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:

	Dust mask, surgical gloves, long sleeve shirt, long pants (safety first!)
	Thin CA with small tip
	Sandpaper ranging from 80 grit to 320 or 400
	Small sanding block and rubber sanding pad (available at automotive paint store)
	Fiberglass cloth, carbon fiber tow, slow-setting epoxy resin
	Scissors & brand new single-edge razor blades
	1" foam brushes
	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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