Why so stable?

Discussion in 'Multihulls' started by catsketcher, Feb 16, 2010.

  1. catsketcher
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    catsketcher Senior Member

    I was amazed to see the AC boats on the net. They had me hooked. One of the interesting things was the stability of the tri. In the stern on shots it was very obvious that James Spithill was keeping BMW much straighter than Alinghi. It seemed that the tri was on rails -at least downwind.

    So was it that the wing was much better at quickly powering up than the normal rig. Sadly I didn't see any shots of the top of the wing working. I would not be surprised to see that the top was wiggling back and forth (reducing and increasing twist) a great deal. Unlike a normal rig you may be able to fiddle around with the upper elements totally separately from the top. Certainly I did not see any traveller moving - I didn't see any traveller. How did they sheet the wing?

    Another thing that I noticed was the huge camber of the wing on deep reaches. On leg 3 of the second race it looked as though the thing had a camber of about 30%. Not only that but it seemed as though both boats had the clew of the main above the centreline. That seems amazing. The apparent must be doing some strange stuff.

    Interestingly I am more intrigued by the whole thing being done again in multis than I used to be. Although we can lament "proper yachts" and the lack of ability of these particular boats to race in heavier conditions, as the French have shown superfast multihulls are capable of handling the most dreadful of conditions. We now have another chapter in the multihull racer book and the same designers anmd builders could certainly build boats to handle the winds of the bay area. It certainly was good multi **** to see boats designed for the light winter winds of Valencia. 30 knots downwind in 10 knots true, a 100ft boat only twice as heavy as my 38 footer - fabulous.

    cheers

    Phil
     
  2. jehardiman
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    jehardiman Senior Member

    It is important to remember that this was the first truely open technology race in a long time. If you look at Spithill while driving you will see that he is wearing a HUD and a CPU backpack. From what I have read, he was getting a quickened nav and performance display to drive to. This would be a huge help in driving.

    As for the wing, it was in two sections. The Fwd section rotated at the step which takes all the load, and the aft section was cambered in 9(?) sections by the bellcranks you see protruding from the wing. This is old technology, at least 15 years as far as I know. There is also a similiar electro-hydraulic 3 section design out there with less windage though heavier. The big thing is the semi-elastic covering that allows twist up the trailing edge.
     
  3. yipster
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    yipster designer

    that may be explaning why BMWO drove so straight but its wingsail really did it eh
    alinghi was experimenting before and wonder if its possible to set cord with inflatable battens
     
  4. Chris Ostlind

    Chris Ostlind Previous Member


    Perhaps it is important to note that the entire wing aft section is actually made up of many unique and independently controlled sections that do not connect to one another at all. The surface of each segment might be marginally stretchable, but there is no uniform, stretchy surface covering the entire wing's leech. It's more like a whole series of independently controlled flaps contributing to a whole... much like a man-made version of the individual feathers of a bird's wing.

    The photo below illustrates this construction.
     

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  5. catsketcher
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    catsketcher Senior Member

    I still love to hear how they trimmed the thing - the ability to power it up and down must be amazing OR

    the rig (and whole boat) is a bit heavier than the cat and it has extra rotational inertia so that it doesn't go up and down as quickly.

    Does anyone have specs on the two boats?
     
  6. Chris Ostlind

    Chris Ostlind Previous Member


    I'm not aware of the precise manner for trimming the various sections of the mast. If I had to guess, it would be a series of electric motors operated by the crew in charge of the main trim. Cam Lewis mentioned the name of that person several times in the AC.com coverage. I did not take notes. I would guess that he had an assistant who used a strong lens to eyeball the tell tales of each sail section on any given point of sail and trim accordingly. I would also assume that they had some serious wind speed readout instruments at various points on the wing, along with wind direction indicators. All that would have been fed into a display so that pin point trimming could be done.

    The BMWO tri, according to many spec comments on Sailing Anarchy during the runup to the regatta, indicated that BMWO was two tons heavier than the Swiss boat. Whether that is true, or not, is still on the table for me as any and all specs released by the teams prior to the racing was highly suspect as misdirection. Still, I'd be inclined to accept a significant weight difference in the two vessels based on platform layout alone.

    The latest on the wing weight by BMWO indicates that it was lighter than Alinghi's soft sail rig, even though much taller. They (BMWO) also said that if Alinghi were to field a wing of their own this summer and issue another challenge, that they already had a better and lighter wing in the works that was much more powerful than the existing unit.

    Again, many grains of salt later, who knows what constitutes the real deal?

    What I do know is the big tri absolutely waxed the white cat from Switzerland and it wasn't even close. Ernesto has to eat that one, go home and lick the many ugly wounds and take stock as to how his arrogance painted the AC as a common street-walking chippy.

    There's a lot of work for the Golden Gate Yacht Club to do in order to get this thing back on track and respectable in the eyes of the boating and larger, sporting world.
     
  7. tspeer
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    tspeer Senior Member

    That's not quite right. Each flap is pinned at the trailing edge to its neighbors. The control arm at the base of each flap thus controls that flap and the one below it. The gaps between the flaps were covered with a stretchy material to seal the gaps, forming one continuous surface from an aerodynamic perspective.

    In order to reduce weight and maintain stiffness, the spar was straight and located at the maximum thickness of the wing. That meant the hinge line had to be curved. Which meant that there had to be some gap between the flaps because they would come together when deflected.

    Really, the wing is not that much different from the soft sail rig. There's a rotating mast and a mainsail attached to that. For the wing, the mast is larger in chord and the mainsail is thicker. But the cross section topology is the same for both rigs - a teardrop shaped mast, a small gap, and a much thinner mainsail that is articulated to be cambered relative to the mast. The hinge points where the flaps attach to the main element/mast are just like batten cars that are fixed in position instead of being on a track. The flap ribs are just thicker battens. The covering is also a woven material, so USA 17 could be said to have a more traditional sail material than A5's molded 3DLs!

    Despite its size, the main (forward) element of the wing can be considered to be a leading edge device for the mainsail, just as the mast is for the soft sail rig. Deflecting the flap one degree while holding the main element fixed relative to the apparent wind will produce more than 80% of the lift one would get by rotating the whole wing one degree while holding the flap fixed relative to the main element. Which also means that rotating the main element while holding the same orientation of the flap relative to the apparent wind will only change the lift by less than 20% compared to rotating the whole wing. So qualitatively, it's where the flap points that matters.

    That's one reason why twist control on USA 17 is all done with the flap instead of by twisting the main element as was done with Cogito. It was a simpler and lighter way to go for a wing that size. This is another way that the wing is similar to the soft sail, where twist is all in the mainsail, too.

    It may look different, but it really is a sloop.
     
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  8. Chris Ostlind

    Chris Ostlind Previous Member

    Thanks for the clarification, Tom.

    Were the stretchy gap fillers, as you describe them, added after initial testing and after the photo that is shown above was taken?
     
  9. tspeer
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    tspeer Senior Member

    Could be. The first attempt at covering the gaps ended up being shed on the first sail. After that, most of the sailing in San Diego was done without seals. They came up with a better solution when the boat was in Valencia.
     
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  10. sailor2
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    sailor2 Senior Member

    Tom, can you explain a bit more why you wrote it had to be curved rather than it was curved.

    Why wouldn't it be possible to have a straight hinge line, and curvature on the leading edge of the main element to produce the same distribution of area along the span, but a different sweep back ?
    Wouldn't that allow still placing the structural spar at the max thickness point at the bottom and at mid height, and reasonably close to that position everywhere below 75% up the mast, above that the mainspar could have reducing diameter to fit in the profile, since there is less loads at top part above the hounds. Wouldn't that approach lead to very close to the same weight with equal stiffness, and avoiding the flaps coming together while deflected could have other benefits.

    If you compare that method and the one that was used in USA17, what do you think would have been the advantages & disadvantages of those ?
    Any quantitative info available of weight differences or what other important differencies there might be ?

    Ps. What was the maximum upvind VMG that USA17 has acheaved so far and in which configuration was it done ?
     
  11. tspeer
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    tspeer Senior Member

    Since the chord distribution is curved, you can only line up one chordwise point at each spanwise station. They chose to line up the maximum thickness points, which made the main element trailing edge curved.
    Your method would not put a straight spar at maximum thickness at all stations.
    They chose to not compromise on weight and to go with the curved hinge line. It really wasn't that big an issue with regard to articulating the flap.
    No.
    I don't know what maximum they've achieved so far. Mike Drummond has said he thinks the boat was at around 80% of its ultimate potential, and I think that's about right. I do know that we could have raced with a configuration that would have doubled the margin of victory over A5, but performance isn't the only consideration to winning the regatta.
     
  12. Gary Baigent
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    Gary Baigent Senior Member

    So Tom, and thanks for the great information, it would be possible to construct a more sophisticated, refined version of the wing, even in this gigantic sized version, with correctly curved, airfoil chord (no flats), cross section shapes - or would that be too time consuming and expensive to achieve. Better for such application be for the smaller C Class and the like wings.
     
  13. tspeer
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    tspeer Senior Member

    Had the AC been delayed by several months, BOR may well have built a second wing. It's likely it would have had full fly-by-wire control of the flaps, and may have had additional elements.

    The flat panels were not that much of a problem - in the pressure-recovery segments the contours wanted to be fairly flat or even hollow anyway. And, with a built-up structure and fabric covering, I don't think there was much in the way of cost savings, unless it simplified the tooling needed to assemble the wing. (I didn't have any insight into that area.)
     
  14. sailor2
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    sailor2 Senior Member

    It seems we agree then, they chose to do that instead of having to do that.

    No, but max diameter for the spar is only needed where stiffness must be maximized in order to avoid buckling, it can be reduced elsewhere towards the ends with no penalty in weight as far as I can see.
    That method would allow having max diam. structural spar halve way between the ball & hounds.

    I still can't see from where there would be any difference in weight if hingeline were a straight line.

    Reliability over best theoretical performance ?
    Or is there some other reasons you had in mind here ?
     

  15. peterraymond
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    peterraymond Junior Member

    wing design

    "No, but max diameter for the spar is only needed where stiffness must be maximized in order to avoid buckling, it can be reduced elsewhere towards the ends with no penalty in weight as far as I can see.
    That method would allow having max diam. structural spar halve way between the ball & hounds."


    First, who said the spar was round? I imagine more of a wing spar design, since the main element is not twisting. Also, unlike a normal soft sail, the spar is carrying relatively more bending load, or at least relatively less compression load, so buckling is less of an issue. Regardless, a thicker spar requires less reinforcing material, so it can be stiffer and lighter.

    One more interesting difference I noted is that there don't seem to be any articulated flaps on the trailing edge of the first element. In the C-Class tuning this flap, and the size of the slot, made a difference.
     
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