Luna Rossa Challenge. AC 36
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nav
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Just found this paper that seems highly relevant here:Thanks Mike, our SAS team is on the job!Couple of interesting things:
- The foil appears to have a quite a "sharp" Leading Edge, and a relatively thin section. That's "fast" - low drag - but it needs to be very carefully angled during flight so the AoA doesn't cause boundary layer separation starting right at the LE, destroying the foil's lift. LR will need to be precise.
- The "initial" AoA of the foil (to the perpendicular) appears to be much lower that I'd originally guessed - perhaps less than 1°. This suggests that LR have designed B2 to fly quite level at all wind strengths. It may also require them to deploy a quite a bit of draggy flap to take-of. Not using hull AoA to substantially set foil AoA also has CoE and weather/lee helm ramifications - flying "bow down" moves the CoE slightly forward.
- The 400kg or thereabouts of ballast weight is presumably mostly buried in the black lower portion of the foil arm. I don't think they would want too much extra weight in the foil wings, and there's probably not much volume available anyway after allowing for structural components.
View attachment 404952
https://www.taoxing.net/web_documents/jhd_2020.pdf
This is the relevant fig and text:
Hydrofoil profiles used in common surface-piercing foil systems are usually thin and often have flat lower surfaces (Fig. 1(a)). Although such shapes are not the most efficient for single-phase flows, their characteristics at high speeds in water are adequate, and these low-profile sections are less susceptible to air ventilation, when suction of the atmospheric air to the lifting foil surface reduces the generated lift. These hydrofoils are also beneficial for delaying cavitation to higher speeds. At lower speed, however, thin low-camber profiles cannot produce enough lift, which leads to higher operational speeds that require much greater propulsion power.
This next part is also interesting:
Another hydrodynamic consideration important for hydrofoils operating near a free surface is the effect of the shallow submergence on the occurrence of stall and specifically on the maximum lift coefficient of the foil, which is a vital property for elevating the boat hull out of water and overcoming the boat hump drag. It is well known that lift of hydrofoils generally recedes with approaching the free surface at moderate attack angles and typical speeds.
...it was noticed that the stall occurrence was delayed to higher attack angles and the maximum lift coefficient could be even higher in the presence of the free surface and at low Froude numbers
In other words, because these foils operate close to surface, they may be less prone to stall even with the sharp leading edge.
GeeJay
Member
Heard a snippet on radio news this morning where Max Sirena accused ETNZ of supplying shoddy design foil arms which don't work as intended. What's the background to this? Haven't seen any rumblings reported before other than the early problems when they were being developed. Anyone got an inkling other than Max just starting the excuses for poor performance?
nav
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ETNZ were responsible for the FCS and there have been stories of either mechanical or software problems with that system off and on for months. Those 'issues' have not been openly discussed or clarified that i know of. Bound to have been some improvements and updates along the way though as the inevitable bugs have become apparent.
rh3000
Super Anarchist
Ironically the foil arms themselves were the responsibility of LR who shopped it to Persico who totally fucked it up and ETNZ had to come in and redesign them.
This added extra work to ETNZ and caused a delay for all teams getting their arms.
Either Max is referring to FCS (in which case glass houses) or he is truly referring to the arms (in which case he is just trolling).
WetHog
Super Anarchist
Not again - LR did the structural design (and evidently got it wrong) and Persico was only responsible for the execution. Is it too difficult to understand?
MaxHugen
Super Anarchist
Interesting, thanks. After looking at my estimate of the foil shape, I guessed this:
Ii seems to have max chord width a fair way back, thought I'd see what CL and CD looks like for such a section, plus at various flap angles:
The CL/CD ratios look OK so far, but haven't looked into it much yet. What sort of CL and CD numbers are you using for the foil, and stabilator?
As we're not seeing so much "bow down" sailing - even at higher wind speeds - should we assume they're concentrating on mostly "level" hull AoA?
pusslicker
Super Anarchist
This is all on ENTZ. They're running this cup and responsible for the design of the boat. Who let's Italians handle something this crucial?
Priscilla
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All four teams have been fanging it on the sparkling Waitemata with no evident foil arm failures looks pretty sorted to me.
Considering the radical design of the AC75 the reality is the designers engineers and builders have produced a versatile and very robust flyer.
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weta27
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Max is referring to the FCS:
Challenger of Record Luna Rossa is unhappy with a key design component provided by Team New Zealand for the upcoming America's Cup regatta - with skipper and CEO Max Sirena describing the foil cant system as something from a regular car being put into a Formula One car.
"I can tell you the two components are not on the same level - it's like putting in a component which is coming up from a Formula One car together with one from a car you use every day, so it doesn't really match. We are not happy because it doesn't work as we wish."
"It would be a shame if we're going to lose a race because the system doesn't work. We didn't complain in reality as much because we have made it work but I know the others are complaining quite a lot."
Challenger of Record Luna Rossa is unhappy with a key design component provided by Team New Zealand for the upcoming America's Cup regatta - with skipper and CEO Max Sirena describing the foil cant system as something from a regular car being put into a Formula One car.
"I can tell you the two components are not on the same level - it's like putting in a component which is coming up from a Formula One car together with one from a car you use every day, so it doesn't really match. We are not happy because it doesn't work as we wish."
"It would be a shame if we're going to lose a race because the system doesn't work. We didn't complain in reality as much because we have made it work but I know the others are complaining quite a lot."
Forourselves
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