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Composite Tube Fabrication Methods that Work!

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wsimpso1

Super Moderator
Staff member
Joined
Oct 18, 2003
Messages
11,359
Location
Saline Michigan
I had two reasons to build composite tubes:

Control Push-Pull Rods that won't mess with my internal antennas;

Pass tubes built into the wet wings for control pushrods;

For my pass tubes, whatever I built would have to allow a 1.64" diameter tube to move within it and not touch. I started out with a hotwired foam mold, and vacuum bagged 2 plies of 15 oz Biax, but I could not convince my self that they would be durable in the flexing wings surrounded by fuel. So I went for the next easiest trick. I bought a box of thinwall 2-3/4" polycarbonate shipping tubes from Clear Plastic Mailing and Shipping Tubes from VisiPak. The drawbacks are that they only stock 4 ft length, want big charges to make some longer ones, and you have to fill them with something.

First, I had to couple up tubes to make my 76" long mold, and cap the ends. It turns out that there are PVC pipe fittings that are just about perfect, once you spin them on a lathe. I spun down the caps and spun the OD and ID for the couplings to give me enough room inside - I figure that the couplings will be there forever. This worked great.

To hold the tube in shape under vacuum bagging, I had high hopes for large circus balloons based upon tests I had done with paper towel cores and small circus balloons. It turns out that the balloons stick to the wall so you can not get them positioned. The size difference is just too big...

Plan B was to fill the tube assembly with sand. Stand it on end, stand on a chair, pour in sand until it is full, insert cap. Now the cap has been further modified to have a NPT pipe plug in it. So, you fill and you tamp, and rap on the sides, and get the sand to settle, and eventually you say it won't take any more sand, and install the plug, then you gently lay the assembly on the table, and roll it back and forth, and now there is looseness... After several cycles of standing it on end and spoon feeding it sand and rapping in the edges, and capping it and rolling it on the table, it finally stays tight.

Then set up enough 15 oz BIAX to give me two plies thick on the tube (plus a little), placed it on a piece of visqueen with enough peel ply to go around it once, wetted it out, and rolled my sandfilled mandrel across it. Because this layup has to be fuel tight, I did not apply perf ply to remove all excess epoxy. I just let the visqueen go around the outside of the tube, then put on a layer of batting to get the vacuum well distributed, and bagged it to 20 in Hg. Because of its wieght and low stiffness, it is as straight as the table, and you have to eyeball it straight in the other plane, but that was easy. Let it cure. Nice looking parts. I pulled the end caps OK, but if you are coupling tubes together, just plan on leaving the PC tubes and PVC coupling in. The coupling will never come out, and the ID was bored with that in mind. 40%; epoxy by weight.

For my aileron tubes, an 8 foot fluorescent lamp tube is a great mold. Cheap, dead straight, and comes out easily when done. I waxed it twice and buffed it out. Then I used 3" braided tubes from A&P Technologies Sharx Braided Biaxial Sleevings | A&P Technology - Braider.com | Braided composite, fabric, sleeving & reinforcement. They pull down to about a 30 deg braid angle on these 1.5" tubes. I anchored one end of the braid on the tube with tape - I did not need a glass tube full length. I then tensioned the other end over the end of the the light tube, and put on zip ties to hold it tight. Wet out was conventional. I anchored and stretched and wet out each ply in turn, and then wrapped with perforated ply and batting - a little masking tape to hold things in place helped. Set this light assembly on a piece of bag film and closed it up. It went to 25" Hg, and I set the switch to cycle there. When it had cured, I peeled the expendables off, sawed off the ends of the fluorescent tube, whacked the thing on the table a couple times and poured the remains of the tube into the trash. Nice tube. At the thickness of this thing, I suspect it will be pretty stable, but I am inclined to make four foam-glass sandwich plugs to press into it and pour a touch of thickened epoxy on to fix them permanently. Oh, and they came in at about 36% epoxy by weight.

For my elevator push-pull tubes, I really needed a larger core than a fluorescent tube, and 1 3/4" was very close to minimum weight for an adequate strength tube. And thinwall 1-3/4" tubes 8' long are in the rack next to the 8' fluorescent tubes at Home Depot. I have not built tubes with these yet, but I suspect that the small circus balloons will work fine, and if not, I can always modify some PVC caps and fill them with sand. I have some 3.5" braid sitting here ready to go.

Some ideas that I got here and noodled over but I did not proceed with:

PVC pipe slit with an smaller PVC pipe inside. Starting with the fiberglass ID I needed, the pipe sizes did not nest tightly. Likewise with metal pipes. The folks at Home Depot thought I was nuts walking around with my dial calipers measuring pipes;

Veneer molds per Mark Stull. My father would have loved them. Probably too much to ever cover them with fiberglass... With my fluorescent tubes and PVC tubes, I spent much less time per mold... so I did not go there;

I thought about the thin wall PVC tube, but decided to try the shipping tubes first. Since they worked and an were much lighter, I never needed to go here;

Paper tubes were $125 per set up, and I would have needed two different setups. My thinwalls worked for less money and less weight.

Anyway, that is the story so far.

Billski
 
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