Like I stated earlier I'm using an Irvine 72 in mine with no flaps and I don't have a long takeoff. I also had to add alot of weight to the front of mine to get it to balance (almost a pound), this was after I went back and moved some things around to help the ballance. If you would go with a bigger battery and mount it out on the engine beam and try to mount your throttle servo on the firewall, anything to get the balance weight down I think the 91 f.s. will fly it fine. I think my Irvine is more powerful than a 90 size f.s.