Best Foil Shape for Rn 250,000 to 750,000

Discussion in 'Sailboats' started by Guest, Apr 30, 2003.

  1. Guest

    Guest Guest

    Anybody have any recommended shapes for foils in this renoylds number range.
     
  2. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    For what pupose?

    There is no single "best" shape. The modern approach is to use a program like XFOIL (http://raphael.mit.edu/xfoil/) to design a custom section for each application. One often uses an existing shape as a starting point.
     
  3. Guest

    Guest Guest

    Foil shapes

    Its for a 12ft Skiff centerboard, 300mm and speeds from 5-7 knots
     
  4. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    In the NACA sections, the venerable 0012 is actually a pretty good choice. I wouldn't use one of the NACA 6-series sections without modifying it with XFOIL - too subject to leading edge stall at low speeds.

    Other more modern choices would include the Wortmann FX L V-152 K25 (nice wide drag bucket - out to Cl = 0.5, min drag Cd = 0.009 @Re=1M) and FX 71-L-150/20 (narrower drag bucket to Cl=0.4, but lower min drag Cd = 0.007 @Re=1M). These were actually designed for the vertical fins on sailplanes. You can use XFOIL to examine them over your Reynolds number range.

    The sections, along with wind tunnel data, are documented in Wortmann, F. X, "Stuttgarter Profilkatalog I", Friedr. Vieweb & Sohn, 1981.
    Coordinates can be found at

    FX L V-152 K25 http://www.aae.uiuc.edu/m-selig/ads/afplots/fxlv152.gif
    http://www.aae.uiuc.edu/m-selig/ads/coord/fxlv152.dat

    FX 71 L-150/20 http://www.aae.uiuc.edu/m-selig/ads/afplots/fx711520.gif
    http://www.aae.uiuc.edu/m-selig/ads/coord/fx711520.dat

    There are also some Eppler sections that might be appropriate. Check out his book, "Airfoil Design and Data," Springer Verlag, 1990.

    Paul Bogataj published some modern symmetrical sections designed for keels and boards in a Sailing World article in recent years, but I don't have the precise reference.

    You can get coordinates for just about every airfoil known to man at http://www.aae.uiuc.edu/m-selig/ads/coord_database.html. Another good site is http://www.nasg.com/afdb/index-e.phtml - it has section data as well as shapes.
     
  5. Guest

    Guest Guest

    I have been recommended the Eppler 836 and have been analysing using the Design foil software. XFOIL has been consistantly recommended but it seems to have a very step learning curve.
     
  6. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    The E836 isn't a bad choice either - Eppler designed it for use as a keel section. He puts a lot more emphasis on cavitation that I would, given the speeds these boats go. Although a skiff probably needs to consider it more than most monohulls. The drag bucket on the E836 goes out to a Cl of 0.5, so that's comparable to the Wortmann sections, but the Eppler code shows bubble warnings on the suction surface outside the bucket.

    Other Eppler sections to consider are the E520 (15% thick) and E521 (13.8%). These are more vertical tail sections. But they have a more gradual movement of the transition point and don't show bubble warnings until much higher lift coefficients. You could proably scale them to the thickness you want.

    Yes, there is something of a learning curve to XFOIL, but it's nothing like learning to use the Eppler code! It's really not that bad. You really need to use a code that can handle laminar separation bubbles and has been validated against test data. XFOIL is the only download-able code that meets these requirements.
     
  7. nico
    Joined: Jan 2003
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    nico Senior Member

    I am interested in the article from Paul Bogataj, published in Sailing World, does anybody has an idea about precise reference?

    Thank u
    Nico
     
  8. Triman
    Joined: Aug 2007
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    Triman Junior Member

    Hy to all,
    I found this interesting thread, but unfortunately most of the links provided by Tom are no more available. Sure you can give me some tips about keel profile: we are building a 7m (23') Sportboat (similar kind of Melges 24) and are looking for the best keel profile (this is a classic vertical blade with a lead bomb, 2m long, 350mm chord). We are considering a NACA 63009, but like suggested by Tom, this can give stall problems; the Eppler could be an alternative. Any suggestion?
    Thanks and good wind
     
  9. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    This site works ok: http://www.ae.illinois.edu/m-selig/ads/coord_database.html
    It contains .dat files and pictures of nearly all of the most commonly used airfoils.

    In addition to Tom Speers' always inspiring words, I'd just like to add that airfoils with a cusped trailing edge will at some point have to be cut, round or beveled along the trailing edge for manufacturing reasons. That operation will increase the drag of the foil.
    Airfoils with thin trailing edges will also be less structurally resistant, so take account of that fact too, particularily if you need to carry a lead bulb. I'm referring to FX series here.

    The increase in base drag due to cutting, beveling or rounding the trailing edge can be approximately evaluated with this formula taken from Hoerner's "Fluid Dynamic Drag:

    [​IMG]

    As for the drag bucket, it will exist and will give you benefits in terms of drag reduction as long as you make sure that the foil surface is clean and smooth. In that way, there will be a laminar flow zone over the forward 30-50% of the airfoil surface, which is the reason for the drag bucket.
    If the surface is not smooth or if it becomes contaminated by a marine growth, the laminar flow area (and the relative drag bucket) will disappear and will leave you with a conventional foil, with an ordinary (smooth and higher in value) drag curve.

    In practice, it means - if the boat will stay in water for long periods, there will probably be no advantages of using airfoils with a drag bucket. Then you could as well opt for a simpler foil with a thicker trailing edge, like the said NACA 0012, NACA 0015 or similar.

    If you intend to haul the boat out of water and clean the hull after every (or so) use, then you will see an advantage of laminar foils' reduced drag.
     
  10. tspeer
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    tspeer Senior Member

    Eppler E836
    [​IMG]

    E837
    [​IMG]

    E838
    [​IMG]

    Comparisons between a NACA 63a010, E836 and NACA 0012 are attached. If the laminar drag bucket can be achieved (in water, with bottom paint), the E836 has the same minimum drag as the thinner NACA 63a010. The NACA 0012 has drag above a lift coefficient of 0.5, and a higher maximum lift at low speeds.

    So it comes down to which is more important to you - laminar flow for less profile drag when sailing downwind, or a keel that is less likely to stall out in a downspeed tack?
     

    Attached Files:

  11. Ben G
    Joined: Jun 2013
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    Ben G Junior Member

    Hi,
    Ten years after the OP and I'm contemplating making a new centreboard for a 12ft skiff.
    I've been looking around and have done a couple of quick calcs to try and help.

    Problem: At the moment the foil I'm using is nominally a NACA 0012 but with a LE rad about half of what it should be. I feel that it's stalling out of tacks and accelerating off the start line, and limits our ability to pinch due to excessive leeway.

    After looking through some information, lift-aoa plots and a few simple calcs, the theory also points to this. Current section probably stalls out around Cl=0.8 or less

    Existing board:
    Chord is ~300mm, span ~1350mm, area is 0.29m2, actual section shape probably resembles a NACA 0012-34 with a highly tapered planform (reflecting a bending moment optimised planform a-la Munk)
    I feel the area and section thickness is about right.

    Here's some numbers I've estimated:

    Side force is 718N in most conditions (two on trapeze, 7-30 kts TWS), so the Cl varies wildly depending on windspeed (i.e. boat speed) and whether we're going upwind or downwind.
    Drag at High Lift is not too important, so long as the section doesn't stall.. but low drag at low lift is important

    V(kts) Cl
    4 1.2 (accel. out of tacks, start etc.)
    8 0.3 (lighter upwind conditions)
    10 0.2 (heavier upwind conditions)
    11 0.16 (fastest common upwind speed)
    17 0.06 (offwind)
    17 0.045 (off wind, one only on trapeze)

    These values are only relative to planform area, I haven't adjusted them for effect of the finite aspect ratio. Is it true the section shape needs to accommodate slightly higher CL values to achieve this global lift target?

    So the sweet spot for upwind efficiency would be in a Cl range of 0.3 - 0.16, with Re of 750,000- 1,000,000 ish
    Low drag at higher Re and low Cl will be helpful.
    We can lift the board a little in stronger winds to reduce surface area.

    I'm wondering if there's a section that has not so much a drag 'bucket' like the 6-series foils, but a gentler low drag 'cup' - a forgiving foil that's not going to stall early, and where the transition point from laminar to turbulent will gradually move forward along the chord as the board loads up (yet maintain a reasonably robust laminar BL at low Cl, for offwind speed)

    I took a look at
    http://airfoiltools.com/compare/index
    to compare some foils.

    It seems that the Eppler 836 stalls relatively early at around Cl = 0.8, as would our existing foil.
    The Wortmann sections that Tom Speer suggested won't fit at 15% t/c, and I'm not entirely convinced.

    Looking at the actual NACA 0012, it seems to be reasonably suitable.. but I'd really like something more modern with, hopefully, better boundary layer control and less drag. It's been quite hard finding any decent information out about Eppler or Wortmann sections although there is a mass of information about NACA 4 digit and 6-series section. Especially any notes on their intended use or special properties.
    I've heard Eppler sections are good for low Re applications and Wortmann are centred on medium Re applications.

    Anyway, I went comparing a few foils and came up with the following list of reasonably similar foils:

    NACA 0012
    Eppler EA 6-1-012
    Wortmann FX-76-120
    Wortmann FX-79-L-120
    12% Joukowski

    The FX-79-L-120 looks promising, but I've no further information on this section.

    Now before everyone says 'use Xfoil', I'd prefer to use a section shape that someone with actual expertise has come up with!

    Can anyone shed some light on these sections? Are they any good and what might be better??
     
  12. johnhazel
    Joined: Jun 2008
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    Location: Michigan

    johnhazel Senior Member

    I wonder what settings should be adjusted in Xfoil to account for the water turbulence that these fins would typically see. Hopefully Mark is lurking and might suggest values (Ncrit? and?) that would represent some typical wind and wave states seen in a sailing regatta? (wind needs to be accounted for because of the energy flowing into the wave system)

    That being done I am guessing the Xfoil drag polars are going to be a bit illuminating regarding the amount of laminar flow to expect.\

    http://www.ocean-sci.net/9/597/2013/os-9-597-2013.pdf

    http://www.terrapub.co.jp/journals/JO/pdf/5604/56040473.pdf
     

  13. fng
    Joined: Feb 2009
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    Location: new zealand

    fng Junior Member

    I've had some very good results with the Eppler 171
    Could be worth a look with the other foils suggested.
    The are several foil data bases that you can compare foils against others.
    But with X-foil you set your parameters and they are compared in the same range
     
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