Newbe could do with a bit of advise

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 From:  Bazzer (BARRIELEVER)
8094.1 
Hello All

I am a very raw begineer at MOI, I have had some success and got things right through to being machined.

However I am trying to model a simple tailplane for a model aircraft, my problem comes at the wing tip, I have scaled a common section with seemingly plenty of stations near the tip, but I still get a nasty looking hard edge in the surface. I am lofting down through the sections.

I have attached a red lined image showing where the problem is.

Any pointers to getting a nice surface would be appriciated.

Regards

Barrie
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 From:  Michael Gibson
8094.2 In reply to 8094.1 
Hi Barrie - it often happens if you try to build a single surface that has a lot of shape change in a small constrained area (in this case the bend in your leading edge) that you'll get that type of effect. The change condensed down tends to result in a type of bump which if narrow enough will look like a hard edge.

Then also things that collapse down to a single point can also be tricky as well. With a wingtip you have both of these things happening at the same time so it can be a difficult thing in general.

You can get a more gradual shape transition by using the "Loose" option in loft but that will come at the expense of accuracy because the generated surface will only be generally guided by the curves rather than going directly through each section.

I would probably attempt to do a sweep or network and see if that behaves any better, they can sometimes be better for things that collapse down to a point.

- Michael
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 From:  Karsten (KMRQUS)
8094.3 In reply to 8094.1 
Hello Barrie,

I have tried to find a simple solution for your problem. It is not perfect, but maybe a startpoint. I have extended the main surface (blue). Then I have made an extrude (red). After that I made a curve for the extrusion oft the cyan surface. Then trimmed the blue one. I have add a trimmpoint to the edge of the blue and made a blend between blue and the red surfaces segment (bulge 0.6). The result of the blend is green. After all I have made a sweep with profiles from the edge of the green and your main surface (don't forget to insert also a trimpoint) and two rails from red and blue. The resulting magenta may have some deviations in continouity, but I thing very small. You have to play with the shape of the cutting surface and the bulge to find a solution that fullfill your needs;-)






Have a nice day
Karsten

p.s.: You should avoid the small patches in your base surface, that would make your life easier;-)

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 From:  Michael Gibson
8094.4 In reply to 8094.1 
Hi Barrie, I played around with this for a while with no good luck and I think you may not be able to get rid of that lump with your sections in their current shape.

I think the problem is in these sections here:



This one is I think too high for this particular surface construction:



To get rid of the bump the sections will need to make a more gradual transition in the area of the lump, I think that will mean either lowering that indicated section a bit, I'd try just slightly scaling it down in z a bit, or adding in even more sections so that there is more of an even transition between them if you need super high accuracy to some existing design. Too many sections can also easily add bumps as well as it adds a lot of pressure on the generated surface, if the transition between sections is wiggly in any way.

It does not take very much compression of change to result in a visible bump is basically the problem.

- Michael

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 From:  Michael Gibson
8094.5 In reply to 8094.1 
Hi Barrie, so the way Karsten writes about above helps to make higher quality individual surfaces because each surface is more relaxed and isn't trying to go through a significant change of shape in a small area. For a technique like that areas that are tightly curved would be constructed as fillet or blend surfaces, and fillet or blend surface cross section stays with that tight bend as it travels along its length, so even though it has a tight bend in it, its cross section does not rapidly change.

The tricky thing about that technique is that the big broad lightly curved areas need to be built out of larger quadrilateral sheets that then are going to be trimmed to cut off excess area where you want to have a transition in shape. It can be difficult to get this extended shape if you're working off of something like digitized physical data because the extended areas just aren't present in the physical model itself.

If you are constructing something from scratch it can be good to have it set up that way instead though, where each broad section of your shape starts out with an extended, simplified and relaxed surface sort of defining the ideal extended shape of that zone of the surface, and then some type of intersection (either through filleting which has an implied intersection and cutting with it, or by direct manual trimming) cuts away some portion of that extended broad shape to get the final result.

Anyway that's usually the preferred way with NURBS modeling to generate the most relaxed and highest surface quality.

- Michael
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 From:  Bazzer (BARRIELEVER)
8094.6 In reply to 8094.5 
Hello Michael

Thanks for the advise, one of the things that I often find when working with CAD systems, I also use Geomagic Design and BobCAD is understanding what the CAD system is requiring behind the scene.Of course this is where MOI is exceptional in that we can communicate with you with your intimate understanding of what is going on within the programme.

Many Thanks

Barrie
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 From:  Bazzer (BARRIELEVER)
8094.7 In reply to 8094.3 
Karsten

Thanks for your advise, you got a nice result.

I think part of my problem is that I set out boundaries for the plan view and sections and basically I am thinking that I will stretch out a surface over those wire boundaries.

You have no worry about flying extended surfaces past these boundaries and trimming back.

Many thanks

Barrie
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