Can tip and bottom end vortices be made beneficial?

Discussion in 'Boat Design' started by lunatic, Feb 16, 2008.

  1. lunatic
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    lunatic Senior Member

    These detached flows can be made attached by planform without any end plates or any other drag producing additions,at least in my primitive flat plate flow studies. A typical sailboard's swept back curved leading edge produces a vortex below tip on the surface. Some booms are pitched down at aft end, I assume to gain area or headaches, at 20 degree angle the bottom vortex rises up as attached flow onto surface, seems even better than end plate where some flow is on horizontal surface.
    Would be curious how removal of a lower triangle of area to create a greater slope aft effects performance. Is there any penalty for using this potential, gaining lift from an already existing vortex, might balance the loss of area and be beneficial?
     
  2. tspeer
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    tspeer Senior Member

    Vortex lift is draggy because it causes low pressure on the backward facing side of the surface. Instead of using the separated flow of a vortex there, it is better to have attached flow so leading edge suction is working for you.

    A downward sloping boom changes the planform shape and acts as a leading edge. The optimum planform shape to produce minimum drag with a given heeling moment is egg shaped - it looks like a sailboard rig, with the maximum chord approximately 40% of the luff length up from the foot. Instead of sloping the trailing edge up from the foot to the clew, you could slope the leading edge down to the clew to get the same chord distribution.

    This leads to a highly swept mast with a very high gooseneck and a strongly swept boom as it drops down from the gooseneck. Both mast and boom are rotating wingmast sections. I've called this concept a "lambda rig" because of the shape of the mast and boom. See http://www.boatdesign.net/forums/showthread.php?t=2595&page=4 for more discussion.
     
  3. lunatic
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    lunatic Senior Member

    In sailing and model telltails, detectable vortices form on lee side. Flat sail and small diameter sweptback LE spar have no indication of separate bubble or vortex on backside as in your Lambda rig, but my studies are quite primitive, http://proafile.com/view/weblog/entries/C12
    In some of my rigs, the only streamline flow is on windward side, and up into LEV on lee, maybe this is the suction force,but modelling shows vortex attached at least 1/2 way along LE. Something is moving me forward, but what and at
    what efficiency?
    Hoping for less variables and a speed ratio with an iceboat, but plagued by warm winds or cold calms, not a good choice on thin ice. There are some amusements sailing these rigs; with CP close to mast pivot point, rig can be rotated 360 by hand for either front or rear steering. Rear nice to observe trailing telltails but need rotating seat.
    Do not fully understand optimum sailboard planform but in a past reference from you, the 1976 Princeton report on sailwings there is a 5X power coefficient in rotors,and improved L/D in semisail wings with D section spars over round. Why still round masts on sailboards?
     
  4. lunatic
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    lunatic Senior Member

    Your Lambda rig could be a delta wing with one LE extended to the deck as the mast, the other LE as the boom. Depending on sweepback and camber, resulting flow might be LEV top and bottom with center streamline flow. I assume this was not your approach, yet we seem to have some convergence of geometry. Would be interested in more details of Lambda rig or any guidance in the questionable direction I have taken. I realise it might not result in a practical sailing craft but I'm in for ride no matter where it leads,as long as there are interesting results worth analyzing.
     
  5. lunatic
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    lunatic Senior Member

    Returing to speed sailing design: the high lift needed in the start up phase could be created by developing LEV on small flat Lambda rig and, at higher speeds, increased camber could reduce drag by transition from vortex to conventional flow. If idea has merit, seems simple enough to build a prototype, but what improvised hull to put it on? Iceboat seems ideal but my ice is deteriorating rapidly leaving plenty of time for further thought.
     
  6. lunatic
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    lunatic Senior Member

    Sailing Experiments

    See Attached For Text And Drawings
     

    Attached Files:


  7. tspeer
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    tspeer Senior Member

    I think the differences between your approach and mine is I'm not trying for a leading edge vortex on either leading edge. I'd much rather have attached flow, so the sweep angles I have in mind are not as great and the leading edges are thicker than you'd use if a LEV was your goal.

    I was more motivated by the desire for a vertical leech, and to approximate the chord distribution for low induced drag with a given heeling moment. The idea behind the vertical leech was to minimize the spanwise pressure gradients there. By putting all the sweep due to the planform shape in the leading edge, the greatest spanwise gradients occur where the boundary layer is thin and the streamwise pressure gradients are favorable. At the leech, where the boundary layer is thick and more likely to separate, minimizing the spanwise pressure gradient helps to keep separation that occurs at one location from propagating to other spanwise stations.

    I was working on a center cockpit design at the time. The lambda rig resulted in generous headroom under the boom for the cockpit, and the clew was to sweep just above the aft cabin. The rake of the mast also increased the forestay tension compared to an upright mast. So there were system design considerations as well as aerodynamic ones.
     
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