34th America's Cup: multihulls!

That sounds like they have a very narrow range of rudder foil angles and main foil positions relative to the CoG allowed for a stable flight.

Does anyone have a reference to methods for estimating lift/drag forces of surface-piercing vee-type hydrofoils? I would like to perform a few calcs, but I don't have the necessary data for a correct modeling of the surface effects.

 

Here's a cologne that you should like, Leo!

 

Apropos the rudder lift question, here's a remark that Darren Bundock recently put on his Facebook page:


"The wheel stand shows how much lift we generate from the rudder or lack of when its gone."

 

Is that what Draper uses to tell the time?
No wonder he screws up most of the starts!

(At least most of the people who wear that are so far downwind you wouldn't smell them.)

 

lol

They should send a bottle to Dean Barker, so they can better smell their way to the start line?

 

Prada "Luna Rossa" Cologne - when you want to smell like you're behind.

 

^^^

Excellent!

 

LOL!!!

 

Rudder Forces

Hi Daiquiri - As drawn the rudder is always pulling down to balance the forward moment of the sails CE so your conclusion is correct... But... to explain. The diagram is of the boat in flying mode. Plus it is drawn in a static equilibrium condition or consider it as an average condition. ie on average the rudder has to pull down when flying. Why on average? Because the boat is moving in a dynamic condition so it is heaving and surging (plus rotational effects, pitching, yawing & rolling) These dynamic or inertial forces need to be considered as well. So if the boat is heaving the rudder and daggerboard could be lifting or sinking (for a small time) and then the forces could be up or down. The water and masses involved act as a damper plus the rudder produces forces counter to the pitching direction damping the motion. Once we start thinking about inertial conditions it can become complicated. Hope this helps. The other condition to consider is lift off, then the rudder must be trimmed to provide an upward force, but once up must be trimmed to provide a downward force otherwise the boat pitches back into the water. Early in the AC72 flying development you would have seen the boats "kangarooing or hobby horsing" a lot until they figured out the timing of various actions and settings. Plus it would not surprise me if the boards had force control mechanisms in them (springs or hydraulics) to limit or "snap on or snap off" various trim angles in the transition, this is called self activating or self energising systems, we use the foil reactions to provide forces to change the mechanisms (drum brakes are a good example of this). Now your comment about controlling rudder pitch. Yes it would have been much easier if we could have "active" trim tabs on the rudders but the rules don't allow this. So the answer is to control boat pitch by using the daggerboard (lift and sink of daggerboard changes pitch of boat). The rudders are set up according to the sea conditions so as to provide a slightly sinking condition ie as stated above on average we want the rudder to pull down. The rudder is a symmetric (or very close) section so it can provide lift or sink depending on its angle of attack. Its AoA is defined by the hull pitch angle which can be controlled by the daggerboard trim. Harder this way but they have figured this out. Its not new there have been many foilers that have a passive rudder elevator in the past 30 years. Peter S

Just reading this opens the problem of pitch stability. Just like we have to set the "lead" of our boat so that the boat remains in a stable condition as it moves around we have to set the rudder in an average stable condition. If we aim at setting it a zero (sink or lift) then we have an unstable condition as the rudder will hunt and try to find find equilibrium. If we set it to a range that is in lift we run the risk of allowing the boat to pitchpole easily, so the answer is to set the rudder to sink slightly according to the sea conditions, the rougher the water the more the sink. This means that the boat will be stable in pitch and resist pitchpoling. We acheive this by moving the CG around by the crew. In the design phase we would have moved the daggerboard and rudder and boat masses around to the best posaition. Then on the day we trim with crew if possible. cheers Peter s

 

34th AC

When Dr. Bradfield was teaching me about foilers, he said that with a boat where the mainfoil supports 80% of the load and the rudder supports 20% of the load at takeoff, that the rudderfoil will gradually unload until at "X" speed it will automatically begin to pull down. Thats how his Rave, Osprey, Skat and my F3 work. I doubt that the 72 foils are designed to give constant vertical lift up-they are bound to pull down when a certain speed is reached, I would think. Bradfield didn't feel that an adjustable rudder foil was necessary even thouh he experimented with it. On my rc foiler the F3 it had no rudder adjustment by radio and worked perfectly. I experimented with a servo adjusted rudder flap but it made no significant difference.
If I was involved with the 72's I would definitely have an adjustable rudder angle of incidence if ony to guarantee that the setup was right.
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As far as the Oracle and TNZ pitchpole incidents go ,I'm more than convinced that the problem was lack of forward lift at a critical moment. If they had been able to drop the windward foil just as they started the rounding the bow would never have gone down and there would have been no incident. It would be legal under the rules and would not have slowed them as much as the near pitchpole did.

 

34th AC

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I'd bet this bow up attitude was caused by the dramatic increase in downforce on the rudder foil as it broke away. The leading edge would have dropped increasing downforce until/as the foil broke off.

 

Hi Doug,
I think you mean a certain acceleration which corresponds to a certain pitch forward condition. The boat will reach a stability condition irrespective of speed. If the boat accelerated at a very very small rate the rudder would never angle down and create sinkage. This brings me back to the point bthat we need to think in terms of the inertial(or dynamic) state not the equilibrium state. Cheers Peter S

 

Unfortunately I think this condition cannot be accepted on face value. The rudder could be lifting or sinking yet the boat could fall backwards when its gone. When flying you have to consider the AC72 as a seesaw with the fulcrum being the daggerboard. The rudder elevator provides trimming of the seesaw, whether it is up or down depends on the dynamic condition at the time. As the boat CG is behind the daggerboard if the rudder is not there gravity wins (when going slowly) and the boat falls backward, but in a bear away the sail drive may win and you pitchpole. Cheers Peter S

 

34th AC

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I was not talking about a generic situation but about that specific situation where the boat a hit a bouy hard a little while before the rudder broke off. It seems to me likely that the breaking off could have caused the bow up attitude in this case.
Unless , you put the windward foil down just before you start the bearaway...

 

34th AC

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Tom Speer has a page on his site that compares v foils ,t foils, ladder foils.
And here it is: http://www.tspeer.com/Hydrofoils/generic.pdf