RV-9 tank building experience writeup

Kind of a long post...

I recently finished building my RV-9A fuel tanks.
I started this work knowing that many consider this the most arduous and messy
part of the build.

First off, thanks to all of the builders who have gone before me and have contributed
write ups, posts and pictures about the tank building process on this and other web sites.

It is invaluable to be able to get a mental picture on the steps before you plunge in.

I started by getting all of the ribs and skins prepped. Take off the blue plastic from
the interior of the skins. Smooth out all of the edges with a file, then polish the edges
with a scotchbrite wheel.

Build the cradle. I used some scrap lumber from the wing kit crating.
Leave some space at the bottom of the cradle for the clecos to poke down
from the skin. I cut an area out of the middle of the base in order to be able to
get the rivet gun to angle up to the rivets on the nose of the tank.

Straightened all of the ribs by fluting. As long as the line of prepunched
holes is fairly straight, that is good enough.
As many have said, the first time clecoing the skin to the ribs is very tough.
The tank skins are thick and the bends seem wrong, but as you wrap the skin
from the nose around to the aft sides, things start to line up. I ended up
rounding out and smoothing the curvature of the small nose rib flanges on
the tank ribs with some sand paper. This made the skin fit easier.

Make all of the stiffeners. One of the stiffeners is cut shorter, and make sure
to make a left and a right version of the short stiffener.

Match drilling all of the parts helps everything start to align better.
I posted a tip about centering the fuel drain. I also sprung for the deluxe fuel caps.
Very pricey, but wow, they work like you would expect, unlike the stock
caps which seem to require brute force and a pry bar to open and close.
I took my soldering iron to all of the rivet lines, drain and filler cap on the
outside of the skin to remove the blue plastic. Deburr everything.

Countersink the rivet holes for the back baffle. You don't want to go too
deep, because the tank skin isn't that thick, but remember when you seal
these there will be sealant in there. After I completed the tank, my line of
skin/baffle rivets was just a little bit proud of the skin surface compared to
the dimpled rivets that used the tank dimple dies.

Clean the interior with acetone, then tape off the surfaces where the
stiffeners and rib flanges will join. Scuff both the ribs, stiffeners and skins
with a maroon scotchbrite pad. I did a line at a time on the skin, and just
moved the same pieces of blue painters tape around. I'm frugal and that
tape is expensive! Clean it again with acetone.

Get the skin dimpled with the C-frame, and dimple the ribs and stiffeners with
the squeezer. I used the tank dimple dies and was very happy with the results.

More cleaning and then tape off the areas of the interior skin that you don't
want to get proseal on. First session with the sealant is just enough to
butter up the stiffener flanges. Cleco them onto the skin, make a fillet around
them with the end of a popsicle stick (bought a package of these at Michael's).
Let the sealant set up for a day, then come back and extract the clecos, dab a
tiny amount of sealant in the rivet hole with a toothpick and then place a
rivet in each hole, cover with rivet tape and back rivet. Be sure to clean
the rivets in MEK before rivetting. I had a terrible time trying to back rivet
these. Even using the pneumatic squeezer on the rivets near the edges was a
challenge with the slippery sealant.

After the stiffeners were rivetted, I covered every rivet head on the interior
with a good glob of sealant. I used a veterinary syringe as an applicator and
that worked great (local feed store had them for $1.99 each) ended up using 5
of them for the whole tank project. The sealant was easy to clean up out of the
syringe, just let it cure for a couple of days, then push it out with a pick
or thin screwdriver.

After the stiffeners are in, get the fuel drain and caps installed. Be sure
with the deluxe fuel caps to figure out the rivet lengths in advance. The
machined flange has differing thicknesses around it, and you also have the
vent line tab to deal with.

part 2

Next session was the 5 interior ribs on one tank. Use the syringe to lay down a
small bead along the rib flange, spread it out with a plastic knife (free from
your local fast food joint). I liked how the serations on the knife acted like
a notched trowel to provide an even amount of sealant across the flange
surfaces. Cleco everything in, and come back the next day and rivet.
Before doing the end ribs, get the vent line fitted and installed. This is
much easier to get in before the inboard rib gets put on. Also get all of the
nutplates and reinforcing ring around the tank access plate on the inboard rib.
I also got the fuel pick up lines fabricated. Van's doesn't give you much tubing
here to play with, so be careful. On the RV-9, make your bend about 3 inches
down the tube. Cut the other end flush where the tank end baffle contacts. I
finished off the end by nipping 4 little V's evenly spaced around the end of
the tube, then bent them into the center. This made a nice rounded closed
end on the fuel pickup tube. I cut the slots on the band saw and cleaned out
the slots as best as I could. Next comes the flaring of the other end.
Be sure to put the collar and nut on there before you flare the end!

I used the standard fuel gauges. You have to bend and cut the wire to length
for the floats. Easy to do on the bench vise with a little gentle persuasion
(hammer). I did the ohmmeter check and verified that the guages were
within spec at empty and full.
Test fit everything and put it aside, then get the inboard and outboard ribs
sealed and riveted. It helps to use some tape across the
back side of the tank where the baffle will eventually be in order
to keep the tank skin from flexing out and away from the aft most rib rivets.
Clear packing tape works great, and you can see through it.

Once the inboard and outboard ribs are attached, rivet on the forward tank
attach bracket and the small nose reinforcement plates. Cover all of the
exposed shop heads, and start hooking up the plumbing. The anti rotation
bracket gets fitted after the fuel pickup tube is torqued down tight. I had
to buck the 2 rivets for the brackets -- too tight for a squeezer to get in
under the nut on the tubing. Again, make sure everything is really clean
before sealing up the various fittings.

I cleaned out the interior of the tank thoroughly prior to putting on the
tank baffle. I mixed up about 110 grams of sealant and filled up a syringe.
A thin layer of sealant was applied to all of the rear flanges of the ribs and
spread out with a plastic knife. I did the same along the lower section of the
baffle flange just to assure a good seal. Then a long thick ribbon of sealant
just under the line of rivets on the skin. The syringe worked great to get the
sealant on there. I dropped the baffle straight down and started clecoing
everything in place. A look inside the access hole showed a nice bead of
sealant between the baffle and skin.

The brackets get pop riveted with some special closed end rivets. I dipped the
rivets in sealant then pulled them in place. I also put a very thin smear of
sealant on the bracket. That part went pretty fast. Just make sure you have the
orientation of the brackets all correct! I ended up using pull rivets on the
end brackets. I probably should have used solid rivets here, but pulling was
much easier than bucking. I had to order some more of these rivets for the
other tank.

Next were the skin/baffle rivets. The skin was countersunk, but it looks like
there is probably enough wiggle room to dimple these if you wanted to. I
used a toothpick to dab a bit of sealant in each of the countersinks before
I squeezed the rivets. Lots of rivets here, and this took quite a bit of time.
I went back and dabbed a bit of sealant over each of the rivet shop heads.

Lastly, I put the access plate and fuel sender in place. I skipped the cork
gasket and just sealed them up with sealant. I did use the thick rubber
gasket under the fuel tank sender. I used stainless steel hex socket head
screws on the access plate and fuel sender. A small bead of sealant just under
the head of the screw went on before tightening them down.

After a couple of days to let the sealant set up, I did the pressure test.
I used Van's tank test kit. I bought some clear tubing and a small hose
clamp at Home Depot. The hose went over the smaller vent line fitting and was
held tight by the hose clamp. I taped over the fuel caps. The tubing was
partially filled with water and then the tank was pressurized to about 1 psi.
The water level did rise and fall over the course of the day, mostly due to
temperature changes. Both tanks tested out with no leaks!

Overall, the time to build both tanks was just under 2 months. I took many
days off of building during the holidays, and also during that time I
finished the leading edges for both wings, and installed the leading edge
landing lights. I had to wait for a weekend day in order to do the final
baffle sealing, since that took about 8 hours per tank.

The tanks are now mounted for good on the spars. Time to get the top skins on!

Thanks for a great write up.

Bruce,

Thanks for the great write up! I really appreciate the time you spent to document the process.

Rob

Gonna do it just like that. Thanks!

dave

Countersink tip

Excellent write up Bruce. You may consider adding it to the "Tips" section.
Quick tip on the baffle.
Cleko the tank together including the baffle then use the holes in the baffle to keep the countersink cutter pilot aligned. Then when you set the baffle use a few rivets in the holes keep everything aligned. The combination of countersink holes and clekos allows some movement. A few rivets will align the parts and subsequent riveting will be easier.