All of the responses are right. To summarize, there is no technical reason that a model of any size with any power plant would require more than a single channel. In the 40’s-50’s some pretty large aircraft were flown with rudder only. For instance, the original “Big Guff” was almost as large as some of the giant aircraft today. The issue is one of safety and performance. The large old rudder-only aircraft were slow and were limited as to what maneuvers they could do (and required a pretty large field to do them in). Generally aircraft of moderate size and speed can be effectively controlled with three channels, where the rudder is used as the roll control. They can perform such maneuvers as loops, snap rolls, immelmans, cuban eights, stall turns, spins, and with skillful thumbs, inverted flight. Rudder-only turns tend to be not very well coordinated with a skidding entry. Addition of aileron control allows an unlimited range of aerobatics and more precise control, which especially for larger aircraft, also increases safety.

Many glow powered trainers both kits and ARF’s are designed to operate with three channels only. There is nothing wrong with learning to fly with one of these. The transition to an aircraft with ailerons is pretty simple. Of the eight aircraft I currently have, three are equipped with three channels and I still enjoy flying them for a relaxing time.