The problem really isn’t with OpenVSP, but with how you’ve used it.
Years ago, OpenVSP (v2 and VSP before that) used the OpenNURBS library to write out native 3dm files. When we got IGES and STEP files working, we dropped 3dm support because Rhino was just as good at reading our IGES and STEP files as the 3dm files – and everything else would prefer to work with IGES or STEP directly. So, dropping OpenNURBS was one less software dependency to wrestle and we didn’t loose anything. The point here is that the file format didn’t really matter, (when using the untrimmed surfaces export under the File… menu) OpenVSP is sending Rhino a piecewise quilt of Bezier patches. File conversion and interoperability is not the problem – the problem is with non-ideal surfaces.
This wing is severely over-defined, with twenty ‘hard’ cross sections – each defined as file airfoils.
Blending is not used effectively to create a smooth LE and TE – so most of those cross sections are actually G1 discontinuous. Some of the airfoils are clearly not correct – in particular, the 5th from the root (#4) is clearly not what is intended.
I would suggest you choose the option ‘Rotate foils to match dihedral’ when lofting a wing with a strong winglet like this one.
From what I can see, many of the sequential foils are identical, so perhaps you are trying to specify a complex twist distribution. If not, deleting most of the sections and using blending to recover the planform shape will give you much better surfaces.
Some of the airfoils are varying, but you would get a better surface definition if you specified less and let OpenVSP’s lofting handle the interpolation where it can.
How did you go about coming up with the airfoil stack, planform details, and twist distribution? I’m assuming you used some other program – does that program have more planform information? This model is not doing a great job of reflecting the intent of a well designed wing.
I know it sounds crazy, but If you really believe you need complex and smooth control of twist and other wing design parameters, you might actually try modeling the wing with a propeller blade. However that takes an experienced hand and I would like to see you understand and master the mistakes made here before taking a significant jump in complexity like that.
Certainly following the advice to turn off the ‘Split Surfaces’ option is a good one if that is not what you want.
You might try using the ‘Trimmed Surfaces’ option under ‘Analysis’ to write out a BREP solid STEP file if you would prefer that. There might be file compatibility issues there, but it should result in a solid from the start with nothing needing to be done in Rhino. That said, your geometry is a single part, so it should be easy either way.
If the root and tip caps are giving Rhino problems for some reason, they can be easily turned off in OpenVSP. The root caps aren’t doing anything for you - and the tip caps will get better if you ‘Rotate foil to match dihedral’.