I just finished modifying the door on my FC40 to accommodate a Multistar 4000 mah battery. It slides in and out with ease and the door closes. I have a great deal of add-ons and was not sure if the extra weight afforded by the battery would be a problem. I am now weighing in at 1175 grams without blade guards nor the PMU that I am about to install. Anyway, I began a sort test to see if the FC40 would fly. It has no problem taking off. I also am running T-Motors, that too is a help. But when I want it to hover it starts to move backwards. When I replace the battery with a standard one, the FC40 stays in place perfectly. I repeated the process and it only occurs with the new battery. I assume that the weight of the battery is just enough to "tip" the quad of center. I re-set my compass. Since there are no trim controls on these controllers, do any of you out there have a suggestion for possibly adjusting the gain through the Naza software ? For now, I have no plans to back to 2200mah batteries. The Multistars are only $19.00 at Hobby King and if I can get 15 min of flight time I will be a happy camper. Any thoughts ?