Firewall Installation

Next on my list of tasks was the firewall installation.  It's nice to finally be installing some of the larger parts; the little detail stuff is all necessary, but visual progress isn't very obvious when you step back and look at the project.  The neighbors constantly drive by while I'm working in the garage and yell "I see you working on that thing all the time, but it never looks any different!"  Then I yell back, "naive fool, clearly you know nothing about building an airplane."  Well, not really, but I'm sure that's what they're thinking.  Now that the bigger pieces are coming together, progress is more apparent.

A little trimming was required to get the firewall to fit in the back of the airplane, but that's normal.  Overall it fit without giving me any fits.  Once it was clamped into position, I filled the gaps and radiused the inside corners with potting compound and glassed it in place with 2 plies of S2 glass.  Glassing the joint between the firewall and the lower surface of the spar wasn't too fun.  It was awkward and kind of hard to get to, much like doing a layup on the underside of your desk.  Do you lay on your back and work over your head, or kneel and reach around?  I think I did a combination of both. 

To keep things purely chronological, you would skip ahead to the NACA scoop installation chapter now, and then come back here.  I installed that before continuing on with the aft side fiberglass work.  But it really doesn't matter... there are plenty of chronology violations on my website, and I haven't received a ticket yet.

Access to do the aft side firewall layups was much better, but there was an added complication: I had to incorporate the cowling flange into the joint layup.  And to compensate for the thickness of the cowling, the flange had to be goggled.  To accomplish that, I obtained some 2" wide strips of plastic and secured them in place with pieces of rigid foam and hot glue.  Because this, too, was a compound curve, I had to cut the strips into 18" lengths.  Anything longer and they wouldn't play nice.  Once tacked into place, I covered the plastic strips with duct tape and release agent, then layed up 3 plies of S2 glass to complete the joint.

After the fiberglass had cured, I removed the plastic strips and trimmed up the flange.  You can see the result in the photo on the right.  At some point in the future, I'll install some nutplates or ¼-turn fasteners on this flange, and the engine cowling will attach here.

To complete the fiberglass portion of this chapter, I added a bunch of plies to locally strengthen the engine mount attach points.

While structurally sound at this point, there was one important feature that my firewall installation was lacking: Fire resistance.  Making that happen involves bonding a fire-resistant ceramic felt (Fiberfrax^®) directly to the firewall followed by a metal face sheet.  Most people use steel for this, as the melting point of aluminum is too low.  But steel is heavy.  Indeed, the SQ2000 construction manual also called for steel.  This was a pretty big part to be building out of steel, I thought, so why not use titanium instead?  That's what Boeing and Northrop would do.    Titanium is light weight, has a very high melting point, and doesn't corrode.  The main drawbacks to using titanium are that it's expensive, and it's fairly hard to work with.  It also doesn't get along well with cad plated hardware, which is typical of AN and NAS parts.  But I was determined.  My first stop was Boeing Surplus down in Kent, WA to look for titanium sheet.  No luck.  All they had was thin strips, and a bunch of Ti plate that was way too thick.  What I needed was a 36" x 48" sheet, .016 - .020 thick.  My next stop was the McMaster-Carr website.  McMaster-Carr is great--I buy lots of hard to find things from them--but this time they let me down.  Yes, they had .016 thick 6AL-4V titanium sheet, but it would have cost me over $1400 to cover my firewall.  Way too much.  Suddenly steel wasn't sounding too bad.  But I didn't give up.  

I surfed the internet a while, and eventually landed on Titanium Joe's website.  Ti Joe is a scrap metal dealer, but he had what I was looking for: at 48 x 48 x .020 it was a bit larger than I needed, but it was only $230 including the cutting and shipping fee, so I bought it.  I also bought a pneumatic shear that had no trouble cutting the thin titanium sheet.  The shear wasn't too fond of cutting curves, but I was able to coax it.  Finishing up with a little rotary- and straight-file action gave me a perfectly fitting titanium firewall.  The photo at left shows the finished product.  Well, it's not actually finished yet, because I haven't bonded the Fiberfrax in... I figured I'd hold off on that task until I have the wing strakes installed, just in case I need access to the fiberglass side of the firewall.