Well, first off, it's all "RC fuel". Or you can use it for control line, free-flight, planes, etc.
At least if you're talking about common model airplane fuel -- nitromethane, methanol, and some form of lube.
Heli fuels generally differ from "normal RC fuel" in their oil content. Heli fuels generally have more lube in them as the motors run close to wide open most of the time in a poorly cooled enviroment. The extra lube keeps your motor healthy.
The "percentage" that you hear/see many refer to is the nitromethane percentage .. 10%, 15%, 30% are common values, there are some 12-1/2% and 20% fuels out there, as well.
The other percentage number you may see on the bottle (but rarely do people mention it when talking about the fuel they use) is the percentage of lubricant. If you're going to use the fuel in a heli, the minimum percentage of lube you should consider is 18%.
I've been flying Sig Champion 15% Heli fuel for years and years, and always have had great performance from it. It's 22% synthetic oil, a bit messy, but my motors have been happy. I've used this fuel in motors of various displacements over the years -- from as small as 0.10 cu inch up to 0.91 cu inch. And I've used in in both helis and planes, all with very good results.
I've been using Magnum and Wildcat 30% in one of my Caliber 30's for awhile, the OS 37 seems happy with it. Wildcat claims 18% lube, Magnum says its has 22%. As I started flying a Trex 700 recently with an OS91, I've started running 30% fuel in that bird, as well. I currently am running Sig's 30% heli fuel in it.
Lubes can be full synthetic (Klotz, for example), castor-oil, or a mixture of synthetic and castor. Lubes also vary in viscosity, another factor that can complicate the decision of which fuel to use.
Without wishing to start another debate, castor oil is a great lubricant and can protect your motor's innards in the event of a lean run somewhat better than many of the synthetic lubes. It tends to burn in lean runs, taking large amounts of heat with it out the exhaust. Synthetics can break down at those higher temperatures. The downside of castor-based or castor-oil mixes is that when the castor oil burns, it forms a gummy residue (varnish) on the piston skirt and cylinder walls. This varnish, when it forms, causes the motor to work harder, and hotter, causing more varnish to form. It's a nasty cycle, eventually making you pull your hair out trying to tune the needles to get the motor to run well. The solution is to tear down the motor, clean off the varnish, reassemble, and run just a couple of clicks richer....
Most heli fuels use a synthetic lube exclusively, perhaps a mixture of synthetics. They work well, leave behind little, if any, residue, and if you take care of your motor, they will take care of you.
Your OS 37 in that Century Hawk will be more than happy with a 15% nitro heli fuel. As your skills improve, you may want to switch to 30% and a better muffler on that heli to really let that 37 do its thing.
If you look at RC car fuel, you'll find that it has quite a bit less oil in it -- some as low as 8% lube. Stay away from a very low oil content fuel. Heli motors run a bit hotter than other motors due to their environment, and need that 18% or more lube to be happy.
I fly planks as well, and have been using that same 15% Sig Champion heli fuel in those motors. Due to the extra oil content, I end up wiping a bit more slop off my planes at the end of the day, but it sure beats having to haul different kinds of fuel to the field for different motors.
Higher nitro percentages require that you run the needles a bit richer than you would with lower percentage nitro fuel. If you switch from 15% to 30%, richen the needles a few clicks. If you switch from 30% to 15%, then lean them out carefully a few clicks.
All that being said...
...My advice to you for now, since you maybe have less than a couple of minutes of flying time under your belt
, is to not fiddle with different fuels at all. Learn to fly first.
Buy some decent 15% nitro heli fuel (no less than 18% lube) and STICK WITH IT until you really know how to fly and how to tune your motor.
If you start messing around with different fuel at this stage, you're going to spend a lot of time with a poorly tuned motor that will really discourage you. And in the process of dinking around with your fuel, you run a very high risk of toasting your motor.
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