Foiler Design

RVELL,
I am interested to know how actual loadings change on both main foil and canard when a forward pitching moment is applied.

I am assuming that as the forward pitching moment is applied, the canard initially takes a small load, but as it is depressed, the main foil has less incidence and takes less load, this in turn places extra load on the canard and the process continues until an equilibrium is achieved. What loading would be on the canard at this point?

 

What happens when a hydrofoil boat pitches forward?

Wardy, You have asked a good, thoughtful question. The answer is rather lengthy.

Perhaps we should start with some definitions. “Load or loading” I would define for our purposes as the amount of weight a foil supports. This is expressed in pounds, kilos, etc. The load of a given foil can also be expressed as percent of the total weight of the hydrofoil boat. This is designed into the boat. For example, if you design the Center of Gravity to be behind the front foil 90% of the distance between the foils, the front foil will carry 10% of the load and the rear 90%. Of course, center of gravity can shift, as every Moth sailor knows, but for the purposes of explaining how the boat corrects for pitch, let’s assume the sailor is snoozing at the helm and the small changes due to geometry are to be overlooked.

“Wing loading” is the lift (or weight) per unit of area. This is expressed in pounds per foot^2, pounds per inch^2, kilos per meter^2, etc. It is the ratio of lift over area

Lift is the amount of upward force a foil creates when traveling through the water. A state of equilibrium is reached when the hydrofoil is neither climbing nor diving. Then lift will equal load.

To answer your question: let’s assume an example, to make the math simple, starting with 100 units. We will assume:

Boat weight, total = 100 pounds
Front foil load (lift) = 10% or 10 pounds
Rear foil load (lift) = 90% or 90 pounds

The point Tom Speer has made (correct me, Tom, if I misinterpret you) is that the areas of the front and rear foils should not be 10% and 90% respectively. The wing loading of the front foil should be higher than that of the rear foil. So the front foil might be, say, 5% and the rear foil 95% of the total area. This would give the front foil a wing loading of roughly double that of the rear.

Refer to my graph posted 2/17/04 “Percent Change in Lift vs. Wing Loading”. Assume you design the rear foil to be loaded at .25 lbs / in^2. The front foil would then carry a wing load of .50 Lbs in^2 – or in this example, twice the wing load.

So if you experience a disturbance from equilibrium, say a pitch forward, what will be the dynamics?

First, the load shift is more or less temporary. Something caused the pitch forward, but the boat’s Center of Gravity is designed into the boat and remains relatively unchanged. The CG is behind the front foil 90% of the distance between the foils, so the load on the foils remains 10% / 90%.

What happens to the lift? Assume a pitch forward of 1 degree – the same amount I used in my graph. If you had equal wing loading the lift would be reduced by the same percentage for both the front and rear foils. The result would be a rapid dive followed by a crash.

If the foils had unequal wing loading as in our example, the graph shows the front foil would lose 50 % of its lift and the rear foil would lose 100 % of its lift.

It is at this point that I disagree with previous assertions that this is a stable state. I believe you are still diving because both foils have lost lift, but your rate of dive is not increasing, and you are not diving as badly as if you had equal wing loading. Probably in a canard configured airplane the unequal change in lift would lead to an eventual pitch up. The airspeed will increase and lift will change non-proportionally and this may result in a pitch correction. However, in a hydrofoil we do not have the luxury of altitude excursions.

We need something that generates a strong pitch up, and we need it fast. One or two or three inches of altitude (heave) are all we have in which to recover.

This is why we need a front foil that rapidly increases its lift as it increases its submergence. Examples of this type are front foils with a surface following wand that changes the angle of incidence or actuates a flap. Another example is the ladder foil or the “V” foil. They both increase their area as they increase their submergence.

Your idea of a surface following planing foil has merit as well, however the whole discussion of relative wing loading then becomes irrelevant. When you put a large foil up front to dance on the water you will be depending on only the lift generated by the underside of the foil. About 2/3 of the lift of a submerged hydrofoil is produced by the relative vacuum created on the topside of the foil. Therefore the lift of a planing foil will be reduced to about 1/3 of the lift of a same-sized submerged hydrofoil. In other words, a “planning” foil needs to be about 3 times as large as a submerged foil.

I advocate using a supercavitating type foil for this purpose. It is designed to generate lift from the bottom surface and not from the top, so you do not experience the surge in lift experienced when the “normal” foil submerges deep enough to collapse the topside air bubble created while planning.

This is a long answer to a short question, but I do not know how to make a brief answer that is comprehensible on its own. The brief answer is, following a disturbance that causes a forward pitch the load does not change. In our example, it remains roughly 10% / 90%. The lift changes, and not for the better.

Ray Vellinga

 

mr

im sorry to reply to an old message (as you guys have probably moved on) but i tried to move the whole lifting wing on a rudder. the 2 struts were based on the front wing of the jordan f1 racing car (two struts angled at 45 degrees that changed angle at the top surface of the foil to enter into the pivot point of the foil at 90 dregees, anyhoo i made it (what i thought) pretty strong, shaped the struts in to little foil shapes themselves (sad) and was fairly proud, it looked as it was a goer! after a little pressure testing they failed mainly due to the fact it was a laminated form and the laminations are the problem. if it was metal maybe it works.

just a note: im a big fan of moving a foil instead of a flap but i agree with rohan its too big a variation at speed and its too sensitive. all you could do is make the foil smaller to compensate but then in light conditions more variation would be required increasing drag.. blah blah blah.

i now reakon you should get flapped!!

 

i agree with your points, here but it seems somewhat counterintuative.


i recently read an article titled "flight annalysis" longitudinal stability by martin simons. this guy is a guru of aerodynamics of model aeroplanes.

in this he talks abut the neutal point and the relationship with the centre of area. the neutral point as he calls it is effectivley the centre of area of the combined main wing and stabiliser.
the condition he states for static stability is that the centre of gravity is in front of the neutral point. this reqires that a rearward stabiliser needs to be pulling down at much of the time. though this applies to a conventional aeroplane i felt sure that the same effect would apply to a canard.

obviously im no scholar in fluid dynamics. but i would have though that the bow foil would behave better with lighter loadings. remember that the bow foil is planning and the lift which comes at the surfae is much less efficient than a submerged foil. if the loading is too large it will just send itself downwards if the pitching moment submerges the leading edge of the canard. the lift which could come while submerged it will take up to 5 secconds to shed the air it drags down with it.

 

glen ive conceded that point. dad and i have just put hinges in the back of the illet sections we bought. it aso makes it a hell easier to keep it attached as you would need a big hinge. with the section that john uses it seems to be not such a big variation in the section shape by flapping. it only the gap in the bottom that has me thinking of alternatives.

 

Hello Ray,
It is very good of you to take the time to respond in this forum, I think this is a really crucial discussion as longitudinal stability is the one area where sailing dinghy foilers have a unique difficulty to overcome compared with aircraft, powered foilers etc. due to the very large changes in pitching moment due to the rig.

I agree with your observations and conclusions and think you are on the right track here..... but have not taken the argument to its full conclusion yet.

By this I mean the following:
1) Let us assume my own situation where I am foiling with the centre of gravity effectively over the main lifting foil on the centreboard and therefore 100% of the load is supported by this foil. As I am using a small stabilizing foil on the rudder, which helps maintain a constant attitude with small pitch variations and I have found that the boat will foil in a stable condition like this, with the front canard clear of the water altogether for around 10-15 seconds at a time.

2) The real problem comes when the boat pitches bow down due to a gust, sheeting in, bearing away etc. In effect the centre of loading moves forward. Not only does the canard now hit the water surface, but also the main foil now has a negative attitude reducing its loading and the boat begins to dive.

In order to correct this situation, the canard firstly has to balance:
a) the pitching moment and then, in addition it has to
b) share that load being shed by the main foil, in order to support the weight of the boat. It then has to
c) apply a correcting moment to bring the boat back into stable attidude and height.
It is these three moments to be overcome which result in such very high loadings on the canard, which for me was unexpected. Not only that, this has to be done, as you correctly point out, within just a few inches of height loss, before the boat hits the water surface!

The canard therefore needs to be very large if surface running, or actively sensor controlled if submerged.

The reason Miller can make the canard work on his sailboard is that the pitching moments are very small and can be quickly and easily corrected by moving his weight, rig position, trim etc.

3) If I take the converse situation, with the boat fully in the water, supported entirely by hull boyancy but moving forward, then a very small angle of attack on the canard will provide sufficient upward pitching moment to change the attitude of the boat. From there it can "fly" stably upward and launch into foiling mode, with little or no further support from the canard.
Unlike the former case of pitching down, the canard does not have to share the load of the main foil and hence its loading is quite small. This is akin to an airplane taking off by "rotating" ie, pitching the back downwards to change the attitude of the main wing at takeoff.

4) I have found it absolutely necessary to have a small stabilizing foil on the rudder to counter any aft pitching moment which occurs when you ease the sheet, round up or get hit with a severe gust which drags the rig aft.
This foil can also act to reduce forward pitching moment as it is lifted up, by exerting a negative downward force. This in turn reduces the load on the canard and increases the load on the main foil.

It begs the question, why not simply have a sensor operating the rudder stabilizer and remove the canard altogether?

One reason of course is that in fat sterned boats it is not possible to sink the stern so easily, and so take off is difficult. Also, negative lift is not as good as having positive lift at all times as with a canard.

Perhaps the negative loading from the rudder foil will place a commensurately higher loading on the main foil, resulting in a better overall efficiency than relying soley on the canard to correct the pitch?


5) It occurs to me that a combination of both a forward canard and rear foil can provide a good balance against forward pitching and provide a more efficient solution by reducing the canard loads, allowing a smaller canard to be used. At the same time, this maximises loading on the single, high efficiency main foil.

Perhaps there is an optimum relationship between the sizes, loads and loadings of all these foils. It would be very useful to calculate the full scenario to determine the best arrangement of foil areas to give the most efficient result. I hope you can help me with this.

I am currently trialling two separate configurations. One is a large surface running canard, some 3 times the area of my original canard, and also a trailing wand sensor controlling a submerged foil......
Results this weekend I hope!

regds, Ian

 

About Bow Steering

ABOUT BOW STEERING

{But first many congratulations to Wardi, P and A Stevo for going out and getting wet. And much respect to Pstevo for his personal commitment to Moth class democracy.}

Posting #67 outlining my design (plywood, 2ft wide moth, Miller foiler, bow steering, twin leg main foil, rakish aft mounted rig). There are a couple of aspects that I have had to review and change: The position of the rig and the design of the bow rudder. This post is about the rig position.
This bears on the Stevos' bad experience with bow steering. Just to separate this from a discussion of canard foiling THE FOLLOWING IS A DISCUSSION OF CANARD TRIM AND BALANCE IN YAW ONLY i.e. the bow steering problem.

The conventional centreboard (or keel) and rudder configuration we are all used to can be thought of as being analagous to a conventional aircraft configuration (think of the model gliders we made). The main wing is like our centreboard, the tail plane is the rudder (we don't need the tail fin) and the centre of gravity is representative of the centre of pressure CP of our rig (actually the centre of area). You will remember that the glider CofG has to reside somewhere on the main wing position. And if you work out where your CP is relative to your centreboard position it is the same. This means that the wing/ centreboard supports the CofG/ CP with the tail plane/ rudder serving to point the wing/centreboard in the desired direction to get the required lift. A recent trend on skiffs has been to move the CP aft of the centreboard by a few % in order to load up the rudder a bit and hence ensure that both the centreboard and rudder are lifting in nominal trim. I suspect that is the accepted trim on Moths?

Now consider the canard configuration. If you have a model glider about you, try hand launching is backwards (having adjusted the angles of attack to be +ve on both wings). The result is instant leading tail up, stall and reversion to normal 'forwards' progress (if you give it enough height). Because the CofG is in the wrong place. I think this is what happened to the Stevos. For a cannard, both the foils are 'wings' and share the lifting task of the main wing or centreboard. The right place for the CofG/ CP is around the centre of area of the combined lifting surfaces.
Suppose you have a bow rudder of area 1 unit and a main foil of area 2 units; the balance point should be 1/3 of the distance between them, measured from the main foil. That is where the CP of the rig should reside. So on a Moth with these area proportions, you would need to move the rig forward about 0.5 m from the conventional position. Or move the centreboard aft by an equivalent amount to get the combined cenre of lateral resistance under the CP. You may also wish to take the hull lateral area into account, especially if the board is aft of midlength.
So my scheeme of having an aft mounted rig with the CP over the main foil at 75% of length and a bow rudder was clearly wrong. The rig needs to remain in the conventional position to keep its CP over the CLR. Sorry for that brainfade!

It should be said that I have never seen this in a text book or paper and I was only able to find this info on an aeromodeller's personal website a few years ago, so if you can comment with authority please do so.
As a reality check I looked at how balance is being achieved on the new generation of CBTF (canting ballast, twin foil) yachts. In these the lateral resistance is provided by equal sized linked rudders at bow and stern (which is a canard of sorts). They are positioned at 25% and 75% of length, giving a CLR about 50%. This is about where the keel they have replaced used to be positioned, so agrees with the above.
Most aircraft designers shy away from canards because, whilst theoretically more efficient, they have generally proved difficult to get to behave as predicted and a few have spectacularly fallen out of the sky. Burt Rutan however seems unfazed and has reaped the benefits.


For a Miller foiler with bow steering there are three cases to be balanced:-
1) The displacement mode.
2) The full speed foiling mode.
3) The intermediate range with bow foil at the surface and the main foil between displacement depth and full speed foiling.

For a conventionally steered foiler (miller or otherwise) the CLR stays with the centreboard irrespective of its immersion because the change of area in the rudder is not relevant to balance, only to assured steering.

That said I am still planning to try the bow steering configuration but am building the boat to allow a change to conventional stern steering if it all goes wrong!

I hope this essay is of use.

 

modle glider

ive got 2 model gliders here. flat plate wings made from depron sheet. based on our observations you can make a carard fly quite happily. the required cog is about 1/3 rd of the distance from the main wind to the stabiliser. we also experimented with moving the centre of gravity around and the canard handles changes much better than the conventional form. interestingly the conventional glider recovers from stall much better? this is due to the light stabiliser loadings i think. maybee i should make one with froward and rear stabilisers. essentialy you can build a conventional plane with lighter stabiliser loadings...draw your own conclusions

in moths we all rig our boats to have zero weather helm when dead flat or slightly to windward. this means that there is no sideways load on our rudders.

the problem is not with the centre of gravity placement on that configuration. experience has shown, and wardy would confirm, that the bow foil has to be very lightly loaded in order for it to track the surface. to get this light loading you have to sit along way back. once it is up you rush forward half a metre and if the canard dosent dive you are sailing along comfortably. at this point you want as little weight on the bow foil as possible as it will track much better. here the lifting mechanism is much different to a lifting foil. you do not get the changes in lift due to angle of attack as rvell spoke about. with a submerded foil you will though. ill try this in a week when i get my main foil built. as i have a submerged foil with surface sensor already built. im hoping it will have greter response to variation in pitching moment effects as it has the ability to carry higher loads. ive also built it with a little dihedral and sweep as i hope this will fascilitate the air being shed after ventilation/cavitation.

the canards which wardy and the stevos have built are designed for minimum lateral resistence. on saturday the bow foil with the steering mechanism was not quite freely rotating and so it did funny things to the helm. other than thet the helm should have been only effected by removing the lateral area of the hull.

in developing foiling moths we have tried to not move the rig and therefore keep the lateral resistence fixed as it is one less ariable to consider.

 

pitching moments

I tried to calc what the real pitching moments are.

Does anyone have agood idea what the real Lift/Drag ratio of a foiling moth might be. I know a big performance glider is like 1:200 and a basic hang glider is 1:5, but without a better estimate I punted on 1:20 for a total moth package.

If boat and crew weigh 135kg (OK I am fat), lift must be 135kg or 1350 Neutons (Sorry US readers but kg is mass and forces in SI units are in neutons = mass x g , g = 9.8m/^2 = accelleration due to gravity , rounded to 10 for this exercise)

So drag is lift/20 = 57.5N. This also must = force in the direction of motion from the sail (at constant speed.)

The sail force is applied about 2m above the deck and the drag is applied about 1m below the deck, so the bow down moment due to the maximum sail force is 57.5N x 3M = 172.5 NM.

If all the lift is taken by the main foil as Wardi supposes, an 80kg skipper (800N) needs to move only 172.5/800 = 0.21m aft to balance the pitch down effect.

OK maybe at extreme accelleration the sail force might be greater, but not much. And conversely when decellerating in a lull there might be no sail force and the drag couple might be lower, but the point is that very little body movement should be needed to respond to changes in sail force and drag.

This corresponds with my experience last weekend. Once flying in smooth water very little movement was needed, and most of that was done for height control in the absence of any other controlling devices.

Subject to some very different L/D ration being verified, I do not think we have a serious longitudinal stability problem.

WE do of course have a height control problem because we need to operate in such a small vertical dimension. I believe when we can manage height well we will find the other perceived problems will disapear. Other than ladder and V foils, John Illet has done this the best way so far, but there must be other simple automatic means available.

Start thinking and get building.

Phil Stevo.
AUS 9324

 

oh one more think the bow rudder would sail quite happily, it was just tacking that made it unworkable. with the current system we need a stern foilr so really is it worth all the hassle of the mechanism and tubes through the boat? especially when we are not actually reducing the number of appendages

 

I think Tom has made a key contribution here and has been prepared to admit his prior error....it seems we have now all been through this stage!!

Tom has highlighted that for canard steering to work, the CE must be moved way forward of where it is in a standard boat, or the centreboard needs to be moved well aft.

In the experiments of Phil Stevo last week, I do not think that the CE was moved forward. Phil, perhaps if you give this a go, it might work...before you scrap it altogether!

 

Phil, If I continue this calculation....
In order to keep the boat balanced and sailing level when hit by a gust, without moving my body weight, the canard must instantly apply an upward force of 172.5Nm/1.5m = 115N at the bow which is about 1.5m from the main foil.

If the canard cannot provide such a force immediately, then the boat will head down. The angle of attack reduces and the lift decreases. According to the graph supplied by RVELL, at a wing foil loading of 1.5, a 1 degree reduction in angle of attack will result in about 20% reduction on lift. For the boat to stay foiling, it will be necessary for the canard to provide this 20 % lift also. ie: 1350 x 0.2 = 270N. That is 270+115 = 385N in total.

I believe it is this requirement to provide 38.5Kg lift at the bow, which is causing the canard to bury so easily. That is why I am now trying a BIG canard in one trial and a submerged surface sensing canard in another.

If the aft foil takes some of the load and produces say 50N down force x 1.5m = 75Nm, then the canard only has to produce 172.5-75 = 97.5Nm at the bow, ie:97.5/1.5 = 65N. lift.

Clearly the larger the rudder foil, the smaller the canard can be.....until you look at the take off situation, where the canard must override the aft stabiliser to get the require angle of attack to take off.
Somewhere in between is a best ratio of canard to stabiliser area. At this stage I am working with 2:1. ie: the canard is double the size of the stabilizer.

On the other hand, using the foil on the centreboard as in the bifoiler arrangement, to provide this restoring moment against pitch requires even more lift, as the moment arm is much smaller. This means more drag, which is the core reason for developing the canard arrangement.

All comments welcome!

 

Canard

Ian,
No the rig was not moved before the bow rudder test. My previous logic was that the rudder was irrelevant to balance, that it was a matter between the sail and the fin. I am prepared to ammend that opinion based on National?'s recent post.

It would be hard to test with the existing hull. I can not move the rig significantly and the fin case is fixed. The hull is very tired. It has many cracks before last weekend and a lot more after crashing down 1m to the water on numerous occasions.

I have to decide very soon what configuration to put in a new hull. At present I am favouring a forward position for mast and fin and a rudder hung a bit behind the transom. I should get at least the separtion which John has proven.

I am at present building a main foil from John's sections which will have the flap trimmed by a surface sensor. By changing flap this alters both the lift coefficient and the angle of attack.

I am also builsing a deeper rudder which will have a symetrical foil (the one from the canard) which can be trimmed manually or maybe later by a surface sensor. Trimming the symetrical foil can provide both up and down forces when needed.

If these two work on the old hull I will build the new hull with their positions as far appart as I can, moving rig accordingly.

The force and balance calculations I did are for a constant speed, an equilibrium position. When accellerating there will be extra forces which I have not calculated. So your first line taking my number for a gust is wrong in principle but maybe not wrong in magnitude. The rest of your logic looks right.

But you seem to be leading to a bigger tail and smaller canard, like I did on sunday to the extent of 100:0

 

Phil,
Some big decisions to be made if you are building a new boat. Looks like you have given up the bow rudder idea altogether. I think you should be careful not to compromise displacement sailing performance just to enable efficient foiling.

Although the argument I put provides some logic to using a big rudder foil, in fact my own thoughts are only to make it big enough to assist the canard.

Another reason for persevering with the canard is that previously I have sailed successfully with a submerged bow canard and rudder foil only. No centreboard lifting foil. This worked quite well downwind. So I am sure a canard can give me the lift I require, it may just be that a surface running canard is not the answer.

Will test the new canard configurations this arvo or tomorrow and let you know!

 

forget the canard?

Ian,

This is a little late getting back to you; I was out splashing in the water. Remember, weeks of building and testing can save you hours of bookwork.

Briefly, I will share with you what was learned. The canard ladder foil is working well—see post #89 for photo and description.

To make it work I have done the following: reduced the span of the lower NACA 63-421 foil to 9 inches. The new area is smaller by 25% and now measures .19 ft^2. My sitting position has been moved forward 9 inches. The effect has been to reduce the effectiveness of the statically and dynamically unstable submerged front foil by over-loading it and keeping it deeply submerged. This has increased the reliance on the high dihedral “supper-ventilating” upper canard (V-foil). This upper foil is dynamically stable in small excursions and has made life easier for the over-worked pilot. Because the upper foil provides a fraction of the lift, the ride is softened. A bonus is the small spray that comes from the foil as it dances on the surface. The spray gives the pilot an indication as to the heave of the bow and indicates how the control stick needs to be moved.

I was pleased to experience rather slow demands for stick movement. In smooth water and with constant speed the stick input is more like “trimming” the foil’s angle of incidence as opposed to “chasing the heave”. You kind of set the angle of the bow foil and let the upper “V” foil skip on the water.

The next change will be to replace the surface piercing rear foil with a submerged foil with modest dihedral on the wing tips. The present foil ventilates with any slight disturbance. Ventilation results in a quick reminder: our mid-winter bay water is 56 degrees F. In California we call that cold.

I quote from your #126: “It begs the question, why not simply have a sensor operating the rudder stabilizer and remove the canard altogether? One reason of course is that in fat sterned boats it is not possible to sink the stern so easily, and so take off is difficult.”

In response to this and your other ideas in #128 I shall throw something at the wall and see if it sticks:

1. Eliminate the canard.
2. Use a bow sensor to sense bow heave (altitude). Link this sensor to the REAR, small foil.
3. For lift off, provide the main, amidships, foil with variable lift.

To me, canards are suspicious. Arrows don’t fly backwards, the genius Wright brothers could hardly make canards work and canard hydrofoils have problems. Even ducks, for whom canards are named, don’t fly that way.

Maybe canards, little foils up front, are popular in hydrofoils because that’s where the heave sensors belong. It seems natural to place the controlled foil near the sensor, but this may not be best. Ask Christopher Hook.

I say leave the sensor (surface follower) at the bow and link it to a deeply submerged rear foil. I am thinking of a trailing wand pivoting about a point placed forward on a bow sprint. Push rod(s) and bell crank(s) would link the wand to the rear foil.

For lift-off, create a surge in lift from the front, main foil. To accomplish this, use either flaps, variable incidence or a small super-ventilating ladder foil on the center strut. To take off the helmsman would spin a crank, pull a lever or push a pedal to increase the main lift. After take-off the lift would be returned to the cruise setting. The ladder foil would be automatic—no helmsman’s input required,

A word about center of gravity location: Astevo addresses this subject in post # 40. He refers to the need of placing “the center of gravity …in front of the neutral point”. In a conventional configuration this would place the CG ahead of the main foil. You and most of your boat would be cantilevered out front. You will be depending on the negative lift of the rear foil to keep it all from diving to the surface.

To avoid ventilation and broaching the rear foil must run deep. It may also be necessary to separate it from the rudder to avoid ventilation during sharp turns. To use an airplane analogy, you may need the rudder to be attached to and trail a vertical stabilizer.

I am agnostic about designing 100% of the load to be supported by the main foil. Because you and perhaps others are doing it, I cannot say it is wrong. The exceptions may involve the surface--planing canard foil you now use or the unique ability of the experienced Moth sailor to balance his weight to supplement the boat’s stability.

However, as Astevo has pointed out and as all long-lived aviators know, the center of gravity must be kept within strict limits to avoid divergent instability. This is another way of saying that correct wing loading is essential. Tom Speer has been instructive here. To quote him: the aft foil [the small foil in this case] must be more lightly loaded (carries less weight per unit area) – possibly even negatively loaded – than the front foil. [my words in brackets].

My feeling is that the small foil needs some definite loading, probably negative. This prevents excursions between negative and positive loading. That simply sounds unstable to me. I visualize a foil flapping up and down, push rods rattling and a surface follower flailing out of control.

Comments, anyone?