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Brushless motors how to choose the correct size

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Paul Harris 509/08/2015 23:11:08
62 forum posts

Hi all

Being rather new to all things electric flying I have been searching for information and although I have found plenty of building hints and tips, one thing that is often glossed over is the choice of motor size needed for a given Wight and size of plane.

Are there any online guided to help the newbie pick an appropriate sized motor and matching battery pack.

Thanks for any help with this.


Richard Wood10/08/2015 10:00:26
1098 forum posts
164 photos

Hi Paul,
Motor choice also depends on the type of model - a high revving motor for an EDF
or slow revving for a vintage type or similar. A big factor here is kV rating or 'revs per volt'.
Changing battery size, prop size etc can alter a brushless motor's performance
considerably so selection isn't always straightforward.

There was a handy guide to power system selection on this forum a few years ago
which will still be relevant:

Do you have a particular model in mind to pick a motor for?

Pete B - Moderator10/08/2015 10:11:19
7702 forum posts
735 photos

This thread is worth a read, Paul, as are the other 'stickied' threads in this section.


Geoff S10/08/2015 10:42:44
4034 forum posts
68 photos

As I've mentioned before, selection of a suitable motor would be greatly helped if the published data by suppliers bore some resemblance to the actual motor performance. My personal experience recently has resulted in motors which were wildly outside the specifications published on the web sites. Both motors would have been destoyed had I relied on what the supplier told me rather than testing for myself. A so-called Watt meter (actually its most important function is current measurement) is absolutely essential for anything other than ready to go electric models (eg Wot 4 E) and even then, a useful tool if you want to use a different propeller or a different number of LiPo cells).

It's a jungle out there in electric land I tell you


Ernie10/08/2015 11:40:49
2550 forum posts
24 photos

Hi Paul, have a look at a series of publications called Gibbs Guides. They are very good. You can have a paper copy, or, instantly, an online version. There is one that covers this very, and common problem


Bill_B10/08/2015 11:55:12
1146 forum posts
11 photos
Posted by Richard Wood on 10/08/2015 10:00:26:

Hi Paul,
Motor choice also depends on the type of model - a high revving motor for an EDF
or slow revving for a vintage type or similar. A big factor here is kV rating or 'revs per volt'.
Changing battery size, prop size etc can alter a brushless motor's performance
considerably so selection isn't always straightforward.

A high Kv. motor can actually be used for a glider or vintage model by using it through a reduction gearbox. This then allows the use of a large propeller. There's always more than one way of skinning a cat etc.

NormB10/08/2015 12:00:10
57 forum posts
5 photos

Hi Paul,

the 4-Max site is worth a look too - there are some useful downloadable PDFs. One of them covers motor selection...



Dave Hopkin10/08/2015 12:27:00
3672 forum posts
294 photos

I use Ecalc - I know the models weight, wing area, I know what batteries I want to use, select a generic ESC of about the right size, guess the motor cooling, choose a prop size thats about right for the model aethetically - then use the motor search guessing at the KV value and gradually "tune" into a motor

Mowerman10/08/2015 13:29:42
1569 forum posts
105 photos

A trawl through The Electric Flight Database as in Richard Wood's post . If the actual model you havw is noy listed there will be something very close.

DH 82A10/08/2015 16:53:49
199 forum posts

Also try Web-O-Calc (Google the name). I've used it for years.

MAD Dave10/08/2015 17:20:42
97 forum posts
9 photos

Ecalc has worked very well for me......

john stones 1 Moderator10/08/2015 17:30:10
11924 forum posts
1531 photos

I tend to buy 3 or 4 and ones usually right embarrassed

John...hasn't a clue dept

kc10/08/2015 18:05:15
6946 forum posts
175 photos

The easy way is to use the same setup that someone else has used successfully in a similar type of model. That's what the database is for but unfortunately motors go out of production quickly so it may not help much.

The important thing is to use exactly the same motor with exactly same Kv and the same prop size if you want the same performance.

A currently available motor setup that I have used for an Avicraft Moronic ( roughly equivalent to a 52 inch balsa Wot 4 ) which weighs just over 4 pounds is a Turnigy 3536/9 910kv with APC type 11 by 5.5 prop and 4S3000 lipo. 60 amp ESC. This produces about 500 watts at 33amps to give about 6 minutes of full throttle fully aerobatic flying. A fair compromise between cost / performance / weight / flight time.. Given 465 satisfying flights in the last 15 months (must be about 45 hours! ) A larger Lipo could give longer flights at the expense of slightly reduced vertical performance. Everything is a compromise with electric flight.

Bill_B11/08/2015 11:31:46
1146 forum posts
11 photos
Posted by kc on 10/08/2015 18:05:15:

Everything is a compromise with electric flight.

Everything is a compromise with any form of propulsion for model aircraft. wink

Bruce Austin30/09/2015 09:13:14
242 forum posts
1 photos

Hi guys and Mr Moderator, My question is maybe at a tangent to what this thread is about, so apologies, in advance.

I have been using a Propdrive 2836 1000kv on a 3S Lipo for my P2K glider with the factory 10x6 folding prop with great success.

This motor is rated that it can also be used with a 4S. I have tried this using the standard 10x6 prop, but the motor gets very hot to my fingers. I only want to use the motor with a burst to get to height and then glide. The other day, there was not much lift and so less interval between climbs, and less time to cool down.

I have read that ventilated spinners are available that help cool the motor. How do these work, and do they really make much difference.

Go With the Flow


john stones 1 Moderator30/09/2015 11:26:53
11924 forum posts
1531 photos

Not sure if changing the prop for a smaller one isn't the best solution Bruce but the fluted spinners do work, I think they drive the air onto the motor for cooling, quite a few for sale if you google them here's some at H.K


Keith Berriman30/09/2015 13:00:42
848 forum posts
7 photos

Hello Bruce I once went to a 4 cell operation on a 3 cell motor and found that the power rating on the watt meter went from 300 to over 450 watts so now you can see why the motor is running hotter. Regarding cooling spinners these are useful if the motor is out in an open cowl but if you have a motor in a Glider Nose you will have to look at opening up any cooling holes without weakening the bulk head. If you are looking for a bit more climb out could a slightly larger prop help on a 3 cell battery. You will need a watt meter

Bruce Austin30/09/2015 13:47:33
242 forum posts
1 photos

Thank you guys for those quick responses.

I do have a watt meter and did do some bench checks with the ten inch prop and the figures were within the spec limits.

BUT, ahemmmm, perhaps some manufacturers are being a little "optimistic"???

I will do some more bench experimenting with the watt meter and different props.

I'll also need to look at how the glider nose might be opened a bit for the benefits of a ventilated spinner.

All good exploratory fun.

Go With the Flow


Dave Hopkin30/09/2015 14:12:27
3672 forum posts
294 photos

I have found that most of the cheap motors seldom reach their full power claims, but usually manage 90% of it WITH the right prop, the prop makes a huge difference to the eventual power output so its well worth experimenting

Simon Chaddock30/09/2015 14:41:18
5853 forum posts
3104 photos


I have found the specification figures given on the Hobby King web site are reasonably accurate but not unnaturally seem to represent the very best that can be achieved 'on the bench' and not necessarily in a model.

This applies in particular to motor cooling. The conditions for a motor on a test stand in the open are rather different to being mounted in the restricted space in the nose of a glider.

You not only want to let the air in but also to 'extract' it - ideally into a natural low pressure area.

Edited By Simon Chaddock on 30/09/2015 14:41:43

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