Foiler Design

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

  1. alans
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    alans Alan


    yes I was aware of the shock cord on the moths but I think the wand comes out of the water before the foil does. Some math modelling I have done for my Kooee Model ( the model is about 40% complete) showed that springs on the wands made them bounce badly under some conditions but the heavier low drag wand stays in the water and enjoys a level of viscous damping.

    What is your experience with your Microsail hydrofoil model?
  2. Doug Lord

    Doug Lord Guest


    In waves up to the boat starting to bottom out the wands always stay in contact with the water-they are spring loaded.
    On the Rave there has never been any tendency of the wand to come out of the water or to bounce as far as I know-at least Dr. Bradfield never mentioned it and I never saw it.
    In short choppy waves the ride on a Rave can be like driving a truck over a rutted road because of the wand sensitivity.
    On the Rave and SKAT the wands are curved so that they plane on the waters surface; on the Moth it appears that the wand is flexible.
    When the wands were first made curved they increased sensitivity dramatically . They need to be tried that way on the F3 but so far they are just straight (thick) airfoil shaped tubes. Had a problem with a high frequency vibration using just round tubes so switched to putting a small very short chord airfoil shape out of aluminum over the carbon tube and filing the trailing edge. That stopped it..
    Last edited by a moderator: Jan 31, 2005
  3. alans
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    alans Alan


    Thank you for the response

    Your experience with the wands is consistent with my sums, the aerofoil section being very much low drag than the circular wand, hence slower to respond to short period changes in immersion depth particularly when the wand is near vertical. I have not added wand flexibility to my moth simulation but will do. Long (hard) experience has taught me to avoid trying to eliminate forcing function and instability problems from closed loop control systems with spring, that's not to say they will not work in some cases.

  4. Doug Lord

    Doug Lord Guest

    F3/wand control

    Alan, on the model there was no shock cord available that wasn't too fragile so I had to find an SS spring that worked in a similar fashion...
  5. alans
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    alans Alan


    If you are finding "man in the loop" control difficult it is probable that you have a rudder foil that is too small in area compared to the area of your mainfoil with your CofG too far aft.

  6. alans
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    alans Alan

    Tom /Doug

    Some further simulation runs show clearly that the light weight wand, low drag tip and spring combination is the best option I have modelled to date. Also an independent wand control rudder flap/foil appears to work quite satifactorily provided the gearing of the rudder flap is less than the gearing of the flap on the main foil. The bow mounted wand already provides a very stable robust closed loop control system, amazingly good for a simple system. Your ideas for improvements are all possible. A fundermental issue is that in order to improve the response of the boat so that it come closer to following the wave profiles ( standing high enough to clear the waves and not responding is one solution) requires a significant increase in loop gain and appropriate feed back shaping. This is very difficult to implement without going to an active auto pilot control system. Will keep posting as model developes

  7. Doug Lord

    Doug Lord Guest

    foil control

    Alan,the main foil flap on my 16 footer is controlled using a twist grip on two extension tillers; mechanically they work fine but given the current boat layout they are a pain. I'm going to switch to a wand that can be bi-passed manually in order to jump the boat and then maybe switch back to all manual once I have more experience just to thoroughly test it.
    I don't know if you've seen my foils(on , and ) or not but the ones on the 16 footer and the F3 are designed with a flap in the center one third of the foil; the idea was to dampen the flap response a little and also to avoid the hypersensitivity of the Rave full flap foils.[The shorter flap also avoids some of the flap hinge problems found when the foil flexes.] It works very well on the F3 but I'm not sure about the big boat yet. On the new 12 footer I will have two sets of main foils to experiment further with the partial span flap.
    Both the 16 and 12 foils are designed and engineered for jumping which I believe will be a whole lotta fun once I get it right. The experience on Moths so far,as I understand it (with foils NOT designed or engineered for the extra loads) is that in rough water an inadvertant jump almost always means crash where in smooth water it rarely does. However, that experience is with NO manual control over the wand system which as far as I can tell should cause the boat to be sucked back down hard after jumping. The wand will cause full up flap and when the boat re-enters it may crash before the wand regains control.
    I can see how it would crash in heavy air/waves.I would bet that if the skipper could bypass the wand for a couple of seconds after the boat leaves the water that a crash might be avoided but I'm not sure.

    edit: add info= [----] above
    Last edited by a moderator: Feb 6, 2005
  8. alans
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    alans Alan


    hinge lockup caused by wing bending is as old as flight. one approach is to make the full span flap in 4 pieces with small universal joints between. You pay both induced drag and interferrence drag penalties with partial span flaps when ever you pull high Cls. On paper damping from pitch rate and alpha dot is very adequate even with large full span flaps

    A configuration that I am sure will work for you is to use the aft foil flap for man in the loop control and the flap on the main foil for lift out and trim only as dictated by your speed on the day.

    With either cannard control or more conventional tail plane elevator control the centre of gravity basically needs to be ahead of the CofP of the combination of the foil areas for the system to be stable enough for the man in the loop to manage. With experience and skill this rule can be broken but the pilot work load increases dramaticaly as the CofG goes aft. Automatic control does not suffer this problem to any where near the same extent as the poor man in the loop.

    As for jumping I will see what my moth simulation shows.

  9. Doug Lord

    Doug Lord Guest

    boat control

    Alan, from my experience and talking to various people I'm convinced that manual altitude control should be done via the main flap vs the rudder flap. That is how the wand controls the altitude and is probably better for manual control. Though I think for realy high jumps control of both flaps may be important.
    On both the Rave, Moth and the model F3 the rudder t-foil is rarely-if ever- used.
    On the F3 when I first tested the boat it had servo control of the rudder flap and wand control of the two main foils. At speed the rudder flap could be deflected one way or the other and there would be a barely noticiable attitude change. I thought at the time I could use the rear foil to jump the model but I didn't fully understand the wands at that point.When the rudder t-foil flap was deflected it wanted to change the pitch angle of the boat but the wands prevented that from happening because as the pitch angle changed and angle of attack of the main foils became more positive(or less positive) the wand would raise(or lower) the two main foil flaps to prevent a rise(or fall) in the boat.
    When I talked to a couple of others about manual control of altitude using just the rudder t-foil-if I remember correctly- they all agreed that it was too critical-causing large excursions in altitude and pitch change for very small flap movements.
    At any rate, I tend to think that manual control should be with the main foil.
    In either manual or wand systems it seems to me the best use of the rudder t-foil is to set it and forget it.
    Thats generally the way Moths and the Rave are sailed....
    I'm very interested to hear the results of your simulated jumps! Can you simulate the jump so that the main foil flap is neutral or slightly down on re-entry? To do that in "reallife" the manual control would have to temporarily bypass the wand.
    Can you also simulate the behavior of the boat coming clear with the wand still in "control"?
    That's sort of a misnomer because if the wand was in control the boat shouldn't come out of the water in the first place! Re-entry under those circumstances would be made with the flap full up which would be pretty hairy at speed...
    So I'm interested and looking forward to hear your results.
    A Note about Flap Hinges
    One of the first things I ever learned about flap hinges was from Dr. Bradfield who showed me how the Rave hinge has to be manualy opened up(at least back then) to prevent binding under load. It struck me then as a very labor intensive way to deal with the hinge.
    I then came up with a way to do the F3 model hinges using very thin mylar incorporated into the layup but I figured then that if I made it a full span flap it would probably bind since it was "solid state".So I used a partial flap; I tested the first foils under a simulated load and they didn't bind at all.
    When I asked John Ilet to make the foils for my 16footer he described his "new" technique at the time of using kevlar wet out with flexible resin to make the hinge; I decided then after discussing this a bit that I'd go with the short partial flap(same area as a 20% full flap). The foils were also engineered to be able to jump but for whatever reason they do not bind under load.
    I recently tested a foil with a full length 20% "solid state" flap that DID bind under load-so much so that I'm not convinced a wand could over come the extra load. The point of these ideas was to come up with as simple a method as possible to allow production of a foil/flap combo without it being too labor intensive. Stil more to learn especially whether the labor saving and 100% flap seal is worth the penalty incurred with a partial flap. The partial flap works well on the F3(boat takes off in 5-6mph wind and is estimated to do 2-2.5 times windspeed in those conditions and windspeed in 20mph wind/reefed) and it is a one design but it would be interesting to make some direct comparisons which I intend to do on the 12 footer. I'd be interested in any other flap hinge techniques that others have tried....
    Last edited by a moderator: Feb 7, 2005
  10. tspeer
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    tspeer Senior Member

    If you look at the conditions for stable flight, the forward foil must have a relatively higher heave stiffness than the rear foil, and the rear foil must cause a greater change in pitching moment with a change in angle of attack than the forward foil. This is why wand feedback to the forward foil and a fully submerged aft foil work so well together. The heave stiffness of the aft foil is virtually zero, so it does all the job of stabilizing the craft in pitch. The forward foil then is totally responsible for controlling the craft in heave.

    Manual control of altitude via the forward flap makes sense, since that is the surface that really controls heave. But the big difficulty is having enough control power to handle both the dynamic change in lift needed by the control system, and being able to trim out the change in lift due to speed. With direct gearing between the wand and flap and the boat flying at a constant pitch attitude, the only way for the control system to trim out the increase in speed with lift is to deflect the flap upward, and because of the gearing between flap and wand, this means flying higher.

    If the boat is trimmed bow down as the speed changes, then the reduced angle of attack will compensate for the speed and the boat can fly at its design height with the foil centered (on average). One way to get this pitch trim is with manual control of the stern flap. The speed changes less rapidly than flying through waves or even being hit by gusts. So pitch trim with speed makes sense for manual control.

    It might be possible to get some automatic speed trim by clever manipulation of the hinge moments on the rudder flap. Aircraft use a device called a "down spring" to augment their speed stability. A spring with a low spring constant is used to apply a near constant nose down force to the controls. This has to be trimmed out with the trim tab. But the force from the trim tab depends on speed. So as the speed increases, the tab effectiveness increases and moves the elevator in the nose-up direction. This makes the aircraft climb, which reduces the speed.

    The opposite could be applied to a hydrofoil. If the rudder flap were deflected upward with a spring, increasing speed would reduce the deflection. This would make the boat trim more bow down, reducing the flying height in much the same way as the manual pitch trim described above. You could also divide the rudder flap into two parts - one driven by the spring and the other under manual control.

    Naturally, the spring force would control the amount of automatic trim change. It's better to get the force from deflecting a long, weak spring a lot, than by deflecting a short, stiff spring. This makes a bungee cord ideal for the purpose.
  11. Doug Lord

    Doug Lord Guest

    pitch control

    Tom, in two boat testing of the F3 model using the rudder foil to change pitch attitude was slow.It also changed the attitude very very little.
    It seems to me that if the rudder foil was left alone and the angle of incidence of the mainfoil changed at a speed that "up" flap began to be required that you would have less drag either manually or wand controlled.
    This would be an exceedingly simple set up requiring just the throw of a lever to change the mainfoil to its high speed "gear".
  12. alans
    Joined: May 2004
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    alans Alan

    Most not all aircraft use elevator/tailplane controlled in pitch, nearly all yachts are steered with stern hung rudders, the dynamics are nearly identical. However either system can be configuired to work. Sorry but your thoughts on sensitivity are erroneous.

    On David Luggs 14 we chose to go the conventional aircraft way mainly because implimentation was easier, changing the rake of the centreboard to change the trim angle of the fixed main foil. If you have a copy of the seahorse article on David's skiff that has the steady state static equilbrium equations and you can do you own sums to see the effects of trim all be it by the forward or rear foil flap.

    John Illet's kevlar hinge is a great piece of engineering and less likely to lock up than a 3 or 4 gudgeon metal hinge never the less the WA boys believe they are at times experiencing lock up with their newer high aspect ratio foils.

    We have chosen to use 4 gudgeon metal hinges on the kooee model with a very stiff foil but yet to test it under load.

    Will let you know if I get some meaningful simulated jumps

  13. Doug Lord

    Doug Lord Guest

    pitch/foil control

    Alan, it is quite possible I may have "erroneous thoughts" but there are some serious differences between the airplane case and the hydrofoil-which I'm still trying to understand. One is that the(extremely variable) power is being applied at a distance equal to or greater than the distance between the foils ABOVE the two foils. Along with this is the transition of the rear foil from lifting at takeoff to negative lift at speed. At or near this point-the crossover between positive lift up and negative lift down- has got to be a difficult point of control if you were trying to control pitch attitude(and altitude) of the whole boat manually using just the flap on the rudder t-foil.Especially when you factor in real wind conditions(gusts,lulls) and waves.
    From a practical standpoint, it appears that if the rudder foil is left alone on the Rave ,Moth and F3 this 'changeover' is managed automatically with no human intervention(rudder flap almost never used).
  14. CT 249
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    CT 249 Senior Member

    Results from the UK's biggest dinghy event, the Tiger Trophy, may show the good and not-so-good side of foiling.

    The foiler Moth was given the very fast handicap of 860. For comparison, 49er is faster at 747, the 16' long skiff type R800 is faster at 790, the International 14 foot skiff is a bit faster aat 850.

    Boats that are slower than the foiler the MPS (singlehanded skiff) at 875, the 505 is 902, the Flying Dutchman is 879, the International Canoe (no spinnaker) is 902.

    So on the upside, the yardstick guys reckon the boat is fast (as we all know).

    On the downside, the only foiler Moth (well sailed) finished almost last, with 213th, 218th and a couple of DNCs.

    Simon Payne (3rd worlds) had been proving in earlier regattas that the foiler was sloooow in light winds, with 13th out of 20 Moths (after 3 races) in earlier regattas when the wind stayed less than f3. he also proved that it was incredibly fast in a breeze, scoring three wins when the breeze arrived the next day.

  15. Doug Lord

    Doug Lord Guest


    Chris, what were the winds in this regatta? While Simon Payne is probably an excellent sailor it's too bad Rohan couldn't have represented the class. There is no question that his technique is the best in the class at this point in it's development.
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