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Thread: 'Face cup'

  1. #11
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    Joe:

    Thanks. Although I haven't had a prop in my hand for 30 years I still want to know how they work.

    After your reply I googled airfoil and there was a bunch of info. My math is from the 60's when we didn't have calculus in high school but I still see things I didn't know.

    Again ; thanks:
    John

  2. #12
    Team Member Smokin' Joe's Avatar
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    Default hrdrofoil theory

    Quote Originally Posted by rumleyfips View Post
    Joe:

    Thanks. Although I haven't had a prop in my hand for 30 years I still want to know how they work.

    After your reply I googled airfoil and there was a bunch of info. My math is from the 60's when we didn't have calculus in high school but I still see things I didn't know.

    Again ; thanks:
    John
    John,

    The way a foil (wing, sail, prop blade) works is nonintuitive. There's a 'lift'/side force
    on the foil because of an eddy circulating about the foil. This is the origin of the
    high lift/drag ratios of a wing. Jim Booe is the only prop man I've talked with
    who understands this. I still talk with him, he taught me two invaluable tricks in 2011.
    I learned foil theory from Newman's 'Marine Hydrodynamics' in the late 1970s.
    More than calculus is required.

    Best,
    Joe

  3. #13
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    I have always felt that prop design was a dark art, but like anything else science and math are always or should be involved. I recently read the book We Were the Ramchargers and nothing they did was by trial and error, solid engineering with all the math was done before they ever picked up a tool.

    Your comment on how an airfoil works is non-intuitive is so true. I also sail and the number of people who think the wind pushes on the sail is unbelievable. In fact when going down wind, when this is the case the boat is quite slow. Great article.
    kk

  4. #14
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    Yes, wind behind a big sail is a high drag limit. The way the circulation works is subtle.
    The thin wake/vortex sheet behind the trailing edge is a velocity discontinuity, that's equivalent to
    finite circulation. For engineers and physics students, that's treated clearly in Landau-Lif****z's 'Hydrodynamics'.


    Quote Originally Posted by Krazy Karl View Post
    I have always felt that prop design was a dark art, but like anything else science and math are always or should be involved. I recently read the book We Were the Ramchargers and nothing they did was by trial and error, solid engineering with all the math was done before they ever picked up a tool.

    Your comment on how an airfoil works is non-intuitive is so true. I also sail and the number of people who think the wind pushes on the sail is unbelievable. In fact when going down wind, when this is the case the boat is quite slow. Great article.
    kk

  5. #15
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    man this is all so far over my head, anything with an ology or matically on the end is just way too much info.
    just want to know what to actually do to a prop to get the effect.

  6. #16
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    Quote Originally Posted by filthy phill View Post
    man this is all so far over my head, anything with an ology or matically on the end is just way too much info.
    just want to know what to actually do to a prop to get the effect.
    Ain't it the truth!

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