Although the true size is in hundredths of an cubic inch, such as .40 cu. in., the size is informally expressed as a "forty" or a "sixty".

Trainer-style planes usually fly best with smaller displacement engines (i.e. a .40-size engine will work fine with "40" size trainer). Sport planes usually need a displacement size from the middle of the recommended range. Many aerobatic planes use the biggest engine possible which necessitates more throttle control from the pilot, but can result in better aerobatics.

Glow Plugs

Most model engines have a simple, two-stroke, diesel-like ignition system which uses a glow plug rather than a spark plug. A glow plug will cause combustion by itself once the engine is running. Glow plugs do not require an on-board battery or ignition system and make model engine operation easier.

Carburetor

The carburetor in model engines is very simple compared to full-size cars or planes. All R/C model engines have:

-A venturi to allow air into the engine
-A needle valve to control the fuel flow (and therefore, the fuel to air mixture)
-A rotating throttle "barrel" or valve to open and close the venturi

Correct air-fuel mixture adjustment is important not only for performance, but also for engine life because glow fuel for a model engines contains lubricants. Model airplane fuel is made up of mostly methanol (a type of alcohol) with about 16-20% oil and 5-15% nitromethane (for more power and better acceleration). Consistently flying the engine with a lean setting (very little fuel) will prematurely damage an engine. Most engines come with detailed directions, and they should be read before operating the engine.

2-Stroke vs. 4-Stroke

Two-stroke engines fire on every revolution. They are easier to adjust, less prone to failure from high stress or misuse, and offer more power for their size and weight.

Four-stroke engines fire every two revolutions. This equates to more torque, less propeller RPM, and a quieter running engine. However, four-stroke engines need more maintenance and are harder to adjust. They offer better fuel economy but produce less power for their size. For example, a Magnum XL46A two-stroke and an FS52A four-cycle weigh about the same and produce similar power. However, the .46 uses more fuel. The .46 also swings a slightly smaller propeller at a higher speed, thus giving higher top speed but less climbing ability.

Most trainers work great with a two-stroke engine. Also, because they are easier to operate and less expensive, buying a two-stroke for a first engine is a good place to start.

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