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Old Jul 09, 2009, 04:24 AM
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Data
Measuring motor power output

Power output can be determined by running the motor with a calibrated "stick" fitted in place of the propeller, and measuring the rpm.

The power is calculated thus: power [watts] = B x [rpm/1000]^3

B is the power characteristic of the stick. It depends on the stick length, and it can be read from the attached graphs.

Sticks are made from nominal 6mm plywood, 26mm wide. Precise thickness of those used to generate the graphs was 5.8mm [.232"].
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Old Jul 09, 2009, 06:50 AM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
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Very impressive. Eliminates the hassle of trying to locate "standard" props and dealing with mfg variation and thrust stands.

What is the main resistance factor? Aerodynamic? Better to use a square one? Do holes effect readings? Better to trim one end instead? Can other thicknesses be used? Too many questions?
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Old Jul 09, 2009, 09:27 AM
jrb
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Neat!

How do I find the "B" for my stick; I need a 1KW load?
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Old Jul 09, 2009, 10:38 AM
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I was thinking of matching his "stick" dimensions and use his B values.

But let me ask a question--
Is B only dependent on stick length? Or is it the length of a 5.8mm thick, 26mm wide stick?
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Old Jul 09, 2009, 10:59 AM
Old Submariner
United States, WI, Milton
Joined Dec 2007
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Load ?

I know I am dense but where does the load come from ? The stick being symetric with no airfoil to "grab" air is simply running up to speed at any given RPM and consuming power (load). The same RPM with a propellor of the same diameter would consume more power because of the aerodynamic load as the propellor is actually doing "work", e.g. attempting to move the mass ( airplane or work bench up to theoretical distance per revolution- pitch) .

Please don't misunderstand, I am not being critical of the procedure, I am just trying to understand the application.

Thanks,

Tom
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Old Jul 09, 2009, 04:54 PM
homo ludens modellisticus
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near Nijmegen, Netherlands
Joined Feb 2001
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These 'props' don't give an airflow for cooling. Extra cooling fan needed during testing?
IIR correctly, one of the German firms has/had a set of $$$ calibrated metal dummy props.

Vriendelijke groeten Ron
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Old Jul 09, 2009, 05:27 PM
Carlos Reyes
Albuquerque, NM
Joined Jul 2007
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This is a great idea, but I wonder how repeatable it is to make the sticks of wood all the same size and weight.

My Top Flite Precision Magnetic Balancer says to use a Top Flite 11x8 Power Point propeller as a pre-calibrated load so that it can be used as an anemometer (wind speed meter).

Might such a propeller yield more consistent results?


Carlos Reyes
Author of RCadvisor's Model Airplane Design Made Easy and RCadvisor's ModiFly
www.RCadvisor.com founder - Brainy calculator, so you don't have to be.
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Old Jul 09, 2009, 08:16 PM
Space Coast USA
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It has a 6mm x X area beating the air and to me is like sticking a straw out the window of a car going 60mph....does it present a load? It's trying to move the air radially instead of back.
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Old Jul 10, 2009, 03:48 AM
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On the attached graph, the red spots represent two real propellers, a Master GF 9 x 6, and a 6 x 4.

The green curve represents a "virtual continuum" of datum propellers scaled from the two real ones. All with the same p/d, namely .67.

A 7.7" stick puts a load on the motor equivalent to the Master 9 x 6.

For any one stick length, this provides us with a datum propeller, of known diameter and pitch. Knowing rpm and pitch, we can evaluate pitch speed.

When we come to select real props, we don't have to stay with p/d of .67. A pitch variation of about +/- 15% will be possible for any one "B" value.
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Old Jul 10, 2009, 04:44 AM
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Antony (France)
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Hi
Anyway the first problem is to "calibrate" a stick or a prop
Using a dynamometric bench
In my tests I often noticed that the exponent 3.0 is not the best
Generally 3.1 to 3.2 is a better fit
Louis
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Old Jul 10, 2009, 07:35 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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Sorry Perter, but I still measure motor power as, 'wow', 'not bad', and 'damn, should have fitted the other one'.

No offense to those who love their calculations, and many do these days; long may your calculator batteries last, (nearly wrote, long may your slide rules slide, it's age thing ), but I'd rather be flying with a hand full of propellers.
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Old Jul 10, 2009, 10:00 AM
Carlos Reyes
Albuquerque, NM
Joined Jul 2007
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I went to the Master Airscrew website. This is what I found:

MA0960, 9x6 G/F Series Propeller - Now with narrower blades for a bit more rpm

Of course it has to be the one propeller that they've recently improved! The 6x4 still looks like a safe bet.

Thank you. This is useful information.

Carlos Reyes
Author of RCadvisor's Model Airplane Design Made Easy and RCadvisor's ModiFly
www.RCadvisor.com founder - Brainy calculator, so you don't have to be.
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Old Jul 10, 2009, 11:02 AM
jrb
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It’s interesting!

This subject along with what’s more efficient a geared in-runner or out-runner comes up very frequently.

As a guy who loves measurements, testing, and building things; I find myself getting caught up in these discussions.

But I’ve about reached the point of “who cares”!

The state of E-Flight is quite different from when I started 10+years ago – newbies & the industry have stuff today that owe a debt a gratitude to the many pioneers of E-F. Opinions will very but this list begins with Astro Bob and includes other names like Bourke, Cimato, Neu, Peterson, Wang and others!

And, w/o this great website the pace of E-F development would have not been as fast – Thanks JB!

Back to the key point we’ve got enough good/great stuff that we no longer need every last % of performance just to fly.

There will always be a better battery, ESC, motor, or prop than the one you got!

But, unless you got a real and very specific need “who cares”?

Go fly -- have fun!!!

Tell us what works, like adding to this thread: http://www.rcgroups.com/forums/showthread.php?t=203529 .

If you have a specific need, then there’s probably a type of test or piece of equipment that is better suited than just a generic dyno, thrust stand, or Whattmeter.

After all what average park flyer need to have the most efficient components to enjoy his time in the air.

Even those of us who like flying bigger stuff can find very good and virtually PnP set-ups.

For those that need a dyno/info this article maybe of interest:

BOL!

Jim
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Old Jul 12, 2009, 06:35 AM
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The attached graph shows the performance characteristics of a typical motor, obtained from a dyno test.

The green column is the speed band 70% to 80% of no-load speed. When you fit a prop which gives [at WOT] a speed within this band you can be sure that the motor is operating somewhere near its optimum. This applies to all motors, not just this one.

I have marked a point on the power-out curve which I choose to call the motor power rating. It is, approximately, the the highest power which can be used continuously without overheating [given normal ventilation].

I have never seen this important performance point mentioned in any avertisements, even though it is easily determined. In fact, I've never seen any mention of output power. It is always misleading input power which is given prominence.

You can evaluate this rating for any motors you posess. The only equipment required is a tacho and a few "sticks". And you must apply the data in post #1.
--------------------------------------------------------------------------

We now have a meaningful power rating and a corresponding pitch speed for the motor+prop combo. This is the essential data to match motor-to-model.
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Old Jul 12, 2009, 08:30 AM
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Near Kingston, Ontario, Canada
Joined Dec 2004
378 Posts
How to make a Dynamometer

For those of you interested in playing around with your motors and actually measuring motor performance you can get complete details on how to build your own dynamometer in the August issue of RC Sport Flyer magazine
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