I found the 10 X 6 to let the engine over rev. Also blue lock tite the bolt that holds the carb on, the screws to the firewall and the collars on the wheels. I lost one or more of all these parts. I made it completely through my training on one 11X6 prop. The things i didn’t lock tite I got to replace. I have no complaints with this bird (once i got use to it) it looks good and is fun to fly.