It's 1/5 scale, a Sig kit I modified to an L-4B. The center section is very different from the 'stock' Sig plans. In the 'stock' kit, there are two airfoil-shaped formers in the middle of the center section. I eliminated them, and doubled the thickness of the rib root formers, using lite ply. The main spar in the center section is as the plans call for, with an addition of a piece of triangle stock along the upper forward edge; it is hidden by the angled former that comes up from the forward canin floor, and becomes the 'A' pillars between the windshield and side windows. This triangle piece actually attaches the spar to this angled piece, and makes a very solid crossmember from one wing spar to the other. All the window framing in the greenhouse is basswood or spruce, and the crosspieces are fitted into notches in the wing roots and pinned as well as epoxied. The aluminum wing tubes are also omitted, as they detracted from the scale appearance.

In the full scale L-4, there is a tubular framework "jungle gym", and it is duplicated with 1/4 and 3/16 wooden dowels on my model. These pieces are tightly fitted and epoxied, and the joints have a #0 screw (actually a 2mm titanium screw used in orthopedic surgery to repair finger bones) through them for added strength. The angled braces that go from the crossmember in the top of the center section to the rear deck at the back of the cabin are glued to angle blocks attached to the fuse frame behind the angled bulkhead at the back of the cabin. All this triangulates to form a pretty solid structure that does not flex. The uprights from the floor of the cabin (two on the left side and one on the right) have 1/4 inch dowels glued to them from the floor to the crossmember at the top. This crossmember extends through the root formers, and fits in a hole in the wing roots when you attach the wings; this helps establish the correct incidence. The 'stock' Cub has only one bolt to attach each wing at the root, I added a second one just behind the spar, and used fender washers to distribute the compression load of the screws when you snug them down. The angled former in front has also been reinforced with 1/8 x 1/4 carbon fiber strips along the bottom edge from the cabin floor to the spar.

I had no idea when I started this build how much reinforcement I would need, so it may be a bit over-engineered; rather too much than have the center section come apart in the air from stress. As you may know, the wing struts are functional on this kit, and take a good bit of the inflight wing loads. I have been flying this plane for over a year and a half now, it has been 'crashed' a couple of times. In one of these, I bounced a hard landing,( I had forgotten to hook up my aileron servos when I attached the wings, didn't pay attention to my preflight check, stupid me. I found out a Cub can be flown on rudder and elevator only, but not something you want to do on purpose) and it came down on the nose hard enough that it sheared the root ribs out of the wings, they folded forward, pivoting on the strut attach points on the side of the fuse. There was no damage to either the struts or the center section. In the other crash, I stalled after a deadstick, trying to stretch my glide to the runway, and the plane pitched straight down from about 75 feet into a thicket of kudzu. It was like landing in a big spider web, the vines snagged the wings, and the plane didn't have a scratch on it. I'd say the center section is as least as strong as the original plans.