THE REAR FUSELAGE PAGE 2

(Continued from page 1)

Below left is a shot of the F-711 bulkhead cleco’d back together including the horizontal stabilizer attach bars.  The line of rivets I added in the center removed most of the gap between the two pieces.  Below right is a photo of the riveting in progress.  At this point the forward flange of the F-711 is riveted to the F-779 and the rear is in progress.  This photo also shows that I put a little primer in each rivet hole prior to riveting.  I used a cotton swab to do this.  The larger hole behind (above) the rivet line is a hole I drilled to allow the use of a socket to tighten the nut on the tail wheel spring attach bolt.  I drilled it when the assembly was cleco’d together.  (I did replace the rivet that has the step in it.)

F-711 assembly and assembly tool F-711 installation in progress

Earlier, I showed a photo of the WD-409 with the gap it has at both bulkheads.  This part must be in place when the F-712 bulkhead is attached.  I wanted to put something in the gaps so that I could tighten the attach bolts with out warping the bulkheads (worse than they already were).  I decided to use gray Marine Tex.  This is an epoxy filler that is relatively strong and takes compression loads fairly well.  Why did I use gray and not white?  I had a box of gray on the shelf.  The photo at right shows the WD-409 bolted to F-711 bulkhead.  I barely tightened the nuts until the Marine Tex was cured and the F-712 bulkhead was riveted to the F-779.

Gray Marine Tex epoxy filler WD-409 in place on the F-711

Next, the F-712 was added to the assembly.  A layer of Marine Tex was put between the aft flange of the WD-409 and the E-712.  The assembly tool was cleco’d to the vertical flanges of the F-712 and the bolts of thWD-409 were barely tightened.  After all of the rivets were installed in the F-712 to F-779 skin, the assembly tool was removed.  When the Marine Tex was fully cured, the attach bolts were re-torqued.  I took a photo of the completed sub-assembly three days later (I waited for some reason I can’t explain).  Anyway, this unit is ready to install.   

WD-409 attached to both bulkheads Completed sub-assembly

Riveting the F-711 and F-712 bulkheads to the F-779 lower aft skin was sort of like flossing your teeth with one hand tied behind you back.  It can be done but it is tricky.  Enough of that.  It was time to dimple the skins (below left) and etch, alodine and prime them (below right.  I also dimpled, etched, alodined and primed the skin stiffeners at the same time.

Dimpling the F-778  aft bottom skin Rear fuselage parts primed

During the dimpling process mentioned above, I managed to get careless, not once but twice.  I dimpled beside the rivet hole on the F-778 aft bottom skin (Left lower photo) and also on the F-773-R side skin (Below right).   This makes a total of four times I have done this, the other two being on the left elevator and the elevator trim tab.   On the bottom skin I decided to install a strap over the damaged hole made from the same thickness on material.  The strap extended two fasteners beyond the damage in each direction.  On the side skin I cut the damage out and made a doubler to attack to the back. 

Damage on aft bottom skin Damage on aft right side skin

The picture at left below shows the strap repair on the aft bottom skin.  The picture at right below shows both sides of the F-773-R aft side skin repair.  The damage was cut out with a 5/8” unibit.  The doubler is made from the same thickness material as the skin.  The most difficult part of a repair like this is the manufacture of the 5/8” plug that goes in to the area that was cut out.  The proper name for this repair is “Flush Repair.”  In the early days of aviation it was called a “Nickel-Dime Patch” for the size of the hole that was put in the skin.  

Strap repair on aft bottom skin F-773-R aft side skin repair

The item I tackled next was to make it easy to install the bulkheads on to the aft side skins after the stiffeners were in place.  The cut-out for the stiffeners in each bulkhead wraps around the stiffener in such a fashion that it is nearly impossible to slide a bulkhead past the stiffener and cleco it to the skin if the stiffeners are in place.  The plans have you rivet the bulkheads in first and if you do that there is no problem.  The reason I backriveted the stiffeners in place was to minimize the natural deformation that occurs when a rivet is driven into very thin parts.  If you look at the bulkheads at each stiffener cut-out, there is about 1” of edge distance between the rivet holes above the stiffener cutout towards the cutout.  There is no call-out to put a rivet in this open space so this material is wasted.  I cut the flange of the bulkhead at each stiffener cut-out even with the largest part of the cut-out.  The left photo below shows that I have put tape on the flange to mark the intended cut.  The right photo shows cutting the excess material with o die-grinder and cut-off wheel.  

Excess material to be trimmed Cutting the excess material at stiffener hole

The next step was to install the F-711/F-779 sub-assembly.  It is hard to see in the photos but the F-779 slides between the lower skin stiffeners and the F-773 aft side skins.  The pictures  below show 2 different views of the F-773-R and the F-711/F-779 assemblies attached. 

Bottom view of F-711/F-779 attached to F-773R Bottom view of F-711/F-779 attached to F-773R

The assembly described on the previous page was next attached to the F-773L.  In the photo at left (below) you can see that the clecos neatly extend into the grove in the plywood.  This allows the rear fuselage assembly to lay flat on the wood so that I could back-rivet the assembly together.  I made sure that the 2 2X4’s that I used for a base for the plywood strips were straight.  I kept checking the assembly with a level as I was putting it together to make sure that the assembly did not twist.  A close look at the photo below right shows a gap in the clecos where bottom skin attaches to left side skin (RH side of photo).  This is where the back-riveting bar is located.  I did not start the riveting at the front edge.  I decided to start in or close to the middle.  I thought that this might also help to keep from twisting the assembly.  See “Longerons” for more details.

A view of the clecos “In The Grove” Gap in rivets is position of B.R. bar

Photo at left below shows the back-rivet bar beneath the rear fuselage assembly.  At right is a photo of the technique I used to hold the rivets in place until the assembly was placed over the back-rivet bucking bar.  I put a piece of masking tape loosely over 7 or 8 rivet holes from the inside.

Back-rivet bar beneath the assembly Internal “Rivet Tape”

I picked the assembly up and put rivets in each of these holes.  The tape held them in place until I could place the assembly with these holes over the back-rivet bar.  I then shot these rivets and moved to the next set.  This worked very well and eliminated the need for rivet tape.  The picture below left shows me back-riveting the rear fuselage.  I riveted the left side first and at this point I am on the right.  The stick I have in my left hand was used to hold the assembly firmly in place co that there would not be any gap between the skins and the bucking bar.  Any gap whatsoever leaves a gap between the rivet and the skin.  A close look at the photo at left below also shows the repair that I made for the “botched” dimple hole in the side skin.  The photo below right shows the strap over the other “botched” dimple hole.

Back-riveting & side skin repair Repair strap

The photo below left shows the sub-assembly including the bulkheads.  The installations of the F-707, F-708 and F-710 were all straight forward with no surprises.  The rivets were installed with the help of an assistant (Mary).  The photo below right shows the rear fuselage upside down on 2 sawhorses.  The bulkheads are ready to rivet. The upper longerons are cleco’d in place and the assembly has been leveled in anticipation of the next step.

Rear fus. with completed bulkheads and skins. Rear fuselage turned over on sawhorses

The picture below left shows the rear fuselage after the bulkhead riveting was complete.  The photo below right is sort of a sneaky shot.  I took a picture of the F-709 bulkhead and F-714 aft deck after I installed them as an assembly, however I had not taken a photo from the outside.  I realized this several months later and so I took a picture of the outside and made a collage of the 2 pictures together (lower right).  There was nothing out of the ordinary on this assembly which is probably why I did not take pictures as it was being completed.  I take most of my photos at various stopping points as I am contemplating the next task, so, if I do a task without stopping, I sometimes forget to take pictures.

Skins and bulkheads complete Aft fuselage collage

While I was into collages, I decided to make one of the two repairs that are visible on the outside of the rear fuselage.  The lower left photo shows the repair to the aft edge of the aft bottom skin and also the repair to the right aft side skin where I messed up on dimpling.   Why would I bring up mistakes that I made that are embarrassing?  I just want people to know that when something like this happens it is not the end of the world. You don't necessarily have to order a new part.  Repairs can be made that look sanitary.  If you put some thought into them and lay out symmetrical rivet patterns the repairs might even look like part of the design.

Repairs F-787 stiffener installation

The next item was to install the F-787 stiffener which connects the F-706 and F-707 bulkheads (photo above right).  If you look closely at the photo you can see that the forward side skins are riveted on at this point.  This assembly is discussed in the Center Fuselage section of this log.

At this point the rear fuselage is complete.  The next step is to attach the longerons.  Press "The Longerons" link below to read this narrative.