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Watts delivered:

Old 12-17-2019, 06:08 PM
  #1  
RONALDO_UK
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Default Watts delivered:

Hi all -
I haven't bothered this site with silly questions for some time but here's another!

I have a 10oz depron trainer, 33" wingspan, with a direct drive 2-3S 2100 Kv motor. Using a 6x4 prop and a 2S 20-25C lipo, my wattmeter shows only 27 watts is delivered, and 65 watts with a 3S 20-25C lipo. I'm wondering why the figure is so low with a 2S lipo? An online calculator suggests a 6x4 prop is ideal with a 2S lipo delivering 6 amps and 42 watts power in.

Regards.

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Old 12-17-2019, 10:42 PM
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quorneng
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RONALDO_UK
Its not just the Kv of the motor alone that determines the amps it draws but also it size (diameter and length). What motor is it? Is the ESC rated for the amps required.
In addition the capacity of the battery does make a difference. A small (and old) battery is likely to suffer a bigger voltage drop when delivering amps then a big capacity one.
Finally are you sure you are actually getting full power from the ESC? It may need it throttle range resetting. The Tx stick may be full forward but the ESC may not be reading it as full power.

I like to test a motor set up using a servo tester as then I know the ESC is seeing the maximum possible 'full power' signal. Any less Watts when using the Tx/Rx then I know the full power signal is not getting to the ESC.
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Old 12-17-2019, 11:21 PM
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ron_van_sommeren
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A motor does not behave like a resistor R where current I is proportional to voltage U: I = U/R.

Power drawn is proportional to voltage cubed
Here power wants to increase by factor (3/2)= 3.3

Current drawn is proportional to voltage squared
Here current wants to increase by factor (3/2)= 2.2
current increase with one or two cells added, simple table

Both current and power drawn are proportional to:
#blades, pitch, Kv and diameter⁴
As you can see, small changes in setup can have huge efffects.

'Wants', because the above Ignores the extra voltage losses in battery, wires, controller and motor due to higher current.

Last edited by ron_van_sommeren; 12-19-2019 at 09:57 PM. Reason: added: link to current increase table
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Old 12-17-2019, 11:50 PM
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RONALDO_UK
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Thanks for your reply. The motor is a Hobbyking AX-1806N 2100KV, apparently good for 80W and it's a 10A ESC. I tried a few 2S 850mah lipos, some just a few months old. I'm just surprised by the wide difference between a 2S and 3S lipo.

Regards.
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Old 12-17-2019, 11:56 PM
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Thanks to you too Ron for your reply. As I said I'm just surprised by the wide difference between a 2S and 3S lipo. Perhaps I should stop worrying about it and use what works?! - in this case I think I'll plump for the 3S lipo which gives a lot more power than this plane needs. I know it's not an exact science and by the same token it's worth trying different props or even different makes of props.

Regards.
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Old 12-19-2019, 09:56 PM
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ron_van_sommeren
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Originally Posted by RONALDO_UK View Post
... As I said I'm just surprised by the wide difference between a 2S and 3S lipo. ...
See also this handy table
current increase with one or two cells added, simple table - RCG
a.k.a Why did my controller and/or motor go up in smoke?

Originally Posted by RONALDO_UK View Post
... in this case I think I'll plump for the 3S lipo which gives a lot more power than this plane needs. I know it's not an exact science and by the same token it's worth trying different props or even different makes of props. ...
Yes, you have to prop down if current is too high.

If current at wide open throttle is too high for the controller then reducing throttle to get current below ESC max.current is not a good idea. In fact, it is even harder on ESC than to keep overamping at full throttle. Better to prop down a bit to reduce current.
See:

Last edited by ron_van_sommeren; 12-20-2019 at 04:14 PM.
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Old 12-20-2019, 10:21 AM
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Just to elaborate a bit on Ron's point an ESC throttles the current by chopping the supply on and off. Reducing the throttle just means the off period is longer than the on. This means each individual power pulse can be close to the full power amps.
As far as the ESC is concerned it is too many amps that damages its electronics and it does so almost instantaneously.
Simply adding more battery cells improve a planes performance can prove to be expensive.
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Old 12-20-2019, 05:34 PM
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Thanks to everyone for the additional useful information.

I copied this plane design and the actual suggested motor was a Cheeta 2212-10 1400kv 3S with a 30A ESC, a 9x4.7 slow fly APC prop and a 1300mah lipo - it's supposed to be a docile trainer but that all seems a bit OTT to me for a 10oz plane? I'm reasonably sure it will fly just fine with my configuration - just waiting for our field to dry out which could be next Spring!

Regards.
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Old 01-18-2020, 07:13 PM
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AEAJR
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How is your plane Doing.

I use ecalc prop calculator to model my power system combinations and I find the final results match the prediction very well.
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Old 01-18-2020, 11:23 PM
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Posted in error
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Old 01-19-2020, 06:13 AM
  #11  
RONALDO_UK
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Thanks for asking but I don't actually know! - yet. Since my post we've had a helluva lot of rain here in the UK (I'm just south of London) and our flying field has been unusable. It's approached 200m up a track which tends to get flooded in a few spots after heavy rain. But in fact our entire site has been waterlogged and it may be several weeks before it's usable again. I've thought about going to the local park although it's not strictly allowed - next week is forecast to be calm and dry (but very cold) and I may be tempted.

This hobby sorely tests your patience!

Regards.
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Old 09-01-2020, 09:53 AM
  #12  
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On my initial flight with this plane the wings folded! - the wings were two joined pieces as per the plan but this was simply to maximise cuts from the depron sheet - so I remade them as one piece. A combination of weather and covid meant I waited seven long months (until yesterday) for my second flight! Because I didn't use the suggested (heavy) motor I added 50 gms weight to the nose on a plane that only weighs about 200 gms! - so total weight with lipo was about 300 gms. It flew! - like a docile trainer which is exactly what it's meant to be, but it was still a little tail heavy. I used a AX 1806N 2100kv motor with a 6x3 prop and the suggested motor is a Cheeta 2212-10 1400kv with a 9x4.7 slow fly prop - so a helluva difference, not least the weight. I may change the motor if only to eliminate the ugly weights on the nose.

Regards.
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