P2FModder
New member
- Mar 23, 2023
- 4
Hello everyone,
I have ran into an issue which I have not experienced before while performing a free-free check. I am wondering if this is an indication of a deeper problem or is it something that I should not worry about.
The project is a modification of an OEM aircraft. So, we were able to obtain the FE model (aft fuselage only) from the OEM but we do not have any verification/validation documentation for the model. The modelling is quite standard (Bar elements for frames, rods for stringers, shell elements for skin, bars for crossbeams, seat rails and struts, shear panels for floor panels). As part of the modification, we replaced the crossbeams, seat rails and struts. For the FE model, we amended the crossbeams, seat rails and struts accordingly. In general, the replaced structures are stiffer than the OEM structures. Both models do not have any RBE elements. Not sure if this is an important information, but the OEM model came with rotational SPC at every frame node. We have kept these unchanged in the modified model and retained them in free-free runs. The models would not converge without the rotational SPCs. I have removed all other SPCs that would constrain the model.
I have ran the free-free modal analysis on both OEM and modified FE models. For the OEM model, I obtained 18 low eigen values (cycles < 0.0001Hz) before the jump in eigen value occurred. In the modified model, I obtained 22 low eigen values (First 21 were < 0.0001Hz, #22 was ~0.08) before the jump in eigen value occurred.
My issues are as follows:
1) For structures which are properly connected to each other, we would usually expect 6 rigid body modes, 1 Rigid body motion in each DOF. What could possibly explain the higher number of low eigen values? I am seeing coupled DOFs as well as repeated DOFs.
2) In the OEM model, I am able to identify the displacement/rotational in the 6 DOFs within the 18 low eigen value (some DOFs were coupled in an eigen value). I would think this is okay for a structure as complex as the fuselage (please correct me if I am wrong). However, for the modified model, I was not able to spot 1 DOF within the first 22 low eigen values. I am only able to identify that missing DOF in an eigen value > 1Hz. Is this an issue that I should be concerned of?
Please let me know if you need any further information. I will try to provide as much information as possible without violating confidentiality. Thank you for your help in advance.
I have ran into an issue which I have not experienced before while performing a free-free check. I am wondering if this is an indication of a deeper problem or is it something that I should not worry about.
The project is a modification of an OEM aircraft. So, we were able to obtain the FE model (aft fuselage only) from the OEM but we do not have any verification/validation documentation for the model. The modelling is quite standard (Bar elements for frames, rods for stringers, shell elements for skin, bars for crossbeams, seat rails and struts, shear panels for floor panels). As part of the modification, we replaced the crossbeams, seat rails and struts. For the FE model, we amended the crossbeams, seat rails and struts accordingly. In general, the replaced structures are stiffer than the OEM structures. Both models do not have any RBE elements. Not sure if this is an important information, but the OEM model came with rotational SPC at every frame node. We have kept these unchanged in the modified model and retained them in free-free runs. The models would not converge without the rotational SPCs. I have removed all other SPCs that would constrain the model.
I have ran the free-free modal analysis on both OEM and modified FE models. For the OEM model, I obtained 18 low eigen values (cycles < 0.0001Hz) before the jump in eigen value occurred. In the modified model, I obtained 22 low eigen values (First 21 were < 0.0001Hz, #22 was ~0.08) before the jump in eigen value occurred.
My issues are as follows:
1) For structures which are properly connected to each other, we would usually expect 6 rigid body modes, 1 Rigid body motion in each DOF. What could possibly explain the higher number of low eigen values? I am seeing coupled DOFs as well as repeated DOFs.
2) In the OEM model, I am able to identify the displacement/rotational in the 6 DOFs within the 18 low eigen value (some DOFs were coupled in an eigen value). I would think this is okay for a structure as complex as the fuselage (please correct me if I am wrong). However, for the modified model, I was not able to spot 1 DOF within the first 22 low eigen values. I am only able to identify that missing DOF in an eigen value > 1Hz. Is this an issue that I should be concerned of?
Please let me know if you need any further information. I will try to provide as much information as possible without violating confidentiality. Thank you for your help in advance.
