How Do I Learn About the Ins and Outs of Fuel?

By Mike Philips, North Dallas R/C Club

What is the best fuel to run? I hear this question ring throughout the flightline repeatedly from new pilots and longtime fliers alike. Many wonder what the best fuel is for their airplane, costs, protection, and other things associated with fuel and glow/gas engines for our RC aircraft.

This article will cover a month-long adventure I was on to determine just what fuel I should be using, along with field tests, a lot of reading on the Internet, and conversations with experienced longtime fliers. I hope to be able to explain what might help anyone determine the best type of fuel for them to use.

Being fairly new to this hobby, and absolutely no expert in this field, I will explain this in the best possible way in an effort to have something to point to when somebody asks "What is the best fuel to use."

Disclaimer: This information is provided as is, with no warranty. Use this information at your own risk. Feedback is always welcome; please feel free to contact me if you would like.

Fuel: model aviation fuel contains three elements:

  • Alcohol
  • Nitro
  • Oil

Engine fuel for RC aircraft, known as glow fuel, contains three elements that are determined by the manufacturer and usually printed on the gallon jugs or cans purchased when you buy your fuel.

Out of these three elements, only two are combustible: the nitro and alcohol. In my testing during the past month, my main concern was the oils used in these fuels. Allow me to explain.

I generally run Saito four-cycle engines and these engines require slightly less oil than their two-stoke counterparts. Determining the oil content is what has taken me down this road because of a malfunction on my Saito 100 that is currently flying in my U-Can-Do 60. A deadstick over the runway inverted at about three feet makes you start figuring out things you had not thought of in the past. The airplane survived; however, the rush I got from getting the model flipped over and back on the ground had me thinking.

After further examination, I discovered that the engine had a stuck tappet in the tappet guide; this caused the exhaust pushrod to hang and rip teeth off of the cam gear — a really ugly site, too.

Repairing the Saito 100 (or better said, an attempt to repair) found that any small debris in this motor will cause damage. Finding that this motor did the same thing on the second flight after the repair, I found that microscopic pieces of the cam were lodged in the tappet guide once again.

A full breakdown of this motor and repair once again, replacing the bearings (rusted and pitted), and a complete cleanup and soaking helped put it back in the air.

With this information in hand, I was able to determine that rust had played a part in the first engine malfunction and pieces of the cam gear on the second malfunction.

This all started with rust. Where does this rust come from? There is moisture in the engine. Where does this come from? The adventure was on its way …

Moisture in your engine can cause damage, sometimes catastrophic damage and so, this takes us to the third element listed: oil.

Glow engines run fuel like our everyday two-stroke motors with a combustible (alcohol, nitro) liquid and a lubricant (oil). Oil is an important piece of ensuring that your engine does not rust and also keeps the engine running smoothly and well lubricated to prevent heat.

Rust can build in your engine, whether it is stored for long periods or short periods of time and thus, we simply should have oil in the engine to prevent the rust.

When an engine runs, it takes in moisture from the fuel source and carburetor. Nitro acts as a magnet to moisture and will draw the moisture into your motor. Have you ever noticed when you spill any of this glow fuel it will quickly obtain a milky looking film over the top? This is the nitro pulling in the moisture from the air.

The oil used in the fuel plays a big part in protecting the engine from moisture and preventing rust long term and short term. Many fuel companies list what they use in their fuels.

Many use synthetic oil and this oil allows the motor to run more RPMs than castor-based oil will. Castor is a thicker, natural oil and will protect the motor (long-term) better than a synthetic fuel. If the motor is running a lot where it does not have time to ever be dry from a synthetic fuel, then synthetic fuel may be okay to run with no issues.

Castor being a natural lubricant (hey, this stuff comes from beans) is thicker, and will leave residue all over the motor, which will protect it while in storage helping to prevent rust.

So, I decided that I would want a fuel that had castor to beat the rust, but also wanted a synthetic fuel that would loosen up the mixture so I could produce the RPM range I was looking for.

Let’s look at some of the fuel tested here (percentages are based on volume):

  • Cool Power 2-cycle fuel: 15% nitro/20% oil (10% low viscosity, 10% high viscosity)
  • Cool Power 4-cycle fuel: 15% nitro/18% oil (9% low viscosity, 9% high viscosity)
  • Cool Power 4-cycle fuel (castor based): 15% nitro/18% oil (9% castor, 9% synthetic)
  • Ritch's Brew 2-cycle fuel: 15% nitro/22% oil (known as the 11-11)

The goal was to use like brands to determine the best RPM and change the oil content. And with the findings, the Cool Power 15% 4-cycle, 100% synthetic has proven to provide the most RPM and power; however, running this fuel comes at a cost.

Back to the rust issue. (Note: This is on a four-cycle engine, for a two-cycle; you would want the 20% oil). Running a fuel that is 100% synthetic can prevent rust in a short-term period; however, my feeling and understanding is that the castor would assist in preventing rust.

So, how can you run the best fuel and get away from the worry of rust? If you run a fuel with castor, there is probably nothing to worry about. If you run a fuel without castor you should use after-run oil.

If you read a label on the Cool Power (this was amazing to me), it states "after-run oil not required." After all I had read through and understood, this was somewhat of a mystery to me. How can you run 100% synthetic fuel and not have to use after-run oil? They attribute this to low-viscosity synthetic fuel from what I gathered in the information online at Morgan Fuels.

In short, use my recommendation because this is based on what I know to be the best fuel for me. But, if you’re running a fuel and it does well for you, then that is the fuel for you. In my opinion, all fuel is about the same: different manufacturers are the difference in the production of fuel. I personally like Cool Power; however, another brand with the same mix would probably run the same.

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