centersection tank cover

[sarpysam]

I'm new at this so forgive any ignorance I might display. I'm building a Hatz CB-1 and built an aluminum cover to place over the fuel tank. I've doubled the top capstrips so securing the sides is no problem however the attachment to the spar concerns me. I have 1/4" fillers between the ribs on top of the spars. Can I use a 1/2" X10 metal screw and run it in the top of each spar 1/4" (1/4" filler and 1/4" spar) at about 5 1/2" spacing? Any alternatives?

thanks for any suggestions.

Sarpy Sam

[crusty old aviator]

You shouldn't run sheetmetal screws into wood spars, like you're questioning, because water (rain, dew, melting frost & snow) can migrate down the screw threads into the wood and start rotting the spars. I would suggest wrapping the spar with a 1" wide strap of .040 6061 with the ends overlapping on the entire upper edge of the spar. Dill a #21 hole through both ends and into the filler blocks. Remove the strap and rivet a nut plate, like a K1000 with 8-32 threads, to the bottom side of the lower end of the overlap (strap) and enlarge the hole in the filler block just enough to accommodate the the nut plate. Reinstall the strap on the spar and transfer the hole location to the leading adge of the tank cover. Do this in two or three places between each nose rib, and cut your screws short if it looks like they may protrude through the nut plate, into the top of the spar.

Any of you rocket scientists out there have any better (as in simpler, stronger, lighter) ideas?

[WLIU]

I will suggest that all screws installed over the spar, of any type, will provide a path for water, whether they be sheet metal or machine screws. The best solution is to move that row of screws from over the spar to next to the spar. Gluing a sacrificial block to the front or rear face of the spar to accept the screws would accomplish that. And that block could be a type of wood that better holds screw than spruce.

If we can not do the above, the question becomes how can we use modern materials to seal the spar from the water. I suggest the following. First, put the screws into the top capstrips. Then remove them so that you now have the holes to work with. Mix up some of your favorite structural expoxy, West System, T-88, etc. Put epoxy into the holes and let the wood soak it up and cure. If you put too much into the holes, carefully use a drill, sized for the sheet metal screw to redo the hole. You have now created a nutplate in place. The wood should be sealed from the moisture.

I am told that CA glue can also be used to do the above, but I like epoxy.

I am not a rocket scientist, but I know a couple.

Best of luck,

Wes
N78PS

[sarpysam]

Thanks for the input you guys. I was starting to lean toward Wes's suggestion of securing a block just ahead of the spar and use that as an anchor. Still need to keep the water out of that as well but I think the epoxy will mitigate it.

Sarpy Sam