Cozy

Chapter 15 - Firewall

The finished firewall

Materials

I've read recommendations (including Rutan's CPs) to use stainless steel for the firewall rather than aluminum, so I ordered a sheet of the thinest (0.018) stuff Wicks sell. It seems awfull heavy in a 4 by 3 sheet. I looked up the weights of various materials and, estimating 10 sq ft of area to cover, came up with a table. 
                      Weight
     Plans AL          2.32 lb
     0.018 stainless   7.5  lb
     0.016 stainless   6.7  lb
     0.010 stainless   4.3  lb
     F2000 firewall    5.9  lb
The weights are all relatively close except for the plans specified al. I asked on the mail list and got various responses. Quite a few, including Nat, said that its the fiberfrax that handles the heat, not the AL. On the other hand, if the AL melts, the fiberfrax will become dislodged. So, do I wait a week, order the plans AL, save 5lb in weight and have a firewall that goes against Burt Rutan's recommendation, spend over $200 for F2000 firewall material from ACS with a weight saving of 1.6 lb, or go with what I've got? Decisions. Decisions. I checked Marc Zeitlin's site and saw that he used 0.016 SS. I'm sure I saved some weight on the wings, and I'm hoping the Mazda 13B will come in a little under the weight of a Lycoming, so I decided to give up 5.18 lb (or 12 oz, depending on how you look at it) in exchange for piece of mind. My cowling might melt, but my firewall is going to be a serious fire barrier.

Preparation

After doing the cowl lip reinforcements (Chapter 23) I added the clark foam in front of the spar at the sides and smoothed everything down as best I could. In a couple of places, where reinforcement layups had caused a small bump, I added some dry micro to give a smooth transition. I'd already done the aileron controls, including the brass bearings in the firewall. I decided that these would have to come out and be replaced after the stainless steel was installed. I destroyed them taking them out, so will have to order two more. Next I decided to check the 14 "captive" screws used for the rudder controls. I cleaned any stray epoxy and flox out of the threads as best I could, then used a nut driver with the torque set fairly low to put the nuts on and off. If these screws were going to turn I wanted to know this before doing the final touches to the firewall. 7 out of 14 turned. I dug these screws out, cleaned them up and floxed them back in with 2 BID tape. After cure, 3 of them still turned and I had to redo these once more. The fix wasnt too hard, but there has to be a better way to handle this issue. Perhaps nutplates rivetted to a thin AL plate on the forward side of the firewall would work. Wayne Hicks recommends welding music wire to the heads. If the screws do turn, one way to get around the problem is to use a dremel to cut a slot in the end, then hold the screw still with a screwdriver. Apparantly it says this in the plans somewhere, but I never saw it. It didnt work anyway. The welded music wire solution is probably the best.

My rusted aileron control The other thing I wish I'd done different was the aileron controls. I'd been so anxious to install these that I'd done them before doing the firewall and the canopy. This was a mistake because I'm not a very tidy worker. They got covered with micro and flox drippings, and the Florida humidity had caused some oxidization. Once the firewall is done I plan to remove the entire aileron linkage, clean it up and paint the steel rods with clear polyurathane. Not a major deal, but work I could have avoided if I'd kept the parts in their plastic baggies until after I'd done the firewall.

Fiberfrax

I'd read in the archives that the plans amount of fiberfrax is not enough. I'd got 10 sq feet as this builder recommended, and JUST had enough. Even then I had to piece together a bit of a jigsaw to cover the whole area. Plans say to make a paper pattern, but I found it easier to hold the fiberfrax up to the firewall and cut it in place. I stuck the fiberfrax onto the firewall with high temp RTV silicone. I bought two small tubes and used them both. Had to go out and buy two more to stick the steel on later. I should have bought a large cartridge instead. I'm not sure how well the plans recommended glue works. People say the silicone is better. The fiberfrax isnt very strong, so I don't think it matters much how strong the glue is. Next I cut a 2.5 inch circle around each of the plans engine mount points and drilled a 5/16 hole in the center of each. This was a mistake. I should have drilled 1/4 pilot holes. You see, I'm going to be using 5/16 bolts for the engine mounts, so I was thinking 5/16. But I hadn't thought this through. The Barry mounts I'll be using need 1.5 inch holes in the firewall, and the 5/16 bolts go through them. Now I'll have to find a way to enlarge the 5/16 holes to 1.5 inches while most holes saws use a 1/4 pilot. Ah well. I guess I'll find a way. 
A note came from Peter Militch a few days after I posted this....

Hi John, regarding your note in your on-line log ....

What I do in these situations is take a scrap piece of metal and pop rivet
it or screw it to the back of the sheet that has the offending hole in it.
Just make sure that the rivets or screws are installed around the 5/16 hole
in a circle that is smaller than the diameter of the hole saw you intend to
use.  Now your errant hole is filled in and you can center punch the backing
plate through the old 5/16 hole and drill your new, 1/4" pilot hole.  Then
do the hole saw thing.  When you cut through your firewall, the temporary
backing plate and rivets will just fall away with the piece you just cut
out.

Isn't the internet great!

Installing the steel

The steel being held in place while the silicone dries I got a roll of brown craft paper and made a paper pattern, including all the blind screws and holes, transferred the markings to the steel sheet and then cut the shape out with tin snips. Using a 3 * 4 foot sheet, I was able to get the main firewall in one piece. I needed two small end pieces about 8 inches long for the outboard parts. I was fairly careful to get the cuts right, but still had to fiddle and trim for quite a while to get a good fit. Cutting and trimming the steel with tin snips was no problem. I only cut myself on the sharp edges a half dozen times. The high temp RTV silicon is blood red, so no one will ever know that I bled all over the steel.

I put the 5/16 bolts in place, made and trial fitted the various aluminum rudder assembly parts. Finally, it came time to stick the stainless teel sheet in place. I did the two small outboard sheets first, then the main sheet using criss cross beads of silicone. I put the firewall in place, then realized I was going to need some way of holding up against the firerfrax while the silicone set. (Just like it says in the plans). So much for planning ahead. I ran around and did the job using 2 * 4s, bricks, clamps, an aluminum tube and a broom. Next day all the support stuff came off and I have a cool looking steel firewall. I'll leave the fuel parts until the engine comes and I'm a little clearer about how the fuel will be handled.

Final Touches

Here I'm I adjusted the vent lines and extended them with in-line brass brake line fittings from the auto store. I noticed that one of the lines seemed easy to blow into, the other didnt. Hmmm. I got some strong wire and poked it up the vent line. After about 6 inches I hit a blockage. After a bit of poking some brown dust came out. I think this was the work of insects. I made vent line pipe covers with 1/4 plastic tube and a couple of bolts.

I cleaned up all the aileron controls, assembled the rudder pulleys, made the aluminum rudder adjustment thingys and swaged the rudder cables. At last, I could sit in the cockpit and wiggle the rudders. I need to check that the brakes don't come on with full rudder, but I can't do this until I put the fluid in.

<<< Back | Index | Next >>>