Fuselage fitting: This is too easy

This comes mostly under the "this is too easy, better check" file. I fitted the firewall to the fuse today. Piece of cake, just as mating the center and aft fuselage was a piece of cake and just as fitting the side skins was a piece of cake and jsut as rolling the curve in the side skin was.....ahhh...well..not so much.

Anyway, the instructions advise to check that three conditions exist, not the least of which is "The forward face of the firewall is 5/8" aft of the forward edge of the F-770 skin."

OK, fine, but normally the instructions would say something like "be sure the flange of the firewall is flush with the forward edge of the F-770 side skin" because, well, the flange is 5/8". Right?

Now a harder question.

The next step in the instructions say "When all these things come to pass, drill the F-770 skin to the flanges of the firewall, starting at the top longeron and working toward the bottom. Since only one longeron, the top one, is installed at this time, and the thing is flipped over, I assume this instruction is the same as "start at the bottom where the longeron is and work toward the top" (as presently positioned).

Right?

I'd be flying this damned thing by now if I didn't double-check every little thing.

OK, fine, but normally the instructions would say something like "be sure the flange of the firewall is flush with the forward edge of the F-770 side skin" because, well, the flange is 5/8". Right?

Click to expand...

My flange had some slight variations in width.

The next step int he instructions say "When all these things come to pass, drill the F-770 skin to the flanges of the firewall, starting at the top longeron and working toward the bottom. Since only one longeron, the top one, is installed at this time, and the thing is flipped over, I assume this instruction is the same as "start at the bottom where the longeron is and work toward the top" (as presently positioned).

Right?

Click to expand...

Correct. All references to top/bottom/left/right in the plans and instructions are relative to the plane in flight. You get to translate everything when it's upside down.

Drilling the firewall is a big step. Better to double-check than to have a crooked firewall.

Sure. I don't want a crooked firewall. But it's interesting to me, for example, that Van's only has you drawing a centerline on the bottom angle and then, of course, line the line up through the holes and voila! You've got the bottom distance to the firewall equal across the span.

Similarly, you could draw a line on the flanges of the firewall and have them line up through the holes in the side skin and, theoretically, it would guarantee a straight line relative to the 704 on the sides.

I just found it odd that on this step, Van's abandons the method that it's used to instruct us builders through the Emp and then the wings.

I'm just at this step now, and wondering what others' solutions are to this:

Drew the centerline on the bottom angle, then centered it in the second row of holes on the bottom skin. Guess what? The distance from the edge of the *bottom* skin to the firewall is a little more than 5/8", perhaps 1/16" greater or so. (And the instructions only reference the distance from the forward edge of the *side* skins, and only mention that *after* the instruction step to drill two holes in the *bottom* skin to hold the firewall in place, which I have not done yet).

I'm thinking the right answer is to make the firewall face 5/8" from all (bottom and side) skins and drill it and be done with it.

I have the motor mount now, as well, and my buddy advises using that to check for coplanarity of the firewall mounting positions, as well (which I think is a great idea).

Anybody have a similar deal with the edge distances? Any pointers or advice?

ETA: OK, I just read something that might make this go away...per jmjula's website:

"One thing I learned, which I didn't find out until after I had to work around it, is that I have been marking flanges on the angles incorrectly. This isn't documented, but when drawing a center line down a flange, it is not exactly half the width. It is half of the width minus the thickness of the other flange. So a 1/8" thick 3/4" x 3/4" angle gets a line 5/16" from the edge, not 3/8"."

Hmmmmm....I've been doing the same thing, and up until now it hasn't seemed be much of an issue because nearly everything that has been marked like this is thin aluminum (things like control surface stiffeners, etc., where the difference would be really small), not the thicker AA6 parts, so that might account for the difference here...but now that we have a bunch of thick aluminum angle (1/8" or thicker), it definitely would show up. I'll redo the centerline tomorrow and see...

Of course, now it cheeses me off that I may have a few odd parts here and there that should have had a very slightly different centerline drawn before drilling. I *hate* mistakes like that...

Good catch, I have been doing the same, and will be at the fwd fuse / firewall very soon. Thanks for the heads up !

Yep, that was it! Thanks to Mr. Jula for the tip...once I redid the centerline to be (3/4" - 1/8")/2, it worked out nearly perfectly, with the bottom face of the firewall almost precisely at 5/8" in from the edge of the side and bottom skins. Drilled a few holes on the bottom, then set 5/8" inch distances all around and used the motor mount to check for coplanarity...perfect! Biggest bitch was getting the weldments and longerons all set up so the already drilled holes were in alignment and such.

But, she's done...building on!

F-770 sideskin to firewall edge distance

Hung my firewall and noticed that the top rivet of the F-770 sideskin is not going to have enough edge distance on the firewall flange, because Van's made a notch on the firewall flange too big.

I guess it won't matter because there will be a backing plate for the cowl fasteners here? I'd like to make sure the corners of the side and top skins are well attached here.

Anybody else have this problem?

-T

Me too

I haven't drilled yet, but noticed the same spacing issue for this hole. Anyone else?