I am playing with a design that has a 2.6 inch nosecone and a long (18 inch) transition out to a 4 inch diameter 4 inch long body tube. What I am seeing in open rocket is the stability of this rocket goes negative with the long transition but positive if I make it a 4 inch body tube instead of the transition. This is counter intuitive to me. The taper should move the CP toward the back not move it forward. Can anyone explain this to me?