dynamical behavier of weight-added Post Mod beam

[ABV97]

Hi All,
On a modified beam I have statically proved that is OK. However on the beefed portion of the beam, since the weight of the Post-Mod beam is bigger than the original Pre-Mod beam, I was wondering what would be the dynamical behavior of the beam. I am thinking of getting approach to undamped natural frequency and somehow to prove it is against the resonance?

Thanks in advance for your opinions,

 

[rb1957]

why is dynamics a question ? helicopter ?

you can always do a modal analysis (FEA) of the mod'd beam.

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

 

[ABV97]

...no, it's not a helicopter, and under 6g down flight load, due to that Mod, there is a -0.013" deflection, so from that deflection probably I could get the undamped natural period of vibration - I don't know, just still thinking in that direction. If won't work, yes FEA is the next.
Thanks rb1957

 

[rb1957]

the deflection (0.013") isn't a problem nor an indication of one, in my mind.

possibly there's something novel in the loading ??

did someone ask you "what about dynamic response ?" or are you thinking about what could be an issue ?

how has your company written up this kind of work in the past ?

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

 

[ABV97]

Yes, I was asked that question. I also think, the deflection is quite small and wouldn't be a problem. Last night I was plying with the natural and impulse frequencies showing that they are of different diapason. No history with that type of question unfortunately, thanks again rb1957!

 

[rb1957]

unfortunately knowing the modes is only 1/2 of the problem; the other 1/2 is "what modes are excited in the airframe ?" which, unless you're the OEM, is really hard to find. maybe showing that pre-mod and post-mod beams have similar modes is enough, but unless there's reason for concern, this is "gilding the lily" IMO.

is this beam supporting an antenna ?

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

 

[ABV97]

No, it is a modified beam at the tail, under the APU!

 

[rb1957]

so is the question "how will the new beam behave when running the APU ?" since i imagine the APU will try to excite the beam. if this is a serious question (the properties changed a lot, the thought is this is close to the APU frequencies, ...) i'd suggest the best way to answer it is ground test, and if you really want to kill it, test pre-mod and post-mod (but that may not be an option).

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

 

[ABV97]

Thanks a lot, something to think about!

 

[ABV97]

Thanks again rb1957, appreciate your thoughts and time!