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FEA: Boundary Conditions Help for Multicopter

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Shahid_Ameen_Khan
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(@shahid_ameen_khan)
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Hi,

I would like to analyze the structure of the UAV that I have for different loading conditions. It is a simple quad-copter with co-axial motors at each motor arm (I have 2 motors at each arm, so 8 motors in total). I have a global FEA model where I have all of the structure meshed with proper properties assigned to the elements and also ensuring element quality and connectivity. I have difficulties in choosing the right boundary conditions.

I ran a few test cases to understand the behaviour of the FEA model (basically, checking the reaction loads at constraint points). Among these test cases, the boundary condition which fit right was where I constrained the 3 translational degrees of freedom at the motor-points and applied mass elements for the motor, payload (5 kg), and the battery and ran 'gravity load analysis'. The reaction forces at the constraint points were equal to the thrust I would require to hover.

Now, I would like to understand how will the structure behave for the one-motor-failed condition. What boundary condition can I go for this case? Say, if what I have so far is incorrect and I need to assign loads at the motor points, where do I constrain my model?

Any help is highly appreciated. Thanks in advance!

Regards,
Shahid

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Negative Volume
Posts: 554
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(@negativevolume)
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Joined: 4 years ago

Hi @shahid_ameen_khan

Seems like a cool study! What FEA solver are you using for this? I would just assume that the easiest way to do what you are asking would be to apply your thrust load to each mass nodes where each motor is and then remove the load from the failed motor? I know that's overly-simplistic but that's all I got with the information here. Different solvers will have different techniques of accomplishing this. 

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Shahid_Ameen_Khan
(@shahid_ameen_khan)
Joined: 2 years ago

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Posts: 2

Hi @negativevolume,

Thank you for your inputs.

Yes, what you are saying makes sense to me. I thought the same thing (applying thrust loads at the motor points) but I couldn't figure out where to assign the constraints.

I am using MSC Nastran as the solver.

Thanks again.

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Negative Volume
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(@negativevolume)
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Posts: 554

@shahid_ameen_khan I'm not at all familiar with Nastran but what you are trying to do seems simple enough. Do you have to define all translational and rotation constraints? Something you could do as a simple test would be to have your 4 constraints at the motor locations and constrain the x and y translation, but allow free motion in the z (upward) translation and in x,y,z rotation. That way you would be able to see differences in the case of a motor going out. 

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