Firewall Flange

[nichzimmerman]

In a post on another topic, someone mentioned that kit firewalls have an aluminum flange riveted on rather than just hammering a flange in the stainless. I was just about to fabricate my firewall from .016 material, and this is the first that I heard of an alternative to the plans. Anyone know what the pros and cons are of the these methods? I found myself wishing that the improvements made in the kit found their way into the plans more often.

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[rv8bldr]

That was me. The "pro" is that it is dead simple to do compared to trying to bend/flute/shrink stainless. What a miserable time that was. As far as "cons" are concerned, I can't really think of any.I will be sealing the intersection of the angle and the firewall with high temp RTV.

[nichzimmerman]

Can you give me an idea of how they are made? Aluminum angle or bent sheet? Thickness of aluminum? Shrunk or fluted? Rivet spacing? I didt have any trouble making the wing ribs, but I can see a large thin sheet like this might have some issues being flanges.

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[Mark Goldberg]

At the factory, we use scrap .050 5052 to make the angles and use a shrinker/stretcher (not the easiest job) to get the angles to conform to the curves. I think the bent over stainless flanges are easier/quicker. But we like the more finished look of the aluminum angle. Mark

[rv8bldr]

I use 2024t3 angle. I make it myself with a shear and brake (48" at a time), 1 1/4" wide. This gives me flanges about 9/16". Then, I drill a 3/16" hole every inch or so, right up against the bend, and then snip out a little triangle. Here are a couple of pictures of a piece (that I haven;t cleaned the edges up on yet). NOTE: I didn't drill the rivet holes until I had fitted and clamped the angle to the firewall. In retrospect, I should have marked and drilled the rivet holes BEFORE I cut the little triangle bits out. This would have given me more perfectly aligned rivet holes.

[Battson]

The Bearhawk kitset riveted-on firewall flanges are a fine idea.

They are:
- easier to work with in many regards, than stainless, when finishing the cowling (more metal to work with)
- avoid a few dissimilar metal issues with non-permanent screw joints
- seal perfectly without any extra sealant
- are easier to repair and replace parts of, if things are damaged during the service lift of the part

[JimParker256]

Here's are a few pictures of my Patrol QB kit's firewall, just as it came from the AviPro factory. They do nice work!

[whee]

We are starting on the boot cowl but ran into a snag. We built the flange on the firewall and on the instrument panel using angle aluminum. We didn't think about the fact that the firewall and the instrument panel sit at different angles and so the flanges aren't parallel with each other. If we line up the boot cowl with the flange on the instrument panel then how do we attach it to the firewall flange and have it look right?

[Mark Moyle]

Perhaps.... Identify the angle change, top, upper radius, side and bottom. Remove the firewall. Secure the assembly to a sheet of 3/4" plywood using wood screws and fender washers at the rivet holes. Bend the aluminum angle in conjunction with fluting pliers, dolly and body hammer.
Edit: I'd try what I suggested. I would probably mess up the stainless.... Then make a new stainless firewall and form 6061-T6 .032" into an angle....shrink to get the radius right where needed.

I used .032" 2024T3 for my boot cowl. Used the spring clamps to make the firewall lip fit the boot cowl.

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[whee]

That's pretty much what we've decided to do. Hopefully we don't mess up the SS. We are also going to block the nose bowl in place and make sure the flange angle will work with the engine cowl before we start pounding.

[Battson]

That place at the top of the firewall is a funny one - the angle of the engine cowling and the angle of the boot cowl are not the same, and they merge at that point. So there is always a slight bend there. Although I it looks like that flange pictured is at right angles to the firewall, which will clearly need to be modified regardless of the aforementioned.

I believe the flange needs to angle inward some the whole way around, but more so at the top corners, and less so at the sides. I am pretty certain the ideal angle varies around the periphery of the firewall. If your flange is a little bit flexible, that may not matter at all.

[svyolo]

I found this thread as I am having the same question. With the kit flange, what is the best solution? Flute the flange a bit and bend it inward to match the angle of the boot cowl?

[Archer39J]

I found this thread as I am having the same question. With the kit flange, what is the best solution? Flute the flange a bit and bend it inward to match the angle of the boot cowl?

I found it wasn't an issue when I did mine. Everything looks good when assembled. I'll still be sealing with some high temp RTV as suggested in earlier posts.

[svyolo]

Dave, can you explain? The angle is very different is some locations. I don't see how you could rivet it, and nut plates would put a crease in it if you didn't cross-thread the screws?

[Archer39J]

Dave, can you explain? The angle is very different is some locations. I don't see how you could rivet it, and nut plates would put a crease in it if you didn't cross-thread the screws?

I just trimmed the boot cowl skins by wrapping them around the firewall flange and holding them in place with a ton of those padded clamps from harbor freight.

The boot cowl skin won't sit flush on the flange just as is noted in earlier posts, but for me at least it looked fine when I drilled, countersunk and installed nutplates. The dimpled boot cowl skins sit nicely when screwed in, though I might need to open up some of the countersinks in a couple spots but this isn't a fault of the flange angle.

Here are some pics that kinda show it but I can take some better ones when I get home.

[Mark Goldberg]

The correct angle of the flange at the top of the firewall changes around if you move around a little towards the sides. What the workers at the factory do is put a straight edge on the top of the instrument panel and the other edge on the firewall. They adjust the firewall flange angle as they move the straight edge to get the flange to be in line with how that boot cowl skin needs to lay flat. Pretty easy thing to adjust if you find the angle you have on your firewall is not optimal. Mark