KA-series prop geometry

Discussion in 'Props' started by benha, Nov 10, 2016.

  1. benha
    Joined: Oct 2011
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    benha Junior Member

    Howdy, folks.

    I'm working on a low-budget (ie: don't send me to PropCAD) project that requires me to model a KA propeller (or similar).

    The CAD part is easy for me. Or at least it's easy if I do it in SolidWorks - I may try to figure out CAESES free version so I can do some CFD optimization, but that's another question. What I'm struggling with now is translating the dimensional data for the KA-series props into something that makes sense to me.

    Attached are two files that presumably tell me everything I need to know to understand this prop design. I'm very new to all this, but I've made good headway.

    I pretty well grok everything I need to understand in table 3A, with one exception, which leads to:

    Question 1:
    Why are there no values in the Distance of maximum thickness... row for r/R values of 0.7 and above? I believe I need to know where this point is at every r/R section in order to model the prop, no?



    Moving on, after a few hours staring at it, I believe I'm starting to understand table 3B. If I'm right, the whole point of this table is to allow me to calculate X-Y values for the curves defining the profile of the blade at each r/R section location.

    The percentages values (column headers) allow me to calculate an "X" coordinate value for a point in X-Y space. To do this I would first calculate the blade width at the r/R section as a function of Diameter, Blade Count and Ae/Ao. Then I'd determine an X-coordinate for the max thickness point based on it's percentage distance from the leading edge. And finally I'd add or subtract from that value by the percentages in table 3B to determine for any r/R section where the ordinates are located along the X-axis. (This, by the way, is why I think I need an answer to Question 1. How do I know where I'm working from if I don't have a position of max thickness.)

    Then for each of those X-values, I need to determine a Y-value for both the Back and the Face curves of the blade. I do this by applying the values corresponding to the correct r/R row as percentages of the Max Thickness at that section, right?

    That gives me two curves in X-Y space that I can interpolate into splines and connect to make a perimeter shape for the blade. Or so I'm assuming.

    Question 2:
    Does that sound even remotely correct?

    Question 3:
    The lines described by these curves appear to be in their own coordinate systems. The Back curve starts and ends at Y=0, where the Face curve starts at Y=0.08084 and ends at Y=0.13332. Resolving this isn't a simple transformation since the fact that the Face curve starts and ends at different Y-values means that the length between its start and end coordinates is different from that for the Back curve, which has the same extent along the X-axis but starts and finishes at the same Y-position. If you rotated one curve so all the end points were colinear, the ends wouldn't connect. And besides, if you move them to connect things the total thickness would be off. I see lines like those in the attached graph when I chart things. What am I supposed to do to connect these lines and make a continuous profile?



    Thanks in advance for any help you can offer!

    -Ben

    PS - If anyone has a nicely parameterized KA-series model I'll be your best friend if you upload it :)
     

    Attached Files:

  2. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Get a copy of the 1967 or 1981 edition of Principles of Naval Architecture and turn to Chp 7, sect 15 and all will be revealed. The tabular data presented is for the developed shape not a plan shape like you plotted.
     
  3. benha
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    benha Junior Member

    Thanks!

    Unfortunately that's a rather expensive volume and the San Francisco public library doesn't have a copy.

    I'll ask around and see if I can find one somewhere...
     
  4. jehardiman
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    jehardiman Senior Member

    Or try and google "marine propeller blade geometry" and get things like this. Remember that the 2d foil shape has to be wrapped around a 3d helix generator.

    https://www.youtube.com/watch?v=clyA8Lt4iN8

    Oh, and I won't even begin to warp your head with you correctly generate and measure the root fillets.
     
    Last edited: Nov 10, 2016
  5. benha
    Joined: Oct 2011
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    benha Junior Member

    Well, I think I'm making some progress. The video was extremely helpful, and I got a couple of photos of the relevant pages from PNA.

    Working in SolidWorks I've sketched the Transverse View, and am now attempting the Expanded Blades, where I'm running into issues.

    The attached images show the development of the 0.2R blade profile. Per the video and PAN, I divided the total length into stations based on the percentages in the table, and then for each station placed points for the face and back of the blade at a height above the base line calculated as the max thickness (@ 0.6R) * the percentage from the table.

    There are no percentages provided for the 100% extents on the face edge, so I assumed the back side controls those points. (?) Anyway I then started at the leading edge and connected the points with a spline, which remains open at the leading edge.

    The results can be seen in the attached images, which highlight a challenge I still can't resolve.

    Also attached is the ordinates table with two values circled. The back side's elevation at 100% is higher above the baseline than the face's elevation at 95%. This yields the situation circled in red on the zoom of the leading edge, which I can't make sense of. What am I missing here?

    Finally, using a single continuous spline, and going through all the points yields a somewhat unexpected dip in the trailing edge. Does that look right?


    Thanks!
    -Ben
     

    Attached Files:

  6. benha
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    benha Junior Member

    I tried doing the 0.3R profile as well, and while it's better the leading edge is still clearly not right.

    I'm completely confused. I've watched several videos now, and I'm virtually certain I'm following along, but unless the data in the KA table I'm referring to are errant - which is HIGHLY unlikely - I must be doing something wrong.

    Note that this time I connected the spline all the way around the leading edge so it looks a bit different than the one above, but it's basically the same issue in a less extreme case.
     

    Attached Files:

  7. benha
    Joined: Oct 2011
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    benha Junior Member

    Ah HA!!!!!

    I believe I've got it. In a somewhat unexpected development (and unlike what I believe I understood from the videos of other prop drawing) the ordinate percentages in the Ka-series table appear to be additive. So at 95% back from the leading edge the absolute percentage of max thickness for the Back ordinate is not 27.40% per the table, but 27.40+20.62% = 48.02%.

    This makes everything look a lot nicer.
     
  8. jehardiman
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    jehardiman Senior Member

    Remember, you are still projecting the sectional ordinates onto a curved (and possibly raked & skewed) helical surface.
    [​IMG]
     
  9. benha
    Joined: Oct 2011
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    benha Junior Member

    Yes.

    So my understanding of how one would do that (in CAD) is to create cylinders of the R0.X radii, and then pitch the sections by θ and wrap each section around the appropriate cylinder. The Ka-series props generally aren't raked or skewed so at least I don't have to figure that :)

    Once I've got the profiles all in the right place on their appropriate cylinder I'd loft all of them profiles together.

    This is a miserably involved process, but at least I'm moving forward again :)
     

  10. johneck
    Joined: Nov 2011
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    johneck Senior Member

    It seems that you are doing this the hard way. The easy way is to work in a cylindrical coordinate system. There is a paper titled "A rational approach to propeller geometry" published over 40 years ago that describes how to do this.

    It was a SNAME paper, so a search should turn it up.
     
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