Foiler Design

The download on the stern foil is not necessarily as evil as it sounds. The stern foil acts in the downwash of the main foil, and a negative lift vector on the stern foil can actually point forward, reducing or eliminating its induced drag.

And for positive pitch stability, the canard must carry more load per unit area than the main foil. But the canard has a short span, so this creates excessive drag.

A three-surface arrangement is a good idea. It lets each surface specialize in doing one task well - lifting, pitch stabilization and trim, heave stabilization and trim.

The biggest drawback I can see is additional wetted area. If you linked the bow and stern rudders together, you might be able to reduce the combined area to be comparable to that of a conventional rudder.

 

You have provided a very good explanation of pitch stability using asymmetrical wing loading, etc.

I would very much like to hear your comments on roll stability. Our San Diego group is presently using three systems for roll stability all applied to the rear foil: Surface piercing with dihedral; submerged with dihedral; and submerged without dihedral.

With the surface piercing foil the wing rolling into the water has an increased submerged area while the wing on the high side of the roll has decreased area. The differential in area creates a corrective rolling moment. This system is plagued with ventilation problems

Both the surface piercing and submerged dihedral wings develop asymmetrical lift when yawing . It is a mater of geometry that the advancing wing experiences an increase in angle of attack, AOA, while the receding wing experiences a decrease in AOA. The wing tips on this type are close to the surface and that can lead to problems.

The submerged wing without dihedral is a little more mysterious. It seems that if the span is great enough the vertical wing tip speed in a roll is high enough to dampen the roll, that is, decrease the rate of roll. This allows the pilot time to "turn under the fall" as a bicycle rider would instinctively do. This is an OK system, but the wide wings can be awkward. A nimble stearing system and alert pilot are required

My question is, besides the stabilizing effect of your sails and the hiking of the crew are you folks using other devices to control roll, eg. alierons, variable angle of incidence, spoilers, etc?

Rvell

 

RVELL,
The short answer is NO!
The dynamic stability of the foils provides much better stability than when simply displacement sailing in what is effectively a 350mm wide 3.3m canoe.
Moth sailors are very used to managing later stability problems and there is no doubt that having a sail to balance against is a very big plus.

Also, we do not do much hard turning, except during tacks, when we come to the centre of the boat and contol stability by moving body weight while crouching to get under the boom. So many other balance issues here that the rolling

Ironically it is the fact that your skipper is an inactive passenger, which makes it more difficult for you to balance. Perhaps they should be sitting up on a bicycle seat so they can feel part of the action and provide necessary active balance!

I have tried various dihedral angles and about 6 degrees seems good for turning, but this is probably not enough for roll stability. I have no other ideas at present.

 

I do not see why the canard must carry more load per unit area than the main foil. In normal running mode it has little or no load. As it pitches bown, the stern foil will take some of the load and the canard needs just enough lift to maintain attidute. As I move my weight aft, it unloads the canard again. I have not found exessive drag on the submerged canard, only when it is running on the surface.

While I agree there is a little extra wetted area with my canard configuration, I think you need to look at the entire configuration efficiency. My main foil is large and efficient as it takes all of the lifting load and has no flaps, so it is always at lowest drag, the rudder aft is only a big as necessary and the stabilizing fin is quite small. This fin is almost always lifting or neutral, as is the canard, so there is no drag from flaps and no foils acting with negative lift. The only extra drag is the very small forward struct supporting the canard.
I have tried to reduce the canard and strut drag by running it on the surface, but this is where I am having trouble getting enough lift from it. ...any ideas to solve this?

 

I do not see why the canard must carry more load per unit area than the main foil. In normal running mode it has little or no load. As it pitches bown, the stern foil will take some of the load and the canard needs just enough lift to maintain attidute. As I move my weight aft, it unloads the canard again. I have not found exessive drag on the submerged canard, only when it is running on the surface.

While I agree there is a little extra wetted area with my canard configuration, I think you need to look at the entire configuration efficiency. My main foil is large and efficient as it takes all of the lifting load and has no flaps, so it is always at lowest drag, the rudder aft is only a big as necessary and the stabilizing fin is quite small. This fin is almost always lifting or neutral, as is the canard, so there is no drag from flaps and no foils acting with negative lift. The only extra drag is the very small forward strut supporting the canard.
To make this a forward rudder has some appeal, but from watching AStevo trials with his bow rudder on the weekend, I an now convinced that is a dead duck..it is almost impossible to tack the boat!

I have tried to reduce the canard and strut drag by running it on the surface, but this is where I am having trouble getting enough lift from it. ...any ideas to solve this? Any thoughts on planing surface efficiency, foil shapes etc for this purpose?

 

just a thought

with the canard at the bow, how will this react in choppy seas? and i am not talking about sydney harbour either, more like port phillip bay in 15-20 knots. would you guys think it would still be usable?

i am really interested to see one of these in action tho.

 

rohan at this point i think it would crash and burn but thats not saying we shouldnt continue.

the original idea was that the front foil would track at the surface up and down the waves. given we could be going much faster than waves especially downhill it is not reasonable to expect the bow foil to go go up and down with such high frequency. the foils we have used do not track the surface adequatly, so either they need to be big or submerged in order to get enough lift.

my idea is that the submerged foil will not see the waves. this is what ive just built anyway, im here to waiting for the epoxy to go off.this conceptdepends on a delay in the sensor acting on the angle of attack (like the flexible wand in your system).but my wand is just a peice of plate alloy at this point.

 

I was talking about the static stability of a canard and main wing, with no aft surface and with the weight fixed in position. Say the two surfaces have the same aspect ratio. If the canard is trimmed at a higher lift coefficient than the main foil, then a change of, say, one degree in angle of attack will result in a smaller % change in the lift on the canard than it will on the main foil. This will result in a negative pitching moment, which is stabilizing.

The difference in drag between an efficient canard and an efficient tail is not great. But the best tail configuration can be designed to have slightly lower drag than best canard. Probably the most important point of this is the fact that the canard is lifting and the tail may not be, isn't as important as it might seem.

Absolutely. I rather like the three-surface approach as a way to optimize the whole configuration without having to compromise any of the foils too much.

Deflecting flaps doesn't necessarily add drag, and neither does the negative lift. Ideally the flap gaps should be sealed and the flaps extend the full span of the foil. With full span flaps, the induced drag will be the same whether the lift is produced by the flaps or by changing the angle of attack of the foil. And as long as there's no separation, the profile drag doesn't change for modest flap deflections, either. Deflecting the flap may actually reduce the drag if the desired lift coefficient is outside the minimum drag range of the undeflected section.

Whether the negative lift on the aft foil contributes drag or not depends on the net spanwise lift distribution from all the foils. The lift from all the foils should result in a uniform downwash left behind in the wake. If that is produced by more lift on the middle of the main foil and some negative lift on the aft foil, that's OK.

What is the canard doing for you that the same area on the aft foil wouldn't? Do you have any way of sensing the water surface other than the canard? Or is you canard supposed to act like the planing surfaces on the Trifoiler, which rotate the outer hulls to control the incidence of the foils.

 

There are two different contributions to roll stability here. The dihedral responds to the leeway angle, producing a rolling moment proportional to leeway, for the reasons you point out. This is a contribution to static stability - it makes the boat want to move in the right direction.

The roll damping is in addition to the dihedral effect. This contributes to dynamic stability. It is proportional to the roll rate and proportional to the span-squared. So it's much more of a factor as you extend the span.

Moth sailors are used to sailing boats that are essentially unstable in roll, in that there's virtually no restoring moment from the hull as the boat heels. But the sail provides roll damping, and by a combination of actively sheeting in and out and moving one's weight, the boat is artificially stabilized. The roll damping of the foils slows down the boat's response in roll, making it easier to actively compensate for it.

 

Pretty simple really, to keep the bow up, the canard always provides positive lift, which is assists the main foil by unloading it. If we were to use negative deflection of the aft foil instead to achieve the same result, this produces suck, which places more load on the main foil. It is the same argument in efficiency between conventional and canard aircraft.

The surface running canard is in fact just a fixed angled foil canard. There are no other surface sensing devices, it is the surface sensing device itself and the boat acts as the connection to the main foil to alter the angle of incidence, exactly as Miller does with his sailboard.

 

Two,just two foils

I've been reading with interest the various comments and discussion regarding the Miller foil system vs the system used by John Ilett.I'm convinced that you can't beat a monofoiler with just two foils and an altitude sensor.I think such a boat lends itself particularly well -with a great deal of refinement- to producing not only the fastest foiler but the most allround light air capable foier. A daggerboard mounted foil combined with a hull designed specifically for it seems to me like it would allow the main foil to be completely retractable for lite air. This idea has been buzzing around for awhile and I think it has a lot of merit.
There are still major questions to be resolved: hull beam vs foil span;room under the boom for a retractd "main" daggerboard(also :how high to fly); the feasibility of putting one daggerboard within another to allow the board with the foil on it to retract etc. This presumes that(for now anyway) the rudder t-foil would remain immersed in light air.

 

Hello Doug,
Two foils is of course ideal as all boats need a rudder and centreboard anyway. John Ilett has done a fantastic job getting the two foil arrangement working properly and I am sure we are now only looking at incremental improvements.

Nevertheless, since I first developed and sailed my two foil arrangement, l have not been fully satisfied with its pitch stability, efficiency and surface tracking downwind. These concerns are also borne out by comments by Rohan Veal. The Unifoiler arrangement with surface tracking canard developed by Miller offers solutions to both these problems and that is why I have been persevering to test this solution.

In latest trials with the canard arrangement I have achieved far better surface tracking and speed than I could ever manage with the bifoiler arrangement, so I am keen to continue. From my practical trials, it would seem that a submerged canard with wand sensor and small stabilizing foil on the rudder is the best configuration I have developed to date, perhaps you could call this the "Ward" foil arrangement

There is no intent to criticise the Bifoiler arrangement or imply it is defective. Quite the contrary, I would recommend anyone attampting to foil for the first time to buy a set of Ilett foils and get going straight away. I have even paid for a set from John for a Japanese Moth sailor friend.

As I see it, this is a unique discussion forum for sharing ideas on how to further improve on dinghy foiling, a completely new form of sailing with a very big future. I think we have come a long way in a short time and I am very impressed at the willingness of people all over the world to openly contribute their valuable ideas and information. It is rather like being there with the Wright brothers, but each test result and new idea is shared live and globally. I really hope this discussion can continue and commercial interests remain set aside.

It would be very good to have more contributors working on the bifoiler and any other arrangements as well!.

Your idea of retracting the lifting foil in light conditions is a good one, although I am not sure how drag is reduced if you only lift it high under the boat. Surely it would have to retract into a moulded cavity in the hull, which adds cost, complexity and extra drag when the hull and foil are in the water.

Why not make it possible to tilt the lifting foil to near vertical to provide lateral resistance when displacement sailing?

You have not really said why you prefer the two foil arrangement and how to overcome the problems mentioned by Rohan of sailing in big waves..any thoughts there?

 

Two Foils Just Two

Ian, I think the two foil system is appealing to me because it is simple and works very well.
I think problems in waves are going to be solved with more experience actually foiling -perhaps a better "wand" or even some kind of really simple electronic assistance-don't have the answers but am sure thining about it.
My retractng system would have the foil retract into and flush with the bottom of the hull; when the foil is deployed the gap could be filled with some kind of flexible skin or just a lowering section of hull. I'm leaning toward just making it work then refining it. A true off the beach retractable foiler has a big future commercially which means it will introduce potentially large groups of people to the excitement of foiling.
I've been too involved in business to spend any more time with my 16 footer but that is about to change-hardly any time since the first sailing mid year 2003-and no foiling yet. But she'll fly -its just a question of when.
I think the work all you guys are doing is trully awe inspiring and I definitely feel apart of witnessing and discussing history in the making. So Wardi by all means stick with it!! You may be on to something-time will tell..
I'm going to spend more time working on a boat that can be sailed off the beach in any wind with a retractable foil system and the capability, when the conditions are right, to intentionally JUMP and re-enter safely-a whole new facet of foiling just for the joy of it!

 

Percent lift change with various wing loading

Hello, Tom:

You wrote: If the canard is trimmed at a higher lift coefficient than the main foil, then a change of, say, one degree in angle of attack will result in a smaller % change in the lift on the canard than it will on the main foil. This will result in a negative pitching moment, which is stabilizing.

Attached is a graph that demonstrates the principal you have written about. Notice that at the lowest wing loading (lowest lift coefficient, low angle of attack, big wing for a small load) one degree change in angle of attack results in 200% change in lift. At the highest wing load, as stall is approached, one degree change makes virtually no change in lift. After stall, an increase in angle of attack results in decreased lift . As you say, these dramatic differences can be exploited in the design process.

Ray Vellinga

 

For me, contributing to this thread is on-the-job-training. The "Percent change in Lift..." chart in my last message appeared to be small and unreadable. I was about to replace it with a larger chart when I accidentally discovered that clicking on the thumbnail chart enlarged it to a readable size. I am probably the last to know this, but if there is anyone else out there having trouble reading the chart try this little trick.

rvell