Hi Kevin,
> To be honest I don't believe I am asking the software
> to go through big or radical changes in cross sectional
> shape but a subtle change of curvature. However it is
> still buckling and twisting
Yes I've noticed that you expect for this to work, but from the standpoint of the mechanisms involved that would be considered a big change.
Maybe "radical change" is not the right word for me to use to describe the problem - really the underlying problem is when you look at a shape and use your human intelligence to match different features of it together, the surfacing mechanisms just do not have the same matching intelligence built into them so they are giving you results that you do not expect.
Take these shapes for example:
Then consider these different regions on the curves:
You're expecting for the green region on the left to match to the green region on the right profile, and the blue region on the left to match to the blue region on the right.
You'd like for the blue part to kind of stay nailed down and only the green part to sort of stretch up.
The surfacing methods just do not have this sort of subtle pattern matching between changing parts and automatically knowing which pieces to nail down and which to stretch. Instead they will just average the shapes together more evenly which will tend to produce results like you're seeing.
You can't rely on the surfacing tools to do this kind of "sub region of single curve" pattern matching between changing things for you, you have to break things into separate constructions so that you more directly control the matching. That's why the fin needs to be done as a separate piece.
Does that help to explain it a bit more?
Really your project here is not something that I would call a slam dunk to do with NURBS instead of polygons. With NURBS modeling you definitely have to break things down into components and it can be difficult to identify the components in something that has a totally smooth skin at the end.
Wanting to have an output of a totally smooth blended skin is starting to get quite similar to construction of organic shapes like a face for example, it is moving towards an area where polygon/sub-d type techniques can be easier to work with.
Similar to a car, it is kind of a somewhat gray area that I would not term is automatically easier to do in NURBS. To do it with NURBS will involve using much more of the "low level" and advanced NURBS toolset of creating all custom formed surfaces, trimming, blending, working a lot at a surface level instead of a solid level. The things that go really really much more quickly with NURBS than polygons is when you're leveraging all the solid modeling type tools which is not really the case here.
At any rate, in this case and in the case of doing a car, the strategic approach is very important, some strategies like trying to build things incorporating more than one "feature" all at once is not likely to work very well, it is too hard to control that.
I can understand that it would be cool to just draw a few cross-sections through the whole plane including several features (such as fuselage and fin both) and then just do a loft or sweep and have a perfectly formed result spit out automatically... Unfortunately things do not work that way, the sort of "artifical intelligence" pattern matching is just not in there to make that happen. You've got to break things down into components, and recognizing the components is a big part of the strategy that is needed to make this kind of model work right.
It is definitely a difficult model to work with, it is not something that I expect is going to be a really easy and quick thing to do, although once you get a workable strategy planned out it can get a lot easier...
For things that fall more directly into the solid modeling path like more mechanical parts, stuff that may have edges in it instead of a totally unbroken smooth skin, those are the kinds of things that are much easier to do with a NURBS toolset, this does not really fall into that area though.
- Michael
