Mark Burton
Joined: 10 Jan 2006
Posts: 74
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 Posted: Sat Sep 08, 2007 1:23 am Post subject: Mod 74 experience |
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I have just done Mod 74 and you may be interested in our experience.
To remove the pins we simply cut a piece of the foil covered insulating material about 8" in diameter and made a hole in the center the same size as the pin - with that in place, a short blast with a heat gun on the pin soon made the glue soft enough so that mole-grips on the pin would make it move - work it back and forward a few times and it will come unscrewed. The surrounding structure was barely warm.
I was helped by someone who is very experienced in composite construction/repair (he repairs seriously broken gliders for a living) and he was unhappy with the mod as described due to the placement of the hole so near to the flap plate and the rather short taper of the skin in that region. So we moved the hole 10mm further outboard and used a smaller hole (less than 2" dia).
We didn't use locktite to fix the pins but, instead, bonded the pin/washer/nut assemblies in in one operation with the wings rigged. We thought that this would remove the possibility of a pin becoming un-aligned if done as per the instructions. As this is the third set of pins to be installed in my aircraft, they were a bit loose in the threads and so we thought that this was the safest option to ensure perfect alignment.
Apart from the final tweaking of the nuts, the job can be done with the wings off and then rig, put all the pins in and then tighten the nuts down (not too tight).
To tighten the nuts we used a socket with universal joint drive. That worked fine but if you try the same thing and it won't fit, it's OK to open up the hole in the outboard and forward directions - don't make the hole bigger towards the root or trailing edges. Take care to not damage the original foam under the edge of the hole.
We actually used two-part foam to fill the cavity - it was very quick and produced a perfect job. If you want to do the same, make sure that the expanding foam you have is compatible with the blue foam and doesn't eat it!
The layups were completely symetrical around the hole and round in shape. The layups did not quite reach either the flap bracket or the trailing edge.
After the layups cured, we made good with filler and then, finally, put white Fablon(tm) patches over the top and it doesn't look too ugly at all. Perhaps, sometime in the future, I will re-gel those areas.
I don't know how easy the job would be going through the flap closeout but as we did it, it was very straightforward and I believe the end result to be structurally more sound than if we had carried out the instructions to the letter. Obviously, if you want to stray from the official guidelines you have to have expert advice.
Cheers,
Mark
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