AC 36 Foiling Monohulls

Discussion in 'Sailboats' started by OzFred, Sep 13, 2017.

  1. Doug Lord
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  2. Doug Lord
    Joined: May 2009
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    Location: Cocoa Beach, Florida

    Doug Lord Flight Ready

  3. Doug Lord
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    Doug Lord Flight Ready

  4. JosephT
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    JosephT Senior Member

    Team USA 21 is the name (for now). They're looking for a sailing base. In my opinion they should consider a primary base and some alternates to ensure they don't have to worry about not finding enough wind to train in. More often than not, south Florida can have long stretches of low wind. I would opt for a base on the west coast. That will give them options to train year round along the same coast...less hassle with transport logistics. The east coast would work too, but there's more wind to be had on the Pacific side in my opinion.
     
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  5. Doug Lord
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    Doug Lord Flight Ready

    Where would the conditions be most like Auckland?
     
  6. JosephT
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    JosephT Senior Member

    Both New Zealand & Australia have great wind year round. That's the key. The better wind you have for training, the more experienced the crew. Choosing a training site with good wind = key factor. A good meteorologist can play an important role in choosing the locations.
     
  7. AlexanderSahlin
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    AlexanderSahlin Junior Member

    The best conditions for practising with the new AC-75 will be found in a good VPP-system. As far as I have heard, no prototype of the AC-75 has been tested on the water yet. One nice feature of computer simulations is that you can sidestep such boring things as starting from rest, where the AC-75 will have quite poor righting moment before it reaches foiling speed. In a computer-simulation you can instead start the yacht from 25 knots boat-speed in foiling position. Then you will easily find out that the AC-75 performs about as well as the AC-50 catamarans, -higher righting moment thanks to the wide foil-arms and higher weight, but some extra water-drag because the ballasted foils.
    Furthermore, you can also chose a favourable atmospheric turbulence-level, so the competitors can handle the boats without too frequent rudder-foil spinouts or capsizes.
    You can also chose material strength, stiffness and weight independently of the real world. Much easier to get the rig light enough for "self-righting" and powerful enough hydraulics for handling the foils then.
    So, why not run the America's cup as a computer game instead?
     
  8. JosephT
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    JosephT Senior Member

    Computer simulations are good for optimizing the design before building it (e.g. digital mock-up), but the crew need hands on experience.

    As for video games, totally different market there. There have been a few nice ones released (e.g. Virtual Skipper) but they don’t sell really well compared to many others. Highly competitive market. If a game is built around an adventure story (e.g. sailing a ship and battling pirates, finding cool stuff, etc.) it will sell better. Arghhh!
     
  9. markdrela
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    markdrela Senior Member

    One really big change from AC-62 to the new AC-75 boats is that the latter have no passive roll stability when one of the main foils is retracted.
    In this mode the AC-75 effectively has only two support points in the water, and only the sail can then exert any significant roll moment about the axis connecting the two points. So to stay upright in this operating mode, the sail will need to be actively trimmed like on a Moth. The foils won't work for roll control, regardless of how fast they can be driven by the hydraulics.
     
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  10. tspeer
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    tspeer Senior Member

    I'm not sure what you're getting at here. The AC50 daggerboards provided passive heave stability through leeway coupling but not roll stability, and while differential deflection of the two rudder foils generated a constant righting moment, they were not actively controlled to stabilize the boat in roll. The windward foil was retracted except when tacking or gybing. So effectively the AC50 only had two points of support - the daggerboard and the average of the two rudder foils. They needed the crew to balance the boat with sail trim, too.

    One difference between one vs two rudder foils is the roll damping provided by two foils. There is very little roll damping from a single centerline T foil compared to the two foils separated by the catamaran beam. But there is also a lot of roll damping from the sail rig, and that will be similar between the two boats.

    The AC75 will need the crew to actively control the leeward T foil to regulate heave, but this should be easier to do with a flap than by raking the entire foil.

    One big difference between the boats is the change in center of gravity position when maneuvering. The AC50 daggerboards plunged almost straight down, and with the appropriate choice of rake angle there was little change in heeling moment until the crew wanted to shift the load from the old board to the new board. With the AC75, a great deal of the righting moment comes from using the heavy wing as ballast. As soon as the foil starts to be lowered, there will be a reduction in righting moment, and there's nothing the crew can do about that until the windward wing is well immersed. So they will need to start easing the sails (or turning) as soon as they start to drop the windward board.

    The rate at which this happens may be dictated by the supplied FCS. No specifications for the FCS have been make publicly available, so I don't know just how much control the crew will have over the foil cant during a maneuver. I would hope the teams have been given some hard numbers as to what the FCS inputs are (will there be a proportional cant command, or will it be bang-bang control to a few set positions?) and what the performance of the FCS will be.

    All this is assuming the boats are actually sailed as described in the animations and prescribed by the Design Rule. The event may well be decided by which team is most inventive in getting around the limitations of the Rule.

    Besides the roll stability when flying, a big difference is the roll stability when hullborne. The catamaran had similar stability hullborne and foilborne. The AC75 will have a lot less righting moment available when the foils are lowered. Like a keelboat, it will need to heel to leeward to generate righting moment until the windward foil can be raised. Accelerating from a standing start will need coordinated sail trim, raising of the windward foil, and raising (to leeward from the measurement position) of the leeward foil. Moving the leeward foil from its full-down position will actually decrease the righting moment until it can start to generate more hydrodynamic lift than its own weight.

    Once flying, easing the sail on the catamarans resulted in the boat pitching up and heeling to weather, and the resulting motion was worthy of the Lone Ranger. The pitch-up was because of the loss of bow-down pitching moment from sail thrust. Heeling to weather wasn't a big deal because the windward hull soon hit the water, so while the motion was spectacular it wasn't as dangerous as it looked. But with the AC75, things could get more interesting. There's not much to halt the windward heel until its reached a large value. You could see the boat flying up and plunging back down sideways, with the crew on the low side.
     
  11. Doug Lord
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    Doug Lord Flight Ready

    =======================
    Tom, would it be possible -with both foils down and the boat moving-to use differential flap control on the foils so the windward foil could develop downforce until enough speed (perhaps with windward heel) is developed to allow raising the windward foil?
     
  12. markdrela
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    markdrela Senior Member

    I was envisioning that the lifting rudder foils would give some roll stability because of free-surface effects. This effect, acting on the wingtips, was significant for roll stability of the Flying Fish HP hydrofoil, and probably also the Trampofoil (A. Sahlin can comment on this).
    On the AC-50's, as an upward-lifting rudder foil approaches the surface it will gradually lose lift, which provides a restoring roll moment for the whole boat. This assumes that the average lift of the rudder foils is upward.
     
  13. tspeer
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    tspeer Senior Member

    OTUSA, and I assume other teams as well, ran their rudder foils at maximum differential rake, with the windward rudder pulling down and the leeward rudder lifting up. The mean angle of attack for trimming the pitching moment came from boat's pitch attitude, which was why the boats had a pronounced bow-down attitude when at speed. They even took it so far as to use unstable bend-twist coupling in the rudder foils to effectively get even more differential deflection as the foils loaded up.

    I suspect surface effects would have had a destabilizing effect in roll, because they would have reduced the righting moment from the rudder foils as the load reduced when approaching the surface.
     
  14. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Concerning altitude-stability for hydrofoils near the water surface, I have unfortunately not observed the kind of roll-stability, that was observed for the Flying Fish, on the Trampofoil. This may be because the lower Froude-number for the Trampofoil, that usually travels at some 6 or 7 knots, and has 2.8 m wingspan. But you have a huge roll-inertia on the Trampofoil, that makes it quite easy to control the roll-angle. But I know that such surface-effect has been used for altitude-stability of lifting hydrofoils in other applications, such as the canard of the Flyak canoe.
    For the AC-catamarans, see Tom Speer's comments. There you also have a high Froude-number, and the surface-effect will destabilize in roll, since you have negative lift on the windward rudder-foil.
     

  15. Doug Lord
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    Doug Lord Flight Ready

    From cupexperience.com/Scuttlebutt Europe:

    America's Cup Supplied Equipment: Foil Arms and Foil Cant System (FCS)
    The foil arms and the mechanism to raise and lower them will be identical for all teams and will be supplied by a single source. When the AC75 Class Rule was released at the end of March, two important pieces of information were missing: the specifications for the FCS and the drawings and specs for the one design mast tube. The class rule states that the date for releasing this information is TBA - to be announced. As of last week, the teams had not received these specs.

    The AC75, AC50, and AC45F catamarans controlled lift by raking the daggerboards fore and aft to change the angle of attack of the foil wings. The foil wings were not allowed to have movable control surfaces like flaps. The AC75's will be very different: the foil arms will not be raked. The foil wings will have movable flaps to control lift.

    Foil design was crucial for the 2017 America's Cup. That will be true again for 2021. The teams need the FCS specs to get going on their foil wings designs. -- Jack Griffin

    cupexperience.com
     
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