Old foils with acceptable data

Discussion in 'Hydrodynamics and Aerodynamics' started by revintage, Feb 10, 2019.

  1. revintage
    Joined: Nov 2016
    Posts: 135
    Likes: 20, Points: 18, Legacy Rep: 10
    Location: Sweden

    revintage Senior Member

    Have had this subject up before, but made a new model with more predictions and the pointed leading edge profile still is a mystery to me, as it should indicate ventilation problems. Also tried to round of the nose but that made it worse.

    In chapter V of AYRS74 John Moorwood in 1958 describes a modified ogive hydrofoil section.

    Used JavaFoil for predictions. Seems OK with very good cavitation numbers, close to 40 knots at Cl=0.23, but what are the flaws except for going nuts quite early above Cl=0.8?
    The cavitation in red made in one of Doug Halseys old plots, looks unrealistic to me.

    Could also be JavaFoil does not work properly;).

    IncipientCavitation.jpg ayrs74.png ayrsclcd.png ayrsCp.png ayrsCpmin.png ayrsfoil.png ayrsabove.png
     

    Attached Files:

    Last edited: Feb 11, 2019
  2. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Javafoil has a significant limitation for sections like this. It cannot predict laminar separation bubbles. The sharp leading edge of an ogival section will have laminar separation at the leading edge on one side or the other for all but a narrow range of angles of attack. I believe the use of sharp leading edges to "cut the water" was a leading cause of the unpredictable ventilation that plagued early hydrofoil experimenters.

    Xfoil will do a better job with these sections than Javafoil. Xfoil can also report the minimum pressure coefficient when running polars, which will allow you to calculate the incipient cavitation speeds (cavitation bucket).
     
  3. revintage
    Joined: Nov 2016
    Posts: 135
    Likes: 20, Points: 18, Legacy Rep: 10
    Location: Sweden

    revintage Senior Member

    Yep, have understood that XFoil results, is what you should rely on. Have downloaded the program and a manual. Will learn.

    About XFoil, I have learned it sometimes have problems with sharp leading edges?

    Anyway, do you think JavaFoils prediction in the ”narrow range” you refer to are acceptable?

    For some reason JavaFoil predictions indicate the ”narrow range” is widened when modifying the ogive, by lifting the leading edge and making it symmetric for the initial part of the profile.

    Checking a pure 6% ogive that shows what normally happens. The extremely narrow useful range can be seen. This leads me to believe the sharp leading edge in combination with the flat bottom beginning directly at the leading edge makes it worse.

    If I understand it right my pressure coefficients are shown in one of my previous images. Used them to calculate cavitation speed manually plotted in one of Doug´s old diagrams where also H105 can be seen.

    EDIT: Added a 8% ogive and the useful range moved up, still as narrow.

    8.png 6.png
     
    Last edited: Feb 12, 2019
  4. Doug Halsey
    Joined: Feb 2007
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    Location: California, USA

    Doug Halsey Senior Member

    Here is an experimental comparison of sharp and rounded leading edges, clearly showing the narrow range of effectiveness for the sharp one. (From Hoerner's book, p.11-28)
     

  5. revintage
    Joined: Nov 2016
    Posts: 135
    Likes: 20, Points: 18, Legacy Rep: 10
    Location: Sweden

    revintage Senior Member

    Haha, remember it now. More about ventilation at 11-29. You recommended me to read it some time ago. Have his book as pdf and have actually printed those particular pages. Have to read again when back from my short visit to Norway.
    541D12BE-1995-46EB-974D-1896AC5A4D5B.jpeg
     
    Last edited: Feb 13, 2019
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