Most smaller R/C planes do not need rudder input to complete a clean looking turn but the bigger the plane is, the more rudder it needs to keep from making that sloppy looking sliding turn that ailerons only will produce.

A little rudder (I usually use about 30%) in the same direction of the turn aids the ailerons in the turn and is actually the full scale method of turning an aircraft. Ailerons alone do not do a good job in providing the needed yaw for a turn. Using aileron and rudder together produces what they call a coordinated turn. In larger models it makes a huge difference and computer radios make it very easy once programed correctly.

I'm sure there are many out there who will want to add to my simplistic post here but I hope this helps.

Have fun!