Hi guys

Wonder if anybody knows how to model this windscreen of an aircraft with nurbs.

I've tried loft, sweep and network without success.

This is a seemingly simple object to model with nurbs but it is beyond me how to do it and make all look smooth.

If anybody has any suggestions they would be most welcome.

Network?

Hi Frenchy, thanks for responding

That is what I want to do.

When I use network on the 5 seperate lines I get this result. I'm using the latest v2 beta.

One possibility but there is surely better with another functions ;)
For these curves 1 2 3 4, Network is better than Sweep

Maybe with drawing a section curve between "middle of 1" to "middle of 2" and some sweeps 2 rails
Wait and see :)

HI kevjon here is a way to do it join the curves remove corner points sweep 2 profiles with 2 rails and scaling rail, trim
and trim surface edge to sweep the small window with 2 rail

@lyes You win cool idea!
Except the fact that with your file you can't Shell the result? (bug?)

I have made more simple from your idea ;)

Hi Kevin, yeah really the key concept is like lyes shows there - to build your form as a larger extended sheet initially, then trim or boolean it back to slice out areas. That's instead of trying to build a form to every single final 3D edge directly.

It can be a bit of a difficult shift to get used to if you have spent a lot of time building things with polygons, since you tend to very much more often build directly to the final edges with polygons, and never tend to use many booleans.

With NURBS it is very much the opposite, you'll generally want to build things in a simplified larger structure and then cut or boolean pieces out to make the final outline. Several of the edges in your final model will then be the result of intersections between other surfaces and profiles instead of drawn directly by you.

The general idea is to try and draw more 2D planar curves that are easier to control, and then get more 3D results by intersecting those 2D curves with things. That tends to be how stuff comes together quickly using NURBS.


lyes shows the cutting curve coming from the top, and another option is to do one from the side as well, you can make a flat planar cut by drawing a line as the side profile cutting object.

- Michael

Frenchy and Lyes thanks very much for taking the time to respond and have a go at it.

Michael, yep I fully understand what your saying. The problem I have with creating this shape is that I'm trying to model this windscreen accurately. It is not for a fantasy object but a real aircraft. While Lyes solution at first glance seems to be a good one it is not giving me the flat windscreen that I want to achieve but a curved windscreen that follows contours of canopy shape. In the side view shown it needs be straight line and not curved. Cutting out the windscreen and sweeping it again is only restoring the original shape So Frenchy's very first post is closer to what I want to achieve (although not ideal) due to the extra flow line added to the shape associated with the sharp corner which will require lots of tidy up in the polygon mesh generated.

I guess this is just another limitation of working with nurbs, that I have to accept.

Not very sure that I actually used any of the forgoing methods exactly?

Alternatives/alternatives!

Brian

I wonder why my wife locks the doors so I can not get out and wander the streets!??

Brian

Hi Kevin, sorry I missed your last post here.

You wrote:
> While Lyes solution at first glance seems to be a good one
> it is not giving me the flat windscreen that I want to achieve
> but a curved windscreen that follows contours of canopy shape.
> In the side view shown it needs be straight line and not curved.

If I understand correctly, I would still recommend using the basic approach that was shown there of building a larger and more simple structured surface first and then cutting it.

But if you want to have a perfectly straight line as viewed from the side view, instead of cutting from a shape in the Top, draw a line on the side and use that as the cutting object.

Here are some visual steps.

First sweep the larger shape same as lyes showed - this is with the 2 bottom parts as the rails and the central part as a scaling rail. That produces this:



Now switch to the side view and draw the straight line that you want to have as your exact straight line profile, like this:



You can now select the sweep, and then use either Edit/Trim or Construct/Boolean/Difference to cut the sweep by the line. Here I used boolean difference which cut the sweep into 2 pieces:




Delete that interior part that has the curved region that you don't want to keep, that leaves you with this:



As viewed from the side that edge is completely flat:



Now to surface that interior piece - if I understand you correctly you do not want to have any curvature there, so instead of sweeping a curved profile along it, draw a line at the bottom of it between the ends there:



Then select the line and the edge and use Construct / Planar to build a planar surface there:



The result is attached here as Canopy_flat_side.zip


Is that the result that you were looking for? If not I may need some more details.

- Michael

Hi Michael
Why in your model there are crips?
Could you have shell now?

Curious in my model I have a more thickness of lines?? I don't know why?
Ps Of course I have not the Plan Surface of the little part inside my previous model ;)

Hi Pilou,

> Why in your model there are crips?

That's a display artifact that can happen if you have the mesh density set to lower. With a smaller number of polygons the triangle structure can be revealed.

If you want to eliminate those, go to Options / View / Meshing parameters, and check the "Add detail to inflections" option, which will create a denser mesh and should eliminate the visible triangles there - does that solve the problem for you?

Basically though that is not really any defect in the surface at all, it is just a display issue only.


> Could you have shell now?

I don't think it will shell properly - there tends to be a problem with matching up corners when attempting to shell an open surface that is made up of multiple pieces that have sharp corners in them.

Shell will tend to work best when used on single surfaces that are all smooth, or it can handle corners better when you shell a closed solid. A non-solid that has corners can be problematic, there was some discussion on this previously here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=133.11

I believe this case will run into the same issue as in that link, with the surface extensions not being done properly.

For cases like this you may need to use the Offset command to construct some individual offset surfaces to build the thickened pieces manually. Or possibly a good approach would be to use Edit/Separate to break the model into individual surfaces, and then use Shell on the separate surfaces to get some pieces to work with.

Or actually now that I think about it some more probably the best option would be to shell the original sweep surface before cutting it.


> Curious in my model I have a more thickness of lines?? I don't know why?

Probably you've got some duplicated pieces there, or duplicated some of the edge curves as new separate curve objects.

When you have several curve objects stacked on top of each other they will get a bit darker like that.

- Michael

Thanks Michael,

That is a really good solution to the problem. Just trying to figure out what shape the green line needs to be so my windscreen shape matches the real aircraft when the line on the side is cut is the only problem. I guess it is just a trial and error process.

Hi Kevin, yeah you may need to edit that curve to tweak the result to what you want.

You can use history to help with that - currently history will be broken after you do the trim or boolean operation but you can use it immediately after you do the sweep to help fine tune the results.

The way that it works is just after you do the sweep, select that center curve and turn on its control points by Edit/Show pts. When you move the curve's points around the sweep will update so you can adjust the shape of the curve and see right away what the effect is on the sweep. You have to just visualize what the result of the cut is though, that part does not work with history yet but I do want to improve this in the future.

I mean you still need to edit points for the NURBS construction method, it is just that the focus is a lot more on editing points of 2D curves rather than editing points of a 3D cage mesh.

- Michael

Then another piece is if you want to blend off the sharp edge you can put a fillet on this edge:



I'm not quite sure if you actually wanted a totally flat plane in there or not though.

- Michael

Also Pilou, if you wish to shell this it will shell more easily if you fillet that edge. Shelling of smooth pieces tends to reduce a lot of problems.

- Michael

< Artefact
I had a premonition about that :)

< does that solve the problem for you? "Add detail to inflections"...
Surely but as I am always at same default regulate, "add details" disable, it's the same for your model so pertubating :)
Because mine has not!

< Probably you've got some duplicated
He he
It was just the start generator curve of the original of Kevin's model so resolved :)

And yes with the fillet Shell is some better!

>You have to just visualize what the result of the cut is though
This is the problem working with nurbs when you are trying to model stuff that looks like the real thing and you have to define the overall shape first and then do booleans rather than doing it the way I was on the first post of this thread which is define the shape of the object with curves (get that part looking right) and then create the surfaces.

For example the fuselage of the F9F panther (see pic) is quite simple (perfect circle) but trying to define the shape of the cross section curves so that when you boolean out the cockpit opening so it is the right shape and size is a bit of geometric puzzle due to the subtle curvature extending from the fuselage up to underside of the canopy (particularily around the rear of the canopy).

>I'm not quite sure if you actually wanted a totally flat plane in there or not though.
It very slight (see photo) but thanks for showing the idea of how to address this area.

For the canopy area as you mention, you could build the shape first, then cut out your inside section.