Foiler Design

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

  1. Wardi
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    Wardi Senior Member

    I think there are more problems than just the engineering. If the wand provides complete foil control, then it must be set up with an excellent response. I am not sure this is solved yet.

    From a mechanical point of view, I agree that strength is critical. I currently use 8mm rod inside the Tee foil and a 6mm welded vertical tang inside the centreboard, both in high strength duplex stainless steel. As this seems reasonably strong, I do not see the need for an 18mm shaft, but it could definitely do with some improvement. Agree with your comments on Titanium. I do not see any advantage, only extra cost!
     
  2. Doug Lord

    Doug Lord Guest

    takeoff

    I did some rough calcs and it is enlightening.This assumes that the main foil is a 63412 section and that the rudder foil is symetrical(wrong: see edit at bottom) and that both foils are set to 0° referenced to the static waterline. I figured a .2 lb.per sq' of SA pitching "load" 9' above the center of lift of the main foil for a pitching moment of 153 ft. lb's.
    Now at 8mph the Moth main foil at 1.08 sq.ft. area requires a 5° angle of attack. Since both foils are set at zero the only way to achieve this angle of attack (for now) is to tilt the boat. When that is done the rudder foil is also at a 5° angle of attack which means it is developing around 49 lb. of lift.
    When you factor in the estimated pitching moment of 153 ft lbs. lift on the rudder foil appears to be counter
    productive. This seems to me to show that the idea we've been discusing to stop the rudder foil from lifting at takeoff and for low speed foiling by changing the angle of attack of the main foil could reduce drag quite a bit. It would theoretically allow quicker takeoff and faster low speed foiling at an almost level attitude; these rough numbers seem to back that idea up.
    To be beach sailable and to be able to be jumped it seems that an all moving board/ foil combo would work well but it would require "shifting" as boat speed approached 15mph or so or the angle of attack of the foil would be too great for the flap/wand system.
    I read the Chapman stuff and they and Wardy apparently use a wand to move the fully articulating foil which would eliminate any shifting since the wand foil relationship would develop the correct angle of attack thruout the speed range( I think).
    Which makes the fully articulating system basically hands off whereas my system would have to be manually shifted(so far). But the foil/daggerboard "shift" system seems like it would be more rugged for jumping and off the beach foiling.
    Either system would unload the rudder foil and result in a much more level attitude.
    ===================================
    EDIT:I just want to add that I'm not 100% sure of this at all and I'm hoping (and I know Wardi is to) that someone with insight in this area will contribute their comments/ideas ect. What I want is to learn something and improve my understanding of this segment of foiler design....
    ===================================
    EDIT#2: According to John Iletts website the rear foil is a 63412-same as the forward foil thus the lift created by it at a 5° angle of attack is 77lb.'s -see post ahead for more
     
    Last edited by a moderator: May 18, 2005
  3. jimb
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    jimb Junior Member

    Foiler angle of attack

    Very interesting. Please don't forget that the Moths you are discussing are essentially flat underneath with very little rocker. At a high pitch angle you have a surface about 250mm wide x 2.2m long assisting with lift out. Of course, as soon as the hull leaves the water you lose this lift but you also lose the drag.
     
  4. casavecchia
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    casavecchia Senior Member

    tee foil

    From an engineering point of wiew I dont think the shaft is the main problem. You probably need a 18 mm shaft if you think of a two pieces tee foil interconnected only by the shaft. But think of a one piece tee foil with the shaft at 50% of the chord integrally molded inside and cut off in the aft part. Put the hinge point of the foil on the nose of the vertical strut.Now the main issue becomes the bearing surface inside the strut that could be augmented extending the strut say 2 cm each side at his lowermost part.This part could be acceptably shaped to minimise drag and the strut will remain thin as it needs to be. A side benefit of this arrangement could be the slight increase in foilbase.
    Hope what I say is clear, it's not easy at all for me!
    Marco
     
  5. astevo
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    astevo Junior Member

    re my size calculations, im sure it is quite easy to make it striff enough, my concern is regarding the required toughness.
    when the boat launches out and hits the water, a lot of load goes onto the very tip of the foil, This is where i have found they break. the actual bending under normal foiling is easy to accomodate.

    also the post before me sugested a shaft at 50% chord, that would be megga twitchy read any article taking abbout yacht rudder shaft locations and youd understand better
     
  6. casavecchia
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    casavecchia Senior Member

    Tee foil

    Sorry, I evidently was unclear.
    I didnt mean that the horizontal foil have to be necessarily straight.
    It could be tapered or swept back as often rudders are.
    I was focused on putting a significant quantity of carbon crosswise and in one length forward of the hinge point so to make the horizontal foil one piece and stronger.
    Wardi, are you using a one piece foil or a two piece foil (connected with the shaft)?
    Marco.
     
  7. Wardi
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    Wardi Senior Member

    I use a one piece foil Tee foil with the shaft embedded 20% back from the leading edge. This is close enough to the centre of lift to have small loads on the wand actuator and far enough forward that the foil will trail with neutral angle of attack when there is no load on the wand. I ran several trials to find the optimum position.

    It is very important to locate the pivot point in the centre of the thickness of the foil section and to ensure the foil is symetrical.
     
  8. Doug Lord

    Doug Lord Guest

    rear foil

    Just discovered on Johns website that the rear foil is a 63412 section-same as the main foil.(instead of the symetrical foil I thought it was) That changes loading a lot.
    Assuming that the rear foil is 60% of the main foil(I think I read that ) and since at 8mph the foil angle of attack needs to be around 5° the foil loading is very different from what I posted earlier.
    To achieve that angle of attack with both foils at 0°(angle of incidence) the boat has to be tilted bow up and the lift on the rear foil is about 69.5lbs(107lb.'s per sq.ft.) and on the main foil around 150.5lb.'s( 139 lb.'s per sq. ft.). So the main foil carries 68% of the load and the rear foil 32%.
    As a comparison the main foils on the Rave are designed to carry 80% of the load(+&-RM) and the rudder foil 20%(nominally and ignoring the effect of the pitching moment of the rig on rudder foil loading).
    The effect the pitching moment generated by the rig has on these loadings at takeoff is one fascinating question I'd sure like to know the answer to because it surely has SOME effect...
    And I have a gut feeling that the rudder foil is generating a lot more drag than it should be or would be if the main foil angle of attack was variable and or adjustable and the rudder foil was symetrical. Comments??
     
  9. PTO
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    PTO New Member

    Simple solution?

    Ive just started moth sailing but have had an interest in the class for some time so have been folowing the development of moth foiling systems with interest.
    I have just finished scan-reading the thread (yeap, the whole 30 pages) and the basic issue of height control seams to remain. I am just begining to get to grips with the concepts of foils so forgive my ignorance but it seams the fastacraft 'wand' control system works well in all conditions except heavy chop.
    My reading of the situation is that this is because the wand cannot react fast enough to the change in water level and this results in the boat rising as it meets a wave, only to shoot out the far side of the wave, still trying to rise up and stalling in the open air the foil now finds itself in.

    My instant thought is that this problem could be countered by using a very long very flexible wand and setting the shockcord to give a relatively low ride-height. In my head the wand would now not suffer from not being able to keep up with the waves as it would take an average of the water height along the length of the hull and keep the hull just skimming along at that height.

    I would be very interested to hear what people have to say on this as I am planning of building my own foiling system as soon as ive got the hang of lowriding.
     
  10. tspeer
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    tspeer Senior Member

    What you're describing, in control system terms, is reducing the gain in the height-to-foil feedback channel. As the height relative to the water changes, much of that change goes into flexing the wand instead of rotating it to activate the flap.

    That can work, but it also means the wand doesn't respond quickly to legitimate changes in flying height on the part of the boat. You're lowering the bandwidth of the height control feedback loop. You are also reducing the precision with which the control loop can regulate the flying height, allowing the boat to drift up and down a lot before the flexible wand responds.
     
  11. Speedy
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    Speedy New Member

    I've been following this thread for a little while - great reading.

    I have a Windrider Rave with the trailing wand / bungee system. Frankly, the simplistic wand and bungie system works great! With nearly three years of foiling experience with this rig - I can attest to it's abilities in choppy water. The quick response time is unecessary.

    If I sit out on the ama and control the flap by hand (applying quick flap movements) the foil / flap will cavitate. So a slower response might be a good thing.

    The ultimate foil setup (in my mind) would incorporate an angle of attack adjustment along with the flap adjustment. The limitation of the Rave foil system is upwind foiling. The upwind foil needs to generate more downward force than available from the flap alone when beating upwind. Some Ravers have made a simple modification to their rigs to do this. I plan on modifying my boat next year with a similar modification.

    see this PDF: http://www.doranoster.com/pdf/Incidence-021119.pdf

    I would modify this implementation with a long, stiff control lever to manually set the angle of incidence from the cockpit.

    I look forward to reading more on this thread.. carry on.
     
  12. pleversedge
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    pleversedge New Member

    P.Leversedge

    Would Rich Millar please contact me regarding his foiled sailboards?
    Thank you.
    Regards
    P.Leversedge
     
  13. Penfold
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    Penfold Junior Member

    Can I ask a simple question... Why are you using assymetric foils instead of symmetrical ones? Sure a cambered foil is more efficient at generating lift, but far less efficient when no lift is required and the net AofA is 0deg... When foiling is marginal/not happening and the lowriders are going faster, you're going to get hammered surely?!
     
  14. Doug Lord

    Doug Lord Guest

    Asy vs Symetrical

    As best I can tell most Moth foilers (at least the Ilett boats) have been using asymetrical 63412 sections on both the main foil and rudder foil because that section compared to a symetrical section has the ability to operate at a significantly higher AOA w/o stalling which enables low speed take offs. And at higher speed operates at the same lift coefficient at a lower AOA reducing drag. But off the foils the section is still developing lift at zero degrees AOA depending on the flap setting. Unless the boat has a clutch to disengage the wand the flap would be deflected down to a max lift position when the boat is not on foils-and that would be true regardless of the foil section. I know some Raves are equipped with that kind of clutch-I don't know about Moths.
    The Rave foiler uses a symetrical section designed by Dr. Bradfield mainly because it has to pull down as well as lift up.
    So in very light air non foiling conditions with the wand disengaged the symetrical foil
    might have an advantage especially if the whole foil could be moved to zero degrees AOA. But on the Rave the main foils are set at +2.5 degrees relative to the flight waterline(which is nearly parallel to the off the foils waterline) so even off the foils with the wand disengaged it still develops some lift.
     

  15. tspeer
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    tspeer Senior Member

    Not necessarily - it depends on the amount of camber. Most hydrofoils have camber that is low enough to include zero lift in the low-drag region.

    The H105 section was designed to include a small region of negative lift in its drag bucket for just the reason you state. The figures below show the effect of adding varying amounts of the NACA a=1 camber line to the section. The drag bucket is shifted to higher lift ranges with increasing camber, so it's a matter of selecting the camber that positions the low-drag region to match your operating conditions.

    The problem with a symmetrical section is the narrow drag bucket of the NACA 6-series sections isn't used very efficiently. Chances are, you'll be operating outside the bucket for typical lift coefficients - especially at takeoff. If you go symmetrical, you'll probably be better off to use a NACA 4-digit section (see third figure below).
     

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