I can not attest to this, but my gut feeling as an engineer is that if you are going to shorten the blades, then shorten BOTH upper and lower sets. My reasoning is that if you significantly alter the aerodynamic behavior of one set of blades relative to the other, you distrupt the evenly balanced counter-rotating stabilization. That is, the tail is more likely to swing in one direction or the other, depending on which blade set you shorten. Just thinking out loud. You can always try to shorten one set, then do the other if necessary. You'd better work really hard, too, to balance the blades because you wouldn't need to shave much extra off of one blade to lighten it significanly compared to the other. As I mentioned above, if you shorten the flybar in conjunction with the longer inner/outer shafts, you'll likely not experience a blade strike. I haven't. However, I'm also very careful not to go from full forward flight to full backward. My motors are more powerful than stock, yet still no blade strikes with that setup. I haven't needed to cut/shorten blades. Neall