First post, and sacrilege already

[BobbyBreadwinner]

I'm wondering why go through the trouble to precicely cope the tubes if you're assembling with gussets anyway? 

That has been discussed by the coffee group at our EAA: One could argue that the copes become a secondary load path under compression, or they straddle the tube preventing movement side to side, or if you needed to you could add epoxy to the joint, or it is a starting point for a welded structure, or at the very least if you have the something in the wrong place it will definitely not go together. Final answer: it just looks cooler.

I have used this system to create assemblies that were welded (larger aluminum tube assemblies, engine mounts, non-gusseted assemblies, etc.) so the copes were necessary. However, back to the original topic...

It is kind of inefficient to use a bunch of time to accurately cope small tubing. In my humble opinion, 1/2 inch is the tipping point. Anything smaller and you can just straight cut at the correct angle and fill when welding. At 1/2 inch you need a little bit of cope, but close is good enough. At 5/8 inch tube your TIG welds start to look like ugly MIG welds if you have sloppy copes. At 3/4 and up, coping accurately is necessary and really shows in your final product. Almost the whole fuselage of the Legal Eagle is in the 1/2 to 5/8 inch neighborhood. So whether you CNC cut or hand cut you end up with about the same aircraft. I personally would rather CNC cut tubes because it makes welding a bit easier: I am great at cutting and grinding, I am not so great at TIG welding tiny objects (thank you old eyes and shaky hands).


I hope this helps,

Robert

[rv7charlie]

Charlie,

I am going out on a limb here to say that I am likely the only one who can answer your question with any experience: I have a CNC router, with a tube cutter, of which I use to cut aircraft tubes. I have written specialized software to perform the coping calculations which result in CNC files. And yes, I am the guy who has cut the tubes for the current builds of Barnaby Wainfan's Facetmobile. Those details are here:

https://worktablecnc.us/projects/facetmobile.html

I have also built a Legal Eagle the traditional way: With two angle grinders, one with a cut-off wheel and one with a grinding wheel, I have cut, ground, ground, ground, and ground until the tubes are all coped perfectly together. I did the welding using TIG, and any gaps left I had to bridge with extra filler. Those details are here:

https://worktablecnc.us/projects/legaleagle.html

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If I had to build a second Legal Eagle...

I would probably use the CNC. It would take more time. I am a builder, that does not bother me. I enjoy building, solving problems, being creative, and ending up with a product which reflects the very best I can do.

If I needed to get it done quickly...

I would cut it by hand just like I did it before, and then blast it with a MIG welder.

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* There is space enough in this community that either way is the correct way. You do you. *


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I do have a few additional comments:

+ If you already have a RV-7, then you already have an aircraft with a second seat that most of the time will be empty. If you are looking for a simple after-dinner flying machine then please reconsider the single-seat Legal Eagle. It will be an ultralight, no registration, lots of flying examples, easy-peasy. I have a lot of friends which are RV builders/pilots. A common mantra with those guys is More-Is-Better. Legal Eagle = Less-Is-Better.

+ I previously looked at the VR3 Engineering requirements (for the rest of the group VR3 Engineering is a Canadian company which provides CNC cut tube kits, and they do not exclude aircraft). You can either simply buy one of their kits, or you can send them your own files. However, creating your own files is no small task: You have to take every tube, drawn and coped in 3D, then slit the 3D tube, flatten it, and provide a 2D pattern. I use CAD for a living; this is a mountain of work. Whoever did all of this originally for the Legal Eagle patterns at VR3 deserves a big hug.

+ No, I do not provide CNC services with my router. I only helped out a few of the guys at my local EAA chapter, and Barnaby is now one of them. The time I have spent on their projects I could have completed another aircraft of my own. I have a list of projects I would like to do which exceeds my lifetime, and becoming a tube kit manufacturer is not one of them.


Respectfully,

Robert (BobbyBreadwinner)
Oregon, USA

https://worktablecnc.us

Hi Robert,

First, sincere apologies for not responding until now. I somehow lost track of my own thread here; apparently I haven't set up email notifications for posts to threads I'm supposed to be following.

I love what Barnaby's been doing with low aspect ratio stuff. I helped kill the grass around the Facetmobile at OSH, and I've been wishing for 2 seat plans ever since. I've prowled around your website in the past; truly amazing stuff!

Your advice on hacksaw/file vs cnc matches my neighbor's; he's built several tube/fabric a/c & talks about knocking out a fuselage in a week. ;-) I just need to order the plans and get on with it.
Charlie

edit: On  coping the tubes accurately, I learned the importance when I built a couple of hangar doors from 6" aluminum irrigation tubing. The 1st door was built with just gussets, and it's a bit of a floppy mess. The 2nd door I used printed templates to cut accurate fish mouths, and left material so I could bend tabs. Used sheet metal screws to effectively 'weld' the fish mouths to the adjacent tubes, and ended up with a much more rigid and strong door, with no gussets at all.