I was debating whether to put this into the electric forum or here--so you see what won out!
I made some calculations a couple of years ago using the PropCalc Software (available at
http://www.drivecalc.de/)---at that time I made the calculation for some variations of a 10.5" prop --that's what I thought I'd be using at the time.
Since most of us in electric are using somewhat larger props, I thought I would look at something in the 12" category. PropCalc doesn't have a lot of built in props in the software, so I chose a prop (Aeronaut 12.5x6.5") which was in the ballpark of the APC 12-6 prop that I now use. This is actually a folding prop used in electric gliders, but I will ignore that detail!
Basically I just want to see the thrust and power curves vs airspeed
at a fixed rpm for three simple variants of this prop--one the original prop, two the same prop repitched to 4", and third, the original 6.5" pitch prop, but with 3 blades.
About the repitching---Propcalc provides a prop window which gives the prop airfoil (in this case an E387) and the blade angles as a function of prop station (20%, 30%....). I took the 70% station--which for this prop gave the actual 6.5" pitch number, and recalculated the pitch for this station to 4". Then I scaled the pitches at the other stations (which were NOT 6.5" by the way!) by the ratio of 4/6.5, and then recalculated the angles.
So for the three cases, I adjusted the input rpm to produce 5.0N of thrust at 24 m/s--the speed I fly at. The 5N more or less matches the power my battery is supplying (according to my onboard data recorder) as long as I take into account the propeller efficiency (Prop Calc claims it is about 70% for the 2 blade 6.5" pitch prop) and my motor efficiency (75-80% according to Drive Calc).. Clearly it could be off by some factor, but I am guessing less than 30-40%.
But anyway this provides a base for the comparison.
The fixed rpm comes from the fact that almost all electrics use a motor governor which maintains a fixed rpm throughout the pattern.
Items to note:
1) the Thrust curves for the high rpm/low(4") pitch prop and the 3 blade 6.5" prop lie almost on top of each other. Both are steeper than the "standard" 2 blade 6.5" prop. This implies that as the airspeed drops in the vertical climb, you will get more thrust out of those two props. Also those two props will give better braking in the dive than the 6.5" 2 blade prop ( the 4" pitch is slightly better in the braking capability than the 3 blade).
I have no clue why the thrust curves for the 3 blade and 4" pitch prop came out so close to each other. I simply chose 4" for a pitch because that qualifies as a low pitch in my book!
2) The standard 2 blade 6.5" pitch prop only needs about ~175 watts of input power (roughly 230 watts from my battery) to maintain the level flight at 24m/s. The 3 blade version needs ~195 watts, and the 4"pitch prop needs 250 watts.
So what this tells me is that a 3 blade prop is about the same as a 2 blade low-pitch/high rpm prop in providing extra thrust in the climb, but is a lot more frugal with my battery watts than the 2 blade low pitch.
The 2 blade 6.5" prop wins the frugal award with power, but loses the thrust battle as airspeed drops to the other two props.
I might mention that I am already maxed out on diameter in my particular example. I also guess that a wider prop would tend to mimic the 3 blade prop. In any case I really would like a 3 blade version of my APC 12-6E!