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Thread: Post SOLIDWORKS Designs Here

  1. #71
    SOLIDWORKS Support Volunteer Jeffrey Meyer's Avatar
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    Quote Originally Posted by cwilliamrose View Post
    Tell me about the blue vectors, are they subject to change because they are not fully defined?
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    You have full control of the tangent weighting and direction (the blue vectors), so you can "blacken" everything on the spline. It's usually not necessary to define the weighting because the most aesthetic shapes of splines are usually "relaxed" - aka "minimum energy splines".

  2. #72
    cwilliamrose's Avatar
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    So you're saying the weighting doesn't tend to change on its own? No need to nail it down? I'm clumsy enough to change it by accident.

  3. #73
    Mark Meredith's Avatar
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    [QUOTE=Jeffrey Meyer;... .[/QUOTE]

    Jeff, can you elaborate on "relaxed," "minimum energy splines"? Unclear what it means.

    What if you want a spline to change? I was trying to recreate an elliptical wing discussed in another thread, modeled using a surface sweep, much to my surprise. It used only the root airfoil, LE and TE. Mine would not proportionally"shrink" the root rib height toward the tip rib, only the chord. I tried adding more splines to define a spar shape (worse) then defined the root airfoil lines and handles to turn them black (worse still). Thanks! Mark
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    Last edited by Mark Meredith; 01-07-2017 at 12:33 PM.

  4. #74
    SOLIDWORKS Support Volunteer Jeffrey Meyer's Avatar
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    Quote Originally Posted by Mark Meredith View Post
    Jeff, can you elaborate on "relaxed," "minimum energy splines"? Unclear what it means.
    Mark, permit me to digress a little here with a short explanation of what splines are:
    Splines were invented/used hundreds of years before computers were invented and the only software that was around were woolen socks. However, engineers in the shipbuilding industry did use CAD systems - CAD stands for "Craftsman Assisted Design". i.e. The big-deal engineer designed the ship, and then when things didn't work out right he blamed the craftsman and made him (the craftsman) fix the engineering screwups.
    Seriously though, splines were made of long thin strips of wood lain on the floor of the "loft hall" where the craftsmen drew bulkheads, profiles, plating, and other curves on the floor, at 1:1 scale, by tracing along the splines of wood held in place with weights known as "knots". The coordinates of the knot positions were given in tables known as "offset tables". You can recreate such an offset table directly from the spline node coordinates, and then recreate the actual spline on your table/floor using the ancient but well proven spline technique used by our ancestors.
    By their physical nature such splines would adopt a "minimum energy" shape. For example if you tried to bend the spline to pass through 3 non-linear given points, the bending moment at the middle point would be at a maximum while the bending moments at the ends would be zero. the final result would be some finite minimum radius of curvature at the middle knot and infinite (straight line) radius of curvature at the end knots. Clearly this is completely unlike a circular arc that is uniquely defined by 3 points in space - i.e. there is only one circular arc that can pass through 3 pre-defined points, while there are an infinite number of splines that can pass though those same points. So, we need some extra constraints to uniquely define the spline. Enter tangent directions, tangent weighting, and radius of curvature at the nodes/knots. SW "simulates" the minimum energy of those bent sticks using NURBS - Non-Uniform Rational Basis Splines - they call it "Relax Spline".
    Here are some links that explain everything much better than I:

    Lofting
    https://en.wikipedia.org/wiki/Lofting

    Splines
    https://www.izenda.com/blog/from-shi...of-the-spline/

    NURBS
    https://en.wikipedia.org/wiki/Non-un...ional_B-spline




    Quote Originally Posted by Mark Meredith View Post
    What if you want a spline to change?
    Being a good Jewish boy, I'm going to answer your question with another question:
    What do you want to change in your spline?
    If you want to change the tangent direction/angle of the the spline at any particular node/knot, then you can do that by making the tangent blue arrow at that node parallel to a sketch line (2D or 3D).
    If you want to change the "power" of that tangency you can do so by dragging it or defining it numerically.
    You can also define/change the radius of curvature at the spline end points.

    BTW - a small tip on designing with splines: Use as few nodes as possible. You would be absolutely amazed at the variety of graceful splines you can generate using only 2 nodes and defining their end-point tangents.

    Quote Originally Posted by Mark Meredith View Post
    I was trying to recreate an elliptical wing discussed in another thread, modeled using a surface sweep, much to my surprise. It used only the root airfoil, LE and TE. Mine would not proportionally"shrink" the root rib height toward the tip rib, only the chord. I tried adding more splines to define a spar shape (worse) then defined the root airfoil lines and handles to turn them black (worse still).
    I'll get back to you on this one ...

    Quote Originally Posted by Mark Meredith View Post
    Thanks! Mark
    Don't mention it

  5. #75
    Mark Meredith's Avatar
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    Jeff, thanks for the links! Mark

  6. #76
    SOLIDWORKS Support Volunteer Jeffrey Meyer's Avatar
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    Quote Originally Posted by Mark Meredith View Post
    I was trying to recreate an elliptical wing discussed in another thread, modeled using a surface sweep, much to my surprise. It used only the root airfoil, LE and TE. Mine would not proportionally"shrink" the root rib height toward the tip rib, only the chord. I tried adding more splines to define a spar shape (worse) then defined the root airfoil lines and handles to turn them black (worse still).
    Mark, I threatened to get back to you on this so here it is:
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    This is a solid loft using the root airfoil as the start profile, a point as the tip profile, and the elliptic leading and trailing edges as guide curves.
    I couldn't post the SW part file because it's too big (200Kb), but I can send it to you (or anybody else) by mail (or any other way you like) together with a detailed explanation on how I did it. I don"t want to post the explanation here because I get carried away with my long explanations.

  7. #77
    Mark Meredith's Avatar
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    Thanks, Jeff. Could you post it on GrabCad with the EAA tag?
    I tried building this and it worked with the LE guide curve. But I drew the airfoil with a squared off trailing edge because I've seen and read that SWx does not like shapes that come to a sharp point. This means of course the TE guide curve did not work because the airfoil can't come to a point at the tip. How can this be built using a sweep that ends short of the single point tip, keeping the square TE?

  8. #78
    Mark Meredith's Avatar
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    Jeff, thanks for posting it GrabCAD. Interesting how you built the airfoil. How did you get from the text files to the airfoil drawing? I'm also curious about the extra leading edge points. I thought it was to improve the LE accuracy on the drawing but they show up on the center construction line instead. Thanks, Mark

  9. #79
    SOLIDWORKS Support Volunteer Jeffrey Meyer's Avatar
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    Quote Originally Posted by Mark Meredith View Post
    Jeff, thanks for posting it GrabCAD. Interesting how you built the airfoil. How did you get from the text files to the airfoil drawing? I'm also curious about the extra leading edge points. I thought it was to improve the LE accuracy on the drawing but they show up on the center construction line instead. Thanks, Mark
    Mark, together with your previous post there are several issues here:
    1. Text files to drawing,
    2. Extra airfoil points,
    3. Squared off trailing edge.

    I'll try to answer each in separate posts.

    1. Each .txt file contains a list of X,Y, & Z coordinates of the airfoil. The upper and lower airfoil curves are in separate .txt files.
    To read them into SW, go to Insert>Curve>Curve through XYZ Points.
    In the Curve File dialog box choose Browse, and then in the explorer window on the right of the File Name box, choose file type Text File (.txt). Then pick one of your airfoil .txt files. Repeat for the other airfoil .txt file(s).
    You now have the airfoil curves in your SW part - you can Convert each curve into any sketch, make it "blue" by deleting the "conversion" constraint, and orient/scale/manipulate the resulting blue spline any way you like, simply by dragging it with your mouse.

    Hope this answers your first question.
    More to follow.

  10. #80
    Mark Meredith's Avatar
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    Ok, got it Jeff, thanks. Successfully pulled in points, converted and scaled them.

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