Foiler Design

thanks for that ken, i see your point.



you also wrote: "So what is really needed is a method to properly coordinate both AOA and flap to keep cdf at a minimum all the time."



in my second post i proposed a system of using a manual flap on the main foil and automatic AOA through a wand controlled "elevator" on the rudder. would this address the issue?



arnie

 

i am building an 8.5 x 7 meter sport cat from carbon/epoxy and rohacell foam. the design features a movable center pod that will house centerboard and rudder with "T" foils similar to a moth. the pod can be moved to windward for early flying, then to leeward where, in sufficient wind, the whole boat could be lifted above the water. the system has dynamically variable righting moment, the maximum with the pod to windward and the foil pulling down.

in order to position the front foil lift forward of the cg the lateral resistance is miles out. the rudder area will be increased to compensate. tooling has been made for a 3 x 30 cm NACA 0010 foil section that is envisaged for all requirements?

since the pitch will be dominated by the floating hull, most of the time, we are considering having main and rudder foils synchronized to the wand.

the design is by thai westlawn student, natthavarat titapan, with constant interference from the sponsor (me).

racing weight with crew of two, 900 kgs; sail area including two wing masts, 60m2; anticipated lift foil area, 200 x 30 cm fwd and 120 x 30 cm aft

the boat is about 6 weeks from completion. foils are the main thing that can still changed. only limitation is the center case dim 3 x 30 cm with 3 deg forward rake. comments are invited on foil section, depth, span, flaps, trim-tabs, control systems or whatever you like.

arnie

 

Hi guys, have been following parts of this chat with some interest. I am contemplating a build at 5m x 4.8. foiled at the front of the outriggers and having a manageable aileron on the rudder.

Has anyone ever thought of using a helicopter blade, which cannot (as I understand it) cavitate.


Regards

Daemon

 

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arnie, here is a site you may be familiar with-Bill Roberts Arc 21 uses a concept he calls "shared lift" where the daggerboard is much further forward than "normal" and the rudder is increased in size(relative to the daggerboard) for balance.
http://www.aquarius-sail.com/catamarans/arc21/index.htm
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I applaud your innovative thinking but I'm concerned about your ability to move the pod far enough or fast enough. Do you feel you have that taken care of?
Have you done an analysis that shows that the foils are a net plus in terms of drag? In other words do they "pay" for themselves?

 

hi doug

thanks for your comments.

we had not heard of bill roberts. we could not find much detail about "shared lift" in the link you referred but are encouraged that someone thinks it's a good idea. we have more or less decided to include trim tabs on the vertical foils to address weather helm on the rudder and to minimize leeway with the forward board.

initially, we had planned for the pod to be fixed in the middle with the foils serving like a retractable trimaran main hull but the availability of nice, 3.5 meter, harken travelers and cars won the argument against unnecessary complexity. how fast we can move it from tack to tack, jibe to jibe remains to be seen. also whether it gets used more to weather or to leeward. in phuket, light wind and flat water are the norm so the windward position is more likely to be favoured for early flying. this is where the foil forces will take the pod so it should self tack to adjustable limits. forcing it to leeward will require a winch or maybe a quick pre-tack adjustment?

lift foil section and size will be critical. one benefit of this configuration however is the weight to be lifted in light wind could be less than half that in a moderate breeze. if we get this right it should fly one hull in less wind than a moth can fly.

we will resist boards and rudders in the hulls until we are sure they are necessary.

we had an idea for a larger boat and hired hydrosail to do some foil analysis. the results were encouraging and lead to some refinements of the concept. the new configuration is more versatile and allowed for incorporation into a small race boat design. this boat is supposed to validate these ideas with less risk. hydrosail's calculations indicated that the drag tradeoff would work, even on the less efficient configuration.

the rig has some potential benefits also. certainly it will be easier to sail than a conventional sport cat rig. you moth sailors will be amongst the few that understand, speed to windward is possible without a jib and down wind without a spinnaker!

down wind trim remains somewhat a mystery. apparent on the beam, as favoured by this class of boat, will not be effective with the biplane rig. down wind in the light, the apparent will need to be carried well forward but at some point this will not pay. this may be where the biplane rig shines. since there is no standing rigging to limit trim the windward main can be sheeted out over ninety degrees and the leeward main sheeted accordingly. this will allow laminar flow and slot effect with the apparent well aft of the beam. in this most dangerous condition for a conventionally rigged boat, the biplane rig is only twenty degrees away from a luff.

if all else fails, the outboad stepped masts are foam filled and might prevent inversion in the event of a capsize. a self rescuing sport cat would certainly be unusual.

referrals to flap and trim-tab hinge and control details would be appreciated. we are still waiting for carbon to build the foils so comments on size and section could also help.

thanks, arnie

 

Tom Speer / Mark Drela

Paul, on another forum had a question I'm interested in as well:
"A quick question- I'm wondering why Moths use such short horizontal (lifting) chords- so short that they must be operating in the sub 500,000 re regime more than some of the time. That regime is notorious for non reattaching flow separation, flow hysterisis in extremus, etc.. It seems the main argument for this might be wetted surface(?), but drag from stressed flow must be an issue for sub 500,000 re chords.

For example, the chord re of a 3" chord at 10K in fresh water is (based on the formula: (Speed (knots)) times (Chord (feet)) times (137,250)= chord re). You have to be close to 15K before you get above 500,000 chord re. Which is probably why 0012 was jettisoned pretty early on, as it is horrible in the sub 500,000 chord re regime.

But a 6" chord gets you above 500,000 chord re at 10K, which means you might not have to develop proprietary foils, and use more public domain shapes. Maybe a 8" chord section with a shorter span with endplates might do the trick and keep ws down? (which is kind of what the foil kayak seems to be doing, at least up front.) Given the circulation of waves, a section of high aspect ratio that develops lift at low aoa might not be as important as a section that develops usable lift through broader aoa range? And if the longer chord section pops out of the h20, might it not crash as bad since it might plane a bit instead of stalling? Might work better at sub flying speeds too, since chord re would be more out of the sub 100,000 chord re (aka the strange) range. Like just under 3K. Might be less induced drag at low speeds too? Mav's are going to lower aspect ration at low re's. "

Paul
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Comments would be appreciated!

 

Actually, Re=500k is very "easy". Assuming the airfoil is designed appropriately, real problems don't start until you get below Re = 50k-100k, depending on the required airfoil thickness and Cl range.

The Moth reportedly uses a NACA 63-412 for its wings. At Re=120k, this airfoil very badly needs some turbulation help. In Xfoil, it's a disaster at Ncrit=9, but does OK at Ncrit=3, which may or may not be representative of the actual environment. 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.

 

You have the answer correct - reduced wetted surface.

Certainly there are sections that are not suitable for Reynolds numbers under 500,000 - the NACA 6-series being a good example. But there's no problem if the section is designed for it. Many Moths use the H105 section, which was specifically designed to for low-Reynolds number hydrofoils. It's really a matter of section design philosophy.

For example, take the Eppler E817 section. This was designed to be a high-Reynolds number hydrofoil section. It is characterized by rooftop pressure distributions on top and bottom that have a very abrupt start at their design lift coefficients, and concave pressure recoveries, with a sharp transition between the rooftop and the pressure recovery. This approach works for high Reynolds numbers because transition is via amplification of disturbances in the attached boundary layer of the rooftop, and because the high speed means the disturbances (waves, etc.) are small compared to the mean velocity so the foil can stay within its design operating range. Avoiding cavitation is also a high priority.

But these characteristics are troublesome at low Reynolds number where transition occurs via laminar separation bubble and turbulent reattachment. (Below a Reynolds number of 250,000 it's not possible to obtain transition through amplification of the Tollmein-Schlichting waves before reaching the trailing edge because the adverse pressure gradient that would be required would cause laminar separation first.) The abrupt start to a concave pressure recovery results in laminar separation and a long separation bubble, which causes drag. The abrupt start to the rooftop also results in forming a sharp leading edge suction peak at lift coefficients outside the design range. The rapid increase in pressure on the downstream side of the peak can cause laminar separation right from the leading edge that may not reattach at all, leading to a nasty leading edge stall.

To adapt the same section for low Reynolds numbers, the leading edge suction peak is blunted at higher lift coefficients, which results in a more rounded start to the rooftop at the design lift coefficient. The transition between the rooftop and the pressure recovery is smoothed out into a rounded convex shape. This results in a short laminar separation bubble that forms at the aft end of the transition zone at low angles of attack, providing a long run of laminar flow for low drag, and then moves smoothly forward as the angle of attack increases. This ensures the bulk of the pressure recovery is done with a turbulent boundary layer, which is more resistant to separation. This was the approach used in the design of the H105, which had a lot of attention paid to how the laminar separation bubble moves with angle of attack.

There's a lot of controversy over just how much laminar flow is possible in real sailing conditions. Many America's Cup design teams have failed to achieve the amount of laminar flow they'd planned for in the design of bulbs, etc. It may be that bubbles or biological particles in the water get spun up by the shear in the boundary layer, leading to earlier transition than expected from experiments in air. So the effective Reynolds number of a hydrofoil may be higher than the actual Reynolds number. Or the Ncrit lower - say, 3 or even 1.

In general, one can operate a section above its design Reynolds number(s), but one shouldn't use a section much below its minimum design Reynolds number. The key is to design it for a low enough Reynolds number to begin with.

 

Thanks Doug, I haven't run across this thread before. (Another 44 page thread to read...)

Mark and Tom- thanks for your posts. After reading them it seems that in a general way I might be on the right track, and I'd like to ask you some questions, but I really should read through this thread so I'm not wasting your time, plus I've got to go to work, and the post I was working on was taking way too much time, so I'll do it later.

(New Years resolution- stop composing long intricate posts that make me late for work!)

Paul

 

Monofoiler vertical fin/rudder ventilation

Thanks,guys, for your replies. I have another question. I'm about to install my daggerboard trunk in a 21 foot foiler. In my foil set the vertical is at 90 degrees to the 0 degree chord line shown for the 63412 section on page 523 of "Theory of wing sections". As it stands now the zero degree chord line is parallel to the static(loaded) waterline of the boat and it will be adjustable under sail to an angle of incidence(relative to that waterline) of +3 degrees
and -1 deg.
1)I'm concerned about ventilation of the vertical fin:some Moths angle the daggerboard forward as much as 7 degrees and they say it eliminates ventilation. Since the board/foil relationship on my foils is fixed can you suggest any solutions to avoiding vertical foil ventilation that don't involve angling the vertical fin more than 3 degrees? One other problem: these foils are retractable so a fence is probably out of the question. Any ideas would be most appreciated.

2) The Moth guys also angle the rudder and its pivot axis 2-degrees forward. Is there a better solution to this? Could the daggerboard and rudder installations be slightly angled athwartship so that the daggerboard wake does not impinge on the rudder?
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I can make any changes I have to now but soon it will be too late....
Thanks in advance.

 

Booted and in the Garage?

A large gang awaits the arrival of this long anticipated boat of yours, Doug. There has been so much said, that it must be one hell of a cool boat you got going there.

You will have photos of the first testing session, will you not?

As I have told you in the past, I have friends in that part of Florida who are very accomplished cameramen/directors, in both stills and video and I can ask one of them to be at your initial launching and proof of concept sailing exercise to put it on a digital file. Just say when and where with at least two weeks lead time (they are both busy boys) and I'll do all I can to get one of them to the launch and sail session.


Best of luck in your efforts.

 

Attached is a sketch of one of the two inverted-T units we used on the Decavitator.
Note that the rudder and stabilizer are staggered at the joint. This does a number of good things:
1) Any ventilation pocket which runs down the rudder is too far back to get sucked onto the lifting stabilizer. So the stabilizer can't ventilate, which is the main concern here since it would greatly upset pitch trim.
2) The suction peaks of the stabilizer and rudder are not significantly superimposed, which should eliminate the premature separation and interference drag of such an intersection.

The sweep of the stabilizer is there to move its overall center of lift closer to the rudder to keep its compression load close to the rudder's axis.

The two pieces were molded separately and joined with a small stainless-steel T-insert. The insert doesn't see any significant bending moments, so this worked OK.

 

Mark, if I extended my foil forward say,3", and tapered it back in 6" to 12" on each side would that help my particular foil?

 

Some thoughts for you:

As far as I can see, this is not a real problem yet, but more anticipation of a potential problem. Concentrating efforts on finding fixes for anticipated problems can paralyze your progress. It's better to deal with problems like this once you can duplicate the problematic condition and try things to fix.

The solutions taken to anticipated problems can very easily cause more trouble than the original issue. Your original foil designer is the first guy you should be talking to about this.

I'd also suggest some ideas as well.

1 - Foiling is more important than retractable foils. Although the "feature" of retractable foils seems important to you, if it damages your ability to foil in any way it should be immediately tossed on the heap.

2 - Always opt to simplify instead of complicate. Reducing variables is the easiest path to success and greatly increases chances of solving problems.

3 - You could mock up a "hull" that duplicates foil placement, weight and geometry for tow-testing to see if the conditions you are worried about even exist. Frank Bethwaite's continual testing of things in simplified situations is a great method to follow. Here you have one of the best practical engineers out there and he isn't building full featured sail boats to test - he's doing it in little stages to resolve major issues first before integrating all the components together. Your last effort should have taught you something .... a big complex project tested only at completion has less chance of success than a bunch of small project that make incremental progress towards a goal.

Best of luck with your new boat.

--
Bill

 

Four conditions are necessary for ventilation:
- flow separation
- static pressure less than atmospheric
- a path for the air to get to the low pressure, separated region
- formation of a stable cavity
Basically, if air is introduced to an area of attached flow, it quickly gets swept away and ventilation does not occur.

Fences basically block the path from the surface. But the better way to go is to avoid the flow separation in the first place. Sharp leading edges are a problem because they form a leading edge separation bubble for all but a narrow range of angles of attack, so a conventional section should be used with a leading edge that has a generous stall angle.

Another way to avoid separation is to increase the area. The side force on the foil is determined by the sail rig, so more area will reduce the angle of attack to below the stall angle.

Angling the strut forward causes spanwise flow in the upward direction. This tends to delay stall on the deeper portion, limiting the penetration of the ventilated region. It also means the pressure gradient at right angles to the flow direction is increasing, which also helps to limit the ability of air to penetrate down the foil.

I don't see that angling the pivot axis helps much with regard to interaction between the daggerboard and rudder wakes. Skewing them laterally may help on one tack but may bring the rudder right into the wake of the board on the other tack.