Rotating adjustable hydrofoils

Returning to my original query, is anyone aware of the rotating foil approach having been tried before?

Also, I wonder about the consequence of the inclined axis of rotation. Either the foils will be swept forward in the upwind configuration and perpendicular to the hulls in the foiling configuration, or perpendicular to the hulls in the upwind configuration and swept back in the foiling configuration (or something in between the two). Of these two "suboptimal" (to quote our ex prime minister) situations which would be the least bad?

 

Rotating retraction system rough sketch:
Note: the rudder can be retracted(reduced in depth) from either the foiling mode or non-foiling mode. By "retraction" I mean that the rudder t-foil can be replaced by a normal rudder using the process of rotating the whole assembly.

click-

 

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I'd think swept forward in the upwind configuration if you're sure you have to choose.
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Hydroptere.ch and the "big" Hydroptere both rotate their foils to retract them when not foiling. But I've never seen a system exactly like what you propose and used in the same way.

 

As Doug points out, L'Hydroptere uses a hydraulically adjustable foil but they are highly experimental. One of the reasons the don't go for the J foil is that by widening the dihedral, they increase stability over a J board. They do this a the price of increased complexity and increased drag.

probably the simplest way to do kickup rudders would be with slaved/mirrored servos dedicated to each rudder and then a kickup servo but you are adding a lot of complexity (complexity == weight) for a sailing condition ideally you will rarely if ever be in.

 

Thanks, I will start with this, but it shouldn't be too difficult to modify to try other options

I imagine that the hydraulic system gives them more control.

The simplicity of angled foils (or wide dihedrals, I'm not sure what to call them) seems attractive for an rc boat scale, but the rc boats using this option thatI've seen to date aren't particularly stable. Do you think that this is likely a problem with the implementation, or scale, or something else. Are j-foils likely to be even more difficult to control at this scale?

 

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Surface piercing foils like yours and the ones on Hydroptere and hydroptere.ch(it's baby sister) are ideal for models in some respects-mainly the simplicity of construction. The downside of a surface piercing foil arrangement is that, generally, the foils aren't used to generate righting moment. To compensate for that Hydroptere carries water ballast in the windward ama. She also is oversquare-approx. 80' wide on a 60' main hull in order to have enough righting moment. So models that use surface piercing main foils need to be quite wide for their length or else they simply won't have much power to carry sail. And/or they could use movable ballast like the full size Hydroptere.
The model in your video earlier is ,by the looks of it, way too narrow.
For models it's hard to beat the Bradfield wand controlled main foils-one wand on each side. The dual wand systems create all the righting moment for the boat with the windward foil pulling down and the leeward foil lifting up-and they do it automatically. Greg Ketterman used a similar system on his "Long Shot" record breaker and the Hobie Trifoiler except instead of wands moving flaps on the main foil he had "feelers" out front each moving one of the main foils.
A variation is using an "L" foil like the AC boats except with no inboard "up tip"
(no curve to the daggerboard portion of the "L" foil)and using a wand for altitude control. This system uses a single main foil on the lee side with the windward foil and wand retracted while sailing. Disadvantage for models is that the foils don't develop any righting moment. John Xman is building an 86" catamaran foiler using this system-see rc groups or find the link on page 35 of my MPX thread.
To summarize: an rc model using surface piercing main foils needs to be considerably wider than it is long in order to have enough righting moment and could also benefit from movable ballast.
This is one of the best rc surface piercing foilers I have seen-the ultimate in simplicity:
http://www.youtube.com/watch?v=CKs0EAxEnXA

 

Thanks for the explanation.

Sounds like I need to think about moveable ballast. One advantage of the mast aft wing is that there is plenty of room in the centre of the boat to install some sort of rotating moveable ballast system, perhaps doubling as an endplate?

 

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Don't use movable ballast unless you can make it move all the way to the extremes of the beam or it will require too much weight to be effective-thats why Hydropteres ballast is all the way outboard in the windward ama. Why not design the hulls(displacement wise) for the movable ballast weight so you can add it later, but just go with a very wide hull like the guy in the video. Can't get much more simple and effective for a "sport" foiler.
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Here is my D4Z rc cat with a Trapeze Movable Ballast System installed and I used similar systems on several monohulls. The ballast slides on a "rack" made from carbon tubes and the end of the rack is supported by trapeze wires:
click to get a bigger picture and to read the fine print--

 

Nice approach. I'm imagining something along the lines of the attached image. If the ballast can rotate on a circular platform and move in and out on rails, there will be a lot off flexibility wrt its position.

For this boat I'm not really after a "sport foiler", I'd like to come up with a design that could race around a course with upwind and downwind legs.

 

So the reason surface piercing foils have stability issues is that the lift varies with speed and also with ride height - which theoretically can be set up to be stable, but its a very fine tradeoff and hard to do. Even at full scale, with active controls and a person on board doing the controlling, L-Hydroptere took the better part of a decade and a couple of spectacular crashes to get it right. And their water ballast is more about sail carrying area so that they can set speed records.

And when you go with a dihedral like that (and you do need to think about it as dihedral and look at wing theory on how that works) you need to understand that the flatter the dihedral, the less stability. But of course the steeper the dihedral the less lift.

In model airplanes, trainers will have as much as 20 deg. of dihedral per side and a high wing configuration.

you are instead running a "low wing configuration" (CG well above CoL) and hence you have greater stability issues.

The advantage of 'feeler' based foilers is that they adjust the amount of lift actively based on ride height. But
a) they add complexity to the system
b) they generate more drag.

But adjustable foils are fast - as the Hydros class C Cat showed: with more speed than the "set it and forget it" Groupama boat that did go on to win.

If you want to build a racing boat, the ability to tradeoff upwind foils vs. downwind foils is something the C-Class folks were looking at but have so far not done. Their reason was "crew work load" - with a boat limited to two crew total, the amount of work involved for the crew is already high, add in the need to handle a second set of foils and it gets out of hand.

But on an RC boat, other than the weight issue, I can't see why that becomes too complex:

continue with the system you have but replace the flat foils with "J foils" like those of GroupAma for downwind (and you do NOT want to kick up your rudder in that situation since with two rudder foils you then get a three legged stool which is more stable than a bicycle)

But for upwind, retract them completely and deploy outwardly canted assymetrical straight boards

 

Ok, if I go with j-foils for downwind, should they be deployed simultaneously or individually. If they can work when deployed simultaneously I can get away with one servo, whereas if deployed individually I will need four.

 

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The problem with a circular approach to movable ballast is that in stronger winds, when you tack, the weight will move aft and could cause a capsize end over end-with the bow blowing over the stern. Happens on full size cats if the crew gets too far aft in the right conditions-I know: I did it -once!
You're much better off if you can place the weight exactly where it should be both side to side and fore and aft.
UPDATE-just noticed the track on your diagram-sorry I missed it. Good idea but to go from one side to the other with the ballast you have to slide and then rotate, correct?

 

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Groupama used "up tip" "L" foils with a curved daggerboard. This type of foil was pioneered by TNZ on their cup boat-they are not "J" foils-below is a picture of a "J" foil from Greg Ketterman and the illustration of the TNZ/groupama "up tip" foil configuration. I strongly recommend that you do not attempt to use them on your boat because they require intermittent manual(radio) control at a time when you won't be able to see well.
I recommend the Bradfield wand altitude control system with T-foils or "L" foils both with movable flaps. Or surface piercing foils for simplicity.

Pictures-Left="J" foil from Greg Ketterman, Right= Team NewZealand/Groupama foil configuration-note that altitude is controlled in two ways:
1) as the foil rises leeway reduces the effectiveness of the "up tip" inboard portion of the "V", 2) The curved daggerboard acts as a surface piercing foil and lift is reduced as the foil rises. This configuration requires intermittent manual angle of incidence control. It is more stable in heave than, for instance, the Oracle foil in which the "up tip" angle was significantly reduced, Far Right-Wand controlled "L" foil concept :

 

Steve Clark's Aethon C-class does what you are suggesting, not the inclined angle per se, but it has an adjustable cant angle so he can change the lift vector on the foils. The other input he has is how far the foils are deployed. Steve's goal is not full foiling, although he can achieve it periodically, it is mostly high lit fraction, which allows for reduced wetted surface and better overall heave stability.

Keep in mind C-class cats use assymetric foils, so when a foil is straight down, there is no leeway on the boat, thus as you cant it, you also don't need to add any additional angle of attack to the lifting foil, as it's already built into the foil.

In that configuration, my suggestion for rudders would simply be to have long t-rudders, and take the penalty on additional drag in lighter air, without changing their configuration at all. For all the extra weight an complexity, keeping it simple would be better. just go with long T's all the time.

As for the sweeping back and forth, why would you want to do that? Or is that just a by-product of your inclined axis of rotation?

 

Doug I have a question for you.

Why is it that with every post you make you feel so compelled to always state who you think the particular person or group was that developed a certain foil system?

E.g. "This type of foil was pioneered by TNZ"

Is it possible to simply state a "TNZ style system"?

just curious.