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Bulging Bulkheads

PaulvS

Well Known Member
Bulging Bulkheads and Alignment Details

The fuselage (in the jig) bulkheads bulge outward slightly, by about 1/8 inch, between the back of the baggage compartment and the tail end. This is causing some difficulty in getting the two main side skins to fit properly, because the outward bulge is causing waviness along the edge of the lower curved portion of the skin (the part of the skin that curves over the belly skin) and it won't sit flat.

Does anyone here recall having this problem in getting the side skins to fit? I've searched the forums without any result. The Matronics RV list has some old discussion which mentions that Vans changed bulkheads F-607 and F-608 to deliberately introduce a bulge to accommodate a slight bow in the skin that was caused in the manufacturing process. This was to reduce oil canning, but I have the opposite problem = waviness.

My inclination is to modify and reduce the width of the offending bulkheads so as to make the sides of the fuselage flat, but I don't want to do this if it is not necessary.

Thankyou if someone knows the answer!
 
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Pictures coming

I will get some pictures to upload in the morning.

Essentially the sides of the fuselages and the stringers are not totally flat, and so the side skins curve slightly out along their length, per the bulkhead flanges. This then kinks up the curved over part of the skin where it wraps around the belly. A couple of the bulkheads appear too wide. Dwg 24 Rev 1 indicates F607 and F608 width was increased by 0.125 in 1991. I've checked position of all bulkheads in jig and they are correct.
 
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Paul, you can adjust the fore/aft position of the intermediate bulkheads (not the two aft bulkheads or the aft spar bulkhead) to straighten out the sides. You can use strings run along the sides of the aircraft to establish where those bulkheads need to be before you attach them.
 
Too late

Paul, you can adjust the fore/aft position of the intermediate bulkheads (not the two aft bulkheads or the aft spar bulkhead) to straighten out the sides. You can use strings run along the sides of the aircraft to establish where those bulkheads need to be before you attach them.

Thanks, I'm afraid it's too late for that because I have already drilled on the sides! :(

I noticed the problem when doing the wrap around edges.... I knew the sides were not flat but thought this was deliberate in the design...
 
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It sounds like you don't have the longerons seated all the way. The longeron should sit flush with the flange on the bulkhead.
 
I noticed this problem with my 6. I set the longerons to run straight back (no outward bow) and adjusted the bulkheads to fit. You have to check that the "keel of the canoe" is a straight line, so moving the bulkheads to correct fit in width may not work. I corrected width by re-rivetting the bulkhead overlaps. Everything worked out fine with the fit of the skins, etc.
Stewart Willoughby, 6 paint is next.
 
I think I am pretty close to the same stage of the build as you are right now. I did notice a very slight outward bow when jigging it up, but it hasn't had any adverse effects on the side skin. The issue I DID have was with I believe bulkhead 8. It is too short and needs to be sectioned to bring it "up" so that the bottom j stringers form a straight line. I remembered seeing this in Sam Buchanan's build website and I had the same exact situation. Could this possibly be the case with yours? causing some weird stuff with the skins?
 
Pictures

Thanks everyone for your responses. I must admit that I didn't get much sleep last night thinking about this problem! Here are some pictures to help explain the dilemma.

The fuselage frame has been riveted together and the first 4 skins are drilled and clecoed on.

Before skinning I checked the frame for straightness and the keel of the canoe is dead straight within 0.020 or less.

Both of the fuselage sides between the tail end and the back of the baggage compartment bow out slightly by about 1/8".

DdO15GuGDukxcnW1FR2OTv9NAV3DzsaGKTp9vDFixwHjjxsGLpOiD4lu2PJi-6tGZbxFLNSSnHJQC8DliKx_ijlro63AQhhrh21xj-bPF9SLZGvSYhRhTWC_SL6dvVXhcpVmkzK9_aT-YXKRpB4k86ZRFoIvjz0HBt4Xm6rKmostsyS_hHq0Qems_QdDD8RlleMv-VTGmOn47W7yLERBrpwSMdnW7-cpfoq1GUhu1pw1Dy0oM-na9FEJg0Xeiuc_9eJwa3ztvyFBNuM3MAVn61S_5pmcG1XUr13g_sYII3rodcys1PQvre1eY5k2OZjf0u1sLSoiYPlpmxJmVyMEXomr4pevT-4cXB_06ypfQ_IIXkqHVylruQ_bJhr6UQLp06j2Vmfor7eNHUGQg-tmZIoaQXORkZcwCrfXITBQV86b4gLrhK2vug6mukdx_-WU8xaJFAmkfx-VYbRAPVGdG32br5BFWIg-zb8FPfGYT_IGrQhF1i1kSKf65RcdHpd1dcjhh1pmexnyVP_3_q2lOqOcRW4UtAqS9P9ijOrp3Z3H0_x2L42bGdQiOP99tJ1s5RpfHK0cf_jiKlzaRz4nK1J1O7cIYDOE0XD_nQ3aM2LJ0HSFqaKHQnS1BB-MB1JfDVKvLg9R2jGP_AfizFJ07tKAcvY6UZ9S6p69LQDoQ2deM2eaSf76xh7mzCKwZoAB01nE-Q=w1680-h881-ft


When I try to get the side skin pulled in against the fuselage side and bottom it does not sit flat along the bottom and the wrapped over portion ripples up. This ripple moves around depending on where you push and it is impossible to get it to go away.

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The problem seems to be caused by the sides bowing out. It is a very slight bow so it is not noticeable in the picture however a stringline shows that the bulkhead side flanges are not in the same line.

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On the inside the bulkhead is being distorted by attempting to pull in the skin to get it to lay flat. This makes me think that bulkhead F-607 is too wide. Or maybe F-606 is too narrow. I also noticed on Sam Buchanan's website that he had to split a bulkhead and make it wider.

It's quite difficult to measure the bulkheads against what the plans specify on Drawing 24 but this is what I'm trying to do now.


gxCGRshXOIwWyZN8WnXEQn9JN-Usd9okiT0OcSwHPt3cmZB1EvCjM_gG_HVPn4RG0ADw2mNiMCcg8DrO8M0vatN8LIWsZLqm4Fy0e-2zm_QG7ti6nvqovSZ-58RZiu13uqWZKZE8MRCGJ1q1dgGlU6P8CY5VKDmBQE3b1nWiD0MQcdNKhFvo0bgYJD_apT1e5d8nRRpL6qx26ObPKuE83XT5uPI2vvCiuzyhbzGVuEiTx5jpML_-ZRQr9TcPrnKRDZM4eoJccu-ePvXmfQraKwGYR3o-uvPAg3Zg81wDK1If49WLeBnIIvxqeZR4fOXTzKmTy9i3dlwrp5IkVuuYD5_M72Z-rUzpU7LDJnZUB7Q7WSbPCn-xICfZgqpObJudIOkIHqy-Zer66oOg-Q39W0o3bgJD8udYt2ljGpuiLVvCiQcohUG9pI9mN30mjlZt7E9MYDGTID3EW_PUYA5njCBOalsjfSSgT4shMcW-YFiCWalAC0nJSa9hqT4eO3ZXNWc7rSr66OR7JliHwkRNsk0FWxt4hZC5KJ7RoVDDU54EePK0P1_vfRJTVqtMfT805XfZL9dThn2hjKEIEH1yAEjZCS2hl7Yfnnl1Dc4CTGvFOfVEoUGqLB_0vJ53cNbVWWA5lyGl8WswCSBRvcQCvKhzf6imIYJi6oEeTK4i-rhTHxqPI9bC2TeqqajFzC5tA630cg=w1680-h881-ft
 
Still speaking as a newb here. I had cone to a realization that in many cases on the 6, what is on the plans 'loosely' resemble what you actually see when assembling the fuse. I built my jig per the plans and as i began further into the assembly I realized that from the spar bulkhead to bulkhead 7 the distances are pretty much set by the seat and baggage ribs and the bell crank framework....it all didnt add up to what the plans showed. I mean, some of its really close but its enough to make you question yourself. Some things just have to be tweaked along the way.

Anyway, to your specific issue. You may have already tried. What if you start in the area of that bucked up spot or in the center and work outwards to try and dissipate the extra material throughout the entire length of the skin.
 
Still speaking as a newb here. I had cone to a realization that in many cases on the 6, what is on the plans 'loosely' resemble what you actually see when assembling the fuse. I built my jig per the plans and as i began further into the assembly I realized that from the spar bulkhead to bulkhead 7 the distances are pretty much set by the seat and baggage ribs and the bell crank framework....it all didnt add up to what the plans showed. I mean, some of its really close but its enough to make you question yourself. Some things just have to be tweaked along the way.

Anyway, to your specific issue. You may have already tried. What if you start in the area of that bucked up spot or in the center and work outwards to try and dissipate the extra material throughout the entire length of the skin.

Thank you.

I agree about the plans, I have found many errors and inconsistencies and omissions. And sometimes the kit parts themselves are not right. I've found it generally easy enough to fix a known problem but it is not always apparent what are the knock on effects. At some point you have to cut, drill holes, or rivet and it is frustrating when design problems emerge afterwards.

Re. my specific issue, I have tried pushing and spreading out the bucked up edge but it just moves around due to physics. I think the only way it is going to lay flat is if I straighten the skeleton sides somehow. I have double checked the jig cross piece locations and the widths of the bulkheads and everything is as per the plans. But I did the maths on the design and it is impossible for the sides to to be flat, it just cannot work as designed unless the bow is deliberate, and then it kinks up where the skin wraps over the belly. I think that either F607 needs to be narrower or F606 needs to be wider to achieve this, but I'm not sure of possible complications from hacking things up!
 
Since you have already drilled the skins to the bulkheads, you could add a spacer between the flange and skin on the bulkheads that are narrower, thus creating a straight line and removing the slight bend in the fuselage side. This should let the skin lay flat where it wraps around the bottom. You will have to mess with the stringers a bit as well, but still probably easier than getting new skins, and shifting the bulkhead fore/aft to get a straight line.

Another option is the re-bend the flange on the wide bulkhead to narrow it a bit. But this will shift the rivet line. If the shift is small, you could drill to #30 and use -4 rivets instead.

Option 3, order a new bulkhead for the wide one, if the new one is the same, re-bend and/or shift fore/aft as needed to get it in the sweet spot while still maintaining edge distance with the existing holes in the skin.
 
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Thanks

Since you have already drilled the skins to the bulkheads, you could add a spacer between the flange and skin on the bulkheads that are narrower, thus creating a straight line and removing the slight bend in the fuselage side. This should let the skin lay flat where it wraps around the bottom. You will have to mess with the stringers a bit as well, but still probably easier than getting new skins, and shifting the bulkhead fore/aft to get a straight line.

Another option is the re-bend the flange on the wide bulkhead to narrow it a bit. But this will shift the rivet line. If the shift is small, you could drill to #30 and use -4 rivets instead.

Option 3, order a new bulkhead for the wide one, if the new one is the same, re-bend and/or shift fore/aft as needed to get it in the sweet spot while still maintaining edge distance with the existing holes in the skin.

Thanks Colin for the suggestions, I am trying a variant of the first idea and widening F606 by removing the rivets that hold the three pieces together, adjusting out, and re-riveting.

I can still easily bend some of the flanges on the wider bulkheads because only the flat flanges have been drilled. The slotted flanges at the corners are not yet drilled so I can deepen the slot and bend further.

I'm trying to avoid importing new parts because the shipping costs are formidable and there are delays due to reduced airline freight capacity. Hopefully the above will work out!
 
Before reshaping the bulkhead flanges, consider how that would affect the portion going around the bend radius. and alignment to ribs/stiffners there. Many RVs have been built with shims between bulkheads/ribs and skins to smooth things out. I'd explore that option before resorting to the bulkhead flange bending option.
 
I remember using string to make the aft fuselage straight and having to "adjust" some of the bulkheads in width to fit.

I say it might be easier to adjust the width of the bulkheads to fit.

I added this from the following post...:)

Plywood should be cut to fit and clamped to the inside of the bulkhead to keep the flexible bulkhead straight while you are positioning it and strapping down the skin.
 
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modify the bulkheads

I had to modify the F-608 bulkhead to fit, it was slightly short and narrow. I cut the bulkhead at the two lower corners, widening the bulkhead slightly to fit the skin profile, and added splice plates at the cuts to restore strength of the bulkhead. The goal of course is that the bulkheads are all in perfect alignment so that the skin fits without dips or bulges. Build from the inside out, use string or straightedges to check fit of the skin . Since you cannot move the bulkheads fore or aft at this point, try to cut the bulkhead to fit where it needs to go. Plywood should be cut to fit and clamped to the inside of the bulkhead to keep the flexible bulkhead straight while you are positioning it and strapping down the skin.
For any very minor gaps that might remain between a bulkhead and the skin, you can mix a structural epoxy/flox and with the bulkhead and skin installed and clecoed, force the mix between the skin and bulkhead where necessary, and modify rivet length at that spot as necessary. Result is a skin that lays down perfectly.
Bill Brooks
Ottawa, Canada
 
Update

I've made a start by un-riveting the corners of the three piece bulkhead F606. This is relatively easy and low risk and I can test fit the skins before riveting it back together.

Generally the alignment problem is too much to be fixed by shimming.

After having stared at the plans and skeleton for hours and hours I think the crux of the problem is that the aft sides of the fuselage have been designed to bow out. Maybe this was to prevent oil canning, but it is also the cause of the skin edge rippling up.

Thanks for the suggestions so far, today a TC is going to come out and have a look before I dig myself too deep into a hole.
 
I've made a start by un-riveting the corners of the three piece bulkhead F606. This is relatively easy and low risk and I can test fit the skins before riveting it back together.

Generally the alignment problem is too much to be fixed by shimming.

After having stared at the plans and skeleton for hours and hours I think the crux of the problem is that the aft sides of the fuselage have been designed to bow out. Maybe this was to prevent oil canning, but it is also the cause of the skin edge rippling up.

Thanks for the suggestions so far, today a TC is going to come out and have a look before I dig myself too deep into a hole.

Remember, the 6 was a paper design, so it isn't prefect. The sides need to be straight for everything to work as is should. The plans bulkhead position is just a little off.
 
Remember, the 6 was a paper design, so it isn't prefect. The sides need to be straight for everything to work as is should. The plans bulkhead position is just a little off.

Yes, but if the sides are already drilled the other option available is to "adjust" the width of the bulkheads to fit with the longerons set straight with a string.
 
Yes, but if the sides are already drilled the other option available is to "adjust" the width of the bulkheads to fit with the longerons set straight with a string.

I was referring to his comment about the bulge in the side being and intentional part of the design. There are many little errors like this in the plans (or parts) that require some extra caution before poking holes.
 
Bulkhead adjusted

So far I have adjusted the width of the baggage compartment bulkhead by removing the rivets that hold together the three piece bulkhead (F-606) and moved each side out by about 1/8". This has reduced the amount of bow out as checked with a long straight edge. The skins are not yet back on but the TC (who is a very experienced RV builder) had a look at it and thinks it should be OK now.

If I had tried to adjust the position of the other bulkheads to get the sides flat (before drilling holes in everything) I think it would have thrown out the straight line along the keel and the top deck. These lines are both dead straight right now on both the tops and the bottoms of the bulkheads. This is with the installed bulkhead locations and widths exactly as per the plans, including the revision on Dwg 24 that widened F-607 and F-608 by 0.25 (this change isn't immediately obvious on the drawing as the dimensions are unchanged, but the reference line was moved back by 0.125).

I did some simple maths on the slope of the sides by calculating rise/run for each bulkhead bay and it is not constant, so the sides are designed to be not flat, unless there are errors in the drawing that have never been corrected. Anyway I just have to work with the parts I have got, hopefully when I get the skins back on today there will be an acceptable fit.

Thanks again for the assistance, I'm glad there are still people around who know about the older models.
 
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Glad you are making progress. I definitely think there are some discrepancies in the plans and/or some of the parts that you just sort of have to field modify. With mine, I think it was bulkhead 11(2nd from last) was too narrow, causing an inward bow in that area, it couldn't be moved fore or aft because the distance was locked in by the tailwheel mount. The half bulkhead ahead of those I did have to move to prevent inward bow, and had to cut and lengthen bulkhead 8 to get the belly j stringers to make a straight line.
Its new to you and I, but its good knowing that there are many others who have gone before. Its speaking with some of them and helpful comments on VAF that really sets your mind at ease.
 
Update

Good news; after widening bulkhead F-606 by 0.125 per side the aft skins fit properly and the rolled section that was causing a problem now sits flat over the belly skin and doesn't ripple up any more. It is surprising that such a small adjustment to one bulkhead has made such a bit difference.

For future reference here are some other gotchas that I encountered with the fuselage frame to date:

  1. The aft side skins F-673 came rolled but the roll was not enough at the rear, so I curved it further by clamping and holding the edge between timber boards and then curled them over, it took quite a bit of force to adjust - and not crease.
  2. Bulkhead F-606 needed to be tilted back by about 1/2 inch at the top so as to align with the elevator bell crank support F-628.
  3. F-604E is supposed to have the web bent and not cut all the way through. The flanges need to be cut. These details are not on Dwg 31.
  4. The J-stiffeners along the edge of F-673 needed to be re-positioned after fitting the skins so as to get adequate edge clearance for the rivets.
  5. The attach bars on F-611T are right up against the bulkhead flanges and there is not much space to drill and dimple for the skin rivets.
  6. It worked very well to drill the fuselage skins using small pilot holes drilled from the inside out. Initially I drilled 1/16 holes (all of them) in the bulkhead flanges and also 1/16 holes in the J-stiffeners at the intersections with the bulkheads. Then clamped on the skins and crawled under the fuselage frame to drill 1/16 holes from the inside out using 12" bit. Then crawled out and drilled #40 through the skin using the 1/16" hole, and cleco'd. There was a lot of crawling back and forth to do these one-by-one but it was worthwhile to avoid drilling into a void or a flute or too close to an edge.
 
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