One of the tradeoffs with having composite base tubes is that they can break.
I know of at least 5 pilots who have broken their base tube. It does not
always take a crash or bad landing. I know of two pilots who set there
gliders down and high centered the base tube (had the full weight of the glider
on the center of the base tube) and broke the base tube. I have had
discussions with Felix from AIR and several other people in the composite
industry on the best way to make the repair. The following is the method I
use. I have repaired 3 base tubes with this method and have never had one
break at the repair.
I have worked on both the AIR and Wills Wing base tubes. They are
similar enough that the technique for repair can be used on both. The AIR
base tube has a foam core and no steel cable. The WW base tube is hollow,
but has a steel cable. If the base tube is just cracked and not broken
completely in half, only an external repair is needed. If it is broken in
two pieces, I like to make both an internal and external repair.
The internal repair consists of inserting carbon
fiber rod (extremely strong, much stronger than carbon cloth and epoxy).
A search of the web will come up with many sources for this product. A big
use is in kites. I use .125" diameter rod unless the repair has to
follow a curve, then smaller diameter rod would be easier to use. I coat
the rods with a mixture of milled fiberglass or carbon mixed with epoxy and
insert the rod into the center of the base tube and then put both pieces
together and clamp. This will provide more strength than the base tube had
originally. I use an aircraft structural epoxy. There are several on
the market, West system, Aeropoxy, E-Z Poxy to name a few. A search of the
web with return many vendors that sell these products.
After the internal repair is dry (or if there was just a crack and no need
for internal repair) I start the external repair by sanding the repair area
3" to 4" on either side of the break, on both top and bottom, tapering
from the center of the repair outward so that the outer most sanded area is just
though the gel coat and the deepest area is at the break.
I place about 7 pieces of carbon in both the top and bottom of the base
tube. I use 5.3 oz bi-directional weave cut the width of the base tube.
If the tube was broken in half, then I also add a 1.5" wide piece of
unidirectional carbon tape on the top and bottom of the tube. Each of the
7 pieces are a little longer the piece under it so that the final piece covers
all the other pieces. I place the carbon patches on a sheet of Mylar,
lightly drip epoxy over them, place a second sheet of Mylar on top and then use
a roller to completely wet out the carbon. You want the carbon 100% coated
with epoxy and no more. Any extra epoxy does not make the repair stronger.
See the pictures below. I then take a final piece of carbon that is wide
enough to cover all the previous pieces and long enough to wrap around the base
tube 1 1/2 times. This final piece will hold everything in place and
give the repair a cleaner look.
Once all the carbon is in place I wrap the repair with plastic wrap (Saran
wrap etc.). Then I wrap over the plastic with electrical tape. I
start in the center and wrap towards one of the outer edges of the repair.
This is what I call a poor mans vacuum bag. As you get to the edge of the
repair you will see epoxy being squeezed out, which is good. Start in the
middle again and wrap toward the other edge of the repair. Clean off any
epoxy that oozes out (less sanding later). Care must be taken when taping
that you do not twist or rotate the repair while taping At this point I place
the repair under an infrared heat lamp to cure. Once dry, I unwrap
the tape and plastic. The carbon will be lying nice and flat, but
there will be ridges of epoxy. I sand down the epoxy until the area is
smooth and I am just getting into the carbon layer. I add a final light
coat of epoxy to seal what I just sanded and give it a nice appearance.
|This is what the inside of the AIR base tube looks like. The rods on the bench are carbon rods.||The carbon rods are in half the base tube, ready for the second half.||I placed the base tube in the sun to cure faster. The string is holding the repair in compression.||Internal repair finished ready for sanding. The white stuff is epoxy with milled fiberglass.|
|Base tube sanded and ready for carbon.||Carbon patches cut and ready for epoxy, not shown is the final wrap and two pieces of carbon unidirectional tape.||Wetting out the carbon with epoxy between two sheets of Mylar.||You must be careful when picking up the wetted carbon. It falls apart easily. I am using a plastic putty knife.|
|The final patch in place on the top of the repair. I then repeat the same on the bottom.||This piece will be wrapped around the repair, giving it a nicer finished appearance.||Finished repair wrapped in plastic, ready to be wrapped with tape.||Finished taping. Some epoxy has been squeezed out which can bee seen close to the tape.|
|The repair with tape and plastic removed, ready for final sanding.||Sanded and ready for final coat of epoxy.||Finished repair.||
Repaired Will Wing base tube.