3D Fuse Broken in front of Tail (ARF)  
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All Forums >> RC Airplanes >> Crash & Rebuild >> 3D Fuse Broken in front of Tail (ARF)
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3D Fuse Broken in front of Tail (ARF) - 6/22/2009 1:41 AM   
SeamusG



Posts: 782
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Joined: 6/16/2007
Last Login: 11/6/2009
From: Westminster, CO, USA
Status: offline 		I need some experienced input.

A friend's 3D 40 sized plane didn't fare well after a gust of wind rolled the wings while the tail section remained stationary on the starting table.

Structure: The fuse is made up of 1/8" balsa bulkheads spaced about 6" apart. There are 4 balsa longitudinal stringers (metric dimensions about 5/16 x 3/16) that connect all of the bulkheads. 2 of these are at the bottom corners of the formers with the remaining 2 about 1/2 the way up the bulkhead's overall height. The remaining space above the stringers is covered and provides for a bunch of side area from the cockpit back to the fin (guessing for knife edge performance). There is what I call a backbone of shaped 1/4 x 1/2 balsa that runs from the cockpit to the fin.

The sides from the top stringers to the bottom stringers are sheeted with a single sheet of 3/32 balsa from front to back. The bottom is also sheeted with single 3/32 balsa with the center being removed. There are 2 1/8" ply servo brackets glued to the inside of the side balsa sheeting just in front of the bulkhead immediately in front of the fin.

Damage: Point of damage was between the bulkhead immediately in front of the fin and the bulkhead just ahead of that one. This area provides mounting for 3 servos (2 elev & 1 rudd). Side sheeting on both sides were crunched / splintered. 3 of the 4 longitudinal stringers were broken / splintered. The backbone was broken into 3 pieces and the section between the next 2 bulkheads forward of the damage was cracked. The bottom sheeting was crunched. One stringer and the transparent film kept the pieces in place.

Attempted Repair: I replaced sections of all 4 stringers in the damaged area from the forward bulkhead to the rear bulkhead. The side and bottom sheeting was replaced. Also, the backbone was replaced. This meant that none of the original longitudinal structure remained between the two bulkheads. I butt jointed 4 new stringers sections using epoxy with small holes drilled into the ends of the stock allowing the epoxy to harden "in" the ends of wood. I created tabs on both ends of the side sheeting to tie into original sheeting and lapping the edges of the bulkheads. The sheeting was glued in place with Titebond (alephetic resin).

Needless to say, it didn't take too much stress to pop one of the epoxy butt joint bonds.

How would you approach repairing this "mess"?




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< Message edited by SeamusG -- 6/22/2009 1:55 AM >


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Experience is what ya get when ya don''''t get what ya want - me - SeamusG


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RE: 3D Fuse Broken in front of Tail (ARF) - 6/22/2009 11:11 PM   
aerowoof


 

Posts: 2542
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From: pembroke, NH, USA
Status: offline 		scarf joints are much better than butt joints plus if done in an open area a doubler can be added behind either ply or carbon fiber sheet,also use gussets at the bulkheads if you have to use butt joints

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RE: 3D Fuse Broken in front of Tail (ARF) - 6/22/2009 11:53 PM   
SeamusG



Posts: 782
Score: 113
Joined: 6/16/2007
Last Login: 11/6/2009
From: Westminster, CO, USA
Status: offline 		Aerowolf - thx.

The scarf joint would definitely provide support against lateral shear. Glue only provides stick-em to guard against that kind of sheer. Hind sight (as I had already flush cut the darn longitudinal stringers)

I was considering the use of Gorilla glue's expanding characteristic to "blow up" into a fillet to protect against the sheer forces. Worked surprisingly well on a test piece.

Current state of affairs - I have replicated the two bulkheads using 1/8" light plywood. These are currently being glued to the original bulkheads using aliphatic resin (AR). This will provide 1/8" "mechanical" support against lateral sheer.

There will still be the issue of providing longitudinal support to the replaced side and bottom sheeting. My first cut at gluing up sheeting resulted in separation at the joint. I picked up a couple of sheets of 1/32 aircraft plywood to be used to double the side sheeting on the exterior overlapping both in front of the leading bulkhead and behind the trailing bulkhead. Planning to use AR for glue. A bit of filler on the leading and trailing edges should make recovering a bit cleaner.

Later,


< Message edited by SeamusG -- 6/23/2009 2:51 AM >


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Experience is what ya get when ya don''''t get what ya want - me - SeamusG


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