Rider,

There are some good answers here, and I don't want to take from them. Some of the technicalities are muddled, though.

There are a lot of factors determining how an airplane will turn including the airfoil, dihedral, etc.. blah blah. Be clear on this, though, the rudder is not for turning. Yes, it can be made to turn an airplane if there is enough dihedral, but in a 4-channel airplane that's not the intent. The rudder is for controlling yaw. To visualize yaw, imagine you're standing above the plane. Drive a stake (in your mind!) straight down through the top of it at the center of gravity. Now if you rotate around the stake that's your yaw.

The ailerons are for roll. Look at the plane from the front. Drive your imaginary stake through from nose to tail. Rotation about that stake is you roll.

The elevator is pitch. Looking at the plane from the side, dust off your stake and send it through again (at the CG). Rotation about this one (nose up/down) is pitch.

To turn you use the ailerons to roll in the desired direction. Then you apply some up elevator to pull the plane around. With smaller aircraft, that's all you need to do. You can bank and yank. It's not the "purist" approach but 9 times out of 10 it works fine.

Now, each side of the wing acts differently in the turn. One side has to produce more lift and hence more drag (called induced drag). This causes the airplane to yaw. To conteract that yaw you use the rudder. This is more pronounced in larger airplanes and some scale types (a Piper Cub is a good example). To make a coordinated turn you would apply some aileron followed by a little rudder (in the same direction) and up elevator.

Not all planes are created equal. Some don't require much rudder at all. I have an RV-4 that I can bank and yank with no noticable yaw. Some require a lot. I have a 1/4 scale Fly Baby that almost won't roll into a turn without ailerons and rudder applied simultaineously.

No explaination will tell you how to fly YOUR airplane, but knowledge of the theory will give you a good place to start fine tuning your technique. So take all this arm chair advice to the flying field and play with the controls.