Foiling radio control trimarans

[Doug Lord]

One of the great advantages of wands on the mainfoils on an RC tri or cat is that the dual, independent wands allow the foils to provide most of the RM for the boat.
Dr. Sam Bradfield, who pioneered wand systems in the US felt that an oversquare platform was the best solution because the separation of the main foils reduced the load (due to RM) on the foils.

 

[UpOnStands]

That tri (Electric Lobster) uses a trim tab on the rudder foil. Static setting only?

 

[Doug Lord]

The F3 was set up so that the rudder foil flap could be adjusted on shore. On the first two boats I built servo adjustment was used but turned out to be unnecessary. Once properly set up no adjustment was required.
The mainfoils were set up with +2.5 degrees angle of incidence and the rudder foil at zero degrees relative to the static waterline(which was parallel to the flight waterline). The boat would take off in a 5mph(or less) wind and was clocked at 12mph in that breeze. She was sailed reefed in a 22mph wind and was clocked at 18 mph by me. One of our customers claimed a top speed(reefed) at 20mph.

 

[UpOnStands]

Thanks for the photo.
How much and how rapidly did the boat react to flap movement?
this type of information is gold.

 

[Doug Lord]

On the main foils its instantaneous....... Wands are really an asset on an RC multihull particularly an oversquare tri because of the increase in RM going upwind.
The partial span foils on the F3 were really unique because they allowed the foils to be
thin and yet work real well. Not much movement depending on waves.
Fire Arrow uses a single wand controlled main foil with a wider partial span flap in case it needed the extra power.

Compare the flap length here with the F3 rudder foil above-all three F3 foils were made from the same split mold:

 

[UpOnStands]

Any major problem in getting the force transferred from the wand to to the flap? Slop in linkage, stiction?

 

[Doug Lord]

No. Luckily for me I was taught about wands by Dr. Sam and that was a great experience. You've got to make sure the wand moves freely-the F3 used ball bearings the Fire Arrow uses a carbon rod in a carbon tube. Can't be any binding. You've got to make sure when the wand moves the flap up(for downforce) that the bungee or spring wants to hold the flap up, etc, etc.. The bungee/spring holds the wand against the water. Details are important but it's basically a simple system. The Fire Arrow and F3 both use midship wands :

Fire Arrow midship dual wand mount:

 

[OzFred]

pitch of the hull will determine height, pitch down to lower, pitch up to raise. The control wand alternates between pitch up and down and gives you height.
Yes, its possible to directly raise and lower the main foils but that requires far more energy than controlling pitch.
Controlling pitch of the fwd foils directly would seem to yield a twitchy control system.
It might be a possibility, that is all.
At the moment PerthMini40 is getting interesting results with fixed foils, fwd and aft. Controlling just the aft foil - he already has the pitch system built in - would seem to be easiest.

The trouble with using pitch is it's indirect, you're essentially always over compensating. If the boat pitches down for whatever reason, the rudder has to get down faster than the rest of the boat to increase pitch to stop the drop. Same for pitching up: the stern has to race the rest of the boat to adjust pitch to overcome the lift. And if you run out of vertical freedom (which is very small) in either case, everything comes to an ungraceful stop very quickly.

You also need to consider what happens in less than ideal conditions (which never occur if there's a breeze) and the boat changes attitude and altitude simply due to variances in wind or surface.

In regard to load, it's not really an issue. Moths use vertical pushrods as small as 2mm over a 1200mm length on the main foil and that works extremely well (particularly as the greatest load is in compression), the Stunt S9 uses cables that seem up to the job (but I bet it would be better with solid linkages). If full sized boats can handle the load easily, I can't imagine it being an issue on a model.

I suggest you build a boat and try it. There have been thousands of wand–controlled foilers built using a variety of height control mechanisms. A forward mounted wand driving a flap on the main foil is by far the most common because it has been found to be the best layout (where "best" is judged in terms of efficiency, effectiveness and adjustability with a mechanical control mechanism). Mid–mounted wands are also used, and while they aren't as good at controlling altitude, they can be simpler so have benefits in that regard.

 

[UpOnStands]

Wand adjustment of the main foil directly affects height since the main foil carries all the weight, so a much more direct response.

sorry, I may have misconstrued your statement "directly affect height" ; assumed you talking about changing the foil-hull separation distance. You mean the standard approach of pitch change of the foil (main fixed foil + flap in Doug's configuration).

 

[Doug Lord]

Mid–mounted wands are also used, and while they aren't as good at controlling altitude, they can be simpler so have benefits in that regard.

===========================
Thats probably not true. Dr Sam believed in the midship wand for a trimaran with dual independent wands because there was no chance of pitch coupling. Midship wands were used on his Rave, Osprey and SKAT and are used today on the Whisper foiler cat. Midship wands on RC boats are extremely simple and work exceptionally well.

 

[hump101]

If the main foil is carrying most of the load, as it should be, and the rudder stabiliser is just stabilising, as it should be, then any pitching is always only due to loss of lift, or too much lift, on the main foil, and hence the location of the wand well forward does not cause any coupling issues. The forward wand provides some phasing advance, and hence can compensate for reaction time due to inertia and foil response, hence why it is a better position for a sensitive boat like a moth.

 

[UpOnStands]

Given the variability of wind speed it would seem that lift is forever changing as well as the longitudinal loads (fore/aft) on the sail, so pitching is inevitable?
By "phasing advance" you are referring to wave loading or to positive pitch coupling?

 

[hump101]

By "phasing advance" I mean that the response of the wand to a change in water height is in advance of the foil, which gives the boat time, depending on speed, to commence the required change in ride height prior to the actual change arriving at the foil, which makes for a smoother ride with less amplitude change in regular waves. This doesn't matter too much if the boat is long and heavy, or if the main foil is not carrying most of the weight (>95%), but makes a big difference on something like a moth, which is short and light.

This highlights one of the issues with wands, they respond to all changes in height, even though in many instances it is not desirable to do so, hence the wand and the flap are flapping about, creating drag, even when the ride height isn't changing.

 

[hump101]

Changes in pitch are inevitable, but are automatically compensated for by the rudder stabiliser levelling the boat. Hence the wand only needs to deal with heave variations. This is another reason why you always want the main foil to be taking as close to 100% of the load as possible, so that the stabiliser loads are small and hence the pitch compensation forces are minimised. This is the lowest drag arrangement.

Bradbury foilers had around 20% of the weight on the stabiliser, which is why they had pitch coupling issues with a forward wand and also why they needed to control stabiliser lift dynamically as the requirement changes with speed, whereas the more modern arrangement can fix stabiliser position and reduce its size, reducing drag.

 

[UpOnStands]

Thanks for your replies hump101, seems as if phasing advance is only for wave height response since heave due to speed changes occur "immediately" at the main foil.
reminds me of the Hobicat Trifoiler with its mini pod way in advance of the ama.