negative dihedral foil?

Discussion in 'Hydrodynamics and Aerodynamics' started by fio, Aug 24, 2013.

  1. fio
    Joined: Aug 2013
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    fio New Member

    why do the current foiling kite surfers have a negative dihedral in their main foil? -wouldn't this make the foil set more unstable?
    see http://www.taaroa-kitefoil.com/
     
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  2. Erwan
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    Erwan Senior Member

    Probably to avoid tip-end vortex and downwash of the main foil to affect the rear foil.

    It could have been the opposite, with the front foil curved upward and the rear downward, but probably to maximize lift for a constant aera, it's more efficient to have the tips curved downward, especially when being near the surface.

    It can provide also some specific stall caracteristics, only CFD gurus are able to make an in depth analysis on this point, and I am not, so take my explanations with some care.

    Cheers

    W
     
  3. Crowsnest
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    Crowsnest Junior Member

    At first sight, and from my point of view, it looks simpler.

    At the video, when making a fast turn, the tips of a straight foil with the same span, would get out of the water, decreasing the lift surface and drastically changing the hydrodynamics of the device.
    At the same time, that negative angle helps for minimizing lee during the manouver.

    Regads
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Maybe to improve the response of the higher aspect foil to re-entry when jumping?

    left picture: old style air chair type foil-excellent for jumping and re-entry, right picture: French windsurfer foil about a year or two ago.Better for racing-jumping response unknown.
     

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  5. pogo
    Joined: Mar 2010
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    pogo ingenious dilletante

    The Mainfoil has no winglets, that are Lippisch-ohren.
    I think the rearfoil is a Stabilizer ( negative Lift), also with Lippisch-Ohren.

    These versions also included small. anhedraled aluminium wingtip "droops", reportedly designed by Alexander Lippisch and known in German as Lippisch-Ohren ("Lippisch Ears"), in an attempt to cure the stability problems via effectively "decreasing" the main wing panel's marked dihedral angle.
    Source:http://en.wikipedia.org/wiki/Heinkel_He_162

    pogo
     
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  6. jehardiman
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    jehardiman Senior Member

    Actually, remember that the main foil is pulling DOWN. So it actually has stable positive dihedral. You can see this clearly in some concept studies for 100 knot kite sailers from the 1970's where a much larger dihedral allows the foil to act as downforce and sideforce in a stable manner.
     
  7. Doug Lord
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    Doug Lord Flight Ready

    ==
    I don't think so, Jehardiman. When you see a "normal"
    planing kiteboard there is nothing pulling down. The kite foiler heels to "weather" and the board and strut act as lateral resistance and vertical lift at an angle(to windward).
    The guy in the picture wouldn't be so high above the water if the foil was pulling down. There are paravane -type systems where the "rider" is suspended between the kite and paravane.

    click on rough sketch for clarity:
     

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  8. jehardiman
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    jehardiman Senior Member

    No wonder they have been going so slow...then again the body is made for pushing, not pulling

    But, looking at the foil in the first post, it is either a symetric foil or shaped poorly if the idea is to generate UP force.
    [​IMG]

    Anyway, this is how I expect the 100 knot sailer to look, albeit with posibly a soft kite wing and simplier high dihedral depressor.
    http://patentimages.storage.googleapis.com/US6675735B1/US06675735-20040113-D00000.png
     

  9. fio
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    fio New Member

    there is an interesting parallel discussion happening here: http://boards.straightdope.com/sdmb/showthread.php?t=263498
    it seems that the benefit(?) of negative dihedral is (slightly counter intuitively) in promoting instability so that the board and foil can be cranked over and not want to flip the board back vertical
     
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