What Size Electric Motor/ Prop??

[Tosh McCaber]

Great to get all the info and chat!  Thanks again all!

[leccyflyer]

I think it's a marvellous website and the themes included are all very close to my heart. Especially in the post lockdown world, where there is an increased interest and consideration given to folk's mental health and the obvious benefits which model flying clubs can offer in that regard DMFC's material on the website is superb. I'd love to visit what looks like a superb model flying site at some point in the future.

[Romeo Whisky]

Hi leccyflyer,

Thanks for the feedback.  You'd be made most welcome if ever you'd like to visit our club.

Happy landings in '23

Richard

[Cuban8]

On 29/12/2022 at 08:27, leccyflyer said:

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!

To say that all early electric models had to be hand launched is obviously incorrect but to be fair, the majority of early kits in the 80s and 90s that came with a ready to use power train and claimed it to be a direct replacement for a .15 .20 or .25 glow motor were IMHO  rediculously optimistic.

I had one of the early MFA Hummingbird kits - probably mid/late 1980s...........heavy foam wing, heavy fuselage, aluminium undercarriage and the recommended power kit that couldn't really pull the skin off of a rice pudding. The charger supplied was one of those mad clockwork timer things!

IIRC I paid over £80 for a couple of small Futaba servos and a small RX for the model, which was an awful  lot of money back then (still have them). It did only just fly, but needed an almighty great heave of a hand launch whereupon it transitioned from a ballistic trajectory to wing borne flight at about a metre altitude if you were lucky. A nerve shattering flight then took place on the edge of a stall and flick roll -  the best thing was to not touch the controls unless you had to. The model was hopeless, but eventually flew very well with a small PAW diesel that I had kicking around, minus the battery dead weight when I converted it. Finally sold it on to a friend who campaigned it with the PAW for ages!

Built a few leccy gliders which were always good performers, but I eventually had reasonble success with a Jack Headley plan piper cub that I converted to electric. Quite an expensive buggy motor coupled with eight selected Sanyo cells (reds IIRC?) and the model did RoG from our grass patch if you gave it a bit of a shove. Once away, she flew very well and was easy to handle in the circuit and landed nicely. RoGs became more and more tricky and extended as the batteries lost their initial vigour and eventually a hand launch was required - might have still been ok from a hard surface.

It's not surprising that some electric models had a bad reputation - many were badly engineered in order to quickly get on the bandwagon and must have put loads of people off. Unless you invested a lot of money in high end kit, the more inexpensive end of the market at the time could result in very marginal performance indeed.......despite the advertising hyperbole and I'm afraid the rather questionable reviews in some model magazines. Mole Technology were very good, but their products were very expensive and only really for the enthusiast. I think their Cobalt brushed motor might have been around £100! Remember seeing their Flittermouse model at Sandown which performed very well.

Our club had a demonstration (late 1980s) of electric scale flying by a well known expert at the time whose name escapes me now, but I think was a model shop owner.  Just remembered the chap....Ted Galbraith, used to run Ace Models in Epping.

He came with a lovely P51 Mustang of something like 65" wingspan or so and powered with a very exotic and expensive motor and battery combo - forty nicads seems to ring a bell, but I might be wrong. The model did fly well, but duration was very short. Not really one for the average club pilot methinks!

Thank goodness for brushless motors and lipos when they eventually came along and really did make electric flight a practical and affordable proposition for the ordinary club flyer.

I think the magazine/newsletter that Leccyflyer refers to might be EFUK or Electric Flight UK.....yes, a very useful source of info at the time.

Edited December 30, 2022 by Cuban8

[leccyflyer]

The magazine I was referring to was Electric Flight International, which later became Quiet and Electric Flight International - Traplet publications.

 

EFUK was the newsletter of the British Electric Flight Association (BEFA) - a very useful newsletter, full of useful information.

[Geoff S]

I used to buy Electric Flight International and even had a few copies stored until last year when I binned them.  I remember contributor Derek Woodward making a case for getting out of the '7 cell trap' which was the maximum number you could charge with an inexpensive charger and limited the size and performance of a model.  Apart from my Sonata 'E' the first successful electric model I had was the Multiplex Twinstar.  I flew it a lot (hand launched) in local fields when a trip to my club site wasn't convenient.

 

One problem with small electric models was the need to buy small servos which were very expensive compared to my usual Futaba 148s etc.  Components are so cheap now (though getting less so) and LiPos and inexpensive brushless motors have transformed electric flight.

 

 

[leccyflyer]

Still have every copy of EFI and QEFI which I often look back on. Then again, I've also got every copy of RMC&E back to when I came back into the hobby, every copy of RCMW, Radio Modeller and RCSA - too much useful info in magazines to chuck them away.

[Anton M]

Here's another motor prop combo calculator (estimator) free to use online:

https://rcplanes.online/calc_motor.htm

Edited February 17, 2023 by Anton M

[fly boy3]

Hi all, I too am in the same position as Tosh, after 30 odd years of ic.  Been reading quite a lot about the subject and I am reasonably happy on choosing a power train now.  The only part that gets me is the motor kv.  What is a ball park number for our normal size sports models ? Thank you.

[PatMc]

I go roughly by - 3000 / number of cells = 1000 kv

eg 3000/3s = 1000kv or 3000/4s = 750 kv etc. 

 

Edited March 1, 2023 by PatMc

[Anton M]

59 minutes ago, OLD FLIER said:

Hi all, I too am in the same position as Tosh, after 30 odd years of ic.  Been reading quite a lot about the subject and I am reasonably happy on choosing a power train now.  The only part that gets me is the motor kv.  What is a ball park number for our normal size sports models ? Thank you.

Here’s my two cents:

The motor Kv is chosen depending on certain factors, such as the number of cells in series you’ve access to or are willing to use and the max prop diameter that fits your plane to produce the power needed to enable the plane flying the way you want.

For a chosen power & prop, you may need higher Kv if only using 2S or 3S cell pack compared to using 4S or more.
Or for a chosen power & cell count, you may need higher Kv if using a small diameter high pitch prop compared to a large diameter small pitch prop.

You may find this link useful:
https://rcplanes.online/setup_guidelines.htm

 

[Denis Watkins]

You will find accurate descriptions of kv in various posts on here Old Flyer, but current stock levels can vary on any choice you may have.

Kv in simple terms is revolutions per volt. Most low cost, available motors are mid to high kv, where lower kv can be more expensive.

Low kv, to me, would be 500 - 800kv. Most of my motors are 2000 - 3000kv (lower priced), and motor kv goes much higher.

Scale bods like to turn bigger scale type props at low kv, akin to IC, running nicely at 8000rpm max.

Sport can more agile, and ducted fan running as high revs as they can.

Mid priced sport motors would be your starting point.

Edited March 1, 2023 by Denis Watkins

[Peter Miller]

I will be honest.  After more years than  you have had hot dinners on i.c. I can't master how the electric motors are calculated but I do have an excellent system. I contact George at 4-Max and say "HELP!!!!!" and that solves all my problems.

[Anton M]

As already mentioned, the KV means how fast the motor will try to spin the prop at a given supply voltage.

Lower KV means the motor turns slower for the same voltage.

For the same motor size, prop and cell count (voltage), the motor with the lower KV would draw less current than a motor with higher KV.

The power and current drawn are proportional to Kv³.

[Peter Wedlake]

In 1990 I built a 70" Dornier Do 335 which flew with 2 brushed motors and 35 nicad cells complete with air retracts, we used to attend quite a few electric fly-ins with it. Over 30 years later it has been re-motored with 2 brushless motors and Lipos, performance has been somewhat transformed.

 

The 1990 version

 

The 2022 version

[Anton M]

1 hour ago, Peter Wedlake said:

In 1990 I built a 70" Dornier Do 335 which flew with 2 brushed motors and 35 nicad cells complete with air retracts, we used to attend quite a few electric fly-ins with it. Over 30 years later it has been re-motored with 2 brushless motors and Lipos, performance has been somewhat transformed.

 

The 1990 version

 

The 2022 version

Transformed for the better or for the worse?
 

Could you please provide more details, such as the plane's weight, props size, how many Lipos (voltage), motors' type, power, and KV?

Thanks.

 

 

Edited March 4, 2023 by Anton M

[Peter Wedlake]

1 hour ago, Anton M said:

Transformed for the better or for the worse?
 

Could you please provide more details, such as the plane's weight, props size, how many Lipos (voltage), motors' type, power, and KV?

Thanks.

 

 

The plane was based on a plan by Al Masters instead of being a built up balsa construction its all hot wire foam veneered. Its weighs 10.5 lbs, powered by 2x 3547 800kv motors with 12x6  3 bladed props and 2x 3300 mAh 4S Lipos.