Foil design - scaled down

Discussion in 'Boat Design' started by dale.frahm, Mar 24, 2010.

  1. dale.frahm
    Joined: Feb 2010
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    Location: Southampton UK

    dale.frahm Junior Member

    Given all the wonders and uses of the foil shape , up to what degree do they begin to lose effect , as one scales down a model ?

    I'm building a 1m class sailing yacht , and am just wondering about raked keels for lift, looped keels for righting moment, hydrofoil fins for lift, etc etc .

    Until what degree, dealing with a 1m model and its associated low speed, would a foil shape become ineffective ?
     
  2. landmarine
    Joined: Mar 2010
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    Location: New Zealand

    landmarine Junior Member

    Hi Dale. I probably can't help very much but a few of my friends own these type of model yachts, and what I have noticed is that the foils seem quite thin (certainly not the usual NACA 0012 we are used to in full-scale slow speed boats they model).

    Thinking the problem through, the lift requirements of the foils are scaled down probably to the same degree as the boat is scaled down, however the boatspeed (from my observations) appears not to be scaled down to the same degree (don't quote me on this).

    This observation tells me that the thickness to chord ratio (a NACA 0012 is 12%) should reduce to very thin foils like those used on very fast cats & triamarans. This is consistent with my observations at the local club (an infrequent visitor I might add!!)

    The reason for this is that unless extra lift is needed, you should go for the foil that creates less drag. If you do this, the boat will speed up and as it speeds up it will create more drag & lift anyway. Typically the thinner the foil the less the drag, so long as the shape is consistent and you adhere to the scientifically proven shapes. For a model yacht the percentage error of construction tolerance will mean to get a good result the foil needs to be extremely accurately shaped. These days with specialist laser-guided equipment available, I'd imagine it would be better to produce them using CNC machines, computer controlled injection moulding etc....more $$ I expect

    I'm sorry I can't answer the questions you ask about raked keels, looped keels, and hydro-foils etc.. You can however play around with programs (free demo version) such as Foil 97...and see what you can come up with...

    For slow-speed in light winds I suppose all you need to worry about is whether you have a different set of foils (thicker) for these conditions or you simply compromise the shape to allow the boat to sail reasonably ok in all conditions. Remember lift in a rudder may be quite helpful for feel in light winds / slow speeds.

    Go to the 1m clubs and see what the guys & gals there say...I'm sure you will be surprised how knowledgeable they are!!! Because it's far cheaper to run such boats, you will find it drags out all the mad physicist types, and it will add a whole new dimension to your sailing experience without costing you an arm or a leg i.e. there is far more experimentation & discussion done here than any full-size boat club I can assure you of that!!

    Good Luck Grant
     
  3. markdrela
    Joined: Jun 2004
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    Location: MIT Aero & Astro

    markdrela Senior Member

    What you want is an RC glider tail airfoil.
    The HT13 or HT14 should work well (6.5% and 7.5% thick). You can get them here:
    http://charlesriverrc.org/articles/drela-airfoilshop/markdrela-ag-ht-airfoils.htm

    Within their low-drag bucket, CL = -0.2 to +0.2, these airfoils have 100% laminar flow.
    This gives two benefits:
    1) The drag is nearly the lowest that's physically possible for that airfoil thickness
    2) They are completely immune to low Reynolds number effects, and will generate
    the same CL down to Re = 1000 or so without separation or hysteresis (this is only a few centimeters per second water speed with a 10 cm chord)

    BTW,
    The traditional 12% section is grossly too thick for a 1m sailboat keel. The 7.5% is the absolute upper limit at low Reynolds numbers, and the 6.5% is a bit safer and less critical to shape.

    If making the keel out of wood, or making a keel mold plug out of wood, the sanding procedure in the attached second PDF shows how to sand it a linearly-tapered keel without templates. The first PDF gives the locations for the facet corners.

    If the keel has curved edges, you're on your own.
     

    Attached Files:


  4. dale.frahm
    Joined: Feb 2010
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    Location: Southampton UK

    dale.frahm Junior Member

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