What Size Electric Motor/ Prop??

[Tosh McCaber]

I was brought up on glow engines.  Over the (many) years that I have been in RC, I got a feel for the sizes of engines and props required for different sizes of models that I've flown with- .09, .15, .25, .40, and .60, with their relevant sized models.

However, in the 3 or so years I've been back in the game after a 20 year hiatus, now converted to electric, I still struggle with the size of electric motors/ matching props/ number of cells to use on any of my O/D planes, relating to size weight and style of aircraft.  It's been a case of, when in doubt, ask George (4Max)!!  Whilst extremely grateful for his input, I want to get into the subject further.  I'm sure that there must be articles explaining the subject scattered through our forum pages and elsewhere.  Can anyone direct me to any discussions/ articles on the subject?

[Tosh McCaber]

It's not just the physical sizes of a motor, it's the various options for Kv available that I find confusing.

[leccyflyer]

Tosh

 

You could start by looking at the threads in the electric flight beginners forum here

 

Bear in mind though that for fixed wing models the motor is only 1/3rd of the power train combination and you need to consider the motor, battery and propeller acting in unison. Rules of thumb, in terms of watts/lb for example are also very useful in the first stages of deciding on a suitable power train.

[Piers Bowlan]

There were some really excellent RCM&E articles by the late, great Dave Burton, affectionately known as BEB (Biggles Elder Brother), on the Forum. Some of these covered the mysteries of electric flight. Others, such things as principles of flight and aerodynamics.  I think the archived material is only available to digital subscribers sadly. (I am paper only😪).
Perhaps some of these old articles could be rehashed in future editions of the mag as they would be just as relevant today.

[SIMON CRAGG]

57 minutes ago, leccyflyer said:

Tosh

 

You could start by looking at the threads in the electric flight beginners forum here

 

Bear in mind though that for fixed wing models the motor is only 1/3rd of the power train combination and you need to consider the motor, battery and propeller acting in unison. Rules of thumb, in terms of watts/lb for example are also very useful in the first stages of deciding on a suitable power train.

I was more or less in the same boat as you a few years ago after 40yrs ic flying, I suddenly decided to sell everything ic and go electric.

 

Anything electrical to me was a complete mystery.

 

Anyway, I scoured the internet for "How to convert IC models to electric" and similar, and eventually the fog began to clear.

 

Our club also had a presentation from George recently, from which we all learned a lot.

 

One of the reasons 4 Max has been so successful, is that it has been much easier to ring George than try and work out Watts per pound, volts, amps, battery sizes, prop sizes etc etc.

 

This forum has been very useful, and luckily we have a couple of electrical whizz kids in our club.

 

One tool I have found invaluable, is a programme called "e.calc"

 

It costs about $6 annual subscription for the basic set up, and I have used it countless times to work out which components are best for any given model.

 

Like most things, it look horrendous when you first look at it!.

 

However, after a bit of playing about with the various inputs (and comparing it to other "known" set ups, you should be able to get to grips with it.

 

Very satisfying when you gather all the bits together, go to the patch, and your latest creation flies perfectly.

 

One item which will become another good friend is a "Wattmeter". Again, lots of great articles on the internet as to what an invaluable tool they are.

 

Hope this helps!

Edited December 27, 2022 by SIMON CRAGG

[Brian Dorricott 1]

Another nod for e.calc , always found it reasonably accurate when checked with a power meter . Very good for fine tuning I.e. balancing current/wattage/prop size .

[Outrunner]

Tosh, the most important thing is when you have arrived at a certain size of prop is you check the setup with a Wattmeter. You don't want to damage anything before you even fly. I used to use Motorcalc but lost it many years ago when I updated my computer and never bothered renewing, but I have been keeping a spreadsheet with all my Wattmeter readings that I refer back to when doing a new setup. I record Watts, Amps, Volts, motor, ESC and prop, it's been very useful over the years.

[Frank Skilbeck]

Another vote for e-calc, but there's also drivecalc.de which is free. 

[leccyflyer]

When first embarking on putting together power trains I made extensive use of Motocalc, which was an excellent tool, but found that over time the readily available motors and batteries in the UK were not all that well catered for, requiring creation of custom, user-entered, mathematical models of the motors that I was using. It's many years since I've made use of such software simulation, tending to rely on some very simple rules of thumb in terms of model weights and intended use to determine cell count, prop size and motor specifications - drawing on what has worked in the past. Then those power trains are tested by clamp meter - to ensure that they are within permitted spec for the gear in use - and in terms of flight performance.

[MattyB]

15 hours ago, leccyflyer said:

Tosh

 

You could start by looking at the threads in the electric flight beginners forum here

 

Bear in mind though that for fixed wing models the motor is only 1/3rd of the power train combination and you need to consider the motor, battery and propeller acting in unison. Rules of thumb, in terms of watts/lb for example are also very useful in the first stages of deciding on a suitable power train.

+1 for this, the pinned threads started by Tim Mackey on motors, ESCs and batteries are all well worth reading if you are new to electrics. Most of the crucial content is in the first page or two of each respective thread:
 

https://forums.modelflying.co.uk/index.php?/forum/57-electric-flight-for-beginners/

 

The only bit I haven’t seen mentioned there is the 3W/g rule of thumb for physically sizing (generic, low end) brushless motors that most prop driven electric RTF/PNP/kits/IC conversions will use. For example, if you want to generate a max power of 300W, the motor should generally weigh at least 100g. This ensures it has sufficient size and surface area to dissipate the heat generated, and that bearings etc will have a long life. For higher end (Hacker etc) motors it’s possible to generate more power from smaller motors, and obviously EDF models will generally have much higher W/g ratings because of the enhanced cooling, but as a rule of thumb it’s an excellent one for sizing motors for a beginner.

Edited December 28, 2022 by MattyB

[Tosh McCaber]

Many thanks as usual, all you guys!  I shall use the Christmas break to swat up!!

 

Happy New Year, and great flying to all!

[Romeo Whisky]

I wrote a booklet on the Basics of Electric Flight which can be downloaded from our Club website.

 

The link is here:-   https://dmfc.org.uk/downloads/electricFlight.pdf

[Tosh McCaber]

Thanks RW- I'm looking it up now.

 

Looked it up- excellent- extremely comprehensive!

[Geoff S]

 

1 hour ago, Romeo Whisky said:

I wrote a booklet on the Basics of Electric Flight which can be downloaded from our Club website.

 

The link is here:-   https://dmfc.org.uk/downloads/electricFlight.pdf

 

That's a well written article. You should be proud of it.  I've downloaded and saved it even though I'm quite well-versed in electric flight technicalities.  I seem to recall the esteemed BEB suggested the starting point for designing a power train was the propeller and how fast you wanted it to turn.  That really helps when converting from ic to electric propulsion because the prop doesn't 'know' what's driving it 🙂

 

One minor quibble is your quotation of Ohms Law - Watts = Amps x Volts (which is, of course, correct).  In fact Ohms Law relates Current(I), Voltage and Resistance (V= I R) and the various power equations are derived directly from that.

Edited December 28, 2022 by Geoff S
fast NOT fat!

[GrumpyGnome]

7 hours ago, Romeo Whisky said:

I wrote a booklet on the Basics of Electric Flight which can be downloaded from our Club website.

 

The link is here:-   https://dmfc.org.uk/downloads/electricFlight.pdf

 

Would you mind if I added a link to that on my club website ?

[Romeo Whisky]

Quote

Would you mind if I added a link to that on my club website ?

 

I would suggest you just add a link to our Club website as there are other articles, videos etc there that your members might also find interesting.

 

Check out the Articles webpage.

 

Dumfries Model Flying Club     https://dmfc.org.uk

 

 

[Piers Bowlan]

20 hours ago, Romeo Whisky said:

I wrote a booklet on the Basics of Electric Flight which can be downloaded from our Club website.

 

The link is here:-   https://dmfc.org.uk/downloads/electricFlight.pdf

Perhaps you might consider sending it to Kevin Crozier as a basis for a future article/s in RCM&E? They havn’t had anything like that for a while.

[leccyflyer]

If you do send the article, please consider a little bit of editing to remove the notion that before Lipos it was impossible to get decent performance out of an electric model and that they all needed to be hand launched, because ROG take offs were impossible. That simply isn't true. In the late nineties and early noughties there were marvellous fly-ins all around the country where electric models were routinely taking off and landing powered by NiCds and NiMhs, as Lipos simply didn't exist. I know that several of the prominent electric flight contributors to RCM&E and this forum were successfully flying electric then, because I had the great pleasure of flying with them. 

 

Lipos and brushless models did certainly give a performance boost when they arrived but, just like there was football before the Premier League, there was viable  electric flight before the invention of the Lipo - it was possible to ROG and to convert I/C models to electric flight - we even used to have our own magazine!

[SIMON CRAGG]

A fantastic booklet well done.

 

I will point any newbies in its direction.

 

Thx for sharing.

[Romeo Whisky]

Quote

If you do send the article, please consider a little bit of editing to remove the notion that before Lipos it was impossible to get decent performance out of an electric model and that they all needed to be hand launched, because ROG take offs were impossible. That simply isn't true. In the late nineties and early noughties there were marvellous fly-ins all around the country where electric models were routinely taking off and landing powered by NiCds and NiMhs, as Lipos simply didn't exist. I know that several of the prominent electric flight contributors to RCM&E and this forum were successfully flying electric then, because I had the great pleasure of flying with them. 

 

Lipos and brushless models did certainly give a performance boost when they arrived but, just like there was football before the Premier League, there was viable  electric flight before the invention of the Lipo - it was possible to ROG and to convert I/C models to electric flight - we even used to have our own magazine!

Hi Leccyflyer - I was also flying electric BL (before LiPos) in fact the large club I was with at that time were all electric only, plus slope gliders.  In fact I have never flown an i/c model.  I didn't say decent performance was impossible - the early Multiplex pusher Pico-Jets were virtually mini-rockets - and there were some small Balsacraft warbird models in the club too, but none with retracts.  But we flew off a grass sports field and I never saw an ROG take-off at the Club until LiPos and brushless motors arrived.  Some GWS styrene foam models came with spindly plastic fixed wheels but hardly anybody fitted them.   Clearly personal experiences will vary, but In any case the brief history is not the main purpose and thrust of the article, and it provides an adequate flavour of early electric flight and the essential contrasts between electric and i/c in those early days.

[Mike Blandford]

I first flew electric in the mid 1970s. The model in my avatar dates from 2002. At that time it was powered by a "buggy" motor, geared 4:1 supplied from 10, 2000mAh NiCds.

It did full ROG and was (is) fully aerobatic. The model itself was built quite lightly, the 52" span wing weighs only 9 oz, complete with 3 servos (2 aileron and 1 for split flaps).

I had the flight timer set for 8 minutes.

In 2007 I fitted a brushless motor, and changed to using 12, 3000mAh NiMh, which gave even better performance.

It does now use a 4 cell, 3000mAh LiPo, although being somewhat lighter than when using NiMh it is slightly worse in windy conditions.

 

So I agree that the history in the article is not accurate.

 

Mike

[leccyflyer]

2 hours ago, Romeo Whisky said:

Hi Leccyflyer - I was also flying electric BL (before LiPos) in fact the large club I was with at that time were all electric only, plus slope gliders.  In fact I have never flown an i/c model.  I didn't say decent performance was impossible - the early Multiplex pusher Pico-Jets were virtually mini-rockets - and there were some small Balsacraft warbird models in the club too, but none with retracts.  But we flew off a grass sports field and I never saw an ROG take-off at the Club until LiPos and brushless motors arrived.  Some GWS styrene foam models came with spindly plastic fixed wheels but hardly anybody fitted them.   Clearly personal experiences will vary, but In any case the brief history is not the main purpose and thrust of the article, and it provides an adequate flavour of early electric flight and the essential contrasts between electric and i/c in those early days.

Hi RW - I noted from your article that you were an electric flyer back in the day, but just pointing out that there are some absolute statements in the article about all electric models needing to be hand launched and not capable of take off before lipos which drew my attention. I'm not suggesting a major rewrite of the article, just a wee edit of those absolute statements which, from my experience give a false impression of the state of electric flight in the pre-Lipo days.

[Simon Chaddock]

Col Taplin's electric RC Radio Queen did a ROG in June 1957. OK it was off a smooth runway but it weighed 8 lbs. Just power on/off & a bang bang rudder but it did have a timer to cut off the power if the radio failed. 

[leccyflyer]

Here's my Jamara PT-19 many years ago - easy ROG on a number of different grass fields 12xRC2400mah NiCds, Graupner Geared TurboNeodym Sp700, 13x8" MA wooden prop - 3kg AUW, 65" span ARTF.

 

Also my Wot-4 E-conversion from my previously OS .40SF powered IC model, Twister 60, 16xRC 2400mah Sanyos, 12x8" APC-E on an early electric flight at Shifnal MAC - 6.5lbs, ROG all day off grass, fully aerobatic, no problem.

 

 

Last example shown here is my Aerowatt designed and built by my pal Peter Wilson had a long career flying pre Lipo and Pre Brushless with Mega R7, 16xRC2400mah, built and flown before the BEFA Millennium Bash in 2000 and with stellar aerobatic performance - one of many superb-performing pre-Lipo electric models that Peter was flying around the UK fly-ins in the late nineties and early noughties - DH Hornet, DH Vampire and Venom  EDF, Westland Whirlwind, all more than capable of ROG and super aerobatic performance on round cells. The Aerowatt now flies on 5s1p 5000mah Lipos, more than 20 years later and is my most cherished model, but my point is that this was a superb aerobat before lipos were ever thought of.

 

The e-flight fly-ins in those days were some of the most lasting modelling memories for me. Happy Days.😎

[GrumpyGnome]

On 28/12/2022 at 19:48, Romeo Whisky said:

 

I would suggest you just add a link to our Club website as there are other articles, videos etc there that your members might also find interesting.

 

Check out the Articles webpage.

 

Dumfries Model Flying Club     https://dmfc.org.uk

 

 

Cheers, I'll probably link to the articles that would be most useful for our members, and aren't already covered.

 

It's a nice website!

 

Thanks, GG