FIREWALL AND BULKHEADS
The very first thing that gets assembled on the fuselage, believe it or not, is the firewall. I guess we start at the front and work to the back. I took a lot of pictures of the firewall as I was building it. I am not going to show them all because there are too many. Of course, I haven’t shown all the pictures I have taken on any portion of this project.
The first thing that I did was to take all of the firewall parts to the hangar in Florida on a Saturday. I had to cut two parts on a band saw so I took the whole assembly and cleco’d it together to see what it would look like (two photos above).
What had to be fabricated were two angles that are attached to the lower center of the firewall. They are made from .125” aluminum angle stock. They ultimately will help transfer part of the load carried by the engine mount to the lower fuselage. I promptly screwed one up drilling it and had to make another. Vans must figure you are going to mess one up because looking at the photo at the left above there is just enough material to make three of theses pieces. Anyway, I got it right on the second try and I was on my way on the firewall.
The next thing to do after the attach fittings were made was to dimple the firewall and the stiffeners. I did not show that here because there are plenty of photos of the various ways to dimple skins and ribs in the empennage section. After all of the holes were dimpled it was time to etch and alodine the aluminum parts. I tried something different on the alodine process and it worked.
I poured some alodine 1201 in to a large garbage bag and put the freshly etched pieces in there to keep them wet. After about 3 minutes I took them out and washed them off. The result was more than I hoped for. The parts were a nice gold in color. I washed them off and dried them by wiping which gets the excel film off, and they were ready to prime.
I did the same thing with the smaller pieces and had the same result. I worry more about dissimilar metals corrosion on the firewall than any where else because the firewall is stainless steel and these re-enforcements are all aluminum. If you attach a piece of bare aluminum to stainless steel and get moisture in between, you probably would be able to hear the corrosion process. These metals do not need to be touching each other. It is time to get off the soapbox and back to work. After I alodined all of the parts, I cut a small hole in the garbage bag and put the excess alodine back in the jug. The process wasn't any faster than the normal alodine process so I may not use it again. As well as this worked though, if I had Alodine to do on a daily basis I probably would invest in a couple of large vats filled with etch and Alodine to dip the parts in.
The left photo above shows parts hanging from a pole that was suspended between two ladders. The paint booth is in Tennessee so this is how I primed parts when there weren't too many. In the right upper photo you see a picture I took of the firewall parts hanging up to dry in the garage.
Remember how I said earlier that I was done dimpling the firewall and that I did not take any pictures? Well, I lied. I had dimpled the stringers and countersunk the re-enforcements but I had not dimpled the firewall yet. I am bringing this up because I had left the “C” arm dimpler in Gallatin and initially I was not happy about that. I decided to dimple the firewall with the “Pop” rivet dimpler and I was glad that I did. The stainless material that the firewall is made from is very soft. The hammer dimpling process surely would have marked the firewall at every hole. That is me in the photo at left, obviously taking this dimpling process way too seriously. Lighten up! Weber. You are supposed to be having fun. The dimpling is now done in the RH photo. It is time to assemble the firewall.
I stated my fear of dissimilar metals corrosion earlier. To add one more barrier between the aluminum and the stainless steel, I decided to assemble the stringers wet with sealer. I used PR-890.
I first taped off the area where the stiffener was to be attached. I the put tape on the opposite side of the stiffener from the sealer in an attempt to minimize the potential mess. I mixed the sealer and applied it to the stiffener. I cleco’d the stiffener to the firewall. Sealer isn't that bad to work with if a serious attempt is made to minimize the mess that it can make. Keep some MEK or acetone handy to wipe tools and excess from the parts you are using it on. Stop and clean off the excess frequently. Wear surgical rubber gloves and change them often. Have a chuckle at the poor bastards that are miserable working with this stuff because they don't take these simple precautions.
I riveted the stiffener to the firewall. I prepared two more stiffeners the same way and cleco’d them to the firewall. About this time my friend Bob came over to shoot the …….ah, er, breeze. I immediately put him to work bucking rivets. This was the first time on this project that I accepted help shooting rivets. I really did not need help, but he was there anyway. We had an enjoyable afternoon, solving the world's problems, particularly those that were aviation related, and we got a lot done. All of the rivets in the firewall were shot by the end of that day. I even managed to get the oil filter recess and the engine mount brackets installed, which I figured would be waiting for the next day. Once in a while a day is very productive with a lot of obvious progress. These are the days that this project is fun.
Well, the firewall is done as far as the plans tell you to take it at this point. I decided to put the engine mount on it, just to see what it would look like. It looked good sitting on the firewall. Then I got to thinking, “Why can’t I install the mount now?” The plans don’t tell you to at this point but I could see a lot of advantages to it. What the heck. Let’s do it.
As I was looking at the firewall and the engine mount, one of the first things I noticed was that the gear legs were going to hit the lower flange of the firewall (RH photo). I marked the flange at both gear leg openings and trimmed it on both ends LH lower photo).
I the started on the installation of the engine mount. I centered the mount on the two holes that were predrilled in the steel attach fittings on the back side of the firewall. I drilled one of these holes through to the front side of the firewall and enlarged it with a unibit. I checked the mount for fit again. So far so good. I am glad that I am doing this now because I have the mount sitting flat. It seems to me that this would be a pain in the ___ if it was to be done holding the mount up to the firewall after the airplane is assembled. The thing is not light. It would probably be a two man job at that point. I had to take the mount on and off several times and I even laid in on the back side of the firewall a few times to check hole alignment before I drilled anything.
At upper left is a picture of the first bolt hole I drilled. I very carefully drilled the next four corner bolts one at a time, bolting the mount to the firewall each time. After the corner bolts were all installed, I drilled the two bottom bolt holes.
After I drilled the two bottom holes I noticed a gap between the mount and the firewall at each hole. Furthermore the gaps were not the same amount. I made two shims from 2024 T3 aluminum. One shim was .050” thick. The other was .032”.
I primed the shims and installed them wet with sealer. I only put sealer on the firewall side for now. The firewall and the engine mount are mated, completed and set aside for later.
Moving right along, the plans called for straightening and fluting fuselage frames next.
It did not take too long to straighten and flute the frames the plans called for. These two pictures show all the aft fuselage frames that were straightened and fluted. The center section or Bulkhead F-704 as it is called is next.
This is a massive bulkhead. It is in fact the main structure that ties the wings to the fuselage. The wing attach bolt holes come pre-drilled in this bulkhead. They are actually drilled in a fixture by Vans Aircraft along with the main wing spars. This means that the wings and center section are a matched set. A non repairable screw-up now would be a major catastrophe. The bulkhead consists of two major sub-assemblies, forward and aft. These two pieces sandwich the main wing spar when they are completely assembled. So what would be the first thing that the plans would have you do to such an important sub-assembly as this? Why ream out some pilot-holes for snap bushings of course. That is exactly what I am doing in the RH picture above. Actually, I am glad that this is done first. With everything un-assembled there is much less chance for error. The holes are reamed with a “unibit.” There are 3 basic sizes in the center section. I always put tape on the unibit when I am using it. It is very difficult to shrink a hole that was reamed out too large.
Here is an example of a snap bushing hole that cuts through another piece. This is shown on the drawing and addressed in the plans. If this bracket (above left) had not been cleco’d in place while I reamed the snap bushing hole to the proper size, I would have had difficulty later cutting this out. As it was, I just cut the slice taken out of the bracket at the same time that I enlarged the hole. Speaking of the hole, you will notice a pencil line at the bottom of the bracket. This hole was drilled to a measurement instead of from a pilot. A lot of the snap bushing holes are actually laid out like this. I hope I measured right. Here is a lesser example (above right). This hole is right on the edge of the flange of this re-enforcement. Again, it may have given me a problem later had I not had the bracket cleco’d on while I was enlarging the snap bushing hole. Read the plans, study the drawing.