Cause: Aircraft is rudder only and has significant dihedral

Fix: Fit ailerons and reduce/remove dihedral

Comment: With rudder only models any rudder control becomes down elevator as soon as the aeroplane is banked. If there is significant dihedral in the mainplane, the aeroplane can easily enter a spin.

Ailerons completely cure this tendency as there is no down elevator effect when the ailerons are used with elevator to effect a tight turn.