Fuse side layup problems, solutions, tips

[td_howard]

Hi all.

 

I had some problems with my fuse side layups which I didn't find addressed in the archives. Thought I would post my experiences here (and what Nat said about them) to hopefully assist future builders. This has also been x-posted to the cozy list.

 

1. Longeron gaps

 

Plans say there should be no gaps when you lay the longerons down on the fuse sides. Trouble is, the longeron strips are supposedly .25 thick, x 3 = .75" thick, while the longeron doublers and the long stiffener strip which attach to them on that dimension are both .7", which leaves a .05" discrepancy. In my case the longeron strips must have been slightly thicker than .25", because my glued up longerons are slightly more than .8" thick. I attached the doublers and stiffener so they would be flush with the main longeron structure at the "top" (ie, the side that faces in when the fuselage is assembled), to avoid a joggle there when you glass over the longeron with the 4-ply UND. The result for me was a .1" gap along the longeron bottom (where it joins to the fuselage) in the region of the front doubler and the long stiffener strips. The main longeron structure fits tight against the side, but not the stiffeners or front doublers.

 

I handled this by applying sufficient flox on the underside of the stiffeners and doublers so that plenty was squeezed out when I clamped the longerons down, and by troweling in extra flox to eliminate any gaps.

 

Nat said that was the correct thing to do.

 

2. Fixing air bubbles in the depressions.

 

I did a crappy job of getting the UND to lay down in my control stick and fuel gauge depressions, so I'm going to cut them out and re-do them as per the repair procedure in Ch. 3. (May as well learn that one early, eh?). That procedure specifies that you need 1" overlap per ply of glass, but since the depressions are so close to the top (and the longerons) you don't have enough room for that much overlap.

 

Nat said it would be OK to use only .5" overlap here, and that it would be OK to overlap onto the longerons if necessary. Rather than just remove the glass and glass over the foam, he suggested cutting out everything, laying down a release agent (such as plastic or wax paper) on the (now exposed) masonite so you do not glue your patch foam on to the jig, and putting in a new piece of foam, then glassing. This is approach I am going to take.

 

I did experiment with just trying to get the glass off the foam using a combination of heat and prying, but because of the marginal thickness of the foam in these areas, it's difficult to get the glass off without tearing the foam anyways.

 

I also had air bubbles along the top spacers, which I just injected with epoxy.

 

 

Ch. 5 "big layup" post-game review:

 

a) get helpers. I did mine alone, which probably doubled my elapsed time.

 

b) Work out from the center rather than everywhere at once. (Hmmm, doesn't Ch. 3 tell you to do this?). Get the depressions correct and move outwards from them. Which brings me to....

 

c) Re-read Ch. 3 before you start. You've been building jigs and doing a lot of woodworking-type stuff after chapter 4, so it's probably time for a refresher.

 

d) ensure a good micro fillet exists along the spacer edges. I thought I had done this, but obviously, not well.

 

e) work methodically over small areas. Get each one right before moving on. Don't think "I'll get back to this and fix it later", because you probably won't have the time, the epoxy will have set up, or you will be too tired.

 

f) do each side separately. I was getting very tired at the end, and you tend to rationalize away problems (like air bubbles) when you get punchy.

 

Other things from prior to the layup I just remembered:

 

g) when using the sandwich bag method of applying 5-min epoxy fillets, mix it up first and then put it into the bag. I tried just putting both components in the bag and squishing it around, but in at least one case something was off because it only hardened into a gummy mess, which required me to cut out the joint and re-do it.

 

h) the edge thickness of the 3/8" PVC foam seems to vary by as much as +/- 1/16" from sheet to sheet, so when you 5-min them together, you may get a joggle. Sand them flush prior to gluing them! (This is actually mentioned in Ch. 20!)

 

That's it. This layup took me a long time, so I was a bit disappointed with the results, but the nice thing about composite construction is that everything (well, OK,most things) is/are fixable.

In the immortal words of John Slade "no one will ever know..."

[dust]

OK, the air problems are probably the result of too sharp an angle into the depression, don't have to sand and remove all the glass, a quarter sheet sander will quickly remove bad glass, the white stuff realy sands away fast and the good stuff stays, no need to remove the good stuff.

 

are you using mgs, if so switch to all slow cure, so that you have plentyy of time and always dump out all epoxy FAST, thin film extends pot life dramitically.

 

make all transitions very smooth and get some slaves, they are very handy.

 

Always mix resin and hardener together for a minuite or more and then add micro or flox in the container, if using a bagie, put it in after mixing.

 

your bad layup area will probably be cut out in the strake install area or the spar area or the canard area, he he he make the part perfect and then later you cut it out and throw it away, he he he

 

enjoy the build

 

dust

[td_howard]

Dust, thanks for the input. Alas, I have already cut out the depression (depressing?) areas and micro'd in new foam.

 

Actually, the first ply was not too bad - most of the bubbles were between the two plys. Dunno how that happened.

 

Ah well, no big - won't take that long to fix.