Foiler Design

Discussion in 'Sailboats' started by tspeer, Nov 12, 2003.

  1. tspeer
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    tspeer Senior Member

    It looks to me like you're doing a good job at what I've always recommended - designing a section that meets your specific requirements. This is a good example of how the Mikey section is a better choice than a NACA 6-series section. The 6-series has low drag in its drag bucket, but that's not where you're operating. And with leading edge sweep leading to transition via cross-flow instability, the NACA 6-series section might not have a drag bucket at all.

    And, yes, there is an art to section design - it is design, after all! I'm still pretty inexperienced at it myself. But learning.
    About all I can offer is XFOIL tends to over-predict maximum lift. So once the Cl curve starts to taper off pretty well, don't be surprised if it turns around in real life where XFOIL says it keeps going.
    [/QUOTE]Question: Practically, what range of angles of attack do I have to count with when tacking? [/QUOTE]
    I guess that depends on how good a sailor you are! This is a hard one to quantify. But you can bet that a sharp, early break will be much less forgiving than a gentle break.

    One way you could start to develop some design requirements in this area would be to take your existing keel section and consider adding a sharp addition to the leading edge - an angle iron, perhaps. This will make it stall earlier. You could analyze the modified shape with XFOIL to see what it does, then go sailing with it temporarily attached to your keel and see what it does to your tacks. This might help establish how bad it can be and still be acceptable. Then you'd have a lower bound to use in future designs.
    The lift on the keel isn't determined by the keel design. It's determined by the sail trim. I would expect the angle of attack to be higher at low speeds than at high - a gust when you're slow, for example, means a lower boatspeed/windspeed ratio. And boats seem to stall out a lot easier in light winds.
    That's exactly the kind of thinking you should be doing. You'r e making tradeoffs appropriate to your design.
    I think the transition graphs look OK. For low Reynolds numbers, you want the transition to vary smoothly with angle of attack. This indicates the separation bubble is well-behaved and not likely to burst and suddenly leave the foil stalled. At low Reynolds numbers, the problem is not so much maintaining laminar flow, but getting rid of it gracefully. You want a turbulent boundary layer at high angles of attack because it is more resistant to stall than a laminar boundary layer. By designing for a smooth shift in the transition location, you get more laminar flow and lower drag at low lift coefficients where profile drag is important, and when transition moves forward, you get a section with higher maximum lift and a gentle trailing edge stall. It makes for a robust section that shifts gears on its own.
     
  2. Doug Lord

    Doug Lord Guest

    Froude Numbers and Hydrofoils and more

    This is so good I thought it should be part of this thread for reference:
    QUESTION:

    ANSWER:
     
  3. boogie
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    boogie Member

    hi doug,

    can you post the link to this post from tom speer?

    regards
    boogie
     
  4. sigurd
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    sigurd Pompuous Pangolin

  5. bennn
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    bennn Junior Member

    Wooow, at last I did it.

    I just finished reading the 574 posts, and that was definitely worth it. Better than any book, as somebody said long ago.

    I have an Xfoil question actually. What surface should you consider to calculate the drag in newton from Cd ? The same as for the lift (span*chord) or the front area (max thickness*chord) ?

    I actually searched this answer on the Internet and couldn't find it...

    sorry if I sound dummy.....

    Here is my reward : This airchair video is INSANE.:eek:

    http://www.airchair.com/gallery/GenoYauchler2004MarchAllOneRun.wmv

    Doug was talking about jump re-entry ? holy crap, this guy jumps higher than most wakeboarders, and still lands just PERFECT.... That's definitely something that could be used in regular wakeboarding
     
    Last edited: Jun 9, 2007
  6. tspeer
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    tspeer Senior Member

    Planform area (same as for lift) is the normal convention.

    If you divide the Cd by the thickness/chord ratio you get the drag based on frontal area. This can be very useful if you have to size the physical thickness for structural reasons. Foils with different chord lengths can be surprisingly similar when compared on the basis of frontal area. And extra area can come in handy for lift.

    Another useful practice is to work in terms of lift area and drag area, which are the reference area times the coefficients, or the dimensional lift and drag divided by the dynamic pressure. You can add the drag area's directly from different components to get the total drag area. And it's really convenient if your physical area is changing, like with a surface piercing foil or even the strut of a fully submerged foil, making it difficult to define a fixed reference area.
     
  7. bennn
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    bennn Junior Member

    This is what I thought, the coef are just what you decide to make out of them, but I didn't know what was the common calculation on Xfoil.

    Another Thought:
    You're using a wand but the problem with a wand is that the maximum rate of change is with the wand close to 90 degrees, whereas the maximum rate of change should be like, when you take off, the lift changes very much, & then when you reach the optimal height of cruise, the change should be VERY low. And this is actually the exact opposite of what is going on currently.
    So what would you think of a foiler that would change its Cl most when the hull is close to the water, then would behave like a normal flapped foil afterward
     
  8. bennn
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    bennn Junior Member

    just to explain further what I thought.

    The current wand uses the Cosinus part of the wand motion, whose variation becomes larger as the wand reach vertical position.

    The opposite behaviour would seem much more efficient to me. A foil whose flap's angle would vary very much during the initial situation, and then become much more like a heave controller seems really more efficient. Maybe only because the motion of a parasite flap can be avoided
     
  9. bennn
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    bennn Junior Member

    Let's be very clear.

    To me the only problem with an incidence controlled main foil is that the variatiopn would be very large during the flight phase. Why ?

    Because using the illet's system, the more you go up, the more sensitive your wand would become - which is exactly the opposite as what you're looking after, in my opinion.

    So maybe instead of using the cos part of the wand motion, one should use the sin part of it. I don't give away any part of my concept right now, so that any guy can give its own opinion about it.

    But to me the current setup is kinda not adequate because as the boat rises, the wand system becomes more sensitive (cf. trigonometrical calculations), which means DIVERGENCE to me. But any feedback on this thought is really welcome as I really often mistake with that kind of basic engineering facts.

    benoit Paillard
     
  10. bennn
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    bennn Junior Member

    I would also appreciate your opinion about Teflon Waxed surfaces, because this is what was really making a big difference during the good old days......

    I used to ride a Mistral OD so do you think it was only a placebo, or was it something that was actually worth making a new rule for it ??

    Tried it yet ? I am curious, since it has proved REALLY efficient to me. To the point where I could beat those english guys during Dunkerke's French championship, despite my very high beer consumption, so........
     
    Last edited: Jun 10, 2007
  11. bennn
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    bennn Junior Member

    Okay sorry for that I just went through the bladerider instruction manual, and I saw that the attachement (to the flap) is 90 degrees away from the wand, which solves the problem....
     
  12. Wardi
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    Wardi Senior Member

    Just to be clear, it was John Ilett who originally developed the wand arrangement for the Moth, incuding the 90 degrees offset, later copied by bladerider.

    John found it extremely important to have the flap respond quickly at takeoff, in order to get a smooth transition to foiling. The finer adjustment once foiling gives a smooth consistent flight. This is a key feature of the his original development.

    One observation which still has me intrigued is that when riding quite high, the wand falls vertical, pushing the flap fully up.(high drag!!) It is then possible to ride consistently with the wand completely clear of the water. This means there is no further control via the wand, and yet the boats do not seem to leap clear of the water even if they increase speed.

    Of course a combination of crew body movement and rudder trim can provide control if required, but it seems this is a fairly stable state, which is rather easily maintained. It may have to do with the fact that the foils are nearing the surface and loose some lift, which maintains the ride height automatically. There may be other effects as well... any thoughts appreciated.

    If this is indeed a useful method of automatic height control perhaps we should look into it in further detail.....

    wardi
     
  13. MalSmith
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    MalSmith Ignorant boat designer

    I think this is the most likely explanation. Loss of lift due to free surface proximity is a well known effect and has been used as a height control method e.g. the HYSUCAT foil system (http://en.wikipedia.org/wiki/Hysucat). The problem with this method is that the lift never goes to zero, so unless speed and angle of incidence are limited by some other means, it is not a very positive ride control method.

    Mal.
     
  14. tspeer
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    tspeer Senior Member

    I suspect the lift-loss at the surface is stabilizing the boat. Many Russian powered hydrofoils that are designed to operate on rivers and other sheltered areas use this principle.

    I think the drawback for using the free surface effect in sailing hydrofoils is the reason the lift loss occurs is also the same reason that the induced drag is doubled at the surface. The effective span is reduced 30% when you get right to the surface. So you have increased drag from the horizontal foil and effectively a short stubby board as well. It looks very spectacular, but the performance may suffer.

    A good analog might be flying a hull on a catamaran. The best performance is obtained when the windward hull is just skimming above the wave tops. You can sail with the hull way up in the air, and that may be fun and look cool to the uninitiated, but it's not a fast way to sail.

    If you're sailing very fast and heeling the boat to windward, it's possible that the the reduction in induced drag with speed and the effective vertical span provided by the depth of the windward tip of the hydrofoil (and verical lift from the rig) make the increased induced drag due to shallow depth an acceptable price to pay for reducing the wetted area of the strut.

    But I think for racing, it wouuld be faster to fly lower.
     

  15. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Brigantine NJ regatta photos available at http://phoenix.phanfare.com/album/335465. There was foiling at the outset, but the wind died as the day went on. 3 foilers and a lowrider participating. The lowrider won on light air performance as the wind faltered. No spinnakers were used.

    The other boats in the photos are classic moths, which started separately.

    Have a great summer!
    Stephen
     
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