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GREG DOE: Just a little historical correction. The nitro content in K&B fuels was as follows K&B 100 was 5%, K&B 500 was 12 1/2%, and K&B 1000 was 25%.

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ffkiwi: So I guess-and generalising(your mileage may vary!), my recommendation across the board would be 10% nitro as a minimum starting point, and 35% as a practical maximum, with 10-15% for general sport use, and 25-35% for competition. Unless you're seriously flying 1/2A pylon or doing a speed model, it would be hard to justify the 25-35% fuels for general R/C use from a cost point of view. [from a FF perspective-fuel costs are almost irrelevant-when you're talking 7 sec or 10 second engine runs-I would use less fuel in a day's flying than a 1/2A RC flier would in one flight, assuming a typical 1-ounce or 2 ounce tank in the RC model]

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hllywdb: I do find that my TD's needle a bit more consistently with a bit more nitro, but I live in a hot climate.

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Max-10LA/Max-15LA/Max-25LA "LA Series" Instruction Manual, Page 16, ADVICE ON SELECTION OF FUEL, GLOWPLUG & PROPELLER: Note that even a small quantity of nitromethane (3-5%) will improve flexibility, making the needle-valve adjustment less critical and improving throttle response.

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ORIGINAL: tsmithh

For about $10 you can get a quart sized container of O'Donnell racing fuel. It is 20% nitromethane content and 8% oil (syn/castor blend). The little 049s are designed for higher nitro content like this but too high and your glo-plug life will be cut short (pretty expensive little buggers to replace). The magic to alcohol based fuels is not going overboard on the oil %, best to keep that lower rather than too high to get the best power stroke out of your little engine. many feel to play it safe by adding oil and then wonder why the top end doesn't peak and engine tough to get a reliable needle setting. try it, you'll like it!

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ORIGINAL: MJD

Sorry to say but I entirely disagree with advice to run 8% oil fuel in small engines. We're not talking about car engines with low shaft loads here - I doubt you will find anyone willing to risk a 1/2A aircraft engine on 8% oil fuel. Since when are 1/2A's tough to needle because of sufficient oil content? Too low or high nitro content, yes. Over compressed, yes. Wrong or bad plug, yes. Maybe if one got silly and ran 40% oil, but I think viscous effects might prevail. 18-23% oil in 1/2A's has never been the source of needling problems to my knowledge.

I've observed that some folks think that adding oil to the fuel somehow reduces the amount of energy developed per revolution in direct proportion to the percentage change. This is not true - the intake volume of the engine is predominantly occupied by air, with a very small volume per stroke occupied by atomized fuel - which is still in liquid form. Compare the density of gaseous air to liquid fuel for an idea of the proportional effect of adding oil. In other words, increasing the oil content of the fuel by 5% does not reduce power output by 5%. Open the needle to admit the same amount of methanol and nitromethane, and you get it nearly all back. A small amount of intake volume is stolen by the additional oil content, but it is a small fraction of the oil percentage change in the fuel.

For giggles I attempted to work out the volumetric ratio of fuel to air in a fuel blend of 20% castor, 25% nitromethane. Please check my math if you wish, age takes it's toll.

Methanol:

s.g.=0.79g/cc
Stoichiometric air:fuel ratio 6:1

Nitromethane:

s.g. = 1.137g/cc
Stoichiometric air:fuel ratio 1.7:1

Castor oil: s.g. = 0.956 g/cc
Stoichiometry N/A as we shouldn't be burning the oil

The stoichiometry of this fuel blend works out to about 3.73 fuel:air by weight

Specific gravity of fuel blend = 0.91 g/cc

Specific gravity of air = 1.204 kg/m^3 = 0.001204 g/cc

Therefore the volumetric ratio of liquid fuel to gaseous air is 1:2,815

That means that roughly for every percentage increase on oil content, you lose 1/2815% of potential power output.

If you translate this to a 0.8cc engine running at 16k rpm with 85% volumetric efficiency and running at optimum fuel/air and no wasted fuel:

0.8cc x 16,000rpm x 0.9 = 10,880cc/minute of mixture

Of those 10,880cc, 1/2816 parts are fuel, or 3.86cc/minute.

A Babe Bee has a 5cc tank, so this translates to 5/3.86 minutes running time = 1m 18s running time. While this is not bang on, it is in the ballpark so the numbers can't be all that far off. Can someone remind me how long a Babe Bee on a 6-3 runs on 25% nitro fuel? I think it is a bit longer than that, maybe 1:30, 1:40?

Long and short is, how much does oil percentage change affect the power output due to fuel dilution? Nearly bugger all.

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ORIGINAL: rcguy59

And let's not forget, the oil being spit out the exhaust carries away a fair amount of heat, as well.

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ORIGINAL: vertical grimmace

I always found my 1/2 a engines to like nitro and like to have 35% or so. One thing to keep in mind though, there is a point at which castor oil does not seem to like to mix with nitro. We ran into a problem at the Nats in 93 where our castor did not mix with 50% and found that good high quality synthetic was the trick here. Never had a problem under 50% though. Granted, this is not scientific only experience. I have a jug of Powermaster 35% I bought that I have just for my 1/2 a engines. I trust the oils in that brand.

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ORIGINAL: MJD

I might seem like I went a bit overboard there, but really it was just a catalyst to take 20 minutes and work out some numbers for fun. I knew generally what the situation was regards oil content and power output, but never made the effort to quantify it. All it took was a quick Excel spreadsheet (a piece of paper and a calculator would serve as well) and some numbers from the internet.

I probably have the volumetric efficiency a bit high. I checked out the Holley website again and see that they say a low performance car engine runs about 80%. Given a 0.8cc glow engine is likely not the most efficient pump in the world, the fuel consumption of that hypothetical 0.8cc engine may be 10-20% less than the number I came up with, meaning run time more like 1:30 or more which is more in line with what I recall.

But the real point is, forget about oil content affecting the amount of energy-producing fuel/air mixture in the engine - it is not significant. That is the part I figured was useful. I have seen car folks talk about how the lower oil content gives more power.. I'd like to see someone quanitfy that claim.

I'll fire up Excel again and make a chart showing fuel/air mixture volume loss versus oil content sometime.

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combatpigg: O'donnel's RC car engine fuel with only 8% oil is making money hand over fist if he is charging comparable rates per gallon as model airplane fuel.

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vicman: Honestly, 50 years after these engines were designed and for the most part manufactured and you fools are still argueing nitro content??? **** guys, fly what you are happy with! The OP asked about fuel type...Heli works fine, add some castor if you want to see some smoke. Thats it!! Dang boyz!!!