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Where have my Watts gone?

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Graham Davies 328/09/2020 15:25:26
129 forum posts
49 photos

Afternoon all,

Just putting the finishing touches to a biplane I have very loosely based on an old plan for a Viva. It's scaled up slightly to around 48" span, and lots of things changed to my preference. I went with a very similar power train to my WR Tempest (4250-600kv, 70A ESC 4s 3700MAh pack, 14x6 prop). In the Tempest, I get around 750W, around 50A, plenty of power and 6-8minute duration.

The nearest I could get for this model was a 4250-560kv from Ebay, but otherwise everything is the same. HK did not have the same generic ESC, so I have a redbrick one, but pretty much as is.

Running this up, I can't get more than 400W, even with a 14x8 up front. I have tried re-setting the ESC, changing the timing, etc, but no real noticeable difference. I don't have a tacho, so can't comment on the RPM, but certainly less on the 14x8 than the 14x6, so it is not underloaded.

Should I be checking anything else, or is the motor a Pup?

Cheers all

Graham

Graham Davies 328/09/2020 15:29:46
129 forum posts
49 photos

Picture of the Viva-alike for attention!

viva 270920-1.jpg

Shaun Walsh28/09/2020 15:47:15
440 forum posts
50 photos

Have you tried swapping the ESCs over to see if that is the problem?

Graham Davies 328/09/2020 15:53:21
129 forum posts
49 photos

Hi Shaun,

Hoping to avoid it if possible; it's a bit 'plumbed in' in the Tempest!

Graham

Gary Manuel28/09/2020 16:00:10
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2308 forum posts
1559 photos

You say that you have re-set the ESC. Do you mean calibrated it? If not, try calibrating it.

Graham Davies 328/09/2020 16:09:56
129 forum posts
49 photos

Hi Gary,

Not sure how to calibrate it, as such. I found a manual for it and there is a load of selectable options. It appears to be set to automatically define the high and low throttle positions. I have tried changing timing options for various pole motors. Nothing seems to make much, if any difference.

I agree though; I'd like to see a procedure for setting the power in percentage terms at different throttle positions.

Graham

Gary Manuel28/09/2020 16:29:44
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2308 forum posts
1559 photos

Graham.

It sounds like you already know this but ....

Calibration usually involves powering up Tx and then Rx with throttle at max. Wait for first set of beeps then reduce throttle to minimum. It should beep to acknowledge this. Then power TX and RX down and power back up as normal.

HERE's a link to a generic ESC manual from Hobbyking Red Brick 70A files. Not easy to follow but confirms the process I've described above.

MattyB28/09/2020 16:37:27
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2320 forum posts
43 photos
Posted by Graham Davies 3 on 28/09/2020 15:25:26:

...Running this up, I can't get more than 400W, even with a 14x8 up front. I have tried re-setting the ESC, changing the timing, etc, but no real noticeable difference. I don't have a tacho, so can't comment on the RPM, but certainly less on the 14x8 than the 14x6, so it is not underloaded.

Should I be checking anything else, or is the motor a Pup?

Have you calculated on Ecalc what the output is estimated to be on that setup? If not it might be worth doing to check if something really is amiss or not.

Assuming it is well below what is calculated for me it's much more likely to be an ESC issue than the motor. Do a throttle calibration as per the manual as a first trouble shooting step, and check you are getting a full range of throttle output from your TX. If that doesn't work you will probably have to start swapping out components, starting with the ESC.

Edited By MattyB on 28/09/2020 17:04:00

Simon Chaddock28/09/2020 16:39:08
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5794 forum posts
3063 photos

Graham

The fact that you are using a lower kV motor will 'loose' some of your Watts.

The power absorbed by a prop is roughly proportional the square of the speed so (560 x 560)/(600x600) = 0.87. So the lower kV could account for maybe 13% of your loss.

This of course assumes that the motors are otherwise internally identical and the kV figures given by the manufacturer are accurate in the first place. wink 2

Graham Davies 328/09/2020 16:47:44
129 forum posts
49 photos

Thanks all,

I am suspecting something about the ESC, but may just be a setting.

The motor is rated at 56A max, so should be comfortably good for a lot more than I am getting. I made an RPM/ power calculator, so have a good idea what I should be seeing.

I am going to try to rest the ESC. If that fails, I will swap out the Tempest ESC, but that may have to wait as I plan to fly this tomorrow!

Graham

Shaun Walsh28/09/2020 17:10:12
440 forum posts
50 photos

Have you tried increasing the prop diameter and monitoring the current drawn?

Bob Cotsford28/09/2020 18:34:57
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8794 forum posts
494 photos

Turnigy do a 4250 500W and that shows 410W in Ecalc using your 4S and 14*8. I'd check the motor ID! If it is 500kv then you would need something like a 16*10 to get above 600W.

PeterF28/09/2020 19:31:45
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578 forum posts
740 photos
Posted by Simon Chaddock on 28/09/2020 16:39:08:

Graham

The fact that you are using a lower kV motor will 'loose' some of your Watts.

The power absorbed by a prop is roughly proportional the square of the speed so (560 x 560)/(600x600) = 0.87. So the lower kV could account for maybe 13% of your loss.

This of course assumes that the motors are otherwise internally identical and the kV figures given by the manufacturer are accurate in the first place. wink 2

It is worse than this, power absorbed by a prop is proportional to rpm cubed not squared, standard fan law calcs. So this is (560 x 560 x560) / (600 x 600 x 600) = 0.81 so a small reduction in KV can have a dramatic effect on power and hence current draw.

To do proper troubleshooting you must have a tacho, without it you are just guessing.

You measured the rpm. Then you measure the volts as reported by the Wattmeter. Typically, if the prop is right, it should run at 80 to 90% of the theoretical rpm, say 85%. Let us say at full throttle your Wattmeter says the voltage is 15.2 V with your 560kV motor.

Calculate the ecpected rpm = 0.85 x 560 x 15.2 = 7235 rpm.

If you measure around this then the ESC and motor are running as expected and as long as you are in the current rating of the motor, fit a larger prop.

Graham Davies 329/09/2020 08:35:47
129 forum posts
49 photos

Thank you everyone for your kind input.

I have done similar calculations and also factored both motor and prop efficiency, I may revisit my workings!

Bob, I think you're pretty much correct and I need to prop accordingly. 14x8 is the biggest I have at the moment, I suspect I will see closer to the full berries with something larger.

In any case, as I burned the midnight oil to finish the Viva yesterday, I did a full power check with the airframe fully assembled. It appears to have 'plenty', so maybe I am getting too hung up on the numbers. Certainly the wattmeter tells me I am operating safely; the feel from the seat of my pants tells me it won't struggle to get off the ground!

Graham

Bob Cotsford29/09/2020 11:15:43
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8794 forum posts
494 photos

If it's under about 5lb it will fly on 400W Graham, it all comes down to just how well it flieslaugh.

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