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Jan 27, 2015, 03:17 PM
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Fourdan's Avatar
Thread OP
It is a question of wording
In Scorpion Calc (now v3.63 I am using
in the performances frame
** In power (at ESC input)
** Out power (at propeller level)
in the propellers sub-menu
** Absorbed power (at propeller level) similar to Out power)
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Jan 27, 2015, 03:19 PM
Registered User
Ken Myers's Avatar
Thanks Mike, that's exactly my thinking. Therefore, for electric motors, that's power out to the prop, or power 'absorbed' by the prop.

I've still been researching and can't find anything that is explaining APC's data.

It doesn't match my data for the latest '0.45 inch' thick hubs for the 12x8E, which is much, much lower in power absorption than the data on the APC site.

I tried to find what year the site data is for or what iteration, but I can't find that info. As I posted earlier in the thread, there have been a lot of changes over the years.

I have quite a bit of my own collected data on the various iterations of the APC 12x8E over almost two decades and I can clearly see three breaks in the data created by different iterations of this same designated prop.
Jan 27, 2015, 04:53 PM
Suspended Account
Hi Ken,

The power from the APC prop data is shaft power.

For the APC 12x8E, they give Cp = 0.0741.
If you take air density rho = 1.225 kg/m3 (standard atmosphere),
diameter D = 0.3048 m,
n = 9000 / 60 = 150 rev/s,
and 1 hp = 745 W,

then you have shaft power :

P = rho.Cp.n3.D5 = 1.225 * 0.0741 * 150^3 * 0.3048^5 = 806 W
this gives 1.082 hp.
Jan 27, 2015, 05:56 PM
jackerbes's Avatar
Originally Posted by Ken Myers
Thanks Jack.

I've read and reread your post very carefully. In an obtuse way I believe you are saying the APC HP (horsepower) indication is power in.

My question, Power into what?
Into the prop or the process of turning it? I am completely baffled by and not even concerned with the power out of a prop other than seat of the pants impression as to how it flies. I use the input power as I described it to decide the motor needed. If it needs 300W I would say it needs a 100g motor by the 3W per gram rule of thumb.

That makes no sense to me. Power out is totally relative to what is doing the power conversion from power in.

I assumed, and it appears incorrectly, that the HP (horsepower) indication was shaft horsepower, the power required to turn the prop at a given RPM.
I don't know anything about power out of a prop. As I said, I am not very knowledgeable about the science of it at all, I am just looking for predictions of the thrust at a given RPM and the input power needed for a typical brushless outrunner to attain that RPM. I use that to choose motor weights. And the max RPM desired and input voltage are used to determine the Kv.

Of course it can all be changed to watts for our use.

Using APC's data for the n100W number for the 12x8E, 5281 yields;
9000 RPM / 5281 = 1.7042227 ^ 3 = 4.9497017 * 100 = 494.97017 watts OUT, or absorbed by that 12x8E turning 9000 RPM or about 0.66 SHP (shaft horsepower)
75% efficiency yields 660 watts in
80% efficiency yields 619 watts in
85% efficiency yields 582 watts in

At 7000 RPM
7000 RPM / 5281 = 1.3255065 ^ 3 = 2.328872 * 100 = 232.8872 watts OUT, or absorbed by that 12x8E turning 7000 RPM or about 0.31 SHP (shaft horsepower)

I really appreciate your input, but I'm still confused as to what horsepower APC is referring to.
I guess I can't help you and may have misunderstood your question.

I think APC and are talking about the same kind of power, the input power to the prop as I think of it. Quite frankly, I cannot follow the math in your description above or the logic of it...

Jan 27, 2015, 07:14 PM
Registered User
Ken Myers's Avatar
Thanks for all the input so far to everyone.
Sorry you couldn't follow my math Jack. Guess I didn't do well. My fault.

Here's the problem I'm wrestling with.
The following measured data was gathered on Dec. 14, 2014 in my basement. The elevation here in Walled Lake, MI, USA is ~287m and the temperature in the basement was ~14.2 degrees C. Sorry, I missed noting the barometric pressure that morning.

The spreadsheet screen capture comes from one I use when testing motors. You'll see I use two different equations to mathematically approximate the watts out (Pout).

I used my Emeter II to log the volts, amps and RPM. The Emeter II is calibrated to an optical tach and the volts and amps calibrated with a Fluke multimeter. The Emeter II data should not be off by much.

I change the Emeter II logs to csv files on my computer so that I can view them in a spreadsheet.

I was able to locate logs at exactly 9000 RPM for both props from the Emeter II files. One of the APC 12x8E props has a measured hub thickness of 0.39" and is an older design. The other is labeled exactly the same and has a hub thickness of 0.45".

You'll note that both on the spreadsheet capture and the Drive Calculator captures, the 0.45" thick hub absorbs much less power at 9000 RPM than the 0.39" thick hub.

You can also note that neither of these is even close to the APC data for the 12x8E prop.
APC n100W 5281
0.45" hub n100W 5660
0.39" hub n100W 5404

Comments and help would be much appreciated.
Jan 27, 2015, 07:26 PM
I am a nice guy! Really!
Ken, what is the "n100W" number that you talk about and where is it found in the APC data?
Jan 28, 2015, 02:36 AM
Suspended Account
I have noticed difference in blade thickness between some APC 8x6E props (some are more dark when seen with sun light from behind), but without any other difference I could noticed.

I guess the two kind of 12x8 you noticed may have a small shape variation that can explain the variation in power (did you notice a variation in thrust also ?).

The difference don't mean much, maybe just that one prop is stronger than the other, needing more power but also producing more thrust, with no big difference in efficiency.

It can come from blade shape variation (thickness, chord length and blade area, airfoil, true diameter, or even pitch distribution along diameter).
Jan 28, 2015, 10:14 AM
Registered User
Ken Myers's Avatar
Mike Dubovsky
Ken, what is the "n100W" number that you talk about and where is it found in the APC data?

The n100W number appears in
Antrieb nach Maß. Auslegung und Optimierung des Elektroantriebes zum Flugmodell by Wilhelm Geck, January 1995
Only available in German
Direct Google Translation of the book title:
Drive to measure. Design and optimization of the electric drive for the flight model.

How it is used:
Power Out [Pout] = (measured rotational speed / n100W number)^3 * 100
n100W is the RPM that ‘absorbs’ 100 watts OUT.
Note: there is NO direct relationship to the power in [Pin].
In the n100W notation, n is rotational speed [in this case revolutions per minute RPM] per 100 watts OUT! AKA power ‘absorbed’ by the prop.

Both Aeronaut and APC having been using this number for a long time.
The n100W numbers for APC props are located here:

The exact file is N100.DAT

Jeti has a compilation of these numbers from various sources at

As I previously noted, APC indicates that the n100W number for the 12x8E is 5281. The Jeti compilation notes it as 5243.

Pout for the 12x8E at a measured 9000 RPM, using the APC n100W number is:
(9000 / 5281)^3*100 = about 495 watts OUT to turn the APC 12x8E at 9000 RPM
/ means to divide
^3 is raised to the third power
means to multiply

For the n100W from the Jeti file:
(9000 / 5243)^3*100 = about 506 watts OUT to turn the APC 12x8E at 9000 RPM

There is not a big difference between the n100W number given by APC and Jeti and the difference could easily be explained by elevation and/or air pressure differences.

This small difference does illustrate an important concept. The ‘lower’ or ‘smaller’ the n100W number is, the higher the output power is required to turn at the same RPM, and thus the higher the required input power.

RPM = 8000
APC 12x8E n100W = 5281
APC 13x8E n100W = 4798
Pout 12x8E (8000/5281)^3*100 = 348 watts OUT
That means somewhere between 409 watts in [85%] and 464 watts in [75%]
Pout 13x8E (8000/4798)^3*100 = 464 watts OUT
That means somewhere between 546 watts in [85%] and 619 watts in [75%]

The 75% and 85% efficiencies were picked for illustration purposes as they are somewhat typical for motors that would be turning this size prop.

I think that I confused Jack by converting the watts OUT [Pout] to possible watts in [Pin] using the combined motor and ESC efficiencies of 75%, 80% and 85%.

If a motor and ESC combination is 75% efficient, the Pout needs to be multiplied by the inverse of 75%. If a given number is 4, the Inverse of 4 is 1/4, that is 1 divided by 4, which not surprisingly is 0.25. 75% converted to a decimal number is 0.75. It’s inverse is 1/0.75 or 1.3333333

This 75% efficient motor and ESC combination requires
495 watts OUT * 1.3333333 or 660 watts in to turn the 12x8E at 9000 RPM
proof: 660 watts in * 0.75 [the efficiency] = 495 watts OUT

Using the same math yields the other watts in required for 80% and 85% efficient systems.
At 80% efficiency, 619 watts in is required to turn the 12x8E at 9000 RPM
At 85% efficiency, 582 watts in is required to turn the 12x8E at 9000 RPM

This demonstrates why watts in, the one we normally measure, is not used.

Hope this helps some Jack.
Jan 28, 2015, 12:13 PM
I am a nice guy! Really!
Thank you, Ken, for taking the time to clarify that. I had not noticed that n100w file in the APC data.

I am wondering why the APC data show that it takes 1.082 HP or 807 watts to turn the 12 X 8E prop at 9000 RPM static, and the calculation using the n100W figure is 495W.
Jan 28, 2015, 12:33 PM
Registered User
Ken Myers's Avatar
Yes, that was my question.
To me, the APC data is not holding up well in practice or even theory.

I am also wondering what iteration of a given prop the values are for.

I'm not trying to beat a dead horse, as the saying goes, but since they just recently reposted the data, it would have been nice to know that what is up there is for the most recent versions of the prop.

It is bothering me enough to keep me interested in this. I'm just trying to figure out what is going on.

It has been bothering me for more than a decade now.
Back in 2005, Philip Connolly, Rod Badcock and I exchanged a long series of emails on this 'problem'.
I ended up writing to APC and here is the reply that I received and forwarded to Phil and Rod.

From: [email protected]
Date: Thu, 11 Aug 2005 11:01:17 EDT
Subject: Re: 10x7E Thin-Electric questions
To: [email protected]

The original prop (15 grams) met with disfavor due to interference with the larger spinners. The subsequent design have the blades shifted forward to fix this problem. Other thin electric props have been modified as well. All of our stock is up to date. Hobby shop inventories, (two years or so), might still have the old style.

Fred Burgdorf
Last edited by Ken Myers; Jan 28, 2015 at 12:39 PM.
Feb 09, 2015, 03:16 PM
I ordered a few 12x6 APC style props from hobby King that are supposed to be for electric for my flyzone beaver DHC-2 and they are so much more thick than stock. Even the edges are not sharp. They are the tan colored ones. Does anyone know if these are ok to use. Should I sharpen them up a bit before balancing them???? Any knowledge on these props is welcome.
Feb 13, 2015, 03:00 AM
electric power,what else!
eJets's Avatar

Missing Prop Pitch for Shaft Power


This formula looks interesting for the shaft power - but I am missing the influence of the propeller pitch....

Originally Posted by Fourdan
Power (watt W) = rho Cp (D^5) (n^3)
J = V/(nD)
rho (1.225 kg/m^3), could depend of altitude, temperature, humidity ..
n (rps) .. rev per second or cycle per s or Hz (= rpm/60)
V = air velocity (m/s)
D = prop diameter (m)
Ct could be around 0.1
Cp could be around 0.05
according Chuck Gadd's the shaft power to turn a prop with a certain rpm is

P_shaft [W] = Pconst * (diameter/12)^4 * Pitch/12 * (rpm/1000)^3

Pconst according Motocalc (1.06....1.35)
diameter and pitch in inch
rpm in rev/min

this formula consider the pitch but does neclect the air density roh ...

cheers mike
Feb 13, 2015, 06:37 AM
Registered User
Ken Myers's Avatar
The pitch is included, or a part of, the Cp, coefficient of power.

The formula you note is from Bob Boucher's Astro Flight Electric Motor Handbook published in 1994, page 32. The factor, which you noted as Pconst (constant), changes with the prop manufacturer, series and even size in a given series of props.

The range you noted for the Pconst, which is a Pvariable (Pvar), is not broad enough. Today's APC 12x8E has a Pvariable of about 0.84.
Last edited by Ken Myers; Feb 13, 2015 at 08:41 AM.
Feb 15, 2015, 01:03 PM
Suspended Account
Hi Louis,

Following the recent talks about multi-blade propellers, I wrote an article on the subject.

I hope my english is not too bad !!!

Feb 15, 2015, 03:05 PM
Registered User
Fourdan's Avatar
Thread OP
Hi Guillaume
1) Your english is pretty good
2) My opinion for "optimizing anything" is first to consider a parameter axis (criterion)
Propulsive efficiency versus advance ratio is not always (IMHO) the best 2D space to consider
3) To decide if 2 or 3 or 4 or blades is the best solution, you have to make a complete reasoning and analysis of the "global mission" and to consider a "cost/value" analysis
with all the components (plane, prop, motor, battery ..)

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