Slipstream Effect.

In passing through the propeller the air is accelerated and given a rotational velocity.

The parts of the aircraft that are in the propeller slipstream will therefore have higher speed air passing over them than the parts outside the slipstream. The drag of these parts will therefore be higher, and the effectiveness of any control surfaces in the slipstream will be greater. The rotation given to the slipstream will cause it to meet the fin at an angle and so cause a yawing moment.

 
This effect may be corrected by offsetting the fin or trimming the rudder. The amount of rotation given to the air will depend on the torque of the propeller, and so the yawing moment will depend on the power setting.