If you want to get off the ground, you've got to have an engine. The most common powerplant for radio control airplanes has been a 2-stroke engine burning glow fuel-a methanol/nitro-methane/oil mixture. But, depending on the model you fly, you can also choose a 4-stroke engine, or an engine fueled by regular gasoline. You can even use an electric motor to power your plane.
The size and type of engine is, of course, determined by the size and type of model. And it's important to keep the engine in proportion to the plane. Most trainers, for example, use a .40 size 2-stroke engine. You can boost power by moving up to a .46 or even .48 size, but too much power can be as bad as too little, so it's always best to stick within the recommendations that come with your kit.
As an alternative to the commonly-used 2-stroke, many modelers prefer a 4-stroke engine. A 4-stroke engine has a different rhythm in its operation and produces a distinctive sound (think sports car rumble vs. motorcycle whine). The more "realistic" aircraft sound produced by 4-stroke engines, like Saito, is especially prized by scale modelers.
Giant scale modelers can also opt for engines fueled by regular gas instead of glow fuel. More economical to run, they are also large and powerful. Zenoah, the top name in gas engines, offers sizes ranging from 2 horsepower/1.37 cubic inches to a whopping 6 horsepower/4.45 cubic inches. They are definitely designed for the "big birds."