When taxing or in the takeoff run, the aircraft goes to the left or right making difficult to keep it aligned

Open the aircraft.cfg file and search for the [flight_tuning] section.
At the end of that sections add these lines:
p_factor_on_yaw = 0.0
torque_on_roll = 0.0
gyro_precession_on_yaw =0.0
gyro_precession_on_pitch =0.0

If any of these lines are there just modify the number to 0.0 
Save the file.