|
The fuel tanks are simply a sealed portion of the
leading edge of the wing. There isn't a "separate"
container of fuel inside the structure... the structure IS the tank.
This is accomplished by using sealant called "proseal"
to completely seal around the rivet holes and then installing additional
baffles to hold the fuel.
My construction technique goes something like this: |
|
Prepare all the parts, drill, deburr, dimple... the
usual.
Clean everything with MEK.
Mix up a batch of proseal, and spread a thin layer on the flange of the
part that you are riveting to the skin. I do not put any proseal on
the skin.
Cleco the part in place, and insert the rivets in every other hole. (I
don't add more sealant to the rivets or rivet holes, it will just make
riveting that much more difficult and messy)
Cover the rivet heads with rivet tape.
Rivet, remove the clecos, and repeat with the remaining holes. |
A thin layer of proseal on the flange
of the rear baffle. (it isn't usually this light grey... the flash makes
it look lighter than it really is) |
|
After all the parts are riveted in place, it is
necessary to completely coat the shop head (inside) of the rivets with
more sealant, and put sealant where the flange meets the skin. You
don't want any way for fuel to leak through to the rivet holes.
The easiest way I've found to do this is using the
corner of a plastic (ziplock) bag to hold the proseal and squeeze it out
like a caulking gun exactly where you want it. Then I use small acid
brushes (metal handle with short, stiff bristles) to spread the proseal
around the rivets as necessary.
I mix a little MEK into the proseal to make it thin
and smooth. It is MUCH easier to work with this way. |
Proseal in the bag, with the open
side taped. A small hole will be cut in the point and the excess bag
is cut off to make it easy to get into tight spaces. |
|
This is what the rib looks like after spreading
proseal over each rivet and along the sides of the flange. Neatness
doesn't count... covering everything does. |
 |
|
Before the rear baffle can be installed to close up
the tank, the fuel level sender plates, vent tube, fuel pickup tube, etc..
must be installed.
This is one of the two fuel level sender plates
installed on the rib. |
 |
Pay extra attention to sealing the back side of the
BNC connector or it WILL leak. Even more proseal can be added after
it is mounted in the rib. The metal part of the BNC connector MUST
contact the metal of the rib, so don't put proseal there before you insert
it. |
 |
|
This is the cover plate showing the fuel pickup
tube. I will add proseal around the connectors to make sure they
don't leak.
|
 |
|
Installing the rear baffle is the final step, and it
takes about 2 1/2 hours nonstop to complete.
First, spread proseal on the flange of the rear baffle and the rib
flanges. (see top picture)
Then, using the bag of proseal, put a bead of proseal just to the inside
of the line of rivet holes.
When the baffle is pushed into place, this bead will help seal
everything. Add an extra glob of proseal in the corners. |
 |
|
Attach the tank attach brackets using the special pop
rivets and a gob of proseal on each one. Then squeeze the rivets
attaching the skin to the back baffle.
After this is done, another layer of proseal should be spread on the
baffle rivets, over the tops of the pop rivets, and any other areas where there might
be potential leaks.
The picture at the right shows the INSIDE of the tank
where the back baffle has pushed the bead of proseal into place to seal it
completely. |
The back baffle is on the bottom, the
skin is on the right. |
|
The tank can be tested by putting a couple of
balloons over the fuel pickup and vent fittings, and then using an air
blower on the end of the air hose to gently blow a little air at the quick
drain fitting while pushing it in. The balloons will inflate
slightly. Any more pressure can damage the tank!
I used a bottle of leak finder (bubble soap)
purchased in the aviation section of the corner drug store. Brush
the bubble soap on all the rivets and seams to
test for leaks.
If a leak is found, Van's suggests Loctite 290 (a
thin solution that will be drawn into a tiny hole by capillary action) to
apply on the outside to seal a small leak.
More proseal can simply be brushed on if the location
is on the back baffle or end ribs.
The last resort is to drill out a leaking rivet, and
install a pop rivet with an ample proseal glob on the rivet before
inserting it. |
This
is what it looks like when you 'soap' the tank for leaks. The
balloons show that there is a little pressure in the tank, and you brush
bubble soap over all the rivets and seams. The access panel is just
screwed in with the cork gasket... no leaks. I might add proseal
around it later anyway.
This is what a leak looks
like... large bubbles forming in the soap.
The tank cap always leaks a little (not a problem) so you can see if your
soap method is working. Hopefully, you don't see bubbles like this
around a rivet. |
