Firewall - parallel or according to directions?

I'm at the point where I am fitting the firewall to my -7. The build manual says to mark the center line of the bottom AL angle, center that in the second row of holes on the belly skin and drill two of the holes. Did that, it's all good, and I was careful to mark the exact center of the angle.

Next it says to make sure the firewall is 5/8" aft of the front edge of the forward skins. I can set the top of the firewall to 5/8" aft... but the BOTTOM of the firewall is 11/16" aft of the forward edge of the skins, exactly the same on both sides. So if I set 5/8" at the top edge, the firewall will have a slight downward tilt. Not much, maybe .15 deg or so. But I don't know if the firewall is supposed to be parallel with the front edge of the skin, or be 5/8" at the top and 11/16 at the bottom. I remember when building R/C models with my Dad, we'd have a little down and right thrust. This, however, is not balsa and pine.

My gut tells me make it parallel and set it to 11/16 at the top and the bottom. Anyone else run into this?

Hi Dale,

I just double checked my bird on the factory floor ... both the top and bottom of the firewall are at 5/8s of an inch aft of the skins. I believe I doubled checked this prior to drilling the two alignment holes. I did have to trim the longerorns a tad to get the spacing right for the top end.

I don't know how parallel the firewall needs to be. Consulting the mother ship is always advisable; doesn't seem like a big issue though (correctable with washer under the engine mount if you need to jimmy in an extra 1/16th of an inch)

Yeah... I'm going to make it parallel with the edge of the skin.

I suppose I could have adjusted the bottom edge to be exactly 5/8 before drilling the two holes, but I marked dead nuts on the center line of the angle like it said. I can think of no good reason to drill the firewall at an angle, no matter how slight... and I'm fairly sure the build manual would have pointed out of there were supposed to be any difference between the top and the bottom. It's actually more like 3/64 difference. Would probably be even less if the edges were deburred.

Just second guessing, I guess. Oops! That's third-guessing now. Darn.

Sounds like the judicious application of the deburring tool will fix all... ;-)

Well now I feel a little foolish. When I went back to try and work it out again, I noticed that I could measure the same place several times and get slightly different results. Finally I got out my machinist's square to help measure instead of "eyeballing" it from the side. Apparently I was getting a little parallax. It's actually within a hair of 5/8 at the bottom. I set it to the exact same spacing at the top.