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Longeron repair help please

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Dominic Eller

Well-Known Member
Supporting Member
Joined
May 16, 2019
Messages
333
Hi all, I am rebuilding a lovely but old (1967) Jjungster 1 biplane. I have the nwe wings pretty much ready for covering and am getting into the fuse now properly.
On first inspection the fuse looked in pretty good shape, but as often happens on closer inspection the problems start showing.

I have found a crack in the upper right longeron right up at the engine mount fitting area. Im not sure if this crack has been there for a while and when I test rigged it I've opened it up with wire tension.
I had removed the gussets from a few places earlier to look at what was underneath but didnt notice it then. the fact that I put tension on the structure with out the inner gussets might not have helped things..... oh well live and learn.

Most of the gussets and glue joints look sound and seem to be resorcinol glue with the dark wine colour, however the ones I have removed from the front area in question seem to have something else and not much of it is the joints either, the gussets came away quite easily.
Anyway time to fix it, so a couple of ideas I would like to run by the wood workers amoungst you.

1: The lazy and might be ok way.
So the longeron in this area is doubled on both sides by 1/4 ply gussets and on the top by 1/8 ply so well re-enforced. I could heat the wood and soak and inject epoxy then re glue new doublers and potenially larger ones and call it good............hmmmm maybe not ?

2: Splice in a new short section. Trick here is making a nice cut of both the old and the new. There is splices on the longerons just behind the seat that where maybe done due to not having long enough stock on hand originally. These are about 9" long and have stood the test of time so I figure a length = to or even slightly less will be fine considering the postion of this repair and the substantial ply on 3 sides.
I have a nice japanese Gyokoucho pull saw and have been practicing some cuts to see the result I can get. The trick will be making a jig of sorts (maybe angle extrusion) to guide the cut.

3. A another chap on a seperate forum suggested the possabilty of removing the triangle filler block and running a second slightyly less deep longeron section under the original and taper it off reawards then glue a smaller filler block back in place. Certainly an lot less dramma and potenial for error than scarfing in new section.

The other pain is trying to presserve the ply fuse side fairing while I hackaway at this......I mean perform careful surgery :) I

Interestingly there is a cross piece and gusets missing up front from the original build, I think done to ease the construction of the tank and allow larger capacity.
It's maybe something to change or stick with the status quo? This may have contributed to the slight bow in the very ends of the top longerons. Its hard to know what is going on there as it may have been intenially bent to widen the top longeron spacing out to match the bottom as it appears the longerons tapper a little inwards, which is not as the plans would have. Perhaps needed to align with engine mount better??? who knows. I just want strong and will be having a new engine mount made to suit anyway.

I have toyed with the idea of making the gussets on the inside very large to spread the load and beef up the whole front end a bit...good idea?

The pics should help visulise all this a little better than my description.


Thanks for taking the time to read this all

Cheers Dom
 

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