Here's What Worked For My Tank Skins
I started with outboard leading edge ribs clecoed in place and the tank ribs clecoed and pop riveted to the baffle per the manual. That pop riveting step is important.
Next, I made a set of what I'm calling Howe Fittings for David Howe, who explained how they provide the necessary resultant force vectors to the skin. I made one per strap per side. Since I used four straps, that's four of these on either side. Here's a top one.
David suggested using strapping tape to help hold the ribs and it really, really helped:
With these preliminaries out of the way -
1. Fit the tank skin to the ribs. Tighten the straps and verify that the top side?s aft edge is very close to the main skin?s forward edge. Assuming it is, and both of mine were,
2. Unstrap the skin and remove it.
3. For the top of the wing, drill the rib flanges for the holes. On the inboard rib and the outboard rib, if it hasn't been reversed, drill the bottom flanges too. These are accessible with the tank skin on. Some people reverse the outboard rib to gain a bit more fuel capacity, and if you're doing that then its bottom flange isn't accessible, so don't drill it.
4. Draw a centerline on the baffle flanges for the eventual holes. Mark the spar flange and skins for the maximum inboard point that you can safely drill the baffle from the outside. Tape on tell-tale line extensions so you can find the centerlines after the skin's are on.
4. Hang the skin and position it using those 1? cleco tabs. At this point, the main skin will need to be on to provide location. Also the outboard skin, for the same reason. Tape the joint between the top tank skin and the top main skin in between clecos. Try to get about 12" or 16" of tape, total. This is to ensure that the tank skin can't shift laterally. Once the tank skin is in position, remove the outboard skin.
5. Attach a 2x2 batten partway forward on the top skin, a bit forward of the bottom-most rib flange holes. Clamp it to the inboard tank rib flange and the inboard-most outboard rib flange, which is accessible now.
6. Back-drill through the top rib flanges, one row at a time, clecoing as you go. Start with the aft-most holes and work forward. Move the batten one row forward and repeat until all but the top holes are drilled.
7. Using a block of wood to protect your hands, reach the leading edge from the bottom of the tank and pull it tight to the rib flanges, and drill the top holes. You?ll need to cleco every hole or two. It's a bit awkward but not too bad. I used a 12" drill bit for all this. At this point, all the holes in the top skin have been drilled and clecoed.
8. Reapply the Howe Fittings and the straps and move the batten to the forward bottom of the leading edge, where it's almost flush with the very front, and tighten it down. This involves unclecoing the main skins so that the straps can pass around the spar. The baffle will deform under the strap load with the skin sliding aft a bit.
9. Mark the bottom aft edge of the tank skin for the position of the bottom main skin?s forward edge. This isn't a trim line. It's only there to locate the rib flange hole positions. DON'T trim to this line!
10. Measure from there and lay out the hole positions in the bottom skin.
11. Unstrap the skin and, with it loose on the bottom but clecoed on the top, drill the forward-most holes that clear the batten. Just drill through the skin, not the ribs. These holes are probably the second or third aft of the very front-most holes.
12. Restrap the skin down with its batten and verify that it?s in position and that the leading edge is acceptable, and most importantly, that these new holes line up with the rib flanges. How tight? My straps were scary tight.
13. Drill through these holes, through the rib flanges, and cleco. Back drill through the exposed end rib flanges at this position.
14. Working aft, one row at a time, drill and cleco the skin.
15. Remove the straps and the Howe Fittings. The baffle will relax and the tank skin, at the bottom, will shift forward just a bit.
16. Using the centerline extensions you put on for the baffles in step #4, draw lines on the skin for those holes. Locate the holes, making certain that you don't go so far inboard that the spar is in danger.
17. Drill and cleco the skin to baffle holes.
That's as far as I've gotten so far. Both tank leading edges are acceptable and I'm going to do the outboard leading edges next. I haven't removed the tank skins - warning - there's a huge amount of tension on the skins so strap it down for that.
Dave
