SIG Rascal 110
Preparing the firewall for the battery tray.
The batteries will load through the firewall.
The hole I have in my firewall is 2 5/8" high by 3 1/4' wide. The extra width is to allow for a lot of cooling for the batteries and speed controller. You may not need to go as high with a 10 cell set up (mine is a 12 cell set up). I have an extra cell to get through the hole. The narrow part at the bottom is 2 1/4 wide. The part of the firewall each side of the narrow part is where I put the 2 4-40 blind nuts to hole the tray in. The piece of ply that holds the batteries is Midwest birch plywood 3/32" thick. It is 1 15/16 wide by 15" long. The pieces at the firewall end will have a slight angle do to the firewall offset. I put the tray where I want it and then just trace a line on the wood along the firewall. I use industrial strength Velcro to hold the batteries down. The two screws hold the battery ply down onto the 1/4" piece of ply that the 4-40 screws are in (epoxied also). You will want to put a piece of 1/2" X 1/2" X ? plywood across the inside of the firewall above the opening to reinforce it. It will have 45 degree cuts on each end. The part that the battery tray slides onto that is glued inside the plane. I made it real light because its only purpose is to guide the batteries into position. It is made with Midwest 1/32" plywood and is 2 3/4" wide and approx. 15" long. The side rail pieces are 1/2' X 1/4" balsa glued on vertically.
The piece in the picture below is glued to the floor of the Rascal. You can put some pieces of balsa underneath it to support it. This is the floor of the battery tray that guides the batteries into position.
The picture below shows an arming switch installed The purpose of the arming switch is to open the circuit to the batteries so the batteries can be installed through the fire wall without worrying about powering the motor while your hands are near the prop.
The picture below shows the floor of the battery tray glued into place.
If you look at the first picture below carefully, you will notice that there is and angle at the firewall end of part of the tray that holds the batteries. It is do to the offset of the firewall.