So a foil that has ventilated won't have any cavitation effects around the air pocket interface? From a armchair view there would seem to be zones of aerated low pressure and high pressure outside the envelope, conditions for cavitation to take place. There seems to be bit much discussion or information about how the boats that breach a tip are dealing with the ventilated wing. Any insight appreciated.I think cavitation on the upper side of the leading edge is unlikely in these foil designs at high speed. It is more to be expected as sheet cavitation at takeoff speed. At high speed, I'd expect the foils would be designed so that bubble cavitation occurred more toward the middle of the chord.
Cavitation on underside of the leading edge could be happening at high speed, but I'd think they'd design their sections so that wouldn't happen, either. A little trailing edge up flap deflection, compensated for by a little bow-up change in pitch attitude would cure it.
That is driven by the desire to have the onset of cavitation as high as possible, while also trying to get substantial thickness for structure and to house the ballast. If the forward half produced lift, then that would raise the velocity on the upper surface and lower it on the bottom surface. The result would be the upper surface reaching the cavitation threshold at a lower speed. Both the lower and upper surfaces are hard up against the cavitation threshold, due just to thickness....A question pls: why is the forward half of the lower surface designed so that it produces no lift?
Ventilation is a completely different phenomenon from cavitation. The only thing they have in common is a vapor phase in addition to the water phase near the foil.So a foil that has ventilated won't have any cavitation effects around the air pocket interface? From a armchair view there would seem to be zones of aerated low pressure and high pressure outside the envelope, conditions for cavitation to take place. There seems to be bit much discussion or information about how the boats that breach a tip are dealing with the ventilated wing. Any insight appreciated.
There's a lot that has been learned in the last three Cup cycles. We could be on a Version 3 AC72 by now, and it would be faster than the Version 1 AC72s.So why is the current class of foiling monohull the fastest yet? Size for size the current the class faster than a multihull of similar size/power. Possibly the fastest boat in the world next to Sailrocket.
@barfy, in case you missed a previous post, there's a good YT video on cavitation. It also shows a bubble collapsing which is very cool - video will start at that section:...
The reason cavitation erodes the surface is because it is water vapor that will condense when the local pressure once again becomes higher than the vapor pressure of water. When bubbles of water vapor condense, they don't shrink symmetrically, but instead form a high speed jet of water that shoots through the bubble as they collapse. It is the hammering of the surface by these tiny jets that does the pitting. Air does not condense into liquid as it encounters higher pressure, so there isn't the same kind of erosion cause by ventilation.
I disagree. It didn't give us any LR secrets, but then you can't expect that (plus it looked like somebody from team had final edit).I don't think that interview gave us very much at all. The only interesting bit I thought was about the flap hinge.
Very much a puff piece.
Re-reading your Parts 1-6, with a lot more concentration now.That is driven by the desire to have the onset of cavitation as high as possible, while also trying to get substantial thickness for structure and to house the ballast. If the forward half produced lift, then that would raise the velocity on the upper surface and lower it on the bottom surface. The result would be the upper surface reaching the cavitation threshold at a lower speed. Both the lower and upper surfaces are hard up against the cavitation threshold, due just to thickness.
Each of my posts has a link to the previous post in the series. If you trace back a post or two, you'll see how that situation evolved.
The different types of drag polars in Xfoil are due to the fact that the nondimensional lift and drag coefficients are not the same as the actual lift and drag forces. The performance depends on the actual forces, not the nondimensional coefficients. The type 2 and type 3 polars are intended to take this into account.Re-reading your Parts 1-6, with a lot more concentration now.
Need your advice re XFoil... I've been using Analysis "Type 1", as I don't really understand Type 2, which it looks like you use from the polars in the H143 data. But I'm wondering if I should try to get my head around Type 2. ...
Any recommendations would be appreciated.
There is a big bolt that goes through the end of the foil arm that the teams attach to.Presumably, the FCS reports the cant angle the same way for all boats regardless of the length of the foil arm. We just don't know exactly what their reference point it.
The more I read this, the more intuitive it feels.OK, here is my take on the anhedral foil righting moment debate:
First, we have to clarify whether we're talking about steady-state balance or some dynamic situation. For simplicity, I'll talk about the steady state balance for now.
For this comparison, let's assume that we have two boats that are equally balanced, sailing with the same speed, TWA with the same sail plan. The only difference is that one has a T-foil, the other an anhedral. If we start from here, it's important to realize that the total vertical and total horizontal foil forces have to be the same for both boats. From here, it follows that you can't freely change the force distribution between the wing halves. That distribution will depend on the foil arm cant angle.
It turned out to be quite an interesting geometrical problem. I used Max's measurements for anhedral angle and coordinates of center of effort for the wing halves, and used my VPP for vertical and horizontal foil forces required. The effect of sticking out the outside tip of the foil was omitted.
Here is what happens at TWS=20kts upwind - need for max righting moment (double click to see it better):
View attachment 432975
There's a lot to digest here, I've been staring at these for half an hour now)) Horizontal axis is foil arm cant angle - as we go right, the foil is canted out more and more. Changes in all the forces and arm lengthes were included in these calculations.
For a given sail setup, the T foil can only have one specific cant angle to balance forces. This is indicated by the dotted vertical line. The horizontal dashed line shows total forces and righting moment generated by the T foil. For the horizontal and vertical forces, this is the same as the sum of forces on the anhedral wing halves. Red and green lines represent the inside and outside wing halves of the anhedral foil. RM is referenced to the foil arm rotation axis on the hull for simplicity. We're only interested in the differences anyway.
On the left graph, what you see is that with low cant angle, the outside half contributes minimally to horizontal forces, and the inside half has to carry almost all the force. As you cant the foil out, this balance changes with a cross-over around 64 deg cant angle.
The second graph shows vertical force, and it's quite confusing at first. It shows the same thing that at low cant angle, the outside half contributes minimally, and almost all the vertical lift is carried by the inside. You might think this is wrong (I did first), since the angles should work the opposite way compared to horizontal forces, the outside half is much closer to horizontal - shouldn't it generate the vertical lift? The way it works out is that at those low cant angles, the total lift generated by the outside wing half is minimal.
This can be seen on the third graph. Basically, you can think of it as you need a certain wing angle to counteract the sail forces. At low arm cant angle, the outside wing's angle is just really not aligned well with these forces, so it can't carry much. The two halves become equally loaded at around 62 deg foil cant, which is well below the cant angle that a T foil would use. The load on each wing half is a tiny bit higher than the wing half of the T foil, because of the anhedral angle.
Finally, righting moment is shown on the right side. The blue line indicates the combined righting moment of the two wing halves. As you can see, righting moment of the T foil (dashed line) is reached at a much lower cant angle with the anhedral, and you can generate much more RM with the anhedral foil if you further increase cant angle. If you are limited by how much of the outside tip can stick out of the water, the anhedral foil generates a lot more righting moment.
So what's the downside? Unless you are at the cant angle when the loads on the wing halves are equal (where the lines cross on the third graph), one of your wing halves will carry more lift than the other, so basically you waste some wing area on the other wing half. Plus, if the flap is deflected from its optimum angle, drag will increase further. However, you can see that LR's foil is designed pretty well. The wing halves are loaded equally almost at the same cant angle, when the RM reaches the necessary amount (horizontal dahsed line), around 62 - 63 deg cant.
For contrast, here is what happens at TWS=10kts downwind, when you need less RM:
View attachment 432979
Note that the cant angle is around 63 deg when the total righting moment is at the required value (dashed line). If you look at the lift on the wing halves, you'll see that the inside half is carrying lot more than the outside at this cant angle, so the foil is not balanced out as nicely as upwind. It's also interesting to see how the cant angle barely changes for the anhedral, whereas the T-foil's cant angle went from 67.5 deg (20kts upwind) to ~58 deg (10kts downwind). This explains why in my previous post, cant angles for ETNZ and AM were the highest upwind, and the lowest downwind.
To sum up, the anhedral foil certainly offers more flexibility, but it's top speed performance may be hindered in some conditions.
Finally some of my previous histograms of cant angles supporting the above analysis. This is from the first ACWS race, in about 15-16kts of wind, upwind and downwind cant angles:
View attachment 432983 View attachment 432984
Note how narrow the histograms are for ETNZ, they have to set the cant at a certain angle for balance, whereas LR can play with it depending on how they want to distribute loads between the wing halves. You can also see, how ETNZ's cant angle is much higher upwind, and much lower downwind than LR's
Ventilation drastically reduces Lift, because air is ~1000 times less dense than water.I saw that, great video. I still think the elephant in the room is the ventilated outer wing, with no fences, and what that does to the efficiency of 50% of the wing. I've seen heaps of explanations and examples of vortex and tip ventilating, but other that a lot of boats crashing to surface I haven't heard any of the wise men speaking to the loss of lift. There were early comments here when the first videos showed tip breach and full ventilation without falling to earth, but not much commentary for Awhile.