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Repairing Fuses |
shop talk
Steve Condon
Repairing Fiberglass Fuses
Greetings from the shop. This month I will
describe a technique for repairing those fiberglass fuselages that get damaged from
meeting-up with Mother Earth with a little too much energy. This is a multi-step process,
but when we are finished only your timer will know that fuse was ever damaged.
Assessing the damage
First thing we have to determine is if the
fuse is worth fixing? Believe it or not, a pretty badly damaged fiberglass fuselage can be
repaired to be as strong — or stronger — than before. The hitch, of course, is
that you will add some weight in the process and part of the determination of the above
question is what impact the weight penalty will have. If this is a competition hand-launch
plane, and it needs a lot of reinforcing, I'd say buy a new fuse and keep the plane at a
competitive weight. If it's an unlimited ship, sport plane or your "beater,"
then by all means, fix her!
To determine if it is repairable, see if
you can manipulate it back to its original shape. If it is in multiple pieces, do you have
them all? If you can put it back together like a puzzle and you have all the pieces,
you're in business. If only relatively small pieces are missing, that's OK too.
Materials you'll need:
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Dust mask, surgical gloves, long sleeve shirt, long pants
(safety first!) |
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Thin CA with small tip |
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Sandpaper ranging from 80 grit to 320 or 400 |
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Small sanding block and rubber sanding pad (available at
automotive paint store) |
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Fiberglass cloth, carbon fiber tow, slow-setting epoxy
resin |
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Scissors & brand new single-edge razor blades |
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1" foam brushes |
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3M 77 spray adhesive |
The method described below assumes that
you have a seriously damaged fuselage. If it's not too bad — maybe just a big crack
from a hard landing — you can use the same technique on a smaller scale. I have
busted a lot of Eagle fuselages right in front of the wing with a vertical break that runs
down from the back of the canopy to the bottom seam. In this case, I usually mask-off an
area 1/4" back from where I plan to repair it and protect the good parts of the
finish with masking paper; sort-of like draping a patient for surgery and only leaving the
affected area exposed. This way you won't leave gluey fingerprints on the rest of your
plane.
Step One
Align the parts back to their original
positions. It is really important that you take your time and make sure that it can
be made true. If there are edges that have overlapping fibers (which is usually the case)
that are giving you trouble in achieving good alignment, cut them back a little until you
get it straight. If you can hold it straight with no trimming, you're ready to proceed.
Step Two
Once you are satisfied that all the seams
can be made to fit straight (without being concave or showing an obvious bulge in the
natural lines), get out the thin CA and start gluing. I insist on thin CA because
it wicks into the damaged fibers and really creates a strong bond. The little tube that
comes with Zap or the small tips that come with many other brands make the job easier.
Generally, you want to glue the big pieces first, but it can depend; if you have a nice
puzzle on your hands, glue some of the smaller pieces together to form bigger ones making
sure that things align correctly as you go. Assemble in a sequence that seems logical to
you. Don't worry about little ridges on the seams, just make sure it's straight! Note: a
good overhead light to hold the plane up-to and sight down its length to determine surface
alignment makes the job much easier.
Step Three
After you get it CA'd together, double
check the alignment. If you're not satisfied, this is the time to change it. At this
point, your fuselage should be back to its original shape and reasonably strong. If there
are any really weak spots, try wicking more CA into the joint. It needs to be strong
enough to sand without breaking. The next step is to use some 80 or 100 grit sand paper on
a block to knock down the seams and even the surface. Use chopped fiberglass and CA
fill to any holes between seams. Sand and fair-in.
Once the seams are faired-in, switch to a
piece of 100-120 on a rubber sanding pad to uniformly rough the surface. Don't leave any
shiny spots. If you are repairing an area of the model that allows you to access the
inside opposite your damage, sand the inside as well. At the point where your repair
transitions into the good part of the fuse, try to make a straight, defined seam. It is
important to have the seam at least 1/2" from any damage.
Step Four
Choose your repair materials. I recommend
fiberglass cloth of at least 3 oz. weight (maybe lighter for a hand launch). 4-6 oz. cloth
should suffice for an unlimited ship A light finish layer can be used in addition to
heavier, structural cloth. If you need to add longerons, carbon fiber tow can be run in
the inside of a fuselage along the canopy and/or wing saddle. Cut pieces slightly oversize
for the outside and fairly precise for the inside. For nasty breaks in high stress areas,
I recommend a smaller doubler on the inside with a second larger layer over it.
Step Five
Spray the pieces on one side with the 3M
77 adhesive ensuring a light, uniform coat. Let sit for at least one minute before trying
to apply. Start with the small pieces on the inside and press into position. If a second
piece will be applied over the first, do it now being careful not to loosen the first
pieces. If you have edges protruding outside the fuse, trim them off with sharp scissors.
Now place the pieces on the outside. Make these pieces slightly oversize and trim them
carefully with a new razor blade to the exact size once they're in place .
Step Six
Mix-up as much resin as you think you'll
need to do the job. Using the foam brush, wet-out the cloth starting with the inside.
Ensure the cloth is wet throughout . Dabbing and pushing down with the brush is the
preferred technique. Try to save the brush "strokes" for removing excess resin.
Make sure you wet the entire area. There should be no white areas where the cloth is not
saturated. However, don't use too much resin. Extra resin won't make it stronger, only
heavier! Carefully blot up excess resin with a folded paper towel. Repeat the process on
the outside and let cure.
The Final Step
After the resin has cured, wet sand it
with 220 and apply a finish coat of resin for a smoother look. If you choose to paint, put
the second coat of resin on and let it cure for several days before you start paint prep.
For tips on painting, see the February Shop Talk article.
If you follow the above steps, you should have a
fuselage that is only a little bit heavier and a lot stronger to serve you for
many more flights...but, watch those landings!
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