ATM How to Make a Composite Tube

["Dave Pitou" <dpitou@ix.netcom.com>]

Hi All,
        I originally E-Mailed this to Jack Schmidling because I thought it was too
detailed for the list. Since then a couple of other members have expressed
interest, so here you go.

        Please don't assume I invented most of this, in fact many of the
techniques come from a primer on experimental aircraft building, but I
wanted to be complete enough so you can tell what you're getting into. For
those folks with experience in this area, tell me what you think - I'd love
to update my technique.


Lets assume a 16" OD x 60" long tube, you can scale as needed. 

Bill of Materials:

1 - 15" OD x 72" long Sonotube ~ $30 - for the form, from local concrete
supply house.
8 yds. - 60" wide 8.9oz bidirectional fiberglass cloth @ $5.40 /yd ~ $44
from Aircraft Spruce - Fullerton,CA (800) 824-1930, or local plastic supply
house. This is enough for two wraps for both the inner and outer layers.
You can use 3 if you're paranoid, but 4 is probably too heavy.
2 - yds. Dacron Peel Ply @ $3.65 /yd ~ $8 - from Aircraft Spruce.
1 - 1.25 Gallon Kit of Safe-T-Poxy II $58.25 from Aircraft Spruce - way
more than you need, but you deffinitly don't want to run out halfway
through. Don't try to use polyester resin instead, it's too thick, it
smells, sometimes sets stickey, and worst of all I've found the setting
time unpredictable.
1 - Lb. Glass Bubbles ~ $5 - from Aircraft Spruce.
1 - 6" wide big rubber squeege ~ $5 from Aircraft Spruce.
2 - 1/2x48x48" Sheets of 2lb/Cu Ft Urethane Foam @ $7.88 /sht. - $15 - from
Aircraft Spruce.
Or
1 - Sheet 3/8x48x96" Nomex Honeycomb ~ $100 , get the kind that will bend 
easily in one direction - from Erskine-Johns in Sacramento CA. or Aircraft 
Spruce special order. I think Erskine-Johns is cheeper, but I'm not certain
they mail order.


Preperation - there is no hurry on any of these steps:

1. Cover the floor with a big drop cloth, drops of epoxy will stay on your
garage floor forever if you don't do this.
2. Cut 2 14.5" wooden circles, (try to make them pretty accuratly round), 
to fit Sonotube ID. Cut a 2x4" hole in the middle of each circle. Drive the
circles into the ends of the Sonotube, and mount the tube on a 2x4 between
sawhorses so you can rotate it easily.
3. Find an old truck inner tube and cut it into a 1" wide continous strip.
This will be used to clamp the urethane foam or honeycomb to the inner
layers of fiberglass.
4. Cover the Sonotube with polyethelene plastic. Make sure it's tight and
smooth, and tape it in place.
5. Cover the polyethelene with the Dacron Peel Ply. The Peel Ply isn't
required for the structural integrity of the tube, but it makes a nice
rough light absorbing surface on the inside of the tube when it's removed.
Make sure it's tight and smooth, but try to keep the tape to a minimum.
6. Cut the fiberglass cloth into 4 pieces 65" long.
7. If you are using the Urethane foam you probably will need to score it to
get it to bend aroung the Sonotube. I'm on thin ice here since I have only
used the honeycomb, but the foam is cheep, so you might get an extra piece
just to practice on. Cut the width of the foam so that the ends overlap by
about 3/8" when wrapped around the Sonotube. Use a dust mask since the
particles from this stuff float.
8. Put the fiberglass cloth on the Sonotube with one edge at the top. Tape
the ends, and put a small piece of tape in the middle to keep it in place.
The edge needs to be straight and parallel to the tube axis.


Layup - these steps don't have to be rushed, but keep working and don't
answer the phone. With Safe-T-Poxy you can have about 2 hours of working
time depending on the temperature. I've found it sufficient even for a much
larger tube than this.

1. Mix about a cup of epoxy. Be careful to use the correct proportions, and
mix for at least 1 full minute.
2. Pour a bead of epoxy near the leading edge of the cloth. Work the epoxy
into the cloth with the squeege. Don't stretch the cloth and keep it
straight. You need to use enough epoxy, and work it with the squeege
enough, so that the fiberglass looks "not wet, not white". Mix more epoxy
as needed.
3. Go all the way around the tube wetting and working the cloth until the
first layer is done. The cloth needs to be straight and smooth. The layers
need to overlap themselves by at least 2", so if you scale the tube size
make sure to account for this overlap.
4. Apply the second layer of cloth the same way as the first. It helps to
have two people when you initially lay the dry cloth down since it needs to
be straight and square and it tends to stick. Use less squeege pressure on
this layer since you don't want to form ripples by pressing too hard.

If you are using Honeycomb you can skip step 5.

5. In a big cup mix up about 8 oz of epoxy and add enough glass bubbles to
bring it to a thick buttermilk consistancy. Spread a thin layer of this
stuff over the entire tube, make more as necessary.
6. Wrap the Urethane foam or honeycomb around the tube. Bring the two edges
together and have a friend hold them while you wrap the rubber strip around
the tube to hold the foam or honeycomb in place against the fiberglass.
Continue pulling the strip as tight as you can while your friend rotates
the tube.  Once you have complety wrapped the tube, fasten the strip, take
a deep breath and go washup. Wait a few hours before unwrapping the strip.
7. Once you've Unwraped the strip the foam can be sanded round and smooth
at this point, if needed, and any holes in the foam can be filled with a
putty of epoxy and glass bubbles. After this you are ready to put the final
layers on.
8.  Cover the foam with a thin layer of epoxy and glass bubbles, (skip if
using honeycomb).
         If you are using Honeycomb there is a trick here that I haven't tried,
but that should save alot of epoxy, (I traded secrets with the $800
composite tube guy at RTMC about 2 years ago to get it). When you are
putting the final layers of fiberglass down the epoxy tends to go through
into the honeycomb, which means you end up using alot of epoxy. He uses a
thin layer of Kraft paper over the honeycomb to prevent this. The paper
wets through with epoxy, but doesn't allow it to easily flow through into
the honycomb holes. Like I said I haven't used this yet, so if the epoxy
doesn't wet through, it may be necessary to wet the paper before you put it
down on the honeycomb.
9. Lay the last two layers of cloth down using a little more epoxy since
this is the outside surface. Work the cloth well and keep the layers
straight and smooth.
10. After the top layers have set you can very carefully cut and strip the
Sonotube from the inside with a razor knife. Obviously don't cut too deep.
Strip the polyethelene and Dacron Peel Ply from the inside.
11. Cut the tube to length, I use a sabersaw, (jigsaw), with a guide.
12 Paint: At this point it's taste. I use Carbon fiber, so I don't paint my
tubes since I like the look. If you like a smooth shiny tube you can put on
a few more coats of epoxy, (sand the surface lightly first), before
painting, or fill the surface with epoxy and glass bubbles. using the
squeege, and sand before painting.

        From the length of this message it looks really difficult, but its not as
bad as it seems, and it'll be a way cool tube!

dp2