Orientation of the aluminum bulkhead at station B

[Zzz]

Having a bit of a debate here regarding how to orient the aluminum bulkhead/former that is attached at the station B crossbar (BH 4-place.) The assembly manual makes it very clear that the piece should be oriented aft of the crossbar, underneath the horizontal tabs, with the top flange pointed aft and the bottom flange pointed forward (page 40 photo.)The aft end of the stainless steel tunnel then is fastened to the bottom of the bulkhead and the length is perfect. The manual then mentions that if your kit has the vertical tabs, you should use those for fastening the bulkhead. The only problem is, the tabs are on the front side of the crossbar for this to be an easy swap.

So, we're seeing 2 methods here. I asked this question of Mark Goldberg and he said that if I had the vertical tabs, to use them, as it's a somewhat preferred way of attaching the bulkhead. However, this requires moving the bulkhead forward about 1", which presents a new problem: The side flanges on the bulkhead and the SS tunnel no longer mate together nicely. There is about a 1/4" gap due to the side profile of the tunnel increasing in height.

I had already drilled the tunnel and bulkhead by the time I asked Mark this question, but it would be easy enough to redrill and trim the extra length off the tunnel (even though working with stainless is arduous.)

Here's how I had it originally:


Here's how I could redo it to use the vertical tabs:

Tabs are on the backside of the piece. Don't be disoriented by the AN3 bulkhead fittings.. I didn't bother to flip them around when I flipped the piece.



The new gap on the tunnel flange-- forcing it together doesn't do much to close it up.



Method 1 (aft of crossbar) pros: I already drilled it. It allows nutplates to be installed in top flange to receive floorboard screws. Profile of tunnel matches nicely with no gap. No trimming of stainless.
Method 1 cons: There's really nothing holding it to the airframe except the floorboard screws.

Method 2 (forward of crossbar) pros: More solid as it's fastened to the vertical tabs on the fuselage. Bulkhead not involved with floorboard fasteners.
Method 2 cons: By moving forward on the tunnel, a gap is opened such that the side flanges don't mate well. Trimming of stainless is a pain. Redrilling.

I don't mind redoing work though, so not really a con. Just wanted to use the best practice. Thanks!

[alaskabearhawk]

Why not weld some vertical tabs on the backside of the crosstube to attach the bulkhead?

[Zzz]

Why not weld some vertical tabs on the backside of the crosstube to attach the bulkhead?

I suppose I could, and I've done a fair amount of welding on this quickbuild fuselage as it is, but I'd like to hear from the other quickbuilders about how they did this step. To be honest, having those tabs on the aft side of the crosstube would solve everything.

[Mark Goldberg]

You could also use some Adsl type clamps instead of welding tabs on the aft side of that crossing tube. Without the rubber cushions. Mark

[Steve W]

Why not leave the bulkhead where it was originally and make a second, slightly smaller to fit, that attaches to the vertical tabs, tunnel and the rest of the skins. Holes with grommets for the hoses.

[BTAZ]

Perhaps some standoffs between the existing tabs and the bulkhead?

Or long screws with tube spacers between the tabs and the bulkhead?(perhaps 10-32 screws with some .25" OD by .189" ID with a length equal to the tube)

[alaskabearhawk]

I like the Adel clamp idea too.

[Zzz]

I like the Adel clamp idea too.

Yeah that's pretty good.

I had thought of the long screws and tubing spacers idea too, but it's a weird way to support something with a lever arm like that. Could still work.

[whee]

If it were me I'd leave it where it is and use screws with standoffs as a secondary support. Looks like the fit is better there and if you have to remove the floor screws the secondary screws will hold things in place.

[Mark Goldberg]

Zane, you will not normally be removing that section of floor. You will be removing the tunnel for annual inspection. So using those floor screw attach points is also not the worst thing. Mark

[Battson]

This has been my experience too, it's the tunnel and belly panel which come off.

[Zzz]

Thanks everybody! I think I'll go with it as-drilled and hang it with some adels as Mark suggested.

Do you guys prefer access from underneath because it's easier to get the panels off without having to mess with the control assembly?

[Mark Goldberg]

For me it is accessing most easily where needs inspecting. Mark

[Battson]

Thanks everybody! I think I'll go with it as-drilled and hang it with some adels as Mark suggested.

Do you guys prefer access from underneath because it's easier to get the panels off without having to mess with the control assembly?

Yeah, two reasons:
- it's a lot less work.
- access is much easier.

You are under there anyway, just pull a dozen extra screws and you're in. Lying on your back our crouching under the plane is comfortable and access is unlimited.

When you have the fabric covering on, doors on, seats in, controls in - you find that accessing anything via the door openings is not easy. Especially on 31" tires. Around the back of the instruments or under the floor panels is fiddly at best, or a painful contortionists job at worst. I have spent time curled up like a rat inside the foot-well area, elbowing pedals while I try to rewire some headset jacks or re-route some cables. But I digress.

[Zzz]

I was looking through my photos today and noticed one from Mike Creek. He used what looks like a deep version of a 8-32 monadnock to retain that bulkhead. Pretty simple and elegant! All my monadnocks aren't nearly that deep so I'll have to find these.

Mike's photo:

[Zzz]

The extra deep monadnocks were the ticket:

[22LD]

I can't get to mine easily right now (floor is installed) for a picture, but I attached mine aft using the horizontal tabs to secure it in place. Then I added some plate nuts for attaching the floor along the bulkhead with aluminum spacers riveted to the top the same thickness as the tabs to shim between the bulkhead and the floor.

[Zzz]

I can't get to mine easily right now (floor is installed) for a picture, but I attached mine aft using the horizontal tabs to secure it in place. Then I added some plate nuts for attaching the floor along the bulkhead with aluminum spacers riveted to the top the same thickness as the tabs to shim between the bulkhead and the floor.

I was going to use nutplates initially too, with some adel clamps around the crossbar to attach it on its vertical face, but the monadnocks provide the additional function of holding the bulkhead in place when no fasteners are installed. They are quite a useful little device.