Rigid modes response in structural dynamic analysis

[subbu9394]

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?

 

[GregLocock]

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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[subbu9394]

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

 

[rb1957]

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.

another day in paradise, or is paradise one day closer ?

 

[GregLocock]

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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