Composite Tube Fabrication Methods that Work!

Ok, this thread has drifted far enough.

I posted two methods that have SUCCESSFULLY produced excellent lightweight composite tubes. If anybody has any other PROVEN methods, please post them here.

Billski

Earlier in this thread I mentioned Shrink Tape. Finally ran across it in my hangar:

Hi-Shrink Tape made by Dunstone. The stuff I have is 0.002 x 1 inch.

Dunstone Hi-Shrink Tape

Cool stuff. I bookmarked it.

It works. I used heat shrinkable tape recently. I put an addendum on an existing tube and did a repair on a carbon fiber sailboard boom. Both produced fully consolidated very high fiber content jobs...

Billski

I am thinking of how to produce a tail boom for my (virtual so far) Ultralight Footlaunchable sailplane. Diameter of the tube should be ~125 mm (5"), and its length ~2,6m (102").

The main idea is that it should be done easy as a simple tube (same diameter), with gradually thicker wall toward its root, and with some patches of reinforcing where some forces are introduced from attachment points and tail feathers and skid.

The idea is:

An aluminum tube is purchased and polished as much as possible.
Then it will go to professional guy (friend boat builder who use prepregs and autoclave) where a tube will accept mold release wax and apropriate layers and patches of preimpregnated carbon fabric (prepreg).
Then the asembly will be inserted in vacuum bag and inside of the autoclave, where a curing proces is done under high temperature (variying according to prepreg type) by a program.
The aluminum tube will expand under a heat, so after curing, inside of the tube is filled with icy water. Aluminum tube will shrink now and let carbon tube to pop free from the "mold" surface. Probably with some hammer persuasion it will be pulled down from the aluminum tube.

Of coures unfortunately the shiny surface will be on the inside of the tube, so some cosmetic work will be needed for the outer side of the carbon tube using light filler and paint.

What do you think? Did I miss something cruical? Is it doable?

Regards!

Mitja

Originally Posted by ultralajt

where a tube will accept mold release wax and apropriate layers and patches of preimpregnated carbon fabric (prepreg).

If I remember correctly wax dissolves into epoxy hardener (I've read about it here too, but I can't find it now), so probably it's not the best idea to use wax as a realse agent for structural parts.
Here is a very "Vivid" discussion about wax usage in a slightly different context
Berkut Kit - Page 20 - Canard Zone

Check out also post 302 by Richard Riley (RIP) he was a member of Berkut Engineering, so I believe he knows what he's talking about.

Seb

Mitja,

Tom Nalevanko posted a note with that very method mentioned. It is post #7 on this thread.

Billski

Originally Posted by wsimpso1

Mitja,

Tom Nalevanko posted a note with that very method mentioned. It is post #7 on this thread.
Billski

Upss... I didnt understand first time I read as he mentioned a light pole and missunderstand as yet another variation with a flourescent light... (now I look to a dictionary and see that "pole" is a "mast" for a outdoor light)

I see now, that this methos is possible! Thanks for pointing me that.

@Seb, regarding my mention about mold release way, I was thinking of mold releasing agent that manufacturer use (dont know which tylpe... Formula Five,....or something else...) but it is certainly compatibile.

Mitja

This is an update, and overdue too. Sorry.

I tested my 2 3/4" tubes for fluid tightness, being as they need to keep fuel on one side and air on the other. I put a short cut-off end into a little epoxy in a cup, let it cure, and then leak tested it. It would make a decent filter... I found that I could coat them with epoxy and get them to seal, but decided to just build new ones with 4 plies of BID cloth at 0/90. Everything else is as before, and they are quite tight against leakage. The difference is analogous to having to beef something up vs making it sturdy enough in the first place

One other point. The polycarbonate is reputed to revert to monomer in the presence of fuels... So, I packing taped the PC tubes and they came out with some effort. A 2" tube with one end slit can be used to catch the packing tube and get it rounded over, and then the other end can be used to beat it over and turn it inside out. Much beating and sliding of the aluminum tube inside the fiberglass tube got it out of there.

When I got done with all of that, I rolled several ounces of mixed epoxy diluted with a bit of lacquer thinner around the inside and painted it on the outside to ensure that all passages were filled. With the tube still bonded into the cup, I leak checked them with water, and they were solid.

Bill

Not to sound really silly here but why not purchase the product? I understand the reasoning for building, etc. I just don't have the resources or setup required to make all this.
I found a web site DragonPlate Carbon Fiber Composites - Carbon Fiber Sheet, carbon fiber sheets, Nomex, Square Tubes & Splices, Carbon Fiber Panel, Carbon Fiber Tube & Rod that makes almost anything you could want.
Merle

Several reasons:

First reason is cost. I just checked their prices. I need two pass tubes nearly 6 feet long. A 3" x 48" tube is $266, splices at $69, that's $836 for my pass tubes. Yeah, they if I can get them in glass, they will be cheaper, but not by much, and many of these places won't even talk glass... Then there are my 7' and 8' long control pushrods. Several hundred dollars more. The bucks to buy the tubes you need are pretty impressive. And if you want custom tubes, well, add a lot more money. Let me tell you, I have all of $300 bucks in my tube sets, and $250 in my vacuum bagging gear;

Next is weight - Purchased tubes are usually much beefier than I need, which adds weight. The carbon stuff is thickwalled compared to my stuff, and the pultruded stuff in both carbon and in glass is WAY thicker.

Where I am using tubes, I have internal antennas. These antennas are neat in that they do not drag through the air, don't hiss when flying in snow, and make for a slick looking airplanes too. But if you put big conductive things near them, they get all messed up. So, good antenna placement and RF transparent tubes means glass, not carbon.

One other thing. Many of the composite tubes made from braided socks will make a better filter than barrier. They weep. Try sealing that in a way that is durable over time...

Naw, I will just make tubes. When they have to be fuel tight, 4 BID, no perf ply, they work fine. For pushrods, glass braid socks works great. In both cases, the cost is tiny compared to the commercial products, and this is fun.

Billski

Bill, that does put it into perspective. I am surprised the vacuum bagging cost is so low. Guess I will have to investigate the process.

You say your control arms are fiberglass. That is interesting, had not thought of that. How about strength relative to bending,etc

Merle

There are some basic rules about making dipole antennas work (Jim Wier at RST publishes the book and sells the materials to do these antennas), and one of them is that you can put metal (or carbon fiber) near the vertex of the V, and they work fine. Look at a NAV antenna on a vertical stabilizer. But if you have a big chunk of metal (or carbon fiber) anywhere near the tips of the antennas, signal will get sucked off and the radiation pattern will be all messed up. So metal (or carbon fiber) pushrods for my NAV antennas will mess with an antenna on a wing skin, just about anywhere you put them... So the rods are fiberglass, and the rod end sets and bellcranks are small enough and at the Vertex of the V, so they will not mess with the antennas.

Cool?

Billski

Some basics on vacuum bag equipment. I bought my first two pumps at a garage sale. Old wet pumps. They go pucka-pucka-pucka and emit an oil mist. I built a doghouse around them , installed a box fan and a drier vent, and wired everything to a single switch to keep the oil mist outside. Bought a used industrial oil-less pump used from Applied Vehicle Technology in Indianapolis for minimal bucks (Hit Ebay if they do not have any used pumps), hooked it up with a vacuum switch and hoses from McMaster Carr, an old style 20 lb propane can for a vacuum reservoir, and 1/2" PVC plumbing fittings (couplings, elbows, caps, pipes) to rig up to the vacuum bag film. No bag fittings - just capture the bag with a piece of 1/2" PVC pipe and and coupling, then punch the bag film inside the pipe. The rest of the hardware - a piccolo is made by drilling a chunk of 1/2" PVC pipe, cap it, and wrap the thing with some scrap glass cloth to keep the bag film from choking it. If you get epoxy on the piccolo, toss it and make another in 10 minutes. Sealant and bag film, as well as nylon perforated ply, polyester batting blanket, and peel ply all come form Wicks Aircraft or Aircraft Spruce. If you are doing a bunch of big parts, the investment comes in pretty low on a per part basis, and the weight savings on the airplane can be pretty significant. You won't use less resin, but you will have less resin in the finished airplane. The otehr thing is that it makes the cloth conform to the cores, molds, etc, which means less repairs, much less in the way of trapped air in parts, and sturdy feeling parts, which is tough to put a price on...

Much of my airplane is vacuum bagged. Wing skins were made over simple female molds (search the forum on vacuum bagging and my ID), fuselage skins over male molds, my gullwing doors and roof in a female mold (my most complicated molded parts), and my drag spar in a female mold.

The main spars were wet layup early on, and they would have been better if I had figured out the methods for making simple molds earlier. My stabilizers and control surfaces are even vacuum bagged on hotwired foam cores.

I use "knitted" cloth as it gives lower resin fractions and nice strong parts for simple parts like wing skins and tails and control surfaces. For the turtledeck and aft fuselage, i probably should have used traditional UNI and BID, as it conforms easier to more complicated coumpound surfaces.

Spar caps are about the only sub-part I would still make in open layup. When you do the spar caps right with 3" Uni tape, the resin fraction is really small. Lay in each ply, wet, and brush from center, and squeegee from center - the fibers get straightened out and the excess resin gets run off. I got caps at 18-20% resin by weight.

Billski

Originally Posted by wsimpso1

Spar caps are about the only sub-part I would still make in open layup. When you do the spar caps right with 3" Uni tape, the resin fraction is really small. Lay in each ply, wet, and brush from center, and squeegee from center - the fibers get straightened out and the excess resin gets run off. I got caps at 18-20% resin by weight.

Billski

If you would do the spar caps again would you do them with carbon rods. I figured that I could save over 30 lbs using the rods vs tape. My caps would only need about 8 lbs of Graphlite for both wings.
Did you lay up the caps before incorperating them into the spars?

Originally Posted by wsimpso1

Some basics on vacuum bag equipment. I bought my first two pumps at a garage sale. Old wet pumps. They go pucka-pucka-pucka and emit an oil mist.

Billski

I started out with a conventional vacuum pump. Still have it. Don't use it any more. Switched to a water bed drain tool. Put a cheap sump pump in a 30 gallon plastic tote like you keep winter cloths in and looped the discharge from the water bed drain back into the tote. Had to put a weir between the pump and the return to knock out the air bubbles but it works great and doesn't make as much noise as the "real" vacuum pump. A one way valve from a power brake booster keeps water from being sucked in if someone trips on the chord.