Another tunnel/firewall question

[DBeaulieu]

Trying to evaluate the pros and cons of methods for attaching the tunnel to the firewall and lower boot cowl piece and side skins. I cannot envision (practically) having to take of the tunnel except for a major damage event. Maybe the boot cowl extension to access gascolator and associated fuel plumbing a bit easier versus removing the floorboard? I'm thinking riveting the tunnel except for the rear edge which overlaps the lower boot cowl extension. All thoughts appreciated! Also, spacing on fasteners for the front overlap of the firewall to tunnel - couldn't find that in the plans, but thinking 2" would be more than adequate?

Dave (from Maine)

tunnel.jpg

[jaredyates]

There are lots of ways to do this and it can be a tricky area. Between the tunnel and floorboard you will have fuel and brake lines, maybe an antenna feed or two, and all of those things might need service and inspection. I think you would want either the tunnel or the floorboard to be removable. That section of floorboard probably has the fuel selector, flap handle, control sticks, maybe rudder pedals, etc. I riveted the floorboard in that area and don't consoder it removable. My screw spacing for the tunnel looks more like 3" than 2".

[noema]

As Jared said there are many reasons you would want to have access in that area. Instead of the tunnel it could also be the floor boards or the belly skin behind the tunnel (or a large service opening around the gascolator).

In any case, make sure to also think of how panels overlap (and possibly consider butt joints in some places). It's quite easy to have removable panels and still end up in a situation where access is suboptimal because the overlap necessitate a chain of panels to be removed first. It may sound like a good idea to overlap panels like fish scales from back to front but this also means having to remove the cowl and boot cowl to remove the bottom panels.

[Nev]

On my build I made the interior cockpit floor panels and the centre tunnel fully removable. In practice, most servicing is done simply by lifting the floor panels, which makes access much easier.

I’ve also removed the tunnel several times. On my aircraft that area tends to collect any oil or hydraulic fluid that escapes during servicing. Anything running down the engine side of the firewall eventually finds its way under the floor through the joins, including the occasional drip from the oil breather. The only way to clean it properly is by removing the tunnel, so having it removable has proven very worthwhile.

I added a small access panel under the gascolator as well, which I would highly recommend for maintenance.

Finally, I installed two small removable panels around the flap handle. These make floor removal quicker and provide easy access to inspect the tops of the gear struts and the flap mechanism. As a bonus, they’re perfect for retrieving any hardware or tools that disappear beneath the rudder pedals.

IMG_3032.jpg

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[alaskabearhawk]

FWIW, I have a seven-part series in my kitlog for this area. I used nutplates. Here is the last entry::

http://www.mykitlog.com/users/display_log.php?user=yeeeha&project=527&category=0&log=272647&row=703

This approach allowed me to add a LoPresti "bump" at the firewall to tunnel lip transition:

http://www.mykitlog.com/users/display_log.php?user=yeeeha&project=527&category=0&log=304415&row=251

[DBeaulieu]

Great feedback...thank you all! looks like I'm in for another round of nutplate mania!