ORIGINAL: Baron Johnson
But if the plane is flying a perfect vertical downline, and the incidence is 0, etc. adverse yaw would not be a factor since the wing is not generating a net lift (the lift - and therefore drag - associated with each aileron would be equal and opposite).
Not true- remember lift and drag vs. AOA coefficients aren't the same curves. Why do you say that the wing is generating zero lift? And why, even if that happens to be the case, would deflecting ailerons from zero lift AOA have the same effect up and down? Your assumptions are only true if you have a symmetrical airfoil AND are flying at 0 deg. AOA (those things aren't necessarily true); then your drag would be equal on both sides with the same up and down aileron deflection. But with practically any other airfoil section, or if you are not flying a symmetrical airfoil at 0 deg. AOA, the drag with an up aileron deflection would not be the same with the same amount of down deflection.
ORIGINAL: Baron Johnson
And, if the vertical CG was not aligned with the aerodynamic center, as you put it, there would by a pitch or yaw present when flying the vertical downline without rolling, right? But it only appears when the ailerons are deflected. Otherwise, it tracks a virtually perfect downline.
No, if you are flying at equilibrium, you already have the surfaces in a position to balance everything. You stated an assumption that you were established in a downline- so somehow, you are flying in an equilibrium state. If you then deflect ailerons, the newly added roll forces will act at the AC of the aileron area airfoil sections, which will rotate, or orbit, around the CG. Irrespective of anything else, if the wing is not in line with the CG, you will get a kind of barrel roll effect; if they are in line, you will have a clean rotation around the CG, not withstanding the aforementioned adverse yaw wobble effects, if applicable.