Solidworks Airfoil lofting question

Discussion in 'Workshop Tips and Secrets / Tools' started by Hot Wings, Oct 15, 2018.

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  1. Oct 15, 2018 #1

    Hot Wings

    Hot Wings

    Hot Wings

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    CAD is a tool just as much as a spanner so I'll ask here.

    When I loft an airfoil from ordinates using the Insert Curve Through XYZ Points tool I often get an airfoil that is just a bit longer than the desired chord because the spline extends past the start/origin. When a vertical tangent is added to the nose to find the true most forward point the result then moves the chord line - as defined by the most forward and most rearward point. This isn't much, on the order of 1/3 Degree and 0.001% of the chord, but it tends to bother the perfectionist part of my soul.* Example: 36" chord 23015

    Airfoil.jpg

    Solutions, other than manipulating the ordinates in Excel?


    *it's a pretty small part most times ;)
     
  2. Oct 15, 2018 #2

    addicted2climbing

    addicted2climbing

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    In the past I split the curve as a top and bottom. I tend to run into issues at the trailing edge with airfoils in Soldiworks. You could just draw a new Leading edge profile on the origin and blend it into the curve with a tangent mate. Its a work around fix and likely more annoying than the original problem. Or since the CAD generated airfoil will go to a point anyhow, just cut off .029 from the TE?
     
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  3. Oct 16, 2018 #3

    pictsidhe

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    I would have thought the spline would be an accurate interpretation of the partial details given by coordinates. A cut off trailing edge is where a spline goes wonky.
     
  4. Oct 16, 2018 #4

    Hot Wings

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    Ditto. The reason I split is so I can rotate the whole upper or lower to form a TE with a chosen thickness, rather than a point. For a rib built in wood, like this one will be, .030" isn't a real world problem but as you know CAD doesn't like 'close'.

    I was kind of hoping that there was a different type of spline that used the points in a different way? :ponder:
     
  5. Oct 16, 2018 #5

    Jay Kempf

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    Put in a point at 0,0 at the front of the airfoil. It should be there anyway.
     
  6. Oct 16, 2018 #6

    lr27

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    Can you draw in a vertical construction line at the origin and require the curve to be tangent to that? I forget what the buzzword for that is, but it's equivalent to constraint.
     
  7. Oct 16, 2018 #7

    Jay Kempf

    Jay Kempf

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    Solidworks won't let you do that in that sort of imported curve. The curve has to go through the points you input or grab from the UUIC.
     
  8. Oct 16, 2018 #8

    lr27

    lr27

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    I thought he was just importing the points?
     
  9. Oct 16, 2018 #9

    Jay Kempf

    Jay Kempf

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    He is but they come from a text file which you can add lines to in any text editor.
     
  10. Oct 16, 2018 #10

    Hot Wings

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    The file does contain (0,0,0). I can pull out the ordinates from the created spline, poke them into Excel, and stir them around a bit to get a nice fit at the nose. But it is sometimes as quick and accurate to just manually fit a 3 or 4 point spline on top of the imported ordinates. A couple of thousands from 'true' is still better than any builder will get and better than most hobby type CNC.

    But it's just not right........
     
  11. Oct 16, 2018 #11

    lr27

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    Ok, I think I may have a possible answer. Start a sketch, and make a spline by converting entities and selecting the xyz curve you created. If you click on the resulting spline, you should have a table of properties on the left, including relations. Delete the "on edge" relation. Now you can make the spline tangent to the vertical construction line I mentioned above. However, I don't know what this will do to the rest of the spline.

    It might be safer to move the curve until it's tangent to the vertical plane that's perpendicular to the plane of the curve. Then move the tangent point to the origin, scale as required, and then convert entities. That is, if you need a sketch.

    I tried creating a curve from a bunch of xyz points, from a set of coordinates I had, but the l.e. seemed to be right on the origin. I think this may be what "smoothing" in Xfoil or Profili may accomplish. The coordinates I used had been smoothed. BTW, does SW really need all those zero points for the z coordinate? They're tedious.
     
  12. Oct 16, 2018 #12

    Radicaldude1234

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    I think you guys are overthinking this.

    What I do is:

    1. Download DAT file
    2. Take X,Y points and input it into a .txt file.
    3. Add the Z values (0)
    4. Separate the files into top and bottom points (personal preference)
    5. Adjust the end values (0,0 or 0,1) so that the curve closes
    5. rename the extension on the files to .sldcrv
    6. Import into Solidworks using Insert Curve through X,Y,Z -> Browse -> .sldcrv file
    7. Create sketch on the X-Plane
    8. Use Convert Entities on the airfoil curves to convert to spline
    9. Draw a centerline between leading and trailing edges
    10. Copy sketch

    The airfoil sketch is now scalable through dimensioning the centerline

    If you want the cutoff:

    1. Scale the airfoil to 101% (or however much you want the cutoff)
    2. Start a new sketch on the same plane
    3. Use Convert Entities on the airfoil curves (keep relations between the new sketch and the old airfoil sketch)
    4. Draw trailing cutoff line (x amount from sharp airfoil trailing edge)
    5. Trim the cutoff line and airfoil lines.

    Now you can still scale the original airfoil without having to redo the cutoffs.

    The problem with splines are that they are difficult to define mathematically and fix. A conic is also not appropriate for an airfoil so the best bet is to stick as closely as possible to the coordinates on the DAT file.

    Wing Loft.jpg
     
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  13. Oct 16, 2018 #13

    Hot Wings

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    Yes, SW needs them because it is a 3D CAD. Without the 3rd it has no idea where you want to put the spline. Sometimes a 'wiggly' out of plane spline is what we want. They aren't tedious. Just a click and drag in Excel.

    I do like the idea of smoothing using an airfoil program. I'll have to try that........ Thanks!
     
  14. Oct 16, 2018 #14

    FritzW

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    agahhhhh!!!! I just wrote two paragraphs explaining why that happens and how to work around it. ...and then, of course, I did something stupid and the post never posted.

    Bottom line: We're talking fly poop in the pepper, tweak the the spline point handles a few 1/10,000ths to compensate for 1950 slide rules and french curves and call it good. ;)
     
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  15. Oct 16, 2018 #15

    Hot Wings

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    The #5 part I'd like to avoid. But it does work.

    If you do enough of this an Excel sheet lets you do the scaling before importing and also makes an easy way to move the curve to any plane you desire.

    Shutting down for the night and updates.............
     
  16. Oct 16, 2018 #16

    Radicaldude1234

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    Well, if you make it adjustable in Solidworks and put in the appropriate relations/equations, you can adjust the airfoil chord just from changing the wing plan-form sketch.

    Really helpful for optimization with CFD and/or when tweaking the aerodynamics.
     
  17. Oct 16, 2018 #17

    Norman

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    Computers are capable of some pretty weird, but small, errors due to the vagaries of binary math but the example 23015 may be due to the NACA's funky leading edge slope+radius definition. Since the center of the circle is on a line drawn from 0,0 at some angle the center point is always above the reference line. This ALWAYS results in a slight bulge in airfoils drawn from original NACA tables. It also means that the chord isn't exactly parallel to the reference line.
     
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  18. Oct 16, 2018 #18

    lr27

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    Profili, and I don't know what other software, has a utility for giving a finite thickness to the trailing edge. I don't know if that's better aerodynamically, or to cut off a bit of the trailing edge.

    The curve from coordinates can be open. At least if there's a space between the top and bottom t.e.

    Anyway, unless I messed it up, I've attached coordinates for a 23015 with a 2 percent trailing edge. I meant to blend it in from 60 percent of chord, but got it backwards and blended from 40 percent. I think. Solidworks even liked it. And Excel is nice for adding all those zeros, though it added some characters I didn't want too. Those were easy to search and destroy, however.

    edit:
    I didn't check if this file exhibits the original problem.
     

    Attached Files:

  19. Oct 16, 2018 #19

    addicted2climbing

    addicted2climbing

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    Hows this for a Solidworks airfoil nightmare...

    Swept Sheet Metal profile with a crapload of custom forming tools done one at a time. I also need to add them between each flute on the small 45 degree flange, but I may just forget that and show a dashed line mentioning that there may need to be flutes here too. This took forever as Solidworks was fighting me a bit until I figured out the workflow. This will be used as a representation on how a portion of the wing is laid out in regards to ribs, sub-ribs and support brackets. Subribs not modeled yet.

    This is for the Skylite and I plan to give builders access to 3D pdf's and or E Drawings as a visualization help in hopes it may answer peoples questions without them having to call as often. When I acquired the rights the rib drawing was missing so this assembly will go along with the new full size template I made to supplement the plans. Hoping to launch the website in a week or so.

    Rib Layout - Partial Wing Assy - 1.jpg Rib Layout - Partial Wing Assy - 2.jpg Rib Layout - Partial Wing Assy - 3.jpg

    Marc
     
  20. Oct 16, 2018 #20

    Soliex

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    Nice looking work there ATC!

    HW, if the intent is to accurately locate the airfoil in position, simply add a new coordinate system at a point constrained to a tangent vertical line and the curve.

    Clark_Y_DP_1.JPG
     

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