Gas RC engines, hints, tips and background information. Our gas powered RC plane is taking shape, and the project is ready to progress onto the next stage, fitting a gas RC engine. To anyone outside the US or Canada the terms ‘gas motors’ or ‘gas engines’ can be confusing, it’s a generic term that denotes almost any internal combustion, engine. Gas is of course, short for gasoline (or petrol).
Sometimes we hear the term ‘Nitro engine’. Which is derived from nitromethane, one of the three ingredients that make up ‘Nitro fuel’. The other two being methane and oil. We will get into the role, that nitro plays later on.
Internal combustion, gas engines are a popular choice for any gas powered RC plane. They have been around for decades in one form or another. They are not difficult to operate and maintain and are available as either 2 or 4 stroke engines. It is usual for RC kit plane designers to specify either engine type and give the end user more choice in the build.
2 Stroke engine
A 2 Stroke engine is by far the simplest and least complex of the two engine types. It does not require valves and the mechanisms to open and close them. The crankshaft will complete one revolution for each combustion in the cylinder.
(Diagram showing two stroke cycle)
The 2 stroke engine is the most common type used in a gas powered RC plane and are often found in RC training planes. They offer a reasonably high power to weight ratio and are simple and easy to maintain. Mass production has also helped keep their overall costs down, which make them a very popular choice. It’s also worth noting that 2 stroke engine is physically smaller than a 4 stroke engine but produces more power.
4 Stroke engine
While a four stroke engine is physically larger it produces fewer revolutions but provides more torque. This is useful in a large RC plane with a large propeller taking larger bites of air. A larger engine working slowly. As opposed to a small engine thrashing itself to bits. Suggests longevity and greater reliability. The engine noise on a four stroke is lower and of a less annoying sound frequency than the two stroke engine.
(Diagram showing the four stroke cycle)
For each combustion in a 4 stroke’s cylinder you get two turns of the crankshaft in exchange. The 4 stroke will be physically larger compared to its 2 stroke cousin. You can easily identify a 4 stroke by the pushrods which are needed to open and close the intake and exhaust valves.
Gas RC engines & Glow plugs
Gas RC engines using methanol (Glow fuel) have depended on glow plugs to keep them running for many a happy decade. Where a glowing metal filament (similar to what you would find in a light bulb) ignites the fuel/air mixture in the combustion chamber. This is known as a ‘glow plug’. Glow engines have always been popular, by definition, they don’t need an ignition system which is more weight and complexity and they have fewer parts to break down. The glow plug is fitted on top of the combustion chamber with the filament protruding slightly into the combustion chamber. When the engine is ready to
(Image above shows the gas RC engines glow plug. Note the filament on the underside)
be started, an igniter is connected to the glow plug which causes the filament inside the chamber to glow red at a very high temperature.
When the engine is running the heat generated during the combustion phase keeps the glow plug filament glowing continuously.
At this point, the battery is no longer required and can be disconnected. The engine runs on quite happily and will keep going as long as it has fuel to burn.
The only weakness to consider is that the glow plug filament can (and will) burn out. It’s all down to luck whether it lasts 10 flights or 50. Maybe not just luck, material quality how potent your fuel is may be factors. Either way, it’s a good idea to have some spares handy.
Glow plug frustration
Decades ago, this writer got a control line Hurricane as a Christmas present. The glow plug was a constant source of frustration, the dammed thing burned out nearly every other week.
My allowance didn’t run to more than one glow plug at a time. So, from personal experience, I can tell you it’s a stone ass drag being side-lined, with a dud engine, watching your friends fly.
Check the heat rating on your engine when buying a replacement glow plug or better still, stick to the manufactures instructions.
Ringed glow plug
While we are on the subject, there are two types of glow plug available. The ‘Ringed type’, as the name suggests, has a metal ring which presses against the cylinder wall as it is screwed down into position. This forms a tight seal and ensures that all of the power of the combustion is driving the piston head and turning the crankshaft.
ABC glow plug
The ABC glow plug has a tapered sleeve inside the compression chamber. As the engine heats up, the cylinder (which is brass and an excellent conductor of heat) expands outwards to form a perfect seal. ABC plugs get their name from the materials used. A = Aluminium piston. B = Brass cylinder. C = Chrome lining inside the cylinder sleeve.
The downside of the ABC plug is it can tricky to start on cold damp days. When the engine is really cold, it just can’t form a tight enough seal until it heats up and it won’t heat until it gets running.
That said, many users say the ‘ABC type’ offers better performance in the air. I honestly can’t say I’ve noticed that. As much as possible, I tend to stick to the older ‘ringed type’ they are in no danger of being phased out and at least I can start them quicker.
Breaking in your glow engine
New glow engines need to be ‘broken in’. As the metal expands under heat, microscopic metallic particles will be shaved off as the various moving parts ‘bed in’. This process might take a few hours before you risk flying it. If the engine is going to quit, better it does it on the ground. Tie your RC plane down and ground run it, it’s a safer bet than having the engine quit in the air.
Don’t panic if your new gas RC plane suddenly stops running. Check the glow plug, make sure it hasn’t burned out. It’s the single biggest reason a new engine, previously running well, won’t start.
Read the instructions
Some manufacturers recommend that use a higher nitromethane percentage in the fuel mix. The more nitro you are running, the more expansion you get as the engine will running at a much higher temperature.
What this means is, if you had been using a weaker mix, say for ground running, during the break in period. There exists the possibility, that the cylinder will expand faster than the piston and it could end up being too loose and be losing compression when you eventually fly with a higher nitro percentage.
After you finish flying (or running your engine) for the day, It’s recommended that you lubricate the piston with some ‘After run oil’
Regular gas RC Engines
Which brings me nicely on to ‘regular’ gas RC engines (not nitro). While glow plug engines are by far the most readily available power source for your project. Thanks to advancements in engine technology, pure gasoline (petrol) engines are becoming more readily available (and affordable) in various sizes. You can get a 10cc petrol engine which has a similar power output to a size 40 Glow plug engine.
(Four cylinder gasoline RC engines are very powerful and expensive. This one retails for around $2,000)
Gasoline (petrol) engines are commonly referred to by their cubic capacity (CC’s) but in the RC world the term CC’s means cubic centimetres. Another noteworthy point is that gasoline engines with glow plugs are also an option for consideration. Gas RC engines (running on gasoline) are much easier to start and less expensive to run than their nitro counterparts (gasoline being a less expensive option).
(The image above is a gasoline 7 cylinder radial engine. While it looks and sounds great, it comes with a hefty price tag. You don’t get much change out of $4,000)
Mixing fuel & air
The engine in your gas RC plane is fitted with a device called a carburettor. The name is derived from the term ‘carburet’ which means to combine or mix a gas with carbon compounds. Modern automobile engines, use a fuel injection system to deliver a precise amount of vaporised fuel and air into the combustion chamber.
Whether the engine is ticking over at idle, driving normally or going flat out, a complex computer controlled system of electronics monitors and delivers the exact amount of fuel and air as it is needed. Before we had fuel injection, all engines used a carburettor or ‘carb’ to mix the fuel and air together.
The big bang principle
Gas engines exploit a principle called combustion, which is a fancy term for the controlled explosion, the ignition of vaporised fuel and oxygen, in the combustion chamber (internal combustion). It’s a chemical reaction that releases heat and energy and produces carbon dioxide and water as a by-product.
Enter the carburettor, this mechanical device handles the fuel air mix. The fuel is drawn through a choke point in carb, called a venturi. This causes the fuel to flow faster and vaporise making it easier to mix with the oxygen.
The mix is controlled by a linkage to the throttle which opens and closes the venturi to admit or deny the fuel into the engine. The linkage is connected to the throttle servo which in turn is controlled by your stick inputs on the transmitter.
In a 2 stoke engine, the intake stroke of the piston creates a negative pressure. This sucks fuel into the carburettor and through the venturi where it mixes with air. The fuel air mix is forced into the combustion chamber when the piston is in the downward stroke.
As the piston returns, we have the compression phase which increases the pressure and temperature of the mixture. The filament ignites the glow fuel mixture and we get a controlled explosion which drives the piston downwards. Next, the exhaust gasses are expelled through the exhaust port. This cycle repeats, several times a second.
Some gas RC engines may use a muffler to reduce the overall engine noise. The muffler, helps to draw fuel into the carb by creating back pressure. When the engine is running at idle negative pressure will be at its lowest. Newer gas RC engines benefit from this because their carburettors tend to be larger than earlier models.
A good housekeeping practice that will save you endless problems is to store your fuel in a clean container filled with clean fuel. Make sure it has at least two filters. A clunk filter inside the container and an inline filter.
If you can’t be 100% certain that the fuel is clean or it’s been lying around for a while, get rid of it. Personally, I would rather spend money on clean fuel than spend hours of my time unclogging fuel lines and replacing filters. It is good practice to only use clean fuel. If in doubt, throw it out. The fuel flow will definitely suffer and your engine could lose power and quit mid-air, because you allowed the filter screens inside the carburettor to become clogged.
Where possible, use a gasoline graded fuel tank and lines. Glow fuel and gasoline will degrade them over time and the resultant gunge will clog up your carburettor. Avoid glow fuel leaks, by regularly checking for splits in fuel lines as they can lose some of their flexibility. They are easier to break when they start to harden especially around the connection points.
If you think your engine is running hot or is hotter than usual, check the mix. Engine damage can occur if we allow it to run ‘lean’ for extended periods. This is basic engine fuel management 101, but it costs nothing to check or seek the opinion of others.
Since we are at the front end of your plane and we have been discussing care and maintenance of gas RC engines. It’s also worth mentioning propellers. I’m going to assume you have correct size of propeller in mind for your project. Like a lot of things in life, it can’t be too small or too big, check the manufacturer’s recommendations for prop size as it relates to your RC engine and plane size. Make sure it’s just right.
If your propeller has a few dings on it, if it’s damaged, chipped or bent, it could be out of balance. An unbalanced propeller will set up a vibrational frequency, which left unchecked will produce excessive wear and tear on the bearings.
You can use a prop balancer to identify the heavier blade and remove small amounts of material with sandpaper or a sharp knife until balance is restored. Carrying spare propellers is a good idea. If your prop has seen better days, fit a replacement. Its kinder to your engine and rc plane.
I hope you found this post on gas RC airplane engines interesting and informative, be sure to check out our post on RC electric motors.
The editorial team