Center Spar Supports
They hold the wings up on the ground, and down in the air...
As discussed in my "Starting on the Fuselage!", the gear pockets, as described in the build manual, are not included with the kit, because the mold for them is bad. This left two options, modify the gear pockets till they fit, or do it the "old way". The old way, required flipping the fuselage upside down to do fiberglass lay-ups; well, that wasn't gonna happen, so modifying the bad part seamed best to me. Chad went ahead and sent me a pair of the bad parts.
The area where one side's gear pocket was supposed to go, and the main spar would need its main support structure.
The factory gear pocket was too long, too tall, and did not match the curve of the inside of the gear bulkhead; Therefore, it would need some serious trimming...
The gear pocket, as it came from the factory.
Here is a Spar Support temped into place. Eventually there will be a cut-out opening under it, for the main gear to stick through, and a cut-out above for the main spar to pass through.
The cut-outs the main center spar will eventually fit through.
With the spar supports in place, and the spar openings cut out, you can see how the spar will eventually sit on top of those and stick through the openings. The center spar is where the wings attach to the fuselage, so getting this area right is absolutely essential!
Close up of the outside corner of the original gear pocket part, from the factory. Not exactly professional quality, but fixable.
This was a major blow to my plan. The side absolutely must be attached securely to the firewall, to support the main spar; however, the top must be level too. At this point, I had to completely reconsider using this part; however, I decided this would be easier to fix than flipping the fuselage upside down to do it the old way still. For now, getting the support installed as securely and leveled side to side as possible is my only concern.
Using the bubble level attached to the canard bulkhead, I leveled the spar supports laterally.
After some minor final adjustments, I got the spar supports ready for installation. As always, all bonding surfaces were sanded and prepped. Structural adhesive was applied to both the entire side wall of the spar support and the area of the firewall it was bonding to. This was to ensure a 100% solid adhesion over the entire surface of the spar support. I then pressed the support into the firewall, to squeeze out all the excess adhesive. This excess structural adhesive, was collected and used to create an initial bond to the gear bulkhead and fuselage.
The pilot side spar support initially bonded in place.
I then put 4 large pop rivets through the back of the firewall, to further squeeze and stabilize the spar supports.
The first 3 pop rivets can be seen poking through, on the top half of the support. The fourth will go near the bottom later.
The first three pop rivet heads, from the engine side of the firewall.
The pilot side spar support initially bonded and fully riveted in place.
Next I put a layer of BID fiberglass from the top of the spar support onto the gear bulkhead, to help reinforce the attachment of the support to the gear bulkhead.
The wet fiberglass, is almost clear, but the different texture of the BID on the right side is the top reinforcement. This also started the process of leveling the surface, which would be completed later.
The key to my design for the modified spar support, is to apply 6 layers of triax fiberglass from the bottom of the spar support onto the gear bulkhead. The "old way" was to use 6 layers of triax in a U shape from firewall, across a foam support and onto the gear bulkhead. Due to the extreme importance of this structure, I basically have layer of triax from the gear bulkhead on two of the three sides, but to ensure proper support, it needs to be supported on the gear bulkhead side as well; hence the 6 layers across the bottom and down the gear bulkhead. This will effectively create the solid upside down U to properly support the main spar.
Unfortunately, this was easier said than done! Getting the fiberglass to stay attached to the bottom of the spar support was not easy. Overhead fiberglass layups is much more difficult than overhead welding! After a lot of trial and error, research, and advise, I came up with a technique that made it go very well and smooth every time.
The first step was to go back and apply a fillet of structural adhesive between the spar support and gear bulkhead. This fillet is to allow the triax to have a smooth transition between the two surfaces without any separation, and triax cannot make it around that tight of a corner without the fillet.
Eventually, the technique I used was to do one layer at a time. First, I applied a coat of the fiberglass epoxy to the bottom of the spar and down the gear bulkhead. This is left in place for 4 hours before layup time (times will vary with different environmental conditions). One hour before layup time, the fiberglass piece is wetted out with epoxy, on a piece of plastic drop cloth, in normal plastic transfer fashion. At layup time, I mixed a small fresh batch of fiberglass epoxy, and took that, my tools, and the layup to the plane. After 4 hours, the epoxy is very thick and very very sticky, and it acts a lot like contact cement. From there, it is just a "normal" plastic transfer layup; however, this time it just happens to be upside down! I started sticking the layup to the bottom of the spar, where it meets the firewall, and worked my way to the corner and down the gear bulkhead. Then, just as normal, peel the plastic off and use the brush and epoxy and a scraper to make sure its fully pushed into the spar support. When peeling the plastic, using your free hand hold the exposed fiberglass up, so the peeling doesn't pull the whole thing down! Use fresh epoxy sparingly, and only if the layup is having trouble sticking.
The under side of the spar support, focusing on the triax reinforcement. The 4 rivets can also be seen along the left side of the photo.
Due to the 4 hour time to do these layups, and the 6 layers to be done, it took a few days to complete these layups. The good news is that I had plenty of other stuff to work on while doing this. Mostly, I was installing the front gear doors, while doing this.
With the spar supports fully structurally complete, it was time to work on the fore-aft leveling of them. The plan was pretty simple: add successive layers of BID fiberglass to the spar support, to create a level platform for the spar. The spar is only 3" wide, so I only leveled the first 3 1/2"; the rest then slopes back down slightly.
To create the right angle, to make the surface level, I started 1" back from the firewall, and made each layer 1/2" shorter than the last.
You can see the first area, with no fiberglass, then each layer of fiberglass added to make a level surface.
From above, the leveling layers go all the way back to the gear bulkhead, to make a consistent surface, and extra support for the center spar. I also made the layers wide, so they could be trimmed clean. This was purely aesthetic, for a part that will never be seen after completion, but I will always know!
After sanding, the area the spar will rest on became a flat and smooth surface, perfect for mounting. Now the mounting surface is level in both directions!
The end result of the spar support work. After sanding, you can see the "bend" in the top surface after the end of the spar opening.
With that, the center spar supports are complete and ready for the main spar to be installed...
I will hold off the center spar installation as long as possible, while completing other parts of the fuselage. I am doing this because once the center spar is installed, the fuselage will be much wider and harder to move around as needed. I will also have to walk around the spar sticking way out, to get between the front and back of the plane, so I'm going to finish as much of the fuselage construction as possible before installing the center spar.
Speaking of which, the next step was to install the front landing gear doors...