Dihedral hydrofoil lift calculations

Discussion in 'Hydrodynamics and Aerodynamics' started by vejas, Nov 19, 2018.

  1. vejas
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    vejas New Member

    Hello, I'm new to this forum, I just read Ray's book about hydrofoils.
    Made some calculations, but I'm not fully sure if they are correct.
    So, my question is:
    For example, If my horizontal foil generates 1000kg of lift at 20kts.
    And I put that foil at an angle of 5 degree dihedral. Will it generate:
    z axis lift (up) L=1000 X cos(5) =996kg
    x axis lift(inward) L= 1000 X sin (5)=87kg
    Lets assume that AOA, Cl, submergence, etc.. are constant
    Does that look right?
    V.
     
    Last edited: Nov 19, 2018
  2. philSweet
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    philSweet Senior Member

    Dihedral is usually reserved for the angle of intersection between two surfaces. It sounds like your foil is canted 5 degrees inboard (which on a boat body is usually the y axis: x forward from aft perp, y positive starboard, z positive down, right hand rule).

    Yes, you use the ordinary rotational transform matrix to calculate the force components. The angles are positive x to y in xy, y to z in yz, and z to x in xz.

    Wikipedia covers rotation matrices fairly well.
     
    Last edited: Nov 20, 2018
  3. Doug Halsey
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    Doug Halsey Senior Member

  4. philSweet
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    philSweet Senior Member

    Not wanting to hijack the thread, but I don't think dihedral is the right term here. If you put a plane in a bank turn, does the dihedral of the outboard wing increase? no, of course not. Dihedral is a body angle that doesn't change during maneuvers or orientation changes. The op is asking about the forces of his foil with respect to a topocentric reference frame, not a body frame. Body rotations change these forces. You need a term to relate objects to the topocentric frame. I suggested cant because it historically has a less restricted usage. YMMV.
     
  5. Doug Halsey
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    Doug Halsey Senior Member

    I'm not sure if he's asking about maneuvering, or changing the angle that he's mounting the foil.
     
  6. Heimfried
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    Heimfried Senior Member

    As you can see, the meaning of your question was understood differently.
    Assumed you are - like me - a beginner to hydrodynamics, some questions: is your question regarding a watercraft in an upright position? Refers "angle of 5 degree dihedral" to the mounting angle of the foil (mesured in the plane perpendicular to the forward moving direction of the craft)? Or are "up" and "inward" meant in relation to the surface of calm water when the craft is not upright because it moves in a bank turn?
     
  7. BMcF
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    BMcF Senior Member

    Speaking as someone who routinely delivers foils and fins to various vessel designs, we refer to dihedral as the angle of a foil or fin as-mounted below horizontal, relative the the level baseline of the hull it is mounted on. Always. Dihedral installation angles are nearly always positive, was we define it, meaning the foil or fin is angled tip-down from horizontal. A dihedral angle of 15-20 degrees is typical of the forward foil or fin on a SWATH vessel, for example, while the aft fin/foil might have zero dihedral on the same craft.

    Surface piercing hydrofoils are a clear exception to the "always positive dihedral" rule; the Rodriguez and similar designs illustrating that.
     
  8. BlueBell
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    BlueBell Ahhhhh...

    No, I believe is the answer to your question.
    This is a vector question.
    The 1000kg lift doesn't change, however, vertical (up) lift is slightly reduced while a small value of horizontal (inward) lift is created.
    My understanding is this is sometimes done to increase stability especially under dynamic situations..
     
  9. DCockey
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    DCockey Senior Member

    The equations vejas posted are correct for the vertical and horizontal components of a 1000 kg force at 5 degrees from vertical, towards the center of the boat.
     
  10. DCockey
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    DCockey Senior Member

    So the convention of dihedral angle for marine vessels is the opposite of the convention for aircraft wings.
     
  11. Doug Lord
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    Doug Lord Flight Ready

    Last edited: Dec 20, 2018
    Doug Halsey likes this.
  12. BlueBell
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    BlueBell Ahhhhh...

    He calculates 996kg vertical and 87kg horizontal.
    That totals 1083kg total lift when he only has 1000kg of lift...?
     
  13. Heimfried
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    Heimfried Senior Member

  14. DCockey
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    DCockey Senior Member

    That's how vectors work. The components in two different directions do not simply add to equal the total.
    Vector Components https://www.physicsclassroom.com/class/vectors/Lesson-1/Vector-Components
    Vector Resolution https://www.physicsclassroom.com/class/vectors/Lesson-1/Vector-Resolution
    How to Find Vector Components - dummies https://www.dummies.com/education/science/physics/how-to-find-vector-components/
     

  15. Heimfried
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    Heimfried Senior Member

    FYI:
    The International Towing Tank Conference (ITTC) is publishing a "Dictionary of Hydromechanics" online
    https://ittc.info/media/7939/2017-ittc-dictionary.pdf
    and should have some authority in regard to defining terms (not sure about this).

    This dictionary says:
    So dihedral angle in this definition is bound only to the vessels hull regardless of any heel.
    The term "anhedral" is not recorded in this dictionary, so the case of outward wing tips down is also covered by the term "dihedral angle". Furthermore there is no mention of a positive nor a negative dihedral angle. In my opinion the latter points to the fact, that there is no standard or common use of the sign.
     
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