The boot cowl & tunnel is that area between the instrument panel (station "B/P") and the firewall. It is made from aluminum and wraps all the way around the fuselage and the front door flanges. The tunnel portion is at the bottom of the fuselage and is there to facilitate a nice smooth flow of air from the bottom of the engine cowling.
The tunnel has some complex bends and curves so is made from 5052H-32 aluminum, .032" thick. This type of aluminum is softer and easier to form than the 2024 T3 used elsewhere. The sides and top of the boot cowl are made from 2024 T3 aluminum sheet, .025" thick.
The firewall and instrument panel must be attached to the fuselage before fitting the boot cowl and tunnel.
The tunnel dimensions are given in the July 1998 Bear-Tracks newsletter.
Here is the finished product after cutting forming and several passes
across the metal shrinker:
Notice the pronounced curve at the aft end. It sort of causes a curve
in the whole piece but the front edge will straighten out when its attached
to the bottom of the firewall:
Note that he firewall lip was placed on the outside of the boot cowl tunnel. This will create a better air stream and help prevent fumes from coming into the cockpit (even though I'll seal this seam when it's riveted together).
Here is the tunnel from inside the cockpit looking down:
Next the tunnel was drilled and clecoed in place using 3" spacing.
Next, the firewall and instrument panel were prepared to receive a couple of aluminum angle pieces for braces and to give me something to attach the skins to.
The braces, which are made from 3/4" x 3/4" aluminum angle 1/16" thick, were
then attached to the brackets as shown below:
As previously noted, this will brace the instrument panel and will give me something to attach the boot cowl side skins and top skins to where they overlap each other.
Now with the braces installed between the firewall and instrument panel,
we can make the top boot cowl skin. There is a bit of a challenge here.
You have to make a couple of holes to clear the support tubes that
run from the firewall to the top of the fuselage at station "P". Here
is the top boot cowl skin in place:
Here is the front view:
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