Free-Free Modal Analysis Verification


I was wondering if anyone has any good rules or where of verifying a Free-Free Modal Analysis using the results from the f06 file. As I have learned, a frequency below 0.0001 or 1E-3 is usually a good indicator of accuracy but from the reults included that is clearly not the case.

I was wondering if anyone had any other indicators for determining whether the results are valid aside from the factor between the 6th and 7th frequency being >100.

The link included discussed the sum of the 6th mode being 1.0 but it never mentioned whether that was true, merely that a value close to 0 was good which was generally vague.

I additionally watched a video that mentioned 80 percent participation but I am unsure of what that means either.

All help is appreciated.







Re: Free-Free Modal Analysis Verification



As an example, here is the criteria we consider when performing free-free check as per the European Cooperation for Space Standardization (ECSS). 

The modal analysis of the free model shall show the expected number of rigid body + movements with frequencies equal or less than d Hz. The first six modes should be rigid body modes. These rigid body modes are expected for a three dimensional model without internal mechanisms. A typical value is : d = 0.005 Hz

The ratio between the highest computed frequency of the rigid body modes and the lowest elastic mode frequency shall be less than a value V.

Typical acceptance ratio Is: V = 0.0001


Seifeddine Naffoussi
SafeMecha Stress Engineering

Re: Free-Free Modal Analysis Verification

Gears Phenom Gears Phenom
Gears Phenom

Some large structures like big ships or buildings can have first mode 0,5-2 Hz and therefore difference in frequencies will not be so dramatic. Rigid modes don’t produce deformation - model move like rigid body, you can look and compare element strain energy.

Re: Free-Free Modal Analysis Verification

Siemens Phenom Siemens Phenom
Siemens Phenom

The NASA Goddard FEMAP/Nastran users maintain a good site for FEA modeling best practices, there's section on the Free-Free Dynamics check of a model -


Lots of useful info, from the top -