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Some propeller calculations
Alan Hahn:
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! #^
Dean Pappas:
Hi Alan,
Okay, so inefficiency produces better braking ... that makes sense.
It sure looks like a very wide 2-blader needs to be simulated next! Maybe with thick airfoils?
Well, the 3-blader looks like a great compromise.
Thanks for doing the work,
Dean Pappas
John Witt:
I tried a 13-6.5P cut down to 9.75 and it worked very well. The actual power required appeared to drop a little, as I recall, compared to the 11-5.5 I had been using, and it pulled well out of the corners.
I was at the flying field without the computer so I started cutting the tips off to get the lap times I wanted. Ended up with this clunky looking thing that should be on a crop duster. Hey, doesn't a crop duster have the same kinds of speed control requirements that we have?
I think I don't understand what I know about this subject: the trade between pitch, blade width and diameter. Perhaps there's more than one maximum in this solution space.
John
Alan Hahn:
--- Quote from: Dean Pappas on November 09, 2009, 04:06:17 PM ---Hi Alan,
Okay, so inefficiency produces better braking ... that makes sense.
It sure looks like a very wide 2-blader needs to be simulated next! Maybe with thick airfoils?
Well, the 3-blader looks like a great compromise.
Thanks for doing the work,
Dean Pappas
--- End quote ---
I tend to think about a little differently, although maybe equivalently--we talk about the benefit of a draggy airfoil, which "unloads" as the airspeed drops in the climb, thereby letting more of the prop thrust go toward fighting gravity. In some sense the draggiest item on the plane is actually the prop ---although that drag is being absorbed into the parameter called thrust. So my guess is that is the "inefficiency". Works in a climb the same way as parasitic drag. A little hand waving I agree!
Alan Hahn:
--- Quote from: John Witt on November 09, 2009, 05:47:07 PM ---I tried a 13-6.5P cut down to 9.75 and it worked very well. The actual power required appeared to drop a little, as I recall, compared to the 11-5.5 I had been using, and it pulled well out of the corners.
I was at the flying field without the computer so I started cutting the tips off to get the lap times I wanted. Ended up with this clunky looking thing that should be on a crop duster. Hey, doesn't a crop duster have the same kinds of speed control requirements that we have?
I think I don't understand what I know about this subject: the trade between pitch, blade width and diameter. Perhaps there's more than one maximum in this solution space.
John
--- End quote ---
Here is the original plot but now with a 50% wider 2 blade prop with 6.5" pitch (light green trace). It's thrust behavior is similar to the 3 blade and the low pitch props, but it is a bit more frugal with input power. Just to recapitulate, the dotted traces are the thrust curves (see left vertical axis) and the solid traces are the power curves (right axis).
I note that the force due to gravity is about 12N for a 44 oz (1.25kg) plane, so it is pretty clear that the plane is going to slow down with any of these props, and the differences of prop thrust are not overwhelming compared to gravity ( I am thinking the reduction of drag as airspeed drops is actually a bigger effect). But I suppose every bit helps!
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