Fuselage issues in CompGeom

[Marcos Tejeda]

Hello everyone! I'm contacting you to ask about a fuselage that's giving me a hard time... I developed the complete model and was able to solve it using VLM, but I've never been able to solve it using the panel method. I've attached the VSP3 file. I honestly don't remember which version I created it in, but it must have been around V3.35.

The problem is that when I run CompGeom, it returns a positive theoretical area (which is correct), a zero wetted area (this is the strangest thing of all...), a negative theoretical volume (which could indicate that there are intersecting surfaces), and a -NaN volume (Wow...). Therefore, it doesn't generate a mesh.

The attached model is essentially made up of superellipses (except for the nose, which is a point). The nose doesn't have a flap (because it's a point), but the tail does. What have I tried?:
* Removing almost all the skinning requirements.
* Replace some of the superellipses with rectangles with rounded corners.
* Replace all the superellipses and rounded rectangles with ellipses.
* Delete the sections one by one, from the tail to the bow, until only the initial and final sections remain (to identify the problematic section).
* Export only the fuselage to a new file in version 3.48.2 and try running CompGeom.

None of these actions has helped me obtain a valid mesh.

Sorry for the length of the message, but I wanted to share everything I've tried without success. I'm probably making some rookie mistake that I'm not noticing. Thanks in advance!
Best regards.

[Rob McDonald]

You have 'Negative Volume' turned on for your fuselage.  It is on the first tab of the Geom editing window.

Negative volume is used to make a particular object 'subtract' from the rest of the model.  When that is all you have -- you end up with nothing.

Your fuselage is defined with a large number of cross sections, with the skinning turned 'off' (i.e. linear lofting between cross sections).  This is generally not the best way to go.  Instead, you should try to model your fuselage with as few cross sections as possible and use skinning to get a smooth loft between those cross sections.  For this geometry, I would likely use about six cross sections.

Rob

[Marcos Tejeda]

Thanks so much, Rob!

I feel like a fool. I really don't know why "negative volume" was selected. I checked all the tabs except that one. That was the mistake.

Best regards!

[Rob McDonald]

No worries, it is easy to do.

OpenVSP is a program learned by exploration (clearly not through documentation).  You click on things to see what happens.  Sometimes, you click on things and nothing appears to happen, so you leave it as-is.

In this case, the effect of the option wasn't seen until later -- when you no longer have any association between action and effect.

Rob

[Brandon Litherland]

Fun with negative volume video here from a prior workshop: https://www.youtube.com/watch?v=6d2Fv1AN5lQ

I thought that I covered Negative Volume in the Ground School but it looks like I missed it. I'll have to make a video.

[Marcos Tejeda]

Thanks a lot, Brandon!