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RV12 MOgas and AVGas

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udayaggarwal

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When using either MOgas (91 Octane) non ethanol or AVGas interchangeably on the RV 12 Rotax 100HP engine, can I mix the two if I've filled MOgas initially and then topped off with AVGas? Is an additive required? This is my first airplane and I've no idea whether I should be mixing the 2 and or using any sort of additive as preventative measures.
Any insights into this with directions for use would be very helpful.
 
udayaggarwal said:
When using either MOgas (91 Octane) non ethanol or AVGas interchangeably on the RV 12 Rotax 100HP engine, can I mix the two if I've filled MOgas initially and then topped off with AVGas? Is an additive required? This is my first airplane and I've no idea whether I should be mixing the 2 and or using any sort of additive as preventative measures.
Any insights into this with directions for use would be very helpful.
Click to expand...
You can mix them freely. Many folks add Decalin when using avgas.
 
udayaggarwal said:
When using either MOgas (91 Octane) non ethanol or AVGas interchangeably on the RV 12 Rotax 100HP engine, can I mix the two if I've filled MOgas initially and then topped off with AVGas? Is an additive required? This is my first airplane and I've no idea whether I should be mixing the 2 and or using any sort of additive as preventative measures.
Any insights into this with directions for use would be very helpful.
Click to expand...

-There is no additive required in any scenario. Many people feel better about running Decalin as an additive with leaded avgas. Rotax says that there is no evidence of detrimental effect from field use of the additive. The additive makes no difference with respect to maintenance intervals.

-If you are mixing fuels and using more than 33% leaded avgas, follow the shorter maintenance intervals.

Non ethanol fuel is fine, but not necessary.
 
We ran a 912 for more than a thousand hours, using mostly Walmart 93 octane which had 10% ethanol. Obviously when on cross countries we used 100LL because usually that was all that usually was available. When we removed the gear box at 1200 hours for maintenance, the mechanic doing the IRAN said it was in great shape with no evidence of lead paste on the clutches. Rotax recommends from best to worst: 91octane or greater non ethanol as best, 91 octane or greater, with up to 15% as next best, and 100LL as the least desirable fuel, because of lead contamination to the oil and the gearbox components.
 

You should become knowledgable in the use of auto fuel in aircraft engines… especially Rotax as installed in the RV-12. Auto fuel formulation for vapor pressure rating is changed twice/year, known as summer and winter blends. Extra caution must be taken in spring weather conditions when still using winter blend fuel on a warm day. I had vapor lock on take-off. Search https://vansairforce.net/threads/vapor-lock-rv-12-rotax-912uls.194385/ for my thread on this subject. Also, watch Mogas in Aviation
and SL-00078 REV 1 HTTPS://WWW.VANSAIRCRAFT.COM/SERVICE-INFORMATION-AND-REVISIONS/SL-00078/​


The Rotax 912 is tightly-cowled in the RV-12 and does not get very much airflow during ground ops. Fuel lines on the 912 are routed on top side of engine and get very warm. Restarting a heat-soaked engine is recipe for vapor lock. See… https://vansairforce.net/threads/engine-cooling-when-parked.227809/ and https://vansairforce.net/threads/vapor-lock.132487/
 
You can mix them however you'd like for flight operations. The instructor from my Rotax Maintenance course recommended mixing in 10-20% 100LL/UL94 if you've filled it with mogas and you're going to let it sit for a couple of weeks.

Unleaded is preferred with Rotax engines in general, but if you can't avoid 100LL it's not that big of a deal. Engines below a certain SN lack holes in the gearbox to sling the lead out, and they'll require a more frequent inspection. Otherwise, you'll mostly have to change the oil/spark plugs more frequently.

If you're going to be parked on the ground for <1hr, it's a good idea to shove an exhaust fan into the oil fill hatch to help avoid vapor lock as mentioned above. The voltage regulator will also thank you.
 
NORDO Industries LLC said:
You can mix them however you'd like for flight operations. The instructor from my Rotax Maintenance course recommended mixing in 10-20% 100LL/UL94 if you've filled it with mogas and you're going to let it sit for a couple of weeks.
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This comment is not directed at you NORDO, just quoting this wisdom that we all hear from time to time. Also I understand that mixing of fuels is acceptable, but this is now in the context of fuel that is otherwise not recommended due to age.

I'm always a little curious when I see this type of advice here and there. Usually it's from credible people but without any real explanation or basis as to why or how much. No research to support transitive properties of different types of fuel to fix aging mogas. Is there more to this than, "it sounded right when someone smart said it" ?

I have also had the exact opposite recommended from instructors in the Rotax and LSA world. If your gas is old, replacing it with new gas is preferred over deciding to play petrochemical engineer to fix old gas. Unless of course you are a petrochemical engineer in which case I hope that guy comes along and explains things with some kind of detail.

The 100LL at least seems plausible since you would be getting some amount of tetraethyl lead in the mix. If we assume the TEL distributes uniformly through the mixed gas it makes sense the effective octane would increase. Any assumption about the resulting octane property from a mix would be dependent on volumes and age. I think that Swift uses amines that are common in Mogas and could contribute this benefit, but the octane rating is so close to where we are starting that it seems it would take a large proportion of Swift to improve the properties of old fuel.

Maybe we should just always put a few drops of mystery oil and mouse milk in the tank too. :)
 
One thing I have heard commonly though, is that especially with the Carbureted engines, it’s a bad idea to let Mogas purchased from a local gas station sit in your engine unused, primarily because if you have low-quality fuel sitting in the system, it can deposit varnish/gunk in places it really should not be (like the bowls of your carbs). 100LL/94UL has much tighter QC, and will avoid these problems.
 
NORDO Industries LLC said:
One thing I have heard commonly though, is that especially with the Carbureted engines, it’s a bad idea to let Mogas purchased from a local gas station sit in your engine unused, primarily because if you have low-quality fuel sitting in the system, it can deposit varnish/gunk in places it really should not be (like the bowls of your carbs). 100LL/94UL has much tighter QC, and will avoid these problems.
Click to expand...

The rule of thumb that I follow and encourage maintenance customers to follow is to avoid using mogas more than 30 days old. Fine to fudge it by a few days if necessary, but go fly the thing! This applies to either carb or FI motors, primarily to minimize the risk of detonation from compromised octane ratings. Gunk and varnish is a good reason too.
 
NORDO Industries LLC said:
One thing I have heard commonly though, is that especially with the Carbureted engines, it’s a bad idea to let Mogas purchased from a local gas station sit in your engine unused, primarily because if you have low-quality fuel sitting in the system, it can deposit varnish/gunk in places it really should not be (like the bowls of your carbs). 100LL/94UL has much tighter QC, and will avoid these problems.
Click to expand...

Ethanol when it evaporates becomes a varnish. This is why one reason why to avoid ethanol in fuel for your plane.
I have never asked the question or thought about it. But in my snow blower, I put in Stabil at the end of the season which prevents the ethanol from becoming a varnish.
I wonder about using such an additive in an E-A/B?

Tim
 
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