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Am I getting to complicated?

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SIMON CRAGG30/10/2019 10:51:48
544 forum posts
15 photos

I have just finished completing my first ic to electric conversion.

HobbyKing PT-19

lb10.5

Turnigy SK3 5055-430

6S 5000 40-50C

100A ESC.

Having researched props, I settled on a 17x8.

I paid for the full version of e.calc and put in all my figures.

It came out saying I was okay @ 78.3a / 1631.8w

Connected up my Watt meter = 52.6a / 1237w

Even allowing for the 10% leeway, this is a long way out.

I have tried various permutations on e.calc, but it always comes out high.

Can anybody shed any light on these figures or offer any advice.

I know the model is safe and within limits, but I am interested to establish why the variation.

Gary Manuel30/10/2019 11:27:12
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2195 forum posts
1540 photos

Have you calibrated your ESC to make sure that it is giving you 100% throttle when stick fully up?

See ESC instructions - it's usually the first option.

 

Edited By Gary Manuel on 30/10/2019 11:28:27

SIMON CRAGG30/10/2019 11:36:19
544 forum posts
15 photos

Yes, I am sure the throttle is working as it should. Its a YGE ESC, and the throttle is set via a two page progranmme card. Firstly with the throttle at the low position and then the high position. All confirmed by lights and beeps!.

G194030/10/2019 12:11:09
3523 forum posts
1 photos

That's one reason I rarely bother with electric flight prediction s/w. It rarely produces accurate, real life results. Moreover a lot of the data the programs ask fot aren't readily available from the manufactures/suppliers of either motors or ESCs. For motors, the one characteristic I really like to know - the maximum current, both sustained and burst - is often not quoted. ESCs are usually better specified.

I've done a number of successful conversions relying on experience, copying/adapting known drive trains and my own empirical measurements.

You have a safe power train which looks like it will fly a 5kg model without a problem. Do a short flight (say 5 minutes) land and measure your battery to give yourself a feel for endurance. If you have it, I'd also suggest current measuring telemetry to give you a idea how much energy you're using for different flight phases.

Geoff

Steve J30/10/2019 12:28:57
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1774 forum posts
51 photos
Posted by SIMON CRAGG on 30/10/2019 10:51:48:

Turnigy SK3 5055-430

6S 5000 40-50C

100A ESC.

How well is the battery holding up under load?

Is the motor kv really 430?

How efficient is the motor?

Is the ESC timing optimal?

How efficient is the prop?

+1 on Geoff's post especially the bit about telemetry.

Edited By Steve J on 30/10/2019 12:31:21

SIMON CRAGG30/10/2019 12:40:07
544 forum posts
15 photos
Posted by Steve J on 30/10/2019 12:28:57:
Posted by SIMON CRAGG on 30/10/2019 10:51:48:

Turnigy SK3 5055-430

6S 5000 40-50C

100A ESC.

How well is the battery holding up under load?

Is the motor kv really 430?

How efficient is the motor?

Is the ESC timing optimal?

How efficient is the prop?

+1 on Geoff's post especially the bit about telemetry.

Edited By Steve J on 30/10/2019 12:31:21

Hard to tell how good the battery is until I have conducted flight testing.

430KV? Absolutely no idea.....but says so on the tin.

Efficient motor......same as above really, but good reports.

Timing set to 8% after much research on here and using a suggested formula.

Time will tell on the prop, but e.calc confirmed it was good and others seem to use it ok.

Chris Freeman 330/10/2019 12:41:24
333 forum posts
483 photos

I found that I can never reach the figures recommended on the Turnigy motors, the KV ratings are often way out. I now test the motor with various props to give the required watts. You also need to be sure that you have a good lipo when doing these tests as one with a high internal resitance can be very frustrating when a larger prop of more pitch does not give more watts. I have also found the the air tests do still give the best results as some props of different makes but the same size can show better performance than others.

Dickw30/10/2019 12:41:44
avatar
607 forum posts
89 photos
Posted by SIMON CRAGG on 30/10/2019 10:51:48:...........................

Turnigy SK3 5055-430

6S 5000 40-50C

100A ESC.

Having researched props, I settled on a 17x8.

...................................

It came out saying I was okay @ 78.3a / 1631.8w

Connected up my Watt meter = 52.6a / 1237w

...........................

Can anybody shed any light on these figures or offer any advice.

I know the model is safe and within limits, but I am interested to establish why the variation.

I just put the claimed motor data into MotoCalc and it initially gives 75A and 1600 watts - so similar to your ecalc figures.

I then doubled the stated resistance and got 53A and 1150 watts - so close to your measured data.

One possibility is that the stated resistance is per winding and the windings are Y connected so the resistance needs to be double for two windings in series.

If the windings were D connected the stated resistance would be correct, but the motor kV would be diferent. Manufacturers can play with windings in Y and D conenction as well as the number of winding turns to give different charateristics. Not saying that is what the issue is, but it is a possibility.

Dick

SIMON CRAGG30/10/2019 13:25:23
544 forum posts
15 photos

Dick

Thx for that, you have lost me on this windings bit though. How do I know if the windings are Y or D? This is getting more confusing by the minute?

Bob Cotsford30/10/2019 13:30:49
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8261 forum posts
454 photos

I've had quite good results using SK3 5055/430 motors in Ecalc. Now my maths aren't the best, but 1273W for 53A gives the cells as providing 24v, which I find surprising - only 1v drop from no load voltage when turning a 17" prop.

I'm getting 50-55A draw on 15/8-10 props using 6S 4500 35c packs. That must be a really low drag prop you have!

Dickw30/10/2019 13:32:24
avatar
607 forum posts
89 photos
Posted by SIMON CRAGG on 30/10/2019 13:25:23:

Dick

Thx for that, you have lost me on this windings bit though. How do I know if the windings are Y or D? This is getting more confusing by the minute?

You don't know unless the manufacturer states Y or D wind somewhere in the spec - some do some don't.

Not a lot of help I know, sorry!

Dick

SIMON CRAGG30/10/2019 13:38:41
544 forum posts
15 photos
Posted by Bob Cotsford on 30/10/2019 13:30:49:

I've had quite good results using SK3 5055/430 motors in Ecalc. Now my maths aren't the best, but 1273W for 53A gives the cells as providing 24v, which I find surprising - only 1v drop from no load voltage when turning a 17" prop.

I'm getting 50-55A draw on 15/8-10 props using 6S 4500 35c packs. That must be a really low drag prop you have!

Bob 1237W, not 1273W. Either way , I cannot get e.calc to tally with the Wattmeter figures. Very odd.

Bob Cotsford30/10/2019 14:47:21
avatar
8261 forum posts
454 photos

So my mmaths is OK, observation skills maybe not so goodlaugh. I still find it hard to understand how ecalc can be so far out. The figures you are getting would suggest that the kv is nearer 380!

PeterF30/10/2019 14:49:00
avatar
490 forum posts
675 photos

I have had an issue the other way with a Turnigy motor, pulled 30% more current than expected and the rpm were 10% higher than expected so the motor had a higher Kv than spec. I have had another case with a cheap prop, it was relatively bendy so as the rpm increased, the pitch twisted out and the prop did not give anywhere near as much power as expected. Two lots of info you have not given is the prop rpm nor the prop type, both of which are crucial to a diagnosis.

If the motor is the correct kV of 430, then I would expect the rpm under load at 23.5V (based on your 1273 W and 52.6A to be 23.5 x 430 x 0.9 = 9,100 rpm perhaps a little lower. If you have a much lower rpm then you either have a motor that has a lower kV than advertised, or you have a high resistance somewhere in your connections or your ESC is not giving full rpm but this was discussed and cleared above.

If you are getting something like 9,000rpm then your prop is not as powerful as say an APC, you have not listed which prop you have. An APC 17x8 should be running at about 7,000rpm to draw 1237W, so again, your rpm need to be measured to help diagnose where the issue lies.

One thing that is often not understood is that the power required for a prop is dependent upon the rpm cubed. If you spin a prop twice as quickly, you need 8 times the power (2 x 2 x 2 = 8). If the rpm is down by 10% because of a slightly dodgy motor, then the power demanded will be down by 30%. For ecalc or motocalc to be within 10% accuracy on power, the motor kV has to be within 3% of its correct value.

SIMON CRAGG30/10/2019 14:51:12
544 forum posts
15 photos

I am hoping that somebody much wiser than me, can shed some light on this. Bit disappointed as I paid £6.00 for the full version of e.calc!

Martin Harris30/10/2019 15:34:58
avatar
9171 forum posts
242 photos

The Wattmeter I bought was hopelessly inaccurate so try it with a friend's if possible to confirm your readings.

As per PeterF, an easy test would be to measure the prop RPM - if it's in the ball park as predicted by eCalc then your measurements must be suspect...

PeterF30/10/2019 15:52:31
avatar
490 forum posts
675 photos

I have sometimes measured rpm at full throttle with and without the Wattmeter in line to confirm that the Wattmeter is not giving a high resistance and reducing the volts at the motor below the measured input volts.

Frank Skilbeck30/10/2019 16:55:48
avatar
4610 forum posts
101 photos

Something doesn't tally with the watt meter, if it says it's doing 1237 watts at 52.6 amps that's a battery voltage of 23.5 volts or 3.9v per cell, suggesting that there is only a 0.3v per cell drop (assuming battery is 100% charged). Can you confirm the voltage reading under load, i.e. plug a battery checker into the balance lead.

Also reading the reviews on Hobbyking, several users report the motor on a 6s and 17 x 8 prop giving 65 to 70+ amps

Edited By Frank Skilbeck on 30/10/2019 17:20:07

Stephen Belshaw30/10/2019 17:17:59
96 forum posts
20 photos

I'm glad I'm not the only one who struggles with the black art of motor/battery/prop matching!

After trying to match a selection of props and batteries to various motors on various models I decided to buy a wattmeter and based my "calculations" loosely on the 100 watts per pound formula, give or take depending on the type of aircraft. A little knowledge can be a dangerous thing and I felt I wasn't getting the best results, so decided to "invest" in eCalc in order to save on stockpiling redundant props. I too then found a big discerpancy between my wattmeter and eCalc, the wattmeter always reading much higher.

So I'm back to where I started - that it's a black art .................

Martin_K30/10/2019 18:52:08
134 forum posts

Not a black art, an iterative process. Calculations give a starting point then real world testing of that first cut refines the setup.

You don't expect a new build to fly perfectly first time without trimming, why expect the electric power train to be different?

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