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

[JoeT]

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.

[Cory Puuri]

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.

[JoeT]

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.

[JoeT]

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.

[Cory Puuri]

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 (https://www.dropbox.com/s/rkr9ngah2740gma/Untitled.mp4?dl=0).

[JoeT]

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.

[Jeffrey Meyer]

Just a moment friends (pun intended), a moment is a moment wherever it's applied!
Have I missed something?
Jeffrey

[Cory Puuri]

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

[JoeT]

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.

[JoeT]

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.

[Matt Gonitzke]

Context and precision of terms is important here when there are two similar but very different quantities being discussed here that contain the same word.

Torque = force * distance, and "Moment" in the context of CG location and weight and balance is also force * distance. You have also probably seen this as "weight * arm = moment" on aircraft weight and balance calculations. In this context, torque and moment are functionally the same thing. Torque is not "dynamic"; it is not a function of time.

In your screenshots, "[mass] Moment of Inertia" is is a quantity that relates the torque needed to generate a desired angular acceleration about a rotational axis.

[Jeffrey Meyer]

This is somewhat off-topic, but I couldn't resist the temptation to make this post:
IMHO torque and moment are 100% synonymous and interchangeable. The use of one or the other is entirely part of the beauty of the English language. Let's take the example of tightening a bolt with a torque-wrench. While the bolt is turning (read "in a dynamic state"), we are applying a torque. But when the bolt stops turning having reached the required torque, we have a static torque - a moment. So the tool should now be called a moment-wrench. In both cases we are applying a force on the handle at a fixed distance from the bolt. Nothing has changed except the name of the tool.
At the moment I'm enjoying having this torque.;)

[JoeT]

With all due respect, here is a quick answer to this question (attachment 1). Please note the first comparison.

The second attachment is from a RANS CFD tool I leased for a year. It is 'Stallion 3D.' It is primarily for analysis of models not within a closed flow environment. In other words, airplanes, sailing vessels, cars, etc.

This attachment includes just a small portion of the data for my 207EF FWO model. Notice there is no mention of 'Torque.' Aero weenies want to know about moments, moment coefficients, moments about the X,Y, and Z axes, and the X, Y, and Z forces, along with the appropriate coefficients.

I believe TORQUE is a vestige of the original purpose of the SolidWorks Flow Simulation software - internal flows. In that scenario, it is quite logical to want to know torques. Think about a flow passing through an impeller, vane, etc. The engineer would probably want to know the Torque that is acting on the impeller/vane.

So, the REAL QUESTION is: Can I assume, in this software, that TORQUE can be substituted for moment?

Regards,
Joe T.

[Cory Puuri]

Joe T., Stephen Endersby from DS SolidWorks is developing a step-by-step guide. It may take a bit, but I will post a link out here when I have it. We may also turn this into a bit of a tutorial for others to post out here: http://eaa.org/swu

Thanks!

Cory

[Cory Puuri]

Here are step-by-step instructions (https://www.eaa.org/~/media/8BB354208D9B4866BAA5C0C3D3009B69.ashx). Please let me know if we're missing something.

[Cory Puuri]

That does seem to be a reasonable validation. I may be wrong, but I think the issue Joe was encountering was that SOLIDWORKS uses material weights to identify the CG. His model doesn't have material properties loaded and so he was trying to place the CG where he wanted it to be.

[flyboy2160]

Ok, Cory. I misunderstood his intention, so I deleted my post to avoid confusion.