THE CENTER FUSELAGE
With all of the bulkheads primed and assembled as far as they can go it is time to start assembling the sub-assemblies. But wait, I have that nagging feeling that I am abut to do something over. Is it that I can’t stand the thought that the spar carry through is un primed? That’s it. I just have to prime this piece. Not doing so is akin to squeaking chalk on a chalkboard to me. The picture on the lower left shows the F-704 bulkhead assembly in my office. The next picture shows the F-704 disassembled and primed. There! I feel better. Now I am ready to start.
The first task on assembly was to modify the four inner seat ribs so that the control stick crossover tube has plenty of clearance during its movement from stop to stop. The plans also show two of these ribs need a modification to allow this tube to be removed. I decided to skip this modification and rivet the four inboard seat ribs to the carry through with the cross tube in place. If I ever need to remove the cross tube I can modify these ribs at that time. The picture at lower left shows the first of the seat ribs cleco’d to F-704B prior to modifying the hole. The RH photo shows the four inboard seat ribs installed with the flight control cross tube in place. This photo also shows the enlargement of the lightning hole.
These two photos (below) show 8 of the 10 seat ribs riveted to the F-704B spar carry through (or center section bulkhead if you prefer).
Next, the plans have you drill the flap tube bearing blocks. I didn’t realize until later that I drilled all of these holes crooked. I drilled them by hand 3 feet from the mini-mill which would have drilled them pretty straight.
How could I have done something so stupid? I was using a new drill block that I bought in Arlington at their fly-in. This tool works very good to a depth of about ˝ inch or so.
Next it was time to start riveting the assembly together. I assembled the center section upside down for convenience and also because the plans insinuated that this is the proper way to complete this subassembly. In the photo below left, you can see that the 8 inboard seat ribs have been riveted to the F-704B and the F-705 bulkheads, the F-704A is re-attached to the F-704B and the baggage floor ribs are cleco’d in for a fit check. These ribs are also cleco’d to F-706B which is the lower section of the F-705 bulkhead. In the picture at right the F-776 bottom skin is cleco’d in place to check for proper fit. I modified the workbench with a cross brace to hold the F-705 in place. I leveled F-704 and F-705 before attaching the brace.
With the skin in place, the outboard seat ribs cat be attached. These ribs have to be curved to help form the side of the fuselage which is curved in this area. To get the lower flange curved correctly, a center line is drawn on it. Then the fluting pliers are used on the flange until the line is in the center of each hole. It is also important to mark the approximate position of each hole on the flange. It would not be a good idea to have the flange fluted at a rivet hole. The photo below left shows the curve the outboard rib ends up with. The upper flange is fluted also. The right photo shows the curve of the rivet line at outboard edge, Lots of trial and error here.
The next thing to complete is the task of removing the protective coating from the rivet lines. This is done with a straight edge and a soldering gun. It is not in the plans but a high percentage of RV builders do this to protect the aluminum from scratches as long as possible. The coating is removed from the rivet lines so that it does not interfere with dimpling and riveting. In the photo below left you can see the soldering gun I used. (UPDATE. This type of soldering gun is not ideal for this process. It is for intermittent duty. I actually melted this one and had to through it away.) The photo at right below shows the F-776 skin during the dimpling process. The 2 pieces of wood are clamped on to keep the skin straight. This skin came in a roll and it wanted to stay curled up.
The next step was to etch, alodine and prime the baggage floor ribs (F-725-L & -R,
F-726-L & -R and F-727-L & -R). I did not take photos of the process this time but the procedure is the same as always. Before I started to assemble the pieces, I modified the workbench again. I added 2 2X4’s to the sides of the workbench to take the place of the crossbrace that I installed earlier and to provide a means to support the F-706 bulkhead while the lower skin (F-776) was being attached. The photo lower left shows this modification. I leveled the F-704 and F-705 bulkheads before attaching the 2X4’s with screws. The picture at right below shows the baggage ribs riveted to the F-705 and the F-706B bulkheads. This is not exactly the sequence that the plans called out but I studied the parts and the drawing and it looked like the best way to attack this particular portion of the subassembly.
After the center section rib and bulkhead structure was leveled on the modified workbench the F-776 skin installation process began. Below left the skin is laying on the 2X4’s attached to the workbench primer side up. This is merely a pre-installation photo op and not a needed step. At right the assembly is cleco’d together for riveting. Note the vertical braces screwed to the 2x4’s attached to the workbench. These were then clamped to the F-706B when it was leveled to help hold the assembly straight for riveting.
Now I need help. I can not rivet this subassembly by myself. My wife, Mary, helped on the left wing and she was gracious enough to volunteer her services again. I find that women seem to pick up the riveting process as fast if not faster than men. When I am teaching someone to rivet, I like to have them on the bucking bar for a while first. They learn rather quickly as to the proper shape of the shop head. After they have bucked quite a few rivets, they can transition to the rivet gun easier. More on that later. The left photo shows another view of the lower center section ready to rivet. The right photo shows Mary in the act of bucking another rivet. She had already bucked 390 rivets in the left upper wing skin. She really seems to enjoy doing this if I give her a day or two notice.
After the lower skin was attached to the structure, we moved to the aft fuselage to complete it. That narrative is in another section. When the aft fuselage was complete, the upper longerons had to be formed. That is also in another section. We returned to concentrate on the center fuselage in December. The next item was to start fitting the left and right side skins, F-770 (2 each). The left photo below shows the right forward side skin cleco’d to the rear fuselage. The RH photo shows a modification that must be made to the skin. It has to be rolled at the bottom aft edge. This is a pain in the rear and though the photo doesn’t show it, both sides actually came out lumpy.
Below are 2 shots of the left side skin and the roll at the rear, also lumpy. The plans go into pretty good detail about the proper way to form this little roll but it is still difficult to get it right. The amazing part is that once it is rolled, even though it turned out lumpy, all the rivet holes line up. That is quite an engineering accomplishment for aircraft sheet metal parts.
Time for paint. Below left, side skins in car (Focus wagon). Below right is our temporary booth at the hangar.
Below are the primed side skins.
Below right returning home. More on this in the longeron section.
After the longerons were riveted to the rear fuselage subassembly, it was placed on two saw horses. The rear fuselage was leveled on the saw horses laterally. The saw horses I used were store bought from Lowes. Since my garage slopes towards the door quite a bit I decided to use telescoping roller assemblies to rest the front end of the longerons on. The left picture below shows a length of angle that I used to make sure that the rollers were level with the saw horses. The right photo shows lateral leveling in process. After all was level, I drilled the upper longerons to the F-770 side skins
In the photo above right the upper halves of the F-706 bulkhead are cleco’d in place. These parts were removed prior to mating the 2 sections of the fuselage. Vans suggests that all of the cabin floors be cleco’d in place as much as possible during riveting to keep the assembly as stiff as possible ( photo below left). I checked one more time that the assemblies were still level. Everything was OK so I riveted the two rows of rivets that attach the center skin F-776 to the F-778 aft bottom skin. The forward of these two rows also is attached to the lower flange of F-706B.
Notice in the photo above right and also below left that I have riveted the F-704 Upright Capstrips to theF-704C and F-704D Center Section angles left and right. The plans tell you to leave these parts off and install them later with "Pop" rivets. I decided to attach them now with AN470AD rivets. I have looked at several RV's and these rivets are usually exposed after everything is assembled. I think that the solid rivets will look much nicer so I rivet the pieces together. Skip to the bottom of "The Cabin" page to see the problem that this caused.
Now I believe it is time to dimple the F-770 side skins, But first I have to attach 2 annoying little brackets or ribs if you will. They are the F-623-L and F-623-R corner ribs. The side skin at the conical bend attaches here. The picture at right above shows the F-623-R cleco’d in place (arrow). There is a strap in the front and another in the rear to attach the rib to F-7-5 an F-706 (not shown). They are attached with AN426AD3 rivets (3/32”). The F-724-L & _R attach from the side skin to the lower skin at these ribs. The F-770 side skin preparation is discussed in the Cabin section.
This completes the Fuselage Center Section assembly. The next item in the plans is the Forward Fuselage, however, it was about here that I started to diverge from the assembly sequence in the plans. It looked to me that assembling the Cabin area prior to adding the Forward fuselage made more sense. My reasoning is that the less of the structure that is cleco'd and the more that is riveted the better the chance of keeping it straight. Anyway, I completed the Cabin before moving to the Forward Fuselage.