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Old 11-12-2014, 09:11 PM   #1
KeninAZ
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Default Bench testing vs. ECalc watts

OK, I had a suggestion from a friend more experienced than I to step up to a 14" prop for a new plane I have ordered. I was going to use a 12x8e APC on a Turnigy G60 and bench tests looked good for it.
So I picked up some 14x8.5e and just ran the test on a fresh 6S and my meter is showing me 1455 watts.
EClac says I should have almost 1100 watts.
Why is there such a difference in the power shown?
Yes I am using the correct altitude, cooling, prop maker/model and motor maker/model too with ECalc.

OK, so what am I not understanding?
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Old 11-12-2014, 10:21 PM   #2
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Originally Posted by KeninAZ View Post
OK, I had a suggestion from a friend more experienced than I to step up to a 14" prop for a new plane I have ordered. I was going to use a 12x8e APC on a Turnigy G60 and bench tests looked good for it.
So I picked up some 14x8.5e and just ran the test on a fresh 6S and my meter is showing me 1455 watts.
EClac says I should have almost 1100 watts.
Why is there such a difference in the power shown?
Yes I am using the correct altitude, cooling, prop maker/model and motor maker/model too with ECalc.

OK, so what am I not understanding?
Unloading.

You are doing your tests with the motor unable to move anything but the prop! Later on when it is attached to the plane - the plane will move and when it does the prop (and motor) will unload - dropping the watts from the 1455 down to the expected 1100 in real flight conditions.

Like picking up a weight that is snagged on something - use considerable more energy trying to lift the snagged weight as you do lifting the free weight. Same principle here.

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Old 11-12-2014, 10:40 PM   #3
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Ah so, makes sense and thanks for the excellent reply.
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Old 11-13-2014, 12:09 AM   #4
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Originally Posted by KeninAZ View Post
OK, I had a suggestion from a friend more experienced than I to step up to a 14" prop for a new plane I have ordered. I was going to use a 12x8e APC on a Turnigy G60 and bench tests looked good for it.
So I picked up some 14x8.5e and just ran the test on a fresh 6S and my meter is showing me 1455 watts.
EClac says I should have almost 1100 watts.
Why is there such a difference in the power shown?
Yes I am using the correct altitude, cooling, prop maker/model and motor maker/model too with ECalc.

OK, so what am I not understanding?
Which Turnigy G60 are you using?

There are three different G60's per motocalc: G60-300KV, -400KV, and -500KV.

I ran the G60-500, 6S LiPo, 14X8.5 prop through www.motocalc.com, and got 1430 Watts. Motocalc gives a warning on running this motor at this current level, its going to run danged hot.

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Old 11-13-2014, 12:48 AM   #5
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Actually I regularly had different numbers from the calc programs. Sometimes quite dramatic as you show. They are very much just approximations as there are just too many variables to be super accurate.

Mike
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Old 11-13-2014, 12:51 AM   #6
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Originally Posted by kyleservicetech View Post
Which Turnigy G60 are you using?

There are three different G60's per motocalc: G60-300KV, -400KV, and -500KV.

I ran the G60-500, 6S LiPo, 14X8.5 prop through www.motocalc.com, and got 1430 Watts. Motocalc gives a warning on running this motor at this current level, its going to run danged hot.

That's almost funny. I am running the G60-500kv with a 6S and with the APC 14x8.5e prop eCalc shows me @1092.6 watts. But I don't think this took into consideration the fully charged battery?

Lots to learn still.
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Old 11-13-2014, 01:23 AM   #7
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Originally Posted by KeninAZ View Post
That's almost funny. I am running the G60-500kv with a 6S and with the APC 14x8.5e prop eCalc shows me @1092.6 watts. But I don't think this took into consideration the fully charged battery?

Lots to learn still.
Yup

This is why you absolutely need to read your Amps/Volts/Watts with a wattmeter after you've decided on a prop and battery pack for your motor.

There are a LOT of variables that programs like motocalc and ecalc can not account for. One of them is the prop itself.

Don't take much difference in the prop to make for a 30% difference in watts input. Or for that matter, the battery itself, not to mention just how accurate the motor specs are.

If you should pick up one of the Castle Creations ESC's with data recording, you can actually graph out everything on your computer after a flight. This is interesting information to have, but for the most part, if it flies well, who needs the graphing???

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Old 11-15-2014, 07:58 PM   #8
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Some calculators give you an input field for the charge level of the pack. You can put in fresh or full, partial, etc.

Not all 14X8.5 props are the same. If you picked a prop from the list that is a different brand you can expect a different result.

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Old 11-15-2014, 08:44 PM   #9
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Originally Posted by AEAJR View Post
Some calculators give you an input field for the charge level of the pack. You can put in fresh or full, partial, etc.

Not all 14X8.5 props are the same. If you picked a prop from the list that is a different brand you can expect a different result.
I just ran the numbers on my Hyperion Zs3020-10 922 KV motors.

On a five cell A123 pack, and an APC-E 10-7 prop, results measured out as 35 Amps, 450 Watts, 13.6 Volts, 10800 RPM.

Motocalc predictions are 39 Amps, 503 Watts, 13.3 Volts, 11300 RPM. That's within some plus/minus 10%, which IMHO is pretty good.

Before retiring, we had analog high voltage circuit breaker controls that were rated at plus/minus 10% tolerance on trip out values, timings, Instantaneous trip on very high fault currents, reclose times and so on. It was a real problem keeping this accuracy on the production lines.

Now those controls are all computerized, making it possible to maintain one percent accuracy.

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Old 11-18-2014, 06:18 PM   #10
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You have to realize that ecalc is strictly a theoretical calculator. It's all a mathematical analog of a physical process and it uses manufacturer published (fictional) specs. MotoCalc and Drive Calculator are based on actual test results and will give you much better numbers. All of them are based on static testing data and in-flight data logging might give you a different set of numbers altogether.

Even the best prediction you can get doesn't begin to replace actual testing as you've done to find out what your system really does.
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Old 11-19-2014, 02:34 AM   #11
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Originally Posted by rcers View Post
Actually I regularly had different numbers from the calc programs. Sometimes quite dramatic as you show. They are very much just approximations as there are just too many variables to be super accurate.

Mike
Originally Posted by Rockin Robbins View Post
You have to realize that ecalc is strictly a theoretical calculator. It's all a mathematical analog of a physical process and it uses manufacturer published (fictional) specs. MotoCalc and Drive Calculator are based on actual test results and will give you much better numbers. All of them are based on static testing data and in-flight data logging might give you a different set of numbers altogether.

Even the best prediction you can get doesn't begin to replace actual testing as you've done to find out what your system really does.
+2

At best the calc programs will get you into the ball park. In my experience your pretty lucky to get within 20% of actual measured numbers.

For starters, the numbers you get from motor mfgs are often just invented for advertising purposes and no testing is actually done. Build quality from one motor to the next can vary hugely. This is especially true for the cheaper motors.

Even name brands have specs that can vary a good bit. Ive seen as much as a 15% variation between two motors of the exact same brand and model number form higer end motors. On the low end stuff the variations can be huge.

Then add in battery performance that can vary dramatically and props that can also be way off from the printed specs.

Its actually pretty amazing that the calcs get as close as they do

I think I need a signature.
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Old 11-19-2014, 03:58 AM   #12
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Originally Posted by Larry3215 View Post
+2

At best the calc programs will get you into the ball park. In my experience your pretty lucky to get within 20% of actual measured numbers.

For starters, the numbers you get from motor mfgs are often just invented for advertising purposes and no testing is actually done. Build quality from one motor to the next can vary hugely. This is especially true for the cheaper motors.

Even name brands have specs that can vary a good bit. Ive seen as much as a 15% variation between two motors of the exact same brand and model number form higer end motors. On the low end stuff the variations can be huge.

Then add in battery performance that can vary dramatically and props that can also be way off from the printed specs.

Its actually pretty amazing that the calcs get as close as they do
Again, that is the reason it is very wise to pickup and use a wattmeter for your particular installation. For what appears to be a minor change in prop diameter, or cell count can make a huge differece in the watts and amps pulled by your motor.

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Old 11-20-2014, 02:06 PM   #13
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Calc programs are like climate, it is what is generally expected or predicted.

Weather is what is waiting for you outside the door.

Climate won't get you wet and Calc programs don't fly your plane.

Actual power consumption is what flies your plane and, if you don't size it properly, burns up you components.

A wattmeter tells you about the weather, not the climate. It tells you what you have based on the components in your hand. A wattmeter can confirm your expectation or correct them.

Calc programs, like maps, tell you how to get there.
The wattmeter, like the GPS, tells you where you really are at this moment.

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Old 11-22-2014, 05:31 PM   #14
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As my friend who graduated from MIT with an aeronautical engineering degree is frequented to say "empirical data trumps theoretical data every time". Which is also why I do not use any programs that calculate power use in model airplanes. Watt meter, tach, servo tester, pulse width displayer, and bench testing for the real information. I have learned that you can count on approximately 30% reduction in power usage when the plane unloads in the air. That's not a guess that's based on years of testing.
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Old 11-22-2014, 05:34 PM   #15
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And that 30% figure for unloading is pretty close to what ECalc comes up with vs. what I read on the wattmeter on the bench.
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Old 11-22-2014, 06:21 PM   #16
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Originally Posted by KeninAZ View Post
And that 30% figure for unloading is pretty close to what ECalc comes up with vs. what I read on the wattmeter on the bench.
This is where those ESC's with data recording works so well. My CC ESC's also show a 30% reduction in flight. Depending on the model onyhow.

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Old 11-22-2014, 08:49 PM   #17
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Ken, I would like to know how you are selecting the prop for your bench test. Here is how I do it. I first check the "continuous current" spec. for the motor to be tested. A lot of motors are advertised with the max current spec. which to me is useless. In general it is only good for 15 seconds before you risk burning up the motor. If you only have a max current spec then multiply it by .8 (or 80%), again over time I have learned that continuous current is approximately 80% of max current. This is the upper limit of where the motor can operate. Now with the continuous current spec start propping the motor and with the servo tester and the tachometer and the pulse width monitor between the ESC and the servo tester start bringing up the RPM. Oh, and also have the watt meter connected between the battery and the ESC. Monitor the current value on the watt meter. If it reaches the max continuous current before the pulse width monitor is at max 2 mils the you are over propped. If you reach max 2 mils pulse width before you reach max continuous current then you are under propped. Ideally you want to get to max continuous current at the same point you reach full throttle as indicated by the 2 mils reading on the pulse monitor. Once you have a prop size that gives you that you can now play with pitch and diameter to get the performance you are looking for on your plane.

In R/C I have learned that getting close counts, but I also fly control line aerobatics and there you want to be on the money for power usage so you can size the battery for the plane and use the smallest (read lightest) battery available. I find R/C fun because it is so much easier to assembly power systems that work and give the performance and flight time I want. In control line aerobatics it's a whole different story. I have a cabinet full of motors, ESCs, batteries and props so I can mix an match to get the right power/weight combination. Hope this helps.
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Old 11-23-2014, 02:50 AM   #18
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Originally Posted by abborgogna View Post
Ken, I would like to know how you are selecting the prop for your bench test. Here is how I do it. I first check the "continuous current" spec. for the motor to be tested. A lot of motors are advertised with the max current spec. which to me is useless. In general it is only good for 15 seconds before you risk burning up the motor. If you only have a max current spec then multiply it by .8 (or 80%), again over time I have learned that continuous current is approximately 80% of max current. This is the upper limit of where the motor can operate.
A very rough rule of thumb on these electric motors is a rating of about 100 watts per ounce of motor weight. Just about any quality brushless motor can handle that and not overheat, assuming the proper number of cells for the battery pack.

So, if your motor weighs 11 Ounces, you can run it at 1100 Watts. If your motor weighs 11 ounces, and the mfg specs it at 2000 Watts, word to the wise.

Again, if you can hold your fingers on your motor for 15 seconds after a flight without branding the motor's brand name into your finger tips, the motor is running safely.

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