Firewall

I held from cutting the firewall until after I finished Chapter 5 (the fuselage sides) so that I could measure the exact dimensions of the longeron cutout locations.

I began by tracing the firewall outline onto the birch plywood sheet. I left a 1" overhang all around the top half so that I could match trim to the cowling outline later (this takes care of LPC #48 which gives an outline to better match a 'standard' cowl; however, I rather leave extra room and match it to the exact contour later).

I then traced the longeron locations (per full scale drawing) and then verified the dimensions with the actual fuselage longerons. Here is a close-up of the bottom left firewall.

Close up of the upper longeron outline.

 

Close up of the center hole outline. 

 

And now to cut all the holes. In hindsight, I probably should have waited to do this AFTER glassing. I first started to do this with my dremel... but it had a hard time going through the 0.25" birch plywood steadily. So I decided to use the all mighty router. I used a drill to make pilot holes, and then cutout the outlines. This worked great! The router has enough power to go through the birch like butter!

Notice how I clamped it down to my bench, counter-levered so my router wouldn't cut through my bench! I then used metal files to square the corners so they would be perfectly 90 degrees.

After all holes were cutout, I test fitted the firewall onto the fuselage sides. A little filing was required to make everything fit. I also had to trim the lower longerons which I had previously intentionally left extra long.

Taaaa-daaaa!!! A firewall ready for glassing. Notice the 1" overhang on the top half (again, this will be match trimmed to the cowling later).

Cutting the fiberglass cloth. The plans only calls for 1 ply BID on the front and 1 ply BID on the back.

Wood does NOT require slurry prep the way foam does. However, I did apply a nice layer of epoxy to assure no air bubbles would develop between the grain of the wood (even though birch is relatively smooth).

 And here is the aft face glassed...

After a full cure, I flipped the firewall over and marked where the rudder pulley mounting screws will go. LPC #47 says to raise the left side holes up 0.6" to avoid possible rubbing of the cables with the aileron pushrods. My OCD had me do this on the right side as well (just so that both pulleys would be at the same height).

After drilling out the holes, I countersunk them on the fwd face.

 I then inserted the screws into place and made sure the heads were not above the surface plane.

I then applied a layer of epoxy...

Notice how I added extra epoxy over the heads to make sure they would bond to the glass and prevent them from turning when the rudder pulley brackets are installed later on.

I then applied the fiberglass over and wetted everything out. I then peel plied all the edges (where they will later bond with the fuselage).

After a full cure it was time to trim all the edges and remove the fiberglass from all hole cutouts.

To do this, I used the dremel. I set it to a speed such that it was powerful enough to go through the fiberglass, but not strong enough to continue eating into the plywood.

And there you have it... the firewall ready to be installed onto the fuselage in Chapter 6. I'll leave the stainless steel sheet for another day.