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Old Nov 01, 2010, 08:33 PM
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If Its a simple wing I do something different to get a sheeted skin:

I take all the curves that form the loft, and offset them: Then loft those as well to form a thin walled 'tube'. Then cap it. Like you I find surface offsetting is a complete lottery. Especially if there are any sharp edges.

Over the year or so i have been using Rhino, I find that curves work better than surfaces, and surfaces work better than solids.

Today I wanted to make an embossing tool. Letters to 'put relief lettering into something. I wanted softish corners.

I had to start with letters as curves, soften the edges with fillet corner, then extrude to make the tool and then, because I had got rid of all the sharp edges, the blend edge command worked to round off the face of the tool.

Likewise I wanted to make a rounded off knurled ring..start with a rounded corner rectangle, revolve that, then draw a circle make a helix round the curve (quick way to get a sine curve, project that back to the Cplane, then extrude as a surface, and split my ring with that..

Working with Rhino is like being in an old fashioned machine shop where you haven't got the exact ones you really want.. You have to be a bit creative.
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Old Nov 01, 2010, 10:23 PM
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Quote:
Originally Posted by cpdude View Post
1) Loft the outer surface of the wing
2) Offset the outer surface to create the inside skin surface (either as a solid or separate surfaces??)
3) Extrude surfaces through the "wing" for each rib.
4) Generate curves from the intersection between the extruded surfaces and the offset skin for the ribs. Brian
I think you've got the right approach. The devil is probably in the details. I just did a quicky favour for a buddy who wanted to make a fuselage turtle deck using formers on an existing model. Somewhat similar to your project. All he had was a desired eliptical-ish front/rear former shape, basic planform dimensions & desired stock dimensions like 1/4" balsa top cap & 1/8" skin on the sides. So I:

- started with the front & rear (outer) former shape
- extruded those together to make a surface
- similarly extruded the inner lightening hole (as a surface)
- drew bunch of surface planes intersecting these 2 surfaces
- found the intersections
- this yielded the 'formers' as line entities
- I did these on different layers for control & selective visibility

Some differences re your wing

- by offsetting your outermost wing surface inward, of course you are going to get 'crossover' trailing edge surfaces. The extending portions would have to be trimmed to a single line/curve representing the TE.

- pay attention to the LE (nose) of the airfoil. I prefer to have a 0,0 type x,y point to align with my wing planform centerlines. It makes subsequent operations easier especially if you have twist/reflex etc.

- starting with a bunch of imported ribs from another cad package... although not necessarily bad, it could be an issue if the curves were not clean to begin with. Maybe you can elaborate on this. If its the same airfoil just tapered/conformed to the planform I'd recommend just importing the coordinates as x,y points, draw your own rib curve using (much better) rhino tools which can smoothen & many other tricks. The develop the surfaces from there. Yes, you still want 'ribs' placed for surfacing control, but Im saying better yet is to use the rhino 2D scale command to develop them down the wing panel from a 'good' rhino curve vs trusting imports.

- even if your wing is a blend of different airfoils at different span staions, rhino surfacing tools are much more powerful to work them into the desired shape. But if you have ill-defined 'un-smooth' airfoil datapoints, that can cause some headaches in surfacing.
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Old Nov 02, 2010, 12:02 AM
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United States, CA, Cameron Park
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@Vintage1 - Curves...that reinforces my experiences with Rhino. I'm thinking that it'll also be easier to offset the ribs if they are split into top and bottom halves--they start out that way from Compufoil anyway.

@ptxman - Yes, you are going to get 'crossover' when you offset toward the inside of either a curve or a curved surface. That I can handle! It's all of the broken-up and overlapping surfaces that, for me, are a real pain to deal with. The ribs...I'm pretty certain they are okay! Meaning no overlaps or breaks. Every rib is different (airfoil, cord and twist). I could do this in Rhino. That's something to think about.

Thanks for feedback. I really appreciate it!

Brian
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Old Nov 02, 2010, 09:30 AM
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United States, CA, Cameron Park
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@Kent - I've been thinking about what you said about not using "intersection". By that did you mean the Intersect command (Curve>Curve From Objects>Intersection)? With two exceptions the Intersect and Section commands seem to do the same thing. The Section command only creates a temporary intersecting plane and the resulting curve is always planar. I would think that creating "permanent" intersecting planes (surfaces) and putting them on a different layer would be a good practice. Thoughts?

Brian
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Old Nov 02, 2010, 09:47 AM
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Quote:
Originally Posted by cpdude View Post
@Kent - I've been thinking about what you said about not using "intersection". By that did you mean the Intersect command (Curve>Curve From Objects>Intersection)? With two exceptions the Intersect and Section commands seem to do the same thing. The Section command only creates a temporary intersecting plane and the resulting curve is always planar. I would think that creating "permanent" intersecting planes (surfaces) and putting them on a different layer would be a good practice. Thoughts?

Brian
We are talking about the same 2 commands.

Section is simpler and QUICKER. It creates new curves (your part) at the specified location.

Intersection can create massive amounts of new curves and points, many of them un-wanted, through out the entire model. My machine crashed yesterday after waiting 10 minutes for an intersection command to complete.


Try this. Create dozens of curvy surfaces that intersect with the copy and rotate commands, then run intersection on them. Back-up your file before your do....

then try cutting a section thru the same surfaces.

Kent
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Old Nov 02, 2010, 09:58 AM
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Section from objects

When things start to get complex, use section rather than intersection.
There are about 120 layers to keep track of everything.

Kent
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Old Nov 02, 2010, 10:08 AM
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Quote:
Originally Posted by cpdude View Post
@Vintage1 - Curves...that reinforces my experiences with Rhino. I'm thinking that it'll also be easier to offset the ribs if they are split into top and bottom halves--they start out that way from Compufoil anyway.
Yep. If you have - say - a wing section or series thereof, try this.

Project the whole lot back to the Cplane.

Offset each curve to get sheeting outlines.

Add in lines for spars and LE and so on.

curve boolean various bits - LE is one , spars are another, sheeting is a third, ribs themselves are a fourth.

extrude the ribs and pop them in the right place.

pop the various loftable entitites in the right place, and loft or sweep them.

I find this is faster than trying to offset sheeting:

Oh, and if doing compound curve sheeting, don't be clever. Do a straight loft between two places only, so the curve is not compound, and unroll that surface ..If you are feeling lucky, join the unrolled bits together afterwards.if they dont line up exactly, well you know you have a problem in sheeting the thing anyway.

.
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Old Nov 02, 2010, 11:04 AM
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Sorry but some further questions ... did you close each section spline before extruding ( the different meshing sequences seem strange to me) and the problem with the trailing edge should also not occur then
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Old Nov 02, 2010, 11:38 AM
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United States, CA, Cameron Park
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@Harpye - I'm not sure that I understand your question. What do you mean by "section spline"? Are you talking about the airfoil shapes (curves)? If so, I believe they are closed. Is there a good way to check? Also, I'm lofting the surface using the airfoil curves. You mention extruding? Back to the closed curves...I don't believe I could loft if some of the curves where open and others where closed. I'm very certain that they are all closed.

By the way, I'm able to offset the individual ribs (curves) today...and the trailing edges don't overlap. That's good! Offsetting he surfaces are still a problem though.

I now think that offsetting the skins is a bad idea!

Brian
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Old Nov 02, 2010, 01:46 PM
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It is about the airfoil shaped ribs ( sorry ) ... When I normally extrude the lofts are interconnected with many "parallel" lines see pic 1

Doing a network from curves different areas may have different density of parallel lines see pic 2


regarding the offset surface I have to say that it absolutely has its limits, when it comes to narrow curves or sharp edges in relation to a relatively big offset value !!
the problem is that the algorithm seems to move the surface's control points along the points normals and so would create a new surface.
Doesn't work when the normal vectors cross each other what finally could be the reason of the fishtail of your wing.
The 2 D algorithm is better - so get 2d Offset curves from the ribs and afterwards create a second loft.. picture 3
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Old Nov 02, 2010, 02:09 PM
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Originally Posted by Knoll53 View Post
Section is simpler and QUICKER. It creates new curves (your part) at the specified location.... .Intersection can create massive amounts of new curves and points, many of them un-wanted, through out the entire model. Kent
Yes, good point(s).

But I think by pre-displaying only the surfaces of interest prior to intersections via layers on/off (say the skin surfaces in the case of a wing) you can potentially minimize the "massive unwanted new curves" to those that are meaningful to you. It may take some planning, but nothing saying you have to intersect the entire model. I would think the 2 operations are closely related in what happens behind the scenes processing wise? For example, there may be times when you want to slice through a servo or spar, or times when you dont. But I think thats controlled by the exposed layer, no?

Correct me if Im wrong, but I think sectioning in rhino terms means 'as a single plane' + 'from a particular view perspective'. It extends a desired section line placement through the surface model to yield the 'section'. Thats fne, but you have to confirm the plane of reference also makes sense to the desired end result. If for example you had a wing with dihedral & sectioned from the top view perspective, those sections would not equate to ribs because they slice through the wing at its dihedral angle. Intersections can be between any type of plane surface existing at any desired orientation. So like most cad things, depends on what you want to accomplish.

/pt
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Old Nov 02, 2010, 03:40 PM
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CP: got an Email but your host refuses to take it ... would you have an alternate address?
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Old Nov 02, 2010, 06:10 PM
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You guys want to play with the contour command.
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Old Nov 03, 2010, 11:52 AM
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actually want to play with this .. just guess what plane...
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Old Nov 03, 2010, 04:08 PM
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Quote:
Originally Posted by Harpye View Post
actually want to play with this .. just guess what plane...
The back looks like a Focke Wulf Flietzer to me
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