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10 minutes ago, MaxHugen said:

I think you would definitely get an effect from the jib if they haul in on the traveller too far. Why wouldn't it affect the main?

I agree re the mast and twist issue, which I've brought up before. My bet is that they match mast rotation to the area where they want the most power, in the lower sections where the CE is.

The jib affects the main for sure, but I wouldn't necessarily expect to visibly affect the windward side. I wish I could find those videos. It was really just a subtle luff behind the mast. Plus I doubt that at this level they would overtrim the jib. They were sailing like that on purpose. Maybe they were close to the top of the range of those sails and had to ease them more.

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When I was landsailing, tactics didn't play as big a role as they do in the America's Cup.  Speed is everything, so it didn't pay to mix it up in close quarters.  The yachts tended to do more of their

"With most single-luff mainsails, leech tension is the only means of controlling twist." Actually, not so... For one, sideways bend of the mast will induce some "non-linear" twist into the sail - the

https://www.sailingworld.com/story/racing/sails-of-the-americas-cup/ The Sails of the America’s Cup Between the skins of the AC75 mainsail lies the secrets to powering the latest generation

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10 hours ago, erdb said:

Thanks, that 20 deg twist sounds good to me. I measured 23 deg on that mull photo, and I also increased the allowable twist in my model to 20. The boats definitely like a lot of twist. Maybe that's the main reason why they went with the double skin main vs wing mast + single skin main. These mains allow a lot more control of twist and CoE.

Yes, I heard a designer (forget who) discuss the issues they had with solid wings, where they could only "twist" the entire top section of a solid wind. Drag was nasty. I think the F50s have gone with 4 separate sections to somewhat alleviate this, and also get a more gradual gradient of twist.

One thing I haven't considered is the "uplift" of the airflow as it approaches the luff of a sail. In effect, this increases the effective AoA of the sail.  This may account for some of the low AoAs we're seeing in the top sections. I have no real idea, but that uplift might increase AoA by perhaps 2-3°?

Just thinking out aloud, but in the example above the 3/4 twist is around 20°. If AWA is 15° to the boat that suggests the sail at that chord is at an AoA of -5° - not likely.  But if we add in some uplift, say 2°, that increases the chord's AoA from -5 to -3°, and that's about where my AC75 profile gets CL=0.

So... I'm now adjusting my twist angles from these observations by assessing the top chord angles as being at CL=0, and then working out the lower angles accordingly.   Can't yet say how that will work, it's still in progress.

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21 minutes ago, MaxHugen said:

I think you would definitely get an effect from the jib if they haul in on the traveller too far. Why wouldn't it affect the main?

I agree re the mast and twist issue, which I've brought up before. My bet is that they match mast rotation to the area where they want the most power, in the lower sections where the CE is.

OK I found a video, not the best, but kind of shows the spot. Around 00:45 - 00:55, then again @10:00. On this one, you can't really see the sail luffing, but you can see the telltales go a bit wobbly under the Pirelli sign:

 

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3 minutes ago, MaxHugen said:

Yes, I heard a designer (forget who) discuss the issues they had with solid wings, where they could only "twist" the entire top section of a solid wind. Drag was nasty. I think the F50s have gone with 4 separate sections to somewhat alleviate this, and also get a more gradual gradient of twist.

One thing I haven't considered is the "uplift" of the airflow as it approaches the luff of a sail. In effect, this increases the effective AoA of the sail.  This may account for some of the low AoAs we're seeing in the top sections. I have no real idea, but that uplift might increase AoA by perhaps 2-3°?

Just thinking out aloud, but in the example above the 3/4 twist is around 20°. If AWA is 15° to the boat that suggests the sail at that chord is at an AoA of -5° - not likely.  But if we add in some uplift, say 2°, that increases the chord's AoA from -5 to -3°, and that's about where my AC75 profile gets CL=0.

So... I'm now adjusting my twist angles from these observations by assessing the top chord angles as being at CL=0, and then working out the lower angles accordingly.   Can't yet say how that will work, it's still in progress.

Yeah who knows. I wouldn't completely rule out the possibility of a few degrees negative AOA at the top. There is a reason active control is allowed there, and the boats just love to lower CoE on the sails.

I think I understand what you mean by uplift, kind of the lower, higher AOA segments increasing and propagating pressure up, but I'm not sure it's happening. On the other hand,  deflection of the wind or downwash caused by the sails reduces effective AOA. That's why the vortex analysis gives a lower CL (even with zero twist) than what you would expect from the xfoil curves on 2D profiles.

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13 minutes ago, erdb said:

The jib affects the main for sure, but I wouldn't necessarily expect to visibly affect the windward side. I wish I could find those videos. It was really just a subtle luff behind the mast. Plus I doubt that at this level they would overtrim the jib. They were sailing like that on purpose. Maybe they were close to the top of the range of those sails and had to ease them more.

I think we've probably seen the results of over-trimmed sails in the videos.

When they do a maneuver like rounding a mark, they briefly go through a reach. Both main and jib must be eased out via the travellers. And then seconds later, hauled right back in. I recall in the AC35 where getting that wrong made for spectacular results!

Or the wind changes direction by a few degrees during a gust. Or...

In any event, the crew has to react very quickly on these boats, so I imagine they don't always get it perfect, especially if they're also concerned with other things, like getting hydraulic pressure back up.

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4 minutes ago, erdb said:

Yeah who knows. I wouldn't completely rule out the possibility of a few degrees negative AOA at the top. There is a reason active control is allowed there, and the boats just love to lower CoE on the sails.

I think I understand what you mean by uplift, kind of the lower, higher AOA segments increasing and propagating pressure up, but I'm not sure it's happening. On the other hand,  deflection of the wind or downwash caused by the sails reduces effective AOA. That's why the vortex analysis gives a lower CL (even with zero twist) than what you would expect from the xfoil curves on 2D profiles.

I may not have used the correct term.  By uplift, I'm referring to the change in direction of the airflow ahead of the sail. In relation to our sail, this is in the horizontal plane.

image.png.73332fde19b68dfed61ec18435dce645.png

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26 minutes ago, MaxHugen said:

I may not have used the correct term.  By uplift, I'm referring to the change in direction of the airflow ahead of the sail. In relation to our sail, this is in the horizontal plane.

image.png.73332fde19b68dfed61ec18435dce645.png

Hmm, the lifting line theory for 3D wings actually states the opposite.

Here is a good explanation: https://aviation.stackexchange.com/questions/21799/why-does-the-vortex-created-by-wing-affects-its-own-angle-of-attack

It says:

The wingtip vortices create both upwash and downwash; The downwash lies within the wingspan and affects the wing angle of attack while the upwash region lies outside the wingspan and can be utilized by another aircraft (or bird) flying behind and above the wing.

The flow induced by the downwash reduces the effective angle of attack of the (finite) wing and causes the induced drag.

WEBH6.jpg

 

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1 hour ago, Kiwing said:

Mast twist

b11.thumb.jpg.9dda3ce646abfdf942348e3ff0ccd5d6.jpg

If you look immediately after the mast the sail leaves at a very similar angle?  By the way I read about 25-27 degree twist?

I'm not sure what you are measuring?  My measurements:

image.png.8ec295852446c35c2ae1b9378d80a936.png

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20 hours ago, MaxHugen said:

Probably OK for calcs, but I would point out that the twist is likely non-linear. This is probably due to the ability to control the top section of the mainsail.

From some measurements I managed to draught from a few image collections, it looks like the crews have a fair bit of control over where they initiate twist, as well as by how much, from top 1/4 section right down to 3/4.

The headsails are different.   Here, I believe they are twisted mainly to manage the slot to the mainsail. So a low aspect sail may show a lot of twist, but when you see the slot, it's closely following the twist of the main - as far up the main as the headsail goes that is.

Camber is another significant factor in twist and resultant force calcs.  I've observed that the mast tapers and bends mostly in the top 1/4.  This flattens out the sail more in that area (and maybe the 1/2-3/4 section) than lower down, thus also contributing to a force reduction in upper sections.  Again, this effect is non-linear.

 

In the lifting line spreadsheet, camber and twist have the same effect.  Both rotate the zero lift line of the section.  You can put in a nonlinear twist (or camber) if you want.  The spreadsheet just uses a linear twist for simplicity.  Although it's not 100% the same, you could also add the shear in the apparent wind angle as an equivalent twist.

A vortex lattice code, like AVL, is more accurate than a lifting line and you could model the jib and mainsail separately.  I find it's convenient to set up a spreadsheet with the key geometric parameters and have the spreadsheet write out the AVL input file.  The vortex lattice can also give you the yawing moment, which the lifting line cannot do.

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33 minutes ago, Kiwing said:

Not sure what to call twist?

b12.thumb.jpg.4ead614dcc6a108a4f6e0b3bec2385c1.jpg

To be clear what I mean.

I see... I take the twist to be the difference between the entire chord of the sail at one height (eg the foot) to another, eg the head, &/or varying heights in between such as "1/4, 1/2. 3/4".

image.png

image.png

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7 minutes ago, Basiliscus said:

In the lifting line spreadsheet, camber and twist have the same effect.  Both rotate the zero lift line of the section.  You can put in a nonlinear twist (or camber) if you want.  The spreadsheet just uses a linear twist for simplicity.  Although it's not 100% the same, you could also add the shear in the apparent wind angle as an equivalent twist.

A vortex lattice code, like AVL, is more accurate than a lifting line and you could model the jib and mainsail separately.  I find it's convenient to set up a spreadsheet with the key geometric parameters and have the spreadsheet write out the AVL input file.  The vortex lattice can also give you the yawing moment, which the lifting line cannot do.

OK, if I understand correctly (?), AVL uses a panel method to calculate forces for all 3 axes?

Also, how do you account for increased airflow (etc) on the mainsail as a result of the jib in your spreadsheet? Or is it not significant enough to include?

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8 minutes ago, MaxHugen said:

OK, if I understand correctly (?), AVL uses a panel method to calculate forces for all 3 axes?

Also, how do you account for increased airflow (etc) on the mainsail as a result of the jib in your spreadsheet? Or is it not significant enough to include?

Yes, AVL uses vortex panels on the camber line.  They are distributed in the chordwise direction as well as the spanwise direction.

For the lifting line, the interaction between mainsail and jib is included in the section characteristics.  A multi-element section has a linear lift curve slope until separation begins, just like a single element section.  You could use Javafoil to get the combined section numbers.  You might have to give the jib some physical thickness, like 0.2% of the chord, in Javafoil for numerical reasons.  

 

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Here is my best attempt so far for polars. I've done a few runs to find the best sail combinations for each wind speed. Didn't go crazy to find every 0.1 m2 change, but like 5 m2 steps. The code 0 only works better than a jib in 7 kts of wind. For all other conditions, it's jib and main.

Are there any public Sail GP or AC50 polars out there? It would be fun to compare.

Speed polars:

speed.JPG.caf8e43f65ce06cf0d30ec059f03eaaf.JPG

Upwind and downwind targets:

upwind.JPG.3d29b9b62408415101b5406f752a17a8.JPG

downwind.JPG.42c1e569fe944ad597a4f396d438b454.JPG

Highest speed I got was 55.5 kts in 24 kts of wind at TWA 100 degrees. Cavitation was not considered ;)

Things to further experiment with: best heel angles, foil and rudder wing areas, changes in crew position, foil position etc.

 

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27 minutes ago, Basiliscus said:

Yes, AVL uses vortex panels on the camber line.  They are distributed in the chordwise direction as well as the spanwise direction.

For the lifting line, the interaction between mainsail and jib is included in the section characteristics.  A multi-element section has a linear lift curve slope until separation begins, just like a single element section.  You could use Javafoil to get the combined section numbers.  You might have to give the jib some physical thickness, like 0.2% of the chord, in Javafoil for numerical reasons. 

Thanks.

I found that XFoil also could not handle a zero thickness, eventually settling for the Eppler 376 profile for jib calcs.  Maybe a "divide by zero" issue.

Struggling to keep up with all the new apps, equations etc, that I've tried to come to grips with in 2-3 months! Wish I could learn faster. :rolleyes:

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15 minutes ago, erdb said:

Here is my best attempt so far for polars. I've done a few runs to find the best sail combinations for each wind speed. Didn't go crazy to find every 0.1 m2 change, but like 5 m2 steps. The code 0 only works better than a jib in 7 kts of wind. For all other conditions, it's jib and main.

Are there any public Sail GP or AC50 polars out there? It would be fun to compare.

Upwind and downwind targets:

upwind.JPG.3d29b9b62408415101b5406f752a17a8.JPG

downwind.JPG.42c1e569fe944ad597a4f396d438b454.JPG

Highest speed I got was 55.5 kts in 24 kts of wind at TWA 100 degrees. Cavitation was not considered ;)

Things to further experiment with: best heel angles, foil and rudder wing areas, changes in crew position, foil position etc.

Good stuff!  Note: mainsail is restricted to 135m2 - 145m2.

image.png.5560e137601a02f00af1a354fae9aaed.png

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1 hour ago, MaxHugen said:

I see... I take the twist to be the difference between the entire chord of the sail at one height (eg the foot) to another, eg the head, &/or varying heights in between such as "1/4, 1/2. 3/4".

image.png

image.png

Sorry, don't know what happened here, these diagrams were supposed to be in response to @erdb's comment re upwash/downwash - when my internet connection briefly went down!

I thought the article was contradictory - IMO. With upwash at the LE and downwash at the TE, doesn't that increase AoA as far as the sail is concerned?

An increase seems to account for the angles measured of NZ's mainsail at 24+ knots...

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9 hours ago, MaxHugen said:

Good stuff!  Note: mainsail is restricted to 135m2 - 145m2.

image.png.5560e137601a02f00af1a354fae9aaed.png

Yes, but that's only the sail portion of the main. I add the mast to that since they form an airfoil together. I measured ~ 0.65 m cord for the mast's D profile. I think it tapers on the top, but with 26.5m mast length it easily adds another 15 m2 to the sail area. Plus depending on how you measure the area sealing the main to the deck, there may be a little more there, too. I figured the max mast + sail area together could be as high as 160m2 and then chose 155 to be safe. Plus I've never measured more than that from pictures. But yes, strictly speaking, the sail area of that is only 145 m2.

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18 minutes ago, erdb said:

Yes, but that's only the sail portion of the main. I add the mast to that since they form an airfoil together. I measured ~ 0.65 m cord for the mast's D profile. I think it tapers on the top, but with 26.5m mast length it easily adds another 15 m2 to the sail area. Plus depending on how you measure the area sealing the main to the deck, there may be a little more there, too. I figured the max mast + sail area together could be as high as 160m2 and then chose 155 to be safe. Plus I've never measured more than that from pictures. But yes, strictly speaking, the sail area of that is only 145 m2.

Fair comment!  Maybe I should update my numbers too...

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14 hours ago, Kiwing said:

Does the two bumps on each side of the mast look like attachments for a structure inside the sail for shaping it? It does to me?
In which case this might make the mast rotation fixed to that level about 1.5m above deck?

They're cameras.

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30 minutes ago, MaxHugen said:

Fair comment!  Maybe I should update my numbers too...

If anything, I have to make sure I don't use a main area below 150, but since I combine the main + jib, I can always reduce the jib area to compensate.

Just read Gladwell's article on SW: https://www.sail-world.com/news/233677/Gladwells-Line-Americas-Cup-racing-starts

He says the code 0 crossover appears to be 9 kts, which seems a little high to me, and that boats can go as fast as  3 to 4x wind speed.

My best boat speed to wind speed ratios are 28.9 kts of boat speed in 8 kts of wind and 25.9 kts of speed in 7 kts of wind at 100 deg of TWA. That's 3.6x and 3.7x wind speed, so we're pretty close there:))

 

 

 

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I think it's pretty clear from this video that they can force the sail into negative AOA. At least that's the only way I can explain that sail shape around 2:05.

The actively controlled top and bottom parts are pushed against the wind while the passive mid section is blown in by the wind.

 

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33 minutes ago, Kiwing said:

2082150312_TRleech1.thumb.jpg.9bc839a4381e33e3563903de5adbfea7.jpg1870427144_TRleech2.thumb.jpg.ac801cb53b3b0b26f23c74c2e09c5cc9.jpg614328748_TRleech3.jpg.2eead94f8a52b689fb0e395b31ee8c8b.jpg

That Magic is beginning to show?

Thanks for clipping those images. So that hooked middle section cannot have any active control. Do they have runners causing that maybe? 

I also looked at the class rules, but couldn't figure out whether the point of AM's batwing is to get away with a smaller main but still comply with the girth requirements. The rules just state 

20.12 For a specific mainsail measurement length or girth, the greatest value from all sail skins shall be taken. 

Can they satisfy the girths just by using longer battens?

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On 9/11/2020 at 3:16 PM, RobG said:

only if someone in authority wanted to be a prick.

And when did anyone in authority ever want to be a prick, right?

Phil Goff's band of happy little hitlers will be happy to oblige I am sure.

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Well you are crunching figures "pedal to the metal", honostly I can't keep up, but I read.

Id like to come back to VORTEX, and try to share my rookie experience.

The Downwash is the key. To get familiar with this concept, it is useful to start with the classic elliptical case, for 2 reasons:

1-According to the theory, the downwash is constant (same value at the head and at the foot)  you just need to iterate one value of downwash to find the righting moment you want to get, and the AoAttack is the same along the span, the Lift Coef too.

2-As the CoE of an elliptical "surface" is known a-priori (42% of the span) it makes the  "manual" iteration easy.

Then when you get this constant downwash value for your "case" ( ie 2 °), then you can try a linear decreasing downwash from 4° at the     foot to 0° at the head.

In this case you might notice (AFAIR) VORTEX will create -4° twist at the foot, in order to keep effective AoA constant or at least it is what I guess, and incidentaly, it can provide an explanation to the mainsails or wingsails' foot which seem to go windward relative to the boat centerline.

You can also try a negative downwash at the head with a positive at the foot, you will then mimick the Bell shaped lift distribution.

To make it simple, AFAIR, the theory for minimun induced drag, with or without bending moment constraint states that The downwash is always a straight line.

So if you specify a straight downwash distribution which meets your AWS and righting moment, VORTEX will make the solver job for you.

Cheers

 

 

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4 hours ago, Erwankerauzen said:

Well you are crunching figures "pedal to the metal", honostly I can't keep up, but I read.

Id like to come back to VORTEX, and try to share my rookie experience.

The Downwash is the key. To get familiar with this concept, it is useful to start with the classic elliptical case, for 2 reasons:

1-According to the theory, the downwash is constant (same value at the head and at the foot)  you just need to iterate one value of downwash to find the righting moment you want to get, and the AoAttack is the same along the span, the Lift Coef too.

2-As the CoE of an elliptical "surface" is known a-priori (42% of the span) it makes the  "manual" iteration easy.

Then when you get this constant downwash value for your "case" ( ie 2 °), then you can try a linear decreasing downwash from 4° at the     foot to 0° at the head.

In this case you might notice (AFAIR) VORTEX will create -4° twist at the foot, in order to keep effective AoA constant or at least it is what I guess, and incidentaly, it can provide an explanation to the mainsails or wingsails' foot which seem to go windward relative to the boat centerline.

You can also try a negative downwash at the head with a positive at the foot, you will then mimick the Bell shaped lift distribution.

To make it simple, AFAIR, the theory for minimun induced drag, with or without bending moment constraint states that The downwash is always a straight line.

So if you specify a straight downwash distribution which meets your AWS and righting moment, VORTEX will make the solver job for you.

Cheers

I actually started out with the vortex sheet you posted, which helped a lot in understanding how it works. I plugged in some data I had from xfoil, and used four different sail plans, main+ code0, jib#1, jib#2, jib#3 and plugged in the chord dimensions accordingly in the analysis tab. Then wrote a macro in excel, which basically does what you're suggesting, changing the foot and head AOA step-by-step, to produce hundreds of combinations of AOA and twist and use the vortex sheet to calculate cl, cdi and center of effort numbers for each AOA-twist combination. I use these tables in the model to see what sail trim is needed to achieve balance in a certain condition. It works pretty well now, and I'm just playing with the model plugging in various numbers to see what comes out.

For example, I did a bit of analysis on the question of how maximum twist in the sails affects performance, and the result is that for upwind and downwind vmg sailing it actually doesn't make a huge difference. The biggest difference is at high wind speeds and reaching angles, which they are not going to spend much time doing.

Here is the data for 24 kts wind (upper wind limit) with 20, 15 and 10 degrees maximum twist (indicated as tw20, tw15 and tw 10):

speed_cr.png.90e29f5ce69180aff480e0c68d060e4d.png

At upwind and downwind angles (left and right ends of the lines) there is basically no difference in speed. Upwind speed difference is like 0.3 kts between 20 and 10 deg twist. However, with only 10 deg twist, I couldn't balance the boat at 90 and 110 deg TWA even with the smallest jib. That's where the red arrows are. They are really only going to sail those angles at mark roundings or maybe in a dial-down situation, but I have a feeling no one will risk that in 24 kts of wind - even though it would be spectacular playing chicken at 110 kts closing speed!:o So for those brief moments, they don't actually have to reach a static balance that's modeled by my calculations, they could just depower for a moment by letting the jib luff or something and once they are out of the "death zone" they can sheet in again.

Just for fun here are some details that the model spits out:

Angle of attack at the foot; twist of the head, and height of the center of effort.

1610353082_sailaoa.png.c379c98b1e3029b736c52851f0414e07.png  twist.png.0d6c23740f81830cfea89343736adbb1.png    coe.png.115f366c2b2ffcaee804791658482fff.png  

The model always chooses the highest boat speed where things can be balanced and it's interesting that except for upwind / downwind vmg angles, in strong winds, it always goes for the max twist allowed. What happens is that if I force the model to only accept maximum 10 deg twist, it will just reduce the AOA to compensate - make sense. However, it has an effect on the height of the center of effort (shown on the right). The lower the twist, the higher the center of effort, which in turn affects foil cant angle etc - not shown here, and that's where the speed differences come from.

Finally here is the actual AOA of the head, and this is where we were speculating how much negative AOA can be achieved with active sail control at the head of the sail. In 24 kts of wind, even with only 10 deg of twist, the AOA at the head goes a few degrees negative to balance the boat when sailing higher than 120 deg TWA.

1080499264_Headaoa.png.531233ee79290fae956faca7e57ea919.png

There are of course quite a few limitations with these calculations, because the vortex sheet looks at the sail plan as one airfoil so it's hard to judge what these few degrees of negative AOA mean in practice where you have separate jib and main twist and the possibility of depowering by other means such as feathering. It's just a simple model:)) 

 

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@erdb, at what sort of Vb are you getting the transition of the stabilator from positive to negative lift? Since I've been adding twist, I'm seeing a lot less downforce (negative) required, as expected. Just like to get a comparison, see if I'm in the ball park.

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55 minutes ago, MaxHugen said:

@erdb, at what sort of Vb are you getting the transition of the stabilator from positive to negative lift? Since I've been adding twist, I'm seeing a lot less downforce (negative) required, as expected. Just like to get a comparison, see if I'm in the ball park.

I only get positive rudder lift when boat speed is ~24 kts or less. This only happens upwind in 10 kts or less wind.

501650879_Posrudder.thumb.png.e47f294cb957b4e058673dc0942c940e.png

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On 12/9/2020 at 9:14 AM, erdb said:

I think it's pretty clear from this video that they can force the sail into negative AOA. At least that's the only way I can explain that sail shape around 2:05.

The actively controlled top and bottom parts are pushed against the wind while the passive mid section is blown in by the wind.

 

That is clearly the runner. 

Furthermore, the rigid wing always wins because of the slot.  Let that be the end of it.

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31 minutes ago, Kiwing said:

Having a discussion with @Stingray~ about solid wing verse this soft wing.

I am suggesting VMG comparison FGP verse ETNZ.

What are your thoughts.  Do these soft wing play in the same ballpark as solid wings?

The AC 75's will monster the AC 50's on VMG becasue of the speed of the turns that they are achieving. Refresh your memory of how an AC50 tacks then come back.

This was talked about 6 or more months ago when TNZ issued a Video showing multiple gybes into a leward mark which was rounded smartly and then immediately tacked away - all with little appreciable reduction in boatspeed. 

It's not a Soft vs Hard wing issue - its the entire paradigm change of the package.

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8 minutes ago, Kiwing said:

@Boink 

It is soft verse hard that @Stingray~ and I are discussing.

What is your opinion these soft wing ?% of solid wings in similar foiling conditions?

Most people were suggesting around 60% in the beginning. Now I think that has risen to 70%-100%

I hear you. Some smart cookie will run the numbers - but that will be post Cup. I will say that I have always supported the concept - as being a much more manageable solution than solid wings - and give these 2 or 3 more cup cycles or development and iteration and you will see trickledown into other boats. At that point, the comparison will be entirely moot.

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1 hour ago, Kiwing said:

Having a discussion with @Stingray~ about solid wing verse this soft wing.

I am suggesting VMG comparison FGP verse ETNZ.

What are your thoughts.  Do these soft wing play in the same ballpark as solid wings?

OK had to download the SailGP app to get some numbers. Downwind VMGs were also all over the place, but upwind VMGs were relatively steady.

Anyway the approximate results vs my AC 75 polars:

Reaching:

under 10kts of wind, the AC75 should be faster by 8-9 kts

15 kts of wind: 5 - 8 kts faster

20+ wind:  7-8 kts faster. 

 

Upwind:

At the very low end, 7-8 kts, VMGs are similar (~10kts), but the AC75 would point lower and sail much faster.

9-10kts of wind: AC75 is 7kts faster, VMG 5kts higher

13-15kts: speed is 2-3 kts higher, VMG 3-4 kts higher

17-18kts: speed 2-3 kts higher, VMG is 5-6kts higher (much tighter angles than the cat)

20+ kts, 5-6kts faster, VMG 6-8 kts higher. 

 

Downwind (have no idea about angles and VMG for the cats, but I compare speeds to what would be best VMG for the AC75):

under 10 kts, cats were all over the place depending on who could get on foils and I bet the AC75 would be the same

7-8kts, AC75 0 - 3 kts faster

10-15kts: 0 - 3 kts faster

18 - 20 kts: 3 - 5kts faster

20+kts 2 - 10 kts faster (huge variations in the cat's speed - survival mode / puffs)

 

 

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My thoughts on the soft main is that beyond the practical reasons, it may be better for the AC75, because the sail needs to be twisted out a lot.

The way I see it, the canted foil on these boats fundamentally changes how the forces are generated. On basically every boat, your righting moment is given, and you adjust the sail to balance that righting moment. On the AC75, as you twist the sails and lower the center of effort, the foil can be canted out more, which means you can increase the sail forces further.

I know the C-cat wings can be twisted, and as I recall, the NZ wing could also be twisted in San Francisco, but Oracle chose a simpler wing with no twist in the main element. I think the AC50 wings didn't twist either, only the flaps. 

So it's possible you could make the AC75 faster with a wing, but it would have to be a more complex twisting wing making it even less practical.

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Question for those of you familiar with how sails are measured. The AC class rule specifies min and max girth measurements for the main.

Can AM get away with making a smaller sail with only the batten ends satisfying those girth measurements?

Team New Zealand and American Magic on the America's Cup practice course during Tuesday's official session.

I found nothing in the class rules on how the girths are measured. In strong winds, it might pay to use a smaller sail instead of twisting it off.

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54 minutes ago, erdb said:

Question for those of you familiar with how sails are measured. The AC class rule specifies min and max girth measurements for the main.

Can AM get away with making a smaller sail with only the batten ends satisfying those girth measurements?

Team New Zealand and American Magic on the America's Cup practice course during Tuesday's official session.

I found nothing in the class rules on how the girths are measured. In strong winds, it might pay to use a smaller sail instead of twisting it off.

I looked into this when the "bat wing" was first seen.  AFAIK:

The area is defined in the Rules as :
image.png.032599bf65f55234f7f6016bd639a164.png

I understand that the idea of the Rules was to prevent concave leech measurements from being used to achieve a higher sail area than technically allowed.

The US has taken advantage of this, I suspect, by making the sail actually go out to the batten ends (from a couple of close up shots). Not much in terms of area, but I suppose enough to call it "part" of the sail. It looks something like this:

image.png.3c1bb63846dfab90e845a14e4dbd9bdf.png

It would certainly help to reduce actual sail area in strong winds - twisting is all well and good without better options, but it brings with it unwanted drag. Top battens look like they are lining up with the Rules diagram.

image.png.d90bd8e2729d3ea21cec7c04a47104b1.png

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18 minutes ago, Mozzy Sails said:

ENTZ look like they're getting a lot of bend in to their mast, certainly compared to the italians in the latest video. Aren't the masts on design? Or just one design in shape, but not one design in function? 

I believe ETNZ have noted a few times that unlike the true one design parts, just the mast's dimensions are specified - but the manufacture and layup, thickness, etc is up the the teams.

 

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40 minutes ago, rh3000 said:

I believe ETNZ have noted a few times that unlike the true one design parts, just the mast's dimensions are specified - but the manufacture and layup, thickness, etc is up the the teams.

Per the rule:

20.1  A drawing package for the mast will be issued according to Rule 34. This mast drawing package will include:

(a)  the mast surface;

(b)  the minimum required mast tube laminate and construction details;

The mast must comply with minimum laminate and construction details, so if ENTZ's mast is bendier, it's likely because it's closer to the minimum requirements than the others, or more rig tension is being applied. As far as I know, the mast drawing package is not public.

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6 hours ago, erdb said:

I only get positive rudder lift when boat speed is ~24 kts or less. This only happens upwind in 10 kts or less wind.

OK, good. Ever since I started balancing the transverse moments, the stabilator had to provide lift - no downforce - until the boat hit about 40 knots, then needed a tad.

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5 hours ago, Kiwing said:

Having a discussion with @Stingray~ about solid wing verse this soft wing.

I am suggesting VMG comparison FGP verse ETNZ.

What are your thoughts.  Do these soft wing play in the same ballpark as solid wings?

It might be horses for courses. For the AC75 design, which is inherently unstable as it balances on 2 points, the ability to have infinite control over both twist and camber gives the twin skin some advantages over the solid sail.

At a different extreme, an ice yacht would probably be better off with a solid wing, or wing mast+narrow single skin, as these can maintain a decent L/D ratio at very narrow AoAs which is needed at their SOG.

I think.

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Sorry ERDB, my comment came a bit late,  I did't notice you had already incorprate VORTEX in your VPP.

I guess that very soon, if all sailing datas will be disclosed on screen during the first races, you and Max will get some reliable elements to calibrate your VPP.

I would dream to be a VBA 's Mozart like you are, but unfortunatly, that is not the case that is why cannot provide significant contribution.

Cheers

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8 hours ago, MaxHugen said:

I looked into this when the "bat wing" was first seen.  AFAIK:

The area is defined in the Rules as :
image.png.032599bf65f55234f7f6016bd639a164.png

I understand that the idea of the Rules was to prevent concave leech measurements from being used to achieve a higher sail area than technically allowed.

The US has taken advantage of this, I suspect, by making the sail actually go out to the batten ends (from a couple of close up shots). Not much in terms of area, but I suppose enough to call it "part" of the sail. It looks something like this:

image.png.3c1bb63846dfab90e845a14e4dbd9bdf.png

It would certainly help to reduce actual sail area in strong winds - twisting is all well and good without better options, but it brings with it unwanted drag. Top battens look like they are lining up with the Rules diagram.

image.png.d90bd8e2729d3ea21cec7c04a47104b1.png

Nice little loophole. Interesting the rule didn't consider this option or they weren't really concerned about someone showing up with a smaller sail. But then why specify a minimum value for those girth measurements? On the other hand, while this may be advantageous upwind in strong winds, they may lose some performance during acceleration and downwind.

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7 hours ago, MaxHugen said:

OK, good. Ever since I started balancing the transverse moments, the stabilator had to provide lift - no downforce - until the boat hit about 40 knots, then needed a tad.

By 40 kts, I have substantial rudder downforce,  5 to 7.5 kN. A lot depends on the foil cant angle, since that determines how much vertical lift you get for a given lateral force on the foil, and that foil vertical lift needs to balance out with the rudder force.

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4 hours ago, Erwankerauzen said:

Sorry ERDB, my comment came a bit late,  I did't notice you had already incorprate VORTEX in your VPP.

I guess that very soon, if all sailing datas will be disclosed on screen during the first races, you and Max will get some reliable elements to calibrate your VPP.

I would dream to be a VBA 's Mozart like you are, but unfortunatly, that is not the case that is why cannot provide significant contribution.

Cheers

Yeah soon I'll find out how wrong I was:))  I'm confident that the model describes the general forces and their relationships properly, but there is quite a bit of guesswork involved in calculating hull,  foil and sail drag components. So a couple of knots error here and there is absolutely expected. And again, your vortex file was a big help in figuring out  how to use that worksheet and how to incorporate it in the model.

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4 minutes ago, erdb said:

By 40 kts, I have substantial rudder downforce,  5 to 7.5 kN. A lot depends on the foil cant angle, since that determines how much vertical lift you get for a given lateral force on the foil, and that foil vertical lift needs to balance out with the rudder force.

I had a closer look at your diagrams, and realise I misread your post - thought you were referring to TWS!  I now have the transverse moments balancing reasonably, given a limited number of data points, and the stabilator and foils at target levels:

image.png.b1daf1629c2f29323eb3f580c3cadb47.png

This is just at a fixed AoA of 15°, but it shows no downward force until ~22 knots TWS.

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23 minutes ago, erdb said:

Nice little loophole. Interesting the rule didn't consider this option or they weren't really concerned about someone showing up with a smaller sail. But then why specify a minimum value for those girth measurements? On the other hand, while this may be advantageous upwind in strong winds, they may lose some performance during acceleration and downwind.

I think AM just figured out a loophole in the Rules. Question is, is it worth it, for as you said there are also downsides?

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^ Heeling and righting moments:
image.png.c1aa399f83e0956321d5149407a16c77.png

From about 55 knots AWS (upwind) I can no longer balance these. That's using the smallest main (135m2 skin area) at just 4% camber, and so much twist only the lower third is probably producing any driving force. CE is right down to 6.8m from the foil axis!

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53 minutes ago, MaxHugen said:

I had a closer look at your diagrams, and realise I misread your post - thought you were referring to TWS!  I now have the transverse moments balancing reasonably, given a limited number of data points, and the stabilator and foils at target levels:

image.png.b1daf1629c2f29323eb3f580c3cadb47.png

This is just at a fixed AoA of 15°, but it shows no downward force until ~22 knots TWS.

What's the TWA and how do you come up with the target forces?

On my polar plots, TWA is on the X axis and plots for different TWS are shown in different colors.

It seems the baseline righting moments are similar in your calcs and mine, but then I get a huge increase in righting moment from simultaneous increases in foil lift and rudder downforce.

If you keep these forces at fixed target points, then you won't have the necessary increase in righting moment. This is what I keep saying - just probably not very convincingly - that these boats are unique in this that the righting moment is not a fixed value. The more you push the foil sideways, the more lift it generates therefore righting moment can keep going up as long as you can balance it with rudder downforce.

Here are some of the other parameters:

Speed, heel and pitch moments:

  speed.png.be68d473377417d04b2705728f3a7b95.png  958813874_heelandpitchmom.png.3d1789fe0f7f42003241f0fae908e9b9.png

 

Foil lateral force (Foil_y, negative means pointing to windward), foil vertical force (Foil_z) and rudder force (Rudder_z; positive is up):foil_x_z.png.2dcddea95267e767b6a219070f3d7e14.png  rudder.png.d6c9152a421edd25e9b15980f7557233.png

The difference in foil vertical lift and rudder downforce is the weight of the boat+crew.

Sail CoE measured from mast base; driving force (sail_x) and heeling force (sail_y):

893557985_sailcoe_forces.png.c83533c4531ea8842d988da013daadf0.png

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3 hours ago, erdb said:

Nice little loophole. Interesting the rule didn't consider this option or they weren't really concerned about someone showing up with a smaller sail. But then why specify a minimum value for those girth measurements? On the other hand, while this may be advantageous upwind in strong winds, they may lose some performance during acceleration and downwind.

No measurer I have ever worked with would accept the proposition that the leech hollow would be measured to the artificially poked out batten ends. ERS is pretty clear on the subject:

image.png.0bfa6c0c00ca5f67177e9353e3e70330.png

The AC & Olympic measurers I worked with would use the intersection of the projected actual sail edges to determine the points for the straight to bridge the hollow. 

 

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1 hour ago, erdb said:

What's the TWA and how do you come up with the target forces

I'm using a TWA of 45° for now, and AWA of 15°.  Later I'll see if I can bend and twist these in a spreadsheet!

My methodology:

  1. Calc basics, like AWS, Vb etc
  2. Use moments to calc Boat Mass supported by Foil and Stab (close to 80% : 20%)
  3. Add Sail Drive Force moment and Stab Boat Mass moment = Stab Target
  4. Adjust Stab Angle until ±Lift Force vs Stab Target = Zero.
    Boat is now balanced longitudinally.
  5. Add Foil Lift required to support Foil Boat Mass + Stab Force = Foil Target
  6. Adjust Foil Flap Angle until Lift Force vs Foil Target = Zero
    Boat now can fly
  7. Calc RM from Boat Mass ± Stab Force (modified by Foil Cant y-axis distance) = Available RM
  8. Adjust Sail Sizes, Camber and Twist until Heeling Moment (modified by Foil Cant z-axis distance) vs Available RM = Zero
    Boat is almost balanced transversely
  9. As any change to Sails then affects Sail Drive -> Stab Lift -> Sail Heel, rerun calcs 3-8. I found that 2 iterations balanced it out.
    Boat is balanced transversely
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34 minutes ago, A3A said:

No measurer I have ever worked with would accept the proposition that the leech hollow would be measured to the artificially poked out batten ends. ERS is pretty clear on the subject:

image.png.0bfa6c0c00ca5f67177e9353e3e70330.png

The AC & Olympic measurers I worked with would use the intersection of the projected actual sail edges to determine the points for the straight to bridge the hollow.

If I put on my two bob lawyer's hat, I would point out out that H.5.2 above refers to a measurement point that falls between battens. From the rough and ready diagram I posted earlier, it looks like the critical top two battens could be right at the measurement points!

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26 minutes ago, MaxHugen said:

If I put on my two bob lawyer's hat, I would point out out that H.5.2 above refers to a measurement point that falls between battens. From the rough and ready diagram I posted earlier, it looks like the critical top two battens could be right at the measurement points!

They may well be, but projecting the actual sail edges from either side will still end up inside the batten end, and if they are trying to make a small sail measure big, it fails. 

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5 hours ago, erdb said:

Nice little loophole. Interesting the rule didn't consider this option or they weren't really concerned about someone showing up with a smaller sail. But then why specify a minimum value for those girth measurements? On the other hand, while this may be advantageous upwind in strong winds, they may lose some performance during acceleration and downwind.

The Italian Guys made a video discussing this loophole soon after the cut down main appeared. 

Edit: here

 

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2 hours ago, barfy said:

The Italian Guys made a video discussing this loophole soon after the cut down main appeared. 

Edit: here

 

These "Italian Guys" try your patience, but I love them. Refreshing, yet educated, view of the AC. Wish them well.

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2 hours ago, barfy said:

The Italian Guys made a video discussing this loophole soon after the cut down main appeared. 

Edit: here

 

Another AC innovation that will be consigned to the dust heap to history me thinks. 

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4 hours ago, MaxHugen said:

I'm using a TWA of 45° for now, and AWA of 15°.  Later I'll see if I can bend and twist these in a spreadsheet!

My methodology:

  1. Calc basics, like AWS, Vb etc
  2. Use moments to calc Boat Mass supported by Foil and Stab (close to 80% : 20%)
  3. Add Sail Drive Force moment and Stab Boat Mass moment = Stab Target
  4. Adjust Stab Angle until ±Lift Force vs Stab Target = Zero.
    Boat is now balanced longitudinally.
  5. Add Foil Lift required to support Foil Boat Mass + Stab Force = Foil Target
  6. Adjust Foil Flap Angle until Lift Force vs Foil Target = Zero
    Boat now can fly
  7. Calc RM from Boat Mass ± Stab Force (modified by Foil Cant y-axis distance) = Available RM
  8. Adjust Sail Sizes, Camber and Twist until Heeling Moment (modified by Foil Cant z-axis distance) vs Available RM = Zero
    Boat is almost balanced transversely
  9. As any change to Sails then affects Sail Drive -> Stab Lift -> Sail Heel, rerun calcs 3-8. I found that 2 iterations balanced it out.
    Boat is balanced transversely

You make quite a few assumptions that may or may not be true. You assume a boat speed, and the AWA that goes with it, and you assume the proportion of foil lift and rudder lift, but there is nothing that says these are the right values for that TWA and TWS. If your equations are correct, but they don't balance out with your input parameters, you have to accept that your input parameters were not the right combination and have to test a new set of input parameters.

Basically this is what my algorithm does. For every wind strength and TWA, I run hundreds of combinations of boat speed, rudder downforce and foil cant angles. Only a very small number of these combinations will balance out all the equations (forces in 3 dimensions + pitch and heel moments), and from those combinations, I choose the one that has the highest boat speed. Those polar curves I posted only show data points where everything was balanced for the highest possible boat speed for a given TWA and TWS.

Some additional notes: the 80 - 20% foil - rudder wing lift ratio is very far from what I see. Even when the rudder lift is positive, the highest number I get is like 3% rudder lift, 97% foil lift. In most situations the rudder exerts downforce, so the foil is more than 100% of the weight. Also, the foil cant angle and how that determines the ratio of lateral and vertical foil forces is a key factor in my equations.

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5 hours ago, A3A said:

No measurer I have ever worked with would accept the proposition that the leech hollow would be measured to the artificially poked out batten ends. ERS is pretty clear on the subject:

image.png.0bfa6c0c00ca5f67177e9353e3e70330.png

The AC & Olympic measurers I worked with would use the intersection of the projected actual sail edges to determine the points for the straight to bridge the hollow. 

 

Interesting thanks. I wonder when it's going to go to the Arbitration Panel. Or maybe the other teams are happy to see AM sail around with a smaller main...

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24 minutes ago, erdb said:

Interesting thanks. I wonder when it's going to go to the Arbitration Panel. Or maybe the other teams are happy to see AM sail around with a smaller main...

First, it's conjecture they are trying to build a sail under the minimum girths; it may be they are legal either way. Second, they haven't raced anything yet, and you can test whatever you want. As I've said before, my guess this is an older main recut to test a smaller size heavy air concept that doesn't burn a rule limited set of skins or the cost of a full set of battens and controls by using existing ones. If it tests well, then you build it all proper and go race. 

Since AM is the only team looking this way, and the other three teams are North powered with North designers responsible for rig aero, it could be the North guys are all misled. Or maybe the Quantum/Doyle membranes can't achieve as much range of control as the 3Di Raw membranes, so they have to go for smaller size;-)

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2 minutes ago, A3A said:

First, it's conjecture they are trying to build a sail under the minimum girths; it may be they are legal either way. Second, they haven't raced anything yet, and you can test whatever you want. As I've said before, my guess this is an older main recut to test a smaller size heavy air concept that doesn't burn a rule limited set of skins or the cost of a full set of battens and controls by using existing ones. If it tests well, then you build it all proper and go race. 

Since AM is the only team looking this way, and the other three teams are North powered with North designers responsible for rig aero, it could be the North guys are all misled. Or maybe the Quantum/Doyle membranes can't achieve as much range of control as the 3Di Raw membranes, so they have to go for smaller size;-)

Yeah, you could be right. This is kind of what I thought first, just testing a different sized main while keeping the internal control structure and battens of their standard main, but I was surprised to see it again. I figured if they liked the idea they would have made a properly sized structure for it by now.

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47 minutes ago, erdb said:

Some additional notes: the 80 - 20% foil - rudder wing lift ratio is very far from what I see. Even when the rudder lift is positive, the highest number I get is like 3% rudder lift, 97% foil lift. In most situations the rudder exerts downforce, so the foil is more than 100% of the weight. Also, the foil cant angle and how that determines the ratio of lateral and vertical foil forces is a key factor in my equations.

The 80-20% is not a lift ratio. It is the Boat Mass distribution between the two foils.   For example, at 15 kns TWS, the net force distribution I get between foil and stab is ~94.9 : 5.1 %.

I am calculating one data set for now:  given 3 parameters: TWS (as a range), TWA and AWA,  at what point do all forces balance out - if they even can balance out.  These parameters are not assumptions, they are inputs to determine  an outcome. If I ran hundreds of calcs, by varying 2 of those params (TWA and AWA), some would not resolve, but I would be eventually able to determine the best VMG for one of the params - TWS.

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1 hour ago, A3A said:

First, it's conjecture they are trying to build a sail under the minimum girths; it may be they are legal either way. Second, they haven't raced anything yet, and you can test whatever you want. As I've said before, my guess this is an older main recut to test a smaller size heavy air concept that doesn't burn a rule limited set of skins or the cost of a full set of battens and controls by using existing ones. If it tests well, then you build it all proper and go race. 

Since AM is the only team looking this way, and the other three teams are North powered with North designers responsible for rig aero, it could be the North guys are all misled. Or maybe the Quantum/Doyle membranes can't achieve as much range of control as the 3Di Raw membranes, so they have to go for smaller size;-)

That seems pointless unless AM's lawyers are confident they can circumvent the clear intent of the Rule, and get away with a sail that is factually smaller than the minimum stipulated.  :angry:

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9 minutes ago, MaxHugen said:

The 80-20% is not a lift ratio. It is the Boat Mass distribution between the two foils.   For example, at 15 kns TWS, the net force distribution I get between foil and stab is ~94.9 : 5.1 %.

I am calculating one data set for now:  given 3 parameters: TWS (as a range), TWA and AWA,  at what point do all forces balance out - if they even can balance out.  These parameters are not assumptions, they are inputs to determine  an outcome. If I ran hundreds of calcs, by varying 2 of those params (TWA and AWA), some would not resolve, but I would be eventually able to determine the best VMG for one of the params - TWS.

I don't quite understand the 80-20 and the 95 - 5. The vertical forces include weight, foil vertical and rudder vertical. These three together have to add up to zero. If both rudder and foil lift up, and the weight is distributed 80-20, then the lift forces also should be 80-20.

Anyway, my point is that both this lift distribution and the AWA you start with are just guesses/assumptions/predictions. You don't know when you start if it's possible to achieve balance with these parameters. That's why you run it through your calculations, and if they do balance out, then it's possible to sail the boat like that. If they don't balance out, it just tells you that you have to choose different input parameters. 

For every round of calculation, I start out with one TWA and one TWS value (not a range of TWS, that comes later). So for one given TWA / TWS pair, I run the calculation through all the equations with a range of input parameters (boat speed, rudder down force and foil cant). In an earlier version, my input parameters were boat speed and sail AOA. So there are several ways to do this, but because there are so many different possible combinations, you have to guess some of the parameters and then try to calculate the rest. If everything goes through and balances out, then your guess was good, it's a valid data point. Now you can go back and calculate another TWS/TWA situation. Once you do a whole range of those, you get polars.

If you set up all your calculations in cells of a single row, you can create one worksheet for each TWA/TWS pair you'd like to test. The first few columns are your input parameters (assumptions) and just pull down the formulas next to them to calculate balance in each row. You can have a column that gives you an overall balance of forces and moments. If that's close to zero, you found a combination of input parameters where things balance out.

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3 hours ago, uflux said:

Another AC innovation that will be consigned to the dust heap to history me thinks. 

Sportsmanship has been reduced to "in case we can't win fair and square, let's legally beat them first, so we can cheat".

The critical top section of that sail has been reduced by 17% in area!

image.png.18d18f0d953b1d31a4df6a09f29445b0.png

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