904 bulkheads/976 center skin

Joe Parish · Post by **Joe Parish** » Wed Nov 13, 2013 10:52 pm

Looking for input from the crew. I am working on the center section and about to tear it down for priming. Trying to decide what to do with the 976 center skin and 904 bulkhead. I am leaning toward countersinking the 976 center section bottom skin which will also slightly countersink the flange on the 904 bulkhead. There are four spots that will require this option to allow stiffeners to sit flat against the flange on the bulkhead.

What option did you chose and were you happy with the results? The bulkhead flange is .040 which will be hard to dimple I would think? If I countersink the 976 skin and into the 904 bulkhead, then dimple the 972 skin which will then nestle into the countersunk holes in the 976 skin should work nicely I think.

I checked several sites but did not see where anyone spelled out what they did. Thanks for the help. Here is a link to a PDF from vans site. The last two paragraphs talk about the options.
http://www.vansaircraft.com/pdf/7_9_fuselage.pdf

Thermos · Post by **Thermos** » Thu Nov 14, 2013 6:52 am

Joe,

I went back and forth on this one, but in the end I fell back on the "dimple where you can, countersink where you must" philosophy. The 0.040" bulkhead flange is right on the edge of acceptable thickness for dimpling, but it can be done with a quality set of dimple dies.

I dimpled the bulkhead and both skins with Cleaveland dies (can't buy better, IMHO) and everything came out fine when I riveted them. I know others - Captain John for one - who countersinked (countersunk?) one of the skins, and that worked well too - but he can tell you more about that.

Dave

captain_john · Post by **captain_john** » Thu Nov 14, 2013 8:32 am

Yup,

As i recall, I dimpled the floor skin.

Pretty sure I countersunk the spar and nested the middle skin into it with a dimple.

I would have to go back in my records to be sure.

CJ

Joe Parish · Post by **Joe Parish** » Thu Nov 14, 2013 8:56 pm

I believe I will try the dimple route. I will try to post my results when I am done. Thanks for the input.

Thought I would leave an update. I talked with a multi RV builder at our EAA pancake breakfast this weekend and he steered me to the countersink route mainly due to the thickness of the flange on the 904 bulkhead. That is what I did, turned out good I believe. I took a scrap piece with a dimple on it and checked each one as I progressed. Thanks