Question about wingsail foil shape

Discussion in 'Hydrodynamics and Aerodynamics' started by satrams, May 6, 2014.

  1. Erwan
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    Erwan Senior Member

    Hi Everybody,

    Interesting comment regarding AC72 wingsail section, it reminds me the workpaper attached. Hope it is relevant.

    Cheers

    Erwan
     

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  2. markdrela
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    markdrela Senior Member

    Not that much.

    For the low Re airfoils in the paper, a major goal is promoting transition to avoid massive transitional separation bubbles. At the higher Re of a wingsail, transitional separation bubbles are not much of a concern.
     
  3. satrams
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    satrams Junior Member

    Little confused about RE

    I'm a little confused about the Reynolds Numbers that I properly should be dealing with for my project, and what is considered 'high' and what is considered 'low'. Or, also, seems to me there is a big difference between what is considered 'high speed' or 'low speed' between airplane wings and sailing wings.

    I already noted that my land-sailer probably will generally be sailing at (and designed to sail at) slower speeds than a lot of the type of land yachts that have become the norm (ie: apparent wind will be less).

    That is why my intuition was that the performance I was after might be a little more similar to an AC45, although obviously I would be able to surpass the speeds of these boats. But a concern I had was about the initial acceleration power from rest, which I was feeling I would want more of as opposed to ultimate top speed.

    All of this seems to point more towards lower RE numbers, doesn't it? (And also a thicker cross-section)

    Btw- a really basic and important question I have is- for a wing with an equal flap such as I am planning to use, is the Reynolds number the combined chord of the forward element and flap? (I guess that's why it's called a 50% chord flap?- the terminology gets a little confusing- since the hinge point is considered 90%??)

    My initial plans were for approximately a 2 foot forward foil and a 2 foot flap at the biggest point.

    What type of range is usually looked for or analyzed?- all the way from a medium wind speed (10-15) with the vehicle at rest to theoretical full speed with high wind? Or something more narrow than that? (if I went with what I'm talking about, RE would be starting out at around 400,000)

    Thanks guys.
     
  4. markdrela
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    markdrela Senior Member

    One reasonable definition of "low Reynolds number airfoil" is one where significant compromises of clmax,cl range, or max thickness had to be made to coax the boundary layer to undergo transition at a good location over the operating cl range. Usually this is anywhere from 50k to 250k, depending on the cl and thickness requirements. If not done right, the result will be strong transitional separation bubbles and high drag.

    Above 250k or so, preventing strong bubbles is relatively easy, with little or no compromise of clmax or cl range.

    Above 500k-1000k, the focus shifts to delaying rather than promoting transition.
     
  5. Erwan
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    Erwan Senior Member

    Thanks Mr Drela for your comments and for the low Reynolds breakdown, I had the false assumption that bubble were still a concern around 1 million Reynolds.
     
  6. tspeer
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    tspeer Senior Member

    The issue with laminar flow and wingsails has to do with environmental factors. Is there so much turbulence in the free stream that transition will happen very early? Will a halyard hanging in front of the wing, or along the wing surface, prevent laminar flow? What about when the wing is covered with fog and rain droplets?

    And then there are practical construction issues. Are the branding decals thick enough to cause transition at their edges? Will waviness of the wing structure cause transition? To some extent, these become self-reinforcing. If people don't believe laminar flow is possible or practical, they won't take the care necessary to make the surface smooth enough and to maintain it in a condition that will support laminar flow. So of course, the flow won't be laminar.

    In the end, it turns out wing profile drag is a small contributor to the total drag. The difference around the race course between a fully turbulent design and an aggressive laminar flow design is basically the same distance lost as one bad gybe. It could definitely make the difference between winning and losing in a close race, but there are much bigger gains to be made elsewhere.
     
  7. Erwan
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    Erwan Senior Member

    Thanks Mr Speer,

    In fact it's not the section drag which is a concern, but the max lift.

    If you consider a high lift wing section with XFOIL/ nCrit=9 everything is fine
    If you decrease Ncrit to mimic natural turbulences of wind BL, max Cl decrease accordingly and if you force transition at 5% leeward and 10% windward, you get a 20% drop in max Cl.

    That is why NLF as presented in the workpaper, can look attractive for a rookie.

    Cheers

    EK
     
  8. lohring
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    lohring Junior Member

    The idea of the slotted flap is to provide maximum lift. The early Patient Lady wing designs used a three element section. The current C class designs use a flapped trailing edge on the front section. It all depends on how much lift you actually need. The faster the boat, the more lift to drag matters over maximum lift. Monohull sailboats always operate at the maximum lift point. Land yachts like the Greenbird operate at (or maybe below) the maximum lift to drag point and don't need flaps. A velocity prediction program can help decide on the best solution for a particular case.

    Lohring Miller
     

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  9. quequen
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    quequen Senior Member

    Regarding the image posted by Mr. Speer on post #2, wich role plays the Mach number? (since compressibility should be almost zero at this speeds). Is it an AWS indicator?

    0.15 x 340m/s = 51m/s seems a pretty high AWS for an AC45

    On the other hand, Re = 1.37x10^6, with 2.2mts of chord at the top of an AC45 wingsail = 9.4m/s = 18kts seems Ok.

    -
    lohring: could you please mention the source of your attachment? I'd like to read the full article.

    -
     
  10. satrams
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    satrams Junior Member

    Coming back to this and also to what is being referred to above about RE and the AC45 wing, I hate to contradict anything that Mr. T would say, and actually it doesn't really contradict it anyway.
    It's just the interesting observation, that from what I now have been able to observe and compare, the top section shape shown in the plot is pretty much an exact match for the drawing version of what the bottom section shape is.

    This is what I had been inferring from the drawing, and this plot seems to verify it; that the sections are just exactly proportional versions of each other (at least the top and the bottom), ie: the thickness percentages are identical.
    This would make sense- again, as far as Tom talking about the simplification of aspects of the boat for easier construction.
     
  11. lohring
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    lohring Junior Member

    It's from Marine Technology 1974 at the beginning of the wing era. Otto Scherer and Dave Hubbard are pretty much responsible for the thinking that started the modern wing design you see in the current AC multihulls. Credit also should go to the late Tony Di Mauro, the quiet force behind the Patient Ladies, and his wife, the patient lady who put up with this foolishness. I was fortunate to know all of them.

    Lohring Miller
     

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  12. satrams
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    satrams Junior Member

    Thanks a lot, Lohring, for putting that up.
     
  13. quequen
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    quequen Senior Member

    Thanks lohring. After forteen years papers like this, and people involved, are definitely part of the history.
     
  14. tspeer
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    tspeer Senior Member

    The lower sections actually had a higher thickness ratio in the main element. I showed the top section because I thought it was the most relevant for your purposes. The thicker sections were to give more structural stiffness. The aerodynamics weren't much different, other than a little higher drag.

    If you assume the flow is fully turbulent, the profile drag is not at all sensitive to the precise shape of the main element. The turbulent profile drag correlates almost exclusively with the thickness.
     

  15. satrams
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    satrams Junior Member

    Yes- OK, thanks, Tom. I think I have enough to go on now, and now understand the point that you have been stressing- that the flow will be turbulent.

    It seems there is probably a little too much discussion of theories of aerodynamics and flow that end up pertaining a lot more to aircraft than to boats, and too much in the theoretical world as opposed to the more chaotic, imperfect world of wind and dynamic bodies of water, and the much slower travel through them.
     
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