Foiler Design

[rustysunner]

Gents,

Perhaps not pertinent to your present discussion. If not, sorry. (The Windrider Rave forum is rather quiet right now.)

As a Windrider Rave owner I'm obviously fascinated with this discussion.

I won't pretend to understand all the engineering you're so obviously comfortable with, but it does promote thought none the less.

I've witnessed lots of "boiling" (cavitation?) at the foils ends at even 6 or 8 kts. The boat usually foils at maybe 11 kts, but due to control issues ventilates and "crashes" alot. I recall seeing figures on the foils curvature shape and numbers but don't have them handy if it's helpful.

Wondering about any adaptation to that boat to improve controllability maintaining submerged foils (whether with original wands and bungees or hands on trim controls. Also what efficiency improvements might be made to reduce the speed it foils at.

If at all helpful, I'd attempt to adapt those foils with any reasonably likely improvements to it's performance.

Mark I

 

[rustysunner]

foils,Contols

PS,

Manual controls on the trim was completed on my boat at the end of last years season so a full trial on that improvement is still pending.

Mark I

 

[Doug Lord]

Rave

Have you seen this?
http://www.doranoster.com/pdf/Hand-Controls-for-Rave.pdf

And this is Dr.Sams new url-you might e-mail him: http://sites.google.com/site/hydrosail/HydroSail-Home
----------
Dr. Sam told me that the Rave was set up with +2.5 degrees angle of incidence on the main foils and 0 degrees on the rudder foil . This was referenced to a line between the bow/bottom intersection and stern/bottom intersection. Have you checked this? Good luck!

 

[Gary Baigent]

Also check the fairing of the foil body to flap - IMO you need an almost perfect blend with no gaps or bumps - and some of the efforts I've observed are pretty agricultural. Also foils have to be polished. Better still, have you considered a pure foil with no flap, set at a compromise angle of attack, 2-3 degrees? That way you have cut ventilation and cavitation attacks simply by not having any clumsy junctions.

 

[rustysunner]

Rave foils

Doug,

Thanks for the responses. Doran Osters controls are basically what I built as a replacement for the wands with bungees which seem to oscilate the boat alot, and as I said my technique's not perfected. (Yet more to do while sailing of which I'm no expert)

Angle of incidence (fixed and variable through rotating ama mounts) seems to be as much art as science. More experimentation come spring.

The foils do have 2-3 mm gap between foil body and trim. Careful sealing could improve this I think. The photo's I've seen do seem to show a smooth seal type transition to the trim on the Moth's.

I will attempt to contact the Doc himself. Who knows?

Thanks again for the responces. No other Rave owners here in Seattle I know of to compare notes, but I'll leave you to your own and the designing, but follow along non the less.

(There were 6 Moth's off Seattle's Shilshole last fall all zooming in and out, and beating the pants off everything else at that race. Hopefully some other potential Seattleite foilers are enjoying this forum.)

Mark I

 

[alans]

Ventilation

Several years ago when we were sailing a large model tri-foiler we found that ventilation was occurring via the control push rod and hence into the control surface pivot area. This was inhibited by drilling a small hole in the leading edge of the "centre" boards and rudder blade just above the foils and allowing water fed at dynamic head pressure into the push rod cavity.

 

[Doug Lord]

Alan, great to hear from you-thanks for the info! Working on any new projects?
--
Thanks to Tom ,Mark and Gary for your thoughts on ventilation.

 

[Karl Wittnebel]

The Moth reportedly uses a NACA 63-412 for its wings....My guess is that something more like one of the low-camber RC glider airfoils would be better I think. These airfoils can go to lower Re's, and are also less dependent on turbulation.

Moths have moved on. Ilett may still be using that section, but everyone else is using custom stuff or sailplane sections as you suggest.

 

[Karl Wittnebel]

Arnie,

From a purely theoretical efficiency view, I think a simple fully flying main foil is not the way to go. A foil must provide high Cl to give a low take off speed yet be as small as possible in area at high speed. The problem of course is for a small area to achieve a high cl requires a high cambered section to both extend cl max and minimise cdf at high angles of attack. However once you are foiling at high speed in high wind, you need the foil to operate at a low cl, low AOA but your high cambered section will now produce high cdf for this cl. A flapped foil tends to do this automatically as the flap adds both camber and AOA at the same time. However the effect is hardly coordinated to give the ideal min cdf over the whole cl range. So what is really needed is a method to properly coordinate both AOA and flap to keep cdf at a minimum all the time.

Regards,
Ken

I dunno; mine goes pretty well. I'd say there are practical considerations that offset the theoretical drawbacks to some degree. Precisely how much is the (as yet unanswered) question.

 

[markdrela]

Karl W is correct. You get less benefit out of a camber flap than you might think. We looked at the pros and cons on the Decavitator, and concluded that a trim flap doesn't make sense.

The plot shows the polars of the Decavitator's big-wing airfoil, which is 13.9% thick. The operating range is perfectly adequate without a flap:
Roughly CL=1.1 or 1.2 for takeoff,
and CL = 0.2 to 0.25 at max speed.

With a flapped section, one could increase the takeoff CL to 1.6 or so, in theory permitting at reduction of wing area by a factor of 1.2/1.6 = 0.75x. But there are downsides to this:
1) The high-camber reflexed-flap airfoil has a non-optimal shape and less laminar flow at low CL's, and has maybe 5-10% more Cd at the high-speed condition than the unflapped airfoil. This eats into the wetted area benefit.
2) A 0.75x smaller airfoil will have only 0.31x as much bending stiffness, which will almost surely require a significant aspect ratio reduction and a CDi increase. This chews up more of the wetted area benefit.
3) It's mechanically complex.
Overall, it's tough to justify the flapped hydrofoil wing strictly from a drag viewpoint alone.

 

[Karl Wittnebel]

Karl W is correct. You get less benefit out of a camber flap than you might think. We looked at the pros and cons on the Decavitator, and concluded that a trim flap doesn't make sense.

The plot shows the polars of the Decavitator's big-wing airfoil, which is 13.9% thick. The operating range is perfectly adequate without a flap:
Roughly CL=1.1 or 1.2 for takeoff,
and CL = 0.2 to 0.25 at max speed.

With a flapped section, one could increase the takeoff CL to 1.6 or so, in theory permitting at reduction of wing area by a factor of 1.2/1.6 = 0.75x. But there are downsides to this:
1) The high-camber reflexed-flap airfoil has a non-optimal shape and less laminar flow at low CL's, and has maybe 5-10% more Cd at the high-speed condition than the unflapped airfoil. This eats into the wetted area benefit.
2) A 0.75x smaller airfoil will have only 0.31x as much bending stiffness, which will almost surely require a significant aspect ratio reduction and a CDi increase. This chews up more of the wetted area benefit.
3) It's mechanically complex.
Overall, it's tough to justify the flapped hydrofoil wing strictly from a drag viewpoint alone.

Well, it's good to know someone with some actual training in this field doesn't think I'm completely mad.

I think my foil section maxes out at Cl of 1.4 or so, where it has optimal L/D.

People tend to ignore real-life features like hinge gaps, what the vertical strut intersection looks like with a flap eating up 30% of the chord (ugly), how much easier it is to build a strong foil intersection without a flap, how much less hassle it is to not have to cut the trailing part of a wing off and reattach it somehow, how nice it is to not have to put little bellcranks and rod tubes through the foils for flap control, etc. Getting rid of the hinge also gives much more design control over planform, section thickness, sweep and anhedral than a flap permits.

IIRC, drag varies with square of section thickness, and building an 8 or 10% section that will hang is much easier without a flap sinking your battleship. So far I have not done it, but it's only a matter of time before we push the materials to their logical limits, and flapless is stronger - probably not much, as it is so thin back there, but still.

Regarding the foil size, my foil is about the same area as the flapped foils currently being used. I think I could go a bit smaller actually, but it feels pretty good in takeoff the way it is.

On bending issues, cutting the last 30% of the foil off doesn't improve spanwise stiffness any, and the foil is much more sensitive to bending with a hinge, which will bind if the foil bends too much. Practically we are running aspect ratios of around 9 on mainfoils, so not as extreme as decavitator, mainly because the tips hit the surface if span is increased too much.

I just cannot believe my flapless foil with a nice intersection fairing has more drag at just about any lift coefficient than a similarly-sized foil with a flap on it. Newer flaps are hinged at the lower surface, which is better in my experience, having done some experimentation on the water. But still there is a gap somewhere. And that nasty wedge cut from the foot of the vertical, which must induce some cavitation at speed.

Mechanically, active control of the entire foil is also complex, which is presumably why there aren't more boats with a stabilator-type setup.

Functionally, on a not so obvious level, having a boat that can and does get itself down out of the sky pronto when you need it to can be a huge advantage offwind, and a tilting foil seems to have far more authority and less drag than a flapped foil in this mode, say Cl zero, where the flapped foil's flap is way up high and not looking very much like an airfoil at all. In fact the tilting foil has more authority than one can use at times, but that is an easier problem to solve than too little authority

The last point I will make is that my foil routinely recovers from ventilation before the hull hits the water, which is something that never happens with a flapped foil, period. I did not anticipate this behavior, but I am not complaining.

 

[chabrenas]

If you folks consider this off-topic, I'll start a new thread...

I grew up watching early foil development (including the Hook Hydrofin), and I have crossed the English Channel on a hydrofoil ferry, but I was out of the loop long before the 1990s when the Moths took a big leap forward by becoming foilborne.

What was the enabling technology that kicked this off? I'm sure that the number of people playing with them speeded up knowledge about efficiency, control and stability, both at transition speeds and at top speed, but was it materials technology - in the foils and/or the mast & sails - that made the sudden surge possible? If not, what was it?

 

[bistros]

What was the enabling technology that kicked this off?

Doug Lord's unsuccessful aeroSKIFF. It never has verifiably flown, but it sure has been well promoted.

I'm sure that the number of people playing with them speeded up knowledge about efficiency, control and stability, both at transition speeds and at top speed, but was it materials technology - in the foils and/or the mast & sails - that made the sudden surge possible? If not, what was it?

Mr. Lord's ceaseless distribution of other people's Internet foiling photography and duplicate blog posts has brought significant new attention to foiling. Doug has become the world's most prolific foiling cheerleader, attracting new folks to the idea of foiling faster than he repels and irritates the people who actually foil.

Although my response is tongue in cheek, Mr. Lord has single handledly generated a lot of controversy, conflict, interest and drama for foiling - probably more than any one other individual. He does truly deserve recognition for his promotion efforts, and perhaps corporal punishment for his methods. In addition to recognizing his on-line promotion of foiling, the American Medical Association and Pharmaceutical manufacturers should award him honors for raising national blood pressure and increasing profits.

 

[sailor2]

What was the enabling technology that kicked this off? I'm sure that the number of people playing with them speeded up knowledge about efficiency, control and stability, both at transition speeds and at top speed, but was it materials technology - in the foils and/or the mast & sails - that made the sudden surge possible? If not, what was it?

Just a guess, but how about computer power allowing analysing these ideas with PC instead with supercomputers causing major difference in computation cost. Add that to advancing material technology. And rise in general standard of living allowing people spending extra money in stuff like that. And computer aided manufacturing to get accurate foilshapes matching that of models accurately enough. But like I said this is guessing.

 

[bistros]

Just a guess, but how about computer power allowing analysing these ideas with PC instead with supercomputers causing major difference in computation cost. Add that to advancing material technology. And rise in general standard of living allowing people spending extra money in stuff like that.

I don't buy it.

In reality, more eyes on the problem is directly proportional to solving the problem, hence my thesis that promotion is the major factor in reaching breakthrough.

Materials technology hasn't seen major improvements in the past ten years, and computing horsepower has flattened out as well. Carbon pre-preg is carbon pre-preg. You could build a Linux cluster with world class supercomputer horsepower for cheap five years ago using x86 technology. The general standard of living at the top end of the spectrum has decreased - allowing the mid to low end folks the opportunity to raise theirs.

This is a shade tree mechanic's game still - small projects developed by small individuals and teams rule - hence Andrew MacDougall's successes, John Iletts's successes etc. Since the science of foiling isn't static and proven, trial and error shows faster results (success or fail) in a cheaper development cycle than theoretical computing and simulation.

More people throwing more and different **** at the wall is showing what sticks and what falls off.

Evolution, not revolution.