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random

Wing Tip Reverse Camber

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I have seen this mentioned multiple times, but I am yet to see one instance of the AC50s inverting the wing tip to provide righting moment.  I am sceptical.  Recent reports where OR found that larger Tees on the rudders cost more in drag than they were worth reinforces my doubts.  In order to get reverse camber a section of the wing has to be in transition, producing nothing but drag.

If anyone has seen this, please point me at please, I am wiling to be convinced that this happened.

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I guess your reference to rudders is in relation to their ability to provide RM through downforce on the windward rudder (presumably through negative angle of attack)?

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From this article and others ...

Paul Bieker Moves On from the 35th America’s Cup

Another area was the rudder wings, which were at the maximum size to help with righting moment. It turns out that cutting them down to reduce drag a bit was more helpful than the extra righting moment.

Finally, Bieker added to the tips of the high-speed foils to extend their range into the lower winds.

The changes made Oracle a bit faster and noticeably more skittish. It wasn’t nearly enough to keep pace with the well-oiled Kiwi machine.

Did You Know

Did you sailish.com readers know that righting moment was provided by the wing sails?

Huh?

Yes, it turns out that in higher breezes up to the top third of the wing sail was “inverted” so that instead of providing force on the windward side of the sail, force came from the leeward side of the sail to provide righting moment.

Toto, we’re definitely not in Kansas any more.

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^ Paul Bieker is da man, especially when it comes to the broader concepts - he's telling it like it is. Here's another way to unpack it - the rig that powers up the boats in 6 kts is still the same size when the boats are hitting 44 kts in 14 kts of wind. That's an apparent wind change on the order of 800% - so something big has got to happen.

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Bieker's stuff abut the rudder foil is really interesting.

But I think the OP is talking about the wind sail? I recall talk about it before the cup in reference to San Fran. It came from some boat club talk where the AC sailors were asked whether they inverted the camber in to top of the main wing when over powered to produce righting moment. 

It's something you struggle to ever do with a soft sail, the best you can do is blade the top of the leech off to release power. Skiffs with a flexy mast do this somewhat automatic and is discussed by Bethwaite in his books. However, with a hard wing sail a can actively push negative camber in to the sail. This will of course create drag, but as its right at the top of the rig you don\t have to invert much to create a lot of force due to the leverage. 

That was the theory anyway (I was sceptical), and never really saw it in the cup, but it's a pretty suitable thing to pick up by eye. 

Gravy Waltz post from 10 March here

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Yes I saw that post then and you might see my questions near it also.  I watched the races and saw no wing tip inversion.  It would not be subtle at all for those of us used to watching sail shape, even a flat wing would be very noticeable.

There was some great footage during the races were the camera panned up to the wings on both boats and they looked normal.

So with no evidence that this actually happened, on what was a very well recorded event ... I'm thinking bullshit, but I want to see it if indeed it was there.

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29 minutes ago, random said:

Yes I saw that post then and you might see my questions near it also.  I watched the races and saw no wing tip inversion.  It would not be subtle at all for those of us used to watching sail shape, even a flat wing would be very noticeable.

There was some great footage during the races were the camera panned up to the wings on both boats and they looked normal.

So with no evidence that this actually happened, on what was a very well recorded event ... I'm thinking bullshit, but I want to see it if indeed it was there.

I think it would be very difficult to see. The top of the wing is already operating at both a camber and an AoA approaching zero once the boats are at speed, so you would only need a very small further adjustment - perhaps less than a degree - to create significant righting moment thanks to the 23m moment arm. 

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On 10/2/2017 at 0:24 PM, mozzy656 said:

Bieker's stuff abut the rudder foil is really interesting.

But I think the OP is talking about the wind sail? I recall talk about it before the cup in reference to San Fran. It came from some boat club talk where the AC sailors were asked whether they inverted the camber in to top of the main wing when over powered to produce righting moment. 

It's something you struggle to ever do with a soft sail, the best you can do is blade the top of the leech off to release power. Skiffs with a flexy mast do this somewhat automatic and is discussed by Bethwaite in his books. However, with a hard wing sail a can actively push negative camber in to the sail. This will of course create drag, but as its right at the top of the rig you don\t have to invert much to create a lot of force due to the leverage. 

That was the theory anyway (I was sceptical), and never really saw it in the cup, but it's a pretty suitable thing to pick up by eye. 

Gravy Waltz post from 10 March here

We used to do that in breeze with my former boat, a masthead rig, with a #3 up front and a main with top two full battens.  Trimmer would get the main sheeted in, drop the traveler and when the battens popped in reverse, he'd traveler up and sheet in harder.  Couldn't always get them to pop but when they did, the boat was definitely quicker and flatter.  Worked best in smoothish water. 

Haven't been able to get my frac rig boat to do it.

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Thanks for the reply.  Sounds like you had something going on the old boat but I remain doubtful about the drag V RM issue.  I can understand why it would sail flatter though, some boats do like that.

For something mentioned many times around the wing masts in the AC boats, I have not seen one piece of evidence that it actually happened.  We had full coverage from multiple angles, drones and helicopters even and not once was reverse camber visible.  I was watching for it.  Thinking about it now, it would be something that I would try al the top end of the equipment wind range to bleed off power and flatten the boat, not something that would create more boat speed in normal conditions.  If it works, why did we not see it in all the hours of race footage?

 

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2 hours ago, Hitcher said:

Reverse camber was why T2 capsized in the Solent.

Uh Huh.  But I'm looking for evidence of go fast, or even of productive use.

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