Allowing negative material density

[Lukas]

Hello everyone :D

I am currently working on a 3d printed plane with a current focus on predicting the final mass. I have noticed that a combination of a volume density and a surface density allows for a very accurate estimation of the mass of printer parts using openVSP. 

However now I want to predict the mass of a hallow fuselage, which is to be filled with components. It turns out that combining a "bulk volume" of the fuselage, from which the core is cut with a smaller "core model". The inteded approach of setting this core to a negative volume removes the component inside as well and is therefore unfit especially for later cg calculations, but setting the core modells density to the negative value of the bulk volumes density, and therefore having the densities cancel each other out works well.

The problem is, that after every mass calculation run the negative density value is set to 0, which I assume is done since most of the time setting a negative value will be non sensical.

Is there any workaround for this which doesnt include me modifying the code?

And if not I would suggest removing this automatic correction or maybe adding a toggle button to manually allow negative densities (although implementing this is more work)

Thanks for any advice and have a great day

[Rob McDonald]

Overlapping volumes do not sum their densities, the 'priority' score selects which density to use for that volume.  I.e. set the priority of the things 'inside' the fuselage to a higher number to count them instead of the lower priority density.

To me, the most simple approach would be to use the surface area based density for the fuselage and set the volume density to zero.

Another approach would be to perform the mass properties calculation in two steps.

In the first step, you model the fuselage and a negative component inside to hollow it out.

In the second step, you model all the components.

Then do the math to combine these results.

Rob

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[Rob McDonald]

To be clear, you should never need to set a 'negative volume' for mass prop.  Just stack components and use priority appropriately...

Fuselage - non-zero density, lowest priority

Hollow - zero density, +1 priority.  Likely made from a conformal component based on the fuselage.

Components -- non-zero density, +2 priority

Rob

[Lukas]

Thank you so much for you answer. Your recomendation of the Priority method works perfectly :D