Gasoline Engines for RC Airplanes

Before Ray Arden invented the modern hobby glow plug in 1947, spark engines powered by gasoline were the norm for all sizes of model flight. Glow plugs enabled small engines to be more practical without the weight, complexity, and electrical interference from ‘sparky engines,’ as they were called. Sparkys used a battery, coil, and contacts to produce spark ignition. They produced enough radio interference (RFI) in normal operation it was virtually impossible to use them for RC power.  It wasn’t until weed eaters become popular in the 1970’s that hobbyists began experimenting with modified weed eater engines to power RC airplanes, with their magneto-based ignitions. Magneto based ignitions could still introduce significant RFI, but with modern spark plugs and ignition wires hobbyists were able to find solutions that enabled them to take flight.

The CDI Revolution (Capacitor Discharge Ignition)

Magneto ignition systems with 72 MHz radios were a delicate combination where flyers had to be extremely cautious to isolate the engine from the radio system. Much like the automotive industry, the transition to electronic ignition systems was a technological breakthrough. For models, CDI (Capacitor Discharge Ignition) systems became the norm starting in the late 1990’s and improved on magnetos because they could offer lighter weight, and flexible timing that delivers easier starting, more power, and better performance throughout the RPM range. These electronic circuits get the crankshaft position from a magnet and hall effect sensor and are commonly powered from a 4.8-8.4-volt battery. While they do the same basic function as the old point and coil systems, they are far more precise and able to contain the RFI with a metal case and shielded ignition cables.

A Note for New Flyers

We recommend having some experience with RC systems and flying before diving into your first gasoline model. You should understand how the aircraft needs to be set up on your radio, and how it should behave on the ground and in the air so you can focus on engine setup and tuning. Be prepared for the aircraft to move on the ground at idle, especially if you are flying from pavement.

Glow Engine Flyers

Flyers who have experience with glow engines (engines with a glow plug) will find it reasonable to dive into gasoline power without too much trouble. You will need a new fuel jug, fuel pump and compatible fuel line. You will need to learn about carburetors with a diaphragm pump system, they are a bit different than muffler pressure-based carbs. The engines will sound a bit sharper than glow with more of a crack in the exhaust note. The normal operating temps are a bit higher, and needle valves tend to be more sensitive although they don’t need to be adjusted as frequently. You won’t need to carry as much fuel as a similar sized model with a glow engine for the same flight time. Even with the added cost of oil, the price of mixed fuel will be much less than for glow engines!

Gasoline, Ethanol and Oil, Oh My!

One of the biggest benefits of these engines is being able to easily source fuel. However, just like your lawn equipment it’s best if you can source ethanol free fuel. If you need help finding it, contact your local hot rod club for advice. Just like your model engine and lawn equipment, hot rods don’t do well with ethanol added to the gasoline because it can gum up rubber seals and clog tiny passages in the carburetor. It is possible to purchase a special gasket and diaphragm set to rebuild your carb to be compliant with ethanol in the fuel, but this is generally not something hobbyist pursue.  

These are simple engines; they don’t have separate oiling systems. It is imperative to mix in the proper amount of oil as per the manufacturer’s recommendation. It may be from 15:1 to 50:1, so be sure to read the manual for your engine! You need to source a quality synthetic oil to mix into your fuel. Talk to your local gas RC flyers, dirt bike racers, lawn equipment maintenance companies, chain saw operators, or others who frequently deal with mixed fuel for suggestions. Stick with a quality option that won’t gum up your engine’s carburetor if you leave some fuel in a model.

Ignition Battery

The older, proven method to isolate any interference from getting into your radio is to use a separate battery for the ignition. IBEC circuits have become popular over the past decade and are designed to share receiver power with the ignition while filtering RFI. You will want to use an ignition cutoff to be able to shut off ignition power from your radio anyway, and IBEC circuits integrate this function as well. Regardless of your system, always do a thorough range check with the engine running.

The Spektrum ignition cutoff is a basic option that works well for airplanes with a dedicated ignition battery.

Fuel Systems

Use a quality hobby tank and fuel line rated for gasoline. You cannot use silicone tubing or seals as you do for glow engines, these will be destroyed by gas and make a mess of your carburetor. Most hobbyists will use a three-line setup that has a dedicated line run to the bottom for fueling and draining. Leave some extra length on the fill line so you can pull it out for fueling. It can also be helpful to have the line exit the fuselage above the fuel tank to avoid a siphon. The vent line should be exited out the bottom of the fuselage but be sure to add an extra loop of line behind the fuel tank before venting it in front of the firewall to avoid leaks. The fuel line will be connected to a clunk in the tank, so it always stays in the fuel regardless of the model’s orientation. It should be as short and direct as possible, and you should use safety wire to secure the fuel line on all connections to ensure the lines cannot slip off or leak.

DuBro offers high quality Tygon® fuel lines and fuel tanks.

Two-Stroke or Four-Stroke

Four-stroke engines are less common but offer a more realistic sound with a softer exhaust note. These engines often require more oil in the fuel, so be sure to follow the manufacturer recommendations. Two-stroke engines offer more power and less weight but also need more substantial mufflers to subdue their naturally louder and sharper tone. Some airplanes will seem to fit a four-stroke naturally, like a Piper Cub. Two-stroke engines tend to have a less appealing sound in general so it’s hard to say if there is a type of airplane that sounds best with a two-stroke, but warbirds are a close fit.

Saito offers some of the most iconic four-stroke model airplane engines on the market.

Multi-Cylinder vs Single

Most of the small engines you see will be single cylinder, but as the engines get larger it becomes more practical to have multi-cylinder engines. Airplanes with a narrow cowl will work best with an in-line twin, which are rare but are available from some engine manufacturers. Opposed twins are great for a lot of models because they fit well in wide cowls, have low vibration, and sound great!  They can be found as small as 20-40cc, but they don’t become common until 60ccor larger. As you get into 100cc or larger, twin engines become the norm, and at 200cc there are even 4-cylinder opposed engines available. For airplanes with round cowls radial engines can be found in 3, 5, and even 7 cylinders. While these engines can consume a lot of oil and require a significant investment, they sound incredible! For airplanes modeled after something with a radial engine, a real radial engine is very hard to beat! While prices tend to be rather high for radials, 3-cylinder radials are a bit more practical while maintaining the incredible sound.

Saito radial model airplane engines offer incredible sound.

Cowling and Baffling

There are entire articles to cover how to baffle and cool a cowled engine, but the basic idea is you need to force the air to work for you and not allow it to just flow through the cowl. Baffling forces the air to travel through the cooling fins on your engine and helps draw the air out of your cowl so the engine can stay cool while enclosed. It is critical to address the cooling needs of your engine if it is enclosed in a cowl.

Gasoline Carburetors

Most gasoline model engines use a diaphragm pump style carburetor. These carbs are often made by Walbro, and are the same style used for weed eaters, lawn mowers, dirt bikes, and watercraft. Once the diaphragm is primed with fuel in the line, the crankcase pressure pulses will cause the carb to pump a measured amount of fuel with every rotation of the crankshaft.

Tuning Needle Valves

When tuning your gasoline engine, it can be very helpful to use actual data instead of just tuning by ear. You will be setting the needle valve to find the peak RPM. Follow your engine manual to get the engine running and bring it up to full RPM. Follow the engine manufacturer recommendations as a baseline for your needle valve settings and then adjust the high needle to find the peak RPM, and the low needle to deliver a smooth transition. The needle valves interact throughout the RPM range, so you may need to go back and forth and make small changes between them to find the optimal settings. Monitor the RPM when trying to get a peak, and monitor the temps when verifying engine baffling and the tune. You can only cool an engine so much with the cowl or the fuel, you need to have both systems working correctly for your engine to make power and stay cool.

Cleaning/Rebuilding Carburetors

When storing your models up to a month during the flying season you should leave fuel in the carb so it is primed and ready to run (This is assuming you have ethanol free fuel, DO NOT leave gasoline with ethanol in your carb for any length of time on purpose). When you are planning to store for longer, you should run the fuel system dry. If you get any debris in the fuel or gummy seals from alcohol in your fuel, you can gum up the tiny passages in the carb and it won’t run or will be impossible to tune. If your engine has been sitting for any significant period of time with fuel in it, you may need to rebuild your carb with new seals and a diaphragm. The first step is to clean your carb with carb cleaner spray, remove the needle valves and clean all the passages. If you are lucky this may be sufficient. If not, you will need to remove the carb and take it apart for a full cleaning. Do a web search on how to rebuild a Wabro style carburetor; there is a plethora of good information from the lawn care industry.

A fuel tubing cutter can be helpful to ensure you always get a straight cut. A razor blade is a close second but avoid using scissors for a clean cut. Safety wire pliers used to be a luxury with a steep price aimed at specialized AMP work, but now you can find similar tools from discount tool stores for as little as $10. Use leather gloves when working with safety wire, it will easily poke your skin if you aren’t careful.

A temp sensor connected to your telemetry system can be a very useful tool to help tune your gas engine. Locate the sensor on the cylinder head, near the exhaust port. A temp gun can be helpful as well but is much more difficult to get accurate readings when the model is in the air and with cowled engines. Gas engines will run hotter than glow, it is normal to see 300–350 degrees (F) / 150–175 degrees (C) on the cylinder, where glow engines would roast at those temps. A tach can be a useful tool to help you find the peak RPM when setting the needle valve; this may be optical, an LCD plugged into the CDI, or telemetry plugged into the CDI. Deep socket nut driver tools and wrenches to fit your engine and spark plug are another worthwhile investment, avoid using pliers or adjustable wrenches. An ultrasonic cleaner can be a useful tool to clean gummed carburetors if you find yourself regularly rebuilding carbs. You might consider a kitchen gram scale for mixing oil, it can make it easier to mix odd quantities of fuel and oil and ensures you will always get accurate measurements.