Since this is my mess, I'll try to explain.
Many people are using aluminim tanks, and that is clearly the best choice. Sometimes there are constriants for design or fabrication, but it is clearly the best choice.
Second best choice, and still a good one, is to fabricate the tanks using vinylester resin as it is also resistant to all the various nasty chemicals that can be found in MOGAS. A fair number of KR builders have also done this.
Third choice would be to build them of using Epoxy resin, then slosh with a chemical resistant slosh compound. I did this when I built my KR and it has held up quite well; However, if the tanks are not prepped correctly or allowed to fully cure properly, the slosh compound may flake or peel causing a mess similar to the photos of the inside of my Cub tanks.
Another choice would be to build them using Epoxy resin and use as is. Some KR builders have done this as well. However, that restrict you to using 100LL only, or being very selective about the MOGAS you get. In my case, I thougth Alcohol was the only damaging chemical currently used in MOGAS. That clearly is not the case, so while I was using a fuel I thought was safe, it was dissolving my tanks.
I have also heard of guys using PVC pipe, but I can't vouch for the chemical resistance of PVC when it comes to various fuels.
My absolute last choice would be to use PolyEster Resin to build the fiberglass tanks. However, Piper built the tip tanks for the Cherokee Six and Cherokee 235 lines using Polyester resin, so you will actually find this as an FAA approved construction process. I have overhauled some of those tanks after the owners used mogas in them, effectively dissolving the tanks from the inside out.
Another issue nobody seems to be discussing is that we don't have a clue as to what we are going to have for AVGAS even as soon as 2 years from now. 100LL will go away at some point in time. The FAA's program for a drop in replacement fuel is all but dead. Now we have multiple fuel vendors pursuing STCs for their fuels so they can start selling them as is. That's great if you have a plane that falls under the STC. However, some planes could be excluded. When pursuing a STC for their fuels, the vendors are going to pursue large market share aircraft first. That meant the typical Cessna with either an aluminum tank, or a rubberized bladder. How thoroughly will they test against long term degradation of various composite materials? I wouldn't expect in depth testing, if at all. So when the new fuels start hitting the market under STCs, we need to be very carefully watching our fuel systems. Especially those of us with composite tanks, regardless of the resins used in construction.
As I have found, there are a lot of unknown contaminates in Mogas beyond Alcohol, and after my recent experience, I would conclude that if you want to run MOGAS, which I still intend to do, you may want to do a much more thorough testing of the fuel before putting it into your tanks.
Understand that I bought my Wag Aero Cub as a project plane and the tanks were already built into the wings as an integral part of the wing. A real Cub would have had Aluminum tanks. But this is a strange one off build with a lot of non-standard modifications. I don't think I could even fit in a set of Cub tanks without a major rebuild of the wing, and I just don't want to do that now for reasons too complex and numerous to list here. I leak tested them and found them to be sound, but also knew I was taking a risk since I didn't know what materials they were built with. Well, sometimes when we take a risk, we loose. In this case, I lost.
My plan moving forward will be to pull the wings off my Cub and strip out all but the framework of the fuel tanks. The new tanks are probably going to be fabricated using VinylEster resin. From where I'm at now, that seems like the only reasonable path forward. For me, this is likely to be a months long project instead of the few weeks I had planned.
For those that are wondering why I am discussing a homebuilt Cub here on the KR Forum, it's because the composite fuel tank issue does directly relate to the same issue that could come up in a KR with composite tanks, which makes the discussion pertinent. For those that want to keep abreast of the repair work, I'll be posting it on my web site as I go at
<http://jeffsplanes.com/Cub/MOGAS/mogas.html>
-Jeff Scott
Cherokee Village, AR
Sent: Saturday, September 29, 2018 at 11:04 AM
From: "Ken Henderson via KRnet" <krnet@list.krnet.org>
To: krnet@list.krnet.org
Cc: "Ken Henderson" <dckenhender...@gmail.com>
Subject: KR> Mogas Blues
From: "Ken Henderson via KRnet" <krnet@list.krnet.org>
To: krnet@list.krnet.org
Cc: "Ken Henderson" <dckenhender...@gmail.com>
Subject: KR> Mogas Blues
New to this discussion but I have a question. Why don't more people use
aluminum fuel tanks? I remember reading of people making wing tanks that
laid along the spar out of irrigation pipe of 4-6" diameter. A header tank
similar to the stock glass one could be fabbed up easily as well. It would
probably be lighter to use aluminum also.
If Tig welding is not available or affordable, non-structural aluminum
components such as fuel tanks could be made of 20 series aluminum and gas
torch welded in most home shops.
Just throwing this out there. Feel free to shoot it full of holes. It's all
part of the invention/ education process.
aluminum fuel tanks? I remember reading of people making wing tanks that
laid along the spar out of irrigation pipe of 4-6" diameter. A header tank
similar to the stock glass one could be fabbed up easily as well. It would
probably be lighter to use aluminum also.
If Tig welding is not available or affordable, non-structural aluminum
components such as fuel tanks could be made of 20 series aluminum and gas
torch welded in most home shops.
Just throwing this out there. Feel free to shoot it full of holes. It's all
part of the invention/ education process.
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