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Jul 26, 2010, 12:02 PM
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Question

Model flys fine but wattmeter says no


Help guys,
I am trying to select a setup for a blackhorse Mohawk. I have a Turnigy 35-36c 1100kv in an E-flite Stearman and an e-flite taylorcraft on APC e 10x5 props and 30A esc's and they fly really well on them. I thought I would use two more of the same motors with 40A esc's and that would do me.

As I am doing a bit more in electrics now I thought I would get me a Watt meter. When I checked WOT on the Stearman it told me I was pulling 55A and the watts were up near 500W is this right? I don't fly the stearman flat out often but I have thrashed it sometimes.
Why hasn't the esc blown? and can I really be pulling 55A on a 10 x 5?

Is it because the amps in a static test are more than when flying?

Or have I got a dud meter?

Oh forgot to mention I'm running a 1800 3s in the taylorcraft and 30003s in the Stearman

Gonna go for 5000 3s for mohawk

Help!!!!
Last edited by Ospreyman; Jul 26, 2010 at 12:09 PM.
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Jul 26, 2010, 12:17 PM
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eye4wings's Avatar
Yes, the Amps read in a static test are always more than when flying - for the reason that your prop is trying to accelerate static air to flying speed. In the air the model is moving forwards through the air and meeting the oncoming air with less differential input to output. This is what we normally refer to as 'offloading'.
I couldn't tell you if it would account for that much difference. Only a data logger will tell you that.
Jul 26, 2010, 12:28 PM
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Thanks, That makes sense. I know that I.C motors unload in the air. By the way we are relative neighbours. I live in Cheshunt
i'm beginning to wonder what use a watt meter is ( sorry about the pun)
Jul 26, 2010, 01:45 PM
Registered User
I believe I've seen a figure of up to 10% mentioned for the amount of unloading -- certainly it won't be enough to get your measured 55A down to the 30A rating of your ESCs

I've seen mention on the HK site of people using this motor with a 4-cell A123 pack and an 11x6 prop and only getting "nearly 30 amps", and the motor's only rated for 35A burst, so something seems to be wrong with your reading.

Which wattmeter have you got? Are you connecting the battery to it first, and letting it self-calibrate before you plug into the model and start the motor? Not all require that, but mine does.
Jul 26, 2010, 01:46 PM
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manuel v's Avatar
Your test is only with one motor?

Is very high.

Check how many RPM are.


Manuel V.
Jul 26, 2010, 02:28 PM
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Dont have a rev counter so I cant check that. The Wattmeter is called an ETRONIX "power analyzer" see link

http://www.cmldistribution.co.uk/cml...tId=0000004442

abenn, I tried calibrating to zero with lipo conected first and tried it again.
Volts are showing as 12.08 WOT amps now peaked at 60A Watts peaked at 600 I held it there for 2 seconds . This can't be right can it. Ok the stearman flies nicely in a scale fashion at half thottle. At half thottle the amps show about 20A then it really ramp up as I open it up
Last edited by Ospreyman; Jul 26, 2010 at 02:37 PM.
Jul 26, 2010, 03:48 PM
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Bruce Abbott's Avatar
Quote:
Originally Posted by Ospreyman
When I checked WOT on the Stearman it told me I was pulling 55A and the watts were up near 500W... WOT amps now peaked at 60A Watts peaked at 600
500W / 55A = 9.1V
600W / 60A = 10.0V

What was the voltage under load? If your battery is struggling to hold 10V then the motor probably is drawing 55-60A!

However that combo should not pull more than 30A, so something is definitely not right. What readings do you get on the Taylorcraft? What happens if you try powering the Stearman with your 1800mAh battery?
Jul 27, 2010, 10:01 AM
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I'll try the T craft when I get home and the the 1800 lipo, thanks. I'll keep you posted
Jul 27, 2010, 02:00 PM
Registered User
One way to get some kind of a check on your amps is to see how many mAh you put back into your pack after a timed flight.

The 3000mAh pack in your Stearman, for instance, should be able to provide 30A for 1/10 hour (6 minutes). If your normal flights are longer than that, and if you're only using, say, 80% of your pack's nominal capacity, your average amp draw is less than 25A. If your throttle stick is normally at about 50% during these flights, then maximum amps at WOT may possibly be somewhere near the 55A you've measured. Substitute your actual flight times, and throttle stick position and mAh, in place of my assumptions, and see if that tends to support or disprove your wattmeter reading
Jul 27, 2010, 03:15 PM
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Thread OP
Abenn,
That is an interesting thought, I normally fly the Stearman at around half throttle as you say and I regularly return 13 minute flights. I VERY rarely use WOT as with this motor the speed of the AC is way above scale. I normally fly until the speed bleeds off and I have to start applying more throttle for the same speed. I will time the next flights and make a note of the pack depletion ( with the meter) and see how i get on.

Just out of interest I put the Watt meter on an electric glider I have fitted with a stock 600 can brushed motor, 30A esc and 3300mAh 7 cell NiMh pack. I got the following reading at WOT:

Volts under load 8.8v
Watts 500+
Amps 55 ish

Now I run this model flat out for 3-4minutes to gain height then glide a bit then repeat the process. After a flight I can just feel the slight warmth of the Batt through the ABS fuselage. Surely I would have blown the esc if I was truly pulling 55 odd Amps for 4 minutes? and can a stock 600 can produce 500 Watts. The model is a perkins greensleeve with a 6 x ? folding prop.
Jul 27, 2010, 06:17 PM
Planes from 1oz to 55lb's
simo10's Avatar
Your meter sounds faulty. Also, a 7 cell NiMH battery could never produce 8.8 volts under load.
Jul 28, 2010, 01:04 AM
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Yes thats my thought at the moment too. This would throw everthing else out as well
Jul 28, 2010, 09:04 AM
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just a thought, Could my esc timing be too high or too low?
Jul 28, 2010, 01:12 PM
Registered User
Unlikely, because your motors are running well. But anyway, it shouldn't affect your wattmeter readings.
Jul 28, 2010, 01:32 PM
...design-build-fly-publish...
eye4wings's Avatar
If you're passing through Ware you could drop in and we can check your model with my Astro wattmeter. You would need to get some adaptors if you use Deans as I am standardised on 4mm bullet connectors.
PM me if you want to try it.
It would at least give you an idea of how far out your meter is.

Also you may well know about the NLMFC electric fly-in at Baldock on the 19th Sept. A good meet I always try to get to.


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