If you go from two blades to three, then for the same rpm and power, the diameter will be somewhere around 10% less. For some single engine aircraft this gives a bit more prop to ground clearance, same cruise speed, but less thrust for the climb.
There are several design programs for model aircraft props, where you can make alterations to get the performance you require. Maybe this also works with full size props..... ( edit.. Yes it does.. )
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http://www.godolloairport.hu/calc/strc_eng/index.htm
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