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Thread: Modeling Tubular Frame

  1. #11

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    I have drawn weldments may with curved tubes. Try redoing the curves with a radius or ellipse. As you are drawing, think about what you can build. I have not tried using a spline curve in a weldment and I don't know how I would bend a tube that as a spline.

  2. #12

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    Yes, I do understand the engineering logic behind a truss structure, however the longerons on my Bellanca Cruisemaster are definitely curved tubing members (Guiseppe Bellanca did a lot of things his own way!). All four longerons have been formed into curved members. Now the interconnecting support members are all straight tubes. I would actually say the fuselage structure is over-built. Anyway that's my dilemma. I sincerely appreciate your help... Rob

  3. #13
    SOLIDWORKS Support Volunteer Jeffrey Meyer's Avatar
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    Quote Originally Posted by lathropdad View Post
    I have drawn weldments may with curved tubes. Try redoing the curves with a radius or ellipse. As you are drawing, think about what you can build. I have not tried using a spline curve in a weldment and I don't know how I would bend a tube that as a spline.
    I don't have too much experience with bending tubing but it seems to me the best you can achieve at home is an approximation to the curve you want. The main problem is the ends - you will always have a small straight segment at the end. But here's the thing - if you're prepared to settle for a curve that is approximated by short straight segments, then you can use any curve - circular, elliptic, conic, or spline. You can then use SWx to calculate/measure the lengths of each straight segment and the angle between segments. The shorter the segments the better the approximation, and this applies to any curve.
    My 2 cents worth.

    BTW - if the longerons are bent with a very large radius of curvature then it could be that the tubes are being "bent" in the elastic regime - as opposed to the plastic regime. If this is the case then the curves by their nature will be splines - not arcs/ellipses/conics, a state that SWx simulates very nicely.
    Last edited by Jeffrey Meyer; 06-26-2017 at 05:14 AM. Reason: Afterthoughts

  4. #14
    cwilliamrose's Avatar
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    Jeffery,

    No need to worry about making the bends perfect, once you weld the tube clusters it will all change anyway. The Bellanca used curved longerons to give the fuselage smoother contours and the tube size most likely had to be larger to provide enough strength and stiffness to make a valid structure. It's a trade-off the designer was willing to make. Most airplanes have straight tubes that approximate a curve and that method will yield a lighter structure.

    Rob,

    Splines aren't supported in the weldments tools. You can get close to the spline's shape using a series of tangent arcs. A bit of a PITA but you only have to do two of them (then mirror for the other two). If the spline is in 3D space instead of being on a plane it will be more work but still doable. My engine mount's curved tubes are just single arcs on planes. The curved tubes are thicker wall than the straight tubes;

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    Last edited by cwilliamrose; 06-27-2017 at 05:33 PM.

  5. #15
    SOLIDWORKS Support Volunteer Jeffrey Meyer's Avatar
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    Ok Bill - Question: What tube bending equipment do you have that enables you to bend a tube to some arbitrary radius?

  6. #16
    cwilliamrose's Avatar
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    A piece of plywood (or similar) with a radius cut on one edge with a bandsaw, then clamped to a sturdy table.

  7. #17

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    Bill & Jeffrey... thank you for the ongoing dialogue - all very helpful! Just to give you the "rest of the story"... I have two working Bellanca Cruisemasters plus I also bought a fuselage from a plane that landed gear-up which I have stripped down and am using to reverse engineer various structures and components. My purpose in doing this is to make sure I know every detail of the construction of the aircraft to satisfy my engineering "need to know" obsession. From a more practical perspective I am working with the folks that have produced the Trio autopilot who have indicated that with the appropriate engineering drawings they will pursue adding the Cruisemaster to their STC list. Also designing avionics installations is much easier if the limited space behind the panel is well documented in 3D. So this is my reason for going to this effort. Again, thanks for your help!!!

  8. #18
    cwilliamrose's Avatar
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    Interesting project. We'd love to see screen shots the fuselage model once you have something to show.

  9. #19
    SOLIDWORKS Support Volunteer Jeffrey Meyer's Avatar
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    Quote Originally Posted by cwilliamrose View Post
    A piece of plywood (or similar) with a radius cut on one edge with a bandsaw, then clamped to a sturdy table.
    Hi Bill - This a great idea - I'm really glad I thought of it .

    So, some comments on how to go about doing this:
    1. IMHO the plywood mold should be made with the curve extrapolated at each end by a few cm/inches - this should be done because otherwise your bent tube will have short straight segments at each end. Of course you may be able to get away with no extrapolation of the curve if you cut the tube to be a few cm/inches longer at each end, and insert a straight bar into the extended ends.
    2. Generate the curve representing the centerline of the tube in SW while taking into account that it should be a planar 2D curve. (It may be oriented in 3D, but remember that the mold/plywood is 2D). You can generate the curve using any combination of circular arcs, ellipses, conic sections, and splines.
    3. Offset the curve by half the diameter of the tube.
    4. Make a SW drawing of the offset curve and export it (File>Save As>*.DXF). Include a calibration line of known length in both X and Y axes.
    5. Take the DXF file to your local print shop and have it printed at a 1:1 scale. Measure the actual length of the calibration lines before proceeding.
    6. Stick the print to your plywood sheet using a non water based glue. (Paper expands when wet).
    7. Do your stuff with the bandsaw.
    8. You now have the tooling for mass production of your airframe.

  10. #20
    cwilliamrose's Avatar
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    You forgot one important step Jeffery -- spring back. When you bend a 4130 tube around a form it will not take the shape of the form. The form has to have a tighter radius to produce the needed curve.

    The short straight section at the end is not a problem, simply cut it off.

    For my engine mount the curved tubes are all 3/4" OD and the bends are 12 3/8" radius to the CL of the tube so I'd start with maybe 11 3/4"R for the bending block. To lay out that radius I'd use the Omicron beam compass we have which looks like a modified tape measure, it can produce arc's as large as 72" directly on the plywood -- no need to go the the printer. After cutting my form block I'd bend a section and measure it (or compare it to a template) and adjust the form tool as needed. I'd make long enough bends to get two or three parts from each one -- I need five parts of various lengths for the mount ring.

    If this was more than a simple radius, say the trailing edge tube of the rudder which has multiple tangent arcs of different radii, I'd probably make a full-size template as you describe above but it really only needs to be close since it's a weldment. I could just lay it out by hand and be plenty close enough but since I have a SWx model of the rudder getting a full-size print is the easiest way to go.
    Last edited by cwilliamrose; 06-28-2017 at 05:36 PM.

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