Firewall to Tunnel Transition

[spinningwrench]

Wondering how others have handled this transition. The flap on the firewall is flat but needs to assume the compound curve of the tunnel when bent over.

[JimParker256]

A picture would sure help me visualize what you're talking about.

[Archer39J]

The firewall flange and tunnel are straight on my kit, though I recall this might be specific to kits and plans might have a curved tunnel but I'm away from home and can't check. I think the bearhawk book mentions this.

[alaskabearhawk]

I happen to be working on my boot cowl today. FWIW, This is my exhaust tunnel...flat front to back. The plans have a curved bulkhead at Station B, Bearhawk Aircraft uses an aluminum bulkhead with angles. I adopted that approach in my build. Eric Newton used the plans approach and details his exhaust tunnel in his build manuals.

[JimParker256]

That's what was throwing me off. On the Patrol QB kit (which is the forum subsection where this thread was created) uses a flat tunnel like AlaskaBearhawk's above. I was having trouble picturing something "rounded" in that area...

[spinningwrench]

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The former at station B in the Patrol is a continuous curve as pictured in the middle picture. That is different than the flat former of the four place which is what Eric Newton is detailing. The tunnel as supplied has a continuous curve in it as pictured on the left. It would take a great deal of pressure to force the front of the tunnel to be flat while the rear is curved. Is that the design intent? The drawings are silent on this.

[alaskabearhawk]

The BH 4-place that Eric built is like that...curved in back and flat up front. Just put pressure on the corners up front and it will flatten out. That's what he did.

[Bcone1381]

edit....See post #9 for photos.

The front of the exit is square and I think meets the design intent. It never crossed my mind that the leading edge of the exit area might be anything but square. I understand that the tunnel part is difficult to fabricate. So some curvature on the leading edge of the tunnel part from the factory that we builder are required to give additional attention to is very reasonable . I don't recall, but may have done some additional forming on the leading edge by carefully rolling it on a coffee can size cylinder.

The leading edge of the tunnel extends up under the fuselage cross tube. The firewall material is formed and wrapped around this cross tube. Its secured it to the tunnel using five AN526 truss head 6-32 machine screws and nut plates. On the first photo (seen in post #9) you might see about 5 cleco tips sticking through which are holding the firewall material to the tunnel.

In the future I intend to improve the exit area by adding a round lip to the front of it like the RV-8 and some of our colleagues have incorporated to help direct airflow into the tunnel.
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[Bcone1381]

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[spinningwrench]

I get it. The difference from the 4 place was throwing me off. I’ll probably rivet in a small C-channel on the inside of the tunnel to really hold it flat as it really wants to bend.

[spinningwrench]

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I ended up riveting a small aluminum channel to the inside of the tunnel. It makes the opening dead flat and eliminates the need for the firewall wrap to do the work of flattening the tunnel opening.

[Steve W]

I used a piece of .032 or .050 5052 with a big radius and joined the two parts together which added the strength to the corner and cleans up the area. These are the only pictures I have with me that shows the corner angle.

[JimParker256]

Hey, Steve, in the middle picture, it that your gascolator just to the right of the big hole in your firewall? The item that has metal lines sloping down and to the left? (And speaking of which, why the big hole through the firewall?)

[spinningwrench]

Nice, I find your fuel arrangement interesting.

[Steve W]

The hole is for the prop governor. The back cover is removable for adjustment/removing the governor.
My fuel lines meet at the lower forward door corner each side and move forward to an on/off valve, then forward to the firewall. The lower firewall items are covered/shielded with a removable outflow fairing.

[JimParker256]

I hadn't thought about the governor – that makes sense.

I'm still curious about having a gascolator that is not mounted at the lowest point in the fuel system... Do you have drains at the low points, and the gascolator is just for catching particulate matter? Forgive all the questions – just trying to learn as much as I can.

[davzLSA]

Hey guys I want to thank all of you for the great pictures and explanations, Im getting to the point where Im going to have to start figuring out how to do this tunnel and transition. This thread is a great help.