depending on what your trying to do ,the 40 size mills will fly the Four star fine.the plane has a fair amount of lift / drag so the smaller mills properly proped will work ok.i had an intresting experience with one some time ago. Fox 40 BB , 10/6 . plane flew but just did not respond well &felt some what sluggish. switched to a 11/5 & it was like a whole new aircraft.much better response overall and speed actually increased.