Drilling aft fuselage

Ok what the heck is going on with my fuse?
The manual is specific about making sure the aft fuse is straight before drilling. I've got three saw horses per the manual. I hung four plumb bobs from the first three bulkheads and off the end. A center line was drawn on the inside of the bottom skin.
Front is easy to plumb. Rear falls right in line. The second one is about 1/32" off starboard. The third about 1/16" off starboard. If I try and get the third one on the line, front and rear go off to port. Correcting fore and aft forces the center back off and I end up in a Mobius Loop. Holes seem to line up fine. I've moved saw horses around fore and aft. I've tried 100% clekos and also pulled almost all the clekos and started over. No holes have been match drilled yet because I don't want to drill in a twist. Two days chasing this. Maybe I should not use the second and third plumb bobs.
Got any ideas?

Maybe your center line wanders?

I dunno. I hung four plumb bobs and sighted down the center on the river lines. I suppose things could have been off a millimeter or so, I'm not certain I would have seen it. But it's close enough that if there was any misalignment, it wasn't big enough to be seen. I did the same after dimpling before riveting, and things still seem to be quite straight and twist free.

Oh, and make sure the plumb bobs are actually hung in the center. If you do what I did and hang them over a cleco, they'll be off center by about a sixteenth.

I had the same problem. Was able to correct and remove twist by installing (cleco'ing) the rear top F-775 skin. I use straps to pull the skin down around the bulk heads. Kept the skin on while riveting, eventually had to remove the top skin to get the last few rivets.

Cheers

Idea

DaleB said:

Maybe your center line wanders?

I dunno. I hung four plumb bobs and sighted down the center on the river lines. I suppose things could have been off a millimeter or so, I'm not certain I would have seen it. But it's close enough that if there was any misalignment, it wasn't big enough to be seen. I did the same after dimpling before riveting, and things still seem to be quite straight and twist free.

Oh, and make sure the plumb bobs are actually hung in the center. If you do what I did and hang them over a cleco, they'll be off center by about a sixteenth.

Click to expand...

Anything is possible. The plumb lines are hanging from the alignment holes. A wandering line would would explain it. I'll pull it all apart and double check it. I'm not moving forward now anyway. Thanks Dale.

I built a jig to do the rear cone. Waste of time if you ask me, when you go to lock in the tail cone with the f 714 - aft deck, you can twist the fuse l and r an inch or more when you go to get it level.

Twist

bret said:

I built a jig to do the rear cone. Waste of time if you ask me, when you go to lock in the tail cone with the f 714 - aft deck, you can twist the fuse l and r an inch or more when you go to get it level.

Click to expand...

Thanks Bret
I'm going to try one more thing then go with it. Fore and aft align. It's the middle I can't align.

Everything level?

I chased mine until I went back and

1. double checked each saw horse to make sure they were level where the fuselage would rest.

2. put the tail section back on and hung the plumb bobs

3. I slid a wooden shim underneath one side of a bulkhead to get it level. The shim was the kind you would use when hanging a door. The bulkhead was off, but not by much. Probably like yours but I do not remember. That fixed any twist.

As I drilled and clecoed I stopped every few holes and check to make sure nothing moved. Later on you will flip the canoe and rivet the aft deck on. At that stage you check for a twist again. Mine was straight so this worked for me.

Level

Thanks Scott
Same method. I think the bulkhead is the issue. It's not flat. I fluted it some but don't want to go too far and cause an upper skin hole alignment. It may be causing the tooling hole to be off center which in turn throws off the plumb bob.

Got it

For others in the future.
It was a combination of several factors.
Mainly the first plumb bob string was not perfectly in line with the tooling holes. I checked all the others and put them in place. Once I got the strings right and pulled most of the clekos, everything else fell into place. It takes almost no shimming to stay straight while drilling. I also match drilled the J-stiffener holes #42 so the clekos would hold things tighter as the line was drilled. I worked symmetrically, back and forth on either side. First every 4th hole, then the middle ones, then the remaining. She's as close to straight as I can get.

Good deal -- rock on!! I'm going to try not to slow down enough for you to catch up with me.