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Author Topic: Propeller Power vs. RPM  (Read 2912 times)

Offline Jim Moffatt

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Propeller Power vs. RPM
« on: March 12, 2010, 03:14:30 PM »
My current understanding is that the power input to a propeller scales with the cube of the RPM.
Is this correct, at least as an approximation?
The power I'm talking about is the power delivered to the propeller, not motor input power.
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Offline Howard Rush

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Re: Propeller Power vs. RPM
« Reply #1 on: March 12, 2010, 09:23:14 PM »
That's an interesting question.  Power required for an airplane flying level and not accelerating goes up with the cube of airspeed.  In this case, though, I take it that you are holding airspeed constant and allowing thrust to vary.   Sounds like you are figuring a delta RPM command for a power-compensation circuit, you rascal.  I would guess that for constant blade angle of attack, thrust losses would be proportional to the square of RPM, but angle of attack is sorta inversely proportional to RPM, so torque would be approximately proportional to RPM, hence power to the prop would be approximately proportional to square of RPM.  Igor or somebody could give you the real lowdown.

This reminds me of the time I was driving a truck across Wyoming on I-80 with a strong crosswind.  I had to hold in a significant steering wheel torque to keep the truck going straight, meanwhile trying to calculate whether the force would get greater if I sped up or slowed down.   
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Offline Igor Burger

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Re: Propeller Power vs. RPM
« Reply #2 on: March 13, 2010, 01:27:16 PM »
Depends on conditions. If you mean static test (motor on test stand) then yes, it is aproximately cube of rpm. You can also have power absorption curves for common props, for example measured by Dr. Demuth. I had several of them linked on my hompage in past, unfotunatelly somebody wrote me that I am breaking copyright, so I had to delete them and I also cannot link it here. But for sure you can find constants for aeronaut props here: http://www.aero-naut.de/fileadmin/download/luftschraubenvergleichswerte.xls

That table says how many rpm you can get at 100W on shaft. Then you can recalculate by 3rd power. But anyway, it is only aproximate value, precise efficiency caldulators uses constants from 2.9 to 3.2 (I think E-meter has or had table of constants fro some props)

In flight power is another story and as Howard wrote, it depends on airplane, it can make strange things as airfoil can change drag in more complex way (changing AoA, point of polar, RE number etc.) and that change leads to changes in slippage and that loads prop. It could somewhere close to square of rpm.
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Offline Igor Burger

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Re: Propeller Power vs. RPM
« Reply #3 on: March 13, 2010, 01:32:03 PM »
Quote from: Howard Rush on March 12, 2010, 09:23:14 PM
whether the force would get greater if I sped up or slowed down.   

depends what car you have, if you can control also rear wheeles and make some yaw, the wind can push you forwadr  VD~
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Offline Jim Moffatt

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Re: Propeller Power vs. RPM
« Reply #4 on: March 13, 2010, 07:17:02 PM »
Thanks Howard and Igor.   H^^  I meant the power in a static test. I am trying to make a spread sheet. So I can use FlyBrushless.com motor test data to predict how a given motor will perform with a different prop. Most of their data is for pitch speeds well below what is needed for CL.

I am scaling by the cube of rpm, 5th power of diameter, and linearly with pitch. This is not meant to be exact. It is to give me a good starting point for choosing a motor and bench testing.

Right now I am modeling the motor as a Kv generator in series with the winding resistance. Again this is an approximate model and I plan to add more to the model if it doesnt work well. I'm sure it wont be as good as moto-calc or other programs but it is a learning experience for me.   n~
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Offline Igor Burger

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Re: Propeller Power vs. RPM
« Reply #5 on: March 15, 2010, 03:36:08 AM »
some years ago I did such a spread sheet which can also replace motor calculator, and it has lot of such prop constants ... if you want ... :-)
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Offline Jim Moffatt

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Re: Propeller Power vs. RPM
« Reply #6 on: March 15, 2010, 02:43:03 PM »
Igor
I would very much like ot see your spread sheet!

my email is:
jpmoffatt@aol.com
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Offline Paul Walker

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Re: Propeller Power vs. RPM
« Reply #7 on: March 15, 2010, 04:58:25 PM »
Quote from: Jim Moffatt on March 15, 2010, 02:43:03 PM
Igor
I would very much like ot see your spread sheet!

my email is:
jpmoffatt@aol.com

Yes, as would I.

Paul Walker
go_stunt@comcast.net
« Last Edit: March 16, 2010, 01:02:18 PM by Paul Walker »
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Offline Dean Pappas

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Re: Propeller Power vs. RPM
« Reply #8 on: March 15, 2010, 08:26:33 PM »
Me too!
Thanks, Igor,
Dean Pappas
deanf3af2b@pappasfamily.net
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Offline Igor Burger

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Re: Propeller Power vs. RPM
« Reply #9 on: March 16, 2010, 01:34:53 AM »
hmmmm ...  n~

I must translate it, because it is in slovak language  >:D ... ok, evening (slovak evening)
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Offline Igor Burger

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Re: Propeller Power vs. RPM
« Reply #10 on: March 16, 2010, 03:44:56 PM »
so xls is here:
http://www.kdesombol.sk/madar/igor/ec.xls
I hope I used clear words and did not forget to translate something :-)
it is not classic calculator, it is spread sheet for playing and seeing what is happening there

second folder of the spread sheet is database of constants

it has data from Emeter
http://www.aircraft-world.com/prod_datasheets/hp/emeter/hp-propconstants.htm (lot of info is also on its "see prop talk" link)

and also data for graupner gray props from book of prof. Demuth
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Offline Dean Pappas

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Re: Propeller Power vs. RPM
« Reply #11 on: March 16, 2010, 06:06:18 PM »
dakujem.

Dean
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Offline Igor Burger

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Re: Propeller Power vs. RPM
« Reply #12 on: March 17, 2010, 04:12:11 AM »
 LL~ ... Great Dean, I didn't count that first Slovak word on this forum will be from your keyboard  H^^
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Offline Dean Pappas

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Re: Propeller Power vs. RPM
« Reply #13 on: March 17, 2010, 08:38:42 AM »
I was taught, as a child, that it is very usefiul to be able to say please, thank you, and "go to #@!* " in as many languages as possible.  LL~
Dean
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Offline Jim Moffatt

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Re: Propeller Power vs. RPM
« Reply #14 on: March 17, 2010, 07:03:12 PM »
Igor
Thank you for the spreadsheet. I am studying it.
How are the prop power constants obtained, measured or calculated?

Best regards,
jim Moffatt
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Dave Adamisin

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Re: Propeller Power vs. RPM
« Reply #15 on: March 17, 2010, 08:22:41 PM »
Hi Igor. Thanks.
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