I suspect that the smartest minds hang out in this forum so here goes ...

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.

I have come across prop loading calculations that multiply the D x P to give a value but I think this breaks down for larger diameters.

If there isn't such a formula then what makes this such a difficult problem to solve theoretically ?

Try this........ http://www.hoverhawk.com/propspd.html

Sorry MustangAce,

Am I missing something ? This just gives tip speed and predicated air-speed once you know your RPM.

I am wanting to predicate static prop speed (RPM)

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.

No you can't figure it out like that. First off there's the blade shape and twist distribution between various brands. But even if you stick with the same brand you're loading down the engine in a different way and that means it is wandering around on the HP output curve. So if the first prop just happens to spin the engine up to one HP setting then the second one may or may not let it spin at the same HP setting. Instead it will find a balance at some point that results in an RPM and HP setting that is different from the first. And since each engine will have a different power curve depending on many variables it's impossible to predict what value any given engine will spin those two props.

It's just way easier to buy the props and try them. Keep in mind too that each airplane will run with varying slippage rate on the props. A "dirty" biplane with lots of rigging and hig drag may be flying with the props showing a 60% to 70% true speed to prop/rpm speed while a slippery skinny racer model may be able to run with a 75% to 85% slippage ratio that lets the prop unload more in the air to a higher speed. At least you specified ground RPM to try to avoid muddying the waters that way.

I would hope the different props being used would at least be in the engines suggested rpm range. The term "ballpark" was used because of the hp curve. The prop load factor provided by the program would also help with an educated guess. Of course the actual test with a particular prop would give the real answer.

I thought that 10% prop slippage would be optimal on a clean aircraft. Not confusing slippage with efficiency.