Gas engine, nitro / glow engine or electric motor is what you need to get your model airplane in the air. Most modelers use a 2-stroke glow fuel engine which is the most common and efficient power plant for radio controlled airplanes. Glow fuel is a mixture of methanol, nitro-methane and oil combined to power up the engine. Depending on the type and size of the model you wish to fly, you can also go for a 4-stroke glow engine or a big “gazer” (an engine fueled by regular gasoline)
Choosing the correct engine depends on the size of your plane. Most trainers for example, are recommended to fly with a .40 2-stroke engines due to their size. You can add a bit of more power to your trainer by going for a .46 2-stroke engine but not higher. Over powered airplanes can be as bad as underpowered ones, so it is always a good choice to go with what is recommended for the model you choose.
The larger the airplane, the larger the engine. This is normally how you choose the right engine. This is where 4-stroke engines come in to give a little twist to this ratio. 4-stroke glow engines have a different rhythm to its operation than the 2-stroke ones. 4-strokes have a more realistic airplane rumble while the 2-stroke engines give out a motorcycle-ish sound. Scale modelers usually prefer the realistic sound of a 4-stroke engine while sports flyers go for 2-strokes.
Some modelers like to go for the “Big Birds” the giant scale airplanes. These airplanes are powered by regular gasoline fuel. RC gas engines are larger in size and more powerful than the nitro ones and are more economical. These days, gas engines range from 15cc for a 1.5m wing spam plane to a 400cc for a giant 5m wing spam airplane.