Foiler Design

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

  1. National3434
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    National3434 Junior Member

    Interesting Questions

    Perhaps I should explain my hull design philosophy. It is a sharpie style cross section made of a single bottom plank and two sides but torturing the ply will ensure there are no straight lines. The waterline beam is 600 mm for two reasons:
    1. Not being a moth sailor already I do not want to spend the summer swimming and learning to sail a narrow boat; rather learning to foil. So a design with some static stability seemed sensible and since it should be foiling at around hull speed the 'slow' shape should not be too much of a problem.
    2. As I mentioned earlier the wider beam gives the possibility of a bipod mount which allows a long, high aspect main foil with less worries about the structural joints.

    As the hull should never need to sail faster than hull speed, a rockered design with low prismatic should be a good way of getting to hull speed = foiling liftoff. Of course, if there is not enough lift at hull speed it may never lift off at all!

    About the inter-foil distance question that Olav raises. My instinct tells me that a longer foiling base is 'good' because less pitch sensitive and that a short one would be more sensitive to pitching forces. But if you consider the configuration, as Wardi has described it, as a MONO-foiler maybe that is not really true. The canard is only there to provide a pitch reference for the main foil. If that works on a short base then maybe a long base is no better. One difference I can see is that at the speed where the canard is allowed to pop to the surface, the incidence angle put on the main foil is more for the short based configuration. I have just drawn this out and measured the angles on the assumption that the canard is 300 mm (1ft) below the hull at the bow. The effect is that the stern tries to bury and an angle of 8 deg is put on the main foil in my 'long' case and about 10 deg in the short case. I guess hydrodynamics says the lift is generated immediately so the hull should just launch out of the water and maybe the stern does not get a chance to bury. WARDI.. can you help us out here, what is your experience?

    Just a word about development philosophy. It will take several years for moths to stabalise on a 'best' baseline configuration. For me, the idea is to make foiling work well FIRST and THEN to work out how to make a foiler competitive in sub foiling conditions. I for one do not think I understand enough about the foiling game to be able to develop those aspects in parallel.

    Cheers
     
  2. Andy P
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    Andy P Junior Member

    The 6oomm beam should make sailing the boat easier for a new moth sailor.

    My hull will be double ender with a pointy pintail instead of more rocker. The normal low rocker hull has immersed transom - very draggy at low speed, so solutions are more rocker, or a pintail.

    I have started the foils... by reworking my old foils which have been hanging on my wall for a few years.
    The main foil is now a Y-foil, made by cutting off the lift foil of one side, and bonding to the other.
    This foil may need to cant to make the lift foil horizontal for best effect.
    Immersed bow rudder part is NACA 00 with the surface bouncing foils above, set at ~ 10° dihedral, all fitted on narrow strut.
    The foils are all circular arc with sharp leading and trailing edges, flat bottom for the lifting foils.

    pics of foils soon.

    All i need now is a new hull, or maybe a s/hand moth

    att pic of first foiler flight in 1984
     

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  3. Andy P
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    Andy P Junior Member

    Oops! Maybe that should that be 1994!
     
  4. tspeer
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    tspeer Senior Member

    If you're concerned about the strength of the joint between strut and foil, you might consider adding a fairing to the junction. The boundary layer tends to separate at the junction because the velocity gradients are higher - you're essentially adding the pressure field of strut and foil there.

    A bullet fairing might be designed to help counter this. It would have a "Coke-bottle" shape that by itself would have a dumb-bell-shaped pressure distribution with two low pressure zones and a high pressure zone in between. The fairing would be added to the foil junction so as to partially cancel out the interference effects, with the fairing's high pressure region aligned with the low pressure area in the junction, and the fairing's low pressure peaks aligned with the foil leading and trailing edge high-pressure areas. The goal would be to smooth out the changes in pressure along the junction so as to reduce the stress on the boundary layer and prevent separation.

    So the fairing would serve a dual hydrodynamic and structural purpose.
     
  5. Wardi
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    Wardi Senior Member

    An update and some questions:
    I have recently got my Scow foiler going in light 5-7kt winds, at what seem incredible relative speeds. There is absolutely a lot of potential here. Unfortunately no pics yet as I have been trialling on my own, but they will come soon!. It has taken quite some time to get the foil angles right for the very reason Tom has hit on. With a wide flat stern aft, it is very difficult to sink the stern and therefore the only way to get sufficient angle of attack on the main foil is to lift the bow. This of course requires that the lifting canard has sufficient area to lift half the weight of the boat plus me and any forward moment of the rig. I have finally got it right, but wide flat sterns as on most skiff classes is definitely a problem.

    An alternative would be to have a flap on the main foil and use this manually just for take off, but I am trying to avoid such complications as I just want it all to work automatically. It is because of this effect, that I think the narrow stern pintail skiff Moths are ideally suited to foiling as they allow the stern to sink in order to get the correct angle of attack on the main foil while presenting mininal drag.

    I have some new observations which pose some questions to which I would appreciate some assistance:
    1) Heeling the boat to windward by 5-10 degrees significantly increases the ability to take off....why? Has this to do with leeway increasing the lifting force?

    2) If I heel too far to windward, the boat continues to fall in on top of me. Initially this is very fast, but a little too much feels like it could end in a capsize to windward. On one hand I would like some increased stability when heeling in either direction, but not so much that I am thrown in to windward when heeling upwind or manouvering. Should the main lifting foil be a straight Tee, or should it have some dihedral or even anhedral angle? If so, how much?

    3) The deeper the Tee foil runs the more efficient it is, but also the lower the effective centre of lift and therefore the boat becomes less stable when heeled. Initially this gives greater leverage and righting moment and is very fast, but too much is not good.
    There is clearly a trade off here, what is the ideal depth of the lifting foil?

    I agree that it is most important to get the basics of foiling correct before optimising subfoiling performance, but it is important to keep in mind how to achieve this. I am working on the idea of using the lifting foil as lateral resistance in light winds and l already let the canard run free in neutral when it is not required, so the only resistance it offers is skin friction. It can also be retracted flush under the hull where it is clear of the water. This is not so easily done for the main foil though.

    The drawings of Olav's proposed Skiff Moth are quite good. The detail of the relative foil areas and locations may still require refining, but I think it can work. While I agree with Tom Speer that strut drag is to be avoided, I have been truly amazed at how well the Scow foils even though it has lots of struts and is a pretty agricultural construction at this stage. A bow rudder would certainly further reduce drag, but I am not convinced it is worth the extra complication yet, just keep it simple for now!. I am also not convinced it will track straight and would need active steering at all times.

    In other words, it seems that just about any foil section will do in order to prove the concept and get the basic geometry right, we can refine section shapes, sizes, strut configurations and retraction systems etc later. I am persevering with development the monofoiler concept because I felt the original bifoiler arrangement was not good enough, even though I was got this working several years ago. The success of John Ilett in developing the bifoiler thus far is remarkable. I am becoming even more convinced that the surface tracking canard will solve the problems in big waves and also control flying height much more accurately...but this is yet to be proven.

    Hi to Andy P. I remember your amazing performances in your Axeman at the 1990 Ratzeburg Moth worlds which prompted Emmett Lazich to build the first of his "Aussie Axeman" designs which have since dominated the Moth class. I think there is a long way for us to go yet with this foiler development and welcome your contributions. I have experienced consistently poor results with assymmetric foils, they seem to give a lot of extra drag in light winds and seem quite twitchy to get the correct angles for lifting. I would prefer symmetric foils as a better all round solution.

    The joint strength of the main lifting Tee foil is quite critical. Several all carbon foils have already failed, so it is important to make it as strong as possible. I support Toms idea of a fairing, perhaps a bulb at the joint to enable a stronger construction. I am also about to trial an alternative with 45 dgree compression struts to support the Tee, which will be much stronger still. I am interested in any thoughts on how much extra drag I am likely to get.

    Ian
     
  6. astevo
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    astevo Junior Member

    hi wardy
    i think the benefits of the struts supporting the 't' would create much more drag for the structural benefit. i think with this join the 90' angle is maybee not as critical and a little more bulk of a centre fairing would be better off. Parasite drag will really hurt particularly at speed when ventilation will be a concern.

    have you you increased the size of the canard foil in order to skim on the surface at lower speed.? or the submerged foil held at constant height by the sensor arm as you had perviously?

    my take on the increased lift from the roll to windward would be the slightly larger angle of attack due to the leeway. it also possible that by rolling to windward the vertical foil suffer from less endplate loss and so the main foil has cleaner water flow in which it can lift rather than have the flow interfeered with by the flow down the vertical. maybee this is another reson to race the main foil forward even if only at the bottom.
     
  7. mad engineer
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    mad engineer Junior Member

    Just a thought or two on foil sections - I agree with Wardi that a symmetrical section is going to turn out to have some advantages, especially at higher speeds. The downside is a higher take-off speed.

    My reason for saying this is:
    1) A symmetrical section has its lowest drag at zero angle of attack.
    2) It also produces zero lift at zero angle of attack

    Therefore, when flying the range of angle of attack will be from the maximum at take-off, of say 10 degrees, to close to zero at higher speeds, and the coefficient of drag will be reducing the whole time.

    With a cambered section, the minimum drag is not coincident with zero lift, so there will come a point where the Coefficient of drag is increasing while the lift is decreasing, which will lead to a lower top speed.

    Another thing that might help sub-foiling drag is to look at the use of laminar flow symmetrical sections so we can take advantage of the low drag bucket for non-foiling conditions, and also at the low angles of attack at the top end of the speed range. I don't know enough about the lift performance of the laminar sections to know if that is really workable though...
     
  8. Olav
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    Olav naval architect

    G'day Wardi,

    it's both amazing and really encouraging to hear at how low windspeeds your foils still work!

    Sorry, no real answers to your questions raised, but a few thoughts...

    My pictures show the canard with no V, but it will be built with an angle of 20 degrees upwards from horizontal on each half of the foil and probably with a symmetrical section (something like NACA 0009 perhaps?) instead of the supercavitating thing. You're perfectly right in my opinion regarding the symmetrical foil section for the main wing to improve sub-foiling performance. I think I'll do this, too.

    As the canard obviously governs the ride height I will have a strut about 400mm long as I think this is an appropriate height to avoid the hull being affected by chop. The centreboard is around 750mm long below the hull's bottom (because it's just the 1200mm standard centreboard model I use in conventional mode) so there will be around 400mm left below the water surface when foiling (due to the AOA it's more than 350mm deep of course!). Hopefully that's enough!

    As I wrote in my former post I also think it's worth putting some extra compression struts on. These can be very thin with a really low drag section. I still wonder if I should use just a cylindrical strut for the canard or something more streamline-shaped? However I'll have a tube built in at the bow where the canard strut fits into so it can be retracted eventually...

    All the best, Olav
     
  9. Andy P
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    Andy P Junior Member

    Since I am recycling my old foils, i'm stuck with the section and sizes.... but they seemed about right to fly in ~ 10 knots wind.

    My main foil is probably too large, ( 150mm chord x 600mm each arm) but maybe this is the answer to get low speed foiling, but perhaps limited top speed ( but still faster than conventional boats)
    My plan is to have bow rudder on a pole fwd ~500mm of the bow, to increase horizontal separation ( the sailboard foiler has a foil at each end of the board).
    The surface canard foil fitted to this rudder, (but this might cause steering problems with heel). The complete bow rudder / lift foil can then lift/retract so the horiz foil is above the waterline, and the normal rudder section is at normal immersion depth for low wetted surface.
    I am planning to cant the main foil, so that the hull can be sailed level, with the arms of the Y-foils providing lift, and side force. The arms are set at 40 degrees anhedral. For light winds, the foil could retract leaving just the arms out of the hull, and the vertical fin up in the hull ( and fouling the boom!)
    A possibility for reducing the ends of the canard foil digging in is to have V foil with dihedral, so one or the other side works flat to the water surface.
    Or a curved foil, so that the lowest part of the foil is always horiz to the water surface.
    Not sure of the area required for this foil.... the miller board foil was 110mm x 300mm ish.... does it need to be bigger for the moth?
    What size is Wardi's?

    Obviously a lot more experimentation needed.
     
  10. Wardi
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    Wardi Senior Member

    Olav, As the main foil lifts higher than the canard and the canard is always on the surface, it may only need to be 300mm long, I am still experimenting with this height. I recomend a streamlined canard strut, round is too much drag when it is in the water. I currently use an aluminium 470 spreader section, but it needs to be a bit stiffer really and more streamlined still. I made a mini case and can retract the canard, this works well.

    I agree that the main foil diagonal support struts may work and be much stronger construction than the simple Tee. They are not simply parasitic drag as they can provide both lift and lateral resistance, both of which are needed. The main foil could be made commensurately smaller.

    Andy, The large main foil works well at low speeds. I am using a similar size to yours at the moment. Will cut it down as I solve the other problems. I have tried the inverted Y foil and had big problems with helm changing and utter instability. Recommend a straight Tee or some dihedral. Curved foil may be interesting! My canard is 400 x 100, seems adequate, but a little bigger may be better.

    Canard:
    My canard can be operated in three modes. In the first mode, it is fixed, with the angle of attack adjusted manually. In this case I simply set the angle and as the bow rises, it gets more lift due to the increasing angle of attack until it breaks free of the surface. As the main foil then lifts and the boat picks up speed, the boat begins to rise further the faster it goes. The main downside is that at higher speeds the relative angle of attack of the canard reduces due to the boat flying higher and it provides less lift and can actually be forced to go back down under the surface, where it has almost no angle of attack because the boat is still supported by the main foil. This can result in a slow, controlled porpoising.
    I am interested in any ideas how to solve this, as a fixed canard is by far the simplest system to use.

    A second mode uses a simple trailing sensor arm to automatically alter the angle of attack. With a greater angle the deeper the foil runs. This is the same arrangement I used on the original bifoiler in 1998. It works very well on a deep running submerged foil and controls height well, although it has the same inherent problems of the Ilett sensor system in that it can be difficult to keep the foil in the water when tracking over waves downwind. I have also experimented in using this sensor to run the canard on the surface, but once again it has the problem that it reduces the angle of attack when the foil comes to the surface, which is exactly what is not required for surface running.

    In the thrid mode, I simply release the control wire and the canard pivots freely generating no lift. This gives minimum drag for displacement sailing.

    I want to use the surface running canard because it tracks really well, is very simple, reduces drag by removing submerged foils and overcomes the limitations of the bifoiler and sensor arrangement etc. It feels like I need a double action sensor to first give required lift when deep running and then increase it again as the foil breaks the surface and increases further as the main foil lifts the boat higher.

    During writing this I have come up with a new solution involving a double canard, but would appreciate some help with a good, practical simple solution!!

    Ian
     
  11. tspeer
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    tspeer Senior Member

    Not necessarily so. It depends on how much camber the section has. The key thing is to know what range of conditions you are interested in and target the section design to cover them.

    One way to think about it is the designer uses thickness to trade off minimum drag (thinner is better) against being efficient over a wider range of conditions. Then uses camber to shift the center of that range so that it fits the application. For a lifting foil, you want the design range to include zero lift and a moderate range of positive lift coefficients. A symmetrical foil will have much of its design range wasted because its in the negative lift range that's of no interest in a lifting foil. The fact that the cambered foil has to be at a negative angle of attack to produce zero lift is immaterial - just mount it at that zero-lift angle where you'd mount a symmetrical section at zero incidence.

    As for not producing minimum drag at zero lift, take a look at this example:

    [​IMG]
    [​IMG]

    The four sections compared in this figure are the NACA 5313, a highly cambered front-loaded section, 13% thick; the NACA 63-412, a 1940's vintage laminar flow section, 12% thick; H105 a section I designed using the Eppler code, 12.5% thick; and H106 a section I designed using XFOIL, also 12.5% thick.

    The NACA 5313 does suffer from excessive drag at zero lift. It really doesn't come into its own until Cl=0.8. It goes up reasonably well to Cl=1.4, but a hydrofoil probably isn't going to operate there.

    The NACA 63-412 section has the classic 6-series drag bucket, centered at Cl=0.4 (the "4" in its designation). It has slightly too much camber to have minimum drag at zero lift. A NACA 63-312 section would fix this problem by shifting the drag bucket down by deltaCl=0.1

    The H105 and H106 sections were intended from the outset to be used as sailing hydrofoils, so I made sure zero lift was included in their low-drag range. For the H105, I wasn't too concerned about the upper lift ranges because a hydrofoil boat has trouble rotating to high lift for takeoff. Once it's up and cruising, it doesn't need high lift, either. So I was content to have a design Cl=0.3 and a low-drag range that extended to Cl=0.5 - 0.6.

    For the H106, I wanted to see if I could beat both the H105 and the NACA 63-412. So I resorted to the section designer's favorite crutch - aft loading. I built more aft camber into the section, as you can see from the increased pitching moment. I went back and forth between low and medium-high angles of attack to clip the formation of leading edge suction peaks that would have caused premature laminar separation, leading to early transition or leading edge stall. As you can see, I succeeded fairly well at meeting my goal - all exept for the very corner of the 6-series' drag bucket. And there's just a tiny amount of drag creep at zero lift.

    So it looks like the H106 is a far superior hydrofoil compared to the NACA 5313, right? Maybe not. These are all non-dimensional coefficients that have the effect of area divided out. But say you chose a design lift coefficient of 1.2 for the 5313 compared to 0.4 for the H106. You could make the NACA 5313 foil 1/3 the chord of the H106's. This would cut the drag by 60% while producing the same total lift. Picture the NACA 5313 drag polar shrunk in the figure above until its zero lift drag half that of the other two sections! The maximum lift would shrink down to an equivalent Clmax=0.5, too. So you wouldn't have much margin left. But the NACA 5313 may well be the best of the 4 choices because it has the highest section L/D. And if you could stand the fact that structurally it would have 1/3 the thickness of the others.

    I think you get the idea - there are lots of tradeoffs and it's not possible to make any blanket statements. Even with CFD, there's still plenty of art in design!
     
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  12. Olav
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    Olav naval architect

    Tom, thank you very much for your nice explanations and for publishing the graphs!

    I just came to another concern when talking about drag in non-foiling mode (from my point of view as a Moth sailor this isn't such an issue at the moment as we still have no restrictions on gear and are allowed to remove foils eventually, but I guess we aim at an allround solution with the foils fitted at all times...): I found my current existing Moth design goes pretty well in the (very) light stuff with the bow digged in almost up to the forestay fitting. In this case I think any foil section would produce both unwelcomed incredible drag and negative lift... Probably we need a foil arrangement that can pivot around the transversal axis of the boat (so to avoid a hinge at the centreboard's tip which weakens the joint) to have the foil at the right AOA. These adjustments would be done before hitting the water and left there. Doing this it may be possible to take the "ultimate" ;) section with no regards on drag characteristics at 0 degrees of attack???

    All the best, Olav
     
  13. tspeer
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    tspeer Senior Member

    Sure, you could make it pivot to ensure there's no negative lift - probably a good idea anyway, since hydrofoils can have a really nasty crash-dive. I've heard of a powered hydrofoil in Switzerland that flew out of the water in a wave, then pitched down and the front foil entered the water at a negative angle of attack. The boat pitched down so violently that a crewmember inside was thrown through the cabin roof! Maybe an urban legend, but a cautionary tale nevertheless.


    You'd basically have the same issues as designing a flying wing. You'd want zero net moment on the foil at zero lift - this is the condition of trim - and you'd want the change in moment with lift about the pivot to be negatve then - a requirement for stability. You can get this by designing the foil with zero moment and putting the pivot ahead of the aerodynamic center. This will probably require a reflexed trailing edge. Or you can sweep the foil aft and use twist. The latter approach lets you use sections that have at least some aft loading, but not very much.

    It may not be necessary to have it trim for zero lift. A modest amount of negative lift that's still in the low-drag range might be acceptable. Of course, you have to factor in the induced drag as well as the profile drag, so zero lift is still probably the way to go. It all depends on what you have to give up in other areas to get it.
     
  14. RVELL
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    RVELL Junior Member

    Pitch stability & front foil design

    It is a fine discussion you are having at boatdesign.net. Our small group in San Diego, California, is watching with great interest. I have not contributed because our hydrofoils are generally human powered and, forgive me, my latest prototype is motor powered.

    However, we share many problems. In particular I have been carefully reading the comments on front foils and pitch stability. My associates, Dwight Filley and Steve Ball, have used “Shutt Struts” with good success with their human powered machines. The Shutt Strut performs well in smooth water with constant power but may not be adequate for a motor powered vessel under a wide range of water conditions.

    Automatic pitch control, as with the Shutt Strut is ultimately desirable. For my present prototype, “Hifybe” (High Flying Banana), I have chosen to use pilot input to control pitch. After developing some feeling for what control responses are needed, I plan to turn the job over to a surface following wand mechanically connected to the lever that controls the angle of incidence of the front foil. In the meantime, maneuvering is a game of skill.

    My recent experience follows:

    Hifybe, AKA High Flying Banana, weighs 322 lbs including 172 lbs for the rider and 57 lbs for the 8 HP Suzuki outboard. The front foil carries 11 % of the weight varying on seat adjustment fore and aft.

    Initially, the front foil was a 12 X 3 inch (.25 square feet) NACA 63-421 (approximately) wing section of 6005 aluminum. At rest it is submerged 28 inches, and its cruise submergence is between 16 and 13 inches. The angle of incidence is changeable –1 degree to +14 degrees through its linkage to an aircraft style “stick”.

    In the form described it is so unstable that this pilot can just barely control the pitch excursions. To illustrate what happens assume we are cruising at 8 mph, 6.5 degrees AOA, and 13 inches of bow foil submergence (15 inches of heave at the bow). The weight/lift at the bow is .11 x 322 = 35 lbs

    To compute how the required lift (L) is achieved I use the formula L= FUUSCl where U is the velocity in mph, S is the surface area of the foil in square feet and Cl is the Coefficient of lift at the present angle of attack. Cl may be obtained from aviation sources like Theory of Wing Sections, Abbott and Von Doenhoff. See page 544. The F is what I call the “Ray Factor” that adjusts for the type of units used and the density of seawater. In this case we use F = 2.09. For this discussion use the following coefficients preceded by the degrees angle of attack: 4.5 = .843, 5.5 = .957, 6.5 = 1.071, 7.5 = 1.185, 8.5 = 1.255. Requests for complete table may be addressed to: rvell@san.rr.com.

    Using the formula we get 2.09 x 8 x 8 x .25 x 1.071 = 35 lbs.

    Remember, to be dynamically stable if the bow pitches UP the lift must DECREASE. If the bow pitches DOWN the lift must INCREASE.

    In this configuration if the bow pitches UP 1 or 2 degrees the lift INCREASES by 5 to 7 pounds lbs., respectively. Use the formula to solve for lift.

    If the bow pitches DOWN 1 or 2 degrees the lift DECREASES by 3 or 5 lbs.

    This is an unstable configuration. The more the bow pitches the greater the amplifying pitch force. These large and rapid excursions are difficult for a pilot to stay ahead of.

    An effective surface follower can solve this problem by increasing the AOA rapidly as the bow falls or visa versa if the bow rises, but I chose a mechanically simpler solution at least for now.

    I added a second wing 15” above the original foil. This created a two-rung ladder foil. The original foil area was reduced by 17% by sawing off the ends. The upper foil is a wedge shaped section with the sharp edge leading. Dimensions are 9.5 inches horizontal by 2 inches. It has 4.5 degrees greater angle of attack than the lower foil. The wedge section is normally used to supercavitate in high speed craft but in this instance, “super-ventilate” would be more descriptive. It is in effect a small planning form that follows the water’s surface and supplements the lift of the larger submerged foil. Because its lift does not depend on a relative vacuum on its upper surface it has a smoother lift curve as it enters and exits the water. I guesstimate its lift coefficient to be 1/3 that of the NACA 63-421 wing section. To further soften the ride on the water’s surface, I added 25 degrees of dihedral to the upper foil.

    Water trials showed this change to be a significant improvement. When the bow pitches down, the upper foil provides a strong pitch up force and does not depend on being fully wetted like a subcavitating foil would. When the bow pitches up, lift is lost as the upper foil leaves the water. The pilot may better estimate heave by observing the spray as the upper foil “dances” on the water.

    Good luck, fly low & slow.
     

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  15. Wardi
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    Wardi Senior Member

    RVELL,
    Thanks for your good contribution. This is a very useful innovation and may help solve several problems.!

    On the monofoiler dinghy, I am trying to place all lifting load on the main foil. The canard is simply there for surface reference to maintain constant height, not to be load bearing. The biggest problem with sail powered craft however is that the pitching moment of the rig is very large....something that no other foiler or aircraft configurations have to contend with. This results in very large loads on the canard, which it cannot handle once it nears the surface, unless I make it very much larger. It is already 100 x 450mm.

    I have been foiling on my Scow Moth very recently and solved part of the problem by adding a small fixed Tee foil to the bottom of rudder as a stabilizer. I am now able to fly with the canard completely clear of the water for 10-15 seconds at a time in steady breezes. It then kisses the water and takes off for another freeflight. This works very well and any aft pitch is easily handled when moving aft, easing the sheet, rounding up or in strong gusts which backwind the top of the sail.......but when I move forward, power up the rig or get hit by a gust of wind, the surface running canard cannot handle the load and goes under, dragging air with it. It would seem that a fully submerged canard with sensor is a good solution, but may be a problem in waves.

    I think such a stabilizer would significantly improve your powered foiler also!

    Your solution looks to provide the control I need, but has the disadvantage that the upper foil is high drag when submerged in displacement mode. Do you have any suggestions to overcome this?
     
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