This has been discussed so many times on the forum, if the airplane slews away uncontrollably when you enter the editor, the problem is caused by one of your joystick not properly calibrated in the null zone. To fix this, be sure you calibrate all your controls AND set a proper Null Zone for all of them, even those you are not using the most.
However, the better fix would be simply removing any joystick axes while in Slew Mode, using the Control Settings page at the Slew mode tab.