Home  Site map  Search

Model airplane engines (glow plug)

[This page talks about Internal Combustion (IC) engines, not electric motors]

When talking about model airplane engines, by far the most common is the glow plug engine, also called a nitro engine (or motor).

2 & 4 cycle (or 'stroke')

Just like standard gasoline powered combustion engines, there are 2-cycle, or 2-stroke, and 4 cycle, or 4-stroke, model airplane engines. The primary difference between the two types is that a two-cycle engine fires per single revolution whereas a four-cycle engine fires once per two revolutions.
There is also a major difference in physical appearance, as you can see from the picture below (2 cycle left, 4 cycle right):

Two-cycle model airplane engines produce more power for their given size and are generally more 'user-friendly'. They are cheaper to buy and easier to run, and the majority of training airplanes will be designed around using a two-cycle engine rather than a four.

A four-cycle engine will produce 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 four-cycle engines to be used in scale model airplanes.

Engine sizes

All model airplane engines, whether two-cycle or four-cycle, are expressed as a certain size.
The actual size refers to the engines's capacity in terms of 1/100th of one cubic inch, but is usually expressed as just the number; a .40cu in engine would be referred to as a 40, a .60cu in engine would be a 60 etc.
The model airplane itself may also be referred to with the same number ie a '40' size trainer would be a model that would best suit a 40 engine.

Glow plugs

Where a gasoline combustion engine uses a spark plug to ignite the fuel/air mixture within the compression chamber, so a glow or nitro engine uses a small glow plug.
Although the plug needs to be heated up using a glow plug ignitor initially, once the engine is running the heat generated within the combustion chamber keeps the plug glowing constantly and is thus able to ignite the fuel/air mixture on each revolution.

Glow plugs can fail at anytime, so it's always a good idea to have a selection of spare ones with you whenever you fly.

Carburation

Model airplane engines have a very simple carburetor. It is located on the front of the engine body and consists of a venturi where the fuel and air mix, idle screw to adjust the idle speed of the engine, mixture screw to allow fine tuning of the fuel/air mixture and a rotating barrel.
This barrel is linked to the throttle control servo, and rotates in response to your input at the transmitter. As you open the throttle stick on the tx, the barrel turns opening the venturi letting more air/fuel into the combustion chamber. As you shut down the throttle, so the mixture is reduced by the closing of the venturi.

Model airplane engines can be temperamental sometimes and require what seems to be endless adjustments to keep them running smoothly, but it's perfectly normal however for the mixture screw to require regular adjustment before, and even during, each flying session.
Cold and damp weather can cause starting problems for glow plug engines, but the biggest cause of a non-starting engine is simply a burnt out plug!

Other 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 can be found in larger rc airplanes, and these differ to glow engines greatly in terms of physical appearance, size and power output.
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.

Gasoline model airplane engines aren't cheap - even a smaller bore one can cost you a few hundred dollars, and the larger ones can run into many hundreds. But if you have a model airplane that justifies a gasoline engine, then you're not a newcomer to the hobby!
A typical rc airplane that would use a gasoline engine would be a large scale aerobatic model, such as a 1/4 scale Extra. Of course, there are no limits to the type of model that will take such an engine - but it has to be big!

Diesel engines are another choice but are the least common out of all the IC engines.

Diesel model airplane engines, such as the PAW shown right, are typically used in vintage rc airplanes to keep the model as traditional as possible.
Such engines were being manufactured before glow plug technology, and so were the first IC engines to be used in radio control flying.

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 the friction heat generated by the piston moving up and down as the prop is flicked over to start.
The pressure inside the chamber can be increased or decreased by turning a threaded screw on top of the cylinder head; increasing pressure 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 large selection of model airplane engines here

 

Model airplane engine reading

'2-Stroke Glow Engines For R/C Aircraft' is a very informative book on the subject, available from Amazon.

"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

Related pages

The model jet engine - an understanding of model gas turbines