Paul Harris 5 Posted August 9, 2015 Share Posted August 9, 2015 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. Paul Quote Link to comment Share on other sites More sharing options...
Richard Wood Posted August 10, 2015 Share Posted August 10, 2015 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: **LINK** Do you have a particular model in mind to pick a motor for? Quote Link to comment Share on other sites More sharing options...
Pete B Posted August 10, 2015 Share Posted August 10, 2015 This thread is worth a read, Paul, as are the other 'stickied' threads in this section. Pete Quote Link to comment Share on other sites More sharing options...
Geoff S Posted August 10, 2015 Share Posted August 10, 2015 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 Geoff Quote Link to comment Share on other sites More sharing options...
Ernie Posted August 10, 2015 Share Posted August 10, 2015 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 ernie Quote Link to comment Share on other sites More sharing options...
Bill_B Posted August 10, 2015 Share Posted August 10, 2015 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. Quote Link to comment Share on other sites More sharing options...
NormB Posted August 10, 2015 Share Posted August 10, 2015 Hi Paul, the 4-Max site is worth a look too - there are some useful downloadable PDFs. One of them covers motor selection... *Selection* Norm Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted August 10, 2015 Share Posted August 10, 2015 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 Quote Link to comment Share on other sites More sharing options...
Mowerman Posted August 10, 2015 Share Posted August 10, 2015 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. Quote Link to comment Share on other sites More sharing options...
DH 82A Posted August 10, 2015 Share Posted August 10, 2015 Also try Web-O-Calc (Google the name). I've used it for years. Quote Link to comment Share on other sites More sharing options...
MAD Dave Posted August 10, 2015 Share Posted August 10, 2015 Ecalc has worked very well for me...... Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted August 10, 2015 Share Posted August 10, 2015 I tend to buy 3 or 4 and ones usually right John...hasn't a clue dept Quote Link to comment Share on other sites More sharing options...
kc Posted August 10, 2015 Share Posted August 10, 2015 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. Quote Link to comment Share on other sites More sharing options...
Bill_B Posted August 11, 2015 Share Posted August 11, 2015 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. Quote Link to comment Share on other sites More sharing options...
Bruce Austin Posted September 30, 2015 Share Posted September 30, 2015 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 Bruce Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted September 30, 2015 Share Posted September 30, 2015 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 John Quote Link to comment Share on other sites More sharing options...
Keith Berriman Posted September 30, 2015 Share Posted September 30, 2015 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 Quote Link to comment Share on other sites More sharing options...
Bruce Austin Posted September 30, 2015 Share Posted September 30, 2015 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 Bruce Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted September 30, 2015 Share Posted September 30, 2015 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 Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted September 30, 2015 Share Posted September 30, 2015 Bruce 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 Quote Link to comment Share on other sites More sharing options...
MattyB Posted September 30, 2015 Share Posted September 30, 2015 Posted by Simon Chaddock on 30/09/2015 14:41:18: 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. Yep, agree. If you are ever unsure whether the power figures for a given motor are correct, a simple rule of thumb for a low cost motor in a cowl the real world* is 3W/g i.e. multiply the mass by 3 and you will get a good idea of the realistic, sustained (not peak) power in Watts that the motor can deliver without damage. This in turn can be used to work out the max sustained current using V=IR, and the ESC and prop can be chosen to achieve that. More expensive motors can achieve higher W/g ratings, but obviously you still need to get the cooling right. * - 3W/g doesn't really apply to EDFs where the increased cooling means you can normally run them at much higher W/g ratings. Can't give you any figures though, I am not an expert in EDFs. Edited By MattyB on 30/09/2015 15:37:21 Quote Link to comment Share on other sites More sharing options...
MattyB Posted September 30, 2015 Share Posted September 30, 2015 Posted by MattyB on 30/09/2015 15:29:30: Yep, agree. If you are ever unsure whether the power figures for a given motor are correct, a simple rule of thumb for a low cost motor in a cowl the real world* is 3W/g i.e. multiply the mass by 3 and you will get a good idea of the realistic, sustained (not peak) power in Watts that the motor can deliver without damage. This in turn can be used to work out the max sustained current using V=IR... Ahem... Not V=IR, but Watts = Amps x Volts. Schoolboy error - I'll get me coat! Quote Link to comment Share on other sites More sharing options...
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