6 RIGID BODY MODES OF FLYING STRUCTURES ?

[c1a2l3c4]

Can anybody help me about the practical use of frequencies and modal shapes of the 6 first rigid body modes of flying structures (airplane, spacecraft, ...).

Thanks

 

[GregLocock]

Other than indicating that you have free'd the relevant degrees of freedom in your model, I am struggling to think of a single use for them! The frequency of each , of course, is zero, is it not? Cheers

Greg Locock

 

[zuardy]

Hi, to my know, there is no practical use of frequencies and modal shapes of the 6 first rigid body modes of flying structures (airplane, spacecraft, ...). I mean for aeroelastic analysis, it is not needed. The only practical use of them, that i know, is for checking the quality of a static analysis model of a structure (incl. flying structure), i.e. if there is a rigid body mode, then the static model may not be fixed properly -> care must be taken.
Any other suggestion?

regards

 

[idly123]

I belive the modal shapes represent the possible diaplced confgurations starting from the fundamnetal frequency and higher frequecy. a possible knowledge of mode shapes and frequency ranges are essential in controlling damping and frequency of the material used in structure.
i am not clear but i w ish u can see response spectrum analysis and modal analysis chapters in the book by "Mario Paz- structural Dynamics" or the book by " Chopra on structural Dynamics 's should help. Raj

 

[zuardy]

DrRaj, what you said is right, but this is valid only for 'normal' eigen modes, i.e without RIGID BODY MODES. I haven't read about no practical use of mode shapes of RIGID BODY MODES in aeroelastic : structural dynamic, flutter or dynamic responce. I have only heard from my "static" friend, that the RIGID BODY MODES are used to validate a static FEA-Model regarding its boundary conditions (Fixing).

regards

 

[mishra27]

The only thing I can thinks of except model quality check is the distribution of mass in the structure. A well distribute stucture with good symmetry, should show translation and rotation in pure x y and z direction. This is just my guess, comments are welcome.

 

[unafem]

Rigid body modes are quite important in defining overal response of flying structure to external force/load. Example may be aircraft P.S.D. gust analysis where calculated internal loads (i.e. fuselage side/down R.M.S bending) are composed both from rigid and flexible parts. You might think about very stiff plane having rigid body modes only, and that is exposed to random gust load. Typpicaly random gust is defined in frequency domain starting from 0 Hz (Von Karman spectrum). Gust will generate bending/shear loads on fuselage/wing that is result of external loads and mass properties of the aircraft.