forward mast rake?

Discussion in 'Boat Design' started by lunatic, May 16, 2009.

  1. lunatic
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    lunatic Senior Member

    If upward flow on tapered sail plan and/or sweepback of mast bend produces more loading on less area resulting in strong tip vortex, would forward raked mast with horizontal or even downward flow reduce induced drag of tip vortex, heel and create better load distribution? and what might the negative consequences be?
    Not quite a sailing topic but looking for an explanation of how counter-rotating vortices produce thrust and the possibilities of vortex assisted flapping in bird flight. Highly forward cambered inner wing generates high L/D in apparent flow of forward motion. If the source of that motion is in counting-rotating leading edge vortices of the sharp edged sweptback outer wing, the leveraged position of this loading demands high structural and muscular strength, especially since vortices seem to resist stroke direction. I have sailed sweptbacked LEV producing rigs and one characteristic is a very pronounced delay in responding to wind gusts or rapid sheeting.
    This delay, timed right, might produce an assisting force at a very advantageous position reducing structure and muscle. On the down stroke, LEV forms on wing's upper surface with effect delayed til beginning of up stroke. On assisted up stroke, counter-rotating LEV forms on wing's underside and is delayed til down stroke. All this seems too easy to be true yet gulls lope along without falling from the sky exhausted. I am looking for information that might confirm this?
     
  2. yipster
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    yipster designer

    funny,
    was just looking into a delta windsurf sail, rake and vortex lift
    also see Marchai in my post 388 on the sail aerodynamics thread
     
  3. tspeer
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    tspeer Senior Member

    Besides the vortex at the head, there is also a vortex at the foot that rotates in the opposite direction. So stopping spanwise flow at the head does not necessarily help with induced drag.

    Fundamentally, the vortices and induced drag come from the fact that the sail rig deflects a body of air with a finite span, not because of vertical flow along the sail. To minimize induced drag for a given mast height, what you want is for the wake to come off the leech like it was a rigid sheet, with a uniform cross-wind velocity imparted to the wake. If the lift is limited by the boat's heeling moment, you want the wake to have a linear variation in its cross-wind velocity from foot to head, as though it were a rigid sheet that is also rotating about a streamwise axis above the mast as it comes off the leech. This can be done with a variety of combinations of planform shape, twist and camber to suit the rake.

    The modern trend in low-speed airplane wing design is to keep the trailing edge nearly perpendicular to the apparent wind and put the taper into the leading edge. The reason is the boundary layer at the leading edge is thin and robust, but the boundary layer at the trailing edge is thick and easily separated. By making the trailing edge perpendicular to the wind, spanwise flow at the trailing edge is minimized, which helps to delay stall and to keep the stall from progressing spanwise along the trailing edge. The thin boundary layer at the leading edge is better able to tolerate the spanwise flow due to taper or rake.

    This seems to have been borne out in sailing experience, too. Forward rake is has more often been found to be slow, while many fast boats use considerable aft rake.

    Vortex lift, ala the delta wing, sounds attractive, but it comes with a significant drag penalty. For a high performance boat, lift/drag ratio is the name of the game, not maximum lift by itself. One can nearly always get the same amount of lift with less drag simply by adding area.
     
  4. lunatic
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    lunatic Senior Member

    My musings are often unproductive but have built an experimental rig with perdendicular trailing edge and need advise on good telltale system and its interpretation. I now understand the backside drag penalty of vortex lift after sailing many planforms. Curiously, the best all around performance per area was with all surface flow up into leading edge vortex with no streamwise flow. Even with its limited windward ability it was adelight to sail. It was striking how small variations in planform could produce some real dogs.
     
  5. yipster
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    yipster designer

    searching cfd i see these pictures i like to share :)
    [​IMG][​IMG] this beeing the first delta hang glider?
     
  6. gonzo
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    gonzo Senior Member

    You end up pointing less into the wind and with less speed. We tried it many years ago to try to compensate for weather helm when heeling. Everyone else I know that tried it ended up with the same results
     
  7. lunatic
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    lunatic Senior Member

    27 sq ft LEV sail with the verticle flow could surf a leaking Sunfish on a beam reach in 20 kts. Wonder why crab claw fanciers have not rotated LE up verticle for windward work, trim geometry tricky but looks possible.
     
  8. yipster
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    yipster designer

    sorry for hijacking your thread a bit luna and not comming back more argumented but i'm still reading up, secundairy tertiary vortex makers and breakers, leading edges, its actually the vortex that gives better lift couse the delta self dont seem the best planform, yet the vortex (i think) may be seen as a sucktion wing witch can give much more lift. harder to find is drag explained and lift/drag, on a delta. did read somewhere its measured that even at 5 at others at 20 deg sweep allready some vortex lift occurs coming back to the mast rake thread subject

    ok i get it, its the induced drag http://en.wikipedia.org/wiki/Induced_drag still very interesting tho...
    ever seen the coalfed Lippisch P13a fly http://www.youtube.com/watch?v=MvtxjSrImHw prototype test footage]
     
  9. lunatic
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    lunatic Senior Member

    I did some crude LEV sail experiments http://proafile.com/view/weblog/entries/C12 Has potential as an offwind sail, but with suction the backside of an ideally sharp edged flat sail, there is no forward force component.
     
  10. yipster
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    yipster designer

    where to find your LEV experiments at [​IMG]
     

  11. lunatic
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    lunatic Senior Member

    Click site, scroll down past Bamboo Boatbuilding to Crab Claw Tests, click continued.
     
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