Testing a model

Discussion in 'Multihulls' started by Manfred.pech, Jan 8, 2011.

  1. Manfred.pech
    Joined: Apr 2010
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    Manfred.pech Senior Member

    May be I want to test the sailing stability of a pacific proa of 30 ft with a Bruce foil added to the outrigger for wind force 3 (Beaufort).
    If I have a 1/3 model of it similar on all important points following the laws of Reynolds: - At which force of wind has the model to be tested to get realistic stability numbers? Has the wind force to be three times of force 3 ? Has anybody done similar tests ? Thanking in anticipation. M.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    Manfred, that is a difficult question-at least to me. In a model, as you know, SA scales as the square of length, displ. as the cube of length and RM as the 4th power of length. On my model I'm testing almost everything including max RM and I found ,by doing just the normal calculations, that the model, under radio control, will reach max RM at .75lb sq.ft. pressure-and that is an estimate based on the previous performance of a model foiler I designed and built. That compares to the full size boat reaching max RM (sail area max ,not depowered or reefed) at 1.8lb per sq.ft. pressure. I wouldn't even attempt to correlelate the speed of the model to full size-there are just too many variables-at least as far as I am concerned. What tests pretty accurately is the balance of the boat, the foils(but not the foil section), foil locations and performance under sail including tacking, gybing etc., the altitude control system if any and static pitchpole/capsize stability. I'm sure there are others that can lend more expertise to this but be leery of trying to get too much information.
    I worked with Dr. Bradfield one summer as he modified a 6' RC trimaran model of mine as a test platform for his 40' SKAT: all he was testing were the foil area(lifting, rudder and lateral resistance), foil placement, and altitude control system.



    pictures: left Dr. Bradfields test model based on the 6' Flyer Cubed; right two Flyer's sitting on the dock-if you look closely at the upper left corner you can see Bradfield standing there-gives a good size reference of the model:
    (click on image)
     

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  3. marshmat
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    marshmat Senior Member

    If only it were that simple, Manfred. Unfortunately, it's not that simple.

    The problem, in a nutshell, is that the quantities of interest don't scale the same way. If we use a linear dimension (eg. length) for reference, the Froude number (for comparing model to prototype speed) scales with the reciprocal of the square root of the length. But sail area and wetted surface scale with the square of the length, displacement with length cubed, and stability with length to the fourth power. Your foil will be concerned mainly with Reynolds number, which scales linearly with length and speed.

    That scaling with speed is going to be a big issue for you: at one-third scale, your Froude number is 0.58 times the original if you go the same speed, and if you match Froude number, your Reynolds number is (1/3 * 0.58) = 0.19 times the original, which might be enough to change the laminar-to-turbulent boundary layer transition point and render data on the model foil useless. If you match Reynolds number, your model's Froude number will be out by a factor of (3 / sqrt3)=1.73, which is enough to make a huge difference in trim, sinkage and wave drag.

    Stability is even nastier for scaling problems, and you don't see model tests of stability very often. One approach in your case might be to try matching the force on the sails as a fraction of righting moment. If we consider the scaling to be quartic for RM with length, quadratic for sail area with length and quadratic for wind force with wind speed, we'd get an 81-fold difference in RM and a 9-fold difference in sail area, thus we'd need one-third the wind speed for the model. But I would bet that you won't be able to match either Reynolds or Froude number in this case.

    What it all boils down to is that, in your case, I might use the model for drag measurements, perhaps for fiddling around with the balance of the rig and foils, but not to quantify anything relating to stability. For that, a mathematical/simulation approach will likely prove more useful.
     
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  4. Doug Lord
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    Doug Lord Flight Ready

    ===========
    What I found interesting in Dr. Bradfields conclusions was that he was convinced foil area scaled as the square of length. He said that was born out when he compared the model data with full size data on the SKAT. The section doesn't scale at all(necessarily) but apparently area does? There is more to it but I remember that he was surprised at the results. There is a comment by Tom Speer somewhere regarding this very question but I can't find it.

    Comments by Tom Speer regarding scaling hydrofoils here: http://www.boatdesign.net/forums/boat-design/hydrofoil-questions-17688.html
     

  5. Manfred.pech
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    Manfred.pech Senior Member

    Thx

    Matt and Doug, thanks a lot for your kind help. It really kills my naivety and saves a bulk of work. Seams to be the better choice to try a cheap One Off and change different parts of the configuration than to go to small models. Thank you very much! Manfred
     
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