I thought I would make a followup report on the Mogas testing I have conducted and the little bit that I have learned from it.
I attached a photo of the same five jars of fuel after a month of soak testing various parts that might be in the fuel system with some surprising results, but more importantly perhaps an additional test methodology to verify Mogas fuels for airdcraft use.
In each jar, I place a new O-ring. Then I worked some pipe dope (RectorSeal) into the threads of a 1/8" male pipe fitting and allowed to cure for 24 hours before placing into the fuels. Then I laid up a single sheet of tooling glass with aeropoxy and peel ply squeegeed out as thin as reasonably possible and allowed to cure. After curing, I cut the fiberglas into test strips and placed into the fuel jars for soak testing, one strip with peel ply still on it, and one strip without and only one end of the strip submerged in the fuel.
The fuels in the picture are 1) 100LL as a control, 2) local Conoco/Phillips brand 3) local unbranded independent fuel 4) local CITGO branded fuel, and 5) local Murphy oil fuel that damaged my fuel systems as my second control.
Testing:
The O-rings really don't show enough change to quantify at this point in time. I'd like to think I can feel some slight differences, but it is not sufficient for me to draw any conclusions as to damage to the O-rings at this point in time.
The fiberglass test strips in the three yellow colored fuels (3, 4, & 5) and definitely showing signs of softening and degrading. When removed from the fuel, it is visibly obvious which end of the test strip was submerged in the fuel and the submerged end is starting to soften and the resin is thinning between the weave of the glass.
The pipe dope testing was very quick and probably the most telling. Jar #5 dissolved the pipe dope from the threads of the fitting on contact. That test was completed in a matter of seconds. Jars #3 & 4 both dissolved the pipe dope as soon as I swirled the jar, so that testing was also completed in a matter of 20 seconds or so. Jar #1 (100LL) and Jar #2 (local Conoco/Phillips) neither one attacked the pipe dope on the threads of the pipe fittings. After a month and repeatedly swirling the fuels, Jars #1 & 2 still have the pipe dope intact in the threads of the fittings. In the attached photo, I had just swirled the 5 jars. The cloudiness in the three jars to the right is caused by the pipe dope in suspension in the fuel.
After a one month test, I think I have a winner for a Mogas I can safely run in my planes. The local Conoco/Phillips brand fuel seems to be just as innocuous to the various fuel system parts as 100LL. Of course it is 91 AKI fuel with no alcohol and no lead, so will get mixed with 25% 100LL for the time being to maintain the optimal amount of lead in the fuel for my engines.
In conclusion, based on the simple testing I have done here, in addition to testing fuel for alcohol, I would also take a sample of the fuel and test it against a pipe fitting with some dried pipe dope to see if it attacks the pipe dope. If the pipe dope readily dissolves into the fuel, you wasn to consider whether you want to run that fuel in your plane as it may eventually wick it's way through any doped pipe fittings, may attack any composite parts in the tanks, and may cause problems with rubber or neoprene O-rings or seals in the fuel system.
For those of us with Epoxy resin based fuel tanks, this is probably doubly important. I will say the slosh compound I used in my KR tanks some 22 years ago did save them from damage from the aggressive fuel I used, otherwise I would also be cutting the tanks open in it as well.
I have not yet repaired the tanks in my SuperCub Clone. I have been waiting for cooler weather here in AR. I anticipate getting started on the repairs after I return home from vacation following the KR Gathering. I will post photos and descriptions of what I find after I cut the tanks open.
-Jeff Scott
Cherokee Village, AR
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