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Motor / prop calculator?

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SIMON CRAGG16/02/2019 08:24:40
493 forum posts
15 photos

Does anybody know of a simple / free electric motor / prop calculator?.

Most of the programmes I have looked at need a nuclear scientist to work them out!

I think I now know my way round a Watt meter, but I do find the whole subject a bit on the baffling side of baffled.

Robin Etherton16/02/2019 08:31:18
271 forum posts
41 photos

Don’t we all.

Not free but I use ecalc. Very comprehensive.

Masher16/02/2019 08:33:40
1104 forum posts
79 photos

Here's one

Former Member16/02/2019 08:39:44
1322 forum posts

[This posting has been removed]

SIMON CRAGG16/02/2019 10:00:05
493 forum posts
15 photos
Posted by David Mellor on 16/02/2019 08:39:44:

If you want a good basic one that calculates power drawn by the motor for any size prop at any rpm, then this one is good:-

**LINK**

David. Much more like it, thank you so much!.

Stearman6516/02/2019 12:22:36
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769 forum posts
893 photos
Posted by Masher on 16/02/2019 08:33:40:

Here's one

Ecalc isn't free to use the full facilities. I think it cost me £10 for a years subscription. I found it not so easy to use. For my last 2 selections I asked 4Max & Krick.

Steven Shaw16/02/2019 14:39:20
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334 forum posts
182 photos

I still like webocalc

**LINK**

Dickw16/02/2019 16:51:56
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500 forum posts
76 photos

Another free one is Drivecalc.

Dick

Geoff Sleath16/02/2019 20:50:56
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3500 forum posts
320 photos

I downloaded Drivecalc and experimented by inputting data of a drive train I'd already measured and flown on my Percial Mew Gull. I run it on 6S with an eMaxGT 4030/06 420 rpm/volt. The nodel weighs 10lb/4.5kg

I test flew it with a 14x7 apc/e prop which actually drew 46 amps but DriveCalc shows only 36.4 amps. However, the input power in practice of around 1kw flew the model OK (DriveCalc 767.8 watts input).

The recommended prop for this motor on 6S is 16x8 and DriveCalc says 57.8 amps and 1182.9 watts, which is close to the stated spec for the motor. However my actual measurements with that prop is for 65 amps and in flight (via telemetry) up to 70 amps on (very) short verticals (which I've avoided mostly)..

So I think these electric flight drive train programs are only OK to put you in the right area to start measuring. I've yet to find one that fits really well with practice. On my Mew Gull I chose the biggest eMax motor and experimented. I didn't resort to software at all. Because eMax suggested 16x8 as the correct prop I bought a couple and I intend to cut one of them down to 15" to limit the current a bit rather than rely on my throttle finger's restraint

It's worth trying though and kudos to the guy who wrote it and generously put it on the 'net for us to use at no cost.

How we miss BEB (Dave Burton) at these times. His method of starting with the prop we want to turn when doing a glow to electric conversion seems backwards (as it is, literally) and choosing a motor/esc and battery voltage combination to drive at the speed we want is the way to go.

Geoff

Old Geezer17/02/2019 09:02:51
640 forum posts

My starting point is George's list of recommendations on his 4-Max site, also very useful is the suggested Axi motor/prop combinations on the Electric Wingman site, or go directly to the Model Motors site. The Airtek site can be helpful too, indicating what motor for what type and weight of model. The thing is, these will all give you a starting point, but then it's the wattmeter and how does the motor/prop/battery set-up perform for you down at the field - fine tuning slight changes in diameter and pitch is part of the "art" of setting up just like an I/c plane.

Bob Cotsford17/02/2019 10:49:33
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8068 forum posts
444 photos

I've found e-calc to be well worth the subscription fee as I've found it's calculated results are pretty close to what I see on a wattmeter, at least for the 4s-6s setups that I've used to date.

Dickw17/02/2019 10:50:28
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500 forum posts
76 photos
Posted by Geoff Sleath on 16/02/2019 20:50:56:

...........I test flew it with a 14x7 apc/e prop which actually drew 46 amps but DriveCalc shows only 36.4 amps. However, the input power in practice of around 1kw flew the model OK (DriveCalc 767.8 watts input).

The recommended prop for this motor on 6S is 16x8 and DriveCalc says 57.8 amps and 1182.9 watts, which is close to the stated spec for the motor. However my actual measurements with that prop is for 65 amps and in flight (via telemetry) up to 70 amps on (very) short verticals (which I've avoided mostly)..

So I think these electric flight drive train programs are only OK to put you in the right area to start measuring. I've yet to find one that fits really well with practice...............................

Geoff

Hi Geoff

I agree that these programs are initially only good for giving you a rough guide to a starting point for experimenting and measuring, but if you think about how often people here suggest trying a different brand of the same size prop, or enquire about the condition of someone’s battery when they have a problem, it is easy to see why these programs cannot be completely accurate.

My preference is MotoCalc, but I didn’t mention that before because the OP wanted a free prog.

Once I have a starting point, I do some real-life measurements and then adjust the program to match reality – subsequent, and future, “calculations” will be much better - e.g a different size prop on the same model, or even a completely different model.

The most likely parameter to adjust is battery internal resistance, and I now have a selection of my own “packs” set up in the program which give me initial predictions that are much better than when using the nominal basic packs.

Dick

Edited By Dickw on 17/02/2019 10:51:38

Dickw17/02/2019 10:53:30
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500 forum posts
76 photos

oops! - I have no idea what happened to the font when I added the link to Motocalc - sorry everyone.

Dick

PeterF17/02/2019 23:36:01
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454 forum posts
623 photos

The biggest issues I have found is the actual rpm achieved when checked against the motor KV times the measured voltage, assuming minimum voltage drop over the ESC. Normally with a motor running a prop within the motors capability, the measured rpm will be 80 to 90% of that calculated, and ecalc makes a good guess at working this out. However, I have some motors with the prop running at 70% of the calculated rpm where the propeller is in proportion to the motor, so the motor has a KV 10% lower than spec. I also have some recent measurements on a 2.5kW motor in a 2m aerobat, with a 17 x 8 prop taking close to 2kW of power at 8,460 rpm where the calculated rpm from volts x KV was 8,410 rpm, i.e. there was no reduction in motor rpm for load. I can only assume that in this case the actual motor KV is about 10% higher than spec. Therefore, motors as delivered can have a +/-10% variability in KV, and as motor power is proportional to rpm cubed, this can result in 30% discrepancies in any calculations. I have to admit that in these extreme cases I am quoting the results of cheaper motors, for my more expensive motors I find that they tend to operate closer to expectations and the calculations by ecalc and I would not be without ecalc.

Stearman6518/02/2019 08:34:29
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769 forum posts
893 photos

Does nobody use a spring balance any more? I got my leccy powered model back in 2005. as I recall there wasn't all this teccy stuff involved selecting equipment, you either went with what the kit supplier said or tried the DIY method, which I did when I converted a WM Groovy from I/C to electric power. To check if it would fly, a flying buddy at the time connected a spring balance to the model, if the pounds of thrust exceeded the weight of the model, it should fly. Does anyone still use that method before actually test flying?enlightened

PeterF18/02/2019 10:12:11
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454 forum posts
623 photos

Yes, a luggage or fishing scale can be a useful addition to the test kit. However, when dealing with large kit where a good prop can cost £20 or more for a scale 18 x10 or larger then the buy a few props and test them out is not really viable, I wan't to choose the prop with some confidence before I buy. Then I test it out after wards which is usually a confirmation. This becomes even more the case when choosing a set up from new. The suck it and see method is fine for smaller planes where props are less expensive and if the plane does not fly as expected, changing the prop is a small(ish) matter.

Nigel R18/02/2019 11:05:40
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3176 forum posts
487 photos

Simon - what do you actually want to calculate?

Former Member18/02/2019 11:19:39
1322 forum posts

[This posting has been removed]

Former Member18/02/2019 11:22:42
1322 forum posts

[This posting has been removed]

Stearman6518/02/2019 11:34:35
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769 forum posts
893 photos
Posted by David Mellor on 18/02/2019 11:19:39:
Posted by Stearman65 on 18/02/2019 08:34:29:

Does nobody use a spring balance any more? I got my leccy powered model back in 2005. as I recall there

a5557328-36-rimg0822.jpg

Edited By David Mellor on 18/02/2019 11:20:34

That's a hell of a spring balance.wink

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