Sailing Foiler Design: Foil Assist and Full Flying

Discussion in 'Hydrodynamics and Aerodynamics' started by Doug Lord, Dec 9, 2011.

  1. Doug Lord
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    Doug Lord Flight Ready

    leeway coupling/straight daggerboard

    Tom, do you think an "uptip" "L" foil would work with a straight daggerboard?
    In the A class the "uptip" portion of the foil could be longer if a straight daggerboard was used.
     
  2. tspeer
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    tspeer Senior Member

    You mean one like this?
    [​IMG]

    or this?
    [​IMG]

    Yes, I think it could work.

    There are two issues with dihedral on the wing and a straight board. One is you end up with an acute angle between the wing and board, and this can result in more drag at the junction and a lower incipient cavitation speed.

    The other issue is it's been found to be more efficient to allow a curved board to generate part of the vertical lift, even though that does result in moving the wing inboard with a loss of righting moment.

    However, that was for a class that had no restrictions on the span of the daggerboard wing. Given the limitations on the A class, the best solution may be quite different. It might be that an S shaped foil that put the junction further outboard might help.

    Or moving the trunk to the outboard side of the hull. Obviously the Spitfire foil configuration would not fit the maximum beam limit, but I don't see why this solution wouldn't meet the requirement to lower the board from the top:
    [​IMG]
     
  3. Doug Lord
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    Doug Lord Flight Ready

    Thanks Tom-I hadn't remembered those boards. I had an idea that a trunk could be made that followed the curve of the foil and matched the side of the hull. Would be installed by interlocking with a downward motion-making it A Class legal. A cat designer friend said he thought that the danger of foil ventialtion might be greater with the foil close to the outboard side of the hull-does that make sense?
    click--
     

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  4. Doug Lord
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    Doug Lord Flight Ready

    UptiP foils designed to breach the surface.

    =======================
    Given what Tom says above, I'd imagine the designers of the GC32 and Flying Phantom have a somewhat different view of the inboard tip of the main foil breaching the surface. Probably due to different design considerations since they both seem to do it on a regular basis:

    Flying Phantom photo by Pierk Contier and GC32 photos by Sander va der Borch:
     

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  5. Doug Lord
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    Doug Lord Flight Ready

  6. Doug Halsey
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    Doug Halsey Senior Member

    Vive la V-Foil !
     
  7. P Flados
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    P Flados Senior Member

    Doug Halsey,

    I think it is kind of humorous how many people have missed the fact that that many vids show these "new generation" foilers operating with what we could call "round bottom Vee foils" (when the boat is heeled over and the inboard tip is at or above the surface).

    The only real differences are the round bottom and the angles of the segments (more vertical for the outboard and more horizontal for the inboard)

    I am absolutely confident that had more serious efforts been put into foil development for platforms like your Broomstick, we could have achieved the required technology to have "high performance general purpose multihull foilers" long ago.

    Of course a round bottom Vee is a lot like a U and these are definitely not U-Boats.
     
  8. Doug Halsey
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    Doug Halsey Senior Member

    The angles aren't always so different, though.
     

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  9. Doug Halsey
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    Doug Halsey Senior Member

    Another big advantage of these foils over a pure V is the ability to control & adjust them almost like a fancy centerboard.

    For years I've wished I could dispense with the inboard attachment brackets on Broomstick for that reason. But that would go against one of my biggest reasons to use V foils in the 1st place - the rigidity of the triangular structure allows the foils to be shaped from lightweight wood (redwood or cedar) & reinforced with a single layer of lightweight fiberglass cloth (~6 oz.). That's perfect for the home builder, like me, requiring only modest materials & skills.
     
  10. P Flados
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    P Flados Senior Member

    Doug Halsey

    Your choice made a lot of sense.

    If I had not been able to pick up carbon tow real cheap (first a roll of 12K for $16.00/lb x 5 lb = $80.00 and then a roll of 24k on Ebay for $16), I probably would have had to either make a similar choice, or bring myself to cough up the serious cash needed for carbon cloth. I have very much stuck to methods and materials that I consider appropriate for low cash home builds.

    Note that even though the tow was cheap, it is a pain to use and it takes quite a bit of figuring out how to get the results you want.
     
  11. TrevorJack
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    TrevorJack New Member

    Ref Tom Speer's comment from contribution #49: "Because the horizontal lift is constant but the vertical area is reduced as the boat rises, the leeway angle increases. It is the coupling of leeway with heave that is exploited by the L foil to provide vertical static stability."

    I'd like to understand this better and estimate the magnitude of the effect.

    First some assumptions. 30° dihedral; the vertical section ("mast") and wing are both straight for simplicity; both sections are cambered, possibly differently, and designed so that at cruising speed there is zero leeway at cruising height. Assume further that an upward perturbation results in lateral load being balanced now with 2° leeway. So the mast is now operating at AoA=2°.

    Resolving the lateral velocity into spanwise and normal components on the wing one can adduce that the AoA on the wing is now -1° (as this is proportional to the sine of the dihedral) but with spanwise flow close to the lateral velocity (proportional to cosine 30°). So an upward perturbation such as to produce 2° leeway results in lift from the wing reducing due to a 1° reduction in AoA. Is this a reasonable first approximation to the magnitude of the leeway heave coupling of an L board? Can spanwise flow on the wing due to leeway be ignored (to first order)?
     
  12. tspeer
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    tspeer Senior Member

    You have things qualitatively correct. The actual change in vertical lift is more complicated than that, because the horizontal wing acts as a winglet for the vertical panel, and vice versa. So there's a lot of carry-over of the lift from one panel to the other.

    For your example, the fact that the loading (side force per unit area) has gone up on the vertical panel means it is shedding more intense vortices at the bottom, and these will induce an increase in lift on the horizontal panel. So if the two panels were at 90 deg and there was no change in angle of attack on the horizontal wing from leeway alone, there would be an increase in lift on the horizontal wing due to the increase in leeway as the boat flew higher, and the heave stability would be unstable.

    This is why an L foil needs to have some dihedral in order to be neutrally stable. The neutral stability dihedral angle is not at 90 degrees, as one might expect. Just what dihedral angle is neutrally stable depends on the planform shapes of the wing and vertical blade, the cant angle of the blade, etc. Designers look at a lot of variations in geometry to explore the tradeoff between stability and drag, and it's not always clear just how stable a given geometry will be until its analyzed.

    The good news is it doesn't take a sophisticated CFD code to determine the heave stability of a foil. A simple lifting line spreadsheet can do the job, and lifting line analyses have compared well with RANS results for the same configurations.
     
  13. TrevorJack
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    TrevorJack New Member

    Thanks Tom.

    I just wanted to be sure I was understanding the first order effect. I understand that there are plenty of other things going on. But the fact that at least in some conditions the reduction in AoA of the wing produces stability indicates that it dominates other effects, for example the effect of the higher vortex shedding due to higher AoA of vertical mast, in relevant regimes.
     
  14. Doug Lord
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    Doug Lord Flight Ready


  15. Doug Lord
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

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