C foils

Discussion in 'Multihulls' started by GrahamR, Jan 14, 2018.

  1. UpOnStands
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    UpOnStands Senior Member

    Many thanks. Helps to explain why RC foilers are so twitchy.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    All rc foilers are not "twitchy".........
     
  3. UpOnStands
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    UpOnStands Senior Member

    How about "Some RC foilers are not twitchy."
     
  4. John Perry
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    John Perry Senior Member

    Hi Graham
    I have no experience of sailing the kind of hydrofoil catamaran you are talking about, my practical experience of sailing hydrofoils was with what was probably the first hydrofoil (monohull) dinghy to sail on centreline foils, so not the same as a catamaran with C foils. I haven't had the chance to sail on any hydrofoil boat since those distant days (would jump at the chance though!).

    Having said that, it does seem unlikely to me that you could make your C foil catamaran pitch stable with the hulls clear of the water and without rudder Tee foils, or at least without some additional foil(s) capable of providing vertical force. A hydrofoil catamaran with C foils forward and Tee foils aft might be crudely likened to a car with four wheels. But remove the aft Tee foils and you have something more like a car having the four wheels replaced by just two wheels, one on each side of the car. It would be hard to make such a vehicle pitch stable although the Segway scooters show that for a land vehicle it can be done with a clever control system making rapid changes to the forward/backward force transferred from the ground. However I cant see the Segway approach working with a sailing boat. But who knows - once you have watched an expert foiling kitesurfer you may well believe that anything is possible!

    Doug Lord's diagram at start of this thread shows how forces on a C foil change with flying height. As the flying height increases and the upper part of the foil rises above the surface those red force arrows progressively disappear and since all the arrows have some upward component the total upward force at constant speed and angle of attack will reduce, but because of the foil curvature it will reduce more rapidly as the lower down arrows disappear since those arrows have relativity larger vertical components. A straight 'Vee' foil should have a more linear relationship between flying height and lift force than such a C foil but both will loose lift with increase in flying height at constant angle of attack and so have the potential to stabilize the flying height of the part of the craft to which the foil is attached. Combine this with a suitably large Tee foil rudder(s) and there is the potential for a pitch stable craft since the Tee foils on the rudders will tend to make the flying height at the rudders follow that at the forward lifting foils - i.e. raise the height forward and you increase the angle of attack on rudder foils so lifting the sterns to bring the craft back towards the original pitch angle. This is I think another (probably less elegant) way to say the same thing that Tom Speer is saying.

    There is also the effect that even at constant angle of attack, dihedral angle and speed, a fully immersed hydrofoil will loose lift as it rises to within a few chord lengths of the surface. This is shown by tank tow tests dating right back to WWII, this link describes one such test program.
    http://www.dtic.mil/dtic/tr/fulltext/u2/a800646.pdf - there are other papers from this source that may be of interest.
    Because of the effect that proximity to the water surface has on hydrofoil lift, even a horizontal Tee foil that is always fully immersed can give height and pitch stability and I understand that this is how some of the Russian powered hydrofoil ferries manage without having surface piercing lifting foils or angle of attack controls.

    For a sailing craft you also need to consider the need to resist leeway since at the speeds achievable with a hydrofoil boat you are unlikely to have the apparent wind on the stern. As C foils rise out of the water their ability to resist leeway without stalling will reduce, slowly at first then rapidly as you reach the situation where the only part of the foil in the water is a near horizontal part. Combining C foils with a vertical daggerboard to resist leeway is a possibility, but more practical for a trimaran than a catamaran.

    I have a suspicion that you are geographically not far away from me so would be pleased to come and see what you are up to and chat about this kind of thing sometime!
     
  5. Doug Lord
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    Doug Lord Flight Ready

    Hi ,John ! I think when considering foils for a full flying catamaran with rudder t-foils one could find a far better type of foil than a C foil. C foils have been used successfully for foil assist but there are few, if any, production cats or tri's using them for full flying(with T foils).
     
  6. John Perry
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    John Perry Senior Member

    Sure Doug, I dare say you are right, but the OP says he already has a pair of C foils kicking around and he also says that he is not intending to achieve a 'fully foiling set up' at this stage (so presumably he is only looking for a bit of 'foil assist'). Up to him what he does but I can see why after a couple of years of boat building he might want to get afloat this season with the existing C foils. If he also makes a pair of T foil rudders those are likely to be useful to have if/when he tries full foiling.

    One starts to think that pretty well any shape of foils could work, they pretty well all loose lift with increasing altitude at constant angle of attack and speed and they all loose lift completely when they rise clear of the water surface. I suppose the differences are in the shapes of the lift vs. altitude curves, the L/D vs. altitude curves, the way the ability to resist leeway changes with altitude, also any coupling between lift and leeway resisting force. 'C' foils, 'V' foils, 'L' Foils, 'T' foils, 'Z' foils, 'U' foils, 'H' foils (ladder foils), 'J' foils have all been tried and all have worked to a greater or lesser degree - shall we try a few more letters of the alphabet? How about the Greek alphabet?

    As for my own sailing thoughts these days, for several years I have been meaning to try a foil configuration which would be a bit more different than just another letter of the alphabet but I don't know if it would work (its more likely not to work than to work I think) I have been busy with a project to build a conventional small rowing boat, there is something about it in the thread 'Build a fast rowing boat'. This has been great fun both in building the boat and in using it but I still hope to get back to hydrofoils some day.

    Am I right in thinking that 'C' foils were originally developed not primarily for their hydrodynamic properties but as a way to make a surface piercing foil that would not infringe racing rules by extending outside a trimaran float when fully retracted in harbor?

    I suppose that one limitation of a 'C' foil is that at the very top end of the speed range, when only a near horozontal part of the foil is more or less planing on the water surface, there is not much to resist leeway. So perhaps a little vertical tip fin on a 'C' foil - with a bit of inventive caligraphy that makes it a 'G' foil, so there's another letter taken up!
     
  7. Doug Lord
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    Doug Lord Flight Ready

    ==================
    Sounds logical..........
     
  8. bjn
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    bjn Senior Member

    That is more or less what they have used on the A-cats. But with a short vertical section above the curved section (between the hull and the start of the curved section). I guess this is to get some wave clearance between the hull and the lifting part of the foil. J-foil with a winglet? Or maybe the lowercase greek Zeta would fit: ζ
    See this video from 2014.


    The latest A-cat foils doesn't seem to have much curves. Instead there are two pronounced "kinks" with a straight ~45° section which tapers in cord between them.
    [​IMG]
    Z doesn't really fit imo. The runic letter "s" fits better, but its not on our keyboards... =)
    [​IMG]
     
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  9. UpOnStands
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    UpOnStands Senior Member

    My vote is
    Excellent video of foiling systems and details
    Interesting comment at/from 7:24 "can't really get stability with a wand and push rods"
     
  10. Doug Lord
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    Doug Lord Flight Ready

    Thats because wands are illegal in the A Class,not because wands and pushrods are unstable. Wand systems are among the most stable of all foil systems.
     
  11. UpOnStands
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    UpOnStands Senior Member

    Not sure that was what he was talking about as he is discussing "ways of gaining stability" and notes that bottom-inserted boards would be better for stability (but also not permitted by the rules)
     
  12. Doug Lord
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    Doug Lord Flight Ready

    He said something to the effect that" we can't really use wands....". They can't because: 1) they're illegal in the class, 2) if used they would be used with T-foils that have to be inserted from the bottom which is also illegal.
     
  13. UpOnStands
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    UpOnStands Senior Member

    the actual comment "because we can't really do that with wands and push rods"
     
  14. Steve Clark
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    Steve Clark Charged Particle

    The effect of C foils is to reduce the fore and aft water plane inertia of the catamaran hull.
    The volume of the ends is reduced by the amount of vertical lift generated by the C foil, essentially turning the long hull into a see saw.
    For this reason some form of stabilizer is necessary, which is also known as a t foil rudder. Without some form of stabilizer the boat will trim excessively bow up until the foil stalls or ventilates.
    Alternatively, the drive from the rig can be large enough to overcome the weight of the crew aft of the center of lift, in which case the boat will trim violently bow down, and most likely pitch pole.
    The A class went through all this on their way to becoming a hydro foiling class.
    SHC
     

  15. GrahamR
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    GrahamR Junior Member

    Firstly many thanks for your advice everyone, and sorry for my tardy response. I've spent far too much time in the workshop making moulds and performing countless vacuum bagging operations!
    I think what I'm taking away from all this is that I need to put fairly sizeable tiplets on my rudder blades and ensure that the fore and aft rake of the boards has a degree of adjustment. And if I get those bits right I may get partial lift, pull a few wheelies and the odd nose dive.
    John, as you said, at the moment I am just aching to get this thing sailing - and boring old straight centreboards will do fine for now. That would be an "I" board I'm guessing? Please do come over whenever is mutually convenient, I would love to show you what I've been up to and to discuss the foil bit in more detail. I'll be in touch.
    Once again thanks for your advice everybody, will post some photos of the finished article in the near future.
     
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