You know what? I have no idea! The consensus seemed to be that if you were going to use Robart struts, the 550 series were the way to go and the set I ordered are designed for the 7/16" Robostruts so I ordered those too. It just kinda snoballed from there. Since I am using an OS 1.60 for power I'm sure it will fly well but the wing loading may be kinda high. Kike I said I am new to the whole retract idea so I am kinda clueless when it comes to the details.