# Assign Multiple Density Values To Solid Geometry For CG Calcs?

Hello All,

I’m mocking up a glider design and would like to be able to do some CG calcs on it using different density values for different volumes in the overall structure (fuselage, wings, battery/ballast at minimum)- is there any existing workflow, method or general application that anyone knows of that might help me in this endeavor?

If push came to shove I was going to simply cut and paste solids directly over one another to get a relative density multiplier in the given areas that needed it, but I thought there must be a more elegant way to do this while still in the mockup phase without having to go off-platform…

• David

Rhino provides tools to calculate area, volume, centroid and moments of inertia. Look under analysis tools. With this information it is a straightforward matter to make up a weight and balance sheet in Excel, multiplying the volume of each individual component by it’s density to obtain the weight. Or even paper and pencil.

Yes, I think the point was missed- I mean to say that I am very familiar with those tools, but what they do not seem to support is CG calc with anything other than a single (unspecified) density value. For me to break components into multiple parts and put them in an excel sheet would defeat the purpose of calculating the CG in Rhino in the first place (after all I am not looking for weight calculations that excell would provide, I am looking for a CG…) as Excel will not give me a way to calculate a composite CG without me being an engineer. My goal is to have different densities be included in the same model that will ultimately give me a single point output at the location of the CG, just like it does when calculating a single-density object now.

Even if I could simply work in terms of relative rather than absolute densities I could then structure the values to correspond with physical values, and then get an aircraft design with the CG placed accurately enough to allow me to stay within a margin of error that would be less challenging to correct in the physical. Keeping it in the scope of what the ballast can handle would be ideal of course, but even gross design iterations would be much faster. CG is everything in an airframe…

Maybe Pascal would know?

given the volume, apply a density value for the material you have chosen.

i am not sure if my brain works today, but assuming the material (object) has an equal density, the volume centroid should be the exact CG, or do you need several values along the whole geometry? i thought there is only one CG in a volume… sorry if this does not help, i am interested of course.

@cognitdiss wants the CG of an assembly of multiple objects with the individual objects made with different density materials. Presumably each component is a separate “solid” in Rhino. For a object with uniform density the CG is at the volume centroid of the object. However for an assembly of objects of varying density the CG is not at the composite volume centroid of the objects.

Similar requests have been made previously. Rhino does not have this ability. There are plug-ins which include the ability to track the density/weight of individual components and calculate the composite CG location. Orca3D, a plug-in for boat design, is one example.

An alternative is to use a spreadsheet and input the volume, centroid coordinates and density of each component, and have the spreadsheet calculate the composite CG. The spread sheet can also calculate the weight of each component and the total weight. Attached is an example of a simple Excel spreadsheet. CG Calc DC01.zip (5.6 KB) (Uploaded as a .zip file because this website does not allow direct uploading of .xls files.)

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@davidcockey, Fantastic, thank you so much for that spot on reply and the spreadsheet, it looks emminently useable!

p.s. I had expected that this issue would have come up before so I had done a search for “CG” and "Density before posting the topic, but suspiciously nothing was returned…