MotoCalc accuracy?

As you may know, I'm new to the brushless/LiPo world. I have jumped in head first and am floundering around happily so far (and will be happier yet when
spring weather gets here to upstate NY).
I've been playing around with MotoCalc, plugging in the numbers for my RCer Warp 4 4T motor, TP 2100 LiPo, Phoenix 25 ESC, ElectroStreak airframe and various APC and Master Airscrew electric props.
I'm finding that when I run static tests with my Medusa Power Analyzer, I'm getting much higher amperage than the MotoCalc estimates. For example, using a TP 2S 2100mah 15C LiPo and an APC 8x6 electric prop, MotoCalc shows full throttle motor amps at a maximum of 20.7. In my static testing, I got up over 25 amps (over the 22amp recommended max. continuous for my motor) before I get to 100% throttle.
I understand that MotoCalc results are estimates, but I was hoping the figures would be closer than +/- 20%. Do I have unrealistic expectations? Maybe I'm doing something wrong in MotoCalc?
I have a 2S and a 3S 2100mah LiPo, and I've had to order props three times to find a pair that will work well with the two batteries.
Thanks for any insights, desmobob
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desmobob wrote:

It's been ages since I used MotoCalc, but chances are they're giving you the estimated current in the air, after the prop unloads. While I think there are now gizmos that will monitor motor current in the air, you can just fly the thing hard for a few minutes, land, and put your finger on the motor to see if it's getting to hot (hint -- if the end of your finger turns red and shiny, the motor's too hot).
--

Tim Wescott
Wescott Design Services
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On Sun, 19 Mar 2006 03:19:20 GMT, "desmobob"

Without attempting a complete error budget, just look at an obvious factor. I presume the program usies nominal voltage in its calculation, i.e., 3.7V/cell for LiPo. A fully charged pack should be 4.2V/cell, 13+ % more, and current would be higher by that same proportionality. That leaves a bit in excess of 6% error to account for - probably well with the sum of tolerances for the props, resistance of motor windings, and air density, not to mention the accuracy expected in any projection of how much power the prop absorbs while it is mostly stalled (hey, it is a static case you are looking at). Seems to me MotoCalc did pretty good - but then my background is Elex Eng. A tolerance of 20% is par, you want anything better than that, you pay for precision components!
Abel
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desmobob wrote:

No, but there are many tolerances and things you need to understand
1/. Temperature variation. The colder it is the lower resistance the motor will present: This makes it more efficient.
2/. Air temperature, humidity and pressure. May get +-5% on this alone.
3/. Motor data itself. get a slightly weaker batch of magnets and KV will be higher, efficiency a bit less, and current draw a lot more. anyway,where does the data on motors come from? The manufacturers? Ah...well..
4/. Timing. Aggressive timing can add up to 15% to the current draw. All ESC's are not created equal.
5/. Prop constants. Again these are fudge factors. Ive seen some results on SF props that SUGGEST a prop constant of TWO not one point something, is appropriate. The difference between e.g. a speed prop and a SF prop and a folder is huge. I had one model that simply barely flew on an MA 8x6, but soared away like a bird on an APC 8x6E.
6/. Batteries. These can vary wildly in both voltage, depending on the make, the temperature and the point in the discharge curve, and their effective series resistance - its non linear, highly temperature dependent as above, and varies between cell types alarmingly..
That is the bad news. That motocalc can be well out until and unless you set about correcting the _data_ using real world measurements from your setup.
HOWEVER the GOOD news is that once you do, its incredibly effective at predicting the VARIATION when you substitute a different pack, gearbox, prop or motor wind.
Armed with a tacho and a whattmeter, and a few other things, you can cross check predictions and see what you are really getting.
Feed that back into the data, and your initial predictions will be much better next time.
Stefan does his best, but he can't personally test each prop at all RPM for thrust, and power ratings..nor every motor for its constants - manufacturers are often very shy of giving these, preferring to say things like '80% efficiency at 12A' and 'suitable for 10v operation' and the like, all of which is relatively obscure..
Not one manufacturer gives perhaps the most important figure - safe operating temperature, or the second most, thermal resistance in degrees per watt, and associated curves for the variation in that with the airflow over the motor.

Ask on the Ezone and you will find what you seek.
Several people spend a lot of time testing motors, and props and coming up with more accurate and usable combos.

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Thank you, gentlemen.
Good flying, desmobob
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