ORIGINAL: subtleguru
Is there any formula for calculating the rpm for a prop given a single reference point (at least within a narrow range of prop sizes) ? For example given that a engine statically spins a 13 x 8 prop @ 9750 RPM what can one expect a 14x6 prop to spin at.
You can get a ballpark # using ThrustHP calculator. Free download towards the bottom of the page at
http://www.geistware.com/rcmodeling/link_club.htm
You can put the known prop values on the left side and the second prop size on the right. Adjust the rpm of the prop on the right until the horse power #'s match and you will have a pretty good idea.