Tornado-Cat

Boats and foils comparison

Recommended Posts

3 hours ago, Sailbydate said:

 Ha. At least your machine had paper tape. Our mechanical Burroughs calculators in the Actuarial Dept. had crank handles. But then I also travelled to school on a steam train. ;-)

I used those those calculators in my physics II labs they were a real pain to get 7 significant places out of. I rowed my dinghy up river to one school !?  Lucky you, just admired the steam engine in the Earnslaw across Wakatipu to lunch at Walter Peak station, which I highly recommend by the way.

Share this post


Link to post
Share on other sites
6 hours ago, terrafirma said:

AC75 VVP Analysis from a guy called Alan Smith from a Facebook Page https://www.facebook.com/groups/2397006860595933/ for those interested. Too many figures for me LOL

AC75 VVP v2.xlsx

Very nice, and it employs a similar method to what I've been doing, create a spreadsheet with varying speed and see where things balance out for a given TWA and TWS. He incorporated input from votex as well, but what I don't see - maybe missed something is the effect of rudder downforce. For me, that's what makes the whole balancing act unique and complicated, because for every other sailboat, you can come up with a max righting moment that you cannot exceed with sail forces. That makes it possible to calculate backwards from there. However, on the AC75, due to the canted foil, righting moment "automatically" increases as you increase side force on the sail. This works, because the higher lateral sail force needs to be countered by higher lateral force on the foil, which is directly coupled to the lift force on the foil by the cant angle. As that lift increases, the rudder downforce becomes greater, too, and since they are separated laterally, now you have increased righting moment. 

I don't think it worked that way on the AC50, because even though they used rudder downforce, their main foil wasn't really canted out, so an increase in lateral force didn't automatically increase foil lift like on the AC75. Maybe the IMOCAs are similar with the big Dali foils, because as they lift more and more of the hull out of the water, more and more of the boat's weight is available for righting moment, but only till the whole boat is lifted out. 

Share this post


Link to post
Share on other sites

Question for @Basiliscus or @Erwankerauzen : How do you input data on your wing profile obtained with xfoil into vortex?

I see two ways: First, zero lift AoA for at the head and foot, and second checking for the sum error^2 for Cl>Cl_max. The Cl_max part is clear (I think), however, I'm not sure how the zero lift angle affects the calculation and how you'd deal with the soft double-skinned main, which by profile shape would induce lift even at zero or slightly negative AoA, but in reality, it wouldn't work because it'd lose its shape.

Thanks for the help!

 

 

Share this post


Link to post
Share on other sites

In the lifting line analysis, the section shape shows up in two ways.  First, as you mention, is the zero lift angle of attack.  The more camber the section has, the more negative the zero lift angle will be.  A double skinned main is no different from a single skinned mainsail in this regard.  Both shapes would create lift at zero angle of attack (if you could keep them from luffing).

The second way is the lift curve slope.  In inviscid thin airfoil theory, this has a value of 2 pi per radian.  In principle, a thick section can have a little higher value, but the boundary layer thickness tends to offset this.  For viscous flow, a value of 0.1 per degree is a reasonable guess.

You can get both the lift curve slope and zero lift angle from Xfoil.  Just tell it to calculate the pressure distribution for zero lift and it'll give you the zero lift angle.  Do it again for zero angle of attack and divide that by minus the zero lift angle and you'll have the lift curve slope that is good for the linear lift range.  Or you can fit a line to the lift range of interest and use the intercept and slope of that line.

The lifting line method is really concerned with the influence of the wake and doesn't care what the actual angle of attack is.  You can get the same lift by having a symmetrical section at an angle of attack, or a cambered section at a low angle of attack.  The same goes for the chord.  You can get the same lift with a long chord at low angle of attack or a short chord at high angle of attack.  The associated induced drag will be the same.  The difference comes when backing out the planform shape or twist distribution from the spanwise lift distribution.

On the whole, I don't really get the motivation for the double skinned mainsail.  It can eliminate the separation bubble you'd get on the windward side with a single skinned mainsail and rotating wingmast.  But the soft sail rig on the BOR90 trimaran USA17 achieved attached flow on both sides of the mast all the way back to the sail track over 2/3 of the span.  It was only in the lower 1/3 of the span, where the angle of attack was quite low because of the jib that there was a windward separation bubble.  And that might have been eliminated if the mast could have been twisted Crazy Coyote style.  I suppose it helps to fair mechanisms used to control twist, but those are only at the head and foot.  But it seems like a lot of weight and complication to achieve that.  Landyachts quite successfully use large wingmasts with single skin mainsail at low apparent wind angles and higher apparent wind speeds than the AC75s will see.  And the best of the landyacht wingmast/sail combinations are competitive with the best of the rigid wingsail rigs.  So you don't need separate surfaces to go fast.

  • Like 2

Share this post


Link to post
Share on other sites
23 minutes ago, Basiliscus said:

In the lifting line analysis, the section shape shows up in two ways.  First, as you mention, is the zero lift angle of attack.  The more camber the section has, the more negative the zero lift angle will be.  A double skinned main is no different from a single skinned mainsail in this regard.  Both shapes would create lift at zero angle of attack (if you could keep them from luffing).

The second way is the lift curve slope.  In inviscid thin airfoil theory, this has a value of 2 pi per radian.  In principle, a thick section can have a little higher value, but the boundary layer thickness tends to offset this.  For viscous flow, a value of 0.1 per degree is a reasonable guess.

You can get both the lift curve slope and zero lift angle from Xfoil.  Just tell it to calculate the pressure distribution for zero lift and it'll give you the zero lift angle.  Do it again for zero angle of attack and divide that by minus the zero lift angle and you'll have the lift curve slope that is good for the linear lift range.  Or you can fit a line to the lift range of interest and use the intercept and slope of that line.

The lifting line method is really concerned with the influence of the wake and doesn't care what the actual angle of attack is.  You can get the same lift by having a symmetrical section at an angle of attack, or a cambered section at a low angle of attack.  The same goes for the chord.  You can get the same lift with a long chord at low angle of attack or a short chord at high angle of attack.  The associated induced drag will be the same.  The difference comes when backing out the planform shape or twist distribution from the spanwise lift distribution.

On the whole, I don't really get the motivation for the double skinned mainsail.  It can eliminate the separation bubble you'd get on the windward side with a single skinned mainsail and rotating wingmast.  But the soft sail rig on the BOR90 trimaran USA17 achieved attached flow on both sides of the mast all the way back to the sail track over 2/3 of the span.  It was only in the lower 1/3 of the span, where the angle of attack was quite low because of the jib that there was a windward separation bubble.  And that might have been eliminated if the mast could have been twisted Crazy Coyote style.  I suppose it helps to fair mechanisms used to control twist, but those are only at the head and foot.  But it seems like a lot of weight and complication to achieve that.  Landyachts quite successfully use large wingmasts with single skin mainsail at low apparent wind angles and higher apparent wind speeds than the AC75s will see.  And the best of the landyacht wingmast/sail combinations are competitive with the best of the rigid wingsail rigs.  So you don't need separate surfaces to go fast.

Awesome, thank you. Just a final question, once I got the CL, CDi from vortex, is it prudent to then go back to the xfoil data and look at what viscous drag goes with the average CL shown by vortex? I assume I have to add that to induced drag to get total drag.

For the second part of your post, great to get an insight on this. Maybe the reason is the coolness factor? :)

Share this post


Link to post
Share on other sites
10 hours ago, terrafirma said:

AC75 VVP Analysis from a guy called Alan Smith from a Facebook Page https://www.facebook.com/groups/2397006860595933/ for those interested. Too many figures for me LOL

AC75 VVP v2.xlsx

I like this chart that show the relative relationships to the amounts of drag on the AC75's:

image.png.e8281f770121a95e51d8271383defc98.png

Share this post


Link to post
Share on other sites
7 hours ago, Basiliscus said:

On the whole, I don't really get the motivation for the double skinned mainsail.  It can eliminate the separation bubble you'd get on the windward side with a single skinned mainsail and rotating wingmast.  But the soft sail rig on the BOR90 trimaran USA17 achieved attached flow on both sides of the mast all the way back to the sail track over 2/3 of the span.  It was only in the lower 1/3 of the span, where the angle of attack was quite low because of the jib that there was a windward separation bubble.  And that might have been eliminated if the mast could have been twisted Crazy Coyote style.  I suppose it helps to fair mechanisms used to control twist, but those are only at the head and foot.  But it seems like a lot of weight and complication to achieve that.  Landyachts quite successfully use large wingmasts with single skin mainsail at low apparent wind angles and higher apparent wind speeds than the AC75s will see.  And the best of the landyacht wingmast/sail combinations are competitive with the best of the rigid wingsail rigs.  So you don't need separate surfaces to go fast.

You mention the large wing masts of the land yachts... isn't that almost what you effective get using the mast/twin skin of the AC75?

The mast is already 9-10° of the chord profile. Due to it's thickness, I think you get a similar chord profile to the land yachts, no?

If you get a separation bubble, with it's turbulent flow, doesn't that increase drag?

There are other benefits to the twin skin - it is possible to progressively increase twist as you move towards the head, something a solid wing or wing mast can't do.

I feel that this design will in time be adapted by ocean racing yachts as well. There are challenges, such as reefing, but these may be resolved in time.

image.png.70f378b67cd302b467c00b9a535386e4.png

Share this post


Link to post
Share on other sites
11 hours ago, Basiliscus said:

On the whole, I don't really get the motivation for the double skinned mainsail.

I think a big consideration is the very high apparent wind speed. Not really sure if it applies to most boats, but seems like a very small affect would be critical upwind on these boats. They are doing 20kts VMG. And apparent wind speed is probably well in excess of 40kts. With a very tight apparent wind angle. This requires very high efficiency/very low drag/very high power.

  • Like 2

Share this post


Link to post
Share on other sites

Looking forwards to seeing all the teams out tomorrow. The weather is looking a bit sh@t for the next week so no doubt they will try and get time on tomorrow.

The forecast has eased a bit 20 knots  N in the the morning to 15 knots NW in the arvo. See what shenanigans LR have been up to in the shed maybe :blink:

  • Like 1

Share this post


Link to post
Share on other sites
15 hours ago, The_Alchemist said:

I like this chart that show the relative relationships to the amounts of drag on the AC75's:

image.png.e8281f770121a95e51d8271383defc98.png

Like it too. Would like it even more if the author (like 95% of engineers) didn’t have a problem with significant figures. Meaning within the context approximation it makes no sense to write 51.83%, 52% would be appropriate

Share this post


Link to post
Share on other sites
On 11/24/2020 at 9:54 AM, erdb said:

Awesome, thank you. Just a final question, once I got the CL, CDi from vortex, is it prudent to then go back to the xfoil data and look at what viscous drag goes with the average CL shown by vortex? I assume I have to add that to induced drag to get total drag.

For the second part of your post, great to get an insight on this. Maybe the reason is the coolness factor? :)

Yes, the induced drag from the lifting line analysis has to be added to the profile drag from Xfoil.  And all the other drag sources.  It's really a big bookkeeping exercise.  Each source of drag has a different physical cause. 

With regard to the profile drag, in the past I've added a sheet with section data that cover a range of angles of attack and Reynolds numbers.  Then I used the chord and local lift coefficient from the lifting line to look up the profile drag coefficient and add that to the induced drag to get the total drag at each station.  Then integrate the total drag over the span to get the final number.

On 11/24/2020 at 4:40 PM, MaxHugen said:

You mention the large wing masts of the land yachts... isn't that almost what you effective get using the mast/twin skin of the AC75?...

Yes, exactly.  So why go to the bother of two sails?  

 

On 11/24/2020 at 8:53 PM, nroose said:

I think a big consideration is the very high apparent wind speed. Not really sure if it applies to most boats, but seems like a very small affect would be critical upwind on these boats. They are doing 20kts VMG. And apparent wind speed is probably well in excess of 40kts. With a very tight apparent wind angle. This requires very high efficiency/very low drag/very high power.

Landyachts experience 40 kt apparent wind and a lot more.  I've personally clocked landyachts doing 5 - 6 times the true wind speed, and the apparent wind upwind is approximately the same as the yacht's speed is downwind.  You're probably talking around 20 degrees apparent wind angle for the AC75 and a landyacht apparent wind angle is more like 14 deg.

Share this post


Link to post
Share on other sites
1 hour ago, Basiliscus said:

Yes, exactly.  So why go to the bother of two sails? 

As you mention, they try to reduce every scrap of drag they can. Is it not worthwhile to eliminate the slight drag reduction from having a separation bubble formed on the windward side, around the intersection of foil and sail, as used by the land yachts?

Share this post


Link to post
Share on other sites
7 hours ago, Basiliscus said:

Yes, the induced drag from the lifting line analysis has to be added to the profile drag from Xfoil.  And all the other drag sources.  It's really a big bookkeeping exercise.  Each source of drag has a different physical cause. 

With regard to the profile drag, in the past I've added a sheet with section data that cover a range of angles of attack and Reynolds numbers.  Then I used the chord and local lift coefficient from the lifting line to look up the profile drag coefficient and add that to the induced drag to get the total drag at each station.  Then integrate the total drag over the span to get the final number.

Yes, exactly.  So why go to the bother of two sails?  

 

Landyachts experience 40 kt apparent wind and a lot more.  I've personally clocked landyachts doing 5 - 6 times the true wind speed, and the apparent wind upwind is approximately the same as the yacht's speed is downwind.  You're probably talking around 20 degrees apparent wind angle for the AC75 and a landyacht apparent wind angle is more like 14 deg.

I had heard low teens AWA.  Maybe they should put runners on and find a reeeeeally big frozen lake

Share this post


Link to post
Share on other sites
3 hours ago, RMac said:

I had heard low teens AWA.  Maybe they should put runners on and find a reeeeeally big frozen lake

Yes, in the Shirley Robertson interview, Juan K mentioned 13 deg AWA for the AC75s. 

Share this post


Link to post
Share on other sites

13 deg AWA at 45 deg TWA implies a boat speed that is 2.4 times the true wind speed upwind.  Maybe.

13 deg at 140 deg TWA implies a boat speed 3.6 times the true wind speed.  Sounds optimistic to me, but not infeasible.

Share this post


Link to post
Share on other sites
3 hours ago, erdb said:

Yes, in the Shirley Robertson interview, Juan K mentioned 13 deg AWA for the AC75s. 

I just realized that I was using a different definition for apparent wind angle.  I typically use the angle between the velocity vector through the water and the apparent wind as the apparent wind angle.  But I'll bet Juan K was using the apparent wind measured from the yacht centerline.  The difference is the leeway angle.  You'd need to add leeway to Juan K's apparent wind to get the angle I was using.  So if you add a couple of degrees of leeway you get 15 deg.

15 deg upwind at 45 deg TWA is a boat speed that is 1.9 times the true wind speed.  15 deg downwind at 140 deg TWA is 3.2 times TWS.  I think those numbers are more reasonable

20 deg AWA (including leeway) is 1.2 x TWS upwind and 2.5 x TWS downwind.  I'll leave it to you as to which set of numbers are more likely.

Share this post


Link to post
Share on other sites
21 minutes ago, Xlot said:

Let’s split at 18 deg - who was quoting that a few days ago?

 

Could have been me. Tomorrow I will pull some numbers from what the AC50’s were achieving during the Chall semi’s and finals, they should be reasonably close to what we can expect. Iirc it was around 17. 

Share this post


Link to post
Share on other sites

Tip and shaft article about the new boats 

We learn the the challengers think ETNZ have the smallest area foils and rudder foils.

Prada think that they are doing everything perfectly :lol:

https://mailchi.mp/tipandshaft/99what-are-the-differences-between-the-new-ac75s?e=b4f70f3bdf

  • Like 6

Share this post


Link to post
Share on other sites

Oh.. shortest rudder not rudder foils..:huh:

Share this post


Link to post
Share on other sites

https://mailchi.mp/tipandshaft/99what-are-the-differences-between-the-new-ac75s?e=b4f70f3bdf

M.F.: It's very hard to say, because we can't line up with the others. The only thing we have seen is the quality of manoeuvres and I would say for the moment that we have an advantage. Our manoeuvres are carried out perfectly.

Crikey the slickest manoeuvres I have seen out on the sparkling Waitemata from the contenders are by far from Amway first tack sequence almost reminiscent of the Bermuda no look by TNZ cat.

 

 

  • Like 1

Share this post


Link to post
Share on other sites
On 11/24/2020 at 12:31 AM, Stingray~ said:

I remember those too! Lol, was barely a teenager but helped my dad figure out what the problem with his ‘last night’s run’ was :) 

I started with SPSS then FORTRAN, then COBOL then C then C++, etc, don’t know too much about Python but I’m sure it works well. Truth is, all languages compile down to very-fast executables on modern multi threaded os’s over parallel-processor hardware.
 

otoh, a few teams have mentioned running Star/CMM sims on AWS, some of them so big that they still take a week to complete! 

If you do not want to pay for the code, try openfoam. Even a normal gaming machine with a good nvidia card (2070+) is probably good enough for some simple modelling. I'm sure however that the teams use hundreds of GPUs to do their simulation and Star is probably a good choice.

  • Like 1

Share this post


Link to post
Share on other sites

I realized something surprising about how the foil cant angle determines the center of effort on the sails.

I was working on integrating vortex and sail forces into my model. The vortex analysis gives a range of values for the sails' CoE depending on the sails' AOA and how much the sail is twisted. I tried to figure out where the sails' CoE would be in certain scenarios, when I realized, that the CoE height is simply a function of the foil cant angle.

p1.JPG.b1d99d9e7e23a89cc9387a7b5ea7a9fb.JPG

On the above fig, FOIL lateral = SAIL lateral, and FOIL lift = WEIGHT + RUDDER downforce. The combined vectors of the foil lift and lateral (red), and the combined vector of weight+rudder and sail lateral (purple), has to be in line for a stable boat. So the CoE depends on the foil arm cant angle, and also on the center of gravity. As the CG is moved more to windward, that line marked as "axis of combined weight + rudder force" moves left, and the sails's COE can go higher up. But since only 1-3 crew member moves on the AC75s, the CG must be very close to midline.

What's surprising about this is that it means that the more the foil is canted out, the lower the CoE has to be:

If the angle of the foil force is 60 deg to horizontal, the CoE is 8m high.

p2.thumb.JPG.f3b0951f39b914cd197cb973e8f54b84.JPG

 

If however the foil canted out less, and the foil force is 65 deg to horizontal, the CoE is almost 10m high.

p3.thumb.JPG.8c6257a31f33a5065897b9dd70c1f5a0.JPG

It's kind of counterintuitive, because you'd think the more the foil is canted out, the more righting moment you have. However, since the lateral and vertical forces are coupled by the angle of the foil wing axis, the sails' CoE has to move lower as you cant out the foil. It's weird and very confusing, but if you are fully canted out,  you can generate more lateral sail force, but you have to do it at a lower CoE either by twisting out the top of the sail a lot or by a low aspect jib etc. Probably explains those weirdly shaped J3s we've seen above 20kts of wind.

BTW, to incorporate vortex into my model, I wrote an excel macro that uses the vortex calculation and cycles through a bunch of combinations of AOA and twist in the sail. Here, for each AOA between 3 and 15 deg, I twisted the top of sail from 0 - 15 deg off. This gives this sawtooth pattern for the CoE of the sails:

coe2.thumb.png.c35b04c6491d7de9a2add5e2e83e65ef.png

The higher the AOA, the smaller the possible range of CoE that can be achieved by twist. There are situations, when the top of the sail has negative AOA and provides righting moment. For example, if you have a sail that only has 4 degrees of AOA, but the top is twisted off 15 degrees, you actually have a negative CoE, meaning that the sail heels the boat to windward. Makes sense, although not a realistic situation obviously.

Anyway, it seems the CoE is between 8 and 11m high for realistic sail setups. Now I just have to figure out how to match this with foil cant angle, and make sure that pitch moments and other forces are also balanced.

 

  • Like 4

Share this post


Link to post
Share on other sites

^ nice

Yes, Britt said the same when giving reasons for why they are canting the lee foil so close to the surface; to lower ride height and COE, reducing RM and decreasing lift from the wings. 

Share this post


Link to post
Share on other sites

^ I've been taking foil cant into account for a while.

The moments of the boat+windward foil mass is a constant distance (y1) from the centre line, to which I add the variable calculated horizontal distance (y2) of the  leeward foil CE. Added to that is the ±  moment of the stabilator.

Similarly, I add the variable calculated sail force CE vertical distance (z1) to the variable calculated vertical distance (z2) of the  leeward foil CE.

I'm using foil cant of 5° pre-flying, 15° for light wind (AWS) foiling, and a maximum of 25° for most flying AWS velocities.

Transverse balance is achieved by

  1. Reducing sail sizes.  I use:
    - 3 mains:  135, 140 and 145m2, and
    - 4 headsails:   NZ C0 = 132,  NZ J1 = 83,  NZ J2(?) = 57,  guessed J3 = 50,  AM J4 = 45m2
  2. Adding twist as the above sail combinations start overpowering the RM, to reduce both CL and CE-z.

Share this post


Link to post
Share on other sites

@erdb  I'm having issues getting convergence for headsail profiles. The mainsail is no problem, with my estimated profile showing good results at both different camber percentages and at varying AWS, with minimal boundary layer separation.

However, my efforts to model profiles for a single skin headsail have been woeful. I've used a NACA profile with thickness at zero - total fail - then with 1% thickness, at which I got only a few results.

What settings are you using to get CL and CD values for your headsails?

Share this post


Link to post
Share on other sites
2 hours ago, Stingray~ said:

^ nice

Yes, Britt said the same when giving reasons for why they are canting the lee foil so close to the surface; to lower ride height and COE, reducing RM and decreasing lift from the wings. 

Which interview was that, do you have a link? Thanks

Share this post


Link to post
Share on other sites
45 minutes ago, MaxHugen said:

^ I've been taking foil cant into account for a while.

The moments of the boat+windward foil mass is a constant distance (y1) from the centre line, to which I add the variable calculated horizontal distance (y2) of the  leeward foil CE. Added to that is the ±  moment of the stabilator.

Similarly, I add the variable calculated sail force CE vertical distance (z1) to the variable calculated vertical distance (z2) of the  leeward foil CE.

I'm using foil cant of 5° pre-flying, 15° for light wind (AWS) foiling, and a maximum of 25° for most flying AWS velocities.

Transverse balance is achieved by

  1. Reducing sail sizes.  I use:
    - 3 mains:  135, 140 and 145m2, and
    - 4 headsails:   NZ C0 = 132,  NZ J1 = 83,  NZ J2(?) = 57,  guessed J3 = 50,  AM J4 = 45m2
  2. Adding twist as the above sail combinations start overpowering the RM, to reduce both CL and CE-z.

Yes, I've used all these coordinates, too, but what I realized is that knowing the cant angle of the foil, you can tell where the sail COE needs to be to balance roll moments even without knowing anything about how big the forces are. The COE has to be where that vertical dashed line and the line perpendicular to the foil wing intersects. The only uncertainty is where that dashed line is. If the rudder  exerts no force, then the line is where the CG is (including everything, two foils, boat, crew, rig). If the rudder has downforce, than it will pull that line closer to midline, but it can't be a huge effect, because the CG is already pretty close to midline. The relative weight of more crew to windward is very small to the rest of the boat, and the difference in the lateral positions of the two foils isn't that great either.

I realized this when I played with sail sizes, cl, etc to see how it changes the CoE, but it never budged. Only when I changed the cant angle. That's when I realized, it's really just a geometrical problem that has nothing to do with how great the forces are. Now the pitch moment is a different story, and I think the trick to match the pitch and roll moments to each other will be to chose the correct cant angle. 

How do you measure the cant angle? Relative to what?

 

Share this post


Link to post
Share on other sites
54 minutes ago, MaxHugen said:

@erdb  I'm having issues getting convergence for headsail profiles. The mainsail is no problem, with my estimated profile showing good results at both different camber percentages and at varying AWS, with minimal boundary layer separation.

However, my efforts to model profiles for a single skin headsail have been woeful. I've used a NACA profile with thickness at zero - total fail - then with 1% thickness, at which I got only a few results.

What settings are you using to get CL and CD values for your headsails?

Yeah I had the same problem and while searching, I found some old posts on boatdesign.net (form I think the same Erwan who posts here). He said he used the Eppler 376 foil. You can get the coordinates and info here: http://airfoiltools.com/airfoil/details?airfoil=e376-il

Had to massage it to make it work, smoothed it out in xfoil, but then I was able to get convergence to a small range of AOAs, for various camber %s. 

Now that I switched to vortex, I don't use it, because vortex looks at the whole sail plan as one foil, and gives one CL and one CDi value for a given AOA+twist. I guess the jib foil data could be incorporated somewhere down the line where I look at the viscous drag, but I'm not there yet.

 

Share this post


Link to post
Share on other sites
19 minutes ago, erdb said:

How do you measure the cant angle? Relative to what?

I use the angle of the foil base line (the bottom of the foil design box) from the horizontal.

I also use the foil arm angle and it's mass to calc it's additional righting moment, as it's inboard from the axis of rotation.

Share this post


Link to post
Share on other sites
16 minutes ago, erdb said:

Yeah I had the same problem and while searching, I found some old posts on boatdesign.net (form I think the same Erwan who posts here). He said he used the Eppler 376 foil. You can get the coordinates and info here: http://airfoiltools.com/airfoil/details?airfoil=e376-il

Had to massage it to make it work, smoothed it out in xfoil, but then I was able to get convergence to a small range of AOAs, for various camber %s. 

Now that I switched to vortex, I don't use it, because vortex looks at the whole sail plan as one foil, and gives one CL and one CDi value for a given AOA+twist. I guess the jib foil data could be incorporated somewhere down the line where I look at the viscous drag, but I'm not there yet.

That Eppler profile looks very interesting. I'll try it, hadn't considered the leading edge that a luff track would provide - which could be a major issue in XFLR5/XFoil.  Would you mind sending me the coords for the foil as you modified it?

I'm not keen on "combining" CL, CD and twist etc from Vortex, prefer to work these out myself and then incorporate these separately into my calcs. I'm getting there, slowly slowly, but the headsails have held me up for days!

Share this post


Link to post
Share on other sites
49 minutes ago, MaxHugen said:

That Eppler profile looks very interesting. I'll try it, hadn't considered the leading edge that a luff track would provide - which could be a major issue in XFLR5/XFoil.  Would you mind sending me the coords for the foil as you modified it?

I'm not keen on "combining" CL, CD and twist etc from Vortex, prefer to work these out myself and then incorporate these separately into my calcs. I'm getting there, slowly slowly, but the headsails have held me up for days!

Here you go:

jib_foils.zip

  • Like 1

Share this post


Link to post
Share on other sites
Just now, MaxHugen said:

Thanks!

Your welcome. BTW, the bulb at the leading edge is not really to model the luff track, I think you just need it there to get convergence. It probably gives better numbers than a real jib.

Share this post


Link to post
Share on other sites
2 minutes ago, erdb said:

Your welcome. BTW, the bulb at the leading edge is not really to model the luff track, I think you just need it there to get convergence. It probably gives better numbers than a real jib.

Yep, guessed that, after seeing the Eppler profile! 

I'm wondering also, if there is a bit more "camber angle" right at the luff in reality, than that in the profiles we're using for headsails? (Probably a poor way to explain what I mean)

If this is the case, then it would account for some the "bulb" thickness required for convergence? A better, more "rounded" LE in effect.

Share this post


Link to post
Share on other sites
15 minutes ago, MaxHugen said:

Yep, guessed that, after seeing the Eppler profile! 

I'm wondering also, if there is a bit more "camber angle" right at the luff in reality, than that in the profiles we're using for headsails? (Probably a poor way to explain what I mean)

If this is the case, then it would account for some the "bulb" thickness required for convergence? A better, more "rounded" LE in effect.

In xfoil, you can move the high points for both thickness and camber, can you do that? It's tricky, because it's hard to get convergence and some meaningful numbers. Each camber setting has a very narrow AOA range, but that's actually how real jibs work. As you let them out, the camber also increases simultaneously. 

Share this post


Link to post
Share on other sites
3 minutes ago, erdb said:

In xfoil, you can move the high points for both thickness and camber, can you do that? It's tricky, because it's hard to get convergence and some meaningful numbers. Each camber setting has a very narrow AOA range, but that's actually how real jibs work. As you let them out, the camber also increases simultaneously. 

XFLR5 is simply a GUI for XFoil.   So I can position (% of chord) for both camber and thickness, if that's what you mean.

From my extensive calcs on the main, it can perform at much wider ranges of AoA and camber, but as you point out, it sure looks like the headsails have very narrow ranges.

Embarking on a bunch of calcs to see which camber/AoA works... plus I might also try out my "deeper curve immediately aft of luff" guess. Posted a question in Sails topic, perhaps an experienced sail trimmer might be able to advise if this occurs. I think it may, at lower AWS and deeper camber... but I'm purely guessing.

Share this post


Link to post
Share on other sites
On 11/25/2020 at 6:54 AM, erdb said:

Awesome, thank you. Just a final question, once I got the CL, CDi from vortex, is it prudent to then go back to the xfoil data and look at what viscous drag goes with the average CL shown by vortex? I assume I have to add that to induced drag to get total drag.

For the second part of your post, great to get an insight on this. Maybe the reason is the coolness factor? :)

 

On 11/25/2020 at 6:25 AM, Basiliscus said:

In the lifting line analysis, the section shape shows up in two ways.  First, as you mention, is the zero lift angle of attack.  The more camber the section has, the more negative the zero lift angle will be.  A double skinned main is no different from a single skinned mainsail in this regard.  Both shapes would create lift at zero angle of attack (if you could keep them from luffing).

The second way is the lift curve slope.  In inviscid thin airfoil theory, this has a value of 2 pi per radian.  In principle, a thick section can have a little higher value, but the boundary layer thickness tends to offset this.  For viscous flow, a value of 0.1 per degree is a reasonable guess.

You can get both the lift curve slope and zero lift angle from Xfoil.  Just tell it to calculate the pressure distribution for zero lift and it'll give you the zero lift angle.  Do it again for zero angle of attack and divide that by minus the zero lift angle and you'll have the lift curve slope that is good for the linear lift range.  Or you can fit a line to the lift range of interest and use the intercept and slope of that line.

The lifting line method is really concerned with the influence of the wake and doesn't care what the actual angle of attack is.  You can get the same lift by having a symmetrical section at an angle of attack, or a cambered section at a low angle of attack.  The same goes for the chord.  You can get the same lift with a long chord at low angle of attack or a short chord at high angle of attack.  The associated induced drag will be the same.  The difference comes when backing out the planform shape or twist distribution from the spanwise lift distribution.

On the whole, I don't really get the motivation for the double skinned mainsail.  It can eliminate the separation bubble you'd get on the windward side with a single skinned mainsail and rotating wingmast.  But the soft sail rig on the BOR90 trimaran USA17 achieved attached flow on both sides of the mast all the way back to the sail track over 2/3 of the span.  It was only in the lower 1/3 of the span, where the angle of attack was quite low because of the jib that there was a windward separation bubble.  And that might have been eliminated if the mast could have been twisted Crazy Coyote style.  I suppose it helps to fair mechanisms used to control twist, but those are only at the head and foot.  But it seems like a lot of weight and complication to achieve that.  Landyachts quite successfully use large wingmasts with single skin mainsail at low apparent wind angles and higher apparent wind speeds than the AC75s will see.  And the best of the landyacht wingmast/sail combinations are competitive with the best of the rigid wingsail rigs.  So you don't need separate surfaces to go fast.

"On the whole, I don't really get the motivation for the double skinned mainsail."

How can you write all this dipshit pseudo science without understanding the obvious? 

A wing is more efficient than a sail. But managing wings is painful and expensive... have to dismount them for resting or bad weather.

2 skin sail with a broad mast simulates a wing and even more allows adjustment of profile with mast rotation.

Share this post


Link to post
Share on other sites
4 hours ago, uflux said:

 

Chris talks about maximum cant angle and surface piercing to maximize RM, which is certainly true, but I think when sailing upwind this mode also maximizes lee resistance when the boat has power to burn.

Share this post


Link to post
Share on other sites
10 minutes ago, barfy said:

Chris talks about maximum cant angle and surface piercing to maximize RM, which is certainly true, but I think when sailing upwind this mode also maximizes lee resistance when the boat has power to burn.

My calcs show that a cant of 25° is roughly the norm, at which ~10% is lateral force.  No doubt they can increase that slightly if very calm and they can fly really close to surface. However, the increase in RM is quite small, and has to be paid for with less windward lift.

Foil forces are greater going downwind, as their SOG is higher, while sail forces aren't much different - AWS is similar.

 

  • Like 1

Share this post


Link to post
Share on other sites
11 hours ago, barfy said:

Chris talks about maximum cant angle and surface piercing to maximize RM, which is certainly true, but I think when sailing upwind this mode also maximizes lee resistance when the boat has power to burn.

The righting moment issue is really interesting. Earlier I posted that I found that the center of effort on the sails is determined almost completely by the cant angle of the foil. The more the foil is canted out, the lower the CoE has to be. However, this is not the same as righting moment. If you are only looking at lateral forces, the canted foil arrangement actually allows limitless righting moment (in theory). This is because as long as your center of effort is at the right height, as you increase side force on the sail, the lift on the foil increases by just the right amount to counteract the heel. (The relation of the lateral and vertical lift components of the foil is determined by the cant angle, that's why the center of effort of the rig is determined by cant angle).

p1.JPG.41c6e0ba859b6f5db8304f4823193877.JPG

The catch is that the increased vertical lift from the foil needs to be opposed by increasing downforce since the sum of vertical forces needs to be zero. The only thing that can do that is the rudder. Of course, the rudder is at the back of the bus, so if the rudder produces too much down force, it would mess up pitch moments, the boat would rotate bow-up, foil AOA increases, boat jumps out of the water - remember those weird crashes by AM and ETNZ? 

To put it another way, the limit on righting moment is actually the limit on pitch moment. How can you increase rudder downforce and max righting moment while still keeping the boat pitch angle steady? You can move weight forward, move the foil as far back as possible, maybe sail in a bow-down state. All things that we've seen them doing, but for me it wasn't clear why.

What if you increase foil cant? It means that less of the lateral force by the sails is converted to vertical lift by the foil, which is good, because then you don't need as much rudder downforce. So you increased righting moment, and increased max side force by the sails; however, you have to somehow morph the sails in a way that they deliver that force at a lower center of effort. Low aspect jib, crazy twists in the mainsail...

Finally, some interesting differences among the teams how they approach this. Patriot's hull shape may indicate that they wanted to put as much weight forward as possible, and also sail with a more bow-down attitude. In exchange, they gave up on the full-length keel and sealing the gap to the water all the way aft. The other three teams decided that the full keel is more advantageous to lower drag, and as I think @Steve Clark pointed out earlier, closing the gap also lowers the center of effort on the sails, which then allows you to cant the foil out more and increase righting moment that way.

Once I'll have time to put my model back together, I want to test how moving weight fore and aft affect speed and righting moment. Who would have thought that you have to move weight forward to increase righting moment :blink:.

  • Like 2

Share this post


Link to post
Share on other sites
12 hours ago, MaxHugen said:

My calcs show that a cant of 25° is roughly the norm, at which ~10% is lateral force.  No doubt they can increase that slightly if very calm and they can fly really close to surface. However, the increase in RM is quite small, and has to be paid for with less windward lift.

Foil forces are greater going downwind, as their SOG is higher, while sail forces aren't much different - AWS is similar.

 

I think I've seen pictures with as high as 30 deg cant, but it's not a big difference. However, I think you underestimate the proportion of the lateral force. Instead of the angle of the foil wing, I like to look at the angle perpendicular to the foil wing, because that's the direction of the foil lift.

If the foil wing is at 25 deg, the foil arm angle is 90-25 = 65 degrees.

The ratio of the vertical vs horizontal components of the foil: Fvertical / Fhorizontal = tan(65) = 2.14.

So the Fhorizontal  = Fvertical / 2.14 or about 46.6% of Fvertical.

Share this post


Link to post
Share on other sites

Reading and thinking about what you two are doing (along with the other experts) is almost as good as watching the videos!  Especially when you are able to deliver such a cool simplification as the relationship between cant angle and COE, thanks, @erdb.

Thank you two, and keep up the good work (enough of these PMs?)

  • Like 1

Share this post


Link to post
Share on other sites
1 hour ago, Kiwing said:

Reading and thinking about what you two are doing (along with the other experts) is almost as good as watching the videos! 

Agreed, really cool. 
 

I forget though, what’s the general goal? Predictions on polars? 

Share this post


Link to post
Share on other sites
38 minutes ago, Stingray~ said:

Agreed, really cool. 
 

I forget though, what’s the general goal? Predictions on polars? 

A better understanding of the world through physics :D and yes, some polars just for fun to compare with real data once racing starts.

 

  • Like 3

Share this post


Link to post
Share on other sites
5 hours ago, erdb said:

...

p1.JPG.41c6e0ba859b6f5db8304f4823193877.JPG

... .

Why do you think both foil wings produce the same lift and the strut produces zero lift?

  • Like 1

Share this post


Link to post
Share on other sites
5 hours ago, erdb said:

I think I've seen pictures with as high as 30 deg cant, but it's not a big difference. However, I think you underestimate the proportion of the lateral force. Instead of the angle of the foil wing, I like to look at the angle perpendicular to the foil wing, because that's the direction of the foil lift.

If the foil wing is at 25 deg, the foil arm angle is 90-25 = 65 degrees.

The ratio of the vertical vs horizontal components of the foil: Fvertical / Fhorizontal = tan(65) = 2.14.

So the Fhorizontal  = Fvertical / 2.14 or about 46.6% of Fvertical.

You're right.  I had only calculated vertical forces in my calcs so far, for longitudinal moments, which is ~90%.

image.png.7ffd73e50c541f44aba638ba4d21a0e2.png

Share this post


Link to post
Share on other sites
3 hours ago, Stingray~ said:

Agreed, really cool. 
 

I forget though, what’s the general goal? Predictions on polars? 

Nyet....   best VMG, the only goal! 

Polars make nice pictures though. :)

  • Like 1

Share this post


Link to post
Share on other sites
57 minutes ago, Basiliscus said:

Why do you think both foil wings produce the same lift and the strut produces zero lift?

Yes, good question, thought about it a lot. As for the strut, during stable flight, there is very little of it in the water and I can't imagine the boat having enough leeway (against the much bigger and more effective foil wing) to put a high enough AOA on the strut. If anything, the boats seem to crab windward (or yaw bow-to-lee - depending on how you look at it). Of course when the boat is transitioning from displacement to foiling, it's a completely different picture, but there's no way I can figure out what's happening then, so I just focus on stable balanced foiling mode. 

As for the wing - I guess foils with a lot of anhedral could manipulate vertical and horizontal forces somewhat independently, but it seems that would be far from ideal. You know the drag penalty for that much better, but to me it just seems wrong not having symmetric load on the two sides. Plus, ETNZ is running on completely straight wings. So let's say I'm modeling their setup:)

What do you think?

Share this post


Link to post
Share on other sites
7 minutes ago, Franklyspeaking said:

why are you assuming that the camber of the foil wings remain symmetrical about the strut? 

The Rules dictate it.

Share this post


Link to post
Share on other sites

Nope.

pretty sure the foil wing and foil flap has to be symmetrical "structurally" (read - 3D body shape) but can be actuated and twisted independently about foil wing centre plane......

  • Like 3

Share this post


Link to post
Share on other sites

Interesting that two of the challenging teams have gone to foils similar to those that ETNZ first tested on Te Aihe 12 months ago, no bulb and fairly wide. Will be interesting to see if AM go to foils without bulbs as well.

yysw301967.jpg

yysw299692.jpg

yysw275455.jpg

Share this post


Link to post
Share on other sites
13 minutes ago, Franklyspeaking said:

Nope.

pretty sure the foil wing and foil flap has to be symmetrical "structurally" (read - 3D body shape) but can be actuated and twisted independently about foil wing centre plane......

this is true, no restriction on operating the foil flaps in an unsymmetrical way

Share this post


Link to post
Share on other sites
16 minutes ago, Lickindip said:

this is true, no restriction on operating the foil flaps in an unsymmetrical way

Yes, it is a possibility for sure. I just think it must produce higher drag than symmetric lift. Why wouldn't they try to rather adjust the foil arm cant, to line the forces up? Again, at least during stable flight, which I'm trying to model. I can imagine that the flight controller makes continuous fine adjustments on the flaps to keep the boat steady, and those adjustments are not always symmetric, but they average out so the overall force is in general perpendicular to the foil wing. Or I could be wrong...

 

Share this post


Link to post
Share on other sites
1 minute ago, erdb said:

Yes, it is a possibility for sure. I just think it must produce higher drag than symmetric lift. Why wouldn't they try to rather adjust the foil arm cant, to line the forces up? Again, at least during stable flight, which I'm trying to model. I can imagine that the flight controller makes continuous fine adjustments on the flaps to keep the boat steady, and those adjustments are not always symmetric, but they average out so the overall force is in general perpendicular to the foil wing. Or I could be wrong...

 

for the same reason aircraft have ailerons ... it gives them more control

if the team has independent flaps on an anhydral foil then they can change the vertical lift and the leeway independantly 

  • Like 1

Share this post


Link to post
Share on other sites
36 minutes ago, Lickindip said:

for the same reason aircraft have ailerons ... it gives them more control

if the team has independent flaps on an anhydral foil then they can change the vertical lift and the leeway independantly 

The foil at all times has to balance the mass of boat + crew, and also the downforce from the stabilator. I this is primarily done by the angle of the stabilator (via rudder rake), with the flaps providing secondary control.

At any time, drag from the foil and stabilator is a major contributor to total drag. They would surely strive to have both foils at an AOA and flap deflection that keeps drag to a minimum.

Using one of two flaps to increase windward lift independently probably means an increase in drag, and I think substantially.

IMO.

Share this post


Link to post
Share on other sites
2 hours ago, Forourselves said:

Interesting that two of the challenging teams have gone to foils similar to those that ETNZ first tested on Te Aihe 12 months ago, no bulb and fairly wide. Will be interesting to see if AM go to foils without bulbs as well.

yysw301967.jpg

yysw299692.jpg

yysw275455.jpg

It’ll be even more interesting if ETNZ emerge with a bulb and narrow wings.

  • Like 1

Share this post


Link to post
Share on other sites
32 minutes ago, MaxHugen said:

The foil at all times has to balance the mass of boat + crew, and also the downforce from the stabilator. I this is primarily done by the angle of the stabilator (via rudder rake), with the flaps providing secondary control.

At any time, drag from the foil and stabilator is a major contributor to total drag. They would surely strive to have both foils at an AOA and flap deflection that keeps drag to a minimum.

Using one of two flaps to increase windward lift independently probably means an increase in drag, and I think substantially.

IMO.

agreed, im not saying it IS a better option but I'm saying it is a possibility

sailing upwind you will want more windward lift ... sailing downwind you might want to reduce that lift

Share this post


Link to post
Share on other sites

 

1 hour ago, Lickindip said:

for the same reason aircraft have ailerons ... it gives them more control

if the team has independent flaps on an anhydral foil then they can change the vertical lift and the leeway independantly 

I agree, I'm sure they can operate the flaps separately, but I also think they want to limit the difference between the two sides. Also, for airplanes, the alerions are far apart. Here, the foil wing span is 4 m. Compare that to the 26.5m mast - it's not going to have much effect overall. 

2 minutes ago, Lickindip said:

agreed, im not saying it IS a better option but I'm saying it is a possibility

sailing upwind you will want more windward lift ... sailing downwind you might want to reduce that lift

Yes, but you can do that more efficiently by adjusting cant angle. Of course that's much slower, so again, fine quick adjustments are probably done by the flaps, but the overall setup is optimized by the cant angle. (Maybe)

Share this post


Link to post
Share on other sites
40 minutes ago, Ex-yachtie said:

It’ll be even more interesting if ETNZ emerge with a bulb and narrow wings.

They were testing B1 with their narrow wings, but one foil had a "torpedo" bulb, while the other was the "BFB" - blended foil bulb. 

Last pics I saw of B2, they were still testing with what I think were the same 2 foils.

I think NZ will stay with their basically flat foil, with a bulb, to maximise the RM. By my rough calcs, they need about 850 kg of ballast to meet the Rules, and even though lead has an SG of 11.3, it would still need 0.075m3 of volume.

This has to go in whatever space they can scrounge in the foil and foil arm - after allowing for structural components, plus space for controls, flaps, etc. Looked at LR's B1 a while ago, wondering where they put that lead:

image.png.7b401a32acadb6b8d28550ac71383aa7.png

Share this post


Link to post
Share on other sites
13 hours ago, erdb said:

Yes, good question, thought about it a lot. As for the strut, during stable flight, there is very little of it in the water and I can't imagine the boat having enough leeway (against the much bigger and more effective foil wing) to put a high enough AOA on the strut. If anything, the boats seem to crab windward (or yaw bow-to-lee - depending on how you look at it). Of course when the boat is transitioning from displacement to foiling, it's a completely different picture, but there's no way I can figure out what's happening then, so I just focus on stable balanced foiling mode. 

As for the wing - I guess foils with a lot of anhedral could manipulate vertical and horizontal forces somewhat independently, but it seems that would be far from ideal. You know the drag penalty for that much better, but to me it just seems wrong not having symmetric load on the two sides. Plus, ETNZ is running on completely straight wings. So let's say I'm modeling their setup:)

What do you think?

Leeway will affect the wings differently, even for the T configuration, because of the presence of the strut.  The side force on the strut may not be the majority of the total side force, but it can still have a significant effect on how the lift is distributed between the wings.  Unlike a single lifting surface, where the spanwise lift distribution has to be continuous across the span, the lift distribution for the intersecting three surfaces can be discontinuous at the junction

As for what is the minimum drag configuration, it is not at all clear to me that zero force on the strut, or symmetrical load on the wings, produces the minimum drag.  The drag due to lift will be minimized when the wake wash is uniform across each wing span (and the strut) and proportional to the cosine of the dihedral angle based on the direction of the net force vector.  That will correspond to near zero wash from the strut (but not necessarily zero force) because the strut is approximately aligned with the net force, as you've shown.  The ideal wash from each wing will depend on how perpendicular they are to the net force vector.  The more perpendicular, the more load it should carry.

So you can work the problem backwards.  Pick the cant angle and running depth.  Then determine the vertical and horizontal forces needed to satisfy the equilibrium.  Set the level of the wake wash based on the angle of the net forces and calculate the spanwise lift distribution across the strut and wings using a lifting line method.  Scale the wake wash to get the lift forces to match the required forces.  That should give you the minimum lift-induced drag to add to the profile drag, interference drag, and spray drag.  

  • Like 1

Share this post


Link to post
Share on other sites
11 hours ago, MaxHugen said:

Looked at LR's B1 a while ago, wondering where they put that lead:

 image.png.7b401a32acadb6b8d28550ac71383aa7.png

Is this foil a touch unique for the wing being mounted this far forward on the arm? 

Share this post


Link to post
Share on other sites
26 minutes ago, Stingray~ said:

Is this foil a touch unique for the wing being mounted this far forward on the arm? 

 Seems like it has to do with how far forward or aft they want the lift. But also would affect the turbulence around the end of the arm.

Share this post


Link to post
Share on other sites
8 minutes ago, nroose said:

 Seems like it has to do with how far forward or aft they want the lift. But also would affect the turbulence around the end of the arm.

Bingo, that turbulence is what Ward said too. He said the ‘strats’ (X came up with same term) that ETNZ is sporting on the lower arms are a different solution for a similar area. Basiliscus has repeatedly pointed to this area too. 

Share this post


Link to post
Share on other sites
12 hours ago, MaxHugen said:

I think NZ will stay with their basically flat foil, with a bulb, to maximise the RM. By my rough calcs, they need about 850 kg of ballast to meet the Rules, and even though lead has an SG of 11.3, it would still need 0.075m3 of volume.

This has to go in whatever space they can scrounge in the foil and foil arm - after allowing for structural components, plus space for controls, flaps, etc. Looked at LR's B1 a while ago, wondering where they put that lead:

Maybe the gaiters/spats NZB2 tried were a way of achieving extra ballast volume as well as maybe some recycled lift from spray deflection?

Drag and lift wise, better to put it there than at the “t” foil intersection?

Share this post


Link to post
Share on other sites
13 minutes ago, Sidecar said:

Maybe the gaiters/spats NZB2 tried were a way of achieving extra ballast volume as well as maybe recycled lift from spray deflection?

Good one! First time I’ve seen that suggested.

Wrt to lead, Ward thinks the most likely material being used in the wings is ‘milled steel’ but didn’t get more material-specific than that. He may have mentioned it for being good for not only density but also for better cavitation-pitting resistance. 

Share this post


Link to post
Share on other sites
4 hours ago, Stingray~ said:

Good one! First time I’ve seen that suggested.

Wrt to lead, Ward thinks the most likely material being used in the wings is ‘milled steel’ but didn’t get more material-specific than that. He may have mentioned it for being good for not only density but also for better cavitation-pitting resistance. 

ETNZ used milled steel in AC35 to achieve structural integrity for the very narrow and thin foils they successfully used there.

Share this post


Link to post
Share on other sites
35 minutes ago, MaxHugen said:

ETNZ used milled steel in AC35 to achieve structural integrity for the very narrow and thin foils they successfully used there.

Yes but only just lasted long enough. Think its pretty well documented that they were doing ultrasound scans daily on their light wind foils due to a structural issue they were monitoring

  • Like 1

Share this post


Link to post
Share on other sites
53 minutes ago, MrBump said:

Yes but only just lasted long enough. Think its pretty well documented that they were doing ultrasound scans daily on their light wind foils due to a structural issue they were monitoring

Interesting, didn't know that!

Share this post


Link to post
Share on other sites
8 hours ago, Basiliscus said:

Leeway will affect the wings differently, even for the T configuration, because of the presence of the strut.  The side force on the strut may not be the majority of the total side force, but it can still have a significant effect on how the lift is distributed between the wings.  Unlike a single lifting surface, where the spanwise lift distribution has to be continuous across the span, the lift distribution for the intersecting three surfaces can be discontinuous at the junction

As for what is the minimum drag configuration, it is not at all clear to me that zero force on the strut, or symmetrical load on the wings, produces the minimum drag.  The drag due to lift will be minimized when the wake wash is uniform across each wing span (and the strut) and proportional to the cosine of the dihedral angle based on the direction of the net force vector.  That will correspond to near zero wash from the strut (but not necessarily zero force) because the strut is approximately aligned with the net force, as you've shown.  The ideal wash from each wing will depend on how perpendicular they are to the net force vector.  The more perpendicular, the more load it should carry.

So you can work the problem backwards.  Pick the cant angle and running depth.  Then determine the vertical and horizontal forces needed to satisfy the equilibrium.  Set the level of the wake wash based on the angle of the net forces and calculate the spanwise lift distribution across the strut and wings using a lifting line method.  Scale the wake wash to get the lift forces to match the required forces.  That should give you the minimum lift-induced drag to add to the profile drag, interference drag, and spray drag.  

I'm trying to digest this. I guess the part I have problem with is that the drag will be minimized when "wake wash is uniform across each wing span (and the strut) and proportional to the cosine of the dihedral angle based on the direction of the net force vector".

The foil can be canted so that the strut lines up with the force. If that's done, the angles of the wings are symmetric. The wings' shape has to be symmetric by rule, so is there a reason not to choose this setup and set an angle a bit off on purpose? If everything is symmetric, then the foil doesn't really "experience" leeway either, since the flow is perpendicular to the wing's leading edge as well.

As for all the foil-related drags, it's obviously a blurry point of the model. It seems I'm kind of low on overall drag for the foil, and there are components I can't calculate properly like spray drag and wave making drag (is that the same as interference drag?). I was looking at the vortex spreadsheet to see if I could get more accurate induced drag numbers with it. Does the free surface - infinite Froude number situation apply to the foil here? Can this be plugged in into the spreadsheet with a really big offset for the "head" of the foil to account for the cant angle?

Share this post


Link to post
Share on other sites
55 minutes ago, erdb said:

I'm trying to digest this. I guess the part I have problem with is that the drag will be minimized when "wake wash is uniform across each wing span (and the strut) and proportional to the cosine of the dihedral angle based on the direction of the net force vector".

The foil can be canted so that the strut lines up with the force. If that's done, the angles of the wings are symmetric. The wings' shape has to be symmetric by rule, so is there a reason not to choose this setup and set an angle a bit off on purpose? If everything is symmetric, then the foil doesn't really "experience" leeway either, since the flow is perpendicular to the wing's leading edge as well.

As for all the foil-related drags, it's obviously a blurry point of the model. It seems I'm kind of low on overall drag for the foil, and there are components I can't calculate properly like spray drag and wave making drag (is that the same as interference drag?). I was looking at the vortex spreadsheet to see if I could get more accurate induced drag numbers with it. Does the free surface - infinite Froude number situation apply to the foil here? Can this be plugged in into the spreadsheet with a really big offset for the "head" of the foil to account for the cant angle?

symmetric at measurement, however  in use the wing flaps are not operated symmetrically. I think everyone is placing to much emphasis on the strut carrying hydrodynamic loads. Ideally it would be it would carry no hydro loads therefore increasing the span efficiency and reducing overall drag at the wing junction. 
Think moth main foil when the boat is locked in and canted to windward the strut carries virtually no hydro load. IMO

Share this post


Link to post
Share on other sites
1 hour ago, MaxHugen said:

Interesting, didn't know that!

Yeah they got a stress crack from taking them too far up range in a race I think against Artemis. You notice they weren't really doing those rip turns against Oracle in the match - so as to not break the foils.

  • Like 1

Share this post


Link to post
Share on other sites
47 minutes ago, Franklyspeaking said:

symmetric at measurement, however  in use the wing flaps are not operated symmetrically. I think everyone is placing to much emphasis on the strut carrying hydrodynamic loads. Ideally it would be it would carry no hydro loads therefore increasing the span efficiency and reducing overall drag at the wing junction. 
Think moth main foil when the boat is locked in and canted to windward the strut carries virtually no hydro load. IMO

Agree on the strut not producing any significant force, and agree that foil flaps can be manipulated asymmetrically, but my guess is that it will only happen as quick minor adjustments to keep the balance. However, in a "perfect" balanced position, I still see the symmetric load as the best / lowest drag setup. 

Share this post


Link to post
Share on other sites
8 hours ago, Sidecar said:

Maybe the gaiters/spats NZB2 tried were a way of achieving extra ballast volume as well as maybe some recycled lift from spray deflection?

Drag and lift wise, better to put it there than at the “t” foil intersection?

Highly unlikely. Apart from very early on, NZ have concentrated on having ballast mass as far outboard as possible.

I have a slab on a bet that NZ will go with the BFB arrangement at the pointy end of the AC. The guy who will owe me a slab is going for no bulb. :P

Share this post


Link to post
Share on other sites
52 minutes ago, Horn Rock said:

Yeah they got a stress crack from taking them too far up range in a race I think against Artemis. You notice they weren't really doing those rip turns against Oracle in the match - so as to not break the foils.

And they still spanked them

  • Like 2

Share this post


Link to post
Share on other sites