Foiler Design

[philSweet]

I'm just catching up on the thread, so forgive the digression. A few pages back, Terry wrote

Aircraft design practice has been quoted as a source of theory for foiler design. Foilers can certainly learn from aircraft design but the design of airfoil- and hydrofoil-supported craft is not exactly parallel. Here are some examples of differences:

1) Aircraft wings do not operate just below a medium discontinuity (the surface) like boat foils.
2) Boat foils do not have to operate during takeoff and landing just above a medium discontinuity (the ground) like aircraft wings.
3) An aircraft can lose far more height than a foiler without coming to grief.
4) A (foiled) sailor has a better chance of surviving a crash.

-so the same design guidelines may not apply to both aircraft and foiling boats.

I would like to add four other observations to this list. Plane vs boat.

1. The drag of a plane's wing is a higher percentage of total drag (I think)

2. The location of the foil relative to the vessel's CG. A change in foil drag on a plane will not induce a huge pitch moment change like it will in a boat.

3. The location of the thrust vector relative to the vessel's CG. A change in thrust on a plane will not induce a huge pitch moment change in a plane like it will in a sailboat.

4. Relative span. In engineering terms, the relative size of the pitch, yaw and roll moments are very different.

Control of pitch, yaw and roll do not work in isolation. They each have a coupling effect on one another; and after a great deal of trial and error and study, an effective control system has evolved for planes that works with or in spite of these coupling effects. These coupling effects are highly dependent on moments of inertia; and for this reason I doubt that the eventual bifoiler control system will have much in common with that of a plane.

And one question (because you have to ask a Question). Does anyone know what the relative drag values are (aero vs. hydro) as a function of speed. Kind of hard to solve force balance problems without this info. I figure aero is much higher than hydro at speed, but would drop sharply during a tack. The comments on relative foil loading at takeoff and at speed give some clues, but there's simply too much going on to use this to solve for aero drag.

 

[Doug Lord]

I'm just catching up on the thread, so forgive the digression. And one question (because you have to ask a Question). Does anyone know what the relative drag values are (aero vs. hydro) as a function of speed. Kind of hard to solve force balance problems without this info. I figure aero is much higher than hydro at speed, but would drop sharply during a tack. The comments on relative foil loading at takeoff and at speed give some clues, but there's simply too much going on to use this to solve for aero drag.

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I think this paper answers those questions for a Moth hydrofoil. The aerodynamic drag was much greater than anticipated:

 

[waynemarlow]

Ok so rigid wings have now become the new buzz for the moth but the foil is still not fully developed, such is the pace of development.

We acknowledge then that the slot in the rigid wing gives greater lift so why then don't we do the same in the hydrofoil, if we can get greater lift with less size then that would be good.

 

[yellowcat]

For foils enthousiasts, have you checked the "airfish-8" on youtube ? i like sailing but foils and hovercrafts especially flyinghovercrafts at 13k$ is getting my attention for a dingy. ORMA 60 s have been showing small foils, quickly retractable. At the end of the day, i think that foils will go hand in hand with kites sailing, we benefit from uplifting at 11 and 10 o'clock strings angle, we do this every spring on 4 inches of water above the thick ice, snowboards become surfboards and snowblades foils. Spectacular and thrilling. With only little wind, one can reach amazing speeds.
Food for thoughts.

 

[philSweet]

Wayne, The sail area is rule limited. The rigid wing with slot may produce higher total lift, but its L/d may not be so good. That's ok for the sail. The foil area is not rule constrained and the drag is always in the wrong direction, so minimizing drag is more important for the foil than for the sail. Slots are dragy. Just a guess.

 

[laurencet]

Spot on Phil, If your mounting flaps and slats to an aircraft wing you can create massive cl values but correspondingly high drag values. If the aircraft is coming into land this isn't a problem...Similar applies with a formula one car rear wing wing being driven by 800bhp+ Sadly on the moth you don't have this luxury. so probably do better looking at glider wings you also have the fun of cavitation to think about. The challenge is to optimise the foil over a wide range of conditions.

 

[Doug Lord]

Spot on Phil, If your mounting flaps and slats to an aircraft wing you can create massive cl values but correspondingly high drag values. If the aircraft is coming into land this isn't a problem...Similar applies with a formula one car rear wing wing being driven by 800bhp+ Sadly on the moth you don't have this luxury. so probably do better looking at glider wings you also have the fun of cavitation to think about. The challenge is to optimise the foil over a wide range of conditions.

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I'm surprised we haven't seen things like removable foil tips or other forms of variable geometry in the moth. For instance in light air a heavy guy could change his tips to provide a bit more span. A rough analysis indicated it might help.

 

[CutOnce]

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I'm surprised we haven't seen things like removable foil tips or other forms of variable geometry in the moth. For instance in light air a heavy guy could change his tips to provide a bit more span. A rough analysis indicated it might help.

Given the strength needed in a Moth foil set, I'd expect people interested to just have different foils for different days. In the grand scheme of things for people racing at the front of the pack, having a couple foil sets in the quiver isn't as strange as it may seem. Most of the people at the sharp end of the fleet build stuff anyways. If there was a serious advantage to be found, they'd already be building it.

I'd expect a mix & match foil with different tips etc. to fail in use, because loads, forces involved, drag and actually designing a system with both easy change and absolute, positive locking isn't trivial. Simple and solid wins over complex and failure prone.

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CutOnce

 

[waynemarlow]

OK then the split foil is no no, but we have the problems associated with foil adjust mechanisms, their problems with rough and confused waters and beaching / launching.

The early moths had a type of surface piercing foil from the racks pointing inwards until it was banned, lets turn that around and have two assymetric C foils pointing outwards from the normal dagger board well. It would offer ease of beaching and yet offer some form of self leveling like the typical 45 degree bruce foils. Sort of a modern twist on and old theme. The tips being approx. hull width would also give some level of stability across the boat, the vertical part acting like a dagger board.

 

[CutOnce]

OK then the split foil is no no, but we have the problems associated with foil adjust mechanisms, their problems with rough and confused waters and beaching / launching.

The early moths had a type of surface piercing foil from the racks pointing inwards until it was banned, lets turn that around and have two assymetric C foils pointing outwards from the normal dagger board well. It would offer ease of beaching and yet offer some form of self leveling like the typical 45 degree bruce foils. Sort of a modern twist on and old theme. The tips being approx. hull width would also give some level of stability across the boat, the vertical part acting like a dagger board.

The problem with the original surface piercing foils was that two foils (instead of one daggerboard) was interpreted as a catamaran configuration. If I understand your idea correctly, it would also fall under this established precedent.

We are starting to see some push-back from the Moth community and executive against the wings - rumor has it that Phil Stevo is one of the people against the wing concept. If the IMCA is exerting pressure to keep a Moth a Moth and setting some boundaries around the development class rules, I would not expect too many wildly divergent approaches to foil configurations to be accepted.

From a numbers perspective, this coming Moth worlds looks to be a major step forward for a class in terms of numbers of participants. Given that a formerly dying class has turned around, I would imagine the executive is working towards keeping growth on track - and adding major new development(and cost) for every boat on the water may not be in the classes best interest. It will be interesting to hear the rulings coming very soon on the wings - and I would consider that a strong portent for other wild new developments.

One of the most successful designs out there - the 505, has controlled development carefully, balancing the need to keep the design current with keeping the boat a 505. They've been quite successful directly because of managed development, but I don't know how the Moth community would handle such a similar set of boundaries.

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CutOnce

 

[Doug Lord]

OK then the split foil is no no, but we have the problems associated with foil adjust mechanisms, their problems with rough and confused waters and beaching / launching.

The early moths had a type of surface piercing foil from the racks pointing inwards until it was banned, lets turn that around and have two assymetric C foils pointing outwards from the normal dagger board well. It would offer ease of beaching and yet offer some form of self leveling like the typical 45 degree bruce foils. Sort of a modern twist on and old theme. The tips being approx. hull width would also give some level of stability across the boat, the vertical part acting like a dagger board.

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One of the most innovative design concepts I've seen over the last few years is Mal Smiths concept of two surface piercing foils on one strut:
http://www.boatdesign.net/forums/sailboats/peoples-foiler-2010-flying-fun-31954-2.html post17
Another is Jon Howes wand less foil system designed to operate at the surface.
Both these seem to me to have great potential. But don't write off wand systems: the modern re-invention of the wand was by Dr. Sam Bradfield and his planing wand on the Rave(and soon on the Osprey) are extremely reliable w/o the crash scenarios experienced by many Moths.
And don't discount manual altitude control now being used-particularly in rough water- by Mirabaud.

PIX- top two on left-foiling laser surface piercing foil concept by Mal Smith. Others Jon Howes Tomahawk ventilated foils and the boat they were tested on. Click on image and then again on resulting image:

 

[cardsinplay]

If the Mothies are smart, they won't let this Wing Sail development process become an arm's race. They've just begun to grow what was a languishing class back to something that could generate interesting numbers for a World's event.

To further slam the new class entrants with additional expenses (ditching their existing rigs for self-built, or purchased wings) in order to placate the cutting edge of the class, they will, once again, push the entrant numbers back into the dark ages. Not only that, but the class will be far more dependent on self administered repairs, which not all skippers will be suited to perform. The result... more cost to enter a competitive Moth and another drop in fleet numbers when faced with that reality.

Just when numbers of Mothies were getting dialed-in to the costs and realities of running a fully flying, foiling Moth, along comes another big cash hit as they will have to drop their spendy masts, rigging and sails in order to stay in the game.

Sometimes, this crazy pursuit of the ultimate in tech, results in the inevitable death of a species. The foaming proponents will say otherwise, but how many of them actually have a Moth that they enter in serious competitive events, and further... how many of the foamers aren't even Moth sailors in the first place? Balance it all out and it looks to me that this wing sail thing should be pushed back from the Moth arena for another few years, so that the rest of the fleet has a chance to play catch-up financially, as well as in a pragmatic sense.

As for this baloney regarding manual wand control issues.... it's pure foolishness. Go ahead, count the numbers of boats out there that use this blindly suggested manual control system and then tell us how many of those are operating within a competitive class. Yes, it's true, there are no manual flap control systems in play on class competitive boats, Geez.. I wonder why that fact looms large over the entire foiling community with only a few, outside the loop, proponents even considering the technology? For some folks, the fizz around the new and different is everything. Forget that what works is tried and true and utilized by those who race these boats for sponsor money, as well as personal fulfillment, where delivery of the results is far more important than fiddling with the speculative.

Want to set this argument down where it belongs.... Ask the manual control proponents to match themselves, boat for boat, with a craft that uses an automatic system and let's just see how well this suggested world shaking device actually works in race conditions. Our favorite, gadget oriented proponent, doesn't have the substance to produce such a match racing boat and apparently, neither do any of the others out there who are fiddling around in this manual wand arena. Fact is, until someone is willing to hang their public cheese out there in a substantive boat for boat study, all this manual wand hoopla will remain over-blown hype that is going nowhere.

 

[CutOnce]

If the Mothies are smart, they won't let this Wing Sail development process become an arm's race. They've just begun to grow what was a languishing class back to something that could generate interesting numbers for a World's event.

I think that is what I said above in a little more direct manner.

The one thing out there behind the scenes is that there is a lot of money out there lurking in wing design and implementation. AC teams are going to be team building, and one good idea in the wing development arena could well pad the retirement fund of someone. Especially when that idea is proven on the water in an established class.

I think the AC folks would like nothing better than a dirt cheap (by their standards) development platform where wing performance can be assessed. The Moth's velocities put it up into similar apparent wind speeds (albeit somewhat lower at the high end), and wing testing for AC boats needs boat speed.

I would not be surprised to see a bunch of the people pushing Moth wings to also be considering the AC wing gold rush as a profit/employment opportunity.

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CutOnce

 

[P Flados]

I really do not have a reason to choose one side or the other in this discussion. To me it seems that the "wing is too expensive" stuff just does not sound convincing for a "development" class. Especially when they have not even let one compete in a big race yet.

The existing requirement for a uni-rig is a good rule in that it keeps things reasonably manageable for the solo sailor that is already pretty busy just keeping the boat upright. Since any of the wings being considered really do not compromise this basic configuration requirement, using a wing does not change the character of "mothing" that much.

At this point, most competitors are probably still on a learning curve where the biggest potential for improvement is just time on the water. For those that are more experienced, most are prone to occasional hardware changes anyway.

If rules are established as required and with a focus of keeping any wing advantage small and to keeping upgrade cost & effort reasonable, allowing wings would not be the death of the class. You would probably see a transition to mostly wings for top performers and wings as a small "upgrade" cost (if kits prove to be workable, a wing kit might actually be be a no extra cost option) for individuals just starting out.

This would allow the Moth to stay a true development class and the acknowledged hot rod of the single handed dinghies. A lot of guys have gone for the moth just because of the rush of being able to blow past bigger boats. Rules that seem targeted at keeping the boats slower than they need to be just does not seem to fit with the image that has help this class grow so fast.

Again, it would seem that a development class should not to jump the gun on ruling out obvious development approaches until the facts are in. They need to let the wings have a go to see the results. If Bora's 3 element job puts him way out front on the first attempt, that would provided basis for some corrective rule making. Considering rules such as 2 elements only, non-twisting front elements, and a minimum weight for the wing could all be considered. Each of these rules would help minimize any performance gain while pushing cost down & usability up.

 

[cardsinplay]

It's not about the speed thing, P. It's about the fact that unless the established sail and mast makers for the class, or somebody decently qualified, begin to make a nice enough manufactured wing, a goodly number of these guys are going to have to pop for another rig and have someone else make it for them.

That's not a trivial amount of money that will be added to the already heady cost of a carbon Moth. Simply stated, Mothing is rapidly becoming a very expensive solo boat and that issue alone could be a very big limiter in the eventual size of the class worldwide. An all-out arms race is not necessarily good for the class.

In another life, I used to road race motorcycles in the 350 GP class and the cost of fielding a competitive machine started to escalate to the point where I either had to get a sponsored ride from a bike shop, or bag the GP racer and go to the 350 production class where the costs to field a really good bike were enormously less. In a very short period of time, the 350 GP class dwindled to just a few front line machines and the 350 Production class grew proportionately. Now, that GP class is non-existent and has been for quite some time. It is possible to screw-up a good thing with too many go-fast goodies required to get in the game.

Just a word of caution from my experience.