Foiling radio control trimarans

[PerthMini40man]

Thanks for the photos.
Suggested clip line shown on top foil. For strong wind days only?

Hello - yes your suggested modification to the foil is a good one and one day we'll make a set such as you suggest. We have already reached the conclusion that an rc multihull needs multiple sets of foils to suit different conditions, just as we have different rig sizes. I consider that we are at the equivalent stage of development as the Wright Brothers were with their biplane. We've got airborne - now what?! Trouble is, there aren't enough of us experimenting to speed up progress, and the fleet in the UK is happy to race round conventional courses, so they have no interest in foils. Mike Dann got his Ghost Train airborne in the mid 2000s and it was a photo of his tri foiling that inspired me to get into all this in the first place, but Mike focuses now on conventional sailing mode these days. Mike Cooke at Aardvark is building really professional Shinobis with foils, but his boats are designed to be "foil-assisted" and not get airborne. They have very slim floats and need the foils for additional lift. Makes them lightweight and very fast but not idiot-proof. But the good news is that one of our Mini40s is about to be shipped to the US so this will help.

 

[PerthMini40man]

Fair enough, I have some ideas but making them accurately is a challenge. Maybe some questions that you can answer:

1. What section are you using? If you can't reveal that, is it symmetric or asymmetric?
2. Is the "tang" of each foil is the same so you can swap them in and out easily?

Thanks!

The foils are asymmetrical. The shape was initially dictated by our ability (or lack of ability) to make them, and a few years back were quite thick in section, up to 9mm. These worked just fine, and gave us speeds in excess of 20kph. But - thanks to Dave's creativity - we are building thinner foils now. We use asymmetric because we want lift, with minimum drag.

Yes the top of each foil is the same so we can swap them easily. One screw adjuster on the deck fits all. Initially we made complicated cases with notches in the bottom to accept a small "peg" that went through the foil to help with pivot, but then we realised that this was unnecessary (and flipping hard to make) and now the back section of the foil just sits up against the under-side of the hull and the case itself is a simple rectangular case that is tapered fore-and-aft at the bottom. It doesn't matter that the foils are a bit loose in the cases because the pressure from the water is always pushing them to the back of the case so why worry about this? And if they twist a bit because of slop in the case, its only the vertical section that is affected, and this will produce a bit of lift to windward, no real harm. So the foil is just inserted from the underside of the hull, and a plastic pin goes through at deck level to stop them from falling out, though even this is not strictly necessary since the screw adjuster connection means your not going to lose it. Simple! The biggest pain is the very small screws that the screw adjusters are supplied with. Very fiddly to take on and off, and I always do this in an area with a concrete base - such as a road - so that when I drop them I can find them again. I have lost too many in grass.... To be fair, these screw adjusters are made as compression struts for main boom kicking straps (vangs) kits so they were not meant to be undone every time you go sailing. If I lose one here in the Philippines its an overseas order for a replacement ....

 

[UpOnStands]

We have already reached the conclusion that an rc multihull needs multiple sets of foils to suit different conditions, just as we have different rig sizes. I consider that we are at the equivalent stage of development as the Wright Brothers were with their biplane.

Just played around with some CAD.
If the foil can be rotated around an axis that lies on on the longitudinal vertical plane and is inclined 5 degrees rising towards the bow then by pivoting the foil to bring the free end outboard, the lift is reduced as the projected area of the foil is reduced, foil depth is increased, and foil angle of attack is reduced (axis of rotation might require some horizontal skew (5 degrees outboard toward the front) to more directly reduce AoA). Hopefully there could be some additional righting moment on the windward foil.

 

[TANSL]

Can any beneficial effect be generated by turning the foils around the Z-Z axis?

 

[OzFred]

Just played around with some CAD.
If the foil can be rotated around an axis that lies on on the longitudinal vertical plane and is inclined 5 degrees rising towards the bow then by pivoting the foil to bring the free end outboard, the lift is reduced as the projected area of the foil is reduced, foil depth is increased, and foil angle of attack is reduced (axis of rotation might require some horizontal skew (5 degrees outboard toward the front) to more directly reduce AoA). Hopefully there could be some additional righting moment on the windward foil.

For me the issue with rotating on an inclined axis is that it has two effects for one movement, making tuning complex. I think it's better to first work out the best settings independently (i.e. rotate about longitudinal, vertical and lateral axis independently to find the best settings for various conditions), then see if those settings are reasonably approximated by by rotation about one offset axis.

One issue I guess is the scale of the boats. The movements are relatively tiny and may be overwhelmed by other forces, so two (preferably more) identical boats are needed to gauge the effect of changes accurately (or some very accurate data loggers and lots of data).

Theory says that Z foils should be able to produce downforce on the windward hull due to leeway, but that likely requires symmetric foils and possibly tacking the foils with the boat (say between 0° when to windward and 3° "toe–in" when to leeward). How about wand–controlled Z foils that rotate on the Z axis to control lift?

 

[UpOnStands]

One issue I guess is the scale of the boats. The movements are relatively tiny and may be overwhelmed by other forces, so two (preferably more) identical boats are needed to gauge the effect of changes accurately (or some very accurate data loggers and lots of data).
Tank testing for sure

Theory says that Z foils should be able to produce downforce on the windward hull due to leeway, but that likely requires symmetric foils and possibly tacking the foils with the boat (say between 0° when to windward and 3° "toe–in" when to leeward). How about wand–controlled Z foils that rotate on the Z axis to control lift?

With pure Z axis rotation it seems that you would get significant "steering" from the vertical strut before you got any useful reduction in lift.

 

[TANSL]

Could that movement of the foils (around the Z-Z axis) make the ship rotate so that the wind resistance of the "flying" hulls decreases?

 

[PerthMini40man]

Just played around with some CAD.
If the foil can be rotated around an axis that lies on on the longitudinal vertical plane and is inclined 5 degrees rising towards the bow then by pivoting the foil to bring the free end outboard, the lift is reduced as the projected area of the foil is reduced, foil depth is increased, and foil angle of attack is reduced (axis of rotation might require some horizontal skew (5 degrees outboard toward the front) to more directly reduce AoA). Hopefully there could be some additional righting moment on the windward foil.

That's an interesting concept, I see where you are going with this. Conceivably we would adopt this concept by tapering the foil cases so that they are wider at the top than the bottom. We could then add a second screw adjuster to the top of the foil to adjust the foil in the manner you have described here. This second screw adjuster could be linked to a servo. We have found linear servos in Canada that might be suitable for the job. There's a cat being built in the UK with some of these Canadian servos on board.

Actuonics:
PQ12 15g 20mm travel 30:1 28mm/sec 15N @ 15 mm/sec, 8N @ 20 mm/sec
L12 34g 30mm travel 50:1 25 mm/sec 17N @ 14 mm/sec, 10N @ 19 mm/sec

 

[UpOnStands]

Could that movement of the foils (around the Z-Z axis) make the ship rotate so that the wind resistance of the "flying" hulls decreases?

possible, but can you imagine the result of dropping off the foils while doing in excess of 20 knots? Also, most rigs would probably have insufficient sail adjustment. Balestron rigs might work.

 

[UpOnStands]

That's an interesting concept, I see where you are going with this. Conceivably we would adopt this concept by tapering the foil cases so that they are wider at the top than the bottom. We could then add a second screw adjuster to the top of the foil to adjust the foil in the manner you have described here. This second screw adjuster could be linked to a servo. We have found linear servos in Canada that might be suitable for the job. There's a cat being built in the UK with some of these Canadian servos on board.
Actuonics:
PQ12 15g 20mm travel 30:1 28mm/sec 15N @ 15 mm/sec, 8N @ 20 mm/sec
L12 34g 30mm travel 50:1 25 mm/sec 17N @ 14 mm/sec, 10N @ 19 mm/sec

The snap ball and socket arrangement common on RC copters is not strong enough to maintain the correct foil angle? Hate loosing small nuts/bolts etc.
The linear actuators may work but the case looks very non-waterproof and there is no mounting system to bolt the case down? Hopefully the UK project will have some feedback on durability.
Not sure if the actuator rod can be rotated but 100:1 reduction ratio should give some serious force.

 

[TANSL]

possible, but can you imagine the result of dropping off .......

It was just a figment of mine.
I understand that it is not simple but, if the reduction in the resistance of the hulls is appreciable, surely some designer will study how to get it. At more than 20 knots the wind resistance of the hulls must be very large and, probably, creating other disturbing components of forces.

 

[Doug Lord]

I thought I'd post these in case it would help anyone considering a wand /flap foil arrangement. I used a partial span flap to keep the foil as thin as possible -about 7% t/c ratio. These were used on the production F3. One neat aspect was that the flap and hinge were molded together so that when the foil was pulled the flap worked perfectly. The hinge was made of thin mylar inserted during layup.
The foil plugs were hand shaped from thin model airplane plywood and then glued precisely to each side of a .25" X 3" X 15" piece of aluminum. The molds were laid up one at a time in such a way that the two would be able to be clamped together after layup. Each mold was bonded to the same size piece of aluminum as used for the plug to ensure the stability of the mold. I was concerned about the flap but it worked real well every time. I had plans to make a complex mold that would allow the daggerboard and foil to be laid up and bonded in one operation but didn't get to it.
The foil was 17 sq.in. and the flap 20% of that. The main foils and rudder foil were all out of the same mold but the rudder foil had much lower loading.

 

[UpOnStands]

Just for stimulation.
Here is an Aquila 9.5% 12.5mm chord.
If the foil can be pivoted on axis inclined 10 degrees outboard on the horizontal plane you can make the foil completely flat.
Foil rotation is 6 by 10 degree steps
The section planes are horizontal and show how much the profile changes

 

[OzFred]

With pure Z axis rotation it seems that you would get significant "steering" from the vertical strut before you got any useful reduction in lift.

Yes, but the "steering" is to windward, so not un–helpful. And the steering effect depends on the shape and attitude of the foil, I'm only talking of small rotations of perhaps 2° to 4°.

Your diagrams are interesting, is the purpose to "feather" the windward foil? Rather than swinging it 50° to get effectively 0° AoA, wouldn't it be simpler to just rotate it a few degrees on its own longitudinal (span–wise) axis to adjust the AoA directly? That's what I meant by "tacking the foils", so they go from say 0° (or maybe even negative) AoA when to windward, and 3° (or whatever) positive AoA when to leeward.

 

[UpOnStands]

All started with the problem of strong wind gusts overpowering the model boat so it climbs up on the Z-foils and becomes unstable. The first goal was to cut the lift in response to stronger winds. The idea of increasing the RM came later.
We don't want the inefficiency associated with flaps so there are three (?) ideas rotate the foils on their longitudinal-spanwise axis; swing the foils aft like a centerboard; swing the foils outboard so the effective lift area is cut.
Can't see any practical way of achieving the first. The second cuts lee resistance just when you need it it most (and alters the CoE of the swung foil ). The third actually increases lee resistance.
Trouble is, in the real world the third way really suits only trimarans, not catamarans