The firewall is made of a 15-18 mm plywood frame. I chose 18 mm Okoume, but planed the thickness down to 16.5 mm to save some weight.
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Cutouts
The cutouts to save weight are filled with PVC foam.
The cutouts were glued into the frame using micro slurry. Once cured, the PVC foam was first cut to thickness using a Japanese hand saw and then sanded flush to the frame.
Before skinning the assembly, the PVC foam was sealed using dry micro. Once more, a sanding excercise followed once the micro was cured.
TODO: Photo!
Dimensions
Important! The width of the firewall is not cut to final size in the beginning. The frame, with PVC inserts and plywood skins is assembled with 5 mm excess width on both(!) sides. That is, to 850 mm total width.
The reason is that the 840 mm width has to be achieved at the forward side of the firewall! However, the longerons of the fuselage join the firewall at an angle which requires the firewall to be "oversized" at the rear end. The final width of 840 mm is later achieved by sanding the firewall sides at the angle of incident of the longerons. This will be slightly different on each build since the exact angle depends on the exact properties of each ones Oregon pine strips used for the longerons.