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AC72 foil

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Bille
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AC72 foil

Postby Bille » Wed Sep 25, 2013 7:07 am

It's called a J-foil, there efficient enough because Sail-Rocked used one.
Does Anyone know what airfoil they use on the wing part of the AC72 ?

Bille

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Re: AC72 foil

Postby FrederikS » Wed Sep 25, 2013 9:21 am

It is not a conventional J foil sin the foil section attachment to the board is almost at a 90 degree angle. It is an acute L foil based on this http://carbonicboats.blogspot.dk/2013/0 ... -soup.html

From the on-board footage it looks to be a fairly high aspect ratio foil with a shallow profile. The outline is a bit special and I think I have seen two different on the Oracle boat.

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Re: AC72 foil

Postby ozchrisb » Wed Sep 25, 2013 7:18 pm

It's very different to the foil sail rocket uses. The AC72 foil is made so the the tip points "up" when the board is down and canted. When the tip pierces the surface it reduces the lift, and cavitates the top of the L/V on the foil. Sail rockets foils are all about preventing cavitation which happens at around 50 knots. You can see it happening here The water starts to boil on the high pressure side and it's game over.

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Re: AC72 foil

Postby davesails7 » Sat Sep 28, 2013 4:50 am

ozchrisb wrote: Sail rockets foils are all about preventing cavitation which happens at around 50 knots.

The water starts to boil on the high pressure side and it's game over.


Cavitation is from low pressure, not high pressure. The top of a hydrofoil is the low pressure side. High pressure stops water from boiling.

This is the second time I've seen someone state that cavitation happens at 50 knots. Are you saying that the particular foil on sailrocket cavitates at 50 knots? There is a speed when cavitation will happen for a particular foil at a particular angle of attack, but there is no single speed where all foils will cavitate.

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Re: AC72 foil

Postby droffats » Sat Sep 28, 2013 2:13 pm

Maybe they get that from Spithill at a press conference. Spithill said something like that the rudder foil, being fixed, cavitates around 50mph. He said if they could adjust the rudder angle on the fly, they could hit 60mph in the AC72

But yeah, that is a particular foil at particular speed, not a general rule.

- Stafford

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Re: AC72 foil

Postby Bille » Sat Oct 31, 2015 5:18 pm

Post subject: Re: AC72 foil
PostPosted: Fri Sep 27, 2013 8:50 pm

ozchrisb wrote:...
You can see it happening here ... The water starts to boil ...
and it's game over.


I just watched this again , (3 years later) ; never noticed that there
was hardly no boiling at the , (Strut / Wing) interface. The first time i watched
it ; i looked strictly at the tips , and still can't figure out why i'd do that ?

Anyway --- maybe the wing ; maybe it doesn't Need to be out in
front of the Strut , as much as i previously thought ?

Bille

2:33 minutes :
Attachments
cavitation (2).png

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Re: AC72 foil

Postby mig27 » Mon Nov 02, 2015 7:25 am

Around 50kn of water flow around a wing in normal sea-level pressure levels (or fin, that is why the speed kiteboarders are stuck at exceeding this same speed number) the pressure drops so low that the boiling temperature of water is equal to that of the ambient water. This results in a sudden loss of flow along the surface.
That means no lift of grip any-more..

Sailrocket from Vestas appeared to have this issue with the fin only, as this machine skimmed the surface and party hovered above the water.
They claim to have solved it by changing the fin type during the ride from conventional wing profile into a so called super cavitation version.
Basically a wing section with the trailing edge being sharp flat.
Powerboats use these also with their propeller blades for the same reason:

Image

Image
(the rear of the propeller is upward, so you clearly can see the straight, sharp edge)

That is what I understood reading about this.
Here is a small article about cavitation and sailrocket

And here a more detailed story

Disadvantage for sailing craft (versus mechanical horsepower) is the excessive drag at lower speeds: no chance to reach the top speed then.

This is a nice engineering challenge for the near future I guess

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Re: AC72 foil

Postby Bille » Tue Nov 03, 2015 1:31 am

mig27 wrote:...

This is a nice engineering challenge for the near future I guess


I was into fast 2-M, RC gliders for a bit ; common knowledge that
the TE had to be perfectly flat , and 90-deg to the air-flow , and
that was even if the TE were only 0.020" thick . the squared
off edge would make the air pass off it, like it was a longer cord
on the air-foil.

Even the slower 1/4 scale , RC ships of 80" span and thick TE on the
ailerons ; they are more effective if the TE is sanded Flat.

@ mig27
Ya appear to have a good grasp of this ; bin studying it long ?

Bille

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Re: AC72 foil

Postby edt » Tue Nov 03, 2015 1:41 am

Bille wrote:It's called a J-foil, there efficient enough because Sail-Rocked used one.
Does Anyone know what airfoil they use on the wing part of the AC72 ?

Bille


i wouldnt use the ac72 design as an example to follow. that particular design is very highly constrained the weight the shape and size of the hydrofoils, I can't remember which team it was but I believe the kiwis developed a new hydrofoil they wanted to use in the cup but the rules team refused them.

i dont know if we will see the like of those sailboats again. Next america's cup will be relatively tame, 48 foot foilers at Bermuda in June.

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Re: AC72 foil

Postby mig27 » Tue Nov 03, 2015 10:19 am

Bille wrote:@ mig27
Ya appear to have a good grasp of this ; bin studying it long ?


Funny that you mention, been building and flying model air-planes when I was young too! Until somebody developed a way how to surf with a twintip and a kite.... 8)

Indeed I have been studying the subject for a while, pure out of interest.
I am a mechanical & structural engineer, and work with people who have studied hydrodynamics.
Interesting matter, and this so called "sound barrier in water" is still largely undiscovered territory.

Even a last enough flow-tank is not available. I love what those students did in Scandinavia in that video: lower the pressure in the closed flow-tank to cope with the relatively low water speeds.

I think we will see a lot of development in the near future, since also military water-craft are looking into hydrofoils again.
New materials and new insights can make the difference.


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