Tank Alignment help needed

shadrach · #1 08-18-2010, 09:22 AM

I'm currently working on my wing kit. Its an old 97 kit with only the skins being pre-punched. I installed the nutplates on the top flange of the spar per the plans before starting on the skeleton. When I drilled the tank skin to the ribs and baffle I used two straps to cinch the skin down tight to the ribs. I did not drill the #30 size screw holes on the top side out yet as I used #30 clecos through those into the nutplates to hold the skin on the top side. When I cinched it down, the bottom edge overlapped the bottom main skin by 1/16" to get it touching the ribs all the way around. Before anyone suggests it, no it was not over-tight as I could easily pull the straps away from the skin on the bottom side and for most rivet holes had to actually press the skin to the rib by hand. When I disassembled, the foremost edge of rivet rivet holes into the baffle on the top side are within 1/32 of the start of the radius. (Top of baffle to center of hole is actually .25) By my calcs, I think I would be within military specs to place a 3/32 rivet there but I cannot get even the avery close quarters female dimple die in there to dimple the baffle. If I countersink, I know the hole will wind up out of round. On the bottom side, the holes are 5/16 from top of baffle and can easily be dimpled. The bottom also overlaps the main skin by 1/16 which is more than double what the manual says is allowed.

Anyone have any ideas how this happens and how I can avoid it when I replace the parts and redo? Should I redo? Ken at Van's says I shouldn't have to replace the parts but I don't know that I like the idea of making do with it.

Thanks,

Shadrach

sf3543 · #2 08-18-2010, 10:42 AM

You can grind the edge of the female dimple die to a radius that will fit into the baffle so you can dimple it.
I have quite a few "special" tools that were modified for use by the grinder.
Good luck.

airguy · #3 08-18-2010, 12:36 PM

Yep, I ground down a dimple die way back in the empennage section - don't know how you managed to get this far without it...

evmeg · #4 08-18-2010, 08:35 PM

If I am reading this correctly you should be just fine. Look carfully at the plans and you will see that the standard way to build this is NOT to dimple the skins or the baffle at this joint.....the holes are too close to the radius as you note. You are supposed to machine countersink the tank skins along the baffle line and do nothing to the baffle at all. The tank skin is .035" so you have enough material to do this. Call me if you have questions.

Cheers...
Evan Johnson
(530)351-1776
evansaviationproducts.com
400+ sets of RV fuel tanks.....I quit counting

shadrach · #5 08-19-2010, 07:16 AM

Thanks for the responses.

I wanted to dimple the skins and either dimple or countersink the baffle as I like the idea of having as much material as possible for the rivet to hold onto but it appears I will have to go strictly by the plans if I don't rebuild.

If I let it ride, obviously something is still out of alignment as the bottom edge overlaps the main skin by 1/16". The top edge is nicely butted to the top main skin and so one would have to conclude its not a mere shifting of the skin by 1/16" but a compression of something inside the tank. Does anyone think this will adversely affect handling or performance?

WenEng · #6 08-19-2010, 09:03 AM

prior to drilling, I suspect your shims were a tad too low, allowing the tank to compress a little lower than usual. The skin is designed to have a little oversize to allow for flush trimming with a file or other appropiate tool for a perfect fit against the bottome skin.

You don't need to dimple the baffle. I did on mine and it worked out just fine after using a ground down dimple die to keep from denting the baffle curve. Sounds like you might be hesitant to countersink the tank skin. It's quite simple to do. Just practice on scrap first and use a slow battery powered drill.