longitudinal stability of foilers

Discussion in 'Hydrodynamics and Aerodynamics' started by chabrenas, Oct 17, 2012.

  1. chabrenas
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    chabrenas Mike K-H

    An amateur video of the recent pitchpoling of Oracle's AC72 shows that her bearing off manoeuvre started off fine, with the bows up, but then they went down in a manner reminiscent of the way powerboats can 'porpoise'.

    Clearly, once fixed-incidence foils run at negative incidence, you're in trouble, so the subsequent pitchpoling shouldn't surprise anyone. However, I'd be interested to hear from anyone with technical knowledge about longitudinal stability in foil-borne craft, particularly those under sail, where the motive force is applied high up. Does anyone know what, if anything, Hydroptère's designers changed after she pitchpoled in smooth water a long whileago? I assume they have made major improvements before attempting a long Pacific Ocean record attempt.
     
  2. chabrenas
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    chabrenas Mike K-H

    Not sure whether I'm allowed to make a second post on my own thread when no-one has yet responded. Should I edit the OP instead? Anyway, another observation about longitudinal stability of foil-borne AC72s:

    Oracle does appear to have a small foil set low down on each rudder, but Emirates Team NZ has nothing - what provides the feedback required to keep her from pitchpoling or squatting? Is it the airflow over the rear crossmember?
     
  3. daiquiri
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    daiquiri Engineering and Design

  4. chabrenas
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    chabrenas Mike K-H

    No. That's the one I meant. She seems to start rising onto the foils, then pitches nose down.
     
  5. Doug Lord
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    Doug Lord Flight Ready

    There is a thread here discussing these issues: http://www.boatdesign.net/forums/multihulls/34th-americas-cup-multihulls-34612-52.html#post588548
    And also under the "AC anarchy" on SA: http://forums.sailinganarchy.com/index.php?showforum=23
    -----------------------------
    Oracle and TNZ both have rudder t-foils. Artemis does not at this point in time. The Oracle guys have much smaller hulls than does TNZ because they were relying on the foils to prevent a pitchpole. The rudder foils are set at a fixed angle of incidence relative to the hull they are attached to but Oracle has large platform twist that can allow the windward rudder foil to develop downforce much earlier than it would otherwise(and before the leeward rudder t-foil does).
    I'm of the opinion that the Oracle design and team are excellent and what happened was probably a result of the failure of the lee main foil control system. Note that the lee main foil is not fixed at a given angle of incidence-it is controlled by one of the crew. I have been told by a person that should know that Oracle controls their lee main foil with a "joystick" ,in essence, flying the boat manually.
    I think it was a failure of that control system because the foils are extremely powerfull and should have been able to get the windward hull back up before the lee hull went in.
    Go read the stuff in the AC thread here and on SA-on SA you have to wade thru a large amount of junk before getting to the pearls of wisdom sometimes found there.
    Welcome to the forum!
     

  6. chabrenas
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    chabrenas Mike K-H

    Thanks, Doug. I knew you would chip in if I waited patiently. I'm not entirely new here - a few years ago I was learning from the Downwind Faster Than The Wind gang and made a friend or two in the process. I'm happy to be back. [Edit] ...and my apologies for not finding your AC72 thread before posting here. I'm less than a decade younger than Bob Fisher, but he doesn't seem to be getting senile yet, so why am I?
     
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