Foiler Design

[ancient kayaker]

Minor correction, the scales are identical at -40

 

[bistros]

Minor correction, the scales are identical at -40

Absolutely correct, I didn't consult any reference when writing - I just was working from my (somewhat bad) memory.

At anywhere under -30 it all seems so cold it feels the same. Add wind chill to that and your are not able to think straight.

--
Bill

 

[ancient kayaker]

Is it really necessary to use carbon fiber and pressurized construction for foils? Personally I would much rather have wood ...

 

[peterraymond]

wood foils

Is it really necessary to use carbon fiber and pressurized construction for foils? Personally I would much rather have wood ...

Carbon is certainly not a requirement. Wood isn't as strong or stiff though so there will be some performance hit. Wood might match the low speed, or high speed, performance of a carbon foil, but not both.

 

[Doug Lord]

My first foils were designed by me, built by John Ilett and engineered by the same people that did the Aussie Spitfire foiler cat. They were engineered to be able to withstand jumping the boat. Both the main and rudderfoil were built out of wood reinforced with carbon fiber. The hardest part of a set of foils is the "T " joint and these foils used Illets carbon plate system-a series of carbon plates glued together in a t-shape and inserted into the joint. The system is extremely strong.
The foils have the exact same characteristics they would have if built from 100% carbon except that they are slightly heavier.

 

[bistros]

My first foils were designed by me, built by John Ilett and engineered by the same people that did the Aussie Spitfire foiler cat. They were engineered to be able to withstand jumping the boat. Both the main and rudderfoil were built out of wood reinforced with carbon fiber. The hardest part of a set of foils is the "T " joint and these foils used Illets carbon plate system-a series of carbon plates glued together in a t-shape and inserted into the joint. The system is extremely strong.
The foils have the exact same characteristics they would have if built from 100% carbon except that they are slightly heavier.

Doug:

The moderator has respectfully requested that we not cross-post the same things. These photographs have been posted more often on more than one site than the paparazzi post photos of B-list celebrities. Just post a hyperlink to one of the many, many, many previous posts of your photographs of these foils. The same goes for your oft-repeated foiling history collage, including the I-14, the surface piercing Moth and assorted other foiling history/museum pieces.

Reposting the same material again and again rudely consumes disk space for the poor guy paying for the site, consumes bandwidth for everyone viewing it, and slows down the site by forcing download of unnecessary repeats. If someone wants to see your jumping foils from the aeroSkiff(tm), they will follow a link to view them.

Just a thought.

--
Bill

 

[peterraymond]

The foils have the exact same characteristics they would have if built from 100% carbon except that they are slightly heavier.

True, but if they were 100% carbon they would be stronger and stiffer than they need to be. In that case you could keep the same profile and net area, but increase the span and decrease the cord. Induced drag is proportional to span squared and is also the dominate drag component at low speeds. A moderately small increase in span would significantly reduce low speed drag and give earlier liftoff.

At this point, the partially wooden construction probably wouldn't work any more.

If the foils don't have stall problems at liftoff speeds, then the profile of the original design could be made thinner. Low speed performance would be pretty much the same and the reduction in profile drag would give you a better top end.

Again, the carbon design has better performance.

You had carbon reinforcement. If you go with just wood I think you can still make it work, but there are further compromises.

 

[Doug Lord]

True, but if they were 100% carbon they would be stronger and stiffer than they need to be. In that case you could keep the same profile and net area, but increase the span and decrease the cord. Induced drag is proportional to span squared and is also the dominate drag component at low speeds. A moderately small increase in span would significantly reduce low speed drag and give earlier liftoff.

At this point, the partially wooden construction probably wouldn't work any more.

----------------------------
The primary driver on these foils was low speed takeoff and the ability to jump and re-enter safely with manual altitude control. Secondarily, we experimented with a very small flap area . A problem in the design of the Rave hydrofoil system* was that the flaps would sometimes lock as the foil bent. I have a hunch that that has happened on more than one occasion with Moth foils. At any rate, part of the small flap idea was to avoid flap lockup under any circumstances particularly under the high loads of re-entry. Another part was to use the dihedral of the foil in combination with the reduction in lift close to the surface as an "assist" to manual altitude control.
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I think that carbon reinforced wood construction of foils is perfect for many applications today as long as any flexibility of the foil is understood in the way it can interfere with flap movement-particularly with wand systems.
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*Rave foils are subjected to MUCH higher loading than monofoiler foils due to the fact that they also generate all of the RM for the boat. Because of flap lockup during development the full span flap hinges were reworked to allow the foil to flex w/o binding of the flap during high speed flap oscillation from wand movement.
-
pix:
1-better view of the partial span flap on my rudder foil,
2-Fastacraft stock foil with wooden strut reinforcement(avail now in 4.75" chord and 6" chord)
3- Rave foil set-up

 

[ancient kayaker]

I would expect that the torsional stiffness of the foil would be more important in practice than lateral stiffness. The latter would merely act like a suspension spring whereas torsional flexing would severely effect lift with the possibility of a feedback effect leading to oscillation and failure or a crash.

The profile in the pic looks quite sophisticated.

 

[Doug Lord]

Terry, both my mainfoil and the Ilett foil are based on the 63412 section which was the original section on the first foiler Moths. The Bradfield Rave section was a proprietary symetrical 15% section designed specifically for that boat. Torsional stiffness for a 12-15% section is not a problem since lift is either up or down-no ailerons to generate twist-at least in the spans referenced here.

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2/16/10 See this: http://www.boatdesign.net/forums/open-discussion/bernard-smith-dies-31524.html

 

[ancient kayaker]

Torsional stiffness and the way a wing twists under load would be a concern for an aircraft designer and I think if it turned out wrong that would cause problems for a foiler. As an example, if the axis of torsional flexing is behind the center of lift, then increased lift leads to increased angle of attack which leads to increased lift: a positive feedback effect unless the profile's center of lift moves forward at higher alphas.

Whether that actually happens, I don't know, not being a hydro/aerodynamics expert. And obviously the stiffer the foil the less likely the problem is to arise.

 

[Doug Lord]

Variable area foils

Peter: why not use plug in foil tips to change foil span on boats like Moths and a potential Peoples Foiler? For light air, early takeoff you increase the span by manually adding the easily removable tips; for heavy air simply unplug them-mainfoil only.....

 

[peterraymond]

Peter: why not use plug in foil tips to change foil span on boats like Moths and a potential Peoples Foiler? For light air, early takeoff you increase the span by manually adding the easily removable tips; for heavy air simply unplug them-mainfoil only.....

You could do that, but a People's foiler wants to be simple and a Moth, for only a Kilo-buck, would have two completely different foils to optimize for the two different wind ranges.

On the other hand, there has been some question if you can change your Moth once the regatta has started and that would apply equally to swapping complete foils and sticking tips onto a single set.

If you want to get wild you can go with an oblique foil. Straight across for low speed and rotated for high speed. And, to make you really happy, in low-ride mode you can rotate the wing 90 degrees and pull it up into a recesses in the bottom of a skinny Moth hull. I don't think it's a good idea, but is that a handicap when talking about things that will never be built?

Peter Raymond

 

[Doug Lord]

You could do that, but a People's foiler wants to be simple and a Moth, for only a Kilo-buck, would have two completely different foils to optimize for the two different wind ranges.

On the other hand, there has been some question if you can change your Moth once the regatta has started and that would apply equally to swapping complete foils and sticking tips onto a single set.

If you want to get wild you can go with an oblique foil. Straight across for low speed and rotated for high speed. And, to make you really happy, in low-ride mode you can rotate the wing 90 degrees and pull it up into a recesses in the bottom of a skinny Moth hull. I don't think it's a good idea, but is that a handicap when talking about things that will never be built?Peter Raymond

======================
Changeable foil tips might be legal in the Moth class and would be easy to do manually on the water in any class. Even the R Class guys are considering this idea. They allow a configuring of the boat on the beach for the apparent conditions as well as the changing of the configuration on the water. On a Peoples Foiler you might just stick with the configuration you went out with until you come back in since you won't be crashing,capsizing or pitchpoling(!). At 3-5 grand for a set of foils being able to change the whole configuration with removavble tips sounds cost effective and about as simple as you can get.

 

[ancient kayaker]

I have some thoughts and ideas on foiler design that I would like to throw into the mix for your attention. As I am still catching up with this thread, and have only reached the halfway point, pardon me if I mention something already covered.

- a number of issues have been discussed but control of height and pitch (CHAP) still seems to be open to discussion and perhaps improvement. While gifted individuals are able to balance a fast-moving sailboat and manage the rig while simultaneously planning race strategy, adapting tactics to the demands of the moment, keeping an eye on wave and wind conditions together with their competitors and handling CHAP, perhaps this is not for everyone. It is certainly not for the People’s Foiler mentioned once or twice.

There is literally an extra dimension involved in foiling than in sailing a dinghy, which can be complicated enough, and I for one, although not (yet) a foiler, can appreciate the desire of some to automate some at least of the flight control issues on boats with fully immersed foils.



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.

It has been stated that for static flight stability the center of gravity must be in front of the neutral lift point, which is the center of area of combined main wing and stabilizer. This is true for aircraft, canard or conventional, where the rear flight surface must operate at a lower angle of incidence (alpha) than the front. This ensures the front surface is closer to stalling; then if the nose rises and alpha increases forward lift is reduced and the nose drops. Although the rule has been ignored in some aircraft that actually flew in for distant years, few if any of those aircraft survive.

If manual CHAP is intended the rule should probably be obeyed; probably but not certainly, because the theory of foiler flight is not yet mature. However, the rule is ignored in aircraft using fly-by-wire, and a foiling boat that uses feedback from a surface sensor for CHAP has the equivalent of fly-by-wire.



Ideally, automated CHAP should achieve level flight ensuring the foils remain immersed and the hull does not get wet. Wave following and porpoising are undesirable as either can lead to a crash and will drain off forward momentum.

I believe a boat using immersed foils should have a single lifting foil and a single control foil, much like an aircraft. This does not exclude multi-plane designs where several lifting foils are stacked vertically, if it has an advantage. The boat should ideally be balanced on the lifting foil and the control foil should be only large enough to react necessary movement of crew. In a boat perfectly balanced over the lifting foil the movements of the control foil can be very slight, letting the inertia of the boat average out pitch fluctuations and requiring only long-term, slow response control over the average height.

However, this concept does not accommodate the bow-down moment from the rig. Some changes are required for this and several solutions come to mind.

A lifting rig can be used to eliminate the bow-down moment from rig thrust: a mast-aft rig or lateen sail can do this. A separate lifting airfoil can be deployed when foiling -this could even be a small flying jib. I have seen pictures of foilers with gennakers elsewhere in the forum that clearly can provide considerable lift. Or the CoG could be moved aft of the lifting foil, enough to offset part of the rig’s bow-down moment and the control foil enlarged to accommodate the rest. I personally am inclined towards solving the problem of bow-down moment in the air since any such arrangement would also handle gusts.



A foiler must transition from displacement (or planing) mode to flying mode.

In displacement mode it is probably easier to raise the bow than to lower the stern for takeoff. The CoG is generally closer to the stern than the bow when sailing so it makes sense to me to use a forward location for the control foil.

However, a small control foil will not have the power to raise the bow enough for slow-speed takeoff. Here I believe we can learn from aircraft practice, which is to use flaps. The flap need not be nearly as large as those used on aircraft, where the flaps are fully extended for landing where the extra drag steepens the glide slope but only partly extended for takeoff to minimize drag.

I am reminded of a method I once used to automate “liftoff” of a highspeed catapult-launched model glider. It had a delta wing with an elevator that was forced down by air pressure at the high takeoff speed for a smooth climb-out and raised at height by a rubber band to transition into a glide after losing speed. It worked extremely well but was banned due to its takeoff speed estimated in excess of 80 mph; the catapult was about 150 ft (50 m) long! The same principle should work with a flap on the lifting foil to increase lift at low speed for takeoff and reduce drag at high speed, except the flap would be lowered for low speed operation. It might also help to provide a softer “landing” by increasing foil lift again as speed is reduced.

Although manual control of main foil lift by a lever would work, the automated system above would actually be simpler and more compact.



Once flying, if automated CHAP is used it should be effective but with a minimum of added drag. I am not sure at this point whether the control foil should have a flap or be pivoted. Whichever turns out to be optimum, the sensor and feedback system is critical to success.

Most of the successful systems to date seem to use a surface sensing wand with a mechanical linkage. Other than surface piercing foils, there does not seem to be many alternatives in use. I will try to think out of the box and throw out some ideas.

If the forces required for pitch control can be kept small enough, it may be possible to use the variable buoyancy of a tapered strut for fine pitch control, using the control foil for manual, coarse corrections.

Another depth sensing arrangement not yet tried as far as I know uses hydrostatic pressure feedback. In one method, a lightly-loaded hollow foil is designed to vary its lift by changing profile in response to variations of pressure. Alternatively and easier to design, a streamlined bulb made of flexible material is mounted on a rigid wand behind a forward-mounted pivoting foil; if depth increases it compresses and loses buoyancy, so it falls and increases the alpha of the control foil, raising the bow. A rigid bulb with a small hole would do the same and adjusting the hole size would provide variable damping; an internal bladder would prevent expulsion of air if necessary. These concepts are all robust, self-contained, simple, low drag and weed-free, with no linkages to worry about.

The above ideas for controlling pitch are also applicable to automatic heel control, of course.