem than flat water performance with constant wind.....
     
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