34th America's Cup: multihulls!

[Doug Lord]

First Foiling America's Cup

Rule 5.14 is very badly written, imo. Anything one puts into a water flow will alter the velocity and pressure field around it, and will hence alter the boundary layer. In the same moment they have put the horizontal elevator to the rudder, it has modified the flow field around the hull and the rudder. So, the rule 5.14 is practically very hard to enforce in a literal sense.
However, if one wants to read it literally, any device on the surface of the airfoil or of the hull, the purpose of which is to modify the water flow (and hence the boundary layer) around the foil or the hull, is prohibited.

But... What if it is not ON the surface but IS the surface or an integral part of it?

So I can design an airfoil with a shape which mimics the gurney flap (or an interceptor) and nobody could say anything about it. It is not an interceptor, it is the airfoil's shape - as odd as it could be. If we assume that this is legally possible, than an interceptor would have a pretty big effect on the pressure field around the foil.

Since one picture is worth a thousand words, I have prepared this comparative graph:

View attachment 84873

It shows two identical airfoils, the above one in clean configuration, the bottom one with a small (1.5% of the chord) gurney flap set at 65°.
For the same lift coefficient (0.7 approximately) the foil with a gurney works at a much lower angle of attack, has a much lower peak pressure coefficient (Cp) and the distribution of Cp along the chord is nearly flat and low-value. A low (absolute value of) Cp means that cavitation (and a ventilation too) is delayed for a given speed, thus increasing the foil efficiency.
For the same angle of attack, a foil with a gurney flap has a considerably higher Cl (i.e. produces more lift), at a very small drag expense.

I see the most value of gurney at low speeds, where a high lift coefficient is required for a rapid take-off to a foiling mode. At high speed, close to maximum speed, a foil with an interceptor will produce a higher pitch-down moment and will hence require more balancing negative lift by the rudder elevator. That is a straightforward drag. The final result, in terms of maximum speed, would be the sum between less drag due to delayed cavitation and ventilation, and more drag due to the elevator working harder. Hard to say more with no real-world telemetry data from the boats.

A few remarks:
- Please note that when I say "low Cp", it is referred to its absolute value, |Cp|. Example: |-2| = 2 and |-4.5|=4.5
- Also, please note that these Cl values are not related to the actual AC72 foil data (we don't even know for sure whether it was the solution used by the Oracle), but serve to make a visual explanation of what the gurney flap does to the pressure field around the foil.
- Furthermore, this is a numerical analysis made with a computer software, hence based on a mathematical model of airfoil aerodynamics. So don't take the numerical values shown in the graph as absolute truth. I wouldn't be surprised to find out there's a difference of 10-15% respect to experimental or measured values, in this range of angles of attack.
- Finally, the above graphs show a small-scale saw-toothed irregularities, which is probably due to a numerical instability coming from the very high Reynolds number used for the calculations. But for the purpose of a qualitative comparison, they are more than adequate.

Cheers

==================
Great stuff here-thanks Slavi!

 

[Doug Lord]

First Foiling America's Cup

OK heres a quick SAS system. It works exactly like the suspension on a car wheel. A spring and a shock absorber. The trimmer is locked out when travelling but the trimmer can unlock and trim/preset it as required. The spring acts against the foil load and the "shock absorber" is the damper. The systems neutral position is 10deg AoA so it can take off. The spring rate is set to balance at about 5deg and the boats trim creates the dynamic response. Its not fully detailed but it will show the main features required. It has no stored energy. Once they got the spring rate, damper value and preset AoA right it would look after itself just like a car wheel does on bumps. The trimmer would have a chart in front of him generated by training data. It would have speed and AoA ranges. So he can read his speedo and adjust the trim to the chart or by feel and anticipation. But if they got the values right it would nearly look after itself. Oh yes, if the heave (up movement) is too much for the SAS to handle the foil breaks the surface and the area gets smaller, so the lift drops immediately. Doing this exercise shows me that its actually OK to have a smaller foil and optimise for 30-40kn say and use a high lift foil to get it out of the water. Say a Cl=2+ Also we can make a non linear spring/hydraulic to match the load rate better. Cheers Peter S

I've just added an article I wrote for last months Australian Multihull Magazine.

======================
Thanks very much, Peter!!

 

[markdrela]

So I can design an airfoil with a shape which mimics the gurney flap (or an interceptor) and nobody could say anything about it. It is not an interceptor, it is the airfoil's shape - as odd as it could be. If we assume that this is legally possible, than an interceptor would have a pretty big effect on the pressure field around the foil.

The Divergent Trailing Edge is in effect an integral Gurney flap much like the "feature" on your airfoil. I don't see how a DTE could possibly be ruled illegal under 5.14, since you can't even define where the airfoil ends and the DTE begins.

It shows two identical airfoils, the above one in clean configuration, the bottom one with a small (1.5% of the chord) trailing-edge gurney flap set at 65°.

This is really a straw-man comparison. Of course the airfoil with the Gurney flap will have more lift for the same alpha, or reduced peak velocity for the same lift. But that doesn't mean it's the best solution.

A better comparison would be between
a) a sub-optimal airfoil with an added Gurney flap, and
b) an airfoil without a Gurney flap, but designed for the same peak velocity.

Design b) will be surely be better. It's better to properly optimize an airfoil shape than to add a Gurney flap as a band-aid. This is why you don't see Gurney flaps or DTE's on optimized modern jet wings.

 

[daiquiri]

A better comparison would be between
a) a sub-optimal airfoil with an added Gurney flap, and
b) an airfoil without a Gurney flap, but designed for the same peak velocity.

Design b) will be surely be better. It's better to properly optimize an airfoil shape than to add a Gurney flap as a band-aid. This is why you don't see Gurney flaps or DTE's on optimized modern jet wings.

Thanks for the comment. I agree with your remarks. It goes without saying that these things are trivial and obvious to you, but are not so obvious to people who don't have such a good knowledge of aerodynamics. My considerations about the effect of interceptors, and the relative illustration, were intended principally for them.

Looking at how things have gone in the AC finals, it is perfectly possible that Oracle was in the situation A. Losing 8-1, calling a 1-day postponement and applying a series of overnight fixes to the existing foils. If (hypothetically) the foil shape couldn't be any more refined and changed because the through-hull guiding slots didn't allow it, then they might have been left with just the option of applying a band-aid - like an interceptor or something similar.

We might be talking about an imaginary issue anyways. I have searched through tens of the latest pictures of the Oracle boat and couldn't spot any hint of interceptors on their foils.
It is all just conjectures at this point. We might never know the truth. Or perhaps we will know it soon, if the rumors that ETNZ is preparing a legal action against Oracle's presumed SAS are true. In that case, some technical details might become public pretty soon.

By the way, Mr. Drela, for some reason I thought you might be a very informed person about the aero-hydrodynamic development of the Team Oracle boat... I was rather surprised to find out that (as it seems) you have not been a part of the expedition.

Cheers

 

[Doug Lord]

First Foiling AC

You know, theres been a lot of loose talk that "all of a sudden" half way thru the Cup Team USA became more stable than TNZ and able to sail upwind on foils. Watch these two videos taken before the Cup even started!

http://www.youtube.com/watch?v=xxjBS5cahyw


http://www.youtube.com/watch?v=mk-k67O9fjg

 

[Doug Lord]

hey Peter, Mark, Slavi and other interested people take a look at these submissions by Oracle from February 2013:

click--

 

[DCockey]

An example of some of the silly talk going around. This one was published in the New Zealand Herald as "fiction" about an active gyroscopic stabilizer being fitted to the Oracle boat. Given the stringent maximum weight limit I doubt there was enough mass available to fit two stabilizers which were large enough to make a difference. And given that the boats were inspected everyday by the measurers it is extremely doubtful that they would not have noticed. http://www.nzherald.co.nz/sport/news/article.cfm?c_id=4&objectid=11131180

 

[petereng]

hey Peter, Mark, Slavi and other interested people take a look at these submissions by Oracle from February 2013:

click--

The main thrust of this was could they use servo systems to control the component vs direct control by a sailor. The answer was yes. Reasoning was that the servo did not contribute to the propulsion of the boat only the control of the boat. It could be argued there is no difference between a block and tackle and a servo. The leap is in the use of batteries to enable/disable the servo device. Electric servos make logistics and packaging sense. Cheers Peter S

 

[oldsailor7]

In Fridays issue of "The Manly Daily:, there is a two page spread on the AC. and the front page has a full size Pic of Spit hill kissing the Cup, with the heading--DREAMS ALOFT, "Pittwater resident James Spithill finally has his hands
on the Americas Cup after one of the biggest comebacks in history."
The article goes on to say what an inspiration Spithill and this AC win is to the young sailors coming up in the Royal Prince Alfred Yacht Club.
One sailing boy age 7 says, "I especially like catamarans because they go really fast".
This sort of exposure (X 1000) is what we need to expand the sport of sailing, and in particular---Multihull sailing.

 

[daiquiri]

An example of some of the silly talk going around. This one was published in the New Zealand Herald as "fiction" about an active gyroscopic stabilizer being fitted to the Oracle boat. Given the stringent maximum weight limit I doubt there was enough mass available to fit two stabilizers which were large enough to make a difference. And given that the boats were inspected everyday by the measurers it is extremely doubtful that they would not have noticed.

A gyro-stabilizer for a boat that size require a continuous power input of several HPs (check this: http://www.boatdesign.net/forums/stability/anti-rolling-tank-32925.html#post368667). No way it could be powered by grinders. The way I read that article, it says that gyros would presumably be just boat attitude sensors, which would provide visual input to the sailors.

Don't know... The number of conjectures and alternative timelines seem to be growing exponentially in the last hours. Looks like the journalistic fantasy has been unleashed and is running wildly.

 

[DCockey]

Thanks for the comment. I agree with this remark. Looking at how things have gone in the AC finals, it is perfectly possible that Oracle was in the situation A. Losing 8-1, calling a 1-day postponement and applying a series of overnight fixes to the existing foils. If (hypothetically) the foil shape couldn't be any more refined and changed because the through-hull guiding slots didn't allow it, then they might have been left with just the option of applying a band-aid - like an interceptor or something similar.

I remember claims that Oracle was doing extensive on-the-water testing of different foils during the summer, including sailing with different foils on each side. So I expect they knew how their various foils behaved by the start of the match. Also, there was a report yesterday than someone speaking for OTUSA said they used the same foil for all the races.

We might be talking about an imaginary issue anyways. I have searched through tens of the latest pictures of the Oracle boat and couldn't spot any hint of interceptors on their foils.
It is all just conjectures at this point. We might never know the truth. Or perhaps we will know it soon, if the rumors that ETNZ is preparing a legal action against Oracle's presumed SAS are true. In that case, some technical details might become public pretty soon.

.......

18 measurement certificates were issued for OTUSA from September 3 through 24. 11 certificates were issue for ETNZ from September 3 through 24. Links to the measurement certificates can be found by going to http://noticeboard.americascup.com/ and searching for "certificate". I attached a screen capture of the results of the search. The individual measurement certificates just say the boat was measured and certified as adhering to the rules.

My guess is any legal challenge to the legality of the Oracle boat would be difficult to sustain given that the boats were certified by the measurers as adhering to the rules, and were also reportedly inspected by the measurers after each race.

 

[Doug Lord]

Foiling AC

The main thrust of this was could they use servo systems to control the component vs direct control by a sailor. The answer was yes. Reasoning was that the servo did not contribute to the propulsion of the boat only the control of the boat. It could be argued there is no difference between a block and tackle and a servo. The leap is in the use of batteries to enable/disable the servo device. Electric servos make logistics and packaging sense. Cheers Peter S

=================
So,in your opinion was this a viable thing-and do you think it would add to their foiling stability?

 

[DCockey]

Quote from an ETNZ crew member which was published in the NZ Herald:
"We're just so sorry we couldn't do it for New Zealand," said grinder Rob Waddell.

"This isn't supposed to happen, the good guys are supposed to win."
http://www.nzherald.co.nz/sport/news/article.cfm?c_id=4&objectid=11130655

My guess is Rob Waddell's sentiment is widely shared by ETNZ supporters.

What is left but cheating or a conspiracy to explain why your team did not win when it was their destiny to do so? (Rhetorical question, and not to suggest that Waddell has suggested a conspiracy or cheating.)

An excellent opinion column appeared in the NZ Herald. http://www.nzherald.co.nz/sport/news/article.cfm?c_id=4&objectid=11131265

 

[DCockey]

I'll repeat my post #3696 from yesterday:

It appears that ETNZ knew about Oracle's "self-leveling system" well in advance of the regatta but believed that it was inferior.

Gino Morelli was on the ETNZ design team. In an interview with Morelli which was published on September 3 Morelli said (emphasis added):

GM: Oracle have a narrower boat so if we get into displacement sailing, like in 8 knots of wind, they may have a “slippier” hull which may be an advantage if it’s super light. Just watching them sail over the past six to seven weeks you can see that they’re climbing up the learning curve fast. Their daggerboards and control systems have evolved towards ETNZ’s. They have a theoretically better system, but I think they’ve basically thrown in the towel and said, "We can’t seem to make this work, we’re going to have to go to a self-adjusting system, a self-leveling system." So they gave away, I think, one of their potential advantages in a breeze. http://www.sailingworld.com/blogs/racing/americas-cup/writing-the-rule​

 

[petereng]

Doug - The only two variables that can be used to control the AC72 foils in regard to pitch control is 1) The foil area eg if the AoA stays the same and the boat gains velocity then the boat must lift, the inboard tip of the foil breaks the water surface, the area reduces and the boat settles at that speed and height. There are many self adjusting foilers like this. But the AC72 rules limit the length and form of the board to enable this solution to be implemented entirely 2) The AoA is changed to match the required lift at the sailing speed. The question is can a person change the angle of attack (AoA) quick enough to keep a stable flight height? The answer is probably no. This is further complicated by the fact that the by changing the AoA of the daggerboard you also change the attitude of the boat, which changes the AoA and this can lead to instability. The aim is make a self stabilising system. This means the rudder elevators and wings (foils on the daggerboards) need to be coupled in a way that produces a stabilising condition not a destabilising condition. The type of system I drew earlier takes care of the wing trimming once a "stable" condition has been reached. The wing trimmer needs to ensure the wings AoA is correct for lift off at low speed becuase the boat is bow up so the rudder lifts as well (liftoff mode), then once at speed and attitude the wings need to be reset for the level flight mode. The SAS can then self stabilise the boat. The analogy of this is could you sail a Moth without a wand? Answer No... so without some sort of displacement control (wand) or force control (SAS) could someone control the flight of an AC72 by themselves ... probably not so an SAS is needed.

An interesting comment in the final press conference by Larry was that they found the boat was better with a bow down attitude. This indicates that they found its sweet spot. If they had been trying to trim the boat level and faught the idea of having the boat bow down then they could have been chasing something that they could never acheive. But by giving in to the bow down condition they balanced all the boat dynamics and found the spot that gave it stability, I once built a 3wheeled tandem tricycle that initially was unridable. When you rode it by yourself it was "normal" but when you had a passenger and you turned a corner you fell off. This was due to severe oversteer in the system. After falling off so many times and nearly hurting myself and my passengers I nearly gave up. But then I noticed something and figured it out. All I had to do was lean right before I turned right and it behaved perfectly (you can lean steer a bike and handle bar steer a bike. If you get a bike and steer the handle bars you will notice the frame moves in the opposite direction then it is steered. Same thing happens with a powered trik with no differential) Then it became an easy thing to do. So Oracle probably went through a similiar thing. They chased something and toiled and toiled but it didn't want to work then someone said how about this???? and it all fell into place. . Peter