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# Rigid modes response in structural dynamic analysis

## Rigid modes response in structural dynamic analysis

(OP)
How to understand the response from rigid body modes in modal transient analysis of free free aircraft model with landing gear loads. Does rigid modes respond to excitation part beyond 0 frequency?

### RE: Rigid modes response in structural dynamic analysis

Well let's see if you can answer it yourself. If the frequency of a rigid body mode is 0 Hz, and the first dynamic mode is 5 Hz, which mode will participate most when the system is excited at 0.1 Hz?

Cheers

Greg Locock

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### RE: Rigid modes response in structural dynamic analysis

(OP)
Greg, it should be rigid mode which will participate more. But would I get any strain? My main problem is understanding strain from this, constant force produces strains, clear from Inertia relief analysis, but do I get strains from modal transient analysis in this case

### RE: Rigid modes response in structural dynamic analysis

why do rigid modes excite at all ?

I thought rigid body motion was an artifice of FEM, as opposed to a real world thing. eg if I apply equal and opposite loads to a model, I still have to restrain the rigid modes even though they aren't doing anything.

### RE: Rigid modes response in structural dynamic analysis

Well, in experimental modal analysis we see a zero Hz response which we call the residual. It is, in the appropriate units, the spring stiffness of the foundation, for a fixed system, or the mass of an unrestrained system. It is a real effect.

For instance consider a simple spring mass system. At 0 hz you will merely stretch the spring.

Cheers

Greg Locock

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