How to Strengthen Fiberglass Seam in a Tight Enclosed Area?
 

Hello All,

I'm scratch building an aircraft that has a composite fuse with composite twin tail booms that hold the elevator (figure 1). I plan to make the molds in two halves and join them together before the epoxy cures. I usually strengthen the center joint with 1.5!QUOT! fiberglass tape. The problem is that the twin booms are only ~1" wide and completely enclosed so I can't figure out how to strengthen the area where the two halves join (figure 2). I forgot to mention that the width gets even smaller towards the fin area. How do I add fiberglass tape in the area before joining the part halves? If there is an alternative way of strengthening this area, I'm all ears.

Thanks

Mods

RE: How to Strengthen Fiberglass Seam in a Tight Enclosed Area?
 

I've built some really narrow fuses with a method that used a molded joggle (basically an offset, molded-in step). What we did was to lay up the mold as usual and once we had the two halves finished, we'd go back and insert the plug in one half. Wax up the whole assembly like you were going to mold up another half. You then lay up on the mold flange and slightly up the side of the plug (1/2" is fine). Do that to either the top or bottom (lets say bottom for this example). You'll repeat this process with the plug in the other mold half and lay up on the "other" seam side (top). Take the finished pieces and sand the top of the flange to 1/4-3/8" and this will be your "step". These mold "steps" will be used when laying up a new fuse, one bolted to opposite mold halves. You need to locate these mold steps with an offset to the fuse mold to allow for the fiberglass overlap (measure cloth stack up ie 3 layers of 4oz glass= .025 or something like that). Using clamps locate the steps and drill locating holes through both step mold and fuse mold flange. It's best to use the existing fuse mold locating holes/pins if possible.

When you go to lay up a fuselage, these steps are bolted on and are treated as an extension of the wall of the mold so wax the heck out of them. You will need some micro-balloon mix in the sharp crevis created by the offset and lay up the glass normally. Trim like normal when cured and pull off the step molds (be careful not to pull the glass off the wall of the fuse mold to create a potential spot for joining micro-b. filler to squish into). Sand the molded step rough to create good mechanical bond (mask off fuse mold flange so you don't scratch it). Lay up the halves with your micro-balloon joining filler around the edges and on flange and close mold.

This is not as ideal as a wet layup seam but it works when you have no choice but to close the mold without any way to press the seams.

RE: How to Strengthen Fiberglass Seam in a Tight Enclosed Area?
 

Here's a cross section of what I mean:

RE: How to Strengthen Fiberglass Seam in a Tight Enclosed Area?
 

Excellent tip! I was starting to think that everyone forgot about me. When you said the fuse halves would be "mechanically bonded", you mean that I should join the fuse halves after they have fully cured?

Thanks A Lot,

Mods

RE: How to Strengthen Fiberglass Seam in a Tight Enclosed Area?
 

Yep. In order to make the joggle/step hold it's shape, it needs to be partly cured (epoxy kicked off for a couple hours) for trimming and pulling the step mold off. If it's still really gummy, the glass will not release from the step mold and you risk deforming the step when you go to join the two halves.

I use to do this on really light fuselage lay ups (borderline flimsy) so on a more standard, stiffer lay up this should be a bit easier.

BTW, making the steps on opposite mold halves allows you to slip the two mold halves together at a better angle than dropping it straight down and hoping you are lined up. Shown here:

RE: How to Strengthen Fiberglass Seam in a Tight Enclosed Area?
 

Agreed, but it depends on what kind of alignment system you use. Some alignment pins will only allow a "straight down" assembly.

Also, an alternative method for making the molds for the lip (bonding ledge) is to just use a flat plate (e.g. 1/4" thick MDF, plywood or whatever) and cut it to shape, such that you are left with a 1/8 - 1/4" wide gap to the plug. Wax the plug and flange on the mold, and bolt the plate in place, and fill up the gap with epoxy/microballoons. A syringe is good for this. When the epoxy is cured, you can remove the plate, with the epoxy/microballoons now bonded to the plate. The epoxy will now have the same shape as the plug. Elongate the bolt holes as described in post #2, to allow the small mold to be located a small distance inside the edge of the fuselage mold. No laminating involved.

Magne