First you have to understand the difference between a coordinated and uncoordinated turn. The first involves the use of both rudder and ailerons while the second is the use of only the rudder. Here is a brief, but not complete explanation:

http://www.grc.nasa.gov/WWW/K-12/airplane/turns.html

Notice when using the ailerons, the nose of the aircraft has to be lifted up to compensate for loss in vertical lift. The best way to understand what is really going on is to break down the components of lift using vector analysis. In any case, a rudder only turn is simply skidding the aircraft around the turn.

Three-channel models fly just fine but aren't as responsive as those with ailerons. Also dihedral is a stabilizing force that makes turns somewhat more difficult for rudder only models.

Ciao,

Eagle Al