What is a significance of a wing thickness

Discussion in 'Hydrodynamics and Aerodynamics' started by markmal, Nov 16, 2012.

  1. bearfoil
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    bearfoil Junior Member

    Thanks for the kind response. The cool thing about knowing not very much is it doesn't take long to write it down. I am not good with math, and Bernoulli has plenty. So "starting vortex", Prandtl, Navier-Stokes, etc. are fine, but each time I see them, I have to try to understand them, takes too long. Changing the direction of mass is simple to understand, and suits me. I understand what I know, and its kept me airborne for forty years.

    all the best
    bear
     
  2. P Flados
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    P Flados Senior Member

    Ok I did some more wing mast stuff. This post looks at some stuff I did with the USA 35a foil mirrored for the wing mast.

    It shows the relative affects of over-rotation and sail curvature.

    Over-rotation and lots of curvature seems good for more lift in light wind.

    Smooth mast to sail transition and flatter sail seem better for Cl/Cd in more wind.

    Nothing new from the above.
     

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  3. P Flados
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    P Flados Senior Member

    After the above, I went back and generated a new mast portion from my "blend" effort. To get a similar actual thickness, the mast itself becomes a larger fraction of chord.

    The results of this shape were pretty much the same as for the USA 35a based mast.

    To me this effort indicates the mast rotation and sail curvature play a bigger role in performance than the mast shape.

    Either the mast will work. If you want a lower fraction, the USA 35a looks fine, if you want more you might want to try something more like the Blend mast.
     

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  4. P Flados
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    P Flados Senior Member

    Again, here is something to show under rotation and flat for the blend mast
     

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  5. P Flados
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    P Flados Senior Member

    I have done about all I feel I can do with just poking around. The “Reversible camber” concept is probably not a bad choice for many applications. For around the cans class racing, slotted wings are probably best, but I see room for other choices like the “reversible camber” concept where you can add soft sail out front for light wind / down hill.

    Tom Speer's info on slotted wings (dated 1999) is the only reference point I have that seems valid for comparison. The 40% flap case can be found at http://www.tspeer.com/RigidRigs/40flap/S902fa20.htm

    He has a graph of Cl vs. alpha where he indicates that he corrected Cl down say 10% in the region of interest.

    On the polar just below the above, he shows a 'corrected' Cl of 1.6 at a Cd of 0.035.

    Since I am not going to 'correct' any of my results this looks like a target of say 1.76 for Cl at RE of 250,000 with a Cd of 0.35.

    My N30 foil with NACA 30 out front and a thin flat flap is probably best of those I tried. It gets a much better Cl max at low RE values and it does so with decent Cl/Cd. At high REs it has a nice high Cl max. The one concern is that it is not the most efficient foil for the high RE point.

    At the 250,000 RE used by Tom, my N30 foil looks like it should operate at a Cl of 1.85 at an alpha of 11° and a Cl/Cd of better than 50.

    If you were going for something where you spend a large fraction of time “fully powered up”, Better Cl/Cd at reduced alphas would be more important. My GOE 679 based foil does OK at the low RE point, but has the best Cl/Cd at at reduced alphas.
     

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  6. Petros
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    Petros Senior Member

    Good work! what I kind of figured you confirmed. the most critical part of the shape is the lee profile, and the shape of the mast is not that important relative to the shape of the whole lee shape from LE to TE.

    I also considered another shape, a kind of a "D" shaped mast with two fabric surfaces, one comming off each of the aft corners of the D shaped mast (which means two sets of battens and two tracks up the back of the mast), and than when cambering the sail I would allow the windward batten to kind of buckle inward against the lee batten, making a smoother transition from the mast to the windward fabric surface. However the doubling of the complexity did not seem a worthwhile trade-off for a marginal gain in drag reduction. It might be worth exploring later, but I want to try the simpler arrangement that you shows would form an excellent foil airfoil shape.

    Much of the lituratue also indicates as you found, that for light wind, high CL max is more important, but for high wind speeds, best L/D results in the most drive.

    The other thing I have mixed emotions about is the use of a jib. If properly designed it will greatly improve the ablity to point since I suspect it acts like a leading edge device on a wing, allowing much higher angle of attack before stall. However in all other points of sail a single efficient surface will produce the best L/D, so it should perform better without a jib when not pointing high into the wind. I might try it without a jib, and than add one later to see if there is any benefit to it. But I hate adding complexity unless there is a noticable overall benefit.

    do you have any feel for the benefit of adding a jib so such a sail/mast configuration?
     
  7. P Flados
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    P Flados Senior Member

    Two items,

    Use any of my conclusions with more than just a grain of salt until a serious review & confirmation is provided by someone with real foil design experience. Then they can be just treated with the normal grain of salt until proven by good testing.

    I see wing plus jib as more of a flexibility to address the need for two separate goals, Max lift at low wind speed / down wind and good Cl/Cd after available lift leads to overturning moment above available righting moment. This frees up the designer to focus the "wing only" performance to be for the higher RE condition.
     
  8. Erwan
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    Erwan Senior Member

    Hi Everybody,

    Sorry with this very late reply to mid-dec posts. One of my buddies borrowed me F. Bethwaite/High Performance Sailing, and it has been a nightmare to get it back home.

    Thanks Mr Speer for this reference, good to re-read it with a new perspective.

    The genesis of this knife-edge square-back mast section, is very informative and a must read for those interested with rig efficiency, thin vs thick issue ...

    Interesting comment which worth to be reported: it is mentionned that with this new more efficient mast, they had to move the centerboard closer to the mast, in order to balance "the lee helm" effect triggered by more power around the mast.

    Thanks Mikko for your references and experience regarding the Stratford/concave recovery and the S1223, I have a Selig Gugliemo workpaper about the design philosophy which lead to this section.
    I'll try to join the PDF.

    PFlados / Petros, if interested in morphing sail with possibility to add a geammaker for downwind, just have a look at the second vidéo. I found this link some time ago, maybe here on BDN or on SA

    http://www.p28foiler.com/en/media/

    Comments are in French, and what deserve to be translated is:
    1-The basic idea is to design the boat around the hydrofoil
    2-And it's all about minimizing drag.
    3-They have to cut 1000 of small pieces with high pressure water-jet and also 1000 of small tubes with tip renforcements (Swiss are used to make high precison watches, so may be their tropism for perfection is a bit too much, and a cheaper approache can be found)
    3-Results are far beyond their expectations.

    It seems to be a very light wingsail, but with probably limited high lift potential in light wind. The stays have to be attached at the top of the mast


    Cheers everybody

    EK
     

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  9. Petros
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    Petros Senior Member

    P Faldos,

    can you post or PM me the mast shape data points? It would be easier to reconstruct the mast profile with the data file than just scaling up the print outs of the profiles you posted. Thanks.
     
  10. P Flados
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    P Flados Senior Member

    Petros,

    I played around with a thicker foil, NACA 0035, and a thinner foil, NACA 0024, in addition to the NACA 0030 data I previously reported.

    They both did quite well (generally as good as or better than most of my previous attempts with mirroring the top of cambered foils).

    Unless Tom or Mark or someone similar has a better idea, I am at the point where I would just recommend picking your desired chord, your desired thickness & then generate a NACA profile to suit.

    I have attached text files (low resolution and a medium resolution) for both NACA 0024 and a NACA 0030 just in case one of these would meet your needs.

    It is very easy to be more specific if you want. Just give me a chord value and a either a thickness value or a thickness percentage and number of points desired.

    How many points relates to how you plan to use the data (import it into software or just manual scaling on paper).
     

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  11. P Flados
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    P Flados Senior Member


    I had previously read the Selig paper, but it was good to look at it again. I am just no where near far enough along to try to manipulate shapes to achieve a specific "pressure recovery" (I have only the vaguest idea of what this means).

    In general, I was just shocked at how well a thin flat, constant thickness, fat TE flap worked in the XFLR5 study. This flap configuration certainly has benefits when it comes to hinge and build considerations.

    Given that I kept my forward foil thickness at close to 18% of overall chord, the Cl max values and the Cl/Cd values look darn good when compared to most aviation foils. I am sure this is due to the fact that the goals for aviation foils is so different (they almost always want a really low Cd at "cruise" conditions).

    However, just consider that I got similar results to the S1223 at RE = 100,000. The S1223 was quite a bit better at the higher RE values, but this is not surprising when you consider foil thickness.

    As far as other "variable geometry" wing efforts, I have been watching for but have not seen any comparative results on the water. Trying to get flexible surfaces to "morph" into a good reversible cambered foil shapes sounds like more challenge than I want to jump into for now.
     
  12. Petros
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    Petros Senior Member

    okay, it was not a shaped you developed but borrowed from some known profiles. That is what I was going to do anyway, I just thought I would see if you had something different.

    I was also considering putting some taper in the batten, or use some other means of changing the stiffness, to get more camber near the trailing edge of the mast, and have the trailing edge of the fabric portion have less camber. this should create a better shape for the pressure recovery near the TE, less risk of flow separation. Have you looked at the pressure profile on a shape like that?

    BTW, I hope to build this thing in the coming months, I will post some pictures of it here if you like.
     
  13. P Flados
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    P Flados Senior Member

    I have seen a few wingmasts, but do not recall a lot of detailed discussion on advantages of specific sections.

    I noted that Doug is back on his model & has purchased a wingmast. I think I will go poke to see what he knows.
     
  14. tspeer
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    tspeer Senior Member

    The pressure recovery is the region of adverse pressure gradient in the aft portion of the section. The pressure at the trailing edge will typically be a little higher than freestream ambient pressure, and the flow has to be decelerated from the peak velocity to this near-freestream velocity, "recovering" the ambient pressure. The flow is especially prone to separation during this process, so the steepness and length of the pressure recovery is critical to the performance of the section.

    There are typically five segments to the pressure distribution. The first is the leading edge, where the flow accelerates from the stagnation point to a high speed. The second segment is often a flat "rooftop", which has near-constant velocity/pressure. Next comes a transition segment with a rounded shape or mildly adverse pressure gradient that is designed to trigger laminar to turbulent transition prior to the pressure recovery. Next is the pressure recovery, where the majority of the pressure rise occurs. The last is a closure contribution, which is a short, steep pressure rise that joins the upper and lower surface pressures together and ensures the section has a closed shape.

    Not all sections have all five of these regions explicitly represented in their design pressure distributions. A front-loaded section like a NACA 4-digit section doesn't have a flat rooftop, for example. But if you look at the Selig S1223, you'll be able to pick them out. For example, the closure contribution is especially large for this section, as a result of a lot of aft loading. Each segment has a specific purpose in the design.

    The goal of the leading edge segment is to prevent the formation of a leading edge suction peak until the section is operating outside its design range. It is often designed by designing for a higher angle of attack than intended, and rounding or clipping off the peak. That produces a rounded leading edge segment when analyzed at a lower design angle of attack.

    The rooftop segment is used to minimize the maximum velocity. There may be an absolute limit, driven by Mach number or cavitation, that constrains the section. The higher the peak velocity, the harder the pressure recovery region has to work, so there's a natural desire to not make the peak velocity any higher than necessary. There's a tradeoff between the height and length of the rooftop and the pressure recovery. The higher the rooftop, the longer the pressure recovery has to be and the shorter the rooftop can be. For a given Reynolds number, there is a height of the rooftop that results in the maximum area under the rooftop and pressure recovery, and thus the maximum contribution to lift from the suction side.

    A laminar boundary layer separates much easier than a turbulent boundary layer, and if the pressure recovery starts with laminar separation, the result can be a long separation bubble that has increased drag. A transition segment with a modest adverse pressure gradient can trigger laminar separation with a quick turbulent reattachment, forming a short bubble. The resulting turbulent flow on the pressure recovery allows that segment to be shorter and steeper without running into stall problems. A rounded transition segment allows the separation bubble to be a little farther back at low angles of attack, allowing a little more laminar flow, while moving smoothly forward as the angle of attack is increased.

    The pressure recovery can be very steep at the start, but then must flatten out as the boundary layer loses energy. A Stratford pressure recovery has a constant margin from separation along the entire segment, as is the shortest, steepest possible pressure recovery. But it also stalls all at once when pushed past its limit. The NACA 6-series sections used a linear pressure recovery, which results in a gradual trailing-edge stall but requires a greater distance to decelerate from a given height of the rooftop. A flat geometry produces a modestly concave pressure recovery that is in between the Stratford and linear recoveries.

    The closure contribution can be a bit tricky. A large closure contribution can allow the pressure recovery region to run out to a higher velocity, thus not having to decelerate as much. But you need to be really confident of your boundary layer method and ability to shape the trailing edge if you hope to achieve this without separation.

    The pressure side can have the same five segments. In addition, some high-speed sections have a flat, low velocity segment at the leading edge, before accelerating up to a rooftop in the middle of the section. This results in a thin, hooked leading edge that allows the bottom surface to contribute to the lift, while having a fat centerbody for structure.
     

  15. P Flados
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    P Flados Senior Member

    Tom,

    I catch a lot of your points, but little of it has translated into real improvement in the ability of this amateur. I have tried shifting curvature, but I rarely get the desired result:confused:

    I am however pretty willing to play around with stuff to see how the resutls change.

    Although I had previously gotten what I thought was decent max lift, the transition to upwind mode (RE of 1,000,000 to 1,500,000), did not seem so great.

    I spent quite a bit of time playing with the S1223 discussed in the Guglielmo-Selig paper linked above. I noted that a max low RE lift, there was significant flow separation across the top rear. The loss of lift on the top rear appears to be more than offset by the redirection of lower surface flow at the rear.

    I decided to do a S1223 mirror to get the nice top side pressure distribution on the front half. I then implemented a "double flap" to get a reversible camber section that mimics the S1223 as much as possible. This effort led me to discover the importance of force transitions to turbulent. This reversible camber foil needs a trip on the bottom but keeping it as far back as possible helps performance. I found 30% of total chord to work pretty good.

    The info below shows what seems to me be pretty good performance. I am within 10% - 20% of the S1223 even though I am at 18% thick instead of 12% and the S1223 gets full benefit of lots of laminar flow where I can only use 30% max.

    I even put in some discontinuities to reflect flap transitions. These really made very little difference. A double flap seems like a lot of trouble, but not needing real clean surfaces at the rear most joint should help.
     

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