Priming Gear Parts

I've been working my way through firewall penetrations.  There's not much to show in pictures that would make much sense.  Some attachments will just be bolts and nuts, but most will be bolts into nutplates.  The nutplates are either attached to the firewall stiffeners, or for some I have made doublers that then get riveted to the firewall.  It's kind of ridiculous how many times Van's instructions say something to the effect of "if the firewall is already built, drill out this rivet..."  Almost every doubler, bolt hole, nutplate etc has required me to drill out existing rivets.  It sure would have been nice had Van's actually just told me what the end game was way back in the firewall plans.

There are still quite a few penetration points that I need to figure out, but I'm slowly getting there.  I'm trying to get as many of the firewall holes put in before I hang the engine, but there may be some I just won't be able to figure out until I have the engine on.

In preparation for Dad getting here in a few weeks, I'm trying to get as much done as possible so while he's here we can do the two person jobs.  The first thing we'll probably do is take a few days to test fit the wings and put in the leading edge lights.  Then hopefully we will have time to rent an engine hoist, lift the plane, put it on the gear and hang the engine.

I'd like to have two things ready before the gear and engine go on if possible - 1) the gear ready to go, and 2) the firewall insulation done.  Today I prepped and primed all of the pieces that attach to the gear.  It'll be easier to get it all put together before the gear is on the plane.  I went ahead and sprayed Rustoleum enamel on these parts as well.  I don't really care about them being painted for aesthetics, but since they are all housed in the wheel pants, they will be subject to a lot of water and dirt.  The primer is good for corrosion control, but it's not really meant to be used in that kind of environment without a sealer of some sort.  The paint will do that, even though it's not the toughest stuff.

Battery Box

The battery box is a finished steel box that Van's sends already powder coated...and then they tell you to destroy it.  The plans call for 2" holes to be drilled in the box.  These are just optional lightening holes - 4 in the front and 4 in the back.  My 2" hole saw proved to be fairly worthless the other day when I was trying to drill through the firewall, so I just decided to go with 1 1/2" holes instead.  That hole saw was new and worked a lot better.  After drilling the holes in the front side, I decided not to do the ones on the back side though. Cutting into thin steel sheet like this leaves terrible ragged edges on the exit side of the hole.  In order to clean it up I had to grind the edges down and then hit it all with sand paper.  At that point, the nice powder coat wasn't so nice anymore.  Knowing what I know now, I wouldn't have bothered to drill any of the holes.  Sometimes saving that 1/10 of an ounce of weight just isn't worth it.

I also made the battery hold down strap.  It's a 1/8" thick aluminum bar.  For this one I did go ahead and drill all of the lightening holes.  Aluminum is a piece of cake to work with compared to thin steel, so these holes only took a few minutes with the unibit in the drill press.

Because the powder coat was a mess, I scuffed up the box and hit it with some Rustoleum high temp engine paint.  There are two brackets that attach to either side of the box.  I match drilled these, dimpled the holes in the box, countersunk the matching holes in the bracket, then attached the nutplates.

The box attaches to the firewall stiffeners and bolts replace a couple of rivets.  I drilled out the existing rivets in the firewall and upsized the holes for bolts.  I also made two 7/8" spacers that go under the hold down strap.

I think I will probably mount the box over the fiberfrax vs cutting out such a large area of the insulation, but I'll do some research on that before I decide.

Cabin Heat

Time to dive into the firewall forward stuff!  I figured the first few things I could start with are the ones that have definitive locations in the plans (as long as you look at the right plans anyway - they are a total hodgepodge of diagrams for different engines and different variations depending on options, none of which match my setup exactly).  First up, the cabin heat box.  The Van's version of this box is aluminum, which doesn't make a lot of sense to me.  Of all the firewall items I don't want to melt away in an engine fire, the little box covering the single largest hole in the firewall is number 1!  I deleted the Van's box from my FWF kit order and ordered a stainless steel version from Aircraft Spruce.  It's virtually identical, just made from SS.

I needed to somehow drill a 2" hole in the firewall.  That is easier said than done.  Unibits actually work very well on stainless, but I don't have one nearly that big.  Knockout punches would work as well, but I don't have any of those either.  That left me with trying a hole saw backed up by a piece of wood.  All that did was make a shiny ring on the firewall.  Stainless is tough stuff!  What I ended up doing was using one of the tiny cutting discs on my Dremel.  The hole is large enough that I was able to take small plunge cuts and still have it end up fairly circular after cleaning it up with a grinding stone in the die grinder.

The box came with a small tube of fire sealant.  I ran a bead around the 2" hole and around the bolt holes, then bolted the box in place.  The box has a flapper with an arm on it that will be controlled by a push/pull cable in the cockpit.  When the flapper is closed to the cockpit, the incoming air (comes in via scat tubing pulling air from one of the baffles) just flows right through the box and out the side.  When the flapper opens up to the cockpit, it swings over and blocks the side exit and diverts the air inside.  It's not exactly air tight or anything, but nice and simple.

A lot of the items that will go on the firewall will actually go on top of the fiberfrax and stainless foil cover.  Since this heater box actually has moving parts, I decided it should be bolted directly to the firewall.  I'll just cut the insulation to go around it.  I wasn't sure about mounting anything on top of the fiberfrax, but after a lot of research I discovered it's very common.  The fiberfrax does compress pretty easily, so for light items (a lot of the electrical type items), the bolts will compress it enough that it'll still be very solid underneath the bolt head.  For heavier items like the battery box, I'll just cut around it.  I was initially being very anal about all of this, but the reality is that 99.9% of firewalls are completely bare.  That's fine in almost 100% of cases.  The insulation is simply to give extra time in the extremely rare case of an engine fire.  Fire will not breach the stainless firewall, but the heat will.  The insulation makes the cockpit tolerable even with a roaring engine fire (shouldn't roar for long if you recognize it and cut fuel).  That means the ability to pilot a plane down vs not.

Anyway, all of that to say that I was initially wringing my hands over mounting things on top of the fiberfrax or cutting the fiberfrax around certain things and not having insulation there.  But the point is to make the firewall better, not perfect.  If I have good insulation on 90% of the firewall, even though it's not perfect, it's still FAR better than no insulation anywhere.  Why that is hard to accept, I don't know.  When I see the 100% solution, for some reason that makes a "90% good" solution seem like it's just a terrible thing and full of risk. Sometimes I forget that better is still excellent relative to not doing anything at all.