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Motor / prop calculator?

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SIMON CRAGG16/02/2019 08:24:40
295 forum posts
4 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
257 forum posts
41 photos

Don’t we all.

Not free but I use ecalc. Very comprehensive.

Masher16/02/2019 08:33:40
1094 forum posts
160 photos

Here's one

David Mellor16/02/2019 08:39:44
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1254 forum posts
611 photos

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**

SIMON CRAGG16/02/2019 10:00:05
295 forum posts
4 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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746 forum posts
887 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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278 forum posts
157 photos

I still like webocalc

**LINK**

Dickw16/02/2019 16:51:56
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405 forum posts
61 photos

Another free one is Drivecalc.

Dick

Geoff Sleath16/02/2019 20:50:56
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3188 forum posts
247 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
563 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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7728 forum posts
428 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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405 forum posts
61 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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405 forum posts
61 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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396 forum posts
558 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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746 forum posts
887 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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396 forum posts
558 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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2453 forum posts
402 photos

Simon - what do you actually want to calculate?

David Mellor18/02/2019 11:19:39
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1254 forum posts
611 photos
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 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

 

I do.

 

I use a 90 degree lever arm to push against kitchen scales to measure static thrust as a function of power drawn (I plot a graph for each prop/motor/battery combination of interest).

 

I was very surprised indeed how useful it turned out to be (a few years ago) and have used it a great deal since, upgrading the Wattmeter and comparing the static bench test data with in-flight current and voltage telemetry data and the simulator ( **LINK**  ).

One thing I have found is that many motors have substantially under-specced prop size recommendations (i.e. very conservatively rated) by the manufacturers and that it is quite acceptable to run some motors in flight using much larger props than recommended and still stay within current specifications. Another way of expressing the same thing is to say that some motors when fitted with the manufacturer's recommended propeller size don't load the motor enough to draw maximum power in flight that the motor is rated at.

a5557328-36-rimg0822.jpg

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

David Mellor18/02/2019 11:22:42
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1254 forum posts
611 photos

By the way..... if using static thrust from a bench test as a guide, the amount of static thrust needed to ROG the model can be a lot less than the weight of a model.

 

It can be as little as one third the weight in many cases (such as old style/vintage "plodders" ).  So for these models a (static) thrust-to-weight ratio of 0.3 to 1 is a rough minimum for ROG, and most "old timers" will maintain level flight on very little indeed (10 watts per pound of model and less than 1 tenth AUW value of static thrust).

 

With heavier loaded, faster models, though, bench-measured static thrust becomes less use as a guide - at least it is safe to say that a thrust to weight ratio minimum of 1 should definitely ROG most of them.....

 

The slower, more sedate and docile the model is, the more meaningful that bench-measured static thrust values are  for predicting flight.  That is what I have found, at least.

 

 

Edited By David Mellor on 18/02/2019 11:34:05

Stearman6518/02/2019 11:34:35
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746 forum posts
887 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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