Measuring motor power output - RC Groups
Jul 09, 2009, 05:24 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"].

Images

Last edited by peterangus; Jul 09, 2009 at 09:22 AM.
 Jul 09, 2009, 07:50 AM 7000mw of raw power! 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?
 Jul 09, 2009, 10:27 AM Member Neat! How do I find the "B" for my stick; I need a 1KW load?
 Jul 09, 2009, 11:38 AM Space Coast USA 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?
Jul 09, 2009, 11:59 AM
Old Submariner

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
 Jul 09, 2009, 05:54 PM homo ludens modellisticus 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
 Jul 09, 2009, 06:27 PM Carlos Reyes 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.
 Jul 09, 2009, 09:16 PM Space Coast USA 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.
Jul 10, 2009, 04:48 AM
Registered User
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.

Images

Last edited by peterangus; Jul 26, 2009 at 11:05 AM.
 Jul 10, 2009, 05:44 AM Registered User 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
 Jul 10, 2009, 08:35 AM An itch?. Scratch build. 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.
 Jul 10, 2009, 11:00 AM Carlos Reyes 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.
 Jul 10, 2009, 12:02 PM Member 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: https://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
Jul 12, 2009, 07:35 AM
Registered User
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.
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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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Last edited by peterangus; Jul 12, 2009 at 10:32 AM.
Jul 12, 2009, 09:30 AM
Registered User

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