Bart, think of an aircraft as a dart. On a dart, you have heaps of yaw stability because the aerodynamic center (kind of the CG of all of the aero side forces) is way behind the CG. That keeps it stable in yaw (and pitch, but we are talking yaw here) so that the needle nosed end always points forward. In a plane, we want the same basic thing. We certainly DO NOT want the aircraft's tail to suddenly swap around and point forward. We may not want as much positive yaw stability as a dart, but we do not want to be close to neutral stability (that would be where it does not care where the nose points and will spin either way if a force tries to make it). Now, when you lengthen the nose of the plane, it is like adding a little bit of dart fins to the nose. Think of the dart: if you added the same big fins on the front as you already had on the back, it would no longer fly nose forward. So when you extend the nose on the plane, you are robbing it of some yaw stability because you are adding more side area in front of the CG, thereby moving the aerodynamic center further forward. If you already had gobs of yaw stability then you can likely sacrifice some and still be just fine. But if you were getting low on yaw stability before, and you add significant side area up front, you could get too close to neutral stability. Actually, this all applies to pitch stability too. Bob Parks is the rEAL expert on all of this though. He has said several times that an easy way to test this is by making small hand chuck gliders with the same proportions and GC locale.