repairing my large 12' span DG 300

[Frank v B]

A few weeks ago I crashed my beautiful 12' span Multiplex DG 300.  The first flight was perfect, packed it up and went home to check everything out. It has a power 32 on 5 cells.

The following weekend I brought it out again.  It took off beautifully northbound and got to about 30' altitude. It then immediately turned left towards the driveway and I fought to level it.  It then started to do tight loops but would not respond to inputs.  I cut the throttle but no response. I tried to give full "down" and crash it but no response. After 3 or 4 loops it crashed into the lumber yard parking lot.

Mark, Andy and I drove over and found it in the middle of an empty parking lot.

Summary of damage:
- a 2 foot section of one wingtip broke off.
- both ailerons came off.
- motor broke off.  The twisted motor wires indicated the motor was spinning when it went in.
- the battery was still connected.
- the stab/elevator was still intact.
- the wood servo tray was completely loose with servos still connected to the pushrods.
- the nose was crushed, 6-8 cracks appeared along the fibreglass fuse.
- the wing joiner tubes in the fuselage came loose.

The following posts cover the repair process.

Frank

[Frank v B]

Glenn asked me yesterday how the repair was going and said I had not started.  I did not want to disappoint him so started the repair today. 
Thank you Glenn!!

I followed the normal post crash repair steps:
1) place the plane and the parts in a corner.  It starts to repair itself.  Somehow the repair is usually easier than the pieces in front of you.
2) assemble the plane so all the alignments are kept (motor trust line, incidence, etc.).  Do not worry about the strength.
3) strengthen everything.  I use 5 minute epoxy, microballons, fibreglass cloth, carbon cloth, wood.
4) worry about the cosmetics (covering, colouring, painting, sanding, etc.)

Frank

[GuyOReilly]

It will fly again. 
Like you said, epoxy cures everything.

[Frank v B]

reinforcing the cracked fuselage.

It looks like it pancaked so there were 4 significant breaks at the canopy (see arrows).
Used carbon fibre strands embedded in 5 minute epoxy across each break.
The whole thing took 10 minutes.  Glue was applied with a finger.

Once completed, each break will be sanded and coloured.

Photo 32 shows the carbon fibre strands as they were bought.
Photo 31 shows the 4 breaks. (black arrows)
Photo 29 shows the carbon in place across a crack.

Frank

[Frank v B]

Repairing the wing:

This is a white foam wing core sheeted with obeshi wood and then covered in film.
Looks like it torqued the wing when crashed. It caused a 1' split and a 6" split further towards the tip.

Did the 12" split first:
- Squeezed 5 minute epoxy into the foam
- put epoxy onto the edges of the wood
- covered the split with waxed paper so the squeezed out epoxy will not attach to the straight edges.
- used two straight edges, one on top and one on the bottom of the wing
- used as many mini clamps as I could find on my desk.  Death by 1000 clamps.
Let it dry overnight.

Frank

[msatin]

Obeshi Wood?
Was that the way wing came or your choice?
Is it used because it is similar to basswood?

[Frank v B]

Mark,

re: the wood on the wings:
- the wings came covered with the wood.  I just had to join them, add the ailerons and cover them
- It is much stronger than balsa and it does not dent easily.  It is a wonderful wood for the purpose.
- I had to Google the name to see how to spell it.  I always thought it was obechi but then what do I know.

Live and learn.

Frank
ps: it was the same wood as on my Flamingo glider in the Hafu video. The drone filming the flight crashed into the wing (pancacked) and it did not fail.  The dent is minor but visible.  If it were a balsa covered wing it would have dented significantly or failed.

[Frank v B]

Repairing the wingtip.  It had a break at the end of the aileron. 
Used the same process- 5 minute epoxy, waxed paper, straight edge on bot sides, clamps. Photos 07 and 08.

Put the same covering back in place. Photos 09, 11, 13.  The missing areas will be covered when the other wing half is at the covering stage.  That wing will have to be fibreglassed and a whole new section of covering will be needed.  The cutoffs will handle the minor damage on the first wing half. 

Frank

[Frank v B]

The badly damaged wing.  The tip portion (2') broke off completely.  We recovered some small sections of wood and foam
Turns out there was a whole section of wood and foam missing.  Also missing was about an inch of the trailing edge.

Approach:
1) glue tip back in proper place.  Aligned so the leading and trailing edges are straight (up and down, fore and aft).
2) fill in the missing sections.
3) reinforce everything, probably with fibreglass and carbon.
4) re-cover the missing red section.

photo report:
14- the task- the pieces.
15- shows the foam and wood have split open.
17- used a straight edge on the bottom and a flexible straight edge on the top to compare the bulge while the glue dried.
18- shows an aluminum channel taped to the leading edge and a straight edge on the trailing edge. Note the missing foam and wood.
19- the top of the wing shows that the wood pieces fit perfectly.

Frank

[Frank v B]

replacing the missing trailing edge.
There was about 1" missing so I cut about 5" away so the joints are nowhere near the break in the wing.
Used a piece of plywood as a new trailing edge.

Note the 45 degree joints on both the trailing edge and the new plywood insert (both ends).

Photo report:
20- shows the trailing edge cut away and the new plywood insert above it.
21- used straight metal edges and clamps to make sure the trailing edge stays straight while the glue dries.

Next steps- figure out a way to fill in the missing foam.

[Frank v B]

Patching the foam and sheeting:

Order of things:
1) cut the patch first.  Photo 23. I used a piece of mahogany from a Dumas boat kit.
2) lay the patch on top of the wing and cut the wing sheeting the size of the patch.  Photo 24.  Note the silver spatula in the open joint in the foam (weak spot)
3) glue the patch in place
4) sand the patch- sand off the high spots and fill the low spots.

Problem:
- When I cut the wing sheeting a layer of foam came with it (large bead white foam). See photo 25.  It shows foam on the bottom of the cut sheeting.
- there was an open joint (chord-wise) in the foam. Photo 24

Solution:
- filled in a layer of thinned 5 minute epoxy.  Put in 50% denatured alcohol and mixed in micro balloons as a light filler.
- could not get the glue to lay flat so I added a piece of 3/4 ounce fibreglass cloth. Photo 26.
- bent the wood patch by wetting to top side.  It formed the correct curl. Photo 27
- added the wood patch to flatten the micro balloon filler and cloth.  Pinned the edges in place.  Photo 28

Letting it cure overnight.

Frank