Gasoline mix-up

[Mike S]

Originally Posted by dahugo

DAR to arrive soon, and I don't want to do the first flight with full fuel tanks.

Good for you

Good luck with the inspection, and subsequent fledging.

[Buggsy2]

1/17 is 5.88% Mogas.
Assume as a worst case it's Regular (lowest octane) that the mogas' effective octane is only 70 in aviation octane terms. So now your mixed octane is

(16 * 100 + 1 * 70)
------------------- = 98.2 aviation octane, if you trust the assumptions.
16 + 1

What about ethanol? Assume your mogas is E15, that is, is 15% ethanol and 65% straight auto gas. Your mixed ethanol percent is

(16 * 0 + 1 * .15)
------------------ * 100 = 0.88 %
16 + 1

I think you can see why previous comments are not concerned about this.

[Cuben1]

A 1 to 17 mogas/avgas ratio is no big deal.

If you are really concerned just pour half into one tank, half into the other and then fill both tanks to the top with avgas.

The resulting 1 to 35 ratio is of no worry.

I you want to be even more conservative, fill one tank with 8 gallons of the mogas/avgas mix and fill to a 1 to 35 ratio while keeping the other tank pure avgas for take off and landing.

Repeat when the mixed tank is empty.

If you are filling from a plastic gas container you are probably at a greater risk of static electric sparks starting a gas tank fire than you are having a problem with the small amount of mogas in the mix.

[rocketbob]

Originally Posted by dahugo

Rocketbob, I didn't drain it because of the mogas. I drained it completely prior to that because I was calibrating the tanks on my craft with a Dynon FlightDEK 180, and we have not had our first flight yet. DAR to arrive soon, and I don't want to do the first flight with full fuel tanks.

The drainage was really of no moment--the questions is about the inadvertant mixture of some mogas with avgas.

ok well in that case no worries. I typically ran the other way around with no worries.

[johnny stick]

My old Powerplant mechanic teacher warned us about averaging octane. It doesn't work that way he said. Octane is based on a logarithmic scale, so it doesn't average. He claimed that 5 gallons of 87 octane and mixed with 5 gallons of 100 would give something like 98 octane, not 94. JMO