ORIGINAL: winglift
Okay......I get the picture for the smaller aircraft, but when choosing for a larger aircraft, is there a rule of thumb for say ws vs engine or wt etc?
The easiest way to go about it is... The gasser should throw the same size prop at the same rpm. If the rpm is lower on the gasser (usually is) then you need to throw a larger prop by at least one size.
It is also highly dependant on flying style. But the prop technique works well for all types. You also don't want to add to much weight to the airframe. If you are significantly adding weight to go with gas then the airframe will probably not fly very well (not true in all cases).
For me, gassers are really not suited for airframes below 9 lbs. (IMHO)