Check on power calculation

[John Bisset]

Comments and advice/correction please folks.

I am now flying an electric powered motor glider, which I have uprated from its previous Nicad battery and 540 size brushed motor. That arrangement only just flew; the new fit really is impressive, and I am wondering if I have overdone the power a bit.

The set up is – a PO-3535-1090 motor from 4-Max driving a 10 x 5 prop, with a 40 amp ESC and a Turigny 3S 2200mAh 20C to 40C LiPo battery pack. The model is a Merlin which I built many years ago. Total weight is 960 g or 2.2 lbs ready to fly.

Am I right in thinking that I am probably getting around 330 watts power out at the nominal maximum continuous power setting? Or since it is a 20 to 40C, should I atke the average of 30C output,? That would suggest around 450 watts, which would be a significant excess of power!

2.2A x 20 x 0.66 = ~30 amps

11.1 v x 30 amps = ~330 watts

Are my calculations approximately right? I am tempted to buy a wattmeter –the Turigny 180A meter seems like a useful purchase. Does anyone have one and can they comment?

Thanks & regards,

John Bisset

[Biggles' Elder Brother - Moderator]

I'm afraid it doesn't quite work like that John! I wish it did, it would be much easier!

We can't say "well 35C is so many amps, so at X volts that's a certain amount of power".

How much power the system will deliver is a function of how the wholes system works together -but perhaps the single most important element surprisingly is not the motor, the battery or the ESC - its the prop.

The motor's kV value - and the battery supply voltage - will determine the speed the prop will try to rotate at full power. The pitch/diameter of the prop in turn will determine how much power is needed to do that.

We can, generally speaking, select components to give us a target current/voltage combination that will have the capacity to deliver enough power - I recently wrote an article in the mag covering how to do this - but it's much harder to answer the question "how much power will I get from a specific set up" - in the end, in my experience, only a power meter can really answer that! In practice many people (me included!) use the power meter, alongside a selection of props, to finalise and optimise the actual power output.

BEB

[Frank Skilbeck]

As BEB says the main governing factors are prop, motor Kv and voltage, if you want to get an idea of the performance and adequacy of various set ups then there are on-line programs which can help you like ECalc and Drivecalc .

[pete taylor]

Hi John, I agree with BEB and Frank. There are a lot of variables governing the performance of a power system.

However, having run the numbers you've given through ecalc, you'll have no shortage of power with the components you have (approx 1:1 power/weight ratio!) and at approx 18A, none of them will be particularly stressed.

I'd suggest flying the model and be ready to reduce the throttle as needed.

Also, investing in a Watt meter is probably the best £20 you can spent where electric flight is concerned.

[John Bisset]

Thanks all.

I realise I should not have posted late at night without further thought and mulling. I was aware of the effect of prop match and should as a minimum have allowed a factor for prop efficiency. Tsk. I suspected that a poor prop match was part of why my previous Nicad and 540 motor combo was so marginal.

BEB – I shall have a look for your article. Which issue was it in? I don’t get every issue, partially due to my rather random bookshop visits. (Yes, should subscribe!) Somewhere I have an old strobe light, which could be modified for checking prop rpm. Another little project beckons…

Frank – I hadn’t heard of Drivecalc. I shall try it first since it looks more approachable. Maybe Ecalc later.

Pete – Yes, the beast is well overpowered. I am running her around at less than half power, except for the take-off zoom climb. Happy with that, since it eases strain on the system, but I want to get a better handle on all this for later work. I fly full size motor-gliders, some of which have marginal take-off climb rates, so having handfuls of excess power instantly available is a pleasure, as wellas being useful when shaking off years of rust in my model flying.

Which wattmeter do you recommend? I mentioned the Turnigy because I thought 180A would give plenty scope, and it seems readily available.

I have a number of i/c models which I may convert to electric, plus some kits to build or complete. Whee. Those are old style builds and will be heavy compared to much modern stuff. (Original Wot4 Mk1 and Acrowot for example) For the larger ones having good estimates of power values will be helpful. It’s going to be a while before I get down to smaller and lighter weight work; I have a number of Futaba S148s ‘in stock’ plus a few Hitec S101 Mini servos. Those ‘minis’ look quite large when I peek into the modern foam ready builts.

Cheers, and thanks again

John B

[Ray Farrimond]

If you are going to be doing conversions these might be of help.

http://www.flyrc.com/glow-to-electric-conversion-tips-part-i/

http://www.flyrc.com/glow-to-electric-conversion-tips-part-ii/

Ray

[John Bisset]

Thanks Ray. Some good information there. I am pleased to see that my thinking about my conversion of an old Wot4 is mostly confirmed by the author. Those servos look rather familiar too !

Regards,

John

[Frank Skilbeck]

If it is way overpowered then one other thing you could try is to go to a 2S lipo, this will drop the Wide Open Throttle rpm and the power (on the same prop) and hence the max amps too. I did this on a Ripmax Fokker D7 which was way overpowered on a 4s setup and flies lovely (using the same prop) on a 3s lipo.

[John Stainforth]

I find the following approximate, simple conversions useful for model aircraft:

1 hp ~ 10 cc of i.c. engine size

1 hp ~ 750 W

Electric motors are about 75% efficient, so you need a power draw from the batteries of about I kW for I hp.

To summarise: 1 hp ~ 10 cc ~ 1kW of battery power

[pete taylor]

Hi John, I've used a Turnigy one for several years without any problems. It seems pretty bomb-proof and gives all the numbers you'll need. It's also inexpensive and readily available under several different guises from various suppliers, GT Power, Pro-Peak as well as Turnigy. They're all the same meter just different prices

[John Bisset]

Thanks Frank, John and Pete for the latest thoughts.

I shall try the step down to 2S Frank - I want to get some smaller, lighter batteries and motors for later projects.

Good point John - handy the efficiency just evens up the conversion! Useful.

Pete - that will help when checking alternative prices. As a Scot I do like a bargain !

John

[Simon Chaddock]

John

My own view is that a direct power comparison between IC and electric does not give the complete picture.

For a given power electric tends to be more efficient simply because in a single cylinder IC the prop speeds up and slows down between the compression and power stokes (and even more so in a 4 stoke!) whereas an electric motor provides a smooth torque over the complete motor revolution and hence a constant prop speed.

This very smooth torque also allows the prop to be made nearer to the ideal blade cross section over more of its blade length which improves it aerodynamic efficiency, hence you see "e" props..

Difficult to quantify but each electric Watt does seem to provide quite a bit more performance than each IC one, maybe as much as 25%?

[John Muir]

Hi John,

If you feel you've got more power than you can manage I'd suggest fitting a smaller prop, maybe a 9x5, which should drop the watts down from something a little under 200W now to around 150W, on 3S. This is based on my own experiments with an electric glider a while back. A 2S battery might need a bigger prop, maybe 12x6 or thereabouts, for a similar output. A watt meter is well worth having.

John.

[John Stainforth]

I should have emphasized the word *approximate*. Obviously the torque characteristics of ic and electric motors are very different. Still power is power.

[PatMc]

I agree with John Muir, a smaller prop is better than dropping to 2s when the model is overpowered. Or you can always close the throttle a bit or simply climb steeper in order to hold the speed down at the same time get to altitude quicker then into the glide.

I fly several different e-power gliders & mostly use full throttle for about 30 secs climbing to around 200 metres in the proccess. From there I either do a bit of thermal hunting or some aerobatics on the glide back down, depending on conditions/model/mood.

[PatMc]

Posted by John Stainforth on 22/06/2017 22:14:01:

I should have emphasized the word *approximate*. Obviously the torque characteristics of ic and electric motors are very different. Still power is power.

Actually I think that most ic engines are rarely used at their peak RPM power due to noise considerations whereas it's relatively easy to arrange an electric setup to give the required power.

[John Stainforth]

Posted by PatMc on 22/06/2017 23:00:31:

Posted by John Stainforth on 22/06/2017 22:14:01:

I should have emphasized the word *approximate*. Obviously the torque characteristics of ic and electric motors are very different. Still power is power.

Actually I think that most ic engines are rarely used at their peak RPM power due to noise considerations whereas it's relatively easy to arrange an electric setup to give the required power.

Fair point. However, I think there are many exceptions to what you say, i.e., there are ic flyers who seem to scream around the sky at more or less full throttle all the time, and electric flyers who find it desirable to throttle back considerably to prevent batteries and ESC's overheating.

[John Stainforth]

Posted by PatMc on 22/06/2017 23:00:31:

Posted by John Stainforth on 22/06/2017 22:14:01:

I should have emphasized the word *approximate*. Obviously the torque characteristics of ic and electric motors are very different. Still power is power.

Actually I think that most ic engines are rarely used at their peak RPM power due to noise considerations whereas it's relatively easy to arrange an electric setup to give the required power.

Fair point. However, I think there are many exceptions to what you say, i.e., there are ic flyers who seem to scream around the sky at more or less full throttle all the time, and electric flyers who find it desirable to throttle back considerably to prevent batteries and ESC's overheating.

[Martin Harris - Moderator]

Very few IC sport flyers prop their models to achieve maximum power output (particularly 2 stroke engines) due to noise considerations. In addition to any exhaust sound, propellers running at very high RPMs generate an awful lot of noise so loading the engine with a relatively high pitch prop reduces RPM. An overloaded IC engine (within reason) can cope happily with the situation, unlike an electric motor which will overheat rapidly. This is why there is so much more fiddling with factors such as kV figures, cell counts and propeller choice in order to get the required performance within the parameters of the motor, speed controller and battery C rating.

[Denis Watkins]

Posted by John Stainforth on 23/06/2017 00:24:42:

Posted by PatMc on 22/06/2017 23:00:31:

Posted by John Stainforth on 22/06/2017 22:14:01:

Fair point. However, I think there are many exceptions to what you say, i.e., there are ic flyers who seem to scream around the sky at more or less full throttle all the time, and electric flyers who find it desirable to throttle back considerably to prevent batteries and ESC's overheating.

This is interesting John, and highlights the complexity of both set ups. Those IC that actually appear to scream around, increase cooling by doing so.

But as has been said, should be prop loaded enough to keep the noise down

[John Bisset]

Excellent stuff - lots to think about. Thanks all.

Martin, I understand and agree. We have some fun playing with different prop types and pitches in full size as well. It is more of an art than a science at times, though 'cheating' with a VP prop helps ! Sometimes changing the prop to reduce noise level is worth a small sacrifice in climb rate.

The 'scream around the sky' types are always with us.

Personally I like a more relaxed style, both for model and full size. - and Col Aresti never meant people to fly his diagrams as drawn, with square corners. He flew a lovely aeroplane, gracefully. I admire the Extra 300s and similar machines with their extraordinary capability and gut-wrenching maneouvres, but for me 'LAROSA' sums up my aspirations well.

Cheers,

John