I have a OS 91FX in mine and putting the RX and RX pack behind the servo tray balanced it per instructions. The plane flies really good with the 91. Can pull out of hover no problem. Speaking of hover this thing hovers EASY. Very stable flier. Knife edge loops no problem. Can't get it to flat spin. It's close but the tail doesn't wan't to flaten out enough. Maybe more tail weight? Cool plane. I get alot of people checking it out.