Something I haven't seen discussed, can't find much online, didn't cover it in school: what is the effect of prop to cowl spacing on thrust? Certainly most of the prop's work comes from the outer portion of the blades, but the inboard portions are still doing something, if they're not totally blanked by the cowl, but just how much clearance is necessary? Or alternately, how much performance is lost by an exessively close clearance?