View Full Version : Nurbs Modelling an Aircraft

02 February 2007, 06:54 PM
Hey whats up? Kind of new to the CG Talk forum. I am currently modelling a P-51 Mustang from some 3-up views with Nurbs. I'm finding Nurbs to be good at some things bad at others. I am trying to do a parametric model, meaning that all the seams fit perfectly y and fit exactly to size. The reason I'm doing this is because I'm going to be putting every working screw into the darn airplane as a training aid to help people maintain these older birds.

Anyhow, I'm running into some issues and would really appreciate some comments and maybe even some pointers as I go on Nurbs Modelling as I cannot seem to find ANY tutorials online. And NURBS is a difficult thing to master.

The main Problem I'm having is trying to get the smooth transition from one peice of Sheet Metal to another for certain parts.

On the other hand there are other aspects of it I don't like, for instance, this next image wich shows 2 peices of sheet metal that should retain the exact shape here because they are made by the exact same nurbs line, just copied and left in the same place. but for some reason they do not match. Do I need to convert the line of the surface? I've heard joining them can help. Please let me know what you think.

02 February 2007, 11:20 PM
Hi! I would definetly abandon nurbs for polys, it will be all clearer.

Monica :)

02 February 2007, 09:36 AM
This may help:

"mrys" has modelled aircraft with NURBS for many years and has recently changed to polymodelling, so you may want to try that instead.

03 March 2007, 08:11 AM
I've done a fair amount of both nurbs and poly modeling, and I wouldn't say either is better than the other. They both have their place. Pollies tend to be a bit faster, and nurbs a bit slower, but some would argue that nurbs gives you a better model in the end.

Anyway, moving away from the nurbs vs pollies debate, you're going to need to become very familiar with the attach/detach commands, rebuild surfaces command, stitch edges command, and global stitch command. Oh, and you'll be deleting history on your nurbs patches a LOT. You'll probably want to set up hotkeys or shelf buttons or something for some or all of these.

This is a big topic, but here are the main points...

1) When you want to attach 2 nurbs surfaces make sure they have the same number of isoparms. You can change the number of isoparms a surface has by using the
Rebuild Surfaces command. You can use either 0 to 1 parameterization or 0 to # of spans, it's personal preference, but keep it consistent throughout the whole model.

2) To get a seamless flow between 2 surfaces you can use a couple methods. You can use the attach surfaces command, and then detach them along the isoparm where they were attached. You can either select the 2 surfaces, or the 2 bordering isoparms, either works but occasionally the surfaces will get screwy and you'll have to attach via the isoparms. You can also connect surfaces with the stitch edges tool.

3) You don't need to get everything seamless by attaching or stitching the edges like I just described. That's partially because in general you CAN'T! Not without the next step at least. Once all your edges and corners are fairly close you can use the Global Stitch command to snap everything together seamlessly. There are a couple ways of doing it, but in general i select all the surfaces that I want stitched together, then I'll execute the Global Stitch with its default options. You probably won't see much of a difference, so open up the attribute editor immediately after executing the command, and increase the "Max Separation" attribute of the global stitch. You should see your surfaces starting to snap together, but watch that you don't go too high or else you might see surfaces snapping together that you meant to keep separate. Once you do this, you may be able to improve the stitch by executing the command one more time, one again bringing up the max separation. Oh, and one more important thing.... in the global stitch options use the radio buttons to the right. I believe they are 'Closest knot', 'match params', and something else I can't think of right now. But yeah, use all the options to the right, so I didn't mean to throw you off by saying to use the defaults. You can use the defaults for the other options though!

4) When attaching or stitching surfaces/isoparms, always rebuild the surfaces (with the option checked to keep the NumSpans), and delete the history. If you don't, things will get real screwy and you will have big problems hooking it all together.

If you have any problems let me know. It takes some time to get used to the whole process, and there's a lot of repetitive attach/detach/rebuild/stitch/etc, but after a little while you'll get used to it. You can let me know if you have any problems or need any more help.

Good luck!

03 March 2007, 11:21 AM
at the frist i will read the topic

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