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Thread: How to properly place the CG in Flow Simulation for moment calculations

  1. #1

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    Apr 2020
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    How to properly place the CG in Flow Simulation for moment calculations

    All,

    I want to be able to determine moments about my design's CG. How do I do that?

    When I have SW create a CG, it is not where I need it. Alternatively, how do I move the 'Local Coordinate Axis?'

    Regards,
    Joe T.

  2. #2
    EAA StaffEAA Staff / Moderator
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    From Stephen Endersby (DS SolidWorks - Dir. Product Portfolio Mgmt.):
    The easiest way is to can create a custom coordinate system in the SOLIDWORKS assembly (aircraft) at the location of the CofG.
    Then assemble this SOLIDWORKS assembly into a new assembly (call if flow_aircraft) and mate the CofG coord system of the aircraft to the new assembled default coordinate system. That way your flow moments are calculated around the aircraft CofG.

  3. #3

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    Cori/Steve,

    I'll try that. I was doing something similar, but I could not get the CG to move to the new local coordinate axis. I'll try again.


    Also, Cori, I'll have you the parameters that would be very useful for Flow Simulation.

    Joe T.
    Last edited by JoeT; 08-10-2020 at 03:30 PM.

  4. #4

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    I can't get the CG to 'Mate' to the re-located coordinate system. I get the following message:

    "Mating to the Center of Mass of the top level assembly is not allowed because it would create a circular reference."

    Any suggestions?

    Regards,
    Joe T.

  5. #5
    EAA StaffEAA Staff / Moderator
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    Joe, was this after watching the video I shared with you from Stephen?

    For those of you curious about this video, it was a quick screen recording without sound of the process of creating a custom coordinate system relative to the CG stored here.
    Last edited by Cory Puuri; 08-14-2020 at 07:30 AM.

  6. #6

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    Cori,

    I've watched it several times. As stated in your post above, there is no audio. That's a real problem.

    Also, the video is very poor quality. I am not sure what menus. etc., were clicked.

    I plan to watch it again shortly with my SW Flow Sim up and running and will try to follow and understand the logic better.

    I will have you the 'wish list' of functions that are needed to properly analyze aircraft performance today. I will also send you, separately, how I am trying to move the CG to the desired location. Included in that will be some questions for Stephen that may be useful for other folks following this thread.

    Regards,
    Joe T.

  7. #7
    SOLIDWORKS Support Volunteer Jeffrey Meyer's Avatar
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    Just a moment friends (pun intended), a moment is a moment wherever it's applied!
    Have I missed something?
    Jeffrey

  8. #8
    EAA StaffEAA Staff / Moderator
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    I think your main concern was with getting the Origin placed on the CG. Here are those steps from the video:
    With the part/assembly open, click on Tool / Evaluate / Mass Properties in the top menu
    In the window that opens, check the box for Create Center of Mass Feature, click the Recalculate button and close the window
    You now have a Center of Mass in the Feature Manager Design Tree and it should be selected already
    With Center of Mass Feature Selected, on the Features toolbar, select Reference Geometry, select Coordinate System, and then click the green check mark in the Feature Manager

  9. #9

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    Quote Originally Posted by Jeffrey Meyer View Post
    Just a moment friends (pun intended), a moment is a moment wherever it's applied!
    Have I missed something?
    Jeffrey

    It is where it is being applied that is the concern.

    Joe T.

  10. #10

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    Cory, you were close, but no cigar

    Everything worked until you stated "Recalculate." That didn't work.

    You can see the CG (Black and White 'BMW' Circle), the 'Local Coordinate System' (GREEN), and 'Coordinate System 1' (BLUE) in the first image. Coordinate System 1 is where I want the CG. I created the location, and added the axis, for Coordinate System 1.

    The second image shows the 'Mass Properties' menu. Note the 'Override Mass Properties.' That is the 'button' that needs to be selected, not the Recalculate. Also note that the 'Report coordinate values relative to:' has my Coordinate System 1 selected. Very important. You select that from the tree (see 'Coordinate System 1 in the tree, top image. It is highlighted). Also note the CG/Coordinate System 1 is rotated counterclockwise, it seems. That is because the 'CG' has gravity acting through it. The image 2 (and the other images) are calculated at an AoA of 13 Degrees (see the very top of the image 2 and you will find the file name for this calculation. Note the 13 AoA in the file name) . The X axis of the CG (PINK Coordinate system, which is also the CG and Coordinate System 1) is rotated 13 Degrees counter clockwise.

    Now see image 3. It shows the CG has moved to the location of the origin of Coordinate System 1. Also note back in image 2, that I keyed in the new (required) coordinates (X, Y, Z) for the location of the CG. X=-5, Y=0, and Z=-100. That is quite a different from the original CG location, as you can see comparing image 3 to image 1.

    Now, Torque is not the same as moment. Moment is a static term. Torque is a dynamic term. I am looking for MOMENT, not torque, UNLESS SolidWorks uses the two term interchangeably, which they should not. Does the Flow Simulation calculate TORQUE (Moment??) about the CG? I assume so. But, not sure. And, in this case, is TORQUE the same as moment?

    Finally, see image 4. One has the option of selecting which coordinate system is desired for the analysis. I, again, assume this establishes which set of coordinate systems TORQUE (Moment??) uses for its calculation. Since I moved the CG to Coordinate System 1, this should work.

    OK, I am not a CFD expert. I did not play one on TV. And, I did not stay in a 'Holiday Express' last night, or anytime recently. So, like many of y'all, I am just beginning the 'adventure' of learning Flow Simulation. I truly appreciate Cory's help!

    If I have misstated anything above, or I am just plain wrong, PLEASE CORRECT ME! I have pretty thick skin, for a GEEZER.

    Regards,
    Joe T.
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