Bulging Bulkheads

[PaulvS]

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!

[WingsOnWheels]

Do you have a picture, I am having a hard time visualizing the issue you are having with bulging.

[PaulvS]

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.

[Kyle Boatright]

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.

[PaulvS]

Originally Posted by Kyle Boatright

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...

[WingsOnWheels]

It sounds like you don't have the longerons seated all the way. The longeron should sit flush with the flange on the bulkhead.

[Stoo]

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.

[Coaltowngarage]

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?

[PaulvS]

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".



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.



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.



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.

[Coaltowngarage]

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.