Boats and foils comparison

barfy

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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.  
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.

 
...A question pls: why is the forward half of the lower surface designed so that it produces no lift?
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.

 
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.
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.

Ventilation doesn't require as low a pressure as cavitation - the local pressure only has to be lower than atmospheric pressure for air to be sucked down onto the foil.  But ventilation requires something else that cavitation does not - the flow has to be separated to begin with.  Otherwise, the fast-flowing water will simply sweep the air off the foil.  So ventilation is fundamentally a boundary layer phenomenon.

However, the flow separation can come from some surprising ways.  The C-FLY guys told me about how they had thought they'd designed their surface-piercing main foil so it wouldn't ventilate.  But waves breaking over the foil could trap a pocket of air and that would create a separated zone that would propagate down the foil very quickly.  

Ventilation also requires a path for the air to get to the separated zone.  This can be via a laminar separation bubble at the leading edge that acts like a straw when it is filled with air.  It can even happen by air coming up from behind the boat via the low pressure in the core of a trailing vortex.  A common cure for ventilation is to add fences that block the progress of air coming down the foil.  But fences also add drag, and if the fence comes out of the water, it is not effective.  

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.

 
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.
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.  

I think a Version 3 AC72 (or even an F50) could be a very formidable competitor to an AC75 in the prestart.  It could dial up an AC75 until both boats fell off their foils, and then it would be, "Sayonara."

 
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MaxHugen

Super Anarchist
...

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.
@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:




 
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.
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).

However it gave lots of insight into the general design process and just more general development culture of the cup. I like these interviews.

What i did find interesting was the comment about how it wasn't only ETNZ that affairs a lot of design areas to support a specific design ideas. It was revealing to hear how much was affected by the runnerless configuration. We now know why they held onto it for so long long, as it affected most other parts of the boat.

 

MaxHugen

Super Anarchist
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.
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. 

For a start, the Density default in Type 1 may be an issue? Also don't know how the vals for Chord/Span/Mass affect XFoil?

I'm using an Re calculated using NZ's avg foil chord of 0.32m, at a velocity of 50 knots (Mach 0.075), and the same Ncrit as you do. These are my Type 1 params in XFLR5:

image.png

Any recommendations would be appreciated. :)

 
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. :)
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.

Type 1 polars are what you'd get from a wind tunnel test.  The Reynolds number is constant throughout the whole run as the angle of attack is varied. It is what the wing would experience in flight if a plane were to do a wind-up turn at a constant speed, rolling into the turn from level flight and steadily increasing the g's until the wing stalled.

The problem with type 1 polars is more often than not one is interested in the performance under steady state conditions.  When a plane or hydrofoil is flying at different speeds, the Reynolds number is changing with speed and the angle of attack is changing with speed.  So you need to run a whole series of type 1 polars and then interpolate between them in order to get the profile drag vs speed at the correct Reynolds numbers.

You can shortcut this process with a type 2 polar.  The type 2 polar assumes the lift (in N) is constant.  This is not a bad approximation for the lift because the lift for a wing generally has to equal the weight.  The equilibrium lift coefficient will vary  inversely with the square of the speed, with low speed corresponding to high lift coefficients and high speed corresponding to low lift coefficients.  Since Reynolds number varies with speed, this means the Reynolds number will be low at low speed and high lift coefficient, and Reynolds number will be high at high speed and low lift coefficient.  The type 2 polar behaves this way, varying the Reynolds number by the square root of the lift coefficient.  So you can make one run and have the appropriate Reynolds number for each steady-state lift coefficient without having to interpolate between a number of type 1 polars.

FWIW, Xfoil's type 3 drag polar also varies the Reynolds number with lift coefficient, but this is oriented toward sizing the wing area at the design stage.  Induced drag depends on the span, but doesn't depend on the planform area.  Profile drag, on the other hand, does depend on the area.  If you keep the span constant and vary the area by changing the chord, the Reynolds number will also change.  Keeping the lift (in N) constant again but this time also keeping the speed constant, a larger chord will result in a lower lift coefficient and a higher Reynolds number.  This will increase the drag due to extra wetted area, but not quite as much as you'd expect because of the increased Reynolds number and the variation of drag coefficient with lift coefficient.  The type 3 drag polar varies the Reynolds number in this manner.

The ideal planform area is such that the wing operates at the maximum profile lift/drag ratio.  You can size the wing so the lift coefficient corresponds to the maximum lift/drag ratio of the type 3 polar.  If the wing is larger than this size, the lift coefficient will be lower, the Reynolds number higher, and the profile drag coefficient will be lower as well.  But the increased wetted area will overcome the savings in drag from the reduced drag coefficient.  If you make the wing smaller than the ideal size, there is a savings in wetted area but the increase in the drag coefficient outweighs the savings due to wetted area.  Two different section shapes will vary their drag coefficients differently with Reynolds number and lift coefficient and so will have different ideal wing sizes.  Using the type 3 drag polars, you can compare different sections and the one that has the highest maximum 2D lift/drag ratio will be the most efficient section.  This will be true for that section at its ideal size compared to the other sections at their ideal sizes.

That's why I used type 2 drag polars when designing the sections.  The type 2 drag polars corresponded to operating along the red loading line in the cavitation diagrams.

 
With regard to the settings in XFLR5, you can use the actual water density and kinematic viscosity.  You don't need to use the default values for air.

If you are using a type 2 polar, I think the easiest way to determine the Reynolds number is to consider what the speed would be at a lift coefficient of 1.0.  You can look that up on my cavitation diagrams by following your design loading line.  The speed at Cl=1.0 along with the average chord will give you the Reynolds number to put into Xfoil/XFLR5.

Of course, when you're working the nitty gritty of an actual design there will be different sections designed for different spanwise stations along the wing, and for those you'd use the local chord instead of the average chord.  But section characteristics don't change very rapidly with Reynolds number, especially for the kinds of speeds we're interested in, so the average chord is fine for preliminary design.

 

barfy

Super Anarchist
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12 hours ago, MaxHugen said:

@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:


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.

 

Mozzy Sails

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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.
There is a big bolt that goes through the end of the foil arm that the teams attach to. 

I bet it's that. 

But isnt it quite easy to back calculate from Max's diagram?

 

Mozzy Sails

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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
The more I read this, the more intuitive it feels.

Why would force on each side of the arm be equal? 

Even without flap differential the inboard wing would load up more due to leeway increasing its AoA more than the outboard wing. But with flap differential you could produce the same loading difference but with less leeway. 

Result is thay LR can ay around with different CoE heights without canting or can move foil can without having to change CoE height? 

The thing that still has me confused is thay the upwind cant angles seem very high. I'm not sure they cant be achieved without the hull touching down or the foil surfacing. Os thay becuase the VPP was just left to keep getting faster VMG without limit on maximum cant?

 

MaxHugen

Super Anarchist
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.
Ventilation drastically reduces Lift, because air is ~1000 times less dense than water.

Calculating just when ventilation will occur gets a bit complex, as any wave action is also a factor. Looking at the following diagram, it shows how ambient air is pushing it's way into the low pressure area on the top side of the foil.

The Y foil has a shallower angle to the water surface, and with less water above the foil, air is able to force it's way into and along the length of the foil easier. As ventilation occurs, the foil will produce less lift, and sink down until ventilation ceases.

image.png

Does this help any?

 
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