Foiling radio control trimarans

I have a foil/hull test rig on the front of a sailing catamaran hull (hobie 17) powered by an outboard. It uses strain gauges to measure lift and drag for any form, recorded on a DAS, and can apply the driving force at any height via a sliding post, in the same manner as a towing tank gantry. It works very well, with accurate, repeatable results. Because the test sample is ahead of the bow wave from the cat hulls, there isn't any influence from these to any great extent, so by testing in different conditions it is able to give a reasonable representation of resistance in waves or calm water.

I've also towed models with spring balances in the past, but this is a much better arrangement because the height of the thrust does need to be accurately represented to get meaningful results. You can provide corrective moments, as David discusses above, but these need to be changed for every forward speed so this is a right pain to do accurately.

A rig like in the photo allows for relatively rapid testing, as you are not constrained by the length of your tank, so you can do long continuous runs at a range of speeds instead of many multiple runs. Also, the speeds achievable are well in excess of any towing tank, and hence allows for larger scales or higher scale speeds. It also allows long runs at high speeds in quartering seas or beam seas, something that no towing tank can achieve. We are fortunate to live next to the Rance estuary in Brittany, hence have the perfect range of conditions to test in from flat calm to scale waves on a regular basis.

 

Prior to AC 34,Team New Zealand modified one of their SL 33 platforms with a stub mast and had a tow line attached at the height of the CE. When they pulled the boat they had someone onboard to steer the test boat to one side so the effects of leeway could be better understood. There is an excellent video that I've spent some time trying to find but no luck-it's definitely on this forum someplace. If I was to test an rc model I would rig it the same way as the SL 33 and use the radio to steer it to one side to simulate leeway. Important, especially with foils that function with leeway coupling like uptip foils.
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Found the video: see from 22:50 to 23:55 in:

 

I was thinking of towing distances of the order of 50m. Use something like an RC electric car drive system to haul the line in. You should be able to achieve a wide range of constant speeds reliably.
Most effective at the low speeds needed to refine foil lift off. Strong wind is strong enough but very light wind of different speeds is difficult to get reliably.

Sir Hump, too professional for sure

If the stub mast had a spreader, attaching the tow line to the spreader (various distances from the centerline) and then steering the model to compensate should replicate leeway fairly accurately.

 

Had a productive day yesterday, using 3 sets of foils and the centreboard, using B and C rigs, and learnt quite a lot. The forward raked foils with winglets on the bottom showed some promise (sorry, no photos of these on the boat). I'll upload a video later in the week

 

The surface planing created by the windward U-foil looks to be significant and must make the boat very difficult to control. Did you increase the taper on those conventional kinked foils?
one idea. The windward stay is carrying very heavy loads. No possibility of using the tension to drive the windward foil to slight negative AoA?
The lee stay is slack so the lee foil takes its default setting of positive AoA.

 

If I remember correctly,the Monitor foiler in the early 50's used shroud tension to do exactly that.

 

Doug, your 2014 post stated that the mechanical loads on the stays were fed to a computer that controlled the rear foil's AoA.
Hopefully a simpler (but fiddly) mechanical linkage could work to provide more righting force and greater stability
Monitor- 1950's Monofoiler https://www.boatdesign.net/threads/monitor-1950s-monofoiler.50982/

 

That guys comments were not complete. I've since found that both the rudder foil and main foils were adjusted by rig tension on Monitor:
Wind & Water Monitor 1950's hydrofoil http://www.wind-water.nl/contents/monitor_1.php
There is another article that used to be on foils.org(I think) that went into even more detail on the shroud tension controlled main foils but I can't find it.
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I think if you wanted an auto-stable system you'd be hard pressed to beat wand(or "feeler") foil control which is well proven and can provide nearly 100% of the boats RM.

 

ummm, the photos and drawings shown on the site clearly indicate the AoA control of the rear foil but nothing for the main foils. Would love to know how main foil AoA was altered directly? Or only through the rear foil?

What is the largest extent yacht that uses the wand system for main foil AoA control?

 

You should read the Hanno Smitt site I linked to:
"From the patent (US patent office 2,856,879 Oct 21 1958) I know that she had full automatic mechanical stability control, for pitch and roll."
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AOI Main foil control w/shroud tension(pix from Hanno's site) :


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As far as I know the largest wand controlled foil system was on SKAT -a 40ftr designed by Dr. Sam:

 

Thanks for the heads up. The photos are right at the bottom of the page.

 

ummm, this would require one RC channel to lock out the stay feedback until the boat is foiling. From rest, the windward foil would be driven to negative AoA which may prevent foiling.

 

Another advantage of a wand system: the windward wand starts out at max lift on the flap and as heel begins to happen the wand reduces the flap-down input until the zero lift point is reached and then moves the flap up to create downforce-only when the boat needs it. The lee wand then automatically adds lift(down flap) to compensate for the downforce from the windward foil. And as Greg Ketterman says:
" This is the increase in righting moment or stability due to the ability of the windward foil to pull down. The DLA has little affect on the low wind performance, but it essentially makes the top speed of the boat limited to the strength of the boat. Conventional boats with a finite amount of righting moment can only extract so much power from the wind, but with the DLA, the righting moment is virtually unlimited. Which makes this system ideal for rc cats and trimarans-no radio channels needed-dl
Intuitively many people think that the added drag of the windward foil plus the increased induced drag of the leeward foil would offset the gain in righting moment, but calculations show and practice proves otherwise."
DLA=dynamic leveling affect

 

We have another idea - let the boat twist. If the cross beams are not locked solid to the floats, the pull from the windward shroud towards the back of the windward float will angle the windward float to point down at the bows to give less (or negative) angle of attack. So I am going to try sailing the boat next weekend with the front bolts on the cross beam/float connection intentionally loose (just a couple of mm) and see what happens. It won't have any effect on the lee float and foil, and the boat won't rack since the cross beams will still be rigidly attached to the main hull.

 

umm. just make sure the fastenings don't fall out.
Quick and simple test can be done when the boat has mast up and floating. With the "lee" float against a board, pull the mast top to lee and watch for windward float twist.
I'll keep my fingers crossed for good results.
You may get better response by splitting the shrouds, keeping the aft chainplate as is and taking the lower end of fwd shroud (yellow line) to the aft of the foil head, i.e. behind the foil pivot.