In simple terms - as has been stated - you DON'T need rudder once in the turn. You DO need it when rolling in and out of turns in order to counter adverse aileron yaw which occurs whenever the ailerons are deflected from the neutral position i.e. generating lift and therefore drag. Try it in a glider
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Maybe I should read Stick and Rudder again but I lost interest the first time after he called the elevators 'flippers' one too many times.