Dihedral hydrofoil lift calculations

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

  1. Heimfried
    Joined: Apr 2015
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    Location: Berlin, Germany

    Heimfried Senior Member

    A find:
    Tom Speer is publishing a paper with diagrams and sketches about his presentation at The Foiling Week 2014.
    http://www.tspeer.com/Aclass/A-ClassCatamaranFoils.pdf
    At page 9 there is a sketch defining some terms regarding foils at A-Class multihulls. The dihedral is - as I understand it - shown as pointing upwards towards center plane.
    At page 11 there are some instances of wing shapes 3 of them tips pointing down and the dihedral is stated as a negative value.
    So far in effect conform with the ITTC definition: use of the term dihedral only (and the sign of the numbers picked depending on given or personal preferences).
    But at page 31 the caption is "L Foil With Anhedral" showing mostly the same wings as before (with negative dihedral).

    I have to admit, that I don't understand nearly all of that, Tom said in the paper, but as it appears to me his use of "dihedral" and "anhedral" is contrary to the sketches of the airfoils above.
    Did I misunderstand it?
     
  2. Doug Lord
    Joined: May 2009
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    =====================
    His definitions seem to go along with the aerodynamic definition used by Vellinga with the possible exception of "negative dihedral" which is another way of saying anhedral as best I can tell. The sketches he did seem to illustrate dihedral and anhedral well with the possible exception I just mentioned.
    The "dihedral" shown on page nine fits perfectly the aerodynamic definition of the word with the root being the point that occurs at the bottom of the more or less vertical fin.
    ------------------
    Seems like he used "negative dihedral" and anhedral to mean the same thing but in the last page where he says "L foils with anhedral" seems to show dihedral but that is just an illusion-he previously described it as "zero degree dihedral".
    That paper requires a lot of study!
    I'm more convinced than ever that the best solution is to use the aerodynamic definitions.

    Dihedral-anhedral-.png

    Different degrees of dihedral either side of the root OR for simplicity: daggerboard to right of root, dihedral lifting foil to left:
    MPX ama + ama = cat --foil comparison 011.JPG
     
    Last edited: Dec 30, 2018
  3. Heimfried
    Joined: Apr 2015
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    Location: Berlin, Germany

    Heimfried Senior Member

    Thank you for your explanation Doug and your willingness to deal with an old guy who is reluctant to accept your view. I appreciate that very much. While I know nearly nothing about foils and foiling, I know a bit about definitions. May be I misunderstand some aspects of this and will work myselfg slowly nearer to the core of it.
    Thank you.
     
  4. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    You're welcome, Heimfried. Thanks for your thoughts and ideas.
     
    Last edited: Dec 31, 2018

  5. Doug Lord
    Joined: May 2009
    Posts: 16,616
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    Dihedral for a surface piercing foil-according to Grogono:
    Using the aerodynamic definition this would be anhedral since the root is at the top of the foil.
    Dihedral-surface piercing foil-Grogono.png




    Hydroptere pix by Christophe Launay:

    Hydroptere 5-24-13 Christophe Launay-SA.jpg

     
    Last edited: Dec 31, 2018
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