Compression tube construction

[PoconoJohn]

I'd prefer to make my own compression tubes, but I know nothing about them.

Does it require any special tools or equipment?
Exactly how are they built?
In hindsight, would you do it again?

I'm going to buy the fuel tank. If I need to buy compression tubes, I'll buy everything in one order; so I'm waiting to learn about the tubes.

As always, thanks!

[dougm]

There are compression tubes in the wings as well. Some (like the ones in the C/S) have threaded plugs, some have a PEM nut or small threaded plug, some no threaded plug. I have all the pieces for mine, but haven't welded them up yet - they are fairly straightforward. Probably the more challenging piece is making the threaded plugs. It's not that hard to do, but if you don't have access to a lathe then you're kind of stuck. Some folks on the forum may be willing to help you out with those, so it doesn't hurt to ask.

Essentially, they are 4130 tubes cut to a specific length with the appropriate fitting(s) welded on the ends. Take a good look at the plans. While your question is targeting the Center Section, the type of fabrication you will need to do also applies to the compression tubes in the wings, slave struts, Cabanes, N-Struts, push tubes for aileron & elevator controls, etc.

[Bitshifter]

John,
As Doug said there is nothing special about the center section compression struts except you do need to make the threaded inserts as they can't be sourced locally. The compression struts for the wing are only special in that each end needs to be square and aligned with the other. I found that a few welding magnets and a metal plate to weld on will keep it square while you tack them. I have welded the center section struts and the top left wing struts and am happy that I did it myself. You will have a lot of welding to do in the future and they are great to practice on. I should have practiced a little more first as I had to remake one of the struts and only ordered the material that I needed.
I thought long and hard about buying a small lathe to make the inserts but in the end I got a local shop to make them. I looked at the plans and just couldn't justify the lathe, love to have one though.

[mtaylor]

I ordered a 12" piece of round bar stock that was just over sized of the tube ID. Took it to a local fab shop and had them turn it down for a snug fit into the tube end, then had it cut into the length called for in the plans (I think 5/8ths or 3/4"). Then they center drilled and tapped them for the proper bolt size (chamfer the holes). Didn't cost much. I drilled a 1/4" hole through the sides of each tube about a quarter inch in from the tube end (drilled into one side and out through the other side), inserted the lugs into the ends of the tube and then did a rosette weld at each 1/4" hole and welded the end of the tube to the lug. Leave some of the lug hang out beyond the end of the compression tube and make the tube a little long so you can grind/file off a little at a time to get a good snug fit in the wing. Other wise, if you're too short, just use thick/thin washers.

[PoconoJohn]

All very helpful. Thank you. Maybe I'll give it a try.

[jwhanson]

I have a suggestion on making the plugs for the ends of the compression tubes.
Bore the other end of the plug out about 3/8 of an inch deep so you have a wall thickness of about .060. Then locate the holes in the tubes so the rosette welds line up with the thin wall of the plug. It will be much easier to make the rosette welds with two thinner layers to heat rather than having to heat a solid plug. This will prevent your compression tube from burning away before the plug gets to the melting point. If you are TIG welding this might not be a problem. If you are gas welding it will be easier. This worked very well for me using gas.
I would also suggest not making the compression tubes until you have the ribs on the spars because, depending on how accurate your spar thickness and plywood plates are, the actual length could be slightly more or less than the tube length on the drawings. Some operations are 'cut and fit'.
John Hanson HC-012.

[MarkM]

Hello John,

I guess I'll back up what some of the other guys are saying about this. First, about the threaded plugs. I bought some 4130 round bar stock, and had a local machine shop make the plugs, using the drawings on the plans. I also had them make plugs for the cabane and interplane struts, the elevator idler and pushrods, the aileron pushrods, etc. (of course, noting the tube sizes for those other things). Instead of messing with PEM nuts, I just had additional plugs made, identical to the ones for the center section tubes. For the TB-1 and TB-3 tubes, I just rosette welded the threaded plugs in place, and then welded the assembly to the flat end fittings. A little heavier, but MUCH easier, and stronger too. All of the plugs together cost me less than $200. If you have a friend with a lathe or who works in a machine shop, so much the better.

As for John Hanson's comments... this is a good way to do this if you insist on welding with gas. I used TIG, which having tried both I find is infinitely easier and more precise, and ideal for welding pieces of dissimilar thicknesses (like joining these plugs and .090 fittings to .035 tubes.) If you are having any thoughts at all of welding your own fuselage, or just want to try welding to decide if you're up to doing the fuselage, these tubes, the spar fittings, and the aileron bellcranks are a great place to start. They are simple, and not too much material and time is lost when (not if) you screw one up and have to make it over again. By the time you are done with the compression tubes, you'll be making some welds that you'll feel pretty good about. I know I did. And of course, take examples of your work to your tech counselor for critique. A great way to learn!

Attached are pictures of my homemade jig for the compression tubes. It is sized for the longest tubes (TB-2), and I simply used nuts to adjust for a smaller size when welding the shorter tubes. It's made from a length of square tubing from the Home Depot, and a couple of flat scraps. It took a couple of hours to build, and worked great for all of my compression tubes.

Good luck,
Mark

[dougm]

Nice jig, Mark. I think I'll use that when I get around to welding my tubes.

[PoconoJohn]

Thank you for all the replies. Thank you Mark for those photos. I decided to go with gas welding...bought the equipment and took the EAA course in February. It'll be a long time before I get to the fuselage, so I'll be practicing a lot and I'll give the compression tubes a try. I had been thinking about making the fuel tank also, but after seeing other's try it, I decided I better just buy it.

[M Lightsey]

Great looking jig.
On NX1279, we took a different approach. We made up the ends with just under half required tubing length. As the wings were being assembled, the tubes were Bolted to the spars then joined in the middle with a simple fishmouthed outer sleeve.
Really simplified the alignment/length issues.
Mark