Differential Thrust Problem

Hi guys…
I have a plane which has two motors with differantial thrust.It can fly with differantial thrust in stabilize mode. But can not in autonomus mode. The circle it draws is way too wide than it should be. I dont use rudder servo.
How can It achive to turn via motors in autonomus mode ?

Moved to Arduplane from Arducopter.

set RUDD_DT_GAIN to the amount of differential you want. default is 10, which means 10%
And perhaps try adjusting KFF_RDDRMIX.