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F-711A/B holes do not align on the F-773 side skin

idubrov

Well Known Member
I am fitting the F-711 bulkhead to the skin, and two parallel vertical lines of holes on the bulkhead (one line coming from the F-711A, the front part, the other from the F-711B, the aft part) do not align. Pretty much all the vertical holes on the F-711 bulkhead, not just few of them.

My guesstimation is that the worst hole is ~0.030" off, if I align one side "perfectly". Meaning, that an ideal hole covering both will be around, say, ~0.120", an -4 sized rivet (and the side that is aligned "perfectly" will be normal -3 sized rivets).

The holes on the skin are closer to each other compared to the holes on the flanges of the bulkhead. I cannot think of a way to make the holes on the bulkhead closer to each other. The flanges are bent the way the are bent, and I don't think I have any significant gaps between the bulkhead sides either.


There might be some vertical offset happening, too, but I think it is mostly the horizontal.

IMG_6181.jpeg
IMG_6182.jpeg

Before I ruin the holes in the skin (I've already enlarged them a bit with clecoes trying to fit the parts, but nothing too bad so far), what are my options?

1. I can try to "split the difference" and fit NAS1241 rivets (1/64" oversized rivet shank) on both sides. Lot of "ifs", but maybe I can drill (or even file) the holes very precisely and get away with +0.016" enlarged holes.

2. Give up trying and do the -4 sized rivets on one side. Edge distance is probably going to be okay for the most of the rivets, but there are 4 holes that attach the J-channel stiffeners which already have a very marginal edge distance on the top side of the flange. Maybe, I can move a hole a bit and make the edge distance to work both on the flange and on the J-channel.

3. Maybe, there is an option to make a special order for the part without the holes? Replacing the F-711B should take care of the issue (same way the F-712B bulkhead is made). I haven't reached Van's yet, don't know if that's a thing, though

Anything else I can do?
 
711 bulkhead

I remember both 711 and 712 were a pain. I had to buy a new set of 712. Ultimately it was just adjusting of flanges till everything was flush and the holes lined up. Maybe not the same issue you're seeing.
 
Yeah. I had a similar issue with the elevator ribs, where two ribs (counter-balance rib and the end rib) are riveted back-to-back, same as the F-711 bulkhead.

In my opinion, if two ribs or bulkheads are riveted back-to-back, and there are prepunched holes on the skins, it would be better if Van's haven't punched the holes in the flanges of one of the two halves (same as in the F-712 case). These holes only make it harder to fit the parts. An assembly will be perfectly located via the holes on the flanges of the other half, then drilling through the prepunched holes of the skin will perfectly place the holes on the other half.

P.S. The flanges seem to be sitting fine. Not perfect, but I don't get much tension against the bottom skin F-779 nor the side skins F-773. It's the parallel lines of holes that cause me a lot of trouble.
 
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Holes

It's possible, although rare, one or both bulkheads were formed a bit off. It wouldn't take much for the holes to be .015" further from the web. I suppose you could use a forming tool to adjust the flanges but a new set of bulkheads might be better.
Is it the same port and starboard or onlynone side?
 
Both sides have the same issue. Port side is slightly better, but overall has the same problem. In both cases the distance on the skin is shorter; in other words, the flanges are "too long" on both sides.

I guess, I can try a new set of bulkheads. Maybe, try a different technique of bending the flanges. Try to bend them further away from the center of the part (effectively making the flanges "shorter").
 
Option 4

I am fitting the F-711 bulkhead to the skin, and two parallel vertical lines of holes on the bulkhead (one line coming from the F-711A, the front part, the other from the F-711B, the aft part) do not align. Pretty much all the vertical holes on the F-711 bulkhead, not just few of them.

My guesstimation is that the worst hole is ~0.030" off, if I align one side "perfectly". Meaning, that an ideal hole covering both will be around, say, ~0.120", an -4 sized rivet (and the side that is aligned "perfectly" will be normal -3 sized rivets).

The holes on the skin are closer to each other compared to the holes on the flanges of the bulkhead. I cannot think of a way to make the holes on the bulkhead closer to each other. The flanges are bent the way the are bent, and I don't think I have any significant gaps between the bulkhead sides either.


There might be some vertical offset happening, too, but I think it is mostly the horizontal.

View attachment 30995
View attachment 30996

Before I ruin the holes in the skin (I've already enlarged them a bit with clecoes trying to fit the parts, but nothing too bad so far), what are my options?

1. I can try to "split the difference" and fit NAS1241 rivets (1/64" oversized rivet shank) on both sides. Lot of "ifs", but maybe I can drill (or even file) the holes very precisely and get away with +0.016" enlarged holes.

2. Give up trying and do the -4 sized rivets on one side. Edge distance is probably going to be okay for the most of the rivets, but there are 4 holes that attach the J-channel stiffeners which already have a very marginal edge distance on the top side of the flange. Maybe, I can move a hole a bit and make the edge distance to work both on the flange and on the J-channel.

3. Maybe, there is an option to make a special order for the part without the holes? Replacing the F-711B should take care of the issue (same way the F-712B bulkhead is made). I haven't reached Van's yet, don't know if that's a thing, though

Anything else I can do?

Another option is to cut off the flanges on one of the F-711's and then rivet on new flanges that can be match drilled with the fuselage skin. The new flanges are angles bent from .025 same as the bulkhead material. I've done this a number of times where the parts did not fit and David Paule's posts for his RV3 build have good pics for how he did this with some fuselage bulkheads.
 
Got my answer from Van's support, they think that I should cleco and drill the holes. I don't completely agree with that, based on my elevator experience, it won't be too terrible, but there will be elongated holes past the spec limits. Which is not going to bring the plane down, probably, but just does not feel right.

I thought a bit about Paul's suggestion to replace the flanges, and my thought train brought me to the idea of duplicating the whole part. It has relatively simple profile, so I'll try that and see how it goes.

It's called "nerd sniping". I am too excited to form a part from a sheet stock now (until I fail miserably, of course) :D
 
What about making all new holes, respecting edge distance while doing so, then micro-ing (bondoing) the old holes later?
 
Well, I am into the forming the part already, going to try this puppy first, and see how it goes. Waiting for my 2024-T3 0.032" sheet stock.

4-forming-fixture-2.jpeg
 
D'oh, this was stupid. Part turned out okay, but I bent one tab in the wrong direction!

Well, the second one should take less time to build :D

IMG_6203.jpeg
 
Okay, I guess, I will still need some of your advices on fixing the flanges :D

Overall, I am happy how the part turned out, but the edge distance on the flanges under the middle J-channel turned out to be very short. I kind of knew it could be an issue since even on the original part the edge distance is pretty marginal. I should have moved the flange just a little bit up.

IMG_6232.jpeg

Considering three options:

1. Replace the flange (cut off, rivet a new one).
2. Put a vertical doubler across 3 or 5 rivets. However, it won't be sitting flat as flange under the channel sits deeper into the fuselage. Not an engineer, but have suspicion that bent doubler will be less effective (won't resist vertical forces very well).
3. Put a vertical doubler across 5 rivets with two more 0.025" doublers up and down to keep the long doubler flat. Might be an overkill, though.
 
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I’m right at this spot. My holes lined up they just needed some forceful convincing.

I was able to force the clecos in both rows of the holes, but clecos were tearing the holes and were not sitting straight. Meaning oval holes after upsizing.
 
Im just up the road from you in Kyle.
I did this section back in may and had a similar issue. It was a real pia. It took me a week using a pick and a variety of compression methods to get the tabs adjusted to fit.
I assume you have bent the tabs to follow the 773 skins? It takes more bend than it initially looks. The bend radius will change the spacing of the holes on the 711. I put neoprene soft jaws in the vise and put repeated slight pressure on the tabs. It takes a bit of touch but it worked for me. Of course Yrmv….

Nice job building the former and the new part. I might go back to the vans parts and experiment on closing up that gap.
 
Okay, I guess, I will still need some of your advices on fixing the flanges :D

Overall, I am happy how the part turned out, but the edge distance on the flanges under the middle J-channel turned out to be very short. I kind of knew it could be an issue since even on the original part the edge distance is pretty marginal. I should have moved the flange just a little bit up.

View attachment 31433

Considering three options:

1. Replace the flange (cut off, rivet a new one).
2. Put a vertical doubler across 3 or 5 rivets. However, it won't be sitting flat as flange under the channel sits deeper into the fuselage. Not an engineer, but have suspicion that bent doubler will be less effective (won't resist vertical forces very well).
3. Put a vertical doubler across 5 rivets with two more 0.025" doublers up and down to keep the long doubler flat. Might be an overkill, though.

That's looking good Ivan.
I think option 2 is the way to go, with a doubler across 3 rivets. The rivet at the end of the J channel is just anchoring it to the bulkhead. If you make the doubler from .025 x 5/8 strip and ensure it sits flat with no gaps where each rivet goes through I am sure it will be fine.
 
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