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MotionSolve: static + linear

I notice something weird when running static + linear to get the vibration modes of the aircraft on ground. The aircraft is free to move. The contact comes through fiala tires on the road. So the solver calculates the static weight on wheels condition and then calculates the eigenvalues. I have pitch, plunge and other modes. Frequencies look ok but the damping ratio at least for the pitch mode looks way too much, 50%. This values does not depend on the spring damper properties, which at the moment are modelled as expressions of DM and VR. I also tried to switch to a linear damping term but no change. Anything I am missing with the linearization process in MS?

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MotionView/MotionSolve

Accepted answers

Christopher_Fadanelli

Hi Luca,

Could you please create a ticket in Altair One so you can share the model and one of us can assist you there?

Best,

Christopher Fadanelli

Luca Cavagna

Hello Chris, thanks for answering and for the invitation.

I think I got it and I can avoid disturbing the team. The issue comes from the tire model. The fake damping I saw is an artifact of linearizing the Fiala tire equations at zero speed. I was suspecting it was coming from there and it was basically overwhelming all my changes in the spring-damper properties.

I gave a small initial speed to the main body (the aircraft is taxing basically) and I get a 7% critical damping as it should be.

Hint: always make sure to define the friction through UMIN and UMAX in the tire file or you will not see any mode which make sense. The body will slide as it should do.

All comments

Christopher_Fadanelli

Hi Luca,

Could you please create a ticket in Altair One so you can share the model and one of us can assist you there?

Best,

Christopher Fadanelli

Luca Cavagna

Hello Chris, thanks for answering and for the invitation.

I gave a small initial speed to the main body (the aircraft is taxing basically) and I get a 7% critical damping as it should be.

Hint: always make sure to define the friction through UMIN and UMAX in the tire file or you will not see any mode which make sense. The body will slide as it should do.

Christopher_Fadanelli

Hi Luca,

Glad that the issue was resolved and thank you for providing your solution!

Best,

Christopher Fadanelli