is a low aspect speed sailer an oxymoron

Discussion in 'Boat Design' started by lunatic, Jan 27, 2008.

  1. lunatic
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    lunatic Senior Member

    I have done some qualatative sailing and model studies of several low aspect sail and wing rigs for possible overall drag reduction. Coudl drag and weight of supporting fuselage,heel countering beam and or hydrofoils be reduced by horizontal wing becomming main structure trimmed by steering for a one tack speed run? a sailing wing. Trade off might be positive with improved L/D of low aspect wing.

    Marchaj,not everyone's favorite,has some tantalizing studies,but even I was skeptical,so time to sail something different. Modified a Sunfish with 3 variations of swept back sails and 2 wings. Video tape telltails indicated trailing vortex below tip and 1/12 model allowed visualization of vortex attached to lee surface half way along LE, a LEV? Descriptions,aailing characteristics,drawings of rigs, model flow studies,and some analysis are posted at http://proafile.com/view/weblog/entries/C12

    Comparisons were difficult in open water sailing with unique rigs but some direction for development indicated.Constant area would be good in the choas of variables but easier to change planforms by cutting away area I thought was inactive or negative rather than build anew ending with 27 FT2 SA rig planning in 15-20 KTS wind.

    Possible quantative comparison is speed ratio on ice, boat in construction racing global warming, need feedback before the big melt in the melt belt.
     
  2. Guest625101138

    Guest625101138 Previous Member

  3. lunatic
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    lunatic Senior Member

    Good to have company. Quite a base structure but little details.is sail trim by steering? I have been sailing rigs of .7 and 1.4 trying to replace the "end loss" tip vortex with leading edge one, but no idea of drag consequences
     
  4. Pericles
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    Pericles Senior Member

  5. ted655
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    ted655 Senior Member

    Weren't they called Schooners? lots of short masts carrying lots of canvas?
     
  6. lunatic
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    lunatic Senior Member

    Thinking more of sweptback wing tip of MD 80 or Boeing 787 but I am sailing slender wings,one big tip.
     
  7. Pericles
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    Pericles Senior Member

    Don't think I know of those. Any links?

    Pericles
     
  8. marshmat
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    marshmat Senior Member

    Here's one from an Airbus A319 (courtesy Wikipedia) and one from a 787.
    Essentially, what these devices do (there are many shapes that accomplish the same basic thing) is to reduce the tendency for fluid to move around the foil tip from the high-pressure to the low-pressure face. If the flow can't duck out around the tip of the wing, you have a less nasty tip vortex and more lift on the foil. The same device is used on the tip of the keel on many racing yachts, only with a slightly different shape- and of course it is rotated sideways in that case.
    I know of two reasons why the 787's tips are faired into the wing, unlike the Airbus tips. (If anyone knows of others, please add them.) One is that a major design objective for the 787 was that it was to look pretty. No joke. Another is that conventional winglets fall off, break, scrape the ground, get knocked around, etc. and thus need a lot of repair time.
    I see no reason why the same effect wouldn't apply to a sail. There is a difficulty in that such a device would inevitably add weight aloft, generally not a good thing.
    Have you seen Brian Eiland's mast-aft rig? I don't think it needs high aspect ratios to be efficient... http://www.runningtideyachts.com/sail/ and see drawing below from his site.
     

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  9. rwatson
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    rwatson Senior Member

    The reason high aspect works is that it has more leading edge than a low aspect.
    Large sail area will drive a yacht in high winds, but the lift from the curved leading edge is generally how to go fast in all wind speeds.
    There was a low aspect rig developed for a boat that looked like a venetian blind, or other words, multiple leading edges but with the low aspect. They also had flow tabs on each end of each "vane"
    This might be one option to consider
     
  10. Pericles
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    Pericles Senior Member

    Matt,

    Returning to Wikipedia they described your first picture as "Wingtip fence
    Not to be confused with wing fence.

    A wingtip fence is a winglet variant with surfaces extending both upward and downward from the wingtip. Both surfaces are shorter than or equivalent to a winglet possessing similar aerodynamic benefits.

    Wingtip fences are the preferred wingtip device of Airbus, employed on all their airliners except for the A330 and A340 families. The Airbus A350 will also make use of winglets rather than wingtip fences."

    The other comment I noticed was "Such devices increase the effective aspect ratio of a wing, with less added wingspan."

    Taking this thought to a ridiculous extreme, a zig zag or concertina mast, if it could be constructed, could be 20 feet tall and topped off with a wingtip fence, but have the aspect of a 70 foot tall mast. Or am I completely nuts?:p :p :p :p :p

    Pericles
     
  11. lunatic
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    lunatic Senior Member

    If tip vortex is brought onto wing at least there is some benefit of lift for the drag.This can be done by shape of tip without any extention or device. Ahighly sweptback slender wing's LE is quite large and low aspect. The fashion of wing tips is beyond me,I've been sailing rigs of .7 to 1.4 and not sure if powered aircraft design is the best example,experimental biology might be better.
     
  12. Guest625101138

    Guest625101138 Previous Member

    If you want to do some quantitaive analysis of your rig choices then you should have a look at JavaFoil:
    http://www.mh-aerotools.de/airfoils/jf_applet.htm
    It is quite easy to use if you spend a bit of time with it. It has a nice feature in the Options page that allows you to alter the aspect ratio so you get an immediate appreciation of the increase in induced drag. You can also try different foil shapes and determine performance for any given Re#.

    I do not have a quantitive understanding of the benefit of tip winglets but I have read that the same reduction in induced drag could be achieved by simply extending the length of the foil. So why do it? For keels it means you can effectively achieve a higher aspect keel without increasing draft. THere is also a benefit of extra draft when healed (so it is a Ben Lexan rule cheating idea) For airplanes it means you do not have to extend wingspan with resultant parking problems at airport gates - this is what I have read, no idea if valid although I have seen two incidents where aircraft wings have clipped during taxiing.

    If you take the winglet concept to a sail it means you get the same efficiency as a higher aspect sail but you can push the centre of pressure lower. Clearly there are advantages in lowering the centre of pressure on a sail as the overturning moment will be reduced. How far you take this will depend on what you are trying to achieve. The Macquarie speedsailing team have determined the optimum to be aspect of around 2:1 without winglets and maybe they simply aimed to avoid the structural complexity of winglets. Maybe they did not give winglets any consideration. (You might be able to do better than 58kts in 19kts wind)

    With JavaFoil you will see that you get diminishing returns with foil aspect as efficiency is an inverse function of aspect ratio. I do not see much merit in going more than around 4:1 for keels and sails. With a glider you might go as high as 40:1 but this is a different situation.

    I have compared JavaFoil results with actual wind tunnel data for a wide variety of low speed foils (Re# 50,000 to 500,000) and it gives good results. I find it much easier to use than Xfoil.

    Rick W.
     
  13. lunatic
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    lunatic Senior Member

    These sails are unlike anything I have sailed before,telltails,both from sailing and on models indicate no streamwise laminar flow on lee side,but some steady surface flow pulled up into LEV,details and drawings at http://proafile.com/view/weblog/entries/C12 Can JavaFoil handle this chaos? I am building an iceboat to carry these rigs for a speed ratio to get an efficiency comparison,fairly primitive means but more fun for this Neanderthal.
     
  14. Guest625101138

    Guest625101138 Previous Member

    Javafoil is intended for streamline flow although it does give data beyond stall.

    I expect your sail would work best under streamline conditions or on the verge of stall.

    JavaFoil would still be useful for getting some idea of efficiency for the parameters Re#, AR, sail section, sail plan and speed. Much easier to develop concepts if you have a useful theoretical base than having to continue building then trialling with no sound theoretical base.

    My understanding of back sweep in a wing is that it increases the stall angle so the wing is more forgiving. For low speed wings the increase in Re# for a given aspect might also be significant.

    As noted earlier the overall efficiency of a sail goes beyond the L/D ratio. You are trying to generate the greatest forward thrust with the lowest overturning moment so the analysis goes beyond the foil.

    Have you considered multiple higher aspect wings set well apart to minimise interference.

    If you are not already using a good foil analysis tool I suggest you take the time to understand JavaFoil as it will certainly provide insight into many of the parameters of interest.

    Rick W.
     

  15. lunatic
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    lunatic Senior Member

    In actual sailing hardly moved with streamline flow,not until trailing telltails twisted up into a vortex like spiral did it reach best speed, but recognize that LEV generated lift may not be the theoretical base Iam sailing on. If multiples can be reduced,I am there with the simplest. Past the point of no return with the iceboat,but when the ice melts I will bask in the warmth of JavaFoil. Thanks for the interest
     
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