34th America's Cup: multihulls!

Discussion in 'Multihulls' started by Doug Lord, Sep 13, 2010.

  1. groper
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    groper Senior Member

    Of course it could be true.... but not all airplanes are designed this way; look up "canard wing" which is a aircraft configuration whereby the stabilizer is located forward of the main wing, and they BOTH lift rather than the stabilizer producing down force.

    By this nature, canards are generally more efficient. They are not common place in the airline industry as they are seen as "not as safe" for a myriad of reasons...

    Point is, more than 1 configuration can work, all depends on what goals your trying to achieve...
     
  2. markdrela
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    markdrela Senior Member

    It is certainly possible to make a conventional airplane aft tail have an upward load, by moving the center of gravity aft. But this reduces pitch stability, which must be compensated by increasing the tail area, which in turn increases the profile drag. For a given amount of pitch stability, the minimum-drag solution is to go the other way -- with a small tail producing a small download, typically a few percent of the total lift.
     
  3. markdrela
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    markdrela Senior Member

    Actually, the front canard surface is a DEstabilizer. The rear (wing) surface is the stabilizer.

    It's not so simple. Although on a canard configuration both surfaces are lifting, for stability reasons the larger rear "wing" surface must always operate at a smaller CL than the front canard. This means that the total canard+wing surface area must be larger, and hence draggier and heavier, than on an equivalent conventional configuration. This is the major reason you don't see many canards flying -- they are basically inferior. There are many other issues as well, such as poor yaw stability, dangerous pitch-downs on landing flares, etc.

    In any case, a hydrofoil configuration is a more complex beast, because the pitch stability is also influenced by free-surface effects, and by variable area in surface-piercing foil systems. And there's also the issue of depth stability, which airplanes don't have to deal with.
     
  4. Doug Lord
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    Doug Lord Flight Ready

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    On a foiler like the Rave, Osprey or F3 the main foil is designed to support 80% of the total weight at takeoff and the rudder foil 20%. However, as speed/wind increases the load distribution changes to the point that at "X" speed the rudder foil begins to pull down-this happens automatically with a very slight pitch down. The foil set up on these boats is: main foils +2.5 degrees and rudder foil zero degrees relative to the static waterline which was parallel to the flight waterline).
    So it is likely that the AC boats would work in a similar manner since it is proven stable in pitch and pitch control is automatic.
     
  5. Stephen Ditmore
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    Stephen Ditmore Senior Member

    But unless TNZ's center of gravity is dramatically different than Oracle's that velocity point "X" will be different for the two boats (because of the different main foil positions), reflecting two different design approaches. So I speculate the exact angle of incidence and foil design of the rudder tip horizontal stabilizer is probably different as well, reflecting the different expected load vector. Will you go along with me that far, Doug?

    So how does this affect liftoff (transition to foiling)? When Oracle lifts off, if the main foil is as powerful as Doug says, it should take off first. Then as speed builds the up vector on the rudder mounted foils lifts the stern. On TNZ the main foil is doing most or all of the lifting, and as speed increases the down vector on the rudder presumably increases. Agree? Is it possible the reason for Oracle's platform twist is that Oracle is intended to sail with the windward rudder tip out and TNZ is designed to sail with the windward rudder tip in?
     
  6. oldsailor7
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    oldsailor7 Senior Member

    Quote."No matter how strange a foil can be, I can not imagine how, when Downwash is parallel to external flow (ie no Downwash), you can have Lift ." Quote.

    It's called bernoullie's theorem.
    When a symetrical airfoil is at 0deg AOA there is no downwash and no lift.

    When a flat bottomed or asymetrical airfoil is at 0deg, (or even less) there is no downwash, but the curved upper surface the of the wing is providing a drop in pressure which provides lift, although not nearly as much as when the wing has a positive AOA which is deflecting the airflow downwards producing lift via Newtons law of motion. F=MA.
     
  7. Doug Lord
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    Doug Lord Flight Ready

    ----------------------
    There is a rendering someplace showing three AC Cat hulls in profile. I've run across that a dozen times or more. I was certain I had saved it but it's not in my files. It's all over SA except in the places I just looked. Do you have it saved? I need to study that picture to be able to answer your first question.
    The 80/20 proportion I mentioned is typical for any foiler with a forward main foil. Going too far away from that could negatively affect pitch stability. I would imagine that TNZ has close to the same 80/20 relationship right at takeoff.
    I'll add more when I find those renderings.
    ----
    I've thought for some time that Oracle uses platform twist to get around the "no negative lift" rule on the windward main foil. That is, they may generate downforce from the windward rudder foil earlier than would TNZ.
    ----
    As soon as the main foil begins to lift the whole boat is at a slight angle which allows the rudder foil(set at zero angle of incidence relative to the static waterline) to lift its 20%. Most foilers(with the 80/20 set up) have a slight(or greater) bow up attitude right at takeoff.
     
  8. tspeer
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    tspeer Senior Member

    It is not true that canards are more efficient. Aft tails, with their downward force, are more efficient.
    [​IMG]
    (from http://aero.stanford.edu/reports/multop/multop.html)

    And, despite the downward load on the tail, aft tails also have higher maximum lift than do canards:
    [​IMG]

    Actually, canards are safer than aft tails - that's why Burt Rutan designed so many canard airplanes. He wanted the handling qualities and was willing to sacrifice a little performance to get them. The forward lifting surface of any stable aircraft has to be more highly loaded than the aft lifting surface. A canard will stall before the wing, resulting in the nose dropping before the wing has a chance to stall.
     
  9. tspeer
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    tspeer Senior Member

    Balderdash. A symmetrical section and a flat bottomed section will produce the same downwash when operating at the same lift. Their angles of attack will be different, but "bernoullie's theorem" has nothing to do with it.
     
  10. tspeer
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    tspeer Senior Member

    And a sailing hydrofoil is more complex, yet, because the foil has to generate side force as well as vertical force. This leads to coupling between the sway and heave velocities, and between roll and pitch. Side force and pitching moment can be controlled by the crew through sail trim in addition to control of the foils. You have to take the full equilibrium into account when trying to explain the stability of these craft.
     
  11. oldsailor7
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    oldsailor7 Senior Member

    Tom.
    We are talking about NO lift, not the same lift.
    A symmetrical section has No lift at 0 deg AOA.
    An asymetrical section has to have a neg. AOA to produce no lift.
    Your statement, "bernoullie's theorem has nothing to do with it,"
    is very surprising coming from a fellow aeronautical engineer.
     
  12. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I'm not quite sure what you're arguing o7. There are probably times on both boats that the rudder mounted h.s. is producing an up vector and times it's producing a down vector. At times it's AOA will be zero, and at times not. So your point eludes me.

    Tom, I wouldn't want to interrogate you or suggest that either team is locked into a single approach, but is the general statement true that the different fore-and-aft placement of the main foil reflects different approaches with respect to loading the rudder mounted foil?

    Doug: I thought they were posted to this thread, but I could be wrong and I can't find them now either.
    Chevalier's drawing of TNZ's boat is at http://chevaliertaglang.blogspot.com/2012/08/americas-cup-update-etnzs-ac72-plans.html but I can't locate his drawing of Oracle's boat.
     
  13. oldsailor7
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    oldsailor7 Senior Member

    Sorry Stephen. I was not addressing those remarks to rudder foils in water, but rather to airfoils. OT I guess. :eek:
     
  14. Stephen Ditmore
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    Stephen Ditmore Senior Member


  15. oldsailor7
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    oldsailor7 Senior Member

    Whether a foiled surface is moving in air or water, it makes no difference.
    The basic formula for calculating lift is the same. L=c/l S*RoVsq/2, where S is the wing area, c/l is the lift coeficient of the foil and V is the speed.
    As Ro is the density of the fluid it can clearly be seen that, as water is around 8oo times heavier than air, the foil area S can be many many times smaller to lift the same weight as an equivalent airfoil moving in air.
    In both cases the foils have to be completely immersed in the fluid.
    Since the foil in the case of a sailboat will, of necessity, be near to the surface
    any air entrainment could spoil its whole day. :eek:
    Here is some really good reading.
    http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.htm
     
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