Some engines are picky about the prop used. My Irvine 36 for example doesnt like to be loaded down with a heavy high pitch prop. It wants to build rpm fast or it tends to stumble. Iwent from a 10X5 to a 10X4 today and found the take off and vertical performance much improved despite the calculator telling me based on the rpms Iwas seeing the 10X5 was more powerful.