I gather the three blade prop gives higher max thrust, but if I just wanted to fly straight & level for as long as possible on the SP140 would I be better off with the bi or tri?
I just found this comment: SP 140 propeller specifications and technical data - #21 by Pdwhite
I think that answers the question (not significantly different, going by feel), although I guess if anyone has measured a difference by another means that would be cool to hear about.
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Along these lines, has anyone noticed a difference in tone between the 2 and 3 blade? On videos of the Moster 2 vs 3 blade, the 3 seems to have a much higher-pitched more irritating tone from a 3rd person perspective, but from videos of the 3 blade on the SP140 there doesn’t seem to be that same tone. Can anyone confirm? Does the 2 blade sound less annoying or is is roughly the same?
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Pitch is the main factor in thrust, right? If you have more thrust, that will take more energy to create. So for cruising you want lease thrust for efficiency? Under those conditions I want more thrust for sure during take off. The sooner I can get to a safe recovery height the better. Thus the invention of the variable pitch props. Maybe some day we will have inflight variable pitch like the larger aircraft.
Cheers
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I have the 2 blade and sound aside, I don’t feel like you need more thrust!
I would venture to say it depends on your weight, altitude, humidity and wing size. Do you have any video of your flying with the two blade?
More blades give lower efficiency because the blades interfere with each other aerodynamically. It’s not a huge effect, though. The most efficient prop would have one blade, theoretically, but it would be kinda hard to balance.
I hear you but I don’t think we can use that one tidbit to conclude that this particular three blade prop consumes more energy than this particular two blade prop for a paramotor. Do you agree? If the three blade has more power at the same RPMs then it probably turns slower for a given thrust, right? So does the higher speed of the two blade cause an efficiency drop? Enough to make up for the less efficient number of blades? Seems like there would be many other potential factors, too. Pitch, area, aspect ratio, etc. I was hoping someone had some observations from the field so we wouldn’t have to try to figure it out 
Ya 1 blade is best for efficiency but that only if you have unlimited space so more blades can be more efficient if space is a concern
I would read this on e-props site E-PROPS : les meilleures helices de Paramoteurs, hi-tech ultra-legeres
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Thanks, that’s some great info.
There have been singe blade propellers developed, but the efficiency difference between a 1 and 2 blade prop is negligible especially when the single blade prop has a counterweight causing drag while providing no thrust. Like Paul said, If space is not a limitation, 2 blade propellers would be the most efficient choice. On the other hand, when size restrictions come into play 3, 4, or even 8 bladed propellers can be the most efficient choice for the application.
As for paramotors, we are limited by size, so especially with the smaller cage/propeller sizes, we often see 3 or even 4 blade propellers provide the greatest efficiency.
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I stand corrected. I got my info from NACA Report 640 and some other source I forget. NACA Report 640 is dated and about airplanes. E-Props is a good company. I trust experts to be right.