I did a little test today. I have heard the statement once or twice that the more blades a prop has, the less efficient and more of a power hog it is for diminishing returns on thrust. I did some research and found the claim to be true, but with a couple stipulations: besides when they are used for space and clearance concerns, three blade props are used on some trainers(both RC and full scale) because the inefficiency gives them excess thrust at slower speeds, so the plane will have a better speed envelope, better acceleration and punch out(and as I theorized and tested, better high alpha). The battery just might burn out a couple minutes earlier. It's a trade-off.
I tested my V Sliver with 5x5.4.5x2 compared to the normal 5x5.5x3. I needed to power up to full throttle and give a slight shove for a pretty anemic launch, when with the three blade, I'd crank it up to like 75 or 80 and just let go, and it would rocket off. It had garbage punch out, and couldn't do high alpha worth squat. The test would have been better with one of those 5x5.5x2 race props you can get at Graupner USA. I believe this would have made launches better and gave it a bit more punch out and overall thrust, but I bet there would still be some lag and high alpha would still be mediocre. I could be wrong, I may order a couple 5.5x5x2 and see.
All I'm saying is, the statement about prop efficiency is 100% accurate, but when a newb reads somebody say that, there's a danger of "cargo culting" the idea and going "well then three blade props suck then and I guess there's no situation in which I'd want to use one".
