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# Meaning of off-diagonal terms of the Rigid-Body Mass (MO) in the .f06

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## Meaning of off-diagonal terms of the Rigid-Body Mass (MO) in the .f06

(OP)
We are checking out the mass of a model in Nastran (by using the WEIGHTCHECK or PARAM,GRDPNT commands), and the first mass matrix that appears on the .f06 print file is a M O, which looks like the 6x6 rigid-body mass matrix.

We understand the meaning of the diagonal terms to represent the total mass and rotary inertia of the system on each physical direction (translation and rotation), but there seems to be other off-diagonal large terms which we do not understand.

For instance:

O U T P U T F R O M G R I D P O I N T W E I G H T G E N E R A T O R
0 REFERENCE POINT = 0
M O
* 8.000000E+03 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 -2.000000E+04 *
* 0.000000E+00 8.000000E+03 0.000000E+00 0.000000E+00 0.000000E+00 1.745300E+04 *
* 0.000000E+00 0.000000E+00 8.000000E+03 2.000000E+04 -1.745300E+04 0.000000E+00 *
* 0.000000E+00 0.000000E+00 2.000000E+04 2.500000E+05 -1.456625E+05 0.000000E+00 *
* 0.000000E+00 0.000000E+00 -1.745300E+04 -1.456625E+05 9.309703E+05 0.000000E+00 *
* -2.000000E+04 1.745300E+04 0.000000E+00 0.000000E+00 0.000000E+00 1.180970E+06 *

We went through the MSC/NX Nastran documentation in the Dynamic Analysis User's Guide, and we found nothing related to the explanation of the off-diagonal terms of MO matrix.

What is the meaning of the off-diagonal terms?

Thank you!

Regards,
Francesco

### RE: Meaning of off-diagonal terms of the Rigid-Body Mass (MO) in the .f06

Take a look at the remarks of the GPWG module (the module that computes those mass properties) in the DMAP Programmer's Guide. You will find that the 6x6 weight generator output contains the mass, inertia and centre of gravity information in those 36 terms. This is especially useful if, for example, you want to use these data in an optimization run to say, minimize the difference between a target centre of gravity and the current centre of gravity of the model.
DG

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