Tip: getting a tight joint at the conical bend area

A friend and I did a bunch of riveting over the weekend. One area which was the source of much consternation to me came out well enough that I thought I would share. It's the corner where the side skin meets the bottom skin and the conical bend starts.

Here's how it turned out:


This is what I did to get a tight fit:

1. Elongate the bend relief at the vertex of the conical bend until it is at least 1/8" from the bottom edge of the side skin. I found that the bend relief as supplied from Vans is not high enough on the side of the fuselage to allow the vertex of the conical bend to be made with a 3/32" radius (the minimum recommended for this thickness of skin).
2. When making the conical bend, put a 3/32 radius on the clamping angle so that you don't end up with a sharp bend at the vertex and possibly a crack.
3. Arrange the bend so that it starts 1/8" above the bottom edge of the side skin. The 3/32 bend radius in combination with the skin thickness of the conical flange will result in the flange of the conical bend being level with the bottom edge of the side skin.
4. After making the conical bend, it's likely that the prepunched holes in the flange of the conical bend will no longer line up with the prepunched holes in the bottom skin. Since this flange gets sandwiched between the bottom skin and the triangular rib, I simply pushed the flange as far in as possible to tighten the gap as much as possible, drilled extra holes halfway between the prepunched ones, then match drilled through the original prepunched holes. The prepunched holes in the conical bend flange ended up being figure 8's, especially near the forward end of the flange, but that all got trapped under the bottom skin.

Hope someone else finds this useful.

Wow Mike... that looks fantastic! When I see your plane at OSH one of these days I'll have to climb underneath and admire your work.

Helpful

The above info was helpful, I'm doing the conical bends on a -6A and the kit instructions give an overview but not much detail.
What I did so far:
1. Marked out radial lines on the curve to act as a guide while making the bend. This helped.
2. Used a wooden form block with a 1/8 radius to help form the narrow end of the bend.
3. Clamped the form block with the edge 1/8 below the projected line of the belly ribs. This is so that the curved section will meet the belly skin in the right plane.
4. Made a relief hole at the narrow end of the cone to prevent cracking.
5. Trimmed the aft edge of the side skins AFTER completing the bend. Another builder warned against trimming earlier... otherwise the cone may be too short to meet over the bulkhead at the rear of the baggage compartment.

Next thing to work out is getting the side and rear skins to meet at the same angle in the curved area. Some builders use shim wedges and others cut and bend tabs in the skin.