I have an electric plane which currently has a 10X5 2-bladed prop and I would like to go to a 3-bladed prop to improve the ground clearance. As a rule of thumb, for 2-blade props, if I want to change the dia., I first multi the pitch by the dia and try to come close to that figure with the new size prop. For 3-bladed props, I'm guessing that multiplying the pitch by the dia (as with the 2-blade configuration, then multi that product by 1.5, the increase in additional blade and then multi that result by a efficiency loss assumed to be .85. Does this make any sense and if not, does anyone have another suggestion? Thanks

That all sounds like you've got some logic behind it but bottom line is...............

You probably won't find but one 3-bladed prop in 9" diameter and you need some luck to find it. Master Airscrew shows a 9x7(3) in their recommended props list.

The rule of tumb is to reduce the diameter by 1" and either up or down on the pitch (depending on whose thumb is being ruled).

Truth is, simply try any pitch you can find in the diameter that works. And the harder truth is that there aren't many to be found.

Thanks for the feedback. I found and ordered a couple of 8X6 and 9X6 thin props and will see how they work. Thanks again.

I forgot to mention that the props I ordered are 2-blade since, as you noted, 3-blades are far and few to locate.

Going down from a 10x5 to a 9" diameter, it would be worthwhile to try a few 9"s of different pitch. The pitch we see on the props really is just the mfg's advertised best guess. Most people would suggest that a 9x6 would be the closest match moving down from a 10x5. But not many would bet big bucks every 9x6 would give the best performance.

Try a 9x7 or two.

And since you're using electricity, try plugging in a wattmeter to get some really good info. You can tell a lot about props from a wattmeter.

Good idea to compare the power for each prop. Thanks for the advise.

Actually the pitch by R squared would be a lot better ratio to compare. But as da Rock said, you don't have much to select from anyway.

If interested, I will share the power comparison results and rpm's as well. Thanks for your suggestions.

If all other parameters (such as pitch , airfoil and blade shape) are unchanged the diameter of the 3-bladed propeller should be 90% of the diameter of the 2-bladed propeller in order to allow the engine to swing both propellers at the same rpm and power.

The reason for this is that the power requirement scales with the fourth root of the number of blades, i.e. for a 2- two 3-blade conversion the scaling factor is (2/3)^(1/4)=0,9036...

In real life the rpms may deviate slightly because the smaller diameter propeller operates at a lower Reynolds number and also the interference between blades is more pronounced with a 3-bladed propeller.