Fiberglassing a Wing Center Joint - Tips Needed

Good Evening. All the planes I have flown since I restarted a couple years ago have been ARFs, but now I have decided to see how finishing a partially built plane would work out for me. I bought one at a swap meet a couple months ago. The fuselage is partially built, though I have to fabricate the tail and rudder (no problem here). The wing has been built up (all wood), but the center joint needs to be strengthened with fiberglass. I have never done this before, so I purchased some 5 oz. fiberglass cloth, which I will presumably have to cut into strips to wrap around the center joint, and some Great Planes Pro Finishing Resin. Unfortunately, I haven't seen anything lately about techniques for this type of fiberglassing. For example, is the 5 oz. cloth appropriate for a center joint? How many layers of fiberglass do I need for this? What is meant by "feathering" the glass? Any advice or links regarding fiberglassing the joint would be appreciated.

Thanks, Harry Sanchez

Reply to
Harry Sanchez
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The purpose of fiberglassing the center section is simply to reinforce the joint between the planking on the two wing halves so it doesn't pull apart during high-wingloading maneuvers like tight loops. If there were no dihedral joint and the planking were continuous, there would be no need for fiberglassing.

For a typical .40 to .60 powered plane with 1/16" or 3/32" planking, one layer of 5 oz. cloth should do fine. I'd cut it about 2" wide to overlap each half by 1". The epoxy or polyester resin is only used as an adhesive, and with smaller planes and lighter cloth I often just use regular Titebond wood glue.


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Dave Plumpe

The link is an awesome site and it covers just about everything

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On Sun, 12 Mar 2006 23:53:55 -0800, Harry Sanchez wrote in :

That's way overkill, but no harm done--it will work just fine.

Here's where you get to do what YOU feel like doing.

I would make the strip at least as wide as the fuselage, maybe a touch wider. That's overkill, too, but I don't want to do the destructive tests that would establish the real optimum for my wing design and materials.

I'm going to use 6" for grins. Maybe 4" is better. Depends on the size of your plane.

Cut a 6" strip long enough to cover the bottom of the wing.

Put the dry fiberglass in place.

Drip a tiny amount of resin down the centerline of the fiberglass.

Scrape from the center toward the edges with a credit card or epoxy squeegee.

The resin that counts is the stuff that is inside the fiberglass and in contact with the surface below. Anything left on top is wasted weight, so squeegee pretty good if weight is an issue.

Let the fiberglass dry pretty thoroughly. If you have any excess at the leading or trailing edge, you can trim it off with a razor or razor knife when the epoxy is half-set or you can wait and sand it off when it is fully set.

I like to have the top piece overlap the bottom piece about 1/4 to 1/2 inch. It's just an aesthetic judgment. I feel better doing it that way, although there would be no measurable difference in the joint doing it differently.


That means you try to press down the edges and/or leave a little extra epoxy at the edge so that you have less sanding to do to make the edge of the fiberglass disappear smoothly into the surface of the wood. I haven't worried about that much. A lot depends on your own taste.

There are a lot more engineering complications to think about if you like thinking about stuff like that. But if you just want to get the wing together and go fly, almost anything will work just fine. Chances are quite good that something else will break before the wing joint does.


Reply to
Martin X. Moleski, SJ


We must have attended the same school of model building : )

I learned from good ole Carl Goldberg...the kits that is.


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Great site, Mosher. Thanks for pointing it out.


Reply to
H Davis

I like to use lighter weight glass cloth and go out wider than the fuselage as well. This way the fibreglass takes some of the impact/scuffing where the fuselage sits on the wing (or vice versa): I believe there will always be some movement there anyway plus localised loads on landings, though more so with a low-wing models.


Reply to
Mike Shores

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