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Beck's

Stalling the Moth

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Photos of main foil of a foiling moth.

 

Is condition seen in the photo sequence from:

1. Stall?

2. Ventilation?

3. Cavitation?

4. Air bubbles from the tip of the wand?

5. A “DL” induced effect due to a non-Bradford wand system?

 

And why does it happen mostly in cold water? 48F water in photos - it was really bad in 40F water in Jan.

 

Beck’s

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I find i take air down the main foil when i am heeled to windward and the breeze dies and you have to bear off, sending you lower on the foils and tacking air down as you fall lower into the water! Its a pain in the ass when it happens just before you tack.

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I believe it is because your foils are red (hot). Black foils are cooler and consequently there is less of a temperature differential between the foils and the water. FACT.

 

I would love to hear an opinion from someone intelligent though, I have also been sailing in similar temperature water and dont think I have noticed ventillation on the main foil at all really. Color is the only thing I can come up with....

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Start a blog. I've heard that internet venting reduces on the water ventilation.

 

1 2 4 and 5

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Oh crap. Now I have to put off going to work. :lol:

 

Beck's- very nice. Very very nice.

 

Paul

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for the non mothies attempting to make sence of the photos - the bow is to the left in the photo and the splash comming from the left side of the photo is the wake of the wand. the ventilation bubbles along the side of the foil can be seen on the aft half of the foil starting at the water surface, they then migrate down the foil. the condition tends to be on the back half of the foil so i tend to think that it is not due to excessive attack angle.

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for the non mothies attempting to make sence of the photos - the bow is to the left in the photo and the splash comming from the left side of the photo is the wake of the wand. the ventilation bubbles along the side of the foil can be seen on the aft half of the foil starting at the water surface, they then migrate down the foil. the condition tends to be on the back half of the foil so i tend to think that it is not due to excessive attack angle.

 

What sort of speed are you doing at that point do you think?

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I believe it is because your foils are red (hot). Black foils are cooler and consequently there is less of a temperature differential between the foils and the water. FACT.

 

I would love to hear an opinion from someone intelligent though

 

I won't claim to be THAT intelligent but...

 

Black is cooler in temp than red? Black sucks up the entire visible light spectrum where as red reflects the red waves. Wouldn't that lead to black being hotter than red? Solar panels are black not red.

 

For that matter, why aren't all moth foils white if temp differential is an issue?

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I won't claim to be THAT intelligent but...

 

Black is cooler in temp than red? Black sucks up the entire visible light spectrum where as red reflects the red waves. Wouldn't that lead to black being hotter than red? Solar panels are black not red.

 

For that matter, why aren't all moth foils white if temp differential is an issue?

 

Did you bang your head on the boom last week end?

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I won't claim to be THAT intelligent but...

 

Black is cooler in temp than red? Black sucks up the entire visible light spectrum where as red reflects the red waves. Wouldn't that lead to black being hotter than red? Solar panels are black not red.

 

For that matter, why aren't all moth foils white if temp differential is an issue?

 

 

Are you trying say that black foils aren't cool?? Dude.....

 

(Sorry Becks, I was hoping this would turn into an interesting thread too)

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[...]

For that matter, why aren't all moth foils white if temp differential is an issue?

 

I though it was very well articulated by Nige: "Because Black is cooler"

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Are you trying say that black foils aren't cool?? Dude.....

 

(Sorry Becks, I was hoping this would turn into an interesting thread too)

 

 

maybe none of the above.

 

In another thread, someone pointed out that there a significant change in Reynolds number over the range of water temps.

 

Since the chord-based Re is close to critical, it could be that there is a laminar separation bubble that forms due to the lower Re in the cold water.

 

 

 

If that happens, lift coefficient drops. The flow does not reattach until the AOA is reduced to a smaller number.

 

You can see this if find experimental polar plots from wind tunnel results close to critical Re.

 

How that leads to air traveling down the vertical is another question.

 

KP

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maybe none of the above.

 

In another thread, someone pointed out that there a significant change in Reynolds number over the range of water temps.

 

Since the chord-based Re is close to critical, it could be that there is a laminar separation bubble that forms due to the lower Re in the cold water.

If that happens, lift coefficient drops. The flow does not reattach until the AOA is reduced to a smaller number.

 

You can see this if find experimental polar plots from wind tunnel results close to critical Re.

 

How that leads to air traveling down the vertical is another question.

 

KP

 

Peter, you should youtube your video and link it to that thread, it makes it really obvious what causes the ventilation in that case.

Kirk, that doesn't seem to add up ... If temp goes down, viscosity goes up => Re goes down. Wouldn't that be going away from the critical Re?

 

I think we can fix the pb without understanding the physics.

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Are you sure your foil is fair?

 

Also, what does the other side look like? i.e. Are we looking from windward or leeward and is there a difference?

 

Does the problem change if you sail on a different tack?

 

Am I wrong that the issue goes down the vertical strut but doesn't affect your lifting foil until it ventilates from bubbles? or do you immediately lose lift?

 

Do you hear or feel any vibrations as this happens and do you notice them before the bubbles?

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OK Gui - my job in the AM is to maybe learn youtube although i would rather just sail..... so what is the temp of the water going to be for the worlds?

 

Beck's

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ruessell

foils are fair - issue same on both tacks and not dependent on beating or running. I added more mast rake to put more load on rudder vertical foil and reduce load on C/B vertical foil. maybe a little better but not the critical change. You do not hear this comming on - unlike what can happen with windsurfers who can hear the cavitation starting my foil will just suddenly slip sideways and the boat falls back to earth (well unless there is a more ratical sudden bow drop and a big splash). when the rudder vertical foil goes it just makes a big rooster tail and slows the boat down - typically no crash...

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Peter, you should youtube your video and link it to that thread, it makes it really obvious what causes the ventilation in that case.

Kirk, that doesn't seem to add up ... If temp goes down, viscosity goes up => Re goes down. Wouldn't that be going away from the critical Re?

 

I think we can fix the pb without understanding the physics.

 

The idea is that at higher water temp the Re is big enough so that we are above the critical Re. BL is turbulent so the flow stays nicely attached.

 

At lower water temp the Re drops down to the critical region. The BL is in transition from laminar to turbulent. If laminar, it cannot stay attached at high AOA, it separates, a bubble ( water filled at first) forms, etc.

 

Model airplane guys who need to operate near the critical Re intentionally roughen the surface of the foil to turbulate the flow to keep it attached better. They are operating at high lift coeff, which is different that the typical Moth case (at high speed).

But if the "veal heel" is not enough and you are asking for too much lift from the vertical, it may push the AOA up to the point where the bubble forms, if the Re is near critical.

 

KP

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The idea is that at higher water temp the Re is big enough so that we are above the critical Re. BL is turbulent so the flow stays nicely attached.

 

At lower water temp the Re drops down to the critical region. The BL is in transition from laminar to turbulent. If laminar, it cannot stay attached at high AOA, it separates, a bubble ( water filled at first) forms, etc.

 

Model airplane guys who need to operate near the critical Re intentionally roughen the surface of the foil to turbulate the flow to keep it attached better. They are operating at high lift coeff, which is different that the typical Moth case (at high speed).

But if the "veal heel" is not enough and you are asking for too much lift from the vertical, it may push the AOA up to the point where the bubble forms, if the Re is near critical.

 

KP

 

Thanks, that makes sense. Maybe I should make an effort and look a bit more carefully at the pb ... The good news is that I should sand my foils with 40 grit!!! That or more chord, which is it?

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Thanks, that makes sense. Maybe I should make an effort and look a bit more carefully at the pb ... The good news is that I should sand my foils with 40 grit!!! That or more chord, which is it?

 

Personally I like 36 grit - it makes the sanding go SO much quicker. And a belt sander.

 

Becks maybe your wand wake is putting some hurt on the main foil. Perhaps you should flatten out the tip a bit and see it that makes any difference.

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Thanks, that makes sense. Maybe I should make an effort and look a bit more carefully at the pb ... The good news is that I should sand my foils with 40 grit!!! That or more chord, which is it?

 

 

You laugh, but my aero friend (ex Euro champ) told me that the modern A2 planes put tissue paper over the smooth mylar and CF structures that they build to get a rougher surface to keep the flow attached.

 

The easier and reversible experiment is to glue a tripper (fishing line?) near to the leading edge on one side when you are going to sail in the cold water. Then see if the stalling/ventilation/laminar separation bubble is better on one tack than another.

 

Better than more chord, just sail faster.....30% should do it, I think that was about the change in Re between 25C and 10C water.

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I can't see from the photos whether the ventilation is both sides or just on one side. If its just on the low pressure side of the foil then more/less Veal Heel would seem to be a good way to go to this non Mothy, if for no other reason that according to my understanding having a surface piercing foil contributing to leeway is very much the high drag option. I think I'd have to be ~*very* convinced by someone else doing it that hitting the foils with 40grit will help before I vandalised my own foils...

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Have to say I haven't experienced this on the main foil, but definately on the rudder. But less so now the water temp is lifting.

 

Therefore my foolproof solution to the problem is not to sail when the water is that cold. Done. Does that make me a scientist?

 

Water temperature at the Worlds should be sufficient that this won't be happening.

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The idea is that at higher water temp the Re is big enough so that we are above the critical Re. BL is turbulent so the flow stays nicely attached.

 

At lower water temp the Re drops down to the critical region. The BL is in transition from laminar to turbulent. If laminar, it cannot stay attached at high AOA, it separates, a bubble ( water filled at first) forms, etc.

 

Model airplane guys who need to operate near the critical Re intentionally roughen the surface of the foil to turbulate the flow to keep it attached better. They are operating at high lift coeff, which is different that the typical Moth case (at high speed).

But if the "veal heel" is not enough and you are asking for too much lift from the vertical, it may push the AOA up to the point where the bubble forms, if the Re is near critical.

 

KP

 

I've been following this discussion from when it was just verbal around the dock, and I'd like to continue doing so. So ... since I obviously flunked out on aerodynamics (or whatever science is behind this) can you please explain to me what the heck the following stand for:

 

Re

AOA (angle of attack?)

BL

Veal heel (no cows in the water, I hope)

 

Thanks!

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Re - Reynolds Number. A non-dimensional form of fluid velocity, applicable to foils. Re = V(m/sec or ft/sec)*c(chord length in the case of a foil section, metres or feet)/(kinematic viscosity in m or ft^2/sec). Named for Reynolds who did experiments on fluid flows through tubes to determine laminar vs turbulent boundary layer transition speeds. Generally a very large number at any normal sort of speed as kinematic viscosity is of the order of 10^-6 m^2/second. Just think of it as "foil speed" if the above is undecipherable technobabble to you.

 

AoA - Angle of Attack - angle between the direction of motion and the fluid encountered.

 

BL - Boundary Layer - a very thin layer of fluid, directly next to the surface of the foil or body encountering a fluid flow. It influences the lift/drag characteristics significantly.

 

Veal Heel - heeling a foiled boat into windward to gain maximum righting moment and a component of the lift vector from the lifting foil in the windward direction to help drive the boat to windward and counteract leeway. Think of this one as "sailing the boat hiked to windward" and you'll do fine. Named for Rohan Veal (if you haven't heard of him, I'd hate to see the size of the rock you must have been living under for the last 5 years).

 

As for what the foil is doing there Becks... I'd say you have a classic case of ventilation. BR suggests setting your wand up so it 'surfs' away from the line of the foil as one solution, the way to do this is to have the may stick shocky run around the port side of the mast post before going to your clip or cleat of choice. Another is to fence it (but then it'll cause drag and not exactly stop the foil from venting, but it will reduce the worst of the symptoms... which is what I've done on my rudder and has improved it significantly), another is to look at the grit option, try to trip the BL flow to turbulent, resulting in a "hydraulically smooth" foil and hence keeping the remainder of the flow laminar and better attached.

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You laugh, but my aero friend (ex Euro champ) told me that the modern A2 planes put tissue paper over the smooth mylar and CF structures that they build to get a rougher surface to keep the flow attached.

 

The easier and reversible experiment is to glue a tripper (fishing line?) near to the leading edge on one side when you are going to sail in the cold water. Then see if the stalling/ventilation/laminar separation bubble is better on one tack than another.

 

Better than more chord, just sail faster.....30% should do it, I think that was about the change in Re between 25C and 10C water.

 

Well, thanks Kirk, it's been informative ... 30%, ya think that's funny?

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Thank you, Flying Colours!

 

Big-ass rock, but it's never too late to climb out from under it and learn something. Besides, I figure if I'm wondering, chances are pretty good someone else is wondering, too.

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http://www.youtube.com/watch?v=QD-zD4Qr3_c

 

Slo-mo of the foil in action ... well inaction actually as it keeps stalling.

 

NOTES:

1. Turn the volume down - splashing water in slo-mo is deafening

2. It is Upside down - the fin is at the Top of the screen, underwater

3. As stated above, it's in slow motion

 

 

All credits to Videographer ~ Beck's

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It's hard to believe that the wakes from the sensor (in the centerboard video) and the wake from the centerboard (in the rudder video) don't start some of the ventilation.

 

At 40 seconds into the centerboard video, one of the the foil edges for a frame looks like the bumps on the edge of a grey whale's flukes. This is probably partially an optical illusion, or video artifact, but I think it might also be refracted water streams? It happens just before ventilation. Hard to tell what aoa, but I'm guessing it's large. Has anyone tried the grey whale fluke bumps?

 

Has anyone tried a longer chord at the water interface part of the vertical foils? When I look at Moth vertical foils, there's not much chord, which means the Re # is pretty low. Which would get lower in colder water? Flow around a foil at these RE#s gets pretty screwy, as mentioned above, althought the flow does reattach here. Or is the friction penalty just too big on a larger chord?

 

I was looking at the diamond foil (Doug C's page?), and the chord on that vertical foil looks bigger than the vertical foil on this Moth video. What was the ventilation situation there? Or did the bunching up of the water around the junction of the diamond and the vertical foil influence things too much to tell?

 

A designer named Oossanen has done some flow visualization work on leeboards (which are also surface piercing foils), here is a site that might be fun to look at;

 

http://www.oossanen.nl/images/full/06056-04.jpg

 

Work is so overrated. :lol:

 

Paul

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I was looking at the diamond foil (Doug C's page?), and the chord on that vertical foil looks bigger than the vertical foil on this Moth video. What was the ventilation situation there? Or did the bunching up of the water around the junction of the diamond and the vertical foil influence things too much to tell?

 

This is a very interesting thread and great videos.

 

The Diamond had a very complex issue when the top of the Diamond reached the surface. A huge vortex was generated and the drag slowed the boat down, so much she came of the foils. It was ony when the wind was stronger that there was enought power to punch through this drag hump. I do not think the Diamond and the ventilation of these foils is related, or that the Diamond configuration is any better.

 

Raking the foils forward is suppoes to help as it make it harder for the bubble to travel down the foil.

 

Obviously you have to get the foils with a good finish and good section, maybe the trip wire will help and also wand ends that create less disturbance. However I suspect sailing technique also has a large part to play. You can shake off the bubbles from the rudder somethimes and smooth sailing may also help. Knowing how hard you can pust the boat without problems is also something to learn.

 

I do however like the idea of only sailing in nice warm locations best.

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Bowgirl - thanks for getting the vid up - i sail better than i can manage video on a PC - I should have a Mac ....

 

Who knows the water temp expected for the worlds?

 

Becks

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Did you bang your head on the boom last week end?

 

Sorry, I guess I missed the sarcasm... I'll go grab my coat.

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Sorry, I guess I missed the sarcasm... I'll go grab my coat.

 

;)

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I think Karl has some of the problem solved ... maybe. ..... http://mothchronicles.blogspot.com/2008/04...tion-whoas.html

 

still would like to fully understand how the cold water plays into it - but i do know first hand that it does. 40F = really bad, 45F = sort of bad, 52F = not too bad.

 

We will see how the weekend of sailing goes with the wand (or should I say bubble generator) moved far off center........

 

Becks

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I think Karl has some of the problem solved ... maybe. ..... http://mothchronicles.blogspot.com/2008/04...tion-whoas.html

 

still would like to fully understand how the cold water plays into it - but i do know first hand that it does. 40F = really bad, 45F = sort of bad, 52F = not too bad.

 

We will see how the weekend of sailing goes with the wand (or should I say bubble generator) moved far off center........

 

Becks

 

I like kprice's take on it, fits pretty well with our problem. He also provided a few fixes:

1)40 grit ... 36 gor atyp 'cause well he's atyp.

2)Increase chord

3)Increase speed by 30% ... ha ha

 

Now Atyp's got a good case of pulling too many words out of his ass, of course, but he's got a point about the fence thingy. Me think.

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Just a thought.

 

Since the bubbles seem to travel from the top down along the trailing edge, what if you were to simply add a little spine (for lack of a better word) off the trailing edge to blead off the air early? It seems like you could knick up something in a couple minutes and see what happens. If this were to work, you could add them easier, cheeper, and with less added surface than adding fences. I'm kind of thinking of something ony about as big as a coffee straw and maybe an inch or two max in length. If this were to work in a test, you could make something much more elegant in carbon with lower drag.

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I like kprice's take on it, fits pretty well with our problem. He also provided a few fixes:

1)40 grit ... 36 gor atyp 'cause well he's atyp.

 

Now Atyp's got a good case of pulling too many words out of his ass, of course, but he's got a point about the fence thingy. Me think.

 

Hey I got a good deal on 36 grit carborundum a while back and it's all I have in the double digit arena. But you can do some great stuff with a sheet of it on a long fairing board. You will laugh but I actually did fair my new T foil with that as a first pass over some fairing compound. There is no substitute when you want to get rid of material fast.

 

And my ass is always the first place I look for ideas when I am at a loss to explain something. That is why I have a blog - it's like a colonoscopy writ large - the IMAX of capsule endoscopy. But Gui you are simply envious of my blogorrheogenic typing abilities. I know jealousy when I see it.

 

Anyway once you get a sufficiently large gulp of air down there it is pretty much over, unless you can somehow ride it out until the air gets scrubbed off eventually (the advantage of having a fence). Stabbing the tiller to leeward generally hooks the rudder flow up again on a Canoe; I have found it less effective on Le Moth however I feel it does marginally speed up the resolution of the ventilated state. Most of this probably happens as you slow the boat down though - the pressure in the concavity of the strut must increase at lower speed, with similar effect on the upper surface of the lifting foil. Unfortunately, slow is not the new fast so you are still losing time.

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You laugh, but my aero friend (ex Euro champ) told me that the modern A2 planes put tissue paper over the smooth mylar and CF structures that they build to get a rougher surface to keep the flow attached.

 

The easier and reversible experiment is to glue a tripper (fishing line?) near to the leading edge on one side when you are going to sail in the cold water. Then see if the stalling/ventilation/laminar separation bubble is better on one tack than another.

 

Better than more chord, just sail faster.....30% should do it, I think that was about the change in Re between 25C and 10C water.

 

 

I'm more inclined to believe the AOA thing on the rudder than the main foil - I can't see the main strut having much AOA on in that video at that speed with that much strut submerged.

 

The wand can have an effect on the rudder - for me rudder vent only happens episodically on certain headings in certain conditions so I have to think the strut is sailing in the wake of the wand. Of course the water here never gets much below 50 so it's hard to identify w/ Becks

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I'm more inclined to believe the AOA thing on the rudder than the main foil - I can't see the main strut having much AOA on in that video at that speed with that much strut submerged.

 

The wand can have an effect on the rudder - for me rudder vent only happens episodically on certain headings in certain conditions so I have to think the strut is sailing in the wake of the wand. Of course the water here never gets much below 50 so it's hard to identify w/ Becks

 

 

Laminar separation bubble intro:

 

http://aerodyn.org/LowSpeed/lowspeed.html

 

Beck's, the idea for the temperature effect is that as the viscosity goes up with temp, the Re goes down and the flow over the foil changes from a supercritical Re ( BL flow stays attached, no separation bubble ) to a critical Re ( bubble can form and grow).

It is the reported temperature effect that makes me think the transition from supercritical to critical is what is happening....otherwise I would say it is plain old ventilation/stall , i.e. air getting sucked down on the suction side of the foil.

 

How the bubble leads to more likelihood of air ingestion is not clear to me. The pressure is bigger ( less negative ) in the bubble so unless there is more spanwise flow ( suggested in another thread ) there should be less tendency to get air ingestion.

 

Gui, if you do not want to sail 30% faster, then for cold water either try a trip wire or grit glued near the LE, or pick an airflow that is designed for operation in the critical Re region, or increase the chord. All will have a drag penalty, only use in cold water.

 

Since Nige has a reliable supply of cold water, maybe some towing experiments can be arranged during the winter.

 

For experiments, to get the same Re in air, increase the speed by 12 or so...midnight on the autobahn, foil out the sunroof, put some tufts on the foil to visualize the flow, and post results on youtube...maybe an assignment for Doug C?

 

Kirk

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Numbers:

 

If speed = 15 km/hr, chord = 0.1 m, then Re at 5,10,20C are

(kine. visc in m2/sec*10e-6)

Re = v*L/(kine. visc)

 

T kine. visc Re Ratio

5C 1.519 274303.26 1

10C 1.307 318796.22 1.16

20C 1.004 415006.64 1.51

 

seems like enough change to make a difference in which airfoil to choose.

 

Would 20% more chord for the cold-water foil be too much drag penalty?

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Numbers:

 

If speed = 15 km/hr, chord = 0.1 m, then Re at 5,10,20C are

(kine. visc in m2/sec*10e-6)

Re = v*L/(kine. visc)

 

T kine. visc Re Ratio

5C 1.519 274303.26 1

10C 1.307 318796.22 1.16

20C 1.004 415006.64 1.51

 

seems like enough change to make a difference in which airfoil to choose.

 

Would 20% more chord for the cold-water foil be too much drag penalty?

 

"We" mostly race in warmer water ... I'm just interested in a cheap fix for the 5 or so cold months. I have fences on the rudder, which works ok but doesn't prevent ventilation, just deals with it. Maybe I'll try the sand paper stuff on the LE.

 

As for your number, we sail at ~12kts uphill, so about 22 km/h, that would be when I have the most ventilation going on. Downhill is not as bad, but it does happen too.

 

Yes Atyp, I which I had your "blogorrheogenic typing abilities" ...

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OK just to add some more interest - we had two BR's sailing side by side in 20kts of wind - one boat stalled the rudder all the time the other did not. - changed to a new vertical rudder foil on the boat that was stalling and no change - we first thought it might be due to difference in helm weight but we traded and same rudder stall problem with lighter helm - the boat that stalls the rudder is setup to fly higher than the other --- could it just be that the horizontal foil is just close enough to the surface on that boat to suck the air down? Next test is to lower that boat down and see the change, if any....

 

Good news is that the C/B vertical foil is no longer stalling - rotating the wand outboard most likely helped, or did it? The other boat has the wand closer to CL....

 

So I still am attracted to the the best solution is to only sail in warm waters - So who knows what the water temp is at Weymouth.....

 

:)

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OK just to add some more interest - we had two BR's sailing side by side in 20kts of wind - one boat stalled the rudder all the time the other did not. - changed to a new vertical rudder foil on the boat that was stalling and no change - we first thought it might be due to difference in helm weight but we traded and same rudder stall problem with lighter helm - the boat that stalls the rudder is setup to fly higher than the other --- could it just be that the horizontal foil is just close enough to the surface on that boat to suck the air down? Next test is to lower that boat down and see the change, if any....

 

Good news is that the C/B vertical foil is no longer stalling - rotating the wand outboard most likely helped, or did it? The other boat has the wand closer to CL....

 

So I still am attracted to the the best solution is to only sail in warm waters - So who knows what the water temp is at Weymouth.....

 

:)

 

Height issue might lend credence to Kprice's theory regarding LSBs. Of course if your rudder lifting foil tip is hitting the surface you are hosed. JPZ's videos show both methods of rudder ventilation going on.

 

Glad to hear wand setup changed things for the better.

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I have no problem hanging foils out of my car but getting it to go 12 times faster than a moth will involve more than fitting a sunroof.

 

2 good theorys but I am going to go with the Plain old vertilation one for now.

 

The tests with the 2 identical boats are great and seam to indicate ride height is a key factor. The higher you ride the less foil is in the water so the presure difference is greater. This means that getting an area of the foil with a pressure of less than one bar will suck air down. Once attached the air moves down the foil till it gets to the horizontal and then it is game over.

 

So the easy solution for cold water is fly lower. This is easy to change with cable length or wand length. So optimum ride height is a compromise between wave clearance and venilation.

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Re the two boat test:

Are you sure that the two rudder foils were actually mounted to the struts at exactly the same angle.

One explanation for the difference could be that one strut had slightly more rake than the other. The one with the least forward rake would be the one expected to ventilate. (Well within tollerance of BR gear and owner installation I suspect.)

 

I know I have virtually stopped the problem by remounting the foil so that the strut has a couple of degrees forward rake. Not counting weed, jellyfish and plastic bags, and also the water here never gets below 15 deg C.

 

Try some shims under the front of the strut and retest?

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I am guessing that swapping rudders and finding that the boat was the problem eliminates this theory Phil but it would be good to double check this as it could also be a good theory, and part of the puzzle.

 

PS: Pulling words out of your ass and making up wierld theorys is part of the fun. Or atleast I hope it is because it is my speciallity. This thead just goes to show that we need to think about things from different angles before we come to a half way reasonable conculsion or set of theories. What the truth is is probbably a combination of all the ideas. If you have a ventialltion problem then the solution could be a different mix of these issues, which may not always be the same for all boats.

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Maybe a completely silly thought, but the main foil strut has a tube down it for the push pull rod.

Any chance it is gulping air?

SHC

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I thought this as I sometimes get ventilation on my old flap rudder when I move the flap. I was going to put some grease in my tube to stop it. Others report the observation of air traveling down the outside of the blade. The Bladerider rudders and most newer ones have no flat and tube so this is not the issue reported in the 2 boat comparison above.

 

Again I think there could be multiple ventilation causes, but plain old ventilation is the most common.

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Re the two boat test:

Are you sure that the two rudder foils were actually mounted to the struts at exactly the same angle.

One explanation for the difference could be that one strut had slightly more rake than the other. The one with the least forward rake would be the one expected to ventilate. (Well within tollerance of BR gear and owner installation I suspect.)

 

I know I have virtually stopped the problem by remounting the foil so that the strut has a couple of degrees forward rake. Not counting weed, jellyfish and plastic bags, and also the water here never gets below 15 deg C.

 

Try some shims under the front of the strut and retest?

 

Rake should make propagation less of an issue but should not affect incidence of ventilation occurring in the first place.

 

Doug I think all foils operate at pressures less than one bar somewhere on the foil. I have a feeling the reality of initiation is a bit more complicated. Nonetheless seems to make sense to avoid pressure extremes.

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Doug C, rake forward does induce upwash- when Stealth Chicken did it, they had to cap off their kelp cutter because of the water shooting up the hole! My 40er has a forward swept keel, and the bottom paint is always being scrubbed off at the fillet at the top of the leading edge of our keel. My question is that when things go bad, the water seems to bunch up just above the air/water interface of the vertical foil, and then that wave (? I guess you'd call it) collapses back down into a trough, which has to be a low pressure spot. Which also seems to be around the lowest pressure spot on the suction side of the section. 'And if my eyes don't deceive me, there's somethin' goin' on around here'- like a low pressure spot that air can suck into. And if I see it right, the water bounces off the foil a little after the trough and causes another low pressure hole near the trailing edge.

 

I think I just described wave patterns around a hull. :lol: Doh!

 

Does forward rake effectively throw the trough farther back of the point of most suction, or even aft of a high AR foil? Would you see little cyclone holes in the water behind the foil?

 

I think I'll go to work now.........

 

Paul

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Again I think there could be multiple ventilation causes, but plain old ventilation is the most common.

 

 

Doug, how do you see the temperature effect (and the change in Re) affecting "plain old ventilation"?

Density does not change much so the pressure distribution is about the same....

 

Kirk

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this was probably said but i this was a problem that i had with my boat on the C/B and rudder (venting) and i made a new wand with the tip the same taper, very thin, for about 20mm and then have the wand in the mount rotated to the high side of the edge of the flat of the wand is next to the hull. let me know if you wont Pics.

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Doug, how do you see the temperature effect (and the change in Re) affecting "plain old ventilation"?

Density does not change much so the pressure distribution is about the same....

 

Kirk

Actually water density does change pretty dramatically with temperature http://www.csgnetwork.com/h2odenscalc.html

 

ventilation will be affected by water density since denser water will create a deeper wake trough at the trailing edge than less dense water.

 

A deeper TE trough means more ventilation. It also means more lift for a given speed across all the foils, and since air has significantly less density than water, it will be sucked towards that increased lift (low pressure)

 

As mentioned before, forward canting of the surface penetrating foil has the effect of changing the direction of the laterally induced flow on the vertical foil, which reduces the wake trough at the surface and hence reduces ventilation. Classes with ventilation problems like the J-24 specifically controll negative rake because of this.

 

 

While fencing on the vertical foil will help, you probably need multiple fences to deal with waves etc. Seems to me that the main problem occurs when the ventilation travels down to the lifting foils. So what about putting in a fence on the lifting foils a short distance from the vertical intersection?

 

another thought would be to put in some sort of flow scoop at the root of the horizontal foil that scoops water from below the horizontal foil and directs it in an upward flow direction.

 

BTW a hint for the next time someone tries to video this: Use a circular polarizing filter on the lens. This will largely eliminate any surface reflectivity and give you a clearer view of what is happening to the foil

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Actually water density does change pretty dramatically with temperature http://www.csgnetwork.com/h2odenscalc.html

 

ventilation will be affected by water density since denser water will create a deeper wake trough at the trailing edge than less dense water.

 

A deeper TE trough means more ventilation. It also means more lift for a given speed across all the foils, and since air has significantly less density than water, it will be sucked towards that increased lift (low pressure)

 

As mentioned before, forward canting of the surface penetrating foil has the effect of changing the direction of the laterally induced flow on the vertical foil, which reduces the wake trough at the surface and hence reduces ventilation. Classes with ventilation problems like the J-24 specifically controll negative rake because of this.

While fencing on the vertical foil will help, you probably need multiple fences to deal with waves etc. Seems to me that the main problem occurs when the ventilation travels down to the lifting foils. So what about putting in a fence on the lifting foils a short distance from the vertical intersection?

 

another thought would be to put in some sort of flow scoop at the root of the horizontal foil that scoops water from below the horizontal foil and directs it in an upward flow direction.

 

BTW a hint for the next time someone tries to video this: Use a circular polarizing filter on the lens. This will largely eliminate any surface reflectivity and give you a clearer view of what is happening to the foil

 

Problem is the drag - even if it never gets to the lifting foil it is way slow and you lose a lot of steerage control.

 

I don't quite get how rake changes the "laterally induced flow" but I'm sure someone will enlighten me.

 

Amazing to hear that J24s ventilate. They seem like traffic cones.

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Doug, how do you see the temperature effect (and the change in Re) affecting "plain old ventilation"?

Density does not change much so the pressure distribution is about the same....

 

Kirk

 

I do not know but from reading other posts viscosity changes a lot with tempreture and this affects ventilaiton (or Renolds Number).

 

I agree that maybe Plain Old Ventilation may need a trigger. Waves or wash from the wand or other foils could do this or a inperfection in the section shape...?

 

I do not pretent to understand this but unloading the foils and raking them forward and using even pressure distrabution section makes sence to me. Fences are a last resort but clearly are a good damage limitation device.

 

Does anyone know of some studdies about ventilation triggers or how in detail Plain Old Ventilation works. I think tank tests and high speed cameras may be the best way to really observe this and understand it.

 

I have now changed my theory. I think it is a "Complex Case of Plain Old Ventilation".

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I do not know but from reading other posts viscosity changes a lot with tempreture and this affects ventilaiton (or Renolds Number).

 

I agree that maybe Plain Old Ventilation may need a trigger. Waves or wash from the wand or other foils could do this or a inperfection in the section shape...?

 

I do not pretent to understand this but unloading the foils and raking them forward and using even pressure distrabution section makes sence to me. Fences are a last resort but clearly are a good damage limitation device.

 

Does anyone know of some studdies about ventilation triggers or how in detail Plain Old Ventilation works. I think tank tests and high speed cameras may be the best way to really observe this and understand it.

 

I have now changed my theory. I think it is a "Complex Case of Plain Old Ventilation".

 

Mention here (pointer courtesy of atyp) of tank tests and ventilation

 

http://www.boatdesign.net/forums/showthrea...oil+ventilation

 

If someone is near Boston they can go dig around to find the mentioned video. Or enroll in grad school and do the same study for moth foils.

Also interesting is the foil system for "Decavitator" on the MIT history page.

It was the HPV, did about 20 km/hr, main foil was about 40 mm by 900 mm....but they cheated by having another set of foils for lift-off.

 

 

Kirk

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Gui

you are brilliant - it is not so much the temp as the salinity - i have been sailing in water that has more salinity and therefore more density - the salinity of the water is much greater in the winter months than in the summer months..... :)

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Gui

you are brilliant - it is not so much the temp as the salinity - i have been sailing in water that has more salinity and therefore more density - the salinity of the water is much greater in the winter months than in the summer months..... :)

 

gui and brilliant ... doesn't sound very right, but I'll take it.

 

I think the biggest difference in density would show a variation of ~3-4% ... That's be a worst case scenario, from hot fresh to cold salty.

I sail in fresh water, and I've had the same pb ... I'm ruling out density diff. as the cause.

I have zero clue about what's happening, but I liked Kirk's explaination. Mostly because it's simplle.

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Problem is the drag - even if it never gets to the lifting foil it is way slow and you lose a lot of steerage control.

 

I don't quite get how rake changes the "laterally induced flow" but I'm sure someone will enlighten me.

 

Amazing to hear that J24s ventilate. They seem like traffic cones.

 

The issue in all cases of ventilation is that you have a lifting foil that penetrates the surface. A lifting foil works by bernoulli's principle of differentiated pressure. The difference in pressure "pulls" the foil towards the lower pressure. But when you do this in a boundary condition where the other material is far less viscous - and viscosity, not density, is the real issue with cold water - you will "pull" that less viscous material towards the lower pressure as well. Now add to this the slight downward flow vector that you get off the tip of the foil (some airplanes put winglet end plates to reduce this) and you have two forces contributing to "pulling" air down the lifting side of the foil.

 

If you cant the foil tip forwards, then the "bow wave" of the tip can spread both around the tip, and upwards (since the forward cant allows the tip to operate "ahead" of the rest of the foil). that means that some of that pressure flows vertically upwards. and this is true at every section of the foil. This then induces flow from the tip towards the root, rather than from the root towards the tip. Flow in this direction counteracts the viscosity and pressure differential induced flow from the surface and thus reduces cavitation

 

 

BTW, if you look at the tables of Dynamic Viscosity of water WRT Temp You will see that the viscosity at 5degC (41 deg F) 0.001520 - 0. vs 15degC (59degF) - 0.001139 is roughly 25%. IE it takes 25% more energy for water to bend its way around the foil at 5degC than at 15degC for the same speed.

 

Now on the lifting foil, this means you dial back the amount of flap you need, so you get the same lift and drag as in the warmer water. But for the vertical foil you can't change the amount of lift, so what you get is a greater likelihood of cavitation.

 

I wonder, has anyone tried perfectly flat vertical foils?

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... we sail at ~12kts uphill, so about 22 km/h, that would be when I have the most ventilation going on. Downhill is not as bad, but it does happen too.

 

For 20 km/hr, 10 cm chord

 

temp kine. visc Re Ratio

(m^2*1e-6)

5C 1.519 366000 1

10C 1.307 425000 1.16

20C 1.004 553000 1.51

 

Note that this kills my idea of the importance of transition from subcritical to critical Re...critical Re (from wind tunnel data) is around 100k in the books (Selig and Donovan) that I have on hand.

 

So at least at the speed Gui quotes, foils are well away from critical Re, so the basic nature of the flow should be the same in 5C or 20C water.

 

In the boatdesign thread Drela notes that the free surface of the water is pulled down by the negative pressure on the front part of the foil, since the pressure at the surface = 1atm. His speculation is that the resulting kink in the water is unstable, as evidenced in tow tank tests where the foil either ventilated the whole run, or flow stayed attached.

 

But no speculation on temperature or Re effect.

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peter and others,

 

i have had a bit of experience with this problem and have summarised the solution in the Bladerider Troubleshooting Guide available from the link below...

 

http://www.bladerider.com.au/xseries/support.html

 

Basically the wand is not offset enough. I have never had a problem since we redrilled the axle on the correct angle and twisted the wand to ski away from the hull at high speed. The wand should be at least 20cms away from the foils once flying. Every time I have seen someone else having this problem, I can see their wand is too close to the centerline.

 

15.jpg

This photo above was taken at Garda on the day the water temperature dropped to around 13 degrees, and I still did not have a problem with cavitation.

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K Price said-

 

"If you cant the foil tip forwards, then the "bow wave" of the tip can spread both around the tip, and upwards (since the forward cant allows the tip to operate "ahead" of the rest of the foil). that means that some of that pressure flows vertically upwards. and this is true at every section of the foil. This then induces flow from the tip towards the root, rather than from the root towards the tip. Flow in this direction counteracts the viscosity and pressure differential induced flow from the surface and thus reduces cavitation"

 

 

This begs a question I have not had the courage to ask before, so (big breath) when you have upwash on a foil that has it's tip swept forward, doesn't this upwash change what type of foil the water is 'seeing'? The water is not flowing across the foil parallel to the chord, but at an angle, which seems to effectively make the chord longer, which makes the effective (?) section thinner, as well as changing the geometry of the whole section. So if you are going to change the rake, shouldn't you intentionally pick a section fitting the flow direction across the foil that results because of that rake? And this also seems to beg a question; does this resulting thinner section have less tendancy to ventilate because it has less and different lift distribution and intensity? And can you assume in this raked condition that the nose geometry can be, in general, not have changed? And if so, how much of the nose, say by percentage of the chord?

 

 

K Price then mused-

 

"I wonder, has anyone tried perfectly flat vertical foils?"

 

FWIW, My dad had some flat leeboard foils (aluminum plate) on his Grumman Canoe with a sailing rig- about 2-1/2 feet by 8 inches, IIRC. They sucked upwind going anywhere over 1-2 Knots, flat water. They came with the rig. He had a degree in areo, and we made some mahogany 3d assymetric foils that worked a lot better. I don't remember the section. I was 6.

I sanded those suckers.

 

Paul

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K Price said-

 

"If you cant the foil tip forwards, then the "bow wave" of the tip can spread both around the tip, and upwards (since the forward cant allows the tip to operate "ahead" of the rest of the foil). that means that some of that pressure flows vertically upwards. and this is true at every section of the foil. This then induces flow from the tip towards the root, rather than from the root towards the tip. Flow in this direction counteracts the viscosity and pressure differential induced flow from the surface and thus reduces cavitation"

This begs a question I have not had the courage to ask before, so (big breath) when you have upwash on a foil that has it's tip swept forward, doesn't this upwash change what type of foil the water is 'seeing'? The water is not flowing across the foil parallel to the chord, but at an angle, which seems to effectively make the chord longer, which makes the effective (?) section thinner, as well as changing the geometry of the whole section. So if you are going to change the rake, shouldn't you intentionally pick a section fitting the flow direction across the foil that results because of that rake? And this also seems to beg a question; does this resulting thinner section have less tendancy to ventilate because it has less and different lift distribution and intensity? And can you assume in this raked condition that the nose geometry can be, in general, not have changed? And if so, how much of the nose, say by percentage of the chord?

 

 

K Price then mused-

 

"I wonder, has anyone tried perfectly flat vertical foils?"

 

FWIW, My dad had some flat leeboard foils (aluminum plate) on his Grumman Canoe with a sailing rig- about 2-1/2 feet by 8 inches, IIRC. They sucked upwind going anywhere over 1-2 Knots, flat water. They came with the rig. He had a degree in areo, and we made some mahogany 3d assymetric foils that worked a lot better. I don't remember the section. I was 6.

I sanded those suckers.

 

Paul

 

KPrice? I belive it was me BB.

 

You are correct that canting forwards will change the apparent chord of the foil. But remember that canted vertical you already have downward flow so the apparent flow is already "seeing" a longer and thinner chord. Depending on your cant, you might just take it to neutral. I beleive there was a photo in another thread of a "zbox" that allowed for a dynamically adjustable cant.

 

As for flat foils - I realize how much they suck in traditional blade forms, but downwind lift doesn't matter, and I thought (and I'm still waiting to ride one so this is just me asking - feel free to tell me to F'OFF Newb) that the fast mode was to heel to weather and use the lift from the horizontal foils to generate 0 leeway. Thus the lift from the vertical foil would only be of marginal contribution - or so went my reasoning.

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KPrice? I belive it was me BB.

 

You are correct that canting forwards will change the apparent chord of the foil. But remember that canted vertical you already have downward flow so the apparent flow is already "seeing" a longer and thinner chord. Depending on your cant, you might just take it to neutral. I beleive there was a photo in another thread of a "zbox" that allowed for a dynamically adjustable cant.

 

How can there be downward apparent flow when there is a huge horizontal endplate at the end of it, aka the lifting foil, mother of all winglets? Seems like a road to nowhere.

 

It is an F-box, and it is not dynamic.

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It WAS you, BB, not K Price! :blink: When one is avoiding work, the mind plays terrible tricks. And later, one talks about oneself in the third person!(!) But your downwash musings- do they imply that the section was chosen initially assuming downwash? Perhaps Ira H. Abbott and Albert E. Von Doehnoff had some sort of SECRET PACT? That they kept secret in the Dover Edition? Even with the new Preface?

 

Hmmm?

 

Confess, or I'll torture Papa Doc with my voodoo doll!

 

Paul

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How can there be downward apparent flow when there is a huge horizontal endplate at the end of it, aka the lifting foil, mother of all winglets? Seems like a road to nowhere.

 

It is an F-box, and it is not dynamic.

good point about the endplate - DOH. Ok so then the point about lengthening and "thinning" the foil by giving it fwd rake applies. I don't know enough about the foil equations to compare any changes in lift or drag, but I would imagine they are fairly minimal,

 

Fbox - ZBox, my excuse is that I'm old and senile... mea culpa

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OK - bit of a change - but same topic

 

Let’s go on the assumption that the disturbance caused by the wand tip is causing some or all of the foil issues discussed. Let’s also assume that we have “Vealed” the wand by rotating it away from the CL.

 

What do you think is the best tip configuration to have on the wand to reduce wake and the depth of the trench while at the same time creating the same rotational force on the wand?

 

What have your tried - what do you want to try?

 

Beck’s

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good point about the endplate - DOH. Ok so then the point about lengthening and "thinning" the foil by giving it fwd rake applies. I don't know enough about the foil equations to compare any changes in lift or drag, but I would imagine they are fairly minimal,

 

Fbox - ZBox, my excuse is that I'm old and senile... mea culpa

 

BB, you are right, they are fairly minimal- You have to get to about 15 degrees of rake before you get much longer on the chord length. But it's actually wierder than that.

 

Just for fun, I assumed that a NACA 0009 was the same as an 0012 at some indeterminately different rake (0012 vertical, and the 0009 was what the 0012 turned into at some vague pie in the sky forward rake angle) , basically because Theory of Wing Sections doesn't have a 0010 or 0011, and at 2 degrees aoa, the 0012 had less section lift coefficient than the 0009. This may be wrong, but I am using the word 'assumed', which always worked with my Thesis Advisors.

I did this for some other related symmetrical shapes, and the results were kind of the same. Maybe the point of max flow speed moves forward on the lee side of the section with rake because of the change in geometry of the initial curves of the section relative to the shifted flow? And this moves the point of max suction forward even more out of the way of the usual point of ventilation in addition to the upwash moving the hole that can trigger regular old ventilation away from the point of max suction caused by the collapse of the 'bow wave' aft? And brutally forcing the flow to reattach? This is hard to visualize. It may be my imagination playing tricks on me. I have a cardboard airfoil section in my left hand and a pipe cleaner in my right. I'm glad the Terrier is upstairs. He would be giving me grief.

 

:lol:

 

Paul

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It may be that forward rake creates a wave that squishes the hole that could lead to ventilation as that wave is forced to collapse back to ambient pressure by gravity as well as the surrounding ambient water pressure.

 

I hate it when I run out of editing time.

 

:blink:

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It may be that forward rake creates a wave that squishes the hole that could lead to ventilation as that wave is forced to collapse back to ambient pressure by gravity as well as the surrounding ambient water pressure.

 

I hate it when I run out of editing time.

 

:blink:

Interesting. Have to think about it a bit. Your right - its a bit hard to visualize. My dogs are lying quietly at my feet but terrier yapping would make it hard to think :-)

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