Forums > Windsurfing   Gps and Speed talk

Windsurfing slalom/speed sail twist

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Created by sailquik 1 month ago, 1 Apr 2019
decrepit
WA, 9114 posts
20 Apr 2019 9:32AM
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It's also obvious that the bell curve has a much lower CoE, just what windsurfers need to control the power, and reduce the down force on the nose.

So now we go back to Andrew's original question, how much twist is needed for the optimum bell curve?

sailquik
VIC, 4544 posts
20 Apr 2019 3:28PM
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USA46 said..
I was thinking will I post this link or not, because I knew that this old theory is inside text.I hope no one will read all text.
Sailquik you are now open new most complicated topic "ELIPTICAL VS BELL spanload distribution"[/b]


Many thanks. I really appreciate that you did decide to publish these links. I have spent the whole morning going down that rabbit hole and I am beggining to see the light at the end of the tunnel. That is exactly the sort of stuff I was hoping for.

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USA46 said..
our sails has bell spanload,especially with high downhaul trim, page 32 ,figure 19 [/b]


Can you tell us where this is and what it refers to please?

USA46
24 posts
20 Apr 2019 1:57PM
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sailquik said..

Can you tell us where this is and what it refers to please?



I didn't understand you well,how do you mean where this is?

I put link below this text in my post..maybe you didn't see it..open page 32,figure 19

link:
mauiultrafins.com/technology-2/windsurfing-mechanics/


figure 19 show lift distribution over height of sail.You can see in lower part is max lift(power zone),lift is droping with height as consequnece of sail twist and taper,if you trim sail with high downhaul like on slalom sail,head will be floppy ,so lift line in graph will touch zero point even before 5m height on this picture...etc

sailquik
VIC, 4544 posts
20 Apr 2019 6:11PM
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USA46 said..

I didn't understand you well,how do you mean where this is?

I put link below this text in my post..maybe you didn't see it..open page 32,figure 19

link:
mauiultrafins.com/technology-2/windsurfing-mechanics/


figure 19 show lift distribution over height of sail.You can see in lower part is max lift(power zone),lift is droping with height as consequnece of sail twist and taper,if you trim sail with high downhaul like on slalom sail,head will be floppy ,so lift line in graph will touch zero point even before 5m height on this picture...etc


Ahhh, gotcha now. I didnt notice that link has pages.

Te Hau
363 posts
20 Apr 2019 4:46PM
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USA46 said..




sailquik said..


Can you tell us where this is and what it refers to please?




I didn't understand you well,how do you mean where this is?

I put link below this text in my post..maybe you didn't see it..open page 32,figure 19

link:
mauiultrafins.com/technology-2/windsurfing-mechanics/


figure 19 show lift distribution over height of sail.You can see in lower part is max lift(power zone),lift is droping with height as consequnece of sail twist and taper,if you trim sail with high downhaul like on slalom sail,head will be floppy ,so lift line in graph will touch zero point even before 5m height on this picture...etc


USA 46, thanks for those links, very interesting.

sailquik
VIC, 4544 posts
20 Apr 2019 6:53PM
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decrepit said..
It's also obvious that the bell curve has a much lower CoE, just what windsurfers need to control the power, and reduce the down force on the nose.

So now we go back to Andrew's original question, how much twist is needed for the optimum bell curve?


Well, I have not finished reading though and digesting all the links and papers yet, but my initial feeling is that we are not getting it yet, even with the most twisted sails.

The reason is that the sail is a flexible structure (although so is the birds wing) and we cant actually get enough twist to get a reversal of lift at the top (negative lift). But I am thinking birds can't either, so more thinking and study required......

If it is possible to remove the tip vortex from the actual top/tip of the sail to some place lower on the span, we would have it. Maybe we do have it now in some circumstances??? Maybe not. Maybe it would requite us to go back to a more Higher Aspect sail???

I am wondering if the Little AC designers tried to achieve this. If so, there should be evidence of significant twist in their wings.

decrepit
WA, 9114 posts
20 Apr 2019 5:13PM
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I'm not sure you can shift the tip vortex?
I think it's either at the tip or not there at all.
It's at the tip because the high pressure can get to the low pressure there. It can't happen on the trailing edge because the airflow stops that happening.

sailquik
VIC, 4544 posts
20 Apr 2019 8:05PM
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decrepit said..
I'm not sure you can shift the tip vortex?
I think it's either at the tip or not there at all.
It's at the tip because the high pressure can get to the low pressure there. It can't happen on the trailing edge because the airflow stops that happening.




Perhaps I should have said the 'vortex' As I understand it, In the Bell Curve situation, there is still a vortex off the wing, but it is not at the tip, it is where the lift crossed from positive to negative, and I assume it is a lot smaller. And, as I understand it so far, this results in a higher lift to drag ratio, caused by a big decrease in induced drag (from the tip vortex).




Pacey
WA, 107 posts
20 Apr 2019 9:39PM
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sailquik said..

I am wondering if the Little AC designers tried to achieve this. If so, there should be evidence of significant twist in their wings.


I don't know about the Little AC designers, but I believe that both Oracle and Team NZ played with negative lift in the head of their wings for the 2013 America's Cup. This was possible as the wings were a slotted flap design, and it was possible to set the angle of the rear flap (and its twist) independently for the lower, middle and upper sections of the wing. However, my understanding is that this was done for the purpose of increasing righting moment when overpowered as opposed to minimising drag.

What the outcome of these tests were and whether the approach was used in races or in the subsequent 2017 America's Cup I don't know, but I can make some enquiries.

yoyo
WA, 1614 posts
20 Apr 2019 10:12PM
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Pacey said..
..I believe that both Oracle and Team NZ played with negative lift in the head of their wings for the 2013 America's Cup. This was possible as the wings were a slotted flap design, and it was possible to set the angle of the rear flap (and its twist) independently for the lower, middle and upper sections of the wing. However, my understanding is that this was done for the purpose of increasing righting moment when overpowered as opposed to minimising drag.


Been aware of this concept of aerodynamic ballast for several decades now. It is quite interesting but in the past difficult to impliment. The AC cats would be the perfect platform to try it.

USA46
24 posts
21 Apr 2019 1:35AM
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sailquik said..



The reason is that the sail is a flexible structure (although so is the birds wing) and we cant actually get enough twist to get a reversal of lift at the top (negative lift). But I am thinking birds can't either, so more thinking and study required......





Bell spanload don't has negative LIFT at top,lift is positive and fall down to zero towards top but it is NEVER negative, it has NEGATIVE INDUCE DRAG /induced thrust. This is two different terms.
(If top would has negative LIFT,then lift line will fall down below zero (X -axis ) on graph.To have negative LIFT at top,leeward side of sail must become high pressure side,this is imposible to achieve with our flexible soft sails.Negative lift is solution for increase righting moment, not to reducing drag,negative lift will increase drag because lift vector will point more backwards)


look at induce drag picture,from 0.85 to 1.2 ,drag line fall down below x-axis,here drag become "negative drag" and produce some small amout of thrust.



in general:
-lift allways prependicular to apparent wind
-drag allways paralel to apparent wind
-resultant is vector sum lift and drag


AoA1 is sail angle of attack between apparent wind and sail at lower parts(boom..)
AoA2 is sail angle of attack between apparent wind and sail at top(head)
-Alfa is angle between apparent wind and local upwash at top of sail

Lift head vector (blue) is slightly tilted forward ,so we have a little thrust at top or this is called negative induced drag.
sail upper part has different local apparent wind angle ,becasue of upwash,so lift vector is tilted forward thus minimize drag.
The key is upwash,bell spanload use upwash as adventage to make small amount of thrust at top parts.






quote from link:

"For the bell spanload, shown in figure 2(b), the net force vector is such that it varies along the span. Inboard, the force vector is tilted away from the relative wind, like that of the elliptical spanload case, and the parallel component produces induced drag. Progressing outboard, this parallel component reduces in magnitude until it eventually (past b/2 = 0.704) is tilted into the relative wind. This phenomenon is referred to as induced thrust (that is, negative induced drag). It should be noted that the sum total force of this parallel component is still producing a net drag (and this sum total is more than that of an elliptical spanload for the same span - in our case we are able to increase the span and achieve less total induced drag), but locally for the outer 0.296 span, it produces thrust."

TGale
TAS, 284 posts
21 Apr 2019 4:48AM
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^^ ... presumably our backwards raked sails will exaggerate this effect by encouraging more airflow up the sail.

sailquik
VIC, 4544 posts
21 Apr 2019 9:49AM
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Ahh, very well explained USA46. Thank you.

USA46
24 posts
21 Apr 2019 12:25PM
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(I forgot it to write word "local" in discription of AoA2(my picture in previous post), AoA2 is angle of attack between local apparent wind(upwash) and sail head.)




Picture 3a shows downwash distribution behind wing.Arrows pointed down represent downwash and arrows pointed up represent upwash behind wing.We can see that at some point of wing downwash shift in upwash.This is point where induce drag become negative/this wing part produce small thrust component.


People allways ask me how wing can produce positive lift(lift vector poitned up) at outborad wing part if arflow behind this wing section goes upward(upwash)?
Yes it can, because outborad wing part "feel" oncoming local apparent wind("upwash") is coming from different angle to wing...

-Upwash is upward airflow movemet in relation to freestream wind
-Downwash is downward airflow movement in relation to freestream wind
-Freestream airflow/wind is wind that is not disturb by any object or phenomen,in windsurfing it will be apparent wind(vector sum of true and head wind)

In general:
Whenever wing produce lift, upwash is happening in front of wing and downwash behind wing.Because lower pressure on top of wing has influence on oncoming airflow, it " sucks" airflow upward.And behind wing is going down because wing is tilted down at angle(AoA).
But here we have on outboard wing upwash infront and behind wing , BUT both in RELATION TO FREESTREAM WIND.
So this is relative term,If we look at outboard part of wing separately,when freestream wind is not our reference point,than again airflow infront is bend up(upwash) and ariflow behind wing is bend down(downwash) as it should be whenever every wing produce lift.
Beta is angle between freestream and local upwash airflow,this part of wing "feel" local upwash airflow not freestream wind.
I know that all this is confusing but I'm doing my best to make it clearer..








decrepit
WA, 9114 posts
21 Apr 2019 12:57PM
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You're right, complicated and confusing indeed, but you're doing an excellent job of making it less so. Thanks very much.

When Andrew started talking about negative lift. I was very confused, thanks for confirming it can't happen with a windsurfing sail.

sailquik
VIC, 4544 posts
22 Apr 2019 1:33PM
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Yes, spot on Decrepit.

Thankyou USA46. you are indeed making it much clearer.

So it seems that we indeed may be benefiting from an overall reduction in induce drag with a highly twisted, flat camber head, sail.

So back to my initial question. How much twist do we need to take full advatage of this?

These are the questions that come to mind:

Is there a way to calculate the 'Beta' (Upwash) angle for a given sail? If we assume a certain angle of sail and angle of apparent wind?

I can see that the camber of the sail and the agle of attack will influence this, along with the velocity of the airflow. Can we get close enought on what these things would be in a speed sail to get a number for twist?

Also, would it be correct that the deeper the camber/draft of the sail in the lower section, the greater the upwash Beta angle would be at the top, and that we would need more twist?

This also seems to mean that on an upwind sail, which has a finer leading edge angle and shallower draft, the upwash angle beta would be less and we would want less twist for the same benefit.

Pacey
WA, 107 posts
22 Apr 2019 4:52PM
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sailquik said..
Also, would it be correct that the deeper the camber/draft of the sail in the lower section, the greater the upwash Beta angle would be at the top, and that we would need more twist?


As I said in a previous post, the deeper the camber in the lower part of the sail relative to the upper part, the more aerodynamic (i.e. not geometric) twist you will have in the sail because of the differences in their angles of zero lift.

MrFussy
WA, 4 posts
23 Apr 2019 12:48PM
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Boy, a lot of interesting stuff here. As has been shown you can see how lift is generated, what happens at the ends of wings as lift tries to escape the easiest way possible, and what can be done to control it if need be.One thing not really mentioned in here is that windsurf sails are hand held, alot that goes into windsurf sail design is not so much searching for ultimate lift to drag numbers but it's how to be able to get going, once going being able to keep accelerating and ultimately be able to hang on to it in the end. The advent of heaps of twist coming into sail design (along with strong draft forward wherever it exists) enabled more sail area to be carried, flat but twisted off it generates drive forward (because its rotated what lift it does generate is more in the direction you want to go) but once underway and apparent comes forward that power turns off (the head turns into a wind vane) as the rest of the lower sail powers up with speed so you can still hang on. As the head is now more of a wind vane the flow on each side is equal and it now acts like a fence and keeps vortices in check to some degree. So the twists job changes through the range. In building twist in the other effect is that the mast becomes shorter, the head is no longer at the top of the mast but in a sense about a third of the way down, that's where your leach tension becomes directed to. The mast then effectively becomes shorter and stiffer which reduces mast movement and the changing of draft position that usually comes with it, the sail is more stable and once again you can hang on to it better.
What's the right amount of twist? ,depends on what angles you are sailing and how the head matches how the rest of the sail is being set. Keep in mind that downhauling the crap out of your sails is to increases all the above effects and to overpower the material makeup of the sail, to increase its flexibility to deal with mast movement, and all lead to control which gives you speed. Control is often turning things off, not on, you can't hang on to certain kinds of efficiency. To be fast you sometimes have to work with inefficiencies, so don't get carried away crunching numbers looking for magic, good sails have range because they are tuneable to what's happening on the day and you have to set the tune.

decrepit
WA, 9114 posts
23 Apr 2019 1:09PM
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Nice one Bugs

olskool
QLD, 1283 posts
23 Apr 2019 3:24PM
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Mr Fussy, thats a Great easy to understand comprehensive explanation. Agreed there is no one magic number. As with all windsurfing gear its about what suits the individual on the day. But id guess theres a certain scope/amount of twist thatll work for most.
Bring out the smoke flares Sailquik. Lets see how different sails look in reality.

sailquik
VIC, 4544 posts
23 Apr 2019 10:45PM
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Yes Bugs, an elegant description. Practical observations and theories supported by experience, and trial and error experimentation. Many thanks for thanks for your explanation.

What I was hoping for is to find the actual underlying physics princples and actual evidence of them, and/or how thay actually work.

I am sure you will agree that more knowledge of the actual physics can help to focus the direction of further, more rapid advancements. Particularly for different windsurfing disciplines.

I think we have some of the key pieces now and know where to look for the actual evidence that goes with it.

It might indeed be time to get out the smoke generators.

barney831
99 posts
23 Apr 2019 10:12PM
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sailquik said..
I am sure you will agree that more knowledge of the actual physics can help to focus the direction of further, more rapid advancements.


The best way to demonstrate a complete understanding of the physics is to write a numerical model that simulates measured results. The simulations for the first moon landing were so good that the astronauts felt during the landing that they had been there before.

sailquik
VIC, 4544 posts
24 Apr 2019 12:32AM
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Oh come on Barney. Who's is going to do that?

barney831
99 posts
23 Apr 2019 10:57PM
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sailquik said..
Oh come on Barney. Who's is going to do that?


Perhaps one of the internet experts who understands the physics or knows how to use google.

USA46
24 posts
24 Apr 2019 3:13AM
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sailquik said..
Yes, spot on Decrepit.

Thankyou USA46. you are indeed making it much clearer.

So it seems that we indeed may be benefiting from an overall reduction in induce drag with a highly twisted, flat camber head, sail.

So back to my initial question. How much twist do we need to take full advatage of this?

These are the questions that come to mind:

Is there a way to calculate the 'Beta' (Upwash) angle for a given sail? If we assume a certain angle of sail and angle of apparent wind?

I can see that the camber of the sail and the agle of attack will influence this, along with the velocity of the airflow. Can we get close enought on what these things would be in a speed sail to get a number for twist?

Also, would it be correct that the deeper the camber/draft of the sail in the lower section, the greater the upwash Beta angle would be at the top, and that we would need more twist?

This also seems to mean that on an upwind sail, which has a finer leading edge angle and shallower draft, the upwash angle beta would be less and we would want less twist for the same benefit.






Is there a way to calculate the 'Beta' (Upwash) angle for a given sail? If we assume a certain angle of sail and angle of apparent wind?

Aircraft wing "feel" in 99% condition only head wind,so they are optimize for this cruising speed at given AoA.
Our sails are felxibile and this is good for our condition because we have, different courses,AoA,wind speed,shifty gusts,rake, etc all the time
Too much variable.Even if you calculate some "optimum twist angle", when wind load ,sail will open and increase this "optimum" twist angle when go downwind compare to upwind etc...So ther is no optimum angle in this shifty condition.

Even today engineers can't calculate 100% accurate wing lift with Navier Stokes equations.They have correct numbers with experiments in wind tunnels.Calculate upwash angle is far more complicate than basics lift..Aeodynamics is complicated stuff

The key is unload tip, especially for upper end.
I trim all my sails with floppy head when sailing,it that way I know I am in "bell spanload " configuration and this prevent upper part from stall.
So my rule is whenever head is floppy I have correct twist angle..Floppy head is only way how you know that head not produce lift.
If you try trim sail like this with too stiff mast ,you will flatten power zone nad kill the sail.This is reaason why mast must be compatabile.




Also, would it be correct that the deeper the camber/draft of the sail in the lower section, the greater the upwash Beta angle would be at the top, and that we would need more twist?
This also seems to mean that on an upwind sail, which has a finer leading edge angle and shallower draft, the upwash angle beta would be less and we would want less twist for the same benefit.

Yes,more camber down,more AoA(sheet in),more rake and more taper will cause more upwash on upper part.
Yes,upwind need less twist.


It might indeed be time to get out the smoke generators.


When wing increase AoA(our sheet in),streamlines infront leading edge bend upward.This is upwash.



or here,on 21:50 low pressure on top of wing cause oncoming air to shift upwards,this is upwash.
&t=1318s

boardsurfr
WA, 769 posts
24 Apr 2019 6:00AM
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I'm with Daffy on thinking that more experimental data could lead to a better understanding and therefore better sails. I also agree with Barney831 that a successful simulation would be a very good way to "demonstrate a complete understanding". The funny thing is, though, that to call a simulation successful, you need to show that it matches actual measured data (unless you're the marketing guy).

I think numerical simulations have helped in fin design in some cases- the Tectonics Weed Demon comes to mind, and there must be others, too. But these are comparatively easy simulations with a fixed fin shape and constant speeds and angles, as opposed to a constantly changing sail shape in variable wind - and they still required tons of (super-)computing power. Adding all these variables pretty much introduces a combinatorial explosion - the typical "brute force" approaches to numerical simulations don't stand a chance.

This reminds me a lot of computer approaches to playing chess and go. Chess is comparatively simply, and enough CPUs will allow brute-force solutions that beat the best human players. The same approach in Go completely failed - at the time where any computer was able to play chess at the level of a grand master, even the best go programs on high end clusters had no chance against top level amateur go players.

The outcome changed dramatically just a couple years when a new neural network approach succeeded in beating the best human go players. Most amazingly, the software learned to play better than any human only by playing against itself - the only "knowledge" from humans it needed where the rules of the game (which, importantly, include how to determine the winner).

I think a similar black-box, neural-net based approach might be the only feasible approach to simulating the air flow around and forces on a flexible windsurf sail. Since China is leading in both AI research and in windsurfing competition, maybe there is hope! But as Daffy explained, we need data! Smoke generators in front of the mast to visualize the air flow should work beautifully with GoPros to capture the data at 120 fps in real-world conditions. Much cheaper than wind tunnels, too! This could be a great use for all those safety flares required for slalom racing that expire unused. Might be best for places like Shark Bay, though, to minimize the chances that the data gathering is cut short by an early "rescue".

sailquik
VIC, 4544 posts
24 Apr 2019 9:56AM
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boardsurfr said..
This could be a great use for all those safety flares required for slalom racing that expire unused. Might be best for places like Shark Bay, though, to minimize the chances that the data gathering is cut short by an early "rescue".


yep, I think we will need to have a prior chat with the local emergency services!

sailquik
VIC, 4544 posts
24 Apr 2019 9:59AM
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I was talking to Mal Wright about his fin design analysis software (Finmaker) yesterday. I wonder of it could handle the shape of a sail?

sailquik
VIC, 4544 posts
24 Apr 2019 10:02AM
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Why is it that all these great films demonstrating this stuff, were all made 40-50 years ago It was the same with all the hyrofoil stuff that Fangy dug up.

fangman
WA, 843 posts
24 Apr 2019 8:14AM
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sailquik said..
Why is it that all these great films demonstrating this stuff, were all made 40-50 years ago It was the same with all the hyrofoil stuff that Fangy dug up.


Maybe because all the best sailors were made 50 years ago too?



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"Windsurfing slalom/speed sail twist" started by sailquik