Kiwing

The new sailing twin skin setup

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4 hours ago, Kiwing said:

pictures from on top of Britiannia

Britiana.thumb.jpg.41f62709e9de031aaf5fcc713299ef96.jpg

from above !!

Not quite as good, but a bit of a comparison to, Patriot.

Screen Shot 2020-11-27 at 1.13.23 PM.png

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54 minutes ago, Stingray~ said:

The powerful mainsail camber that ETNZ used in the recent GMR video of them lifting out after a bareaway in seemingly only 5 knots of true wind was pretty amazing, that thing was deep. 

Sure was. Now, would you work out the percentage camber for me pls? :)  Oh, and I need that at every "draft  stripe", OK?

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looking at the mast head control lever, too.

b11.thumb.jpg.85e4fa4fca333eae53e89969e87c32b1.jpg

rotating around that black dot pivot?

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3 hours ago, MaxHugen said:

Sure was. Now, would you work out the percentage camber for me pls? :)  Oh, and I need that at every "draft  stripe", OK?

Draw the quadrilateral of the head, the tack, the clew and the end of the top batten. Divide the “verticals” by the number of stations you want and draw a line between them. (Or by proportion if you require a specific height). Stretch the tack/clew length so that it is correctly proportional relative to the head and takes account of mast rake.That gives you the twist changes. You know the profile at the bottom and the top, you can morph/proportion them for the intermediate stations on the windward skin. By guessing the profile of the leeward skin across the bottom, you already have the top leeward skin profile, so repeat, and you should have a pretty good approximation of the whole foil between the skins..

You’re welcome........

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The masthead shots also highlights  something that @Basiliscus touched on earlier. That is, that maybe at AC75 size, the mast section width necessary and skin separation achievable means that a twin skin set up is worth while, especially as you have enough room between the skins to fit controls. At that size a D mast and two skins probably is much lighter than a wing mast and one skin.

On smaller craft, the mast section width required and insufficient skin separation means that a wing mast and single skin sail is the way to go?

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3 hours ago, Sidecar said:

Draw the quadrilateral of the head, the tack, the clew and the end of the top batten. Divide the “verticals” by the number of stations you want and draw a line between them. (Or by proportion if you require a specific height). Stretch the tack/clew length so that it is correctly proportional relative to the head and takes account of mast rake.That gives you the twist changes. You know the profile at the bottom and the top, you can morph/proportion them for the intermediate stations on the windward skin. By guessing the profile of the leeward skin across the bottom, you already have the top leeward skin profile, so repeat, and you should have a pretty good approximation of the whole foil between the skins..

You’re welcome........

It was a joke of course....

image.png.5ec2baa9ff9b7b8af34284503a3ca616.png

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Need some help from experienced racers!

Question re headsail shapes at the luff... is there a "deeper" curve just after the luff, than in the rest of the sail generally?

That is, more like (A) than (B) in the diagrams below?

image.thumb.png.d9d218861e4fc7ad9ce57286c53daf50.png

image.png.857af733ad832c458938897a585b9fa4.png

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7 minutes ago, MaxHugen said:

Need some help from experienced racers!

Question re headsail shapes at the luff... is there a "deeper" curve just after the luff, than in the rest of the sail generally?

That is, more like (A) than (B) in the diagrams below?

image.thumb.png.d9d218861e4fc7ad9ce57286c53daf50.png

image.png.857af733ad832c458938897a585b9fa4.png

Depends on the way you trim it.

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4 minutes ago, nroose said:

Depends on the way you trim it.

I'm not referring to moving the camber forward/aft using the sheet... more if the sail will "naturally" exhibit a slightly deeper curve, only in the immediate area aft of the luff track, and presumably at lower wind speeds and greater camber.

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4 minutes ago, MaxHugen said:

I'm not referring to moving the camber forward/aft using the sheet... more if the sail will "naturally" exhibit a slightly deeper curve, only in the immediate area aft of the luff track, and presumably at lower wind speeds and greater camber.

It depends on how the sail is cut and how much luff tension you put on. In general, the more curvature you have close to the luff, the more forgiving it is for changing AOA, but if you want to sail close to wind with tight AWA, you need low luff tension - "speed wrinkles" and the camber moved further aft.

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38 minutes ago, MaxHugen said:

Need some help from experienced racers!

Question re headsail shapes at the luff... is there a "deeper" curve just after the luff, than in the rest of the sail generally?

That is, more like (A) than (B) in the diagrams below?

image.thumb.png.d9d218861e4fc7ad9ce57286c53daf50.png

image.png.857af733ad832c458938897a585b9fa4.png

Others have pointed out that it depends how you cut and tune it, but generally speaking a rounder entry is more forgiving. You can imagine that a flat entry is very sensitive to apparent wind angle. It can back wind quickly. A rounder entry offers a broader groove, but is higher drag. 
 

In 470s we used to used to talk about having more or less round forward depending on the skipper’s need to steer more (choppy water for instance).  Depending on the skipper’s propensity to get distracted, or have the need to look around (big fleet/small fleet)

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1 hour ago, Ex-yachtie said:

Others have pointed out that it depends how you cut and tune it, but generally speaking a rounder entry is more forgiving. You can imagine that a flat entry is very sensitive to apparent wind angle. It can back wind quickly. A rounder entry offers a broader groove, but is higher drag. 
 

In 470s we used to used to talk about having more or less round forward depending on the skipper’s need to steer more (choppy water for instance).  Depending on the skipper’s propensity to get distracted, or have the need to look around (big fleet/small fleet)

Think I'll ignore the "roundness forward". Since the crew will be monitoring the headsail constantly, I'll stick with the foil section @erdb suggested, and be done with it.

I did modify the Eppler profile a bit, reduced thickness to 1.5% from 2.5, and the max. thickness position from from 4.3 to 0.5% from LE. Looks OK so far...

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I had a bit of a "discussion" with someone, about the high Reynold Numbers I was getting from my calcs... basically, as AWS &/or chord width increase, so does the RE.

I get numbers from 0.7m to around 16m for something like the code zero - depending where the chord is measured, and the AWS - hence the discussion.

Found a comparison of interest which I sent him, thought others might be interested?

image.png.7b2adaf01f9dbc3753d3acc24930992c.png

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8 hours ago, MaxHugen said:

Need some help from experienced racers!

Question re headsail shapes at the luff... is there a "deeper" curve just after the luff, than in the rest of the sail generally?

That is, more like (A) than (B) in the diagrams below?

image.thumb.png.d9d218861e4fc7ad9ce57286c53daf50.png

image.png.857af733ad832c458938897a585b9fa4.png

Look closely and there is a clue from UKB1 mast head aerial photo?

9BF7A7BC-9B88-4E13-B293-57C2598BD212.jpeg

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1 hour ago, Sidecar said:

Look closely and there is a clue from UKB1 mast head aerial photo?

9BF7A7BC-9B88-4E13-B293-57C2598BD212.jpeg

Thanks... pic definition makes it a bit hard to tell when zoomed to the max... but it doesn't look like (A).

I've settled for adding just a tiny smidge of extra curve at the luff and will leave it at that for now, or AC36 will be done and dusted by the time I test every possibility!  :)

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13 hours ago, MaxHugen said:

I'm not referring to moving the camber forward/aft using the sheet... more if the sail will "naturally" exhibit a slightly deeper curve, only in the immediate area aft of the luff track, and presumably at lower wind speeds and greater camber.

I my experience, if you have a very flat, old stretchy sail, you see what "naturally" happens, which is that the max draft goes far aft. I think the wind kinda blows it there. But definitely, what you want is the max draft forward of the middle. And in heavier wind, you want less draft but farther forward. In lighter air, you want more draft but not as far forward. This is all affected by the cut of the sail, the stretchyness of the sail, and the way you trim it - cunningham, sheet, vang, halyard, and the way you bend the mast - in many boats, more bend will flatten out the draft close to the mast. And different sails on different rigs may respond a little differently to the different controls. But definitely the natural world is going to push the max draft aft and you want it forward.

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12 hours ago, MaxHugen said:

I've settled for adding just a tiny smidge of extra curve at the luff and will leave it at that for now, or AC36 will be done and dusted by the time I test every possibility!  :)

Ha!  The advertising at this site shows exactly what I was thinking about, get a load of this luff:

image.png.c38a453aa9284924197b493a8907f8ce.png

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5 minutes ago, MaxHugen said:

Ha!  The advertising at this site shows exactly what I was thinking about, get a load of this luff:

image.png.c38a453aa9284924197b493a8907f8ce.png

But that's not apparent wind, AC75 sailing. Not even close. Imagine trying to get drive off that rag at 45 knots!

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24 minutes ago, Sailbydate said:

But that's not apparent wind, AC75 sailing. Not even close. Imagine trying to get drive off that rag at 45 knots!

Sure.  I was thinking in terms of a Code Zero at app wind speeds hardly sufficient for foiling. One know past that and... yeah, not so much!

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@erdb I'm trying to find some info on the data that XFoil produces from the polars:

alpha     CL        CD       CDp       Cm    Top Xtr Bot Xtr   Cpmin    Chinge    XCp

Apart from the first 3, I don't know what they are/mean, and Google doesn't like me today.

I was especially hoping to get some info on boundary layer separation, which hopefully I would understand.

Do you know of any websites that actually discuss these outputs, in terms that a novice like me could get to grips with?

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1 hour ago, MaxHugen said:

@erdb I'm trying to find some info on the data that XFoil produces from the polars:

alpha     CL        CD       CDp       Cm    Top Xtr Bot Xtr   Cpmin    Chinge    XCp

Apart from the first 3, I don't know what they are/mean, and Google doesn't like me today.

I was especially hoping to get some info on boundary layer separation, which hopefully I would understand.

Do you know of any websites that actually discuss these outputs, in terms that a novice like me could get to grips with?

I'm not 100% sure about them all, but top / bottom xtr are where laminar to turbulent transition happens. You can either let the program figure these out based on Re or force the flow to trip at a certain point with the vpar/xtr command. CDp I think is related to pressure difference between the two sides, Cm I is a coef for  rotation momentum generated on the foil. I don't know about the rest, but I don't use these, only cl, cd and sometime play with the xtr.

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58 minutes ago, erdb said:

I'm not 100% sure about them all, but top / bottom xtr are where laminar to turbulent transition happens. You can either let the program figure these out based on Re or force the flow to trip at a certain point with the vpar/xtr command. CDp I think is related to pressure difference between the two sides, Cm I is a coef for  rotation momentum generated on the foil. I don't know about the rest, but I don't use these, only cl, cd and sometime play with the xtr.

Great, thanks!   I calc all the CL/CDs based on RE numbers,as they are significantly different.

Will be interesting to interpret those xtr vals... I want to eliminate some results when these get nasty if possible.

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When is a Code Zero not a Code Zero?  When it's a Code One... I guess. 

Someone did correct me when I was talking about jibs, and explained that they raced with several "Code" sails, being different from a jib in having a fuller cut (besides being bigger).

Had to add another sail to Te Rehutai's wardrobe, as I'd thought the bowsprit mounted sail on B1 was their C0:

image.png.0f9d611273e0b125a94b9823095142cb.png

I note that they don't use the "near-masthead" point shown in the Rig Plan (Rules 20.3), but the same point where the jib stay/sheaves are.

Now I have 3 Mains (135, 140, 145), 2 Codes as above, and 4 Jibs - although I've only seen AM use the smallest I've included for NZ - 45m2.

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On 11/30/2020 at 6:38 PM, MaxHugen said:

I note that they don't use the "near-masthead" point shown in the Rig Plan (Rules 20.3), but the same point where the jib stay/sheaves are.

Less weight and drag? A special height Code Zero halliard isn’t going to get used much? They have a “spare” jib halliard anyhow? If not, they do now......

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4 hours ago, Sidecar said:

Less weight and drag? A special height Code Zero halliard isn’t going to get used much? They have a “spare” jib halliard anyhow? If not, they do now......

That could be the reason. 

Also, recently read that most racing yachts don't use masthead jibs any longer to avoid jib and main vortices combining - or something like that - and stick with fractional rigs.

Maybe I've answered my own question?

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19 hours ago, MaxHugen said:

Also, recently read that most racing yachts don't use masthead jibs any longer to avoid jib and main vortices combining - or something like that - and stick with fractional rigs.

There is an all round rig efficiency zone around 7/8th-9/10th using the inherent cantilever strength of the mast for above hounds support which reduces mast panel length and shortens stays, shrouds and fore halliards and the drag and weight that goes with it all, with some mainsail (on a cleaner mast top) above to reduce the jib top vortex, whist still maximising foretriangle, and, not that it is relevant for AC75’s, but a (running) backstay (or swept back shrouds) set above the forestay can help stop mast pumping offshore without the need for an inner forestay.

By using the lower hoist position for the Code 0, there is also a slightly further aft CE which might reduce the balance disparity between jib and Code 0? CE is slightly lower as well for reduced RM.

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5 minutes ago, Sidecar said:

CE is slightly lower as well for reduced RM.

Thanks for adding that, it was my impression that it is a major factor for AC75’s.

Slightly OT but it’s interesting that RG wrote in his latest piece an opinion from observations that CZ’s are good only from around 4 to 6 knots TWS, below the race limits for a start signal. 

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@rh3000 said over in ETNZ thread

"The gap between the two skins, at the window cutout, is much larger than I was expecting given the photos."

This is a clue?

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On 11/30/2020 at 5:38 PM, MaxHugen said:

 

image.png.0f9d611273e0b125a94b9823095142cb.png

There are some pretty big differences between B1 and 2:

  1. Much less mast rake (might just be an optical illusion)
  2. Raked bow is now plumb—better aero, possibly better buoyancy when the bow goes in
  3. Pronounced hull airfoil shape with max camber way aft
  4. Much more tapered topsides around the transom, combined with 3 above seems to be about reducing turbulence/vortexes off the transom? Maybe even trying to get a bit of downforce at the back, a la F1? Will there be turbulators or diffusers—probably not possible within the rule on the bottom, but along the top edge of the transom?
  5. Skeg/skirt  separates flow on windward/leeward sides of hull

Seems to me they are greatly optimising aero at the expense of hydro, it's almost like they expect to be flying all the time…

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18 minutes ago, RobG said:

Seems to me they are greatly optimising aero at the expense of hydro, it's almost like they expect to be flying all the time…

Now THAT is a big surprise.........

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11 minutes ago, Sidecar said:

Now THAT is a big surprise.........

Lol! 
Agreed but they do have to make choices about where to put relative importance’s,  across a variety of subjects. For example if you think Spithill could take you off your foils with 3 secs before the gun, well you better be able to bare off and get back into aero mode ‘f’ing fast or get left in LR’s wake! 

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6 minutes ago, Kiwing said:

A Big difference?
leech.thumb.jpg.13f73432c20bcec434a8ee9ef6a61bfd.jpg

Looks even and deliberate.

Agreed.  Effectively larger sail area?

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18 minutes ago, Ex-yachtie said:

Agreed.  Effectively larger sail area?

That is what the 12ft skiff designer and world champion sailor, John Chapple, from 70s thinks

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1 hour ago, Stingray~ said:

Lol! 
Agreed but they do have to make choices about where to put relative importance’s,  across a variety of subjects. For example if you think Spithill could take you off your foils with 3 secs before the gun, well you better be able to bare off and get back into aero mode ‘f’ing fast or get left in LR’s wake! 

Of course. Splashdowns, inadvertent or otherwise and takeoffs are fundamental too.. But as long as there there is enough breeze, it should be pretty quick. It may not be obvious, but AC75’s have Bruce Numbers around 2.5, about the same as the most high powered multihulls, they also only need about 6 knots of breeze to lift a hull.

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2 hours ago, Stingray~ said:

Lol! 
Agreed but they do have to make choices about where to put relative importance’s,  across a variety of subjects. For example if you think Spithill could take you off your foils with 3 secs before the gun, well you better be able to bare off and get back into aero mode ‘f’ing fast or get left in LR’s wake! 

Well, Stinger he didn't look too flash in the start box last time around, camo wheel aside. ;-)

Now you're going to tell me his AC50 was off the pace, right? No wait....BDA sailed in downwind conditions and all...blah, blah.

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52 minutes ago, Sidecar said:

Of course. Splashdowns, inadvertent or otherwise and takeoffs are fundamental too.. But as long as there there is enough breeze, it should be pretty quick. It may not be obvious, but AC75’s have Bruce Numbers around 2.5, about the same as the most high powered multihulls, they also only need about 6 knots of breeze to lift a hull.

The Bruce Number is ~2.2, if the C0 is included in the Sail Area calc. My calcs suggest they need around 8 knots to get on the foils at 45° TWA... so at greater angles the TWS required would be less than that.

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3 hours ago, RobG said:

There are some pretty big differences between B1 and 2:

  1. Much less mast rake (might just be an optical illusion)
  2. Raked bow is now plumb—better aero, possibly better buoyancy when the bow goes in
  3. Pronounced hull airfoil shape with max camber way aft
  4. Much more tapered topsides around the transom, combined with 3 above seems to be about reducing turbulence/vortexes off the transom? Maybe even trying to get a bit of downforce at the back, a la F1? Will there be turbulators or diffusers—probably not possible within the rule on the bottom, but along the top edge of the transom?
  5. Skeg/skirt  separates flow on windward/leeward sides of hull

Seems to me they are greatly optimising aero at the expense of hydro, it's almost like they expect to be flying all the time…

  1. Mast rake is dictated as 5° from perpendicular to MRP.   NZ (and others) tend to sail mostly at hull AoA at -2°, imgs were not rotated back to 0°.
  2. There is less volume forward in B2, but the flare will help considerably in a "dive". Certainly, the increased rounding around the gunwales should help aero.
  3. Max. camber of both is at the mast - which is also where the MRP is defined (mast/deck junction), as 1.5m above MWP.
  4. Topsides and crew pods are well tapered to the trailing edge, trying to minimise draggy vortices. They may be sailing at negative hull AoA for downforce or to at least reduce any lift. This also slightly moves CE forward, might better balance weather helm too?
  5. Undecided by the skegs. Who knows, maybe long after the event, someone will release some CFD imgs or vids that show what effects really were in play!

IMO, of course. :)

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1 hour ago, MaxHugen said:

The Bruce Number is ~2.2, if the C0 is included in the Sail Area calc. My calcs suggest they need around 8 knots to get on the foils at 45° TWA... so at greater angles the TWS required would be less than that.

The BN number I gave was from memory. Just checked my data base and I made it 2.39. But that would be with the larger “topmast” C 0.

FWIW, Beau Greste an MOD 70 tri is 2.55, basically being some 500 kg lighter because of less crew.

I think “fat”skegs or keels are the way to go. In displacement mode, they reduce waterline beam (and hence wave making resistance) and provide better balanced lateral resistance at low speeds, especially with the C 0  up. They also provide softer more controllable splashdowns.

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18 hours ago, Sailbydate said:

Well, Stinger he didn't look too flash in the start box last time around, camo wheel aside. ;-)

Yes, JS lost about half the starts but several were OCS. 
 

The reason I attributed that possibility to Spitball as the opposing helm that could make situations in the start box perilous is for more than his (long) history of aggression but also because several guys at LR have spoken openly about knocking opponents off their foils prestart, they must have it on their minds? 
 

The bottom left corner of the prestart box seems like a coffin corner to avoid too, since if you had to get foiling again you can’t just bare off to get up to speed without going out of bounds. Yikes! 

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9 minutes ago, Stingray~ said:

Yes, JS lost about half the starts but several were OCS. 
 

The reason I attributed that possibility to Spitball as the opposing helm that could make situations in the start box perilous is for more than his (long) history of aggression but also because several guys at LR have spoken openly about knocking opponents off their foils prestart, they must have it on their minds? 
 

The bottom left corner of the prestart box seems like a coffin corner to avoid too, since if you had to get foiling again you can’t just bare off to get up to speed without going out of bounds. Yikes! 

JS is very good 1 on 1, but just like anyone else in the AC he's always looked better when he had the faster boat, not sure putting switching him Burling in 2017 would have done much to affect the result!

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5 minutes ago, RMac said:

JS is very good 1 on 1, but just like anyone else in the AC he's always looked better when he had the faster boat, not sure putting switching him Burling in 2017 would have done much to affect the result!

Sure, agreed, JS helming Aoteroa would likely have also won in Bermuda. Not sure who all were at fault for OR’s OCS’s, JS or the tactician or (what they blamed it on) a slightly-faulty timer. 
 

Again, several at LR have talked up prestart aggression possibilities including Chieffi (sp? the Italian tactician who likely won’t be on board) and Philippe Presti, JS’s guru. I think it’s been also mentioned by both Max and Fischer. 
 

There will not be enough room for timed ‘Vanderbilt starts,’ with them being upwind from a small prestart box and 2 minutes to deal with. 

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3 minutes ago, Kiwing said:

Do Moths use the off foils aggression?

Only when they match race

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21 hours ago, MaxHugen said:
  1. Mast rake is dictated as 5° from perpendicular to MRP.

Where? There are references to mast rake of 5° ±0.5°, but that is only in regard to settings for determining other measurements, e.g. yacht assembly mass and centre of mass in rule 10.11 (b):

the mast and mainsail assembly’s mass mM and centre of mass(uM, vM, wM), with an assumed mast rake angle of 5° as shown in Figure 20.3.

Similarly in 11.20, 17.7 (c), etc. I don't see anywhere where mast rake is fixed while sailing. In the referenced images, B2 appears to be less bow–down than B1, so mast rake should appear greater. The best comparison would be side–by–side floating at a dock, but that's not going to happen.

My theory is that bow–down was used on the multis to extend the range of AoA of the flapless main foils. With flapped foils, they just don't don't need the AoA range so the boat is setup for much more level flight (I think newer Moths have discovered this too). Early liftoff is probably much better achieved with the large volume skegs rather than main foil AoA changes through pitch. Also there is much more freedom in hull design, so pitch isn't needed as much for aero effects either. Once the boat lifts off, then all breeze is +30 kn.

PS
MRP is "mast rotation point", not a suitable reference for angles. I expect mast rake is measured relative to the measurement waterline plane (MWP), which is defined in 3.1 as "the horizontal reference plane of the yacht."

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On another thread?917894514_invert1.thumb.jpg.9ce4d1cdd70b00dc561a2b090be77a4e.jpg

My version

49611430_invert3.thumb.jpg.b204e764ca7597390cede44258605967.jpg

How about this as an alternative?

Actually way back with the mule I pointed out the gap between skins was a helmet width at the window.

@rh3000 just mentioned being surprised by the gap between skins at window, so I personally think these are more wing like than drawn.

And looking at the top of Ineos twin skin it would seem more like the above.

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1 hour ago, Kiwing said:

On another thread?917894514_invert1.thumb.jpg.9ce4d1cdd70b00dc561a2b090be77a4e.jpg

My version

49611430_invert3.thumb.jpg.b204e764ca7597390cede44258605967.jpg

How about this as an alternative?

Actually way back with the mule I pointed out the gap between skins was a helmet width at the window.

@rh3000 just mentioned being surprised by the gap between skins at window, so I personally think these are more wing like than drawn.

And looking at the top of Ineos twin skin it would seem more like the above.

A helmet width at the window easily... in ETNZ B1 fwiw...

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@rh3000 do you think this is a missed observation and hence discussion?

Where are the sail observers, speculators and designers?

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I've always been obsessed with the gap.

As soon as we saw exhaust tell tales in the gap it was obvious that the gap was a feature, not a bug. What it does is still anyone's guess. Most have been saying it will go away when they finalise the clew and the boom. Well, the boom is gone, and the clew looks pretty slick but the gap remains.

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46 minutes ago, barfy said:

I've always been obsessed with the gap.

As soon as we saw exhaust tell tales in the gap it was obvious that the gap was a feature, not a bug. What it does is still anyone's guess. Most have been saying it will go away when they finalise the clew and the boom. Well, the boom is gone, and the clew looks pretty slick but the gap remains.

You're not the only one that's obsessed.  The only guess that seemed to make half way sense to me was a sort of "Kammback" effect, which kind of extends the tail area of the sail, effectively making it larger.

Major prob with that theory is that the boats are often trying to depower the sails in the top half especially, to reduce heeling moments.

So if they're doing the Kammback thing, it would only make sense in the lower half of the sail. My recollection is that the gap extends all the way up?

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10 minutes ago, MaxHugen said:

You're not the only one that's obsessed.  The only guess that seemed to make half way sense to me was a sort of "Kammback" effect, which kind of extends the tail area of the sail, effectively making it larger.

Major prob with that theory is that the boats are often trying to depower the sails in the top half especially, to reduce heeling moments.

So if they're doing the Kammback thing, it would only make sense in the lower half of the sail. My recollection is that the gap extends all the way up?

If you can flick the top control to negative the Kammback effect would be negative...problem is we haven't seen much good footage from astern. And we don't see many airplanes, hang or paragliders, or kites utilizing this effect so..meh.

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32 minutes ago, barfy said:

If you can flick the top control to negative the Kammback effect would be negative...problem is we haven't seen much good footage from astern. And we don't see many airplanes, hang or paragliders, or kites utilizing this effect so..meh.

Gave the sail profile I use a gap, and ran the numbers...  it appears the Kammback effect has increased the virtual "area" as the Lift increased.

BUT... drag increased at a way faster rate.

image.png.cc8f56053cc27c358935fc861be1ffb4.png

I'll wait for another theory...

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1 hour ago, Kiwing said:

@MaxHugen could you run a l/d on this fuller profile which is more like @rh3000 saw?ld1.thumb.jpg.af1dfa33dbb84dfdfb56cbd3304f4643.jpg

Please ?

I'll see what I can do. Took a while to get the original profile well shaped, aero-wise.

[edit] Any pics or references to what rh3000 saw?

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3 hours ago, MaxHugen said:

Gave the sail profile I use a gap, and ran the numbers...  it appears the Kammback effect has increased the virtual "area" as the Lift increased.

BUT... drag increased at a way faster rate.

image.png.cc8f56053cc27c358935fc861be1ffb4.png

I'll wait for another theory...

Then you must doubt your model based on the visual evidence that there exists a second generation working model of the "gap" sail...used by a team not known for following blind alleys.

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2 minutes ago, barfy said:

Then you must doubt your model based on the visual evidence that there exists a second generation working model of the "gap" sail...used by a team not known for following blind alleys.

All I've done so far is speculate.  Unsuccessfully.  Like everyone here, I have no idea really what the gap does.

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1 minute ago, MaxHugen said:

All I've done so far is speculate.  Unsuccessfully.  Like everyone here, I have no idea really what the gap does.

You have done more than speculate with your amazing research, use of available models, and fine hypothesis  'es :o

I've just observed available evidence, and posited questions for the great hive mind that is SA :rolleyes:

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2 hours ago, Kiwing said:

@MaxHugen could you run a l/d on this fuller profile which is more like @rh3000 saw?

Best I could do... of course may not resemble anything in reality! Anyway, it just got worse.

8406-Gap-1.5_v3_SS.thumb.JPG.5ee2f2a2587b8d3044b3ac7890997563.JPG

8406-Gap-1.5_v3.JPG.41e9ac6a619b279596098a19a5f1decb.JPG

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Adding to the unsubstantiated comments: I’d be quite wary of the gap and how one can model its effect without wind tunnel verification ... the reason being my recollection of the Gurney flap, which might be somehow related. With a surface just a 1% of chord perpendicular to the lee trailing edge lift increases substantially - but drag even more

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I snipped a couple of views of Rita from Justin Mitchell's video to get some data on twist, they have just tacked, and I assume they eased the sheets a bit to get speed back up.

image.png.815777d689f0fe2c2fb63c62d4733c45.png

I guessed the AWA by the degree of twist at the 3/4 draft stripe on the main.  This was in "14-18 knots" according to Justin.  Mainsail measured 132m2, so add in a bit for roach, and call it 135.  Jib is 67m2.

What I found interesting was just how far they had to depower a #3 main, and a J2 (which seems a bit large?) at round 45 knots AWS.  They are both depowered significantly right down to the 2nd section (from the foot).

As best as I can judge it anyway. ;)

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1 hour ago, Xlot said:

Adding to the unsubstantiated comments: I’d be quite wary of the gap and how one can model its effect without wind tunnel verification ... the reason being my recollection of the Gurney flap, which might be somehow related. With a surface just a 1% of chord perpendicular to the lee pressure side trailing edge lift increases substantially - but drag even more

 

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23 minutes ago, MaxHugen said:

I snipped a couple of views of Rita from Justin Mitchell's video to get some data on twist, they have just tacked, and I assume they eased the sheets a bit to get speed back up.

image.png.815777d689f0fe2c2fb63c62d4733c45.png

I guessed the AWA by the degree of twist at the 3/4 draft stripe on the main.  This was in "14-18 knots" according to Justin.  Mainsail measured 132m2, so add in a bit for roach, and call it 135.  Jib is 67m2.

What I found interesting was just how far they had to depower a #3 main, and a J2 (which seems a bit large?) at round 45 knots AWS.  They are both depowered significantly right down to the 2nd section (from the foot).

As best as I can judge it anyway. ;)

Funny I just came here to look at your old top-view drawings and the sails to re-evaluate the maximum twist angles and the possibility of negative AOA at the head of the main.

Sorry to mess up your artwork, but I wanted to show what I think is closer to what's happening upwind.

my_twist2.thumb.JPG.8a882b08f590bc903390cfca42d3624f.JPG

AWA is in the 14 - 16 deg range when sailing to 45 deg true in 15 - 20 kts of wind. The sails AOA is around 8 - 12 degrees relative to the apparent wind.

Initially I limited twist to 15 degrees max, but it's clear from the numbers my model spits out that the boat would go faster with more twist. On the above drawing, the jib's AOA is 6 degrees, the main's is 11 degrees (relative to AWA), and the twist in the main from top to bottom is 18 degrees. This puts the head of the main at negative 7 degrees AOA.

Would it be possible?

 

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3 minutes ago, erdb said:

Funny I just came here to look at your old top-view drawings and the sails to re-evaluate the maximum twist angles and the possibility of negative AOA at the head of the main.

Sorry to mess up your artwork, but I wanted to show what I think is closer to what's happening upwind.

my_twist2.thumb.JPG.8a882b08f590bc903390cfca42d3624f.JPG

AWA is in the 14 - 16 deg range when sailing to 45 deg true in 15 - 20 kts of wind. The sails AOA is around 8 - 12 degrees relative to the apparent wind.

Initially I limited twist to 15 degrees max, but it's clear from the numbers my model spits out that the boat would go faster with more twist. On the above drawing, the jib's AOA is 6 degrees, the main's is 11 degrees (relative to AWA), and the twist in the main from top to bottom is 18 degrees. This puts the head of the main at negative 7 degrees AOA.

Would it be possible?

 

"Would it be possible?"  Well, perhaps they could force the top over that far, but why would they? Incur massive drag.

Anyway, they can't do that to the jib as well?  Slightly into negative at the top, OK, those sails would still be in positive lift.  Which is why I settled for ~25° - may have been a tad more.   Makes sense, Rita had just tacked.  From my calcs so far, they have to depower to a major degree to stay upright!

BTW, I checked out turbulent flow, set xtr at 0.25, then at 0 (LE). XFLR5 wouldn't let me set it negative (ahead). As Basiliscus/you mentioned, drag started to look significantly greater:

image.png.4fa843dc03261909da36a54ec6652d73.png

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^ I probably didn't explain myself very well... these angles represent a boat that is off the wind slightly to pick up speed. Then they'd pull in the sails on the travellers and close up that AWA, all the angles would be much closer to what you described for sure!

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28 minutes ago, MaxHugen said:

"Would it be possible?"  Well, perhaps they could force the top over that far, but why would they? Incur massive drag.

Well, the induced drag does increase, but it also lowers the CoE, which let's them cant the foil further out, which increases righting moment, which let's them generate more power. So it's complicated... This is what the model shows:

Speed and AWA:

1801796622_sailsetup1.png.ef8f8dbe366b66d44b84b16c23ab2daf.png

 

Sail AOA (for the whole sail plan) and twist. I'm working on removing the wiggles from the lines but it still shows the trend.

 1355459061_sailsetup2.png.2d92d81fed57c5e151e0826bce9ebcd3.png

Notice how above 15 kts of wind, the twist is basically stuck at the limit I set except for deep vmg running. That's how it works out for highest boat speed considering all the factors including aero- and hydro drags.

22 minutes ago, MaxHugen said:

^ I probably didn't explain myself very well... these angles represent a boat that is off the wind slightly to pick up speed. Then they'd pull in the sails on the travellers and close up that AWA, all the angles would be much closer to what you described for sure!

Yes, I got that. I wasn't saying the drawing is wrong, I just wanted to see how it'd look for max vmg upwind. I'd love to see a nice drawing of that too :D

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17 minutes ago, erdb said:

Well, the induced drag does increase, but it also lowers the CoE, which let's them cant the foil further out, which increases righting moment, which let's them generate more power. So it's complicated... This is what the model shows:

Yes, I got that. I wasn't saying the drawing is wrong, I just wanted to see how it'd look for max vmg upwind.   I'd love to see a nice drawing of that too :D

To depower, the best AoA should be when Lift = 0. The drag should then be quite low. For the main, that's about -2°, or even -3° (if my custom profile is actually realistic?). Beyond that you're just adding unnecessary drag.  The trick is to get the sail to do that progressively down the sail... easy on a computer, but not so much with a real sail.

I can easily fudge those sails in the diagram to show a different AWA, but I use actual pics and do a bit of draughting with them to derive the angles.  For that I need a good shot of the boat perpendicular, and from aft (dead-on) - has to be reasonably accurate. Not too hard with the right video clip though, ever since someone mentioned the frame-by-frame feature in YT!

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Well if you have zero lift, you're not going to sail very fast. The trick is to find the balance of all the factors for the highest speed. I don't think we will see good images of the sail profiles of boat 2s until the AC itself, but from the videos it does seem to me they have quite a bit of twist on the main and they use those ridiculous looking low aspect jibs for higher winds all confirming they don't mind higher induced drag in exchange for more power.

Also, increasing twist is not necessarily depowering in this case. More twist allows you to sheet in more at the lower portions of the sails. It is in fact, increasing  power, but at a lower CoE. 

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I'm only referring to zero lift in the top sections of the sail for depowering of course!

Those low aspect jibs make sense to me: they are helping to power the main in the lower sections, and avoiding unnecessary drag higher up where they'd just have to also depower the jib. This is quite noticeable when you look at the slot between them.

My calcs indicate that they really have to depower the mid to higher sections a lot, esp the main. In fact, as I check out more pics of the boats, I can see that I still haven't depowered low enough - and it shows, when I struggle to balance transverse moments.

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FYI, this is as far as I've gotten with balancing RM and Heeling forces using sail combos and twist:

image.thumb.png.c3593cc87dcb70dcfc1a922fc8b900c6.png

The lower line are the headsails, from a C1 to a J4, and the notation shows the Jib-Twist Level. For headsails , I have 6 twist levels, and the mainsails (#1 to #3) have now expanded to 10 - and as you can see from the jagged line, it's still not enough, but it will probably have to do.

As well as AoA, camber is also altered by sail section, more or less for sail wind range plus twist effects.

I should have started with a databased app, instead of pottering around on a spreadsheet, didn't expect to get so involved! :rolleyes:

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Hey @MaxHugen and @erdb Thank you for letting us look over your shoulder.

With a 70 foot mast the wind must vary in speed and direction up the sail profile?

What amazing enjoyment you must be getting.  Bet you never thought you would be where you are?

By now even the gurus must be enjoying your posts.

Cheers

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2 hours ago, MaxHugen said:

...XFLR5 wouldn't let me set it negative (ahead). ...

XFLR5 uses the Xfoil code, and in Xfoil a negative value for the transition location does not mean it is ahead of the airfoil.  If you enter a positive value, the transition location is measured from the leading edge on both surfaces.  If you enter a negative value in Xfoil, the transition location is measured from the stagnation point.  

At low values of lift, there is not much difference between the two choices, as the stagnation point is near the leading edge.  But at high lift, the stagnation point can migrate well onto the pressure side.  In that case, the flow calculation can be laminar from the stagnation point to past the leading edge, leading to laminar separation and stall before it gets to the specified transition location, due to a sharp pressure peak at the leading edge.  Or there may be more laminar flow than was intended.  Using a small negative value for the transition location will ensure the flow calculations are for essentially fully turbulent flow on both surfaces at all angles of attack.

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32 minutes ago, MaxHugen said:

FYI, this is as far as I've gotten with balancing RM and Heeling forces using sail combos and twist:

image.thumb.png.c3593cc87dcb70dcfc1a922fc8b900c6.png

The lower line are the headsails, from a C1 to a J4, and the notation shows the Jib-Twist Level. For headsails , I have 6 twist levels, and the mainsails (#1 to #3) have now expanded to 10 - and as you can see from the jagged line, it's still not enough, but it will probably have to do.

As well as AoA, camber is also altered by sail section, more or less for sail wind range plus twist effects.

I should have started with a databased app, instead of pottering around on a spreadsheet, didn't expect to get so involved! :rolleyes:

Interesting. Unfortunately it's hard to compare to my data, because I combine sail forces, but will try to get some numbers to compare. I think your method overestimates lift a little, so you may be in the right range. The other day I was searching stuff for Lifting line theory to see if I could actually do the calculations in my software instead of using the vortex sheet and happened to read the wiki page  https://en.wikipedia.org/wiki/Lifting-line_theory that says:

"One might expect that understanding the full wing simply involves adding up the independently-calculated forces from each airfoil segment. However, it turns out that this approximation is grossly incorrect: on a real wing, the lift over each wing segment does not correspond simply to what two-dimensional analysis predicts. In reality, the local amount of lift on each cross section is not independent, and is strongly affected by neighboring wing sections. The lifting-line theory corrects some of the errors in the naive two-dimensional approach ..."

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3 minutes ago, Basiliscus said:

XFLR5 uses the Xfoil code, and in Xfoil a negative value for the transition location does not mean it is ahead of the airfoil.  If you enter a positive value, the transition location is measured from the leading edge on both surfaces.  If you enter a negative value in Xfoil, the transition location is measured from the stagnation point.  

At low values of lift, there is not much difference between the two choices, as the stagnation point is near the leading edge.  But at high lift, the stagnation point can migrate well onto the pressure side.  In that case, the flow calculation can be laminar from the stagnation point to past the leading edge, leading to laminar separation and stall before it gets to the specified transition location, due to a sharp pressure peak at the leading edge.  Or there may be more laminar flow than was intended.  Using a small negative value for the transition location will ensure the flow calculations are for essentially fully turbulent flow on both surfaces at all angles of attack.

Thanks for the clarification. While you're here, how is AOA and twist defined in the Vortex analysis page? Does AOA refers to the mid span or the segment of the CoE? Twist I guess is foot vs head.

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51 minutes ago, MaxHugen said:

I'm only referring to zero lift in the top sections of the sail for depowering of course!

Those low aspect jibs make sense to me: they are helping to power the main in the lower sections, and avoiding unnecessary drag higher up where they'd just have to also depower the jib. This is quite noticeable when you look at the slot between them.

My calcs indicate that they really have to depower the mid to higher sections a lot, esp the main. In fact, as I check out more pics of the boats, I can see that I still haven't depowered low enough - and it shows, when I struggle to balance transverse moments.

I just brought up the low aspect jib, because it's the same issue as increasing twist. You increase induced drag (because of the low AR), but lower CoE so you can produce more power. If the goal was to decrease induced drag, we would see skinnier jibs at high wind speed. 

And yes, my model indicates the same - it's crucial to lower CoE - which is the same as depowering mid to high sections. That's why at higher wind speeds, the model always balanced out at the highest amount of twist I allowed (despite the increased induced drag).

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50 minutes ago, MaxHugen said:

FYI, this is as far as I've gotten with balancing RM and Heeling forces using sail combos and twist:

image.thumb.png.c3593cc87dcb70dcfc1a922fc8b900c6.png

 

OK so some numbers to compare. Boat speed and AWS numbers from my polars:

2022287335_speed-aws.png.bb06d9bbb4bec5697643e9035a53e099.png

Heeling and pitching moments - this is for both sails.

sail_moments_cr.png.c42cedeb377acdae667f2602ce77ebbb.png

So for 15 kts true wind (purple), 45 TWA, AWS is ~42 kts, heeling mom is ~ 450 kN x m. If I look at the highest AWS in 24kts wind (red), at 60kts AWS, ~550 kN x m heeling mom.

Looks like we're in the same ballpark.

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10 hours ago, barfy said:

Then you must doubt your model based on the visual evidence that there exists a second generation working model of the "gap" sail...used by a team not known for following blind alleys.

Again, using the "NZ can never be wrong theory".

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@4pines said "I wonder if this is where they're doing some work and maybe ditching the boom?  I know on the 72s that both Oracle and NZ were adding more width on the mast/wing in the front almost daily in the lower section..."

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6 hours ago, erdb said:

Thanks for the clarification. While you're here, how is AOA and twist defined in the Vortex analysis page? Does AOA refers to the mid span or the segment of the CoE? Twist I guess is foot vs head.

AoA is relative to whatever section is considered to have zero twist.  If the surface was untwisted, then it wouldn't matter - all the sections would have the same angle of attack (excluding local interference effects) and you could choose any section as the reference.  Twist is the variation in incidence from the reference section, and the angle off attack of a given section is the AoA of the reference section plus the twist.  So you can choose whatever section is most convenient and define the twist from there.

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16 hours ago, MaxHugen said:

Best I could do... of course may not resemble anything in reality! Anyway, it just got worse.

8406-Gap-1.5_v3_SS.thumb.JPG.5ee2f2a2587b8d3044b3ac7890997563.JPG

8406-Gap-1.5_v3.JPG.41e9ac6a619b279596098a19a5f1decb.JPG

Thanks.

Sorry to trouble you. I still think this is where ETNZ is for reasons not obvious to us?

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13 minutes ago, Kiwing said:

Thanks.

Sorry to trouble you. I still think this is where ETNZ is for reasons not obvious to us?

No prob.   What makes you think the profile has a lot more depth around that 15-30% area of the section than I'm using?

(Remember what I came up with is just speculation.)

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1 hour ago, Basiliscus said:

AoA is relative to whatever section is considered to have zero twist.  If the surface was untwisted, then it wouldn't matter - all the sections would have the same angle of attack (excluding local interference effects) and you could choose any section as the reference.  Twist is the variation in incidence from the reference section, and the angle off attack of a given section is the AoA of the reference section plus the twist.  So you can choose whatever section is most convenient and define the twist from there.

Thanks, so if I understand this correctly, if I specify Twist at foot = 0, Twist at head = -10, then the AoA setting (Alpha_a) refers to the foot, and the twist would increase linearly to -10deg at the head?

 

 

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10 minutes ago, MaxHugen said:

No prob.   What makes you think the profile has a lot more depth around that 15-30% area of the section than I'm using?

(Remember what I came up with is just speculation.)

Look @MaxHugen I really appreciate what you are doing.  We are all grasping at straws in the dark with the possible exception of one or two on here who give us hints every now and then.  You are shedding light in dark corners and giving lots for us to chew on.  We all thank you for that.

1) the solid wings have a much wider profile (and they are suppose to be the best)
2) some of us are observing a greater profile depth (Me along while ago with the mule),