How do I loft this?

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 From:  gareth.ky (GARETHKY)
6392.1 
Hi MoI users! I discovered MoI a few weeks ago and promptly bought a copy. I've been taking a crash course in using it. I'm building a model of an airplane, one I hope to feed into a CNC router to make a plug mould.

I'd like to make the shape of the air intake you see in the photos:


The is a fairly extreme transition. I found that I could get some curves to feed into the Loose Loft command by:
1) Building the profile curves forward and aft of the intake
2) Move/Copy those curves forward/backward so I can construct a pair of lofts that span the transition area.
3) Intersect each loft with an angled cutting plane and produce a series of curves using the Insect tool
4) Delete the 2 lofts
5) Re-loft everything with the loose loft tool.

Then I can loft the resulting set of curves using the Loose Loft function to create the transition. I found that 2 tightly spaced profiles before and after the transition produce the best results:


Not bad but not quite right. You can see the upper surface has a nasty twist at the transition point. Here with the points turned on you can see why:


I have tried a LOT of other ideas: networks, made cross hatching out of blended lines, rail lofts... I'm out of good ideas. I'm happy to go back to the drawing board. How would you tackle this problem?

3dm attached.

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 From:  Michael Gibson
6392.2 In reply to 6392.1 
Hi Gareth, I think that instead of trying to loft the fuselage and intake both at the same time, you'll probably want to do the fuselage as one big unbroken surface, then use Trim to cut out a section of it and work on forming the air intake as a separate piece from it, possibly using Blend to connect it smoothly to the main fuselage.

Check out Mike Maynard's tutorial on "Transitional Recessed surfaces" here which is kind of similar to what you're trying to do I think:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4880.1

That is generally a pretty difficult kind of shape that you're trying to tackle there. But often times an abrupt transition can be better modeled as an initially bigger extended surface and then cutting away some material from that rather than trying to directly surface it all in one big single loft. Sometimes a big loft can work but it's tricky because the more sections you try to use often times applies more sort of "pressure" on to the forum and can make for tortured surfaces.

- Michael
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 From:  Michael Gibson
6392.3 In reply to 6392.1 
Basically your Loft just has too extreme of a shift in shape from this shape here:



To this next one here:




Loft does not itself understand anything about specific features of the curves involved, it just tries to connect them together into a surface by matching them along the length traveled along each curve. With 2 curves having such suddenly different shapes like that, it's going to create some kinds of twisting and buckling in the surface that's generated from them.

So it can be better to try to only build one of those forms at a time and let them get intersected with one another rather than trying to do it all at once.

When a shape makes an abrupt transition, those are the spots where you would have 2 different surfaces coming together and meeting at a common cut edge, possibly with a fillet or blend constructed between them to make a smooth transition. A couple of demos of that general concept here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2164.2
http://moi3d.com/forum/index.php?webtag=MOI&msg=2164.4

- Michael

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 From:  Mauro (M-DYNAMICS)
6392.4 
Hi Gareth:
Loft-Loose option is a sort of "organic" tool...Andrei Samardac did a lot of examples about
You can't rule this tool but just have an approximate control on it
It's hard to get a good rounded transition between to different shapes,even if using same splines with same number of points
My raw attempt is to loft using first and last set of curves leaving out middle complex part
now delete middle curves and draw two lines (red) to cut the edges just in the complex area,so do a Blend G2-0,25 value




now use blend to connect the rest with same value,then join all surfaces
if you want to check joining process is ok activate-show naked edges-and use Dinos Lightning Plug-in to check reflections on surface




I attached the file if you want to have a look,it's just an alternative way,better to follow Michael's suggestions... ;)

M

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 From:  Hans
6392.5 
Hi Gareth.

The question for me is not "how" it is more "why". You are rigth: from the point of design it looks very good.
But sorry, as an old airplanbuilder I have more the aerodynamic look. My english is not good enaugth to explaine you all the secrets
of laminar flow but a good example to compare is the Northrop T 18. You can google some pictures of it.

The inlet stream is properly separeted from the turbular airflow over the fuselage with a little shield.
The inlet hole cut the airstream like a knife and has no direkt connection to the fuselage.
You can see a distance between the inlet and the fuselage.
The engeneers of Northrop made this to have a straigth nearly uneffected laminar airflow to the engine.
It looks like that there is a sharp corner at the inlet aerea of your plane. It will produce a lot of turbulence inside on the way to the engine.
This turbulence minimize the breathing capacity of your engine. More laminar aerflow to the engine - more power.
In every case the turbulence inside the tube to the engine may not be so bad that the engine quit it´s work.
The sharp corners along the frontside of the fuselage have also it´s secrets. If you fly this plane with a high angle of attack this corners
produce a lot vortexes that may have an influence of the rudderfunktion.

The best message to you is that with Moi you are able to redesign this plane for better and safer flying.

Hope to see you at the EAA airconvention with your private jet in the next years

Hans
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 From:  gareth.ky (GARETHKY)
6392.6 In reply to 6392.3 
Thank you for the suggestions and time Michael! This thread looks particularly promising: http://moi3d.com/forum/index.php?webtag=MOI&msg=4880.1
I'll go back and try to re-build the intake using that idea.
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 From:  gareth.ky (GARETHKY)
6392.7 In reply to 6392.4 
Thanks for the blend solution Mauro! Wow, I didn't think it would come out that nicely. And if forms a joined water tight surface. As you say, I will try it Michael's way. That way has certain advantages because I can construct the initial shape with network and fir the fuselage profiles. But I will not shy away from solving these issues with Blend in the future.
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 From:  gareth.ky (GARETHKY)
6392.8 In reply to 6392.5 
Hello Hans, I know what you are talking about: the air intakes on fighter jets are separated from the fuselage. e.g.


So there is one practical reason why I am not doing that shape: I'm making my fuselage in 2 pieces, split vertically down the middle from nose to tail. That shape needs to be pulled from a mold. Such a mold cant have any parts that overlap when viewed from the side (e.g. right parallel view). I'm optimizing for something easy to build.

You can make separated intakes, you just have to do them as separate parts added on to the basic fuselage shape. My model jet will only be doing 200mph max, its probably not worth the trouble. It would look really cool through!

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 From:  bemfarmer
6392.9 In reply to 6392.4 
What is "Dinos Lightning Plug-in" ?

-Brian
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 From:  Mauro (M-DYNAMICS)
6392.10 In reply to 6392.9 
Hi Brian:
i've been on Mike's site where is the link

http://moi3d.com/forum/index.php?webtag=MOI&msg=5222.43

----------
Thanks Gareth !
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 From:  Hans
6392.11 
Hi Gareth,

thank you for the fast answer.
I am glad that you are an experienced builder of aircraft model.

From your point of view you are rigth.
In my past I was a homebuilder of more antik looking lightaircrafts but I like also modern types.

You can see examples of the types I have made and flown with my friends.

Good luck and a funny time of working with your aircraft.

Hans
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 From:  gareth.ky (GARETHKY)
6392.12 
Getting back tho this after the holidays.

I had one last go at the problem with loose loft. I discovered that if you introduce control points into the curves you can get the loft tool to understand what parts of curve A map to what parts of curve B.

1) Use the trim tool to add trim points to the start and end curves. The curves need to be split into the same number of segments.
2) Use the join tool to merge the segments back into one curve. (My guess in the the join tool merges co-located end points into a single 'corner' point that the curves must go through vs. a regular control point)
3) Loft as normal


In this example I made a very small segment right at the apex of the narrow form on the left of the image, breaking the curve into 3 segments. Then I mapped that to the much larger area on the right hand side. So this tells the loft tool that the small area at the apex on the left should be expanded to cover the vertical area of the form on the right. Add a couple of intermediary coppies of those curves, use loose loft and you get this:


This result is not perfect. I had to chop and blend the area where you see the narrow ping band. I used the technique of sweeping a circle along the curve to produce the cuttaway form. My guess is with some messing around with corner point placement I could avoid this altogether.

This result is about 90% of what I'm after and maybe this idea will be useful to someone else. I have more specific control over the flat area where the intake is so I'm going to try a totally different approach next.

@Michael: could we get some visual indication of which points are regular control points and which are 'corner' points. Like maybe a sub menu for 'Show Points' that would set us see just one type and/or different coloration or iconography when all points are shown?

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 From:  Michael Gibson
6392.13 In reply to 6392.12 
Hi Gareth,

> @Michael: could we get some visual indication of which points are regular control points and which are
> 'corner' points. Like maybe a sub menu for 'Show Points' that would set us see just one type and/or
> different coloration or iconography when all points are shown?

It might be difficult to have a sub menu for 'Show Points' because it currently doesn't have any options stage in it. Like for example right now you can just select a curve, then run Show points, and the command just immediately shows the points and exits.

So there isn't currently any menu to add another option to right now.

In the future I want to have an "extended properties dialog" which might be a good place to add various kinds of specialized show/hide markers on objects.

- Michael
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 From:  gareth.ky (GARETHKY)
6392.14 
OK, I tried all of the suggestions in the thread but noting was an exact blueprint for this situation. But now I have a result that I'm happy with:



The Magenta area was produced from a rail loft. Simple surface-to-surface blending produced creases at the corners. I used several curves produced by putting lines along the surfaces and joining them with blend.

The Magenta form originally went all the way to tho top of the large shape on the left. I messed around with various operation like Boolean Diff and so on to try and cut part of it away. This always ended in something odd happening. I was getting strange surfaces with edges that didn't meet up! The shapes are so close that I think its running into the precision of the underlying math. I also tried the technique of sweeping a circle along the intersection line to make a solid to cut with. No dice.

So I just made a curve in the Right view and projected it onto the surface with Iso Curve and used that for cutting. That works perfectly. With that done I get 3 resulting edges on 3 faces that I join to make a loft curve. Using the technique I detailed in my previous post I chopped and joined the profile of the gray colored form on the right into 3 pieces to match. Then I used Loose Loft to make the Cay form with 1 intermediate control curve. You can see the results of the chop/join in the paths that the Yellow lines take. If you just loft the curves you won't get a result like that.

I still have to do this on the actual model where things are /slightly/ more complicated but I think I have a workflow that will scale to that complexity.

Thanks for all the help everyone!

I've been hacking at this for 3 weeks and I'm super happy I got a solution!!! :)
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