First dabble with electric power

[denis parkinson]

Hi everyone, after being brought up on IC powered planes, I have decided to take the plunge and build a basic plane that I can try electric power for the first time.

It is a 53 inch Meadow Skipper 4S, 3 channel high wing trainer.

On the plan (below), the suggested power unit is a .20 four stroke engine. It seems a straight forward job of putting a brushless motor in its place, but I have absolutely no idea on what size motor, lipo battery or esc I should be looking at.

I have been all the way through the electric flight for beginners, and I'm still no wiser.

I could really do with ordering the motor before I get too far advanced with the build, so any suggestions would be greatly appreciated.

Regards, Den

[Dave Hopkin]

To determine the power train we would need to know

The estimated AUW of the model, desired prop size, we have the span already

Looking at the web I found a picture of one and its sort of Cub-ish looking so I am guessing we are talking about a High wing, trainer style plane thats a draggy, slow graceful floater type? - correct us if thats not the case

The "standard" method is "100 W per Lb" (you dont have to use all the throttle!) and I would suggest going the 3S Lipo route as thats probably the most common type used in leccy planes.

If this is a "one off dabble" in leccy then a 3S will allow you to use a cheaper charger that doesnt go above 3 cell, though if you are going to do a lot of leccy it might be worth investing in a high end charger that will do 4 cells and above.

[denis parkinson]

Thanks for the fast reply and sound advice Dave.

Yes, you are correct with it being "Cub-ish looking and being a graceful floater type"

According to the plans, the prototype weight was 3lb, but that would have been with a heavy four stroke motor up front.

Obviously with electric power etc, the weight will have dropped somewhat, plus I'm trying to make weight savings wherever possible.

The suggested prop size is a 9x6, and I am assuming that would still apply with electric power.

[Peter Jenkins]

Don't forget you'll need a battery hatch so that you can load and unload the battery easily. Also, the battery will be a major determinant in CG position given its weight.

[denis parkinson]

With going from ic power to electric power, I am freeing up quite some space inside the fuselage.

I don't need the bulky fuel tank, or the 3rd servo for the throttle, so I have plenty of choices for the positioning of the battery, and hatch.

Its quite possibly going to be tail heavy with electric power, so I could quite easily put the lipo up front just behind motor itself.

[Frank Skilbeck]

Denis, so based on an all up weight of 3lb you'll need to be looking at around 300 watts, which will give very spirited performance on this type of model, so using a 3s lipo that gives around 11 volts under load, so say 27 amps (P = V x A). Assuming that you want to use a 9 x 6 prop and that the 20 four stroke would have turned this around 10,000 rpm you'd need an electric motor to do the same, at 11 volts you'd need a motor that would give approx 900 rpm per volt under load, allowing for the load factor this would be around 1,000 rpm per volt with no load or a Kv = 1,000. So what you need to look for is a motor with a Kv or around 1,000 and max amps greater than 27 amps (say 30 amps minimum to be on the safe side). If that is too much performance you can always put on a smaller prop.

[Allan Bennett]

Posted by denis parkinson on 02/12/2014 01:31:42:

With going from ic power to electric power, I am freeing up quite some space inside the fuselage.

I don't need the bulky fuel tank, or the 3rd servo for the throttle, so I have plenty of choices for the positioning of the battery, and hatch.

Its quite possibly going to be tail heavy with electric power, so I could quite easily put the lipo up front just behind motor itself.

I've done several conversions of glow models to electric. They all end up slightly lighter than they were when IC powered with a full tank of fuel, and they always need the battery almost as close behind the motor as possible to get the c of g in the right place. So, in practice, you probably won't have much choice for battery location.

Other than that, follow the advice given by the others on motor selection. And don't forget that you really should have a wattmeter or some other means of verifying that the actual amps with whatever prop you choose is within the motor's spec. Props, even of the same nominal size, can vary, manufacturers' recommendations are sometimes off, as are the results from prop-calculation programs.

[Dave Hopkin]

Agree with all of the above, as you should be looking at a current of around 30 amps, get an ESC of at least 40 amps

And just because a motor is rated at 300 Watts with a different prop it will give a vastly different figure, I was testing a motor rated at 850W the other day, with different props I was getting readings from between 550W and 850W

Its down to the way brushless motors work, they "want" to spin at their design speed (the KV rating) and will try and draw as much current as they need to try and get to that speed,so put a small or low pitch prop on (and reduce the mechanical load) and they will spin up to speed with little effort (drawing less current) so output less power = less thrust) - Overload the motor with a over sized or over pitched prop and the current will sky rocket and threaten the ESC and Motor with that lovely little puff of blue smoke

So watt meter (or at least an ammeter is essential - (P = V x A) - as is a LiPo checker (not just a simple overall voltmeter but one that plugs into the balancing port and reads each cell under a load - LiPoGuard is a popular choice)

And when flying don't over discharge the LiPo's it shortens their life span, try not to take them below 30% and if you gently charge them (rather than fast charge) they will last a long time

[IanR]

Denis, may I suggest you have a thorough look through the 4-MAX website (see the "Shopping Partners" box on this page) where George Worley has shed loads of useful information, not to mention all the goodies you'll need to buy.

Ian

[Dave Hopkin]

Posted by IanR on 02/12/2014 09:21:55:

Denis, may I suggest you have a thorough look through the 4-MAX website (see the "Shopping Partners" box on this page) where George Worley has shed loads of useful information, not to mention all the goodies you'll need to buy.

Ian

And George also seems to come up with some pretty good answers if you ask them through the contact form on the 4-Max/Purple Power website too

[ken anderson.]

hello Denis-I don't want to confuse you - But don't get hung up upon the 100 watt/per lb....80 watts will be more than enough for the model as once in the air it will need very little thrust ..... to 'tootle' around -too much will be a waste as the model will be uncontrollable at top speed etc.....food for thought anyway ... save some dosh etc...

ken Anderson...ne..1 food dept.

[Biggles' Elder Brother - Moderator]

I agree with Ken - there is I think a growing tendency to over-power electric models at the moment simply because we can (and I'm as guilty as anyone!). The "100W/lb" rule really is on the generous side for a model like this. A model will fly with as little as 50W/Ib or even less! OK its will be a very sedate flight - but it will fly!

The danger is if you overpower a high-winger you can create a model that is a bit of a handful at full power. Yes I agree you don't have to use all the power, the throttle works both ways etc., but why not customise the power train to the model so that it suits it better.

You sound like a pretty experienced builder - so you are unlikely to build heavy - which is another reason for not over powering it. I would certainly think 70-80W/Ib would be more than enough for nice steady flying characteristics from a model like this - in line with what the original would have got from 20 engine. Also, bear in mind, if you downgrade the power to 70/80W/Ib but stick with say a 2200 3s battery you get longer flights!

Finally, I would not advise simply sticking with the IC prop-size. Electric motors tend to like slightly larger diameter props than they IC equivalents power-wise. So pick the prop that is right for the motor as long as you have ground clearance for it - if that turns out to be 10x6 or thereabouts - so be it. The Watt meter is your final guide on this issue.

Just some thoughts...

BEB (who has shamelessly over-powered far too many electric models!)

[Cuban8]

If you're going down the 9X6 at 10,000 rpm route, you may wish to plug those figures into here http://personal.osi.hu/fuzesisz/strc_eng/index.htm

I find this predictor very accurate, so you might wish to modify your choice of prop to keep the noise down.

[Delta Foxtrot]

dennis,

I can strongly recommend the Gibbs Guides if you are new to electric power. They are very easy to understand and will give you a good appreciation. There are also some very useful tables in there which will allow to make good choices for a wide range of electric power trains.

regards

david

[Delta Foxtrot]

You should also buy a power meter to allow you to test the chosen setup to make sure that the prop you chose doesn't draw too much current leading to esc failures etc.

[denis parkinson]

Thank you very much everybody for some sound advice, and it has all been taken on board.

I have 2 newly built models here that are begging for electric power. They are a Flair Puppeteer and a Flair SE5A, but I would rather get to grips with electric using something cheap and cheerful first, before I commit myself fully to electric..

I have seen one set-up on Ebay that caught my eye and here is the link:-

http://www.ebay.co.uk/itm/UK-2200kV-High-Speed-Brushless-Outrunner-Motor-40A-ESC-Prop-Kit-Combo-Planes-/201060792830?pt=UK_ToysGames_RadioControlled_JN&hash=item2ed02839fe

I'm not bothered about speed and power on this particular model, and from what everybody has said, this set-up seems to fit the bill, but with a slightly bigger prop size

Edited By Pete B - Moderator on 02/12/2014 12:54:07

[PatMc]

As Ken said 80W/lb would be more than adequate for this model. He had/has a Jnr 60 that's nicely powered by an OS26fs. I have a 57" vintage Frankenstien design that originaly flew quite well on 28W/lb with a brushed motor/nicads but has since been uprated brushless/lipo & 60W/lb. IIRC when the Meadow Skipper plan was published in Oct 1990 the only 20fs was the OS which would be lucky to manage 0.2hp max (150W).
I suggest to give a bit more power with 3s on about a 9" prop this 1100kv E-power or if there's clearance for a 10" prop this 920kv version . A suitable ESC would be the Hobbywing30A or for a bit more leeway & the advantage of a switch mode BEC the more expensive Hobbywing 40A .
My advice would be to leave the choice of battery until you know how the weight & balance are turning out then have a look at Hobbyking's range in the UK warehouse.

FWIW, I use the Emax 920 motor in a glider on 3s turning Graupner 9x5 folding blades that extend to 9.75", that makes it effectively a 9.75x5.4 prop. On a freshly charged 3s it pulls 21A for 250W & takes the 950g glider to 200m in under 30 secs.

[kc]

The Meadow Skipper is really similar to a vintage style model isn't it? If so 50 watts per pound may be OK. Prop noise should not be a problem with vintage electric as you only use full throttle for take off and for a moment or two in flight.

My 4lb   53 inch span Electra model -Vic Smeed design- flys well on a BRC 2217/9 with 3S2200. This is about 200 watts on a 10 by 5 APC E. So anything around 50 to 80 watts should be fine for vintage style flying.

 

Edited By kc on 02/12/2014 12:41:28

[PatMc]

Posted by denis parkinson on 02/12/2014 12:23:04:

Thank you very much everybody for some sound advice, and it has all been taken on board.

I have 2 newly built models here that are begging for electric power. They are a Flair Puppeteer and a Flair SE5A, but I would rather get to grips with electric using something cheap and cheerful first, before I commit myself fully to electric..

I have seen one set-up on Ebay that caught my eye and here is the link:-

www.ebay.co.uk/itm/UK-2200kV-High-Speed-Brushless-Outrunner-Motor-40A-ESC-Prop-Kit-Combo-Planes-/201060792830?pt=UK_ToysGames_RadioControlled_JN&hash=item2ed02839fe

I'm not bothered about speed and power on this particular model, and from what everybody has said, this set-up seems to fit the bill, but with a slightly bigger prop size

The motor in that set isn't suitable, it's 2200kv which is way to high. If the 6x5.5 prop was changed for one that would be suitable for the MS it would increase the current way above the max & burn out the motor &/or ESC.

[gangster]

Den. As you see from the above answers there is a lot more to take into consideration than simply watts per pound. At an early stage you need to be considering prop and battery size you wish to use. The max power quoted for a motor may not be suitable. Don't be frightened to put a much larger motor than the watts/pound calculation gives and restrict the power with prop battery and throttle setting

an example. My Dawn flyer is if I remember correctly looks for 270 watts. In my cupboard I found 2 motors both of about 1100kv. One 330 watts the other 440 watts. Looking closely if I tried to prop the 330 watt motor to deliver 270 watts with a3s battery it would exceed the max current for that motor potentially burning it out. The 440 watt motor with 3s and a 10x7 prop is perfect.

Also remember that ESCs are there to reduce the power to the motor so if overpowered just program your throttle throw down a bit

my Junior 60 a model not much different to yours flys perfectly on 250 watts but I have a motor and esc in it capable of delivering about 650 watts on a 13x8 prop with a 4s battery

using a 3s battery 12x6 prop aand throttle throw set to 50percent it is just right

you have also been advised to speak to some suppliers. I can add BRC hobbies to the mix. They are very helpful if you tell them the model and it's weight. Above all as already advised get yourself a watt meter you cannot do anything without one

[PatMc]

Posted by gangster on 02/12/2014 13:13:31:

Also remember that ESCs are there to reduce the power to the motor so if overpowered just program your throttle throw down a bit

If by that you mean use a rate setting (e.g. 80%) on throttle it won't work with most, if not all, ESCs. The ESC will now see the reduced setting as full range & merely still apply full 100% throttle at full stick.

The simplest way to limit power is to use a smaller &/or lower pitch prop.

Edited By PatMc on 02/12/2014 14:01:28

[gangster]

PatMc

Surely that depends on the order you do things

If the transmitter is first set to 100% and the ESC calibrated with max and min throttle settings with that throttle "movement."

If the esc is then down powered and the max throttle limited to a lower value than 100% the esc will never know that the stick hasn,t been moved to full. Just like a servo.

Done the other way round as you say the esc would readjust to max

Its not the puresr way of doing things but it has worked for about 18 months on my Junior 60

I suspect it would work for most ESCs I have only tried it on one of BRCs Pro escs and also did it to a cheap chinese ESC on a friends Diamond 2500 when it was grossly overpowered on a 4S pack ( proper solution of course fit a 3S but it proved my point to him.

I agree it is not the purest method and I suppose the esc could become uncalibrated which would give too much welly

The point I suppose I was trying to make is that you cannot simply do the watts/pound sum without due consideration of prop battery and max current the motor is designed for. and that all is not lost if you go too high with power and cannot rescue with prop and cells.

[PatMc]

Hi Gangster, I see what you mean & your right of course. Sorry I'm afraid I didn't fully think it through.
Though you've proved it can work OK my personal prefererence would still be to limit the power by using a smaller prop.

A few years ago a clubmate had an Easy Glider Pro kitted out with the MPX 2s power set. He found it a bit anaemic so fitted a 3s lipo with the same motor/esc/prop but limited the throttle somehow to keep it just within the current range. I'm not sure exactly how he limited the throttle but I understood that he used the Tx rates. Anyway the extra power made a big difference to the performance & worked well for a few weeks - until - somehow he accidentaly changed it either to full rate or re-calibration (I'm not sure which, it could have been that he'd inadvertantly select high rates & that had been left at 100%). The resulting smoke trail was pretty spectacular - worthy of a Red Arrow. IIRC it cost him an ESC, motor & fuselage.

[gangster]

patMc. Yes that is a recipe for disaster Certainly wouldn't dream of pushing the power train beyond the limit in that way.

[Dave Hopkin]

Smaller prop or finer pitch one is the simplest way to go, but a watt meter is essential otherwise you are simply flailing in the dark - thats the downside of electrics the start up costs are a lot more than IC

Edited By Dave Hopkin on 02/12/2014 22:21:01