MOGAS vs Composite Fuel Tanks


Page 1 - The Cub Project
Page 2 - Fabric
Page 3 - Firewall Forward
Page 4 - Firewall Forward 2
Page 5 - O-320 Overhaul
Page 6 - Final Assembly (2010)
Page 7 - Final Assembly (2011 page 2)
Page 8 - Final Assembly  (page 3)
Page 9 - Completed Project

Ever since this plane was new, I have typically run it on a mixture of 80% Alcohol Free MOGAS and 20% 100LL Avgas.  But in December of 2017, I retired and moved from Los Alamos, NM to Cherokee Village, AR.  In New Mexico, I always bought my MOGAS from Honstein Oil as they were the only jobber that carried Alcohol Free MOGAS.  Fortunately, my new home is in an area with lots of lakes and boats, so every gas station carries Alcohol Free Premium fuel.  As always, I tested and confirmed there was no alcohol present.  However, in June, I took a month away from the planes and the hangar while I had a knee overhauled.  When I returned to that hangar in July, I found the Cub leaking fuel, the O-rings in the primer had failed, the flow meter in my fueling rig on the truck had failed, the hoses on the fueling rig had failed and were leaking, and the flow meter on the fueling rig on the truck had also failed.  Man, that was a lot of failures at once, and the common thread was the fuel.  I worked on the Cub to seal some leaky threads, then topped it off with 100LL only to come back and find it leaking from both tanks enough that it was running down the sides of the plane and dripping off the belly and tail.  It was clear that the tanks in the Cub had been breeched and would require rebuilding or replacement.

Additionally, while this damage was not from Alcohol contamination, there was clearly something in the fuel that was causing damage.  While I may not know what to test for, I could devise a test to see if the fuel would cause problems.  One of the things I notes was that the fuel appeared to attack the Rectorseal (TM) Pipe Dope I used on all the pipe fittings.  So, I made a stop at some of the local gas stations and pumped 2 gallons of premium fuel into my old truck to clear the hoses at the pump, then 1/2 gallon into a clean gas can.  Fuel samples were taken from the gas cans and placed into baby food jars.  Then I placed a new O-ring into the jars.  I laid up a single laminate of epoxy resin and 5.85oz glass and cut into test strips to go into the jars.  Then I took a 1/8" pipe fitting, worked some pipe dope into the threads and allowed to cure overnight before placing the fittings into the jars.


Fuel testing.  I have two controls here.  Jar #1 is 100LL, and Jar #6 is the fuel that damaged the fueling rig and the composite tanks in the Cub.  The other jars are from local fuel stations.  The pipe dope test was a very quick test and apparently accurate.  The fittings with pipe dope on them had the dope dissolve on contact in jars 3, 4, and 6.  The dope did not dissolve in jars 1 & 2.  After 2 months and repeated swirling of the jars, the pipe dope has still not dissolved in either jar.  After 2 months, the fiberglass test strips in jars 3, 4, & 5 are showing significant degradation, while the test strips in jars 1 & 2 are not.  It appears that the clear fuel in Jar #2 is just as safe with composite tanks as the 100LL.  Unfortunately, the fuel I had been using in my planes was from Jar #5.


July 2018 - I've cut the fabric away from the top of the fuel tanks, but I need to give my knee more time to heal, and it's just too hot for this kind of work in Arkansas in July.


September 26, 2018 - It's fall weather and my knee is healed.  So, I cut the top from the fuel tanks.  Wow!  All this debris is apparently a coating of epoxy resin painted onto the inside of the tank to seal it.  It simply broke down and peeled off inside the fuel tank.  It's somewhat horrifying to know this is what the insides of my tanks looked like while I was flying!


The inside was also peeling form the bottom of the top of the tank.


This is the left tank after cutting away the top in three sections leaving the ribs in place.  


This is the right tank after cutting the top off it.  It's not quite as bad, but it's still pretty bad.


This is the right tank after cleaning it up.  It still needs a bit more clean up, and will require laminating some glass on the inside to build it up some before sealing it with slosh compound, but it will be repairable.


Here is the left tank after the initial clean up.  These tanks were built with 3/8" blue foam as a core material.  It delaminated while cutting it away, so I am likely going to need to fabricate new tops for the tanks.  I'm still mulling over how I plan to accomplish that task.

Sept 28, 2018 - I've had a couple of days to consider the repairs on these tanks.  I really don't trust the various penetrations into the tanks to be sound.  As you can see in the photo above, most of the penetrations are at the butt end of the tank for obvious reasons, and I simply do not have access to them with the wings on the plane.  So, the wings are going to have to come off to gain access to the butt end of the tank, and to give me better overall access to build these tanks new.  At this point in time, I am still undecided about whether to build with epoxy resin and slosh to seal, or build a  new tank within the existing tank using VinylEster resin.  Fact is, VinylEster is the only resin that will not deteriorate due to fuel and fuel additives.  On the other hand, I have epoxy tanks in my KR with slosh compound that have served me well for 21 years now.  Additionally, I have a lot of epoxy resin and the slosh compound on hand, and would need to buy the VinylEster.  That's not a huge expense, but the final factor is that I really don't like working with VinylEster, which is probably the biggest factor.  So, for now, it's undecided.  First I need to get these tanks off and do some more clean up before I make the call.  I also have to take a closer look at cutting out the ribs and fitting a metal tank into this space.  That may yet also be a possibility.  But the one conclusion I can make is that this plane is not going to fly again in the near future.  Glad I gave the engine a full treatment of Cam Guard at the last oil change.


October 1, 2018 - Wing tips are off, control cables disconnected, electrical disconnected, and fuel lines are removed.  I've built a set of wing jacks to hold up the wings while I remove them.  Only thing left to do is pull the bolts from the lift struts and wing attach fittings, and the wings will be off.  Then I can get serious about rebuilding the fuel tanks.  


October 3, 2018 - OK.  Here she sits looking like a plucked chicken.    The wings are off and on the bench to have the tanks rebuilt.


The epoxy resin in these tanks is badly degraded.  You can see a lot of bare cloth as well as a lot of peeling resin and cloth.  All the various penetrations into the tank (fuel pickup, fuel site gauge ports, fuel drain) are all compromised and leaking, so they all have to come out.


The baffles are all out.  There are a lot of really rough areas in the tanks that need to be sanded, and some holes that need to be filled with micro before laminating the inside of the tank with vinylester and cloth.  You can see several of the penetrations have now been removed from the tanks.  The tank drain won't be removed until I get the new welding flanges in so I'll know how big or small of an area to cut to install the new flange.


October 4, 2018 - Today I ground away a ton of flox, foam and other materials from the the rib former spanning the top of the tanks.  I also ground down all the stubs and flox points from the various baffle plates and supports that had been in the tanks.  The new tops will fit under the steel rib formers and get floxed to the formers.  The red markings in the tank are areas that will need some repair with micro-balloons before I start laminating with vinylester.  In this photo, you can clearly see the penetrations where I knocked out the fuel pickup and the tank gauge ports.  Those will eventually get aluminum flanges floxed into place for the fuel and vent ports.  Tomorrow... More sanding.  Woohoo!!!


Oct 5, 2018 - I've completed the rough sanding of most of the tanks.  I filled some of the really rough stuff with Micro today.  Will sand more tomorrow.  The idea here is to get the surface smooth enough that I can laminate over it with vinylester resin and tooling cloth to start building a tank within the confines of the old tank.


Oct 8, 2018 - Started fitting up the new tank covers.  With the Aluminum tank covers, I can easily gain access to the new tanks to inspect or repair if there is a problem in the future.  Also cleaned up the filler necks and welded a new flange onto one to prepare for installation back into the tanks.


Oct 9, 2018 - Today's project was to drill and fit the aluminum welding flanges for the various tank penetrations.  These will get floxed into place, then glassed over.  Left to right these are Fuel Sump, Fuel Pick Up, upper and lower ports for the Fuel Sight Gauge, and the far right side at the top is the vent line that returns from the header tank.  These flanges had to be drilled with numerous holes so the flox will have something to bite into, then cut down to fit along the upper and lower edges.  


Oct 10, 2018 - Cheap Vacuum bagging (a.k.a. vacuum garbage bagging).  I am laying up the baffle plates to go into the fuel tanks.  I'll cut them out of flat sheet, so need a sheet of material strong enough for the job, but yet light.  This material is made up of one lay up of 5.85oz glass on either side of a piece of 1/8" divinycel foam core.  To make the piece easy to attach, it will have a peel ply finish on both sides.  The lay up from bottom to top is peel ply (dacron fabric), 5.85 oz tooling glass, 1/8" divinycell foam (perforated from both sides with a porcupine), tooling glass, peel ply,and absorbant breather cloth.  It is covered with a plastic painting drop cloth from WalMart, and sealed using latex caulk.  This is laid up on an old piece of formica countertop I got for the asking, then waxed with floor wax.  This makes for a really inexpensive vacuum bag, and works equally as well for flat lay ups.  This is also the beginning of building this back up using vinylester resin.  It's been 22 years since I last used VinyEster, so I needed to refresh myself a bit on the amount of catalyst to use and the flow qualities of vinylester.  I still don't like the stuff, but it's what I need if I want chemical proof composite tanks.


Oct 11, 2018 - First lay ups of vinylester in the tanks.  I laminated the front and back walls with a single lay up of vinylester and tooling cloth.  The "plyfoam" part I laid up yesterday looks good, so I laid up a second one today.  That should be enough "plyfoam" to fabricate two baffle plates for each tank.


Oct 13, 2018 - Floxed in the various tank penetrations; from left to right - Fuel Sump, Fuel Pickup, Header tank return line, lower and upper ports for fuel sight gauge.  This is just the first pass to get the welding flanges attached in the right places.  They will get more coverage and glassed over before I'm done.
Also laid up more pieces for the tank tops, but really nothing to show picture wise.  They all look the same as the vacuum "garbage" bag lay ups on Oct 10.  


Oct 15, 2018 - Punched a hole for the filler neck in both tank tops today.  Also drilled the finish sizes for all the attach screw holes in the panels.  I need to use a hole duplicator to locate and drill the holes in the butt rib so they match up with the overlying fairing.  I had a hole duplicator before I moved, but don't seem to have one anymore.  Will have to order a new one.  Once the butt rib holes have been match drilled, Some corner clean up accomplished and a little leading and trailing edge work completed with my press brake, the tank covers will be ready to prep and paint.  Also still fabricating pieces for the tank tops.  One per day.  Time to take the next week off for some family travel before I continue with the work.





Page 1 - The Cub Project
Page 2 - Fabric
Page 3 - Firewall Forward
Page 4 - Firewall Forward 2
Page 5 - O-320 Overhaul
Page 6 - Final Assembly (2010)
Page 7 - Final Assembly (2011 page 2)
Page 8 - Final Assembly  (page 3)
Page 9 - Completed Project