RC model airplane engines
When talking about model airplane engines for rc use, by far the most common type is the glow plug engine, often also called a gas or nitro engine.
The name gas is often used generically to describe all types of IC (internal combustion) engines, even though in some parts of the world 'gas' means gasoline (petrol). Nitro is really a name that's been adopted from the rc car world, but it's commonly used to describe glow plug powered planes.
It is fair to say that although glow plug engines are the most common type found in rc airplanes, true 'gas' engines (i.e. petrol powered) have become much more popular in recent years and their availability has increased lots.
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 complete 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). This difference is the presence of two external pushrods on the 4-stroke engine. Internally a 4-stroke engine has valves, whereas a 2-stroke does not.
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, too, and the majority of gas rc training airplanes will be designed around using a 2-stroke engine rather than a four.
But with that in mind, it's perfectly okay to put a 4-stroke engine in any IC plane and indeed the manufacturers of kits usually specify two different engine sizes for their planes - one for each engine type.
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.
Sizes of model airplane engines
All glow plug rc 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.
It's also worth noting that the airplane itself may also be referred to with the same number i.e. a '40 size plane' would be a plane that has been designed to take a 40 - 46 engine.
The size of the engine is usually stamped on the side of the engine casing, making size-identification very easy.
Larger petrol engines are commonly expressed in terms of their cubic capacity in cubic centimeters eg 50cc. Incidentally petrol engines are not currently available in the smaller sizes that 2-stroke glow engines are, although some manufacturers are beginning to produce petrol engines that equate to medium-large size 4-stroke glow plug engines.
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 inside the compression chamber and oil out of it, whereas a more modern ABC engine does not have a ring but instead features a tapered sleeve inside the cylinder.
The letters ABC refer to the materials used; the piston is aluminium, the cylinder is brass and the inside of the cylinder (sleeve) is chrome plated.
The sleeve is tapered inwards towards the top of the compression chamber and expands outwards as the engine heats up. The tolerances between sleeve and piston are such that a perfect seal is created when the engine is at running temperature.
The downside is that when the engine is cold, there is not a good seal between sleeve and piston away from the top of the combustion chamber and this can make starting a cold ABC more tricky.
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!
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.
The plug filament needs to be heated up initially using an external glow plug igniter, but 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.
The 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
Model airplane engines have a very simple carburettor, 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 features an external servo arm that is connected to the throttle servo linkage, and the barrel 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.
A 2-stroke glow plug engine works thus... As the fuel passes in to the carburettor 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 rising back up. This rapid compression greatly increases both the pressure and temperature of the fuel/air mixture, and the red hot filament of the glow plug ignites the mixture when the piston reaches Top Dead Centre (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. Even when correctly tuned and run-in, it is perfectly normal for the needle valve to require regular adjustment before, and even during, each flying session due to changing atmospheric conditions.
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!
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.
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 available.
The main advantages of a gasoline powered model plane engine are the running costs (petrol is much cheaper than glow fuel) and the cleanliness; petrol burns cleaner than glow fuel and doesn't leave behind the oily residue.
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.
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.
Use the search box to the right to search for more model airplane engine books at Amazon: