For a YS91 you probably need to make it a little smaller. I have had a couple of 90 Pattern planes. With a 90 twostroke they flew like a sport plane. I will say it was not the most powerful 90. YS110 fixed it so it could go verticle any time but not a lot of excess. A friends GP Venus II was nice with the OS120AX but even then it was not grossly overpowered. I know my 110 runs about 1" more prop vs the 91 at the same rpm and pitch.