Gurney flaps

Discussion in 'Hydrodynamics and Aerodynamics' started by garydierking, Jun 24, 2011.

  1. garydierking
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    garydierking Senior Member

    View attachment gurney_angle.pdf
    According to the conclusions in this research document, a properly sized and oriented Gurney flap can increase the lift to drag ratio. Is there a reason why we are not seeing these on the new hard wing sails?
     
  2. Doug Lord
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    Doug Lord Flight Ready

  3. Steve Clark
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    Steve Clark Charged Particle

    Signor G had a gurney flap.
    signorg1.jpg
    Wing was a morphing surface controlled by cables.
    Flap was hinged at the aft end of the battens.
    The flap control system put additional compression on the battens, which having been deflected out of column, tended to explode.
    The control system was like tuning a piano with miles of very light wire and hundreds of little turning blocks. All the tensions had to be right for the system to work properly and if a wire jumped a sheave, battens that didn't reverse their camber would cause serial failures throughout the system. Replacing battens and rewiring would take all night.
    It's possible that modern materials and low stretch cordage would now make such a wing practical. Someone should try to figure it out.
    SHC
     
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  4. garydierking
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    garydierking Senior Member

    The best flap in the wind tunnel work was 2% of the chord, much smaller than the one you've shown, so it would not have the bad effects on the battens.
    I've used the flap on a sail sheeted like a junk sail with sheetlets at each batten and had no problems. My flap was also bigger than 2% so I'll be trying a smaller one on the next sail.

     
  5. tspeer
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    tspeer Senior Member

    WRT America's Cup, the half-girth rule would make it hard to use a Gurney flap on the AC72.
     
  6. tspeer
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    tspeer Senior Member

    Are you sure? According to Figure 14 in the paper, the section L/D is much higher without the flap than with it.

    It did increase maximum lift, however. That would suggest that it would be most useful for classes that constrain the sail area. If the area is variable, then it would be better to increase the area than to add a Gurney flap.
     
  7. Hansen Aerosprt
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    Hansen Aerosprt Junior Member

    Time to tighten our leech lines?
     
  8. garydierking
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    garydierking Senior Member

    And all this time we've been worried about the dreaded hooked leech.
     
  9. Steve Clark
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    Steve Clark Charged Particle

    It wasn't the gurney flap that caused the problem, but the larger "#2" that, because the control arms were so small, required quite a bit of tension on the control cables to give adequate deflection.
    SHC
     
  10. wet feet
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    wet feet Senior Member

    Nobody seems to have mentioned the essential difference between racecar wings and sailing rigs.The racecar engine provides the drive and the wing(+Gurney flap) provides the downforce.The sailboat rig has to generate the drive,and in most circumstances,on both tacks.The Gurney flap is normally fixed on one side of a wing element which always generates downforce,a fairly significant difference?
     
  11. daiquiri
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    daiquiri Engineering and Design

    Probably because the conclusions say exactly the opposite (page 496). :)
    Gurney flap doesn't increase the L/D ratio but decreases it, as T. Speer has also noted. It approximately shifts the L/D polar to the right (note: previously was erroneously written "to the left"), so for a given upwind course the keel will have to give more lift (to counter the side force from the sails) and hence more hydrodynamic drag.
     
    Last edited: Jun 30, 2011

  12. Erwan
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    Erwan Senior Member

    High Lift Section with GF

    Hi everybody,

    On the Acrobat file attached, the Gurney Flap issue is addressed with a high lift wing section, on the last page of this workpaper.

    Good reading

    Erwan
     

    Attached Files:

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