Initial Leak Test
1/6/23 - 1.1h
1/10/23 - 1.6h
1/11/23 - 2.3h
1/12/23 - 1.8h
1/14/23 - 0.6h
1/15/23 - 1.5h
I wanted to leak test the tanks before installing the rear baffle. Discovering and repairing a leak after the rear baffle is installed could result in major tank surgery. The idea behind the preliminary leak test is to temporarily seal the tank as well as possible, so the tank holds enough pressure to test the rivets and fillets that are already finished. I wasn't worried about a perfect seal as long as the tank holds a little positive pressure long enough to spray the rivets with soapy water. Some minor leaks near the areas that are not yet finished are fine, and might be beneficial as a safety measure to not over-pressurize the tank.
I decided to start simple to see what would or would not work to provide a temporary seal. The first step was to install plugs in the finger strainer flanges. I used the two plugs supplied with the kit, wrapped each in Teflon tape, and inserted them finger tight. I turned my attention next to the rear baffle starting with the rivet holes for the aft rib flanges. I simply ran a piece of scotch tape over each line of holes on the inside and outside of the baffle. I set the baffle in position on the tank, and then ran a length of Gorilla duct tape down the top and bottom seams with the skin. I clecod through the duct tape to hold the baffle in place. The fuel caps are also installed, and can be covered with packing tape if the caps leak.
The first test failed because I didn't account for air escaping the tank through the rear baffle rivet holes. I sealed the top of the baffle and the holes on the inner flange of the baffle, but I didn't tape off the holes on the skin. The air worked its way between the skin and the baffle flange, and escaped out the rivet holes through the skin.
Test number two also failed. I taped off all of the baffle holes, but still had leaks in the corners formed by the exterior ribs and the rear baffle. The corners are very difficult to seal off with tape, especially where sealant is nearby preventing a good air-tight seal with the tape to the skin.
Success! I went for try #3 and managed to get the tank to hold enough air pressure to test for leaks with the soapy water. The corners and seams that are taped together still leak, but that is a good safety to ensure the tank isn't over-pressurized. I used a small bike tire compressor to provide the pressure, and I left it running while I did the test. I kept an eye on the balloon, which did not inflate, but had just a little air in it the whole time. The soap test confirmed the air leaks in the corners with lots of bubbles there. The areas with proseal (rivets and fillets) did not have any bubbles, so I feel good that I got a good test and that my sealant job is acceptable. There is always a possibility of finding leaks after the rear baffle is installed, but I think that possibility is minimized as much as I can. I will repeat the initial leak test on the right tank to get a good feel that everything there is OK, too.
Final Leak Test
1/14/23 - 1.0h
1/28/23 - 2.0h
I was anxious about this particular step in the build before I even signed up for the empennage kit. I wasn't sure I could build fuel tanks that didn't leak, and I nearly went with the quick-build wing option to avoid having to build the tanks myself. It turns out that building the tanks goes pretty well as long as you understand the process and the steps, plan, prepare, and take your time. The end result for me was 2 tanks with zero leaks!!! I credit a lot of my fuel tank success to the Van's fuel tank video. I highly recommend watching it all the way through before starting the tank build, watching again with the plans in front of you and stopping to make sure you understand what's going on, and watching it again for the step that you are about to start. There are some excellent tips in the video that are critical to build a leak-proof fuel tank. Two enthusiastic thumbs up for the video!
The leak test was pretty easy to conduct with the supplied test kit. The kit comes with some instructions that supplement the instructions in the plans, and the kit comes with a couple of extra items to test RV-10 fuel tanks. The test set-up diagram in the plans shows a hose clamp at each end of the air hose, but the clamps don't come with the kit. I found that I didn't need the clamps since the air-valve threaded tightly into the hose, and the hose threaded tightly on the fuel vent line union. The only other item to take care of before pressurizing the tank was to put a balloon on the end of the fuel pickup fitting. The balloon likely will leak under pressure, so you'll need something to hold it tightly to the fitting. I threaded some wire lacing tape around the end of the balloon, which worked well for me. The balloon is an important element of the leak test. The idea is the balloon will burst before the tank does if you over-pressurize the tank during the test.
After setting up the leak test, I hooked the air valve up to a small electric bicycle air pump. The balloon inflated almost immediately and stayed inflated after turning off the air pump, which indicated a good positive air pressure inside the tank. I then sprayed every rivet and every seam with soapy water to detect any air leaks. The only leak I detected was on the right tank fuel filler cap. That leak was very obvious, and left me feeling good that I didn't detect any other leaks in the tank. Leaks can always appear in the future, but for now, my tanks are in good shape!
[Leak Test Kit, Balloon, Electric Bicycle Pump, Spray Bottle with Soapy Water]
Assemble
1/28/23 - 4.1h
1/29/23 - 4.6h
The completed fuel tanks attach to the wing spars with AN3 bolts and wing skins with AN509-8R8 screws. The bolts go through the holes in the spar web to the nut plates in the tank attach zee brackets. I reviewed Section 5 of the plans for the appropriate torque (20-25 in-lbs + drag) and any relevant notes for inserting the bolts. This is a case where torque is applied to the bolt rather than the nut, so I accounted for the following note:
“When applying torque to a bolt be sure to have a washer under the bolt head and lubricate the bolt shank. Add to the overall torque value the torque required to overcome the friction associated with turning the shank of the bolt within the assembly.”
The lubricated bolts ended up with 2 - 4 in-lbs of drag, so I torqued all of the bolts to 27 in-lbs. I was very happy to see that all of the holes lined up well with the nut-plates on the zee brackets, and the bolts all went in without too much drama.
I clecod the skins in place to check the fit after installing the bolts in the attach zees. The tolerances are very tight, and plans specify a zero to 1/64” gap between the tank skins and the wing skins. I ended up filing a little on the right side to get the tanks to fit properly. I started by installing the screws in the leading edge splice strip from the leading edge back to the spar. The fit between the tanks and the leading edges is excellent. I then worked my way down the spar to the last inboard screw. The “Standard Aircraft Handbook for Mechanics and Technicians” that was provided with the empennage kit has a Standard Torque Table (Figure 6-13), that I used to determine the torque for the screws. The table specifies 12-15 in-lbs for 8-32 and 8-36 screws. Add 7-10 in-lbs for drag, and I came up with a final torque value of 22 in-lbs. The end result was a tight fit between the tanks skins and the spars, and good alignment and fit with the other wing and leading edge skins. Successful building, testing, and installing the tanks is huge milestone in the overall aircraft build!
[Socket Set, Torque Wrenches, Electric Screwdriver, Hobby File, Scotchbrite Pad]