Moth on Foils: 35.9 knots(41.29 mph)

I was one of the protagonists on SA claiming that a slotted wing is a two sail rig. Personally I would love to see the Moth used as a vehicle for the development of slotted wing rigs. My point was that if you are going to allow slotted wings, then you must also allow slotted soft sail rigs, which currently the Moth rules clearly don't.

 

Moth on Foils--------The Wing

Mal, I looked at the rules and don't see anything that would prohibit a slot in any sail. Could you point out where I might have missed something?

http://www.moth-sailing.org/download/IMCA_Rules2010.pdf

I don't see why a slot that is broken by attachment methods of the rear element to the forward element can't be legal. If the forward and aft element are physically attached to each other how can the two elements be defined as two sails?

===============
From Phil Stevo on SA:

The moth class has a very satisfactory method of determining what is in the rules and what is not. The moth class will decide how the wing will be measured and what is legal. There are moth people fore it and moth people against it and they will all get a say in the decision making process. People who are not members of the class can post all they wish in places like this but they will not have any say in the decision.

 

Doug,

It is not completely clear from the photos of Adam May's wing if the slot is intentional, or whether it is a construction issue. It is not the slot per se that is the problem, it is the intent.

If the slot results from a construction issue, such as when using a mast track with slides (soft sails), or the gap in a hinged joint, it is reasonably clear that the intent is to have a continuous surface, but that the slot results from a construction compromise.

If the slot is designed in as a high lift device, then there is a deliberate attemp to gain an advantage. A two element wing (single slot) is, in principle, similar to a mainsail/jib combined airfoil (sloop rig). Under the current Moth rules, a sloop rig is not allowed, so I don't see how you can allow a slotted wing rig.

When a slot is actually a slot, and not a construction issue, may be a subjective judgement. My suggestion, to keep the rules objective, is that any hinge connection in a solid wing rig should be sealed with an impervious flexible material. Alternatively, change the rules to allow slotted rigs, whether solid or flexible. I would prefer the latter for developments sake, but it does change the charactor of the Moth.

One thing that should be considered is, what was the original intent of the rule?

 

OK Doug - we all know you love to hate Phil Stevo for perceived crimes against foiling. This is interesting because from all reports he is directly involved in a foiling class and is something of a foiling innovator as opposed to your position as more of a non-participating commentator (with a penchant for copy pasting foiling content across the internet).

If you review what he has written I think it could be paraphrased as:

The members of the moth class with decide whether the wing rig is inside or outside the class rules.

You'll note tspeer apparently also supports this position and it seems hard to argue against this.

The moth class has a history of innovation and if people don't like the class rules then they are free to set up an alternate class and take it in a different direction.

This also seems pretty sound. Will you take up the challenge or would you rather just tell others how to conduct their business?

 

========================
I see where you're coming from but I guess I feel that if the two are obviously connected together then they can't be two separate things.Tom said USA 17's rig was like a sloop rig as well. And he makes good points in his post here as you do. So we'll see what they decide.
I just don't see a slot-in and of itself-as a rule violation.

 

Actually, I'm in favor of the mast + mainsail interpretation. But it is not a slam dunk case.

And if the ruling goes against the slotted wing, it doesn't mean the end of wing development in the Moth class.

 

I meant only that your quote over the page supported the position that the moth class is the entity responsible for determining whether the wing fitted within their rules. I could have used tighter wording.

 

Seeing Adam May's impressive effort with the Moth wing sail has inspired me to think about building one this winter - especially if his wing shows promise at the races.


The depth of knowledge on this forum, and that people are willing to share it, always astounds me. Any thoughts on these questions?

- Practical considerations aside, would an asymmetric main element be faster than the symmetrical ones, design quality being equal? Particularly a thinnish one along the lines of Liebeck's high lift airfoil, but with less camber.

- With an assymetric main element, thin or not, would a flap/slot still be beneficial? The Moth doesn't need a lot of lift at speed it seems to me. Drag is the killer.

- With it's rules limited luff length is the Moth's AR too low to get enough advantage out of a wing sail?

- And while I have your attention - On a Moth's main lifting foil would we not get better L/D by using a flap with a slot? Or get to make the foils smaller?

 

Moth on Foils!

Welcome to the forum and to the thread, Chris! You might also find this thread interesting -in case you don't know about it: http://www.boatdesign.net/forums/sailboats/foiler-design-2447-65.html

I hope Tom Speer will jump in with some comment on your excellent questions-particulary the aspect ratio question.
Have you read about Ben Halls experience with his A Class wing rig?
Here: http://www.boatdesign.net/forums/multihulls/little-americas-cup-2010-c-class-real-one-32301-6.html post #80

 

I think so, provided there was enough thickness for structural purposes.

Aerodynamically, you can pretty much always design a thin section that will out perform a thick section at any given operating condition. The problem is covering the range of operating conditions with a practical design. That's where thick sections do well. They give up performance at a given operating point but have better off-design performance.

The flap is how one optimizes the camber for different conditions. And controls the twist. The twist is very important because it determines the spanwise load distribution and the induced drag. The induced drag is a lot more than the profile drag.

Whether you need a slot or not depends on what maximum lift you want. I've not tried to compare the drag of a plain flap and a slotted flap designed to the same requirements. But if you never needed high lift, then you have to wonder if you have too much sail area, and perhaps performance could be improved by reducing the chord.

I don't know - you'd have to use a velocity prediction program (VPP) to explore the optimum sail area. I'm not familiar with the Moth rules, but I doubt there's a minimum sail area. You might get better all-round performance by going with a design that has less chord for lower parasite drag, maximum luff length to minimize induced drag, and can camber up for high lift when going downwind so you don't lose out in pure power.

You wouldn't want to make the foil span any smaller. And you still need adequate thickness for strength and stiffness. But it might be a way to reduce the area.

 

Thank you Tom. This is all very helpful. It gives me what I need to begin seriously thinking about the design and contruction of a wing sail.

I am puzzling over how best to do the twist control. I imagine torsional stiffness (or lack of it ) in the two elements will play a roll. If I understand correctly you are saying that the twist control is in the flap. This would make sense because as the top of the flap is twisted the camber is reduced as well, right?

As far as reducing the area of the lifting foil goes if I used a flap with a slot - it would be tough to make the chord smaller, along with the corresponding decrease in thickness, without making them too flexible. Even with high modulus carbon. So I think I'll stick with the attached flaps.

 

Chris,

The point of the slotted flap is to delay separation. Therefore you should be able to use a thicker section (more camber) than you could without the flap, or more to the point, maintain the same section thickness with a shorter chord.

 

Well maybe there is still hope for the slotted flap. But I was thinking of Bill Beaver's towing tank study of Moth foils were he found thinner showed less drag and so I've been pushing for the thin section ever since. I would think reducing the chord but keeping the thickness wouldn't buy you much. Especially since with the slot the main foil's trailing edge now needs to be thin, making the foil relatively thicker still. I would be glad to see that I'm wrong about this.

The possibility of making a small foil with a really high C/L that could then be dialed back to low drag after takeoff really appeals. So any further light you can shed on how that might work would be appreciated.

 

This is the sort of problem where you need to do a lot a number crunching to to assess the trade offs. If there were some benefits to be gained from a slotted flap, I would think that we are probably only talking small percentages. You need to be able to reduce the total chord enough so that the reduced wetted surface and improved aspect ratio will offset the increased form (section) drag.

Another point is that, as opposed to the wingsail problem, a foil section is asymmetrical. Wingsails are usually designed so that the fore and aft elements are both symmetrical full sections. The aft element must be offset below (to windward of) the forward element on each tack. With an asymmetrical section you can do things differently. The early NACA experiments were done by modifying a standard section by simply cutting a slot through it that ran diagonally upwards and aft from the high to the low pressure surface. In other words, the flap system may not need to be much different to what exists now, except that instead of paying a lot of attention to sealing the flap hinge, you would leave a gap which would be shaped to channel the flow up and over the flap through the slot. This might be mechanically better than the current system, where one of the problems is the binding up and failure of the flap hinge.

Another possibility, for thinning down the sections, is that if the flap (or aft element) is offset below the forward fixed element, each element may be designed to act as a flange. This is similar in principle to the biplane aircraft wing, where the two wings, together with the struts, form a truss beam. This could be a tricky bit of engineering, but it would allow the element sections to be much thinner than they are now. The difficulty would be to design low drag connecting struts to separate the two elements. And you still have to get the flap to work!

 

Right. The lift depends on the local angle of attack measured from the zero lift line. Flap deflection/camber changes the zero lift line. Twist rotates the chord. With a 50% chord flap, it is really the angle of attack of the flap that matters most. If you hold the forward element fixed and deflect the flap, you get 80% of the lift that you'd get by holding the flap deflection fixed and rotating the whole section. So qualitatively, it's where the flap trailing edge points that determines the lift.

The big advance of Steve Clark's Cogito over previous C-class yachts was its ability to twist the forward element. On USA 17, the forward element was fixed (it flexed a few degrees in twist, but that wasn't controllable), and all the twist control was done with flap deflection.

There are really three objectives in twist control. The first is accommodating the shear in the apparent wind with height. The second is optimizing the span loading to get the best balance between the height of the center of effort and induced drag. A lower center of effort allows the wing to have more lift for the same heeling moment (obtained by twisting off the head and sheeting in harder) , but the induced drag will be higher. These first two can be done with just the flap and a fixed forward element, or they can be done by rotating the forward element.

The third objective is to optimize the profile drag. This really needs a forward element that can be twisted so at each spanwise station the optimum combination of flap deflection (camber) and angle of attack (twist & sheeting) is used to produce the desired lift at that station.

BTW, these principles of twist control apply to soft sail rigs and rotating mast trim, too. The Mast3 section on USA 17's soft rig had attached flow on both sides of the mast for a respectable range of angles of attack. But there was a separation bubble that formed on the windward side of the lower 1/3 of the span because the angle of attack at the mast was reduced by the turning of the flow by the foresail. It wasn't possible to trim the mast to stay in the attached flow range for the entire length. But if the mast could have been twisted, Krazy Koyote style, then it may have been possible to have had attached flow over both sides of the entire mast.