§29-08, 10, 12-15, 17 Left

 Argh.  I was really hoping to finish off this section.  It's all about attaching the grommets and tie-wrap clips to the D ribs and riveting the ribs to the spar.  Surprisingly, it was the riveting of the plastic tie-wrap clips that was problematic.    5 of the 15 clips didn't hold up to the pressure exerted by pulling the pop-rivets and they ended up getting ruined.   While I could have stolen from the supply designated for the Right wing, the rivets that are called out are LP4-4's, not -3's.   I literally have thousands of -3's, but the supply of -4's is actually only half of what is needed and there aren't any surplus to borrow from.  As a result, I'll have to wait until replacements arrive from Van's before I finish out this chapter and move on to installing the wiring harness.

Ruined tie-wrap clips and LP4-4 rivets

I'm getting close to making a major decision.  Should I continue on the left wing alone, set it aside and then do the right wing, OR should I set the left wing on saw horses to use as a reference and build up the right wing skeleton, and then proceed to put skins on both of them?   I need to decide in a week or so.  Keep reading.

§21 addendum complete, Left & Right

 After 6 weeks of being stuck in the doldrums of sealing the tanks, I have resumed actual construction.  Van's issued an addendum to chapter 21 instructing me to install a bunch of nut plates on the tanks.  Since this had nothing to do with their fuel integrity, it didn't cause much headache.   One set of nut plates is around an access hole on the bottom of the skins.  That took some gentle skill because I had to mildly contort myself in order to see what I was doing and I needed to balance a 426 rivet on one die of the rivet squeezer, then thread the rivet through an upside down hole and finally squeeze it in place.  I only dropped 4 of the 48 rivets that were set that way.

Access hole being equipped with nut plates

The image above probably needs some explanation.   The bulges at the top of the image are the outboard skin of the Left fuel tank.   There's about 18" of skin both above and below the tank and we're looking at the bottom skin with its access hole.  In the foreground, you can see nut plates and set rivets.  in the background are the clecos securing nut plates that have yet to be riveted in place.   This is where I was having to balance the rivet on the squeezer and gently raise it into the open hole.  It was actually a lot of fun.

Airworthy fuel tanks, finally! §21 complete for L and R

(Mea culpa:  I wrote this 4 days ago and never hit the "Publish" button.  Doh!)

The fuel tanks are officially "done."   While I never was able to get them air tight, as some VAF members have been able to do, I have been successful in demonstrating that they would hold water and as of yesterday, that they could hold 100LL.

(recap) As you may recall, I spent nearly a month trying to get the fuel tank sensors installed correctly and without leaks.  The first few attempts were definitely learning experiences for me because it was obvious that I had done a poor job.  Each one got better (e.g., the residual leaks were smaller & slower).  I finally got the obvious leaks sealed and then proceeded to pressure testing.  There were some dumb leaks that were easily corrected, but then I really got serious about trying to make them airtight.   

I gave up on Van's idea of using a kid's balloon because there were some very thoughtful and data supported comments from VAF that it's an exceedingly poor gauge and should best be thought of as a pressure 'fuse' that will blow if you end up putting too much pressure into the tank.  Instead, I bought some clear plastic tubing, added colored water and used that as a simple manometer.   15 inH2O = ~0.5 PSI and is roughly double the pressure of a full tank of fuel, so I figured that would be a good value to use.  Unfortunately, despite everything I could think of, I could never get the tanks to hold pressure for more than 30 min.

I suspect it's the fuel caps that are leaking.  I couldn't prove it, but I just don't like their design.   I even tried sealing them with condoms, but could never find a leak nor maintain pressure.

Lada & I filled them with with water and left them to soak for a long weekend.  No leaks!    100LL is half the viscosity of water, so there might be a hole too small for water to leak through, but might pass avgas.  I drained all of the water and borrowed 29 gal* of 100LL from the Cirrus and tried it again in the Left tank.   Success!  I managed to salvage about 28 gal and put it in the Right tank.  Another success!  Neither tank had any visible moisture after 24 hr of containing 100 LL.

This officially completes section 21, as it was published on the day I received the wing kit.  More about that later!

*I have 3 jugs of 8 gal each that I use to buy premium auto gas and pump into the RV-12.  I also have a 5 gal container.  Together, they are 29 gal. The fuel tanks are 30 gals, so "close enough."  If there's a leak at the very top, it will not be operationally important because fuel won't leak up against gravity.  In addition, aircraft tanks are rarely stored in a completely full state.  I will take pains to ensure that the plane is not parked outside during rain, or will find a way to cover the fuel caps if needed.

Water tight. Not air tight, but water tight.

 I bet y'all thought I had quit and walked away.   Nope.   Thought about it a bit, but nope.

Both tanks remain stubbornly not-air-tight.  I was extremely hopeful that the through-bolt locking mechanism in the cap was the culprit.  I tried sealing the caps with rubber gloves, but there aren't any more latex rubber gloves and nitrile gloves don't have that stretch that is needed to make a perfectly smooth sealing surface.  They tend to fold and bunch up and that makes them leak way more than the cap by itself.  Condoms, on the other hand, are perfectly round in cross section and can be made to fit over the cap.  It typically took 3 or 4 condoms getting torn to lock the caps in place without damaging the condom itself, but I got good at it.

Still, the damn tanks wouldn't hold pressure.  I gave up on the balloons as pressure gauges and got some clear plastic tubing and food coloring to use as a simple manometer.   15 inH2O is about 0.5 psi and so I used that difference in water height as my pressure gauge.  Pumping up the tanks to 15 inH2O resulted in a drift down to about 1 or 2 inH2O in half an hour.  This was fairly repeatable.    Despite being repeatable, I could not find where the air was leaking using the soapy bubble water trick.  (Which is messy, by the way.  At some point, I'm going to have to haul the wing assembly out to the drive way and hose it off.)

I posted my plight on the forum and, man, did I ignite a firestorm!  Two suggestions were worth merit: one was that I had not accounted for the heat of compression and that the declining pressure was actually the heated air cooling off.  The second was to use dish soap rather than kid's bubble soap for my leak detection.   Both of these have been filed for potential future use.

Several commentators had used the logic of "it's a fuel tank, not an air tank" and simply filed their tanks with 100LL fuel and looked for the tell-tale blue stain.  Other's commented that that takes too long to see the stain, and others said balderdash and basically it turned into a debate similar to the holy war of priming vs not priming.

I like the pragmatic approach exemplified by the "it's a fuel tank."   I did a full up water test over the previous weekend.  Literally.   Lada & I filled both tanks to the brim with water and then left for a 3 day weekend.  Upon my return, there was no damp or wet spot on any of the sides or the tops.  I couldn't check the bottoms because of the weight of the water itself, but if either tank develops a leak on the bottom, I can repair those even after the wings are mounted on the plane.

Last night I drained the water and fetched 29 gal of 100LL from the airport.  I didn't have the correct hoses to configure everything (I can't use the cheap PVC tubing with 100LL, so I have to go to the hardware store for the proper fuel hose.).  I'm going to fill each tank and watch for 24 hours.  If no leak, I will declare them airworthy and get back to actual assembly.   This final step will probably take another week or so.  Stay tuned.

still leaking.

 I finally ended up using a condom to seal the fuel cap on the Left tank, but alas, the tank still leaks.  I have this weekend off work and will spend the majority of my time searching for the remaining leaks.  The good news is that the fuel sender and access plates are air tight.  The bad news is that he fuel sender and access plates are air tight...  :-(

might not be so bad after all.

 I have to admit I was pretty bummed the other day.  I really thought that I had a decent chance at not having the fuel float assembly leak.   Maybe I'm still in the running.    I had to walk away for a day to get over my funk, then worked a 24 hr shift, so I didn't go out to the shop until last night.  The kids came over for Labor Day family get together.   Travis is my son-in-law who happens to be an HVAC technician.   As he puts it, "I chase leaks for a living" so he followed me out to the shop and we got to work.

I pumped up the tank to bout 15 cmH2O and the balloon ballooned.  (That's still so weird to watch Starling's Law in action!).   Travis spent about 30 seconds listening and immediately found two leaks. (I have presbycusis and even wearing hearing aids I couldn't hear a thing.)  I  One was the 6D nipple upon which I had attached the balloon. Although I had  applied thread sealant, that didn't work as well as I thought.  That got capped with the appropriate 6D cap and we repressurised.   It still started dropping, but not as fast.   Travis then noted that I had left the bicycle pump on the Schrader valve and pointed out that's just a friction fit and is a likely culprit, so that came off promptly.   Tried again;  still dropping but again dropping more slowly.

The fuel sender seal had no soap bubbles, so that made me hopeful.  I soaped up the fuel filler cap and found a nice leak at the shaft that goes through the cap from the latch to the nylon lock nut on the inside.  I put some thread sealant on it, but it still leaked.  We called it a night and I slept much better even though I haven't proved the L tank is pressure tight.

 

Tiny bubble in the center of the cap

Today, I'm at work and I just did some searching on VAF about this topic and found the following gem, which I'll try it tomorrow night when I get home from work!

 Old trick but for those have never heard it before: Put a rubber glove inside the fill port before installing the cap. That will seal and isolate the cap.

Well, damn.

 4th time was not the charm, either.   Last week we went on vacation. Before we left, I used an extra thick layer of sealant and specifically embedded the star washers and electrical lug in sealant and then covered the stack.  I had such high hopes.

After we got back on Tuesday, I got some clear 3/8" hose to use as a manometer and hooked that up to the Left tank.   I pressurized it to about 13 inH2O which is around 0.5 PSI.  That was the pressure at which the balloon inflated.   I left it overnight and was really disappointed the next morning as the balloon had deflated and the manometer showed only about 1 inH2O.

I haven't had the time nor the heart to try to find the leak yet.  I'm really hoping it's from the filler cap, as that is considered acceptable by Van's.    I really need to get the tanks sealed because I am essentially at a stand-still until they're done.     Alternatively, I might buy another table and start working on the tail surfaces while debugging the tanks.  Stay tuned.