Posted by dannparks on Monday, May 18, 2009 @ 00:24 AM:
I have a couple of small spots where the fabric on the fuselage has cracked and the fabric is delaminating around the crack. In one spot, I removed the delaminated fabric and sanded and feathered the edges, but it has exposed a 1"x3" section of plywood (the plywood is sound). How should I cover this? Fabric, glue and UV block? Or a fiberglass patch? Or something else? Whatever it is patched with needs to be able to be built up, sanded and feathered into the existing surface.
This is the largest spot. There are a few other spots that will need a patch about 1/2" x 1/2". Should the small patches be done the same as the large one? This must be a fairly normal thing for these old planes.
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Posted by eugenio on Monday, May 18, 2009 @ 02:01 PM:
This is a common problem with old coverings. Whatever you do, sooner or later another crack will show somewhere else. If you're doing an "overhaul" of the whole aircraft I suggest to remove the whole fabric from the plywood and cover it with a 50g glass cloth, that is the same weight of the old cotton, in this way you'll have a very smooth surface that will last much more and will look like a plastic glider surface. This is a standard in our overhaul, and the result is very fine.
Eugenio
Posted by dannparks on Wednesday, May 20, 2009 @ 10:29 PM:
Do you cover the horizontal and vertical stabilizer with fiberglass as well?
What do you use to fill the weave and all the pinholes in the fiberglass?
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Posted by Sam M. on Wednesday, May 20, 2009 @ 11:39 PM:
What about 1.7 ounce glider fabric? cotton weighs 4.5 oz per square yard. We covered our rf4 in glider fabric and finished it with dope. To our knowledge our rf4d is the lightest rf4d flying at 583.5 pounds,noted we didnt put cockpit starter in it.
Posted by SteveBeaver on Thursday, May 21, 2009 @ 07:26 AM:
This article from the Sequoia "Falco" builder's website gives a lot of insight into the process of finishing a wooden aircraft with glass/epoxy. It has proven to be a successful system.
http://www.seqair.com/skunkworks/Painting/Notes/Notes.html
Steve
Posted by eugenio on Thursday, May 21, 2009 @ 02:55 PM:
Very interesting the article about Sequoia's Falco. Our procedure is more or less the same, but I use a very thin epoxy resin and in case it is too thick you can diluite it with some thinner (nitrate usually works). Instead of a brush I use a roller like those used for painting, it's better than a brush because it squeezes the excess of resin and the air bubbles. I do not use microballoons, I just use some putty before covering, to have a smooth finish. When doing the process over new plywood (not the case of our RF's) you need to brush over a couple of coats of very diluited nitrate dope to fill the surface, otherwise when you fiberglass it you will have a lots of pinholes. If you do like so, the result will be a very smooth surface without any pinhole, ready for priming and painting. When you finish the fiberglassing, you only need to flush with some putty the overlap, just for not to see the step, then lightly sand the surface and apply a light coat of 2K sealer primer, sand it and finish with polyurethane.
All plywood surfaces are treated the same way, so either the fin and stabilizer and the wing box too. The dacron will be glued with nitrate cement 2 to 3 inches over the wing box (not all of it) and over the ribs without the need of stitching or lacing.
Fabrics: 1.7 oz/sq.y means 48 g/sq.m this is too lightweight, the one approved for covering is 90g/sq.m
4.5 oz/sq.y means 127 g/sq.m. Grade A cotton can be sostituted with medium weight Dacron (90g), heavy weight Dacron (110g) is just useless weight, while lightweight Dacron (50g) is too light.
In any case overall weight is slightly affected by fabric weight (10 sq.y =50oz), but much more by the paint you put on it, usually in the range of 10 to 20 kg (20 to 40 lbs). Using dope, as Sam did, has the advantage to have the possibility to repair paint cracks with a rejuvenator.
cheers
Eugenio
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