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Motors ...for beginners - please keep it simple

All about the whirly bits...for beginners.

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Tim Mackey23/08/2009 12:30:12
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As we learned in the intro section, model aeroplane electric motors are generally divided into two main categories, so lets a take a look at these in more detail.
 
             Inrunners and Outrunners - within each category there are variations of....
1) Maximum power capability,
2) Typical RPM range achievable, and
3) The fuel ( battery ) required to drive it……
 
Sound familiar?
It should do, because these factors are also used in selecting an IC engine.

Inrunner style motors ( where the moving bits are all inside the “can” ) are generally high revving motors more suitable for fast high speed models, and EDFs ( Electric Ducted Fans ) and will be used with small propellers or rotors in the case of EDF. Think of these like the 2 stroke engine.

Most Outrunners ( the actual outside case revolves around a fixed stator ) are slower revving motors with greater torque, and are good for swinging medium and large size propellers. Think of these like the 4 stroke engine.

So, knowing the type of model you wish to power, the first decision is relatively easy - Inrunner, or Outrunner? -and with that out of the way lets look at some more crucial data needed to select the right one. This data should be available with all good motors, and its nigh on impossible to make a proper choice without it.

Kv ( RPM per volt of fuel ) This determines how fast the motor will spin given a voltage from the battery, and the higher this figure, the higher the revs and the higher the current consumption of the motor....... and power ( Watts ) the motor will produce.

Need a motor to power your super fast hotliner? Then choose a high revving model, of around 2000 Kv or more, which will be good for spinning that small diameter, high pitch prop required for top end speed.

Vintage biplane ? You want a relatively larger diameter prop spinning slower to produce more thrust than actual top speed, so choose a lower Kv motor of say 800 Kv or maybe lower.

Maximum Power ( Watts ) or Current. Excessive current will kill motors, so you need to get one which will work at the expected current. Most motor data sheets will tell you the maximum current allowable, and this will be determined by the voltage of the battery used, and also the prop size.
More volts = more current ( and Watts ) and bigger props ( diameter /pitch /or both )  = more current and ( and Watts )

A good general guide as to the power required for different models is

General sports models, older Warbirds and trainers etc 80 - 100 Watts per pound of AUW ( all up weight ).
More modern Warbirds and aerobatic machines etc 120 watts per lb.
Fast models, pattern ships, and jets  150 Watts per lb, with very high performance models requiring around 200 watts per lb.
 
Fairly obviously, larger heavier models requiring good performance will require larger motors capable of producing lots of power ( Watts ) … just as an IC model will also.

So in summary… lets look at the following model.

A high wing Trainer, originally designed for a .30 size IC engine. Its AUW is around 4.5lbs, and would have probably had something like a 10 X 6 prop.

   Outrunner.  450-500 Watt capable. Medium RPM ( Kv ) = around 800 - 1000 Kv.

Now, that wasn’t too hard was it  


Edited By Timbo - Moderator on 23/08/2009 14:13:25

Tim Mackey23/08/2009 16:00:49
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At the opposite end of things, suppose we want a motor for our new small foamy EDF Jet.
AUW is around 1.5lb and being a jet we are seeking circa 150W per lb. As mentioned earlier, small high revving props, and EDF "rortors" or fans need a pretty high Kv motor, probably best to get an inrunner then - especially as most EDF units are designed around fitting an inrunner type of "can".
The model is designed for 3s LiPo so that decision has been made for us
So.... 
 
       Inrunner.  200 - 250 Watt capable.  High RPM ( Kv ) = circa 3500 or higher.
 
Becuse we know the battery will be 3s and therefore about 10.5Volts, we can see that the RPM is going to be very high as follows.
 
10.5v  X 3500 Kv = 36750 RPM.
 
This will, as mentioned, drop due to the loading effect of the fan, but it still means a very high revving setup with lots of speed ( but not much thrust )
This lack of thrust is why many EDF jets need a bungee style launch, to get them up and away.  
The data for the motor should show the expected current draw with the battery you will be using, and also the fan  / prop proposed. This should ensure that you dont exceed the maximum ratings. 
We will discuss how these figures all mesh in with each other back in the general thread, under "Wattmeters" ( Click the link to take you there now )
 
Stephen Grigg24/08/2009 05:18:32
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I havent had an In runner yet,I feel I wont have lived until Ive experienced one.The other thing you cant get your head round initially is that with an out runner the whole motor spins round.Do you notice how whenever you get a new motor the shaft is the wrong way round .You telephone the shop and tell them only to be told you just push the shaft the other way.How do they get those little grub scews so tight.There you are your gleaming new motor and your putting all sorts of pressure on it to reverse the shaft.The relief when it starts moving.Fortunately Ive a 10 ten press at work so its not to bad for me.
Tim Mackey24/08/2009 16:44:09
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Actually, I have only ever had to reverse the shaft on one outrunner - but as you say, it can be rather err umm challenging.  If the motor shaft is still very tight to shift after any grub screws have been removed, then many of the smaller ones will fit within a vice ( use hard wood against the jaw faces to protect the motor ). A suiatable diameter ring such as a large nut, or perhaps a stack of washers , etc can be used to provide a spacer for the shaft to be "squeezed" into IYSWIM. Sometimes they also respond to a good hard whack with a nylon or other softish faced hammer.
Gaz Elliott26/08/2009 21:46:06
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Okay as a beginer I have learnt quite a bit...but not managed to get in the air. So in an attempt to quash that here is my attempt at solving. Does this sound okay??
 
I have a jp 480 si wing
AUW 900g (approx 2lb) 
Battery is 2200mah 3s LiPo
 
To fly the wing easily approx 150watts per lb =300 watts
300 / 11 volts = 27.3 amps
 
Would either of these be okay?
 
Keda Brushless Inrunner 2835 (380S) 2700kv
KV: 2700
Max Amps: 45
 
Or this one
Motor Size: 2445 (370L)

Kv:  2900
Max Amps: 34A
 

I have a 40a esc

 
 
 

Edited By Timbo - Moderator on 26/08/2009 22:38:31

Tim Mackey26/08/2009 22:34:13
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Hi Gaz... I have trimmed some stuff out of your post to keep it simple.
To fly a Zagi style wing - 100 Watts per lb would be Ok TBH, ( but not ballistic ) but lets keep with your plan of 150 watts per lb for now. As this part of the section is about motors, I am not elaborating much on the other stuff. Your choice of Inrunner rather than outrunner is correct, as this type of model was designed around a fairly high revving brushed motor and small prop - usually limited because the ends of the elevons will hit large props. As for mounting bholes, and shaft sizes, well this all depends on the existing mounting holes /your willingness to adapt it etc. Prop shaft size is not that important, as prop adaptors are available in all sorts of sizes.
2700 Kv would be OK, but the 2900 would perhaps be too high -  remember the higher the Kv the higher the revs and the more the current consumed, and the more the noise! powered - flying wings tend to be very noisy as the prop is so close to the wing T/E.
 
The ESC will depend on how much current you pull, and in turn this will depend on the prop you fit.  With ESCs bigger is better see here
 
Your other calcs are about right...well done

 

Edited By Timbo - Moderator on 26/08/2009 22:40:22

Ben Mullins29/08/2009 18:36:41
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SNAP! Nice last name John .
 
Timbo, even for someone who relitivley knows quite alot about electric flight  and electrics in general I've been finding all your little 'guides' quite interesting. Keep 'em coming.
2Bigus05/09/2009 16:48:50
10 forum posts
A phrase sometimes heard is -
'Load the motor with a bigger prop and current increases / use a smaller prop and lower the amps'
Now if we have a small motor say, a 12v and  we conect it to a car battery  ( 12v 70 AH) Surely we might expect this to rev' away to infinity and blow its self apart , but it does not. Why?  Because its "unloaded". It will rev to its maximum, but is not drawing very much current.
Now start to load the motor with a  propeller and the revolutions start to drop, as does, proportionatly, something called  Back EMF ( electro motive force ) and the current  starts to rise, increasing wattage, ( Watts = Amps x Volts).  Remember  that the Amps is what will cause your motor windings to heat up.
Think of your electric fire, 1000W / 240v = 4+amps.
In our 12V motor 1000 Watts will be pulling 83+ Amps! ( 1000W / 12V = 83A )
So consider that thin coat of Shellac covering the winding wires in your motor with some respect. Do not overload it with too much power - or prop it so big that the motor slows  to the extent that the amps rise to heat things back up again up, an almost dead short occurs if you stop it revolving all together, and then you will surely release the magic smoke!
I have here decribed the effects in a direct current system, but our brushless  types are the equivalant of an alternating 3 phase system, but the principle is the same.  
 
 
     
 
 

Edited By Timbo - Moderator on 06/09/2009 15:22:10

Tony Fransens13/09/2009 09:34:03
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Would there be an easy conversion table to convert IC engines to electric? Because I am planning to change a couple of projects , that are still in the pipeline, to electric. Per example the Tony Nijhuis Lysander.
2Bigus13/09/2009 10:55:00
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Equivelence to IC is the difficult part of this, as there are motors = to 4 stroke, slow power, turning large props, then = to 2 strokes high revs, small props. There is unfortunately no comparison tables as its not so clear cut. I find BRC models lists some combos, also,   Hobby city, and R2 models.
A C3548 1100Kv with a 12x8  , prop 60 amp esc, is a good place to start. about = to a good .25 - .35..  2 stroke, light enough to go into a smaller model, but powerfull enough  to pull a larger trainer around. Got my last one off ebay,
 
A Lysander would be nice with a scale 3 blade prop, So perhaps a large motor with a low KV about 800KV. I think a good thing about electric will be the ability to use almost scale size props.
Myron Beaumont13/09/2009 12:44:29
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Timbo
First of all may i say how useful your series of threads have been. Wished they'd been available a while back but brilliant for tying up loose ends to a near beginner to 'lekky stuff!
Here comes the "wobbly" as is my wont.
I'm approaching things from a different direction from most of the contributors
 -That is to say ,I have some spare 'lekky bits ,I know what sort of aeroplane i want to build but am not sure how to plan it all.
 # 1  My spare motor is 2200KV -9-pole-3.5 mm shaft with a prop adaptor (don't know the make-no markings on it )
#2  I have a 4s electrolite prolife series 22C  40C  2200 mAh lipo
#3  The ESC is a Hifie 45 Amp
Now what I have in mind -- An aeroplane that looks like one if you know what I mean- fairly aerobatic - ie mid/low winger -Size irrelevant (transport no problem)-About ten minutes flight time - If you like a sort of Wot 4 type thing (& I have drawn up plans for my version as you know from another thread on the forum) & yes I know it's a high winger but with a good reputation. That's what got me thinking .I worked out that max rpm would be about 28,000
Too much for a prop ?
So  going back to my original query -What sort of weight/prop relationship is possible without smoke .Can I use a biggish prop & not use full "throttle"
The wood is waiting
g-umpy2
!
 
Tim Mackey14/09/2009 17:01:57
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Posted by Tony Fransens on 13/09/2009 09:34:03:
Would there be an easy conversion table to convert IC engines to electric? Because I am planning to change a couple of projects , that are still in the pipeline, to electric. Per example the Tony Nijhuis Lysander.
 No.
PS: I have mentioned the "double up and add zero" system a few times...that sometimes works quite well.

Edited By Timbo - Moderator on 14/09/2009 17:03:30

Tim Mackey14/09/2009 17:07:21
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Posted by Myron Beaumont on 13/09/2009 12:44:29:
Timbo
First of all may i say how useful your series of threads have been. Wished they'd been available a while back but brilliant for tying up loose ends to a near beginner to 'lekky stuff!
Here comes the "wobbly" as is my wont.
I'm approaching things from a different direction from most of the contributors
 -That is to say ,I have some spare 'lekky bits ,I know what sort of aeroplane i want to build but am not sure how to plan it all.
 # 1  My spare motor is 2200KV -9-pole-3.5 mm shaft with a prop adaptor (don't know the make-no markings on it )
#2  I have a 4s electrolite prolife series 22C  40C  2200 mAh lipo
#3  The ESC is a Hifie 45 Amp
Now what I have in mind -- An aeroplane that looks like one if you know what I mean- fairly aerobatic - ie mid/low winger -Size irrelevant (transport no problem)-About ten minutes flight time - If you like a sort of Wot 4 type thing (& I have drawn up plans for my version as you know from another thread on the forum) & yes I know it's a high winger but with a good reputation. That's what got me thinking .I worked out that max rpm would be about 28,000
Too much for a prop ?
So  going back to my original query -What sort of weight/prop relationship is possible without smoke .Can I use a biggish prop & not use full "throttle"
The wood is waiting
g-umpy2
!
 
 Well first impression is that 2200 Kv motor is a fairly high Kv, so most likely its an inrunner ( is it? ) and its suited to swinging small props at high revs.
With that conclusion made... the rest of your questions should be worked through...or simply stopped at that point IYSWIM.
Myron Beaumont14/09/2009 19:14:29
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Thanks Timbo
It's actually an outrunner originally in an EDF unit that I couldn't get a replacement fan for. The only possible solution with what I have seems to be a reduction gearbox ? With their inherent inefficiency I think it's back to the drawing board .(&  keep the motor as a spare)
Total re-think
Tim Mackey14/09/2009 19:20:37
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Oh Ok... there are a few high Kv outrunners knocking about...but as I say, thats a high Kv and suited to small props like a 5 X 4 or summat similar. Indeed you now say its from an EDF originally, so, there you go.
No good at all for a WOT4 type model!

Edited By Timbo - Moderator on 14/09/2009 21:01:05

David RAFster Martin19/09/2009 03:41:24
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Posted by Timbo - Moderator on 14/09/2009 19:20:37:
Oh Ok... there are a few high Kv outrunners knocking about...but as I say, thats a high Kv and suited to small props like a 5 X 4 or summat similar. Indeed you now say its from an EDF originally, so, there you go.
No good at all for a WOT4 type model!

Edited By Timbo - Moderator on 14/09/2009 21:01:05

Myron - I think you might want to consider putting that setup in a small pusher jet similar in size and weight to the jetset44 designs on RCGroups of the depron or FFF line of pusher jets.
With that KV the 5x4 or 6x3 electric prop might be it's max. APC E props (not SF props) would be in order.
 
You won't be likely to see the 28000 rpm when loaded and you need to keep in mind APC's recommendations for their props as well so you don't over rev them and fatigue a blade/blades to the point of having it shed a blade. Definately balance your props just like if it was an IC motor.  If it sheds a blade you'll likely have the motor rip itself out of the plane and  you may not figure out what caused that, if the motor doesn't separate from the plane until impact with the ground as you won't recognize what happened as it isn't uncommon to lose prop blades in crashes...
 
Measure on the ground in a short run up with the props with a watt meter and tach to see what you are getting.  Several motor designs won't tolerate long bench runs on the ground; especially high performance designs.
 
David


Edited By Timbo - Moderator on 19/09/2009 08:44:43

Tim Mackey19/09/2009 08:48:12
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I have moved your other posting into this section David - all good useful stuff, but a bit long and complex for this section
Garry G22/09/2009 12:47:50
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A cautionary tale, I have been using a Speed 600 Race , the one with the brushes outside at the back, in a Playboy Junior, using a 3s LiPo and it has been going like a rocket.
 
No need to check the watts as  it climbs like a home sick angel.
 
Suddenly not working, open up and one brush holder has come loose and a capacitor has come unsoldered, fix this but looks like it got hot !!
 
Better check with Wattmeter, My God it was drawing 43 AMPS, the casing had started to melt and the soldered connection was sprayed around the inside including on the LiPo!
 
Why I didn't have a disaster and the plane bursting into flames in flight only the gods know.
 
The lesson here is ALWAYS check what the load is.
 
Garry G
Erfolg22/09/2009 15:21:20
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Garry
 
I am sure that I have pushed speed 400 Jarama up to 80 amps, without any problems, well not as you have expeirenced.
 
I guess it is a brushless, from now on? Particularly as brushed controllers are few and far between!
Tim Mackey22/09/2009 18:43:30
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Maximum current of the speed 400 series is from 21A up to 40A for the "Race" version.

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