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Interesting mail about rigid sails, snipped from Xfoil yahoo group

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  • Tord Eriksson
    This way of making a Bertrand-style rig woud be interesting - the forward part being either NACA 0012, or the other profilr tested by Tom, and the rear part
    Message 1 of 1 , Nov 1, 2007
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      This way of making a Bertrand-style rig woud be interesting -
      the forward part being either NACA 0012, or the other profilr
      tested by Tom, and the rear part the NACA.

      The problem would be the flap, which I guess would have
      to be rigid - but it should sure be an effective rig!

      Tord


      =================
       Re: airfoils for wingsail
          Posted by: "Tom Speer" me@... tspeer007
          Date: Tue Oct 30, 2007 8:46 pm ((PDT))



      ciolek_andrzej wrote:
      > Could sombody advise me which airfoil will fit as wingsail to the
      > sailing boat???
      Typically, most rigid wing rigs use two (or more) symmetrical sections
      in tandem, forming a slotted flap.  Symmetrical sections are required so
      the boat can sail equally well on both tacks.  A flap is required to
      increase the maximum lift and achieve adequate acceleration from a given
      wing area.   Two elements, both using NACA 0012 sections, with the
      forward (wing) element being 60% of the total chord and the aft (flap)
      element being 40% of the total chord has proven to be very successful in
      landyachts.

      The simplest arrangement has arms extending from the leading edge of the
      flap that are pinned at approximately 90% of the wing chord forming a
      hinge that just allows the flap leading edge to clear the trailing edge
      of the wing.  When deflected, it forms a slotted flap.  By varying the
      hinge location, one can adjust the size of the slot
      (http://www.tspeer.com/temp/ShapeFlapPivot.gif).  Some rigid wing rigs
      have an additional plain flap on the wing trailing edge that allows the
      slotted flap to pass by, allowing more overlap, and adding some camber
      to the wing to start turning the flow ahead of the flap itself.

      The flap section can be comparatively thin because the leading edge
      suction peak on the flap can be suppressed by closing down the slot
      (http://www.tspeer.com/temp/VelocitiesFlapPivot.gif).  It should have a
      front-loaded pressure distribution to accomplish much (most?) of the
      pressure recovery for the whole wing.  The forward element will have
      significant aft loading even though the section is symmetrical, because
      of the influence of the flap.  It can have a fairly mild adverse
      pressure gradient at low to negative angles of attack for significant
      runs of laminar flow.  Since any sail operates at low Reynolds numbers
      in light winds, it should be designed such that the transition point
      (which will be via laminar separation bubble) moves smoothly forward as
      angle of attack is increased
      (http://www.tspeer.com/RigidRigs/40flap/S902fa20.htm).  This will ensure
      the laminar separation bubble is short and the section will not be
      subject to premature leading edge stall.

      Typical flap deflections are around 20 degrees for a high lift/drag
      ratio at moderate to high lift coefficients.  At high speeds (say, 30 kt
      and up) the flap deflection may be nearly zero. Beware of negative flap
      deflections - they cause negative pitching moments that can allow the
      wing to self-trim at an angle of attack, leading to loss of control
      unless the wing is provided with a positive means of controlling the
      rotation (as opposed to a conventional sheet, which only restrains the
      sail in one direction).

      A rigid wing should be mass-balanced about the pivot axis to avoid
      flutter.  Wings can be particularly susceptible to flutter when tacking
      because they can flop from side to side when the steady aerodynamic load
      is removed.  A wing that flutters causes very high drag that can cause
      the craft to pitchpole backwards.

      Unfortunately, this kind of multi-element section cannot be analyzed
      with XFOIL.  It requires a program like MSES.  Or you can analyze the
      wing and flap in 3D using a panel code such as CMARC
      (http://www.aerologic.com/).

      Cheers,

      Tom Speer
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