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Old Apr 10, 2010, 02:58 AM
Rangers Lead the Way
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Interesting results, 2 blade vs 3 blade

I ran some tests tonight that would put in doubt the conventional wisdom that efficiency decreases as the blade count of a prop goes up. My results show exactly the reverse: that the efficiency of the 3-blade prop is nearly twice that of the 2-blade.

First the back story. I purchased a 54" CMP Mustang from Nitroplanes. The model is well-built, but is notoriously heavy for its size, topping out at 6lb lighter end of the build scale. Many guys are engining this bird in the 46-60 class and running on 5S power or more. In an affort to save 1/2 a pound of power system, I purchased a TGY 42-50 600Kv, a motor that should run on 4S. I also purchased a variety of props from TGY including several 3-blade sport props and 2-blade.

The props used in this test were both 12 in, one a 2 blade 10 pitch (12x10) and the other a 8.5 pitch (12x8.5x3). Power was a fully charged Flightmax 4S 5000, which by the way is a real powerhouse at 5300, 16.8V fully charged.

I built a scale test rig to record engine thrust. Full power results were: (3 blade) 49A@15.5V, 778W, and a whopping 6.5lb thrust. Obviously, power will not be an issue with this setup. This P-51 will have plenty of giddyup. Second test w/ the 2-blade: 38A, 15.7V, 600W, but only 3.5lb thrust. Now really curious, I put the 3-blade back on, and advanced the throttle until I was at 3.5lb thrust, about halfway. Amps? 18!!! That's right, less than half the amps for the same thrust. Which means the model can fly twice as long at the same power level on the 3-blade prop versus the 2-blade prop.

So, the efficiency argument is total nonsense as far as I am concerned, at least when it comes to electric brushless outrunner power. Certainly, speed may suffer a bit (I am not even sure this holds true), but with powerplants making 1:1 thrust / weight on a warbird, we're talking about a difference that is totally not scale and matter only to pylon racers. Being able to fly a model at the same power level, twice as long: priceless.

I think the situation for gassers is different, as those motors simply do not develop the torque in any given power class to spin more than a 2-blade efficiently, so prop speeds slow down dramatically with the result being an under powered model with weak thrust. That is clearly not the case with BL outrunners. Thinking about full scale aircraft, the fastest prop planes in the world are multi-blade. The Hawker Sea Fury was the fastest fighter plane built, with a 450mph+ top speed. It ran a 5-blade prop even though there was pleanty of clearance for a larger 4-blade. The Stumax 110 ducted fan (110mm) generates 12-14lb thrust on 12 blades. Conventional 6-7 blade fans in that size are at half the thrust. Of course, a 5 blade fan may fly a little faster, but is in fact less efficient. Food for thought.
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Old Apr 10, 2010, 04:02 AM
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Staffs, UK
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Since you're using two props which are in no way equivalent and only measuring static thrust your results don't really say anything about overall efficiency. The equivalent in power terms to a 2-blade 12x10 is a 3-blade 10.9x10. The equivalent to a 3-blade 12x8.5 is a 2-blade 13.2x8.5. Your 3-blader is effectively a bigger prop with lower pitch and all theories suggest that will be better for static thrust.

But mainly what you've proved is that, for a particular motor/battery, one prop size may be better suited than another, even if it has more blades. I.e. the advantages you get from optimising the motor load can easily outweight the (small) disadvantage of more blades. I'm not sure there are many people round here who didn't already know that. It's interesting to see some numbers though .

Steve
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Old Apr 10, 2010, 04:44 AM
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Slipstick is right. Too many things are different.

It is well known theory and proven many times by personal experience with models and full size ultralights that there are three things responsible for prop efficiency:

1. diameter

2. diameter

3. and last but not least... diameter.
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Old Apr 10, 2010, 05:23 AM
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Hi
Power loading is, at a first degree of approximation, proportional to the
** Product Static thrust x Airflow speed (or pitch speed)
Power loading has a reaction on rpm and so the overall problem is not simple.

Other parameters are the chord of each blade, profile + thickness, twisting ...

A second point is that the ratio "Static thrust / Power out" is just a "ratio" and not an efficiency in the true sense.

Anyway multiblades prop have some advantages in some cases (except the price)
Louis
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Old Apr 10, 2010, 01:07 PM
Rangers Lead the Way
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Originally Posted by rich smith View Post
Slipstick is right. Too many things are different.

It is well known theory and proven many times by personal experience with models and full size ultralights that there are three things responsible for prop efficiency:

1. diameter

2. diameter

3. and last but not least... diameter.
I agree that there are many variables, but this is my point: both props were 12 and the 3-blade with the lower pitch produced the same thrust on half the power. To me, that is efficiency. In any case, my setup is pretty simple and I can set it up any time. I just used what I had, which IMO is pretty close, but I'll try to find a 13x8.5 2-blade and test again. But at this point, the difference is so significant, and the test conditions, prop profiles, and sizes close enough that I have to conclude efficiecies are superior. I'll also test a 4-blade APC at some point. It will be interesting to see how a Zinger 5-blade does on my power 110. Hope I can keep the thing attached to the bench.

To each his own. All I am saying here is that it would be an error to overlook multi-blade props if efficiency is desired, because the conventional wisdom, passed down by gassers, is that only 2-blade props work well. Brushless outrunner power changes all that. Particularly for large scale birds.
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Old Apr 10, 2010, 07:39 PM
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Static testing doesn't tell much. If you were to put the same pitch prop as your 3 blade on the 2 blade, the 2 blade would have much more thrust than it showed. At 0 speed the low pitch will give more thrust but will run out of poop really quick in the air. I tried that on a 1/4 scale Corby Starlet. Worked fine on a 18-8 but I thought I'd try a 20 -6. Both turned close to the same RPM on the ground. Got off the ground real quick but the pitch speed flew it just above the stall speed. Sweating bullets to get it around the patch and back down without tipstalling it in the turns. Made it fine and happily put the old prop back on.

Gord.
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Old Apr 10, 2010, 09:07 PM
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Originally Posted by Fourdan View Post
Anyway multiblades prop have some advantages in some cases (except the price)
Generally the only real advantage is ground clearance all else equal. Aerodynamic efficiency is the major reason for 2 blade superiority but there are also issues with vortex generation in preceeding blades. Here in the real world, specially in full size design (where more than a foam airframe or electric motor shaft is at stake) the advantage of 2 blade over three is well known.
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Old Apr 10, 2010, 09:13 PM
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Originally Posted by TTRotary View Post

So, the efficiency argument is total nonsense as far as I am concerned, at least when it comes to electric brushless outrunner power. Certainly, speed may suffer a bit (I am not even sure this holds true), but with powerplants making 1:1 thrust / weight on a warbird, we're talking about a difference that is totally not scale and matter only to pylon racers. Being able to fly a model at the same power level, twice as long: priceless.
Ummmm.... you do realize that the first bolded statement is immediately contradicted by the second two bolded statements, right?

The parameters you are trying to optimize don't necessarily have to be the same parameters others are trying to optimize.

Chuck
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Old Apr 10, 2010, 09:27 PM
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I'd also like to mention that a one blade prop is superior to a 2 blade. Nearly all planes would be using them today were it not for that vibration problem.

Many years ago I was fortunate to win indoor rubber duration contest with the first 1 blade and next season the only ones to place were those that had followed suit.
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Old Apr 10, 2010, 09:31 PM
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Rich,

Are there toxic fumes being released when you do electronics work?
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Old Apr 10, 2010, 09:33 PM
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I remember the controline speed planes with the single blade props way back when.
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Old Apr 10, 2010, 09:36 PM
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Originally Posted by NoFlyZone View Post
Rich,

Are there toxic fumes being released when you do electronics work?
No but I do miss that ORIGINAL model airplane cement.
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Old Apr 11, 2010, 03:41 AM
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I remember the controline speed planes with the single blade props way back when.
Like the ones that still hold all the C/L World Speed Records you mean ? It's not that long ago .

And there are still plenty of free flight models flying with rubber power and carefully balanced one-blade props.

Sometimes we forget that there's more to this hobby than plastic RTF/ARTF with radios .

Steve
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Old Apr 11, 2010, 04:26 AM
Rangers Lead the Way
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Originally Posted by NoFlyZone View Post
Ummmm.... you do realize that the first bolded statement is immediately contradicted by the second two bolded statements, right?

The parameters you are trying to optimize don't necessarily have to be the same parameters others are trying to optimize.

Chuck
You overlooked my statement " ( I am not even sure this holds true)". Having had now a couple of glasses of wine and feeling no need to be polite, let me be blunt: the whole - more blades less efficient is pure bunk - nonsense - as far as I am concerned. It is created by gasser guys, who accurately have discovered that their motors cannot handle more than a 2-blade prop. That is a motor limitation, not an aerodynamics limitation. Speed? As I well know from race cars, HP/ torque (thrust) vs areo drag is the only thing that counts. Planes are the same. The bottom line is pulling power vs aero drag, and if a 3blade pulls the plane thru the air at the same force that the 2-blade does,, at half the amp draw, then that is the winnner for me, period. I was trying to be polite regarding the speed argument, leaving a bit of argumentative wiggle room as it were. In fact, there is none. Thrust is thrust, period. And the 3-blade is superior, period.

According to the modeling CW, turbofans and the fastest turbojets on earth would run on one blade. Hmmm, I suppose those SR-71 pilots missed the memo. Who know those J-58s could on one counter-balanced blade.

I will offer a challenge to anyone: same plane, same electric motor, same batt and ESC, equivalent prop, defined as follows 12x10 2-blade vs 12x8 3-blade or 11x10 3-blade. The contest is top speed, doppler radar. My brother and others are cops and will do the measuring. $50 Anyone?
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Old Apr 11, 2010, 07:54 AM
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You started off talking about efficiency and now you're looking at maximum speed regardless of power used. They're completely different things.

I particularly like your contest - you rig the rules so that the 2-blade prop that will absorb less power than the 3-blade and then you think the prop using more power might go faster. In-flight loggers are easy enough to get hold of now so why not make it real equivalence i.e. use any props you like so long as they top out at the same power level ? Or even measure speed produced per watt. Now that's a bit nearer to efficiency .

But basically this comes down to the usual thing. You're entitled to your own opinion even if several thousand physicists and aerodynamicists disagree with you. A bit like your ridiculous argument about fast jets. The amount of energy input to one of those is immense. They are fast but they are fabulously, astoundingly INEFFICIENT. Just like an 8000hp Top Fuel dragster is fast but it's not an efficient form of transport .

Steve
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