I ve been flying for over ten years, so I guess this might be a silly topic. I just got back from a great flying session, the wind finally quit blowing here in Kansas, so I had to take advantage of the nice weather. I put together a Super Sportster over the winter just for a "throw in and go fly plane" , The first couple flights a month ago were anything but perfect. The engine that I new was good would not run Right, I checked everything. Finally changed engines a few weeks back, a little better , but still run problems. The fuel I was using I bought right at the end of summer last year. Kept the stopper in the top and figured it was fine. I bought a new gallon of fuel over the weekend, and this engine ran great. Flew two tanks all the way to empty(or close) and had a great time. It is the simple things that we overlook sometimes, Even after weve done this for years . Fresh fuel does make a difference. If youve got an engine problem, Keep this in mind, " How long has that fuel been in the flight box?" Probably longer than you think, Happy flying,

How long has the fuel been setting at the hobby shop if they didn't rotate their fuel?

The real problem is not age, it is storage. I would bet that during use he left the top off a few times and if not that, the bottle was half full or less. Lot of air in a half full bottle or can and that air has a high moisture content. Keep your can as full as possible all the time. Another problem is the fuel caps that companies like Du-Bro sell the little fitting don't seal the can. I have fuel that is over ten years old and it runs just fine. keep it in a cool dark spot and make sure the tops are on tight.

I agree that the problem is storage, not age. I have run fuel that is over 5 years old from a half-full container without any problems. I have also had problems with much newer fuel. This may be a question/comment for the fuel forum but I have been speculating as to what makes fuel "bad". There have been enough tests done with adding water to fuel to discount the posssibility that a small amount of water from adsorption from the air will kill the fuel (a large amount is a different story). Methanol, nitromethane and oil may degrade a bit over time with exposure to sunlight but I wouldn't expect this to be a major problem unless storage conditions are extreme. Methanol is the fuel component with the highest vapor pressure and I wonder if the "bad" fuel is simply fuel where a significant proportion of the methanol has evaporated?? This could effectively increase the oil (and nitro) content to the point where the engine won't run reliably. Next time I see a "bad" fuel situation, I think I will check it for oil content.

Ross

I always thought that the nitro changed chemically if exposed to sunlight and that water absorption made up the rest of the problems we experience.

Like others, I have stored fuel for many years (7) in a constant temperature/no light condition and have not had any problem with unopened containers going bad. It would be nice if we could get some scientific help in explaining what makes "bad" fuel versus good fuel.

I don't care how humid the air is, but there is not enough moisture in a gallon of air to cause any problems, unless maybe there is less than an ounce or so left. Even then I am not sure it would cause any problems. We know that a couple of percent or so of water can be absorbed with no ill effects on our engines. Past that and it is up to debate, except perhaps control line and freeflight operation, those can run well till there is enough water to cause the oil to seperate.

Yes but with exposure to methanol the by products also burn, and the methanol slows down the process also. It also does not change in color. The darkening of fuel is usually only the dye changing color, though it could be some oxidation of the oil.

Nicely put, Hugh!


Avogadro's number and Avogadro's law speak about this.
A gaseous volume of 22.41 litres (~5.927 gallons) are needed for its weight in grams to be equal in number, to its molecular weight.

Water has a molecular weight of 18, so 5.927 gallons of water vapor are equivalent to 18 grams of liquid water.

One gallon of water vapor is about 3 grams of water, which will not have much affect on engine performance, even in 1/4 gallon of fuel.
Even very moist (100% humidity) air contains much, much less water.

The last few days I had the same experience as described by edd540. My OS50 began running a little shabby a few days ago. Sun I brought out my UCD 46 with an OS91 FS. It ran long enough to get airborne then died. After getting it back to the stand it would not run - not a pop. I changed plugs twice, primed it several times, switched glow drivers, used my same electric starter, not a pop. I took off the carb, flushed it and found nothing. Checked the valve lash - OK. The next day I swapped my just- broke-in Saito 91 and it would not fire.
Then I opened a new container of 15% fuel and the Saito fired right up and ran fine. I put the OS91 on a test stand and it started right up. The old fuel was what I used all winter as we tried to get a little flying in and it was a little brown compared to the fresh stuff.

I concluded that the old fuel was the problem. Before this I had never noticed fuel affecting engine operation but it may have, in fact probably did. It looks like that is the case but still could be something else. So what happens to fuel as it ages that would make several engines not run?

I will try the suspect fuel in my OS50 a little more and see if it will run and how well. I will tach it , change fuel to the new stuff and measure top end again to see if that changes.

The next time I have an engine problem one of the early checks will be to try fresh fuel.

Evaporation from a poor cap seal. Less methanol and more oil would require a very hot plug, but then the increased nitro percentage would may detonate.

Perfect example of " a little knowledge can be a dangerous thing". First Avagaro's number (6.02x10^23) has nothing to do with this. It's the number of atoms or molecules of substance at STP in a mole. That is another bit you left off. STP is standard temperature and pressure.

So I 'll tell you this. Go ahead and leave the top off your fuel, especially on humid days, store it in a warm area and subject it to bright sunlight. Then when your engines don't run right you can all write in these forums how all that stuff has no effect. Me??? I'll store my fuel in full containers, in a dark cool spot and keep the top on when I'm not actually fueling or defueling and I will run fuel that is 10 years old and never have problems with my engines.

Crashed a plane once due to bad fuel. Didn't know that at the time. Eventually got it. I don't screw with it any more. If it has been opened and is more than half gone, if it doesn't still look perfect visually, and I haven't used it in a while, I put it in the "Weed killer, bon-fire and grill starter shed. That may not be very scientific, but i figure a gallon of fuel costs 10-15 dollars and the plane costs alot more. Not worth a crash.