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The new sailing twin skin setup


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9 minutes ago, Basiliscus said:

 

I think it helps to consider the wake from several frames of reference.  If you pick a location fixed in the air mass, the yacht passes by and is gone.  There's a momentary disturbance and then an aftermath that continues.  A good example is drawing a canoe paddle through the water and then lifting it out.  It's clear the paddle accelerated a slug of water with a vortex that wrapped around it, and the slug continues to drift through water after the paddle is gone, based on its own momentum.  The wake left by the sail rig is like that.  It pushes the air to the side and then the rig is gone, leaving the air to drift of its own momentum.  This is the side wash in the wake.

Now pretend you're in a hot air balloon at mast height.  The boat just passed under you, leaving its wake behind.  What you see as you study the wake is the movement I just described.  But when you look down at the water, you'll see the water is moving by at the true wind speed as the whole airmass moves over the water.

As the boat passed by, it disturbed successive spots in the air mass.  The previous spot was located back along the apparent wind vector, because since that time, the boat has moved on by the boat speed and the air mass has moved on by the true wind vector. By the time the next spot in the airmass is disturbed, the boat will have moved on by the boat speed and the airmass will have moved by the true wind speed.  

The circles left behind in the Virtual Eye graphics that show the path of the yacht work the same way.  The yacht drops a series of circles that remain in place on the water.  The wake works in a similar way, with the sail shedding blobs of wake.  But now those blobs convect with the true wind.  So the blobs and the circles would initially be together, but with time the blobs will drift away from the circles.  Both the blobs and the circles are being laid down at boat speed, but when you add the true wind vector to the blobs, their trajectory will be angles at the apparent wind vector.

The wake extending at the apparent wind angle is a bit of an optical illusion.  It's like an ocean wave.  We see a linear wave front that advances continuously.  But blobs of water are actually undergoing a circular motion that has no net advance (for deep water waves).  Our eye connects successive blobs of water to perceive the pattern of the wave.  The blobs of wake are moving at the TWD (plus the wash velocity).  But fresh wake is being continuously laid down.  When we connect the older wake blobs with the latest blob, we get a line that is parallel to the apparent wind direction.  

Thanks Mr B!

I shall probably have to create a diagram for myself, with "slugs" of air, and lots of vectors, as you've described. :)

Seems my brain understands concepts better via images.

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Seems like those volvo  pictures are really mostly just showing the tip vortices, rather than the entirety of the affect of the sails.

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43 minutes ago, MaxHugen said:

Thanks.  Was this pic in an article?  I'd like to know what range of TWS they were sailing in...

Trying to think which boats and the year.

Not 2005 as they had 40 knts at the Cape Town start, the first year of the V70's

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18 minutes ago, SloopJohnB said:

Trying to think which boats and the year.

Not 2005 as they had 40 knts at the Cape Town start, the first year of the V70's

2001-2002  VOR
Cape Town, Leg 2 Start

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When they say they can't "stay on their hip" that must be part of the air affected by wing wash as well IMHO?

Love to have your visual skills @MaxHugen I might try in a week or two when I get more time. 

But now is the time to think of AC37?  What changes to the boats, course rules etc?

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Seems appropriate on this day to give a BIG THANKS to everyone who has contributed to the varied technical aspects of this amazing AC75 class, and aero-hydrodynamics in general.

I've learnt so much in so many ways, and it's been a very enjoyable experience. Glad I "tripped" across the SAAC forum.

Thank you.  :)

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6 hours ago, mauriciogfj said:

An interesting snippet from that article:

"In the late 19th century, English engineer Osborne Reynolds found that, with surprising universality, turbulence [post-laminar flow] begins when that dimensionless parameter [Reynolds Number] exceeds about a million."

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6 minutes ago, amc said:

Thank you for your work Max 

Cheers!  Hope my enjoyable hobby is useful at times.

Apart from when I cock it up. :lol:

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9 hours ago, Kiwing said:

When they say they can't "stay on their hip" that must be part of the air affected by wing wash as well IMHO?

In my experience, and in my imagination, the wind "on their hip" is a slight header and a slight lull, compared to the true wind. But no real turbulence.

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14 hours ago, MaxHugen said:

Thanks Mr B!

I shall probably have to create a diagram for myself, with "slugs" of air, and lots of vectors, as you've described. :)

Seems my brain understands concepts better via images.

Try this.  Take two sheets of paper, one opaque and one translucent.  Let the opaque sheet represent the water and the translucent sheet the air.  Draw a line at 45 degrees on the opaque sheet.  This represents the course of the boat sailing to windward with the wind coming from the top of the page.  Put a dot on the line to show the current position of the boat.

Place the translucent sheet on top of the opaque sheet and put a dot on it over top of the first dot.  This represents the wake being shed at that time.  Now put a second dot further up the line on the opaque sheet, say 1 cm for simplicity.  Move ths translucent sheet down 1 cm to represent a wind speed that is the same as the boat speed.  (It could be any distance - just be consistent in the next steps).  Put a second dot on the translucent sheet at the same location as the second dot on the line.

Repeat this process for several time steps.  

Now look at what you have.  There will be a row of dots marching up the line, just like the circles on the water in Virtual Spectator.  The translucent sheet will also have a line of dots.  But they will trail at the apparent wind angle from the final dot on the line.  Each of these dots has drifted with the true wind as you moved the translucent sheet down the page.  But the dots line up in the airmass along the apparent wind vector because of the boat's motion as it shed successive dots into the wake.

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I try to show it here (2:18 video below), with the green dots representing turbulent air exiting the sail and then travelling down screen with the wind. Line representing boats are moving at roughly three times windspeed. 

What's harder to grasp is how quickly the turbulence mixes and decays and how much it spreads. I think that depends on environmental conditions as well as the amount of energy extracted from the breeze. 

 

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Thanks guys but I guess my question relates not so much to the disturbed air but to how far away laterally from the boat the free stream is bent. As nroose mentions sailing on the hip means sailing in a slight header, so how far away (or behind)from the lead boat do you have to be out of that effect?

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@Mozzy Sails where is the clear air they talk about?

Can't live on their hip?
Can't live there, they're out of there?
They crossed their own disturbed air?
Often these places are upwind of the leading yachts trail?

These places are all part of a much bigger area than this vortex trailing behind the yachts(flying machines)? Can you define this area please.

By the way your stuff is really great and we all learn hugely from watching.

Do you have any interest in comparing this twinskin idea against a solid symmetrical wing with say two segments like SailGP's wing?

It will be very interesting to see if someone comes up with a hoistable sail to get them up onto the foils? to be pulled down once they are off? Like 18foot skiffs.

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9 minutes ago, amc said:

Thanks guys but I guess my question relates not so much to the disturbed air but to how far away laterally from the boat the free stream is bent. As nroose mentions sailing on the hip means sailing in a slight header, so how far away (or behind)from the lead boat do you have to be out of that effect?

It's a factor of the amount of power and drag of the leeward/ahead boat. It's an enormous distance for a large schooner or square rigger with all the sails up. It's very small for a boat like a moth. I think it's large for AC75, but they are both powerful and efficient and low drag, so I don't really know how to figure it.

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1 hour ago, amc said:

Thanks guys but I guess my question relates not so much to the disturbed air but to how far away laterally from the boat the free stream is bent. As nroose mentions sailing on the hip means sailing in a slight header, so how far away (or behind)from the lead boat do you have to be out of that effect?

From the pic posted by @Basiliscus, I did a vertical resize correction for PoV at say 45°, and then some calcs as to the possible dimensions of the wake:image.png.b10c403b7989c8c4cee63b1ec81cc601.png
Take this with a grain bag of salt, but it does give some indication of how long a wake might be. Although boat speed is unknown, it could be around 30 knots?

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10 hours ago, Basiliscus said:

Try this.  Take two sheets of paper, one opaque and one translucent.  Let the opaque sheet represent the water and the translucent sheet the air.  Draw a line at 45 degrees on the opaque sheet.  This represents the course of the boat sailing to windward with the wind coming from the top of the page.  Put a dot on the line to show the current position of the boat.

Place the translucent sheet on top of the opaque sheet and put a dot on it over top of the first dot.  This represents the wake being shed at that time.  Now put a second dot further up the line on the opaque sheet, say 1 cm for simplicity.  Move ths translucent sheet down 1 cm to represent a wind speed that is the same as the boat speed.  (It could be any distance - just be consistent in the next steps).  Put a second dot on the translucent sheet at the same location as the second dot on the line.

Repeat this process for several time steps.  

Now look at what you have.  There will be a row of dots marching up the line, just like the circles on the water in Virtual Spectator.  The translucent sheet will also have a line of dots.  But they will trail at the apparent wind angle from the final dot on the line.  Each of these dots has drifted with the true wind as you moved the translucent sheet down the page.  But the dots line up in the airmass along the apparent wind vector because of the boat's motion as it shed successive dots into the wake.

Brilliant!   Now my brain-that-needs-diagrams  finally gets it.   Thank you.  :)

image.png.f651c859afabc721673ab8b06401d27c.png

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Here's a simulation showing wakes behind moving boats in a true wind field, not for the AC but rather a conventional, slower than wind boat (the Finn). This is in the inertial (earth attached) reference frame, rather than the one of the sailor moving with the boat, that we are more used to. As Basiliscus mentions, the wake is shed in the apparent wind direction - the apparent wind varies with height, so down low the turbulence is more in line with the motion of the boat and up at the mast head it's "freer". As the boat moves diagonally along, the wake is dragged along with it across the wind field.

Besides a decrease in wind speed, there’s also the change in direction - the sails are bending the wind, and this persists deep into the wind field, causing a “header” to the boat behind, probably more important a disturbance than the drop in wind speed.

When the boats are crossing, the one taking the stern (crossing behind) takes again in the wind field bent by the boat crossing ahead - in case of the Finn, this is about 70 cm.

You can read about the interference here: https://issuu.com/finn-class/docs/finnfare-november-2018/12

 

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3 minutes ago, Mikko Brummer said:

Here's a simulation showing wakes behind moving boats in a true wind field, not for the AC but rather a conventional, slower than wind boat (the Finn). This is in the inertial (earth attached) reference frame, rather than the one of the sailor moving with the boat, that we are more used to. As Basiliscus mentions, the wake is shed in the apparent wind direction - the apparent wind varies with height, so down low the turbulence is more in line with the motion of the boat and up at the mast head it's "freer". As the boat moves diagonally along, the wake is dragged along with it across the wind field.

Besides a decrease in wind speed, there’s also the change in direction - the sails are bending the wind, and this persists deep into the wind field, causing a “header” to the boat behind, probably more important a disturbance than the drop in wind speed.

When the boats are crossing, the one taking the stern (crossing behind) takes again in the wind field bent by the boat crossing ahead - in case of the Finn, this is about 70 cm.

 

Thanks Mikko, very interesting. Around 0:27, there is a massive amount of turbulence shown from the hull. Have you any idea how it compares percentage- wise to sail turbulence and drag?

That's one area I have no real info on...

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3 hours ago, MaxHugen said:

Thanks Mikko, very interesting. Around 0:27, there is a massive amount of turbulence shown from the hull. Have you any idea how it compares percentage- wise to sail turbulence and drag?

The turbulence from the boat & the sailor is significant, yes, but when it comes to drag, but I'm not sure a boat sailing behind should mind about that. I like to look at sailboat's drag differently than the airplanes: I feel it's more natural to look at drag (or drive) in the direction of the motion of the boat, rather than the apparent wind. In this body-reference frame, the drag of the sails (and the mast) is negative, aka drive. In most cases, including the Finn or the AC 75, the (air) drag of hull is very small or nil. The sailor hiking is real drag, maybe 2-3% of the total drag of the Finn sailing upwind, in this body-fixed frame.   

Think about the turbulence from the sailor &boat in that context: It's like a cyclist drafting behind the other. The hull + sailor drag for the one coming behind can well be less than for the boat ahead. The question, how much bad air is just a heading wind shift and how much the turbulent wake from the sails really harm you, is a good one - I should think about how that could be quantified.

Remember also that natural wind is always turbulent in itself, 10-15% in turbulence intensity is not rare, rather the norm. So how much the turbulence added by other boats influences more, I wouldn't know. In simulations, we can see how increasing turbulence intensity influences very little the average forces, while the instantanious ones differ a lot. Attached an example - red line with no simulated turbulence, black dotted with 15% intensity. 

image.png

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4 hours ago, Mikko Brummer said:

The turbulence from the boat & the sailor is significant, yes, but when it comes to drag, but I'm not sure a boat sailing behind should mind about that. I like to look at sailboat's drag differently than the airplanes: I feel it's more natural to look at drag (or drive) in the direction of the motion of the boat, rather than the apparent wind. In this body-reference frame, the drag of the sails (and the mast) is negative, aka drive. In most cases, including the Finn or the AC 75, the (air) drag of hull is very small or nil. The sailor hiking is real drag, maybe 2-3% of the total drag of the Finn sailing upwind, in this body-fixed frame.   

Think about the turbulence from the sailor &boat in that context: It's like a cyclist drafting behind the other. The hull + sailor drag for the one coming behind can well be less than for the boat ahead. The question, how much bad air is just a heading wind shift and how much the turbulent wake from the sails really harm you, is a good one - I should think about how that could be quantified.

Remember also that natural wind is always turbulent in itself, 10-15% in turbulence intensity is not rare, rather the norm. So how much the turbulence added by other boats influences more, I wouldn't know. In simulations, we can see how increasing turbulence intensity influences very little the average forces, while the instantanious ones differ a lot. Attached an example - red line with no simulated turbulence, black dotted with 15% intensity. 

image.png

Interesting. 

I did some calcs on hull drag by estimating frontal and side areas, and then using AWA vectors. However, I had to use uninformed guesses as to what the Cd might be, and with a resultant 9% of total drag, I suspect this may be well overstated.

image.png.1b6be6430d082cfdda2c9b222b072a26.png

image.png.92137a557d86a8a2d299c945415b7842.png

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17 hours ago, Mikko Brummer said:

Besides a decrease in wind speed, there’s also the change in direction - the sails are bending the wind, and this persists deep into the wind field, causing a “header” to the boat behind, probably more important a disturbance than the drop in wind speed.

I suspect the relationship is quite symbiotic in that as much as the boat behind experiences a header, the boat ahead experiences a lift from the upwash of the boat behind. 

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10 hours ago, Mikko Brummer said:

 The sailor hiking is real drag, maybe 2-3% of the total drag of the Finn sailing upwind, in this body-fixed frame.   

Are there any rules that prohibit wearing some kind of fairing that could reduce the drag of a hiking human body? Sure it would be torture pumping the main downwind in a costume like that - but in theory...

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5 hours ago, erdb said:

Are there any rules that prohibit wearing some kind of fairing that could reduce the drag of a hiking human body? Sure it would be torture pumping the main downwind in a costume like that - but in theory...

I don't think so... but there is a limit on clothing weight, which is rather strict, 5 kg if I recall (?). Another thing is how to implement it, so that it works on both tacks? The sailor tacks his aero-helmet, when  changing sides?

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@Mikko Brummer can you give me opinion please?

Did TNZ (or any other team) reverse the top of their main sail to provide righting moment and lower the COE.

1931626754_00TwinSkin.thumb.jpg.5a1881b3a68a306ecb3d789dc836157d.jpg

some people here think this is just "luffing" the top of the sail

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7 hours ago, Kiwing said:

@Mikko Brummer can you give me opinion please?

Did TNZ (or any other team) reverse the top of their main sail to provide righting moment and lower the COE, whilst racing?

FIFY ;)

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Max we never got low sun angle to see it and we needed stronger winds in the final for them to use this to push a bit beyond cavitation speed having reached max VMG? Only the sailors know, like the no look gybe and a few others that were not shown/seen in AC35.

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1 hour ago, Kiwing said:

Max we never got low sun angle to see it and we needed stronger winds in the final for them to use this to push a bit beyond cavitation speed having reached max VMG? Only the sailors know, like the no look gybe and a few others that were not shown/seen in AC35.

I re-watched a Prada Semi Finals race that had one of the highest wind strengths during the whole cycle. Gusts well over the limit, boat speeds at 46+ knots downwind etc etc. I can't see any sign of luffing/inverting of the main. ( youtu.be/C1rrU79FXGI?t=1522 )

image.png.a34eb767f16954f3e42554b1186b59e2.png

Re the NZ video of them luffing, I suspect they may have tried testing main inversion, like they've tested the batwing etc etc. But my guess is they found they got minimal (if any) RM from doing that, just extra drag. The mast rotation is the problem IMO.

I did mention that I hadn't seen the VO65s invert either... but I take that back.  Been re-watching some of the VOR 2017-2018 races, and these boats clearly do!
image.thumb.png.8eb3aa738efc8ed455700b925ec17a4a.png

youtu.be/5TBSUNnNAI4?t=3608

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3 hours ago, Sailbydate said:

Sail camber 13 - 30%.

Sounds insane!   I struggled to get XFoil to alter camber to such high camber levels... this is what it looks like at "just" 22%

image.png.05e817db25a1e5de9106a8dfd647f72f.png

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19 minutes ago, MaxHugen said:

Sounds insane!   I struggled to get XFoil to alter camber to such high camber levels... this is what it looks like at "just" 22%

image.png.05e817db25a1e5de9106a8dfd647f72f.png

Incredible.

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That would have to be with a wide open leech, as in flat off, would it not, Max?

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I contacted Danial Forster about his incredible pic of the VOR yachts sailing through fog, showing their air wake. Asked about wind speed etc, and he very kindly responded:

"The wind speed was about 12 knots, plus minus 3 knots.
I was in the helicopter at about 250 feet, used a 24 mm lens, and these never been seen phenomena photos went around the world.
 
Included some other angles.
Please sign those photos:
Copyright DanielForster.com if you use them!
I am available for further questions."
 
Have a look at Mr Forster's website, he has a great collection of sailing pics!
 
3yachts-fog01_A.thumb.jpeg.dba457fdde38264661bc32468911da68.jpeg
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DPPI_30000330_000129.jpeg.9fbcde6176717d99e07bae324fde76f1.jpeg
 
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31 minutes ago, Sailbydate said:

That would have to be with a wide open leech, as in flat off, would it not, Max?

Not sure what you mean.   Are you talking about having a large "gap" with such an extreme camber?

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1 minute ago, MaxHugen said:

Not sure what you mean.   Are you talking about having a large "gap" with such an extreme camber?

Sorry. Not explaining myself very well. I was speculating that the only time they could use such a deep camber effectively would be running at a deep TWD angle. Maybe a broad reach or almost flat off - to try and build speed for a lift-off.

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7 minutes ago, Sailbydate said:

Sorry. Not explaining myself very well. I was speculating that the only time they could use such a deep camber effectively would be running at a deep TWD angle. Maybe a broad reach or almost flat off - to try and build speed for a lift-off.

Ah, gotcha. 

We've certainly seen the boats using camber like that with their jibs, as you say at deep AWAs. These are also battened, maybe C-Tech was thinking in terms of those battens?

But I can't even start to imagine them getting within cooee of such massive camber on the mainsail though.

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3 hours ago, MaxHugen said:

I contacted Danial Forster about his incredible pic of the VOR yachts sailing through fog, showing their air wake. Asked about wind speed etc, and he very kindly responded:

"The wind speed was about 12 knots, plus minus 3 knots.
I was in the helicopter at about 250 feet, used a 24 mm lens, and these never been seen phenomena photos went around the world.
 
Included some other angles.
Please sign those photos:
Copyright DanielForster.com if you use them!
I am available for further questions."
 
Have a look at Mr Forster's website, he has a great collection of sailing pics!
 
3yachts-fog01_A.thumb.jpeg.dba457fdde38264661bc32468911da68.jpeg
fogcarving300x2-1.jpeg.30a783d38e6190d3a5341ef3d18b383d.jpeg
DPPI_30000330_000079.jpeg.0969700f41e8e9c68d33b3e47edf711e.jpeg
DPPI_30000330_000117.jpeg.d73d1c7c3707b1d8874e11d183425015.jpeg
DPPI_30000330_000120.jpeg.7a05677d673482fbce0459860d8e57fd.jpeg
DPPI_30000330_000124.jpeg.8dd3bcd4aaaace332cb1bd01d204ed01.jpeg
DPPI_30000330_000126.jpeg.b0009bad81d9af2cb118bad9e054473e.jpeg
DPPI_30000330_000128.jpeg.28f764c3c3d23b63bd27fcb361c7369a.jpeg
DPPI_30000330_000129.jpeg.9fbcde6176717d99e07bae324fde76f1.jpeg
 

Amazing pics!

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A simple take at the AC 75 bad air, upwind and downwind. Upwind, the flow field looks quite normal, there’s the usual backwind/less pressure zone (or wind speed, if you prefer, the blue circle) on the aft hip of the boat, and the increased pressure on the leeward bow (red circle).

In the general plan, the wind is little disturbed, the green color is the true wind speed 10 kn in the simulation, blue colors are about 9 kn and red 11 kn. The big disturbance is limited to the narrow wake back almost in the direction of motion, moving laterally over the field as the boat proceeds along diagonally.The sails are bending the wind considerably, however, over the whole field, an effect probably more important than the change in pressure.

A little disclaimer: the simulation was done in 2D. In 3D, the bad air vane behind the boat would be a little wider, as in the lower part it is directed straight back, while towards the top it will spread out to leeward, as the sails are more twisted and bend less the wind.

The true wind speed is 5 m/s (10 kn), true wind angle 45 degrees and boatspeed 28 kn, or 2,8 times the wind speed.

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Downwind, things look weird at the first glance: The pressure (or rather wind speed) is less on the leeward side/aft of the boat, and higher on the windward side, the opposite of what happens windward, and when sailing at speeds slower than the wind. But if you remember that we are looking at the true wind field here, from the perspective of someone standing still in the water (in the earth/inertial reference frame), it’s (perhaps) logical: The sails are accelerating the flow in front and on the leeward side of the boat, and decelerating it behind the boat.

At the start, there’s a red & blue blob left behind, convecting slowly downwind at 10 kn. You should ignore it, as it’s due to the abrupt starting of the motion, and left behind - but after a jibe, for instance, it could be relevant. For the most part, as in upwind, the disturbance is in a narrow wake more or less against the direction of the motion, and fading out rather fast. In the down wind case, TWS is 10 kn, TWA is 140 degrees and boatspeed 38 kn.

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That is really cool, Mikko.  Especially for the upwind case, its clear how the wake trails at the apparent wind angle even as it drifts with the true wind.

It would be interesting to see plotted the component of velocity in the cross-wind direction (probably the Y component in your plane).  That would give an idea of the header that is imposed on the boat behind.  While the energy deficit in the wake is fairly narrow, the wake wash influence is continuous across the wake and extends for a much wider zone.

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24 minutes ago, Basiliscus said:

It would be interesting to see plotted the component of velocity in the cross-wind direction (probably the Y component in your plane).  That would give an idea of the header that is imposed on the boat behind.  While the energy deficit in the wake is fairly narrow, the wake wash influence is continuous across the wake and extends for a much wider zone.

Yes, I already did this, will have to shrink the video down to the 9 MB limit of this forum. 

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That's really interesting  The zones are almost directly upwind and downwind of the boat.  And the classic lee bow position looks like it's in the null zones for both boats instead of the most affected regions.

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I’m trying to understand how different this is from biplane cell to stagger in that biplane cell,  morphing into a tandem (biiiig stagger?) cell/ into a tandem cell in line, as far as which wing is in downwash or upwash, and what effect different wakes have?

edit, this one is better, but just as one example

oops, left out decalage too

 

 

0239B38D-1828-472B-8B60-A09FE0B1430A.jpeg

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@Mikko Brummer  The pressures displayed in this appear to indicate that the wash pressure is virtually at "neutral" in an area of lower pressure.

image.png.a19c6aad135d7d9ed0b06e71f56f3a9a.png

Mr Forster's pics show a far larger wash, and no sign of that high pressure area to windward of the boat. What am I missing?

image.png.3862e0fa22a99b7a27247e19d824f73e.png

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30 minutes ago, MaxHugen said:

The pressures displayed in this appear to indicate that the wash pressure is virtually at "neutral" in an area of lower pressure.

No pressures shown here, only velocities. I've been using the word pressure here as sailors do in their speech, more "pressure" indicating a stronger wind (velocity).

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32 minutes ago, MaxHugen said:

Mr Forster's pics show a far larger wash, and no sign of that high pressure area to windward of the boat. What am I missing?

These are 2D simulations, so essentially "flat", at about 11 m height, not showing tip vortices, for instance. I hope to repeat these in 3D, but it takes time - while these run in minutes, each 3D run takes days or even a week.

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7 minutes ago, Mikko Brummer said:

No pressures shown here, only velocities. I've been using the word pressure here as sailors do in their speech, more "pressure" indicating a stronger wind (velocity).

Gotcha.  Still, it doesn't seem to correlate with the fog pattern in the photo, where wind strength was around 12 knots?

ps: Just saw your post re 3D, explains things a bit better...  but takes days? Crikey!

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This is going to take my brain a couple of weeks to digest.

Thank you @Mikko Brummer you have answered a lot of peoples questions about "hard to pass, can't live here, looking for clean air" ideas.

Your visualizations make it much clearer.  The best description I have seen.

Wow 3D visualization (even a set of still cross sections) would be amazing.

Your eye for accuracy is great too!

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14 hours ago, Mikko Brummer said:

Animation of the crosswind velocities. I need to re-run this in a wider domain, now the periodical sides influence too much towards the end.

In a true wind field of 5 kn a crosswind velocity of ±0,5 kn (the red & the blue areas) means a wind shift of nearly 6 degrees. So you want to avoid the blue areas... the dark blue blob right downwind of the boat is the one that no-one ever managed to break through in the pre-starts.

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On 3/25/2021 at 11:17 AM, MaxHugen said:

Gotcha.  Still, it doesn't seem to correlate with the fog pattern in the photo, where wind strength was around 12 knots?

ps: Just saw your post re 3D, explains things a bit better...  but takes days? Crikey!

I think what you are seeing in the fog picture is the rollup of the wake into a pair of trailing vortices.  A cross section through that wake would show a mushroom-like structure laying on its side.  Basically the same flow pattern you see after withdrawing a canoe paddle after a stroke.

This cartoon taken from a talk I gave illustrates the structure of the wake.  There are actually two types of 2D calculations one can make.  One concerns the flow around a section, without consideration of the wake.  This is what Xfoil does.  The other is a plane taken at right angles to the apparent wind.  This concerns the flow in the wake.  Between them, one can calculate the 3D aerodynamic characteristics of a wing or sail of moderate to high aspect ratio.

image.thumb.png.9c6abe4ce323d15a711a7fc3ce123d25.png

 

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On 3/26/2021 at 3:35 AM, Mikko Brummer said:

Here's a still pic of the crosswind velocities, blue is a header, red is lift for a boat on starboard.

CrosswindVelo JPG.jpg

 

On 3/26/2021 at 3:52 AM, Basiliscus said:

That's really interesting  The zones are almost directly upwind and downwind of the boat.  And the classic lee bow position looks like it's in the null zones for both boats instead of the most affected regions.

It would be even more interesting to see Mikko’s 3D view in orthogonal plan view, to get a better idea of the distances and angles needed for the optimum lee bow position?

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On 3/23/2021 at 4:59 PM, MaxHugen said:

I did mention that I hadn't seen the VO65s invert either... but I take that back.  Been re-watching some of the VOR 2017-2018 races, and these boats clearly do!

My boat has a (proportionally) larger square headed sails than either AC75 or VO65’s and they happily invert with insufficient leech tension, particularly the mainsail. Sorry I don’t have an inversion photo.

I could well imagine with twin skins and a controlling arm at the head, you could do all sorts of things?

 

2A324E48-188D-4136-8D4E-D02A3D6B45D1.jpeg

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5 hours ago, Sidecar said:

My boat has a (proportionally) larger square headed sails than either AC75 or VO65’s and they happily invert with insufficient leech tension, particularly the mainsail. Sorry I don’t have an inversion photo.

I could well imagine with twin skins and a controlling arm at the head, you could do all sorts of things?

 

2A324E48-188D-4136-8D4E-D02A3D6B45D1.jpeg

That's an unusual looking sail design.  :)

Is the boat a double-ended proa?

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It is a “pacific” Proa, with a sesqui biplane rig, hence my interest in sails to leeward.... It is all about the “stagger”.

I believe the rig has particular advantages for proas. 

Edited by Sidecar
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14 minutes ago, Sidecar said:

It is a “pacific” Proa, with a sesqui biplane rig, hence my interest in sails to leeward.... It is all about the “stagger”.

I believe the rig has particular advantages for proas. 

You piqued my curiosity, so I had to look that up. :rolleyes:

sesqui- (one and a half) +‎ plane, referring to long and short wings as “one and a half” wings.

Did you build this yourself?

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5 minutes ago, MaxHugen said:

You piqued my curiosity, so I had to look that up. :rolleyes:

sesqui- (one and a half) +‎ plane, referring to long and short wings as “one and a half” wings.

Did you build this yourself?

Yes.

Rather that add to thread drift, PM me if you or anyone else, is interested and I can provide some links with more details.

Sesqui biplanes were hi tech fashion at the end of WW1.

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51 minutes ago, Sidecar said:

Yes.

Rather that add to thread drift, PM me if you or anyone else, is interested and I can provide some links with more details.

Sesqui biplanes were hi tech fashion at the end of WW1.

I doubt anyone would complain here about a little thread drift while we're waiting for the next AC :lol:.

I'd love to see more pics of that proa and an explanation of how that rig works.

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6 minutes ago, erdb said:

I doubt anyone would complain here about a little thread drift while we're waiting for the next AC :lol:.

I'd love to see more pics of that proa and an explanation of how that rig works.

I'll second that!   It's about sailing boats and design, instead of... other stuff:)

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A bit OT, but at least on sails per se...

@Basiliscus, some months back you posted an image of a wingsail profile which I saved:

image.thumb.png.432472696ae62fd3d3f8d8f815895486.png

Was this the wingsail from the AC72, or just an example?

Just curious, as I was having a look at the dimensions of the F50 wingsail, and thought I'd compare it to the above pic.  I see that many of the proportions are quite similar, except for a huge difference in the wing thickness!

image.thumb.png.7481c9cf663fa239bbf0b351e6f86c5d.png

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Some views of the AC 75 trailing vortices in 3D. When it comes to the fog photos of the VOR 60s, I'm not sure they are showing just trailing vorticity... as I recall, it was a very special meteo situation, with a strong low inversion preventing the warm humid air rising any higher. It's water vapor, or clouds that we see, and probably air temperature & humidity, sea water temperature, wind gradient etc. are involved. CFD simulations are usually "isothermal", and don't take these in to consideration - we should consult a meteorologist ;-). They use the same equations to predict weather, but in a much larger scale, the cells are in kilometers, while here in this sim, a cell closer to the sails & in the wake is 8x8x8 cm... which is still coarse for CFD.

AC bad air JPG WB.jpg

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13 hours ago, Sidecar said:

t would be even more interesting to see Mikko’s 3D view in orthogonal plan view, to get a better idea of the distances and angles needed for the optimum lee bow position?

Orthoganal view and in a larger domain for less "side effects". The colors are swapped (sorry), red is a header and blue a lift, and the scale is also a little tighter to show more nuances - red & blue represent now a shift of 3,5 deg.

Crosswind JPG.jpg

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43 minutes ago, Mikko Brummer said:

Some views of the AC 75 trailing vortices in 3D. When it comes to the fog photos of the VOR 60s, I'm not sure they are showing just trailing vorticity... as I recall, it was a very special meteo situation, with a strong low inversion preventing the warm humid air rising any higher. It's water vapor, or clouds that we see, and probably air temperature & humidity, sea water temperature, wind gradient etc. are involved. CFD simulations are usually "isothermal", and don't take these in to consideration - we should consult a meteorologist ;-). They use the same equations to predict weather, but in a much larger scale, the cells are in kilometers, while here in this sim, a cell closer to the sails & in the wake is 8x8x8 cm... which is still coarse for CFD.

AC bad air JPG WB.jpg

Thanks for the info Mikko.

The vorticity looks very high from the sail/boat interface (?), does this model account for the sail to be end plated to the deck?

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29 minutes ago, MaxHugen said:

Thanks for the info Mikko.

The vorticity looks very high from the sail/boat interface (?), does this model account for the sail to be end plated to the deck?

Oh yes, absolutely, to the deck and also the sea. If you look at the vortices down at the sea surface, the one most "leeward" is from the keel or "bustle". It breaks down and dissipates fairly quickly, while the one from the head of the main, and from the windward foil, persist.

The big turbulence behind the hull comes from the trenches, where the winch pedestals are modeled but the sailors are missing - I should add them to see if they make things better or worse (probably better?). Towards the top of the sails, you can see the jib tip vortices rolling around the mainsails upper vorticity, the vortex rollup @Basiliscus describes. 

If you zoom in on the jib, it reveals why they want to change into a lower hoist, smaller jib so early - as soon as you start to twist off the upper main, to depower, you get the separation vortex rolling on the windward side of the top of the jib. This is not very efficient, so you are better off with a low aspect jib.

In the individual pictures, I've varied the vorticity intensity (shown in the scale), and also the filtering of weaker vorticity. Without filtering, everyhting will be covered in the blue, low level vorticity "fog". The last screen is showing what you'd see if you released weightless particles (like snowflakes) in the air in a plane in front of the jib.

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3 minutes ago, Mikko Brummer said:

Oh yes, absolutely, to the deck and also the sea. If you look at the vortices down at the sea surface, the one most "leeward" is from the keel or "bustle". It breaks down and dissipates fairly quickly, while the one from the head of the main, and from the windward foil, persist.

The big turbulence behind the hull comes from the trenches, where the winch pedestals are modeled but the sailors are missing - I should add them to see if they make things better or worse (probably better?). Towards the top of the sails, you can see the jib tip vortices rolling around the mainsails upper vorticity, the vortex rollup @Basiliscus describes. 

If you zoom in on the jib, it reveals why they want to change into a lower hoist, smaller jib so early - as soon as you start to twist off the upper main, to depower, you get the separation vortex rolling on the windward side of the top of the jib. This is not very efficient, so you are better off with a low aspect jib.

In the individual pictures, I've varied the vorticity intensity (shown in the scale), and also the filtering of weaker vorticity. Without filtering, everyhting will be covered in the blue, low level vorticity "fog". The last screen is showing what you'd see if you released weightless particles (like snowflakes) in the air in a plane in front of the jib.

Great, you preempted my next question which was about the source of that leeward vortex almost at sea level. :)  I'd often wondering how severe the vortex might be from the boxy Ineos skeg - must be quite a bit worse than TR's.

Very interesting to see how much the jib vortices travel up and increase the mainsail tip vortex - the "rollup" - even several boat lengths aft. The extent of the vortices from the crew pods was also rather surprising.

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7 hours ago, MaxHugen said:

A bit OT, but at least on sails per se...

@Basiliscus, some months back you posted an image of a wingsail profile which I saved:

image.thumb.png.432472696ae62fd3d3f8d8f815895486.png

Was this the wingsail from the AC72, or just an example?

Just curious, as I was having a look at the dimensions of the F50 wingsail, and thought I'd compare it to the above pic.  I see that many of the proportions are quite similar, except for a huge difference in the wing thickness!

image.thumb.png.7481c9cf663fa239bbf0b351e6f86c5d.png

That may have been from the BOR90.  

Here is a section from the AC72:

plot_W3ME01FILMc50p42t15p41W3FLAPr30_r3e6n1fix25e-3_a00_Page_1.thumb.png.1a5b4d678fee8e28e83b82ec10f7f56c.png

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46 minutes ago, Basiliscus said:

That may have been from the BOR90.  

Here is a section from the AC72:

plot_W3ME01FILMc50p42t15p41W3FLAPr30_r3e6n1fix25e-3_a00_Page_1.thumb.png.1a5b4d678fee8e28e83b82ec10f7f56c.png

Thanks for that.  A bit surprising to see Mach = 0.150 (100 knots!), and a huge CL of 2.55.

Although I haven't measured it yet, the AC72 still appears to have considerably greater thickness in the wing than the F50 wingsail. Might that be because the F50 wing is designed for generally lower AoAs?

I see that the AC75 also has a small extra flap on the wing. The F50 doesn't have this, I'd guess that the airflow benefit wasn't worth the added complexity?

This is my first attempt at the two element F50 foil, with 15° flap, at 12° AoA:

image.png.2cf21c43dd8bda7cfc55739d85e70433.png

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The Mach number was irrelevant.  It was just left at at a default low value because the flow is pretty much incompressible.

The tab on the main element is interesting.  It doesn't affect things the way one might expect.  It does add some complexity to the build, so different teams have included tabs or not.  The C-class catamarans have traditionally used a tab because they are trying to push maximum lift to the limit.  I never saw a landyacht wingsail with a tab.  The BOR90 wingsail did not have a tab in order to simplify the construction, based on the success of landyacht wingsails without tabs.  For the AC72, IIRC ETNZ started with a tab on their first wing but then deleted it from their next wing.  OTUSA used a tab on their AC72, and it turned out to be an important tool for tuning the wing during the regatta.  The AC50 and F50 did not have tabs.

Here is the effect of tab deflection for the AC72 wingsail section with 30 degree flap deflection.  Opening the slot with the tab decreases maximum lift.  However, for most tab angles, there is very little influence on either the lift or the drag.  This is because opening the slot with the tab has the effect of decreasing the lift on the main element while increasing the lift on the flap.  But the moment is significantly affected because the load is shifted from the main element to the flap.

plot_W3ME01FILMc50p42txxp41W3FLAPr30_r3e6n1fix25e-3_a-10_Cpx.thumb.png.95ee000b17bb4f3ebac0d690ace3a6b9.png

plot_polar_W3ME01FILMc50p42txxp41W3FLAPr30_r3e6n1fix25e-3.thumb.png.5bc7cba4fd930c632d120711af4a9f33.pngThis turned out to be important during the 34th Match for the AC because OTUSA's AC72 had lee helm, and this impacted the upwind speed.  The traditional fix for lee helm, increasing the rake, was tried during a lay day and was found not to be effective, because when the flap was twisted to invert the wing at the head, the negative lift at the head only added to the lee helm when the rig was raked.

Instead, over the next few races the slot was opened using the tab, starting at the bottom of the wing and then progressively changing the tab angle over more of the span.  The flap deflection was also modified, bringing up the mid leech and decreasing the flap deflection at the head.  These changes moved the center of effort aft, as seen by a significant increase in mainsheet tension.  The increased moment made more work for the crew trimming the wingsail, but alleviated the lee helm and made the boat faster upwind.

And all it took to change the tab angle was to retie the knots in the strings that linked the tab to the flap movement.

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On 3/26/2021 at 1:01 PM, Basiliscus said:

I think what you are seeing in the fog picture is the rollup of the wake into a pair of trailing vortices.  A cross section through that wake would show a mushroom-like structure laying on its side.  Basically the same flow pattern you see after withdrawing a canoe paddle after a stroke.

This cartoon taken from a talk I gave illustrates the structure of the wake.  There are actually two types of 2D calculations one can make.  One concerns the flow around a section, without consideration of the wake.  This is what Xfoil does.  The other is a plane taken at right angles to the apparent wind.  This concerns the flow in the wake.  Between them, one can calculate the 3D aerodynamic characteristics of a wing or sail of moderate to high aspect ratio.

image.thumb.png.9c6abe4ce323d15a711a7fc3ce123d25.png

 

Could these trailing vortexes imply a possible low drag span wise Bell curve lift distribution?  Or do power requirements obviate that even in higher wind speeds because of changing gears with jib selection?

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7 hours ago, Basiliscus said:

The tab on the main element is interesting.  It doesn't affect things the way one might expect. 

That was contrary to what I would have expected. Need to digest this some more. :huh:    Great info, cheers!

"And all it took to change the tab angle was to retie the knots in the strings that linked the tab to the flap movement." :P

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1 hour ago, Amati said:

Could these trailing vortexes imply a possible low drag span wise Bell curve lift distribution?  Or do power requirements obviate that even in higher wind speeds because of changing gears with jib selection?

The minimum induced drag for a given heeling moment is when the wind is deflected with a linear wake wash velocity along the span, including the jib.  The minimum induced drag is when the wake wash is uniform along the span, and for simple geometry this is produced by an egg-shaped lift distribution.  As the spanwise wake wash distribution is given more of a slope, the center of effort comes down and the lift distribution becomes more bell-shaped.  But the drag goes up.  Whether the extra lift for the same heeling moment is worth the extra drag or not depends on the rest of the boat design.  

The Design Team gives the sailors target flap and twist angles that are predicted to give the best performance in different wind conditions, but it's up to the sailors to find out what actually performs the best.

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50 minutes ago, MaxHugen said:

That was contrary to what I would have expected. Need to digest this some more. :huh:    Great info, cheers!

"And all it took to change the tab angle was to retie the knots in the strings that linked the tab to the flap movement." :P

FWIW, those figures were published in the Dec 2013 Seahorse magazine.

I think they are a good illustration that a slotted flap is best thought of as two airfoils in close formation that interfere with each other in a favorable way.  The conventional way of visualizing the flow through the slot as being like flow through a duct is not very helpful.  The reason is because the mass flow through the slot is not constant.  Air can flow through the slot or it can go around the entire section.  

Also, changing the slot geometry by other means than the tab can have different results.  Although the capability was never used, the BOR90 wing had a several possible locations for the hinge axis at different points along the main element chord.  Choosing a different hinge point would have opened or closed the slot by effectively moving the flap up or down at the same angle.  Changing the slot that way doesn't have as big an effect on the leeward side pressures of the main element.

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52 minutes ago, Basiliscus said:

The minimum induced drag for a given heeling moment is when the wind is deflected with a linear wake wash velocity along the span, including the jib.  The minimum induced drag is when the wake wash is uniform along the span, and for simple geometry this is produced by an egg-shaped lift distribution.  As the spanwise wake wash distribution is given more of a slope, the center of effort comes down and the lift distribution becomes more bell-shaped.  But the drag goes up.  Whether the extra lift for the same heeling moment is worth the extra drag or not depends on the rest of the boat design.  

The Design Team gives the sailors target flap and twist angles that are predicted to give the best performance in different wind conditions, but it's up to the sailors to find out what actually performs the best.

I’m trying to figure whether what Bowers (& Maybe Prandtl and the Hortons) would call a 3D Bell curve lift distribution (with twist) would work for a sailboat?  It does seem to bring the CE closer towards the root of the wing, but I think he says the drag is less for the Bell curve than the elliptical.  Is the difference a 3D vs a 2D approach?  Or is the egg shaped solution the best mathematical approximation that the crew can experiment from?  IIRR, Bowers reverse engineers the shapes of the wing from the desired lift distribution.  And he does say that if span is limited, it’s probably better to use a more elliptical lift distribution.  I’ve looked at some papers on this, and I’m a bit fuzzy about where span goes from limited to unlimited, from thrust from winglets to thrust from increased span, in the Bowers/ Prandtl case (Bell curve) to eliminate the vertical tale for flying wing flight control. Or am I conflating a mast/sail with half of a flying wing?

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