Author Topic: Three blade equivalent of the two blade APC electric prop. (Read 5118 times)

Matt Piatkowski · « **on:** December 08, 2015, 08:13:34 PM »

What is the three blade equivalent of the APC 13x4-E prop?
Is there any general formula that can be used to estimate such relationship?
Thanks,
M

Target · « **Reply #1 on:** December 08, 2015, 09:44:05 PM »

Matt-

I'm sure this isn't the "right" way, but why not use Cobra's online prop charts?
I'd assume that an "equivalent" 3 blade prop would draw about the same amps while producing about the same thrust at a similar pitch speed.

http://innov8tivedesigns.com/images/specs/Cobra_3520-12_Specs.htm

On 4S, looks like an 11x5x3 or 12x5x3 would be about the same on the above motor as a 13x4 two blade. I could be totally wrong, but it could be a starting place.

However, the claim is that more blades are in general less efficient at producing thrust than less blades in the first place. Something about the air disturbance.
But, to harness power with limited ground clearance, one must usually go to more blades.
None of that takes into any account any gain in turning efficiency of the plane by using a smaller diameter prop?

Motorman · « **Reply #2 on:** December 09, 2015, 11:13:23 AM »

If you're running a 4 pitch and you want to go the same speed, you need a 4 pitch. So, 12-4 3 blade.

MM

Target · « **Reply #3 on:** December 09, 2015, 11:44:20 AM »

Who makes low pitch 3 blade props?
I'd think the pitch being the same only will work with the RPM being the same, right?
That being said, will the RPM be the same on a 3 blade, 12" prop as a 2 blade, 13" prop?
I think there could be some other factors involved, but certainly not sure.

R,
Chris

Mark Scarborough · « **Reply #4 on:** December 09, 2015, 02:57:18 PM »

Quote from: Chris Behm on December 09, 2015, 11:44:20 AM

Who makes low pitch 3 blade props?
I'd think the pitch being the same only will work with the RPM being the same, right?
That being said, will the RPM be the same on a 3 blade, 12" prop as a 2 blade, 13" prop?
I think there could be some other factors involved, but certainly not sure.

R,
Chris

It most decidedly will not be the same,, factors controling it,, airfoil on the blades,, undercamber,, blade outline shape,
The only way to know is to try and compare in my experience

Howard Rush · « **Reply #5 on:** December 09, 2015, 03:24:36 PM »

Yep. It's nontrivial. JavaProp can give you a clue.

CA (Alan Resinger and Chris Cox) make good three blade electric stunt props, as does Igor Burger, the Champion of the World. Both those props have pitch about 5 to 6 and require more RPM then the APC.

Motorman · « **Reply #6 on:** December 09, 2015, 10:00:39 PM »

Quote from: Chris Behm on December 09, 2015, 11:44:20 AM

I'd think the pitch being the same only will work with the RPM being the same, right?
Of course.

That being said, will the RPM be the same on a 3 blade, 12" prop as a 2 blade, 13" prop?
We're talking electric so the rpm will be whatever you program it to be.

I think there could be some other factors involved, but certainly not sure.
Cons more drag less efficient, pros less procession more focused air blast over the controls.
Who can measure these things IDK.
R,
Chris

Target · « **Reply #7 on:** December 09, 2015, 10:29:06 PM »

The RPM will be the same (programmed) assuming enough excess power available to make that happen....
But it comes at a cost, there is no free lunch. The amp draw may be higher.

One thing that is super intriguing about e power is that I believe there are probably some "ideal" combinations that are more amp economical than others, for each motor and plane combination. Getting the most thrust per watt seems like something that could be possible.
And it does sound like for some people at the top, a 3 blade prop accomplishes that.
Certainly worth experimenting.

R,
Chris

Howard Rush · « **Reply #8 on:** December 09, 2015, 11:15:58 PM »

Quote from: Chris Behm on December 09, 2015, 10:29:06 PM

The RPM will be the same (programmed) assuming enough excess power available to make that happen....
But it comes at a cost, there is no free lunch. The amp draw may be higher.

One thing that is super intriguing about e power is that I believe there are probably some "ideal" combinations that are more amp economical than others, for each motor and plane combination. Getting the most thrust per watt seems like something that could be possible.
And it does sound like for some people at the top, a 3 blade prop accomplishes that.
Certainly worth experimenting.

Depends on whether you want to score in stunt or make money transporting cargo.

Howard Rush · « **Reply #9 on:** December 10, 2015, 02:46:08 AM »

I should elaborate on the last item. Efficiency is nice, and you can use JavaProp or the Adkins-Liebeck paper and airfoil data to find the most efficient prop, but there are other considerations. We have found that the CA props (for example) make nicer corners than the APCs, and it is well worth adding another cell to the battery just for the corner. The APC props also make more noise in corners than the CA or Igor props. Some of this seems to be because of the APC's flexibility, but a carbon APC copy still makes noise. This can ding your score in the wind if a judge listens for time between corners, rather than looking at the shape of a maneuver. Some of the noise may come from the changing prop moment of inertia as you're turning a corner. 3-blade props have a lot less variation in moment of inertia in a corner. The 3-bladers also seem to be easier on motor bearings, but bearing life seems to depend on even weirder phenomena. Then there are the issues of yaw caused by turning corners and by operating the prop axis at an angle relative to the oncoming air.

If you do consider prop efficiency, where do you want it? It might be better to have more efficiency in maneuvers than in level flight. Also, given that you can calculate and maximize efficiency, do you want to maximize efficiency itself or the rate of change of efficiency with airspeed?

Igor Burger · « **Reply #10 on:** December 10, 2015, 03:23:03 AM »

edit:

sorry ... mistake, wrong thread :- ))

Motorman · « **Reply #11 on:** December 10, 2015, 10:43:28 AM »

All of my props are tuned to the strouhal number of the north american ringmaster but can props really be considered oscillatory lift-based propulsion.

MM

Igor Burger · « **Reply #12 on:** December 10, 2015, 11:09:39 AM »

Nice answer is here:

http://www.mh-aerotools.de/airfoils/

However it is valid only if props are "similar" ... if you cut 1" from tip, you will change blade aspect ratio, P/D ratio etc. So those props are not "similar" after all.

And regarding tip stall: Prop even in static regime usually does not exceed critical AoA at the tip, since even in static regime airfoil tip angle does not exceed 14 deg ... unlike airfoil close to hub, so props tend to stall at root (slowed), not at the tip.

John Rist · « **Reply #13 on:** December 10, 2015, 06:40:44 PM »

I have always heard 1" smaller diameter same pitch when going from 2 to 3 bladed. This comes out to about the 10% mentioned in the post above.

Dean Pappas · « **Reply #14 on:** December 10, 2015, 07:11:57 PM »

Hi Chris,
When maintaining pitch and going from 2 to 3 blades, the diameter drops to 93%, based on absorbing the same power at the shaft.
Is that a complete answer? No ... but it makes a good starting point.
The lighter hollow-core carbon fiber props sure improve corners, though!
take care,
Dean P.

Target · « **Reply #15 on:** December 10, 2015, 08:46:48 PM »

Howard-

After I hit the post button, I realized that my comment was a bit off target. Efficiency is nice to have for duration, but not necessary for what the stunt planes are attempting to accomplish.
I need to get out of my sailplane mentality, LOL.

Thanks to all of you for the explanations. Even thought it was originally Matt's question, I find the topic very interesting.

13 x .93 = 12.09, BTW. So, all answers seem to align with a 12" 3 blade replacing a 13" 2 blade for starters.

R,
Chris

Dennis Toth · « **Reply #16 on:** December 13, 2015, 05:48:50 PM »

If the corner is better with the light hollow CF this seems to indicate that weight is a big factor. There are 3 blade wood props made by Xoar. They are not cheap(almost the same as carbon) but they are light, a 12" is 16 grams. They have diameters from 10" to 19" and pitch 5, 6, 7,8 & 10.

Here's the web site: http://www.espritmodel.com/xoar-propellers-3blade-electric-pjie.aspx

I have a few of the 2 blade 10x5's and they are beautifully made and rock hard.

Best, DennisT

Motorman · « **Reply #17 on:** December 15, 2015, 04:49:41 PM »

Do you find the Xoar electrics are thinner than the Xoar gas/glow props? I've got an 11-5 and it's about 1/4" thick. Too bad they don't make any 3 blade pushers.

MM

Mark Scarborough · « **Reply #18 on:** December 15, 2015, 06:26:21 PM »

Quote from: Motorman on December 15, 2015, 04:49:41 PM

Do you find the Xoar electrics are thinner than the Xoar gas/glow props? I've got an 11-5 and it's about 1/4" thick. Too bad they don't make any 3 blade pushers.

MM

the XOAR electrics are in fact thinner than the glow props,,

Author Topic: Three blade equivalent of the two blade APC electric prop. (Read 5118 times)

Matt Piatkowski

Three blade equivalent of the two blade APC electric prop.

Target

Re: Three blade equivalent of the two blade APC electric prop.

Motorman

Re: Three blade equivalent of the two blade APC electric prop.

Target

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Mark Scarborough

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Howard Rush

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Motorman

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Target

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Howard Rush

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Howard Rush

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Igor Burger

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Motorman

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Igor Burger

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John Rist

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Dean Pappas

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Target

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Dennis Toth

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Motorman

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Mark Scarborough

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