Beginners Guide To Flying RC Airplanes ebook

Model airplane engines

When talking about rc model airplane engines by far the most common type is the glow plug engine, often also called a gas or nitro engine, the latter name being a reference to nitromethane which is an ingredient of rc glow fuel.
The name gas is a generic one often applied to all types of IC (internal combustion) even though in some parts of the world 'gas' means gasoline (petrol).

Just like full size IC engines there are 2 stroke and 4 stroke model airplane engines, also sometimes referred to as 2 cycle and 4 cycle. The primary difference between the two types is that a 2 stroke engine fires per single revolution whereas a 4 stroke engine fires once per two revolutions.
There is also a distinct difference in physical appearance, as you can see from the picture below (2 stroke left, 4 stroke right). The primary difference is the presence of two pushrods on the 4 stroke engine...

Browse some 2 & 4 stroke model airplane engines.

2 stroke model airplane engines produce more power for their size and are generally more 'user-friendly' i.e. easier to maintain. They are cheaper to buy and generally speaking easier to run, and the majority of gas rc training airplanes will be designed around using a 2 stroke engine rather than a four.

A 4 stroke engine produces more torque at lower revs and also produces less noise at a lower frequency. Because of this more realistic sound, it is quite normal for 4 stroke engines to be used in scale model airplanes although of course they're not limited to any particular rc aircraft type.

Model airplane engine sizes

All model aero engines, whether 2 stroke or 4 stroke, are expressed as a certain size. This size refers to the engine's capacity in terms of 1/100th of a cubic inch but is usually expressed as just the number eg a .40cu.in. engine would be referred to as a 40 and a .61cu.in. engine would be called a 61 etc.
It's also worth noting that the airplane itself may also be referred to with the same number i.e. a '40' size trainer would be a model that has been designed to take a 40 engine.

Ringed vs. ABC glow plug engines

You'll see two common types of rc glow plug engine; ringed and ABC. The primary difference is in the method of the compression seal - a traditional ringed engine uses an iron ring inserted around the aluminium piston that presses against the steel cylinder wall to keep the fuel/air mixture in the compression chamber and oil out of it, whereas a more modern ABC engine does not have a ring but instead features a tapered piston and cylinder.
The letters ABC refer to the materials used; the piston is aluminium, the cylinder is brass and the inside of the cylinder is chrome plated.

ABC glow plug engines are supposedly the better performers, but each engine type has its loyal band of followers and ringed engines are in no way endangered!

Glow plugs

2 stroke model airplane engine glow plug Where a gasoline internal combustion engine uses a spark plug to ignite the fuel/air mixture within the compression chamber, so an rc model airplane glow engine uses a small glow plug.

Although the plug filament needs to be heated up initially using an external glow plug igniter, once the engine is running the heat generated within the combustion chamber keeps the filament glowing continuously and is thus able to ignite the fuel/air mixture on each revolution, so long as the filament doesn't fail.
Glow plugs for rc model airplane engines can be bought with different 'heat ratings' depending on the engine and flying conditions; it's a good idea to follow the engine manufacturer's recommendations when choosing a suitable plug.

Glow plugs can fail at anytime by the filament burning out and without a plug the engine won't run! So it's a very good idea to have a selection of spare plugs with you whenever you fly. Similarly, always be sure that your igniter is charged because it's sickening to get to the flying field only to find that you can't heat the glow plug to get the engine running!

Model plane engine carburation

Throttle assembly of a glow plug engine Model airplane engines have a very simple carburetter, generally speaking. It's typically located on the top front of the engine crankcase, although newer engines have the carb located at the rear for safer adjusting (to keep fingers well clear of the prop).
The carb consists of a venturi where the fuel and air mix, idle screw to adjust the idle speed of the engine, mixture screw, or 'needle valve', to allow fine tuning of the fuel/air mixture and hence the high-end RPM and a rotating barrel.

This rotating barrel is linked to the throttle control servo and rotates in response to your input at the transmitter. As you push up the throttle stick on the Tx, the barrel turns opening the venturi to let more air and fuel into the combustion chamber. As you shut down the throttle, so the barrel rotates back again and the mixture is reduced by the closing of the venturi.

As the fuel passes in to the carburetter it mixes with air being sucked in through the open venturi, this fuel/air mixture then passes through transfer ports that run vertically up the outside of the piston chamber. These ports exit in to the combustion chamber above the piston. The mixture gets forced up in to the combustion chamber every time the piston completes a downward stroke.

Once inside the combustion chamber the fuel/air mixture gets compressed by the piston as the piston rises back up. This rapid compression greatly increases both the temperature and the pressure of the fuel/air mixture, and the red hot filament of the glow plug ignites the mixture when the piston reaches Top Dead Center (TDC), assuming that the ignition timing is correct (ignition timing can be advanced or retarded if necessary by fitting a different length glow plug, or altering the height of the current plug by using shims or washers).

The resulting explosion forces the piston back down again. As the piston moves downwards, the burned gases are expelled through the exhaust ports, and the cycle begins again.

Model airplane engines can be very temperamental sometimes and require what seems to be endless adjustments to keep them running smoothly. It is perfectly normal, however, for the needle valve to require regular adjustment before, and even during, each flying session.
Cold and damp weather can cause starting and running problems for rc glow plug engines, as can poor quality or old fuel, air leaks in the fuel system and other issues, but the biggest cause of a non-starting engine is often simply a burnt out glow plug!

Browse a good selection of RC model glow plug engines.

Other RC model airplane engines

While glow plug engines are by far the most common type found on the flying field, they are not the only type.

A twin cylinder gasoline model aero engine Gasoline (petrol) engines are often found in larger rc airplanes and these differ to glow engines in terms of physical appearance, design, size and power output. However they are not limited to large scale rc airplanes; as time passes so smaller petrol engines for model aircraft are becoming available, and many fliers are even converting their glow plug engines over to petrol with several manufacturers now supplying conversion kits.

Both 2 stroke and 4 stroke gasoline airplane engines are available and they vary in levels of complexity. For example, some use a traditional magneto ignition system while others might have electronic ignition, and there are even liquid cooled engines for ultimate performance.

The main advantages of a gasoline powered model plane engine are the running costs - petrol is many times cheaper than glow fuel - and the cleanliness; petrol burns much cleaner than glow fuel and doesn't leave behind the oily residue.

A diesel motor from PAW Diesel engines are another choice but are probably the least common of all IC model aero engines.

Diesel model airplane engines such as the PAW (Progress Aero Works from the UK) shown right are typically used in vintage rc airplanes to keep the model as authentic as possible; such engines were being manufactured long before glow plug technology and so were the first IC engines to be used in radio control flying, having seen great success in free flight models.

Diesel engines do not use any form of plug for ignition but instead rely on the fuel/air mixture inside the combustion chamber to ignite from adiabatic heating as the piston moves up and down.
The pressure inside the chamber (compression) can be increased or decreased by turning a threaded screw on top of the cylinder head; increasing the compression aids ignition of the fuel/air mixture. The fuel is a special mix that has a high ether content, which is easily ignited.

You can view a selection of RC model airplane engines here.

Model airplane engine reading

'2-Stroke Glow Engines For R/C Aircraft' is a very informative book on the subject and is an excellent guide to understanding model airplane glow engines.
"This book is undoubtedly one of the best (if not THE best) texts on the subject I've encountered. The section on correct running-in of engines should be included in the box with each and every 2-stroke R/C engine sold...(customer review extract) Read more.