Joe,


I was not referring to a specific engine and its recommended prop size range, but to a specific prop load.


A given engine should spin at a certain target RPM, to show good performance.

Let's take a certain sport .46 engine, with a muffler. It should be set to a ground RPM, between 12,000 and 14,200, where the higher RPM is for best power and the lower range is for best torque (thrust - 3D).

If we target a certain limited range; say 13,200-13,500 RPM, any prop equal in load to a 10x7 will be spun within this smaller RPM range. It could be an 11x5, 10.5x6, 11.5x4, 12x3, 9.5x8, 9x9, 8.5x10... With each of them, the RPM this engine is capable of achieving an RPM between 13,200 and 13,500.

Of course you can increase the load, but then you will get into a lower RPM subset. If you increase the diameter (choose any of the props in the above paragraph) by 1" and decrease the pitch by only 1", RPM will be down to 12K+, which is in a different bracket.
The engine will no longer be able to power this prop into the previous bracket.

And I am talking about props of the same design. If you use narrow blade props, in the new group, you may be able to stay within the higher bracket, but then it is still a prop with the same load as the previous design 10x7...


The general rule remains; 1" diameter is equivalent to 2" pitch.
Increase one and you will have to decrease the other by this ratio, to maintain the same load.

It wasn't me that invented this...