Issue with extra loop when revolving nacelle cross section (Body of Revolution)

[Hanshita Hassanandani]

Hi everyone,

I’m trying to build a nacelle geometry in OpenVSP using the Body of Revolution feature.
I have an .fxs file that defines the nacelle’s cross-section points. When I import it, the nacelle generates fine overall, but there’s something strange happening toward the aft end.

When I convert the generated body to an Edit Curve, I notice that the cross-section seems to have an extra loop or self-intersecting curve near the end (it’s hard to describe, please see the attached screenshots for clarity).

I’m not sure what’s causing this behavior, whether it’s something in my cross-section data or an import/revolution setting in OpenVSP.

Any help or suggestions would be greatly appreciated!

Thanks so much,
Hanshita

[Brandon Litherland]

I plotted your FXS points to get an idea of the desired shape of the curve. I suggest you have a look at the different cross-section types on the OpenVSP Ground School here.

Edit curve must have gotten confused about where the points were and mixed the order up which turned a part of your surface inside out.  I advise against converting to CEDIT in this case.

FXS is not the best format to use in this case and should very rarely be used in any case, if I'm honest.  You'll want to convert these points into an Airfoil File type following the appropriate DAT or AF formatting. 

You're probably seeing that the actual nacelle is much longer than intended given that your points go to roughly pi in X. Both the FXS and AF/DAT formats assume a unit scaling of the coordinates. In your case, if you set the nacelle length to 3 you'll end up with 3 x 3.14... Check with the Ruler tool in Measure.  You are also highly resolved at the leading edge but very coarse everywhere else.  I recommend cleaning up the points somewhat to give a more accurate representation of your desired shape.

Alternatively (and this depends on whether you want the nacelle to be more variable for design studies), you could bring these points in as a PTS file and then use a looped fuselage component and Fit Model to match the shape, or just do it by eye. This would give you greater control over the shaping down the road without having to recreate points and do this over again.