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Radio Control Forum => Electrics => Topic started by: sprogs on October 03, 2015, 01:35:39 PM

Title: Frustrated - Moi!
Post by: sprogs on October 03, 2015, 01:35:39 PM
All I want to do is understand how to decide which motor and which propeller I need for my model. Everyone and every site I've been on say "It depends". Then follows a totally incomprehensible article which, when reduced to it's basics, says "It depends". I F$%£^g KNEW that.
Would it be so hard to just explain WHY it depends?
I think I'm becoming a little hysterical, I might go lay down for a while.
Seriously though, where do I go to learn about eleccy motors and esc's and props?

Title: Re: Frustrated - Moi!
Post by: DaddyO on October 03, 2015, 02:10:51 PM
Well it depends Liz  ;)

What sort of model is it? There were some very helpful articles published in Free flight News about first steps in E36. The alternative is to find someone who is flying a similar design and quiz them about their set up . . .

(It depends because different sized props need different sized motors - same as rubber - different batteries put out different power - same as rubber motors. Esc's and wires absorb a certain amount of power - same as errrr, well we'll just ignore that one for the moment)

I've found that there are a couple of approaches that work for the electrically challenged (such as moi)
1) find out what battery you can use and decide which prop will be most suitable. Use the same size esc as others in the same class; if there's a doubt go up a size
2) Copy a known set up  ;D


Title: Re: Frustrated - Moi!
Post by: danmellor on October 03, 2015, 02:21:42 PM
I know what you mean. I can look at a design and know what size CO2, diesel or glow motor it would require. Not a clue what size electric motor, battery or anything else! It is a black art art practised by the damned...!



Title: Re: Frustrated - Moi!
Post by: slipstick on October 03, 2015, 03:09:46 PM
It is a black art art practised by the damned...!
Well that's told me. Obviously help is impossible so I shouldn't even try.

But you will have to start with a few more details about "my model". My mind reading skills are letting me down and there ain't going to be one simple answer for every possible type of model. You're in the Radio Control forum so if that's what you mean then power to weight is a reasonable place to start. 50W/lb will fly a trainer or vintage model, 100W/lb will get you good aerobatics but you may need 200W/lb or more for a fast EDF model. If you know weight, type of flying and roughly what diameter propeller you want then you're about 90% of the way there.

The only alternative to learning a bit about the black art is either to buy RTF or ARTF models or just copy power setups for similar models to yours.

BTW I'm building a rubber model, what rubber do I need for it....and I don't want anyone telling me "It depends" ;).


Title: Re: Frustrated - Moi!
Post by: Monz on October 03, 2015, 04:30:26 PM
Liz, you do know there is a very simple solution to your problem?

RUBBER!!  ;D ;D ;D ;)

Title: Re: Frustrated - Moi!
Post by: Konrad on October 03, 2015, 04:52:39 PM
First off there is no one way to address the problem. Electric motors are very efficient compared to most other power sources. This means that a wide variety of  motors can yield good results. So don't worry about getting the very best set up. I'm sure you don't  with your IC engines.  

With electric motors the power curve is very linear. Unlike what we have with say a glow engine. For example can you imagine trying to set up this engine for peak performance?

So as has been mentioned before what are you trying to accomplish and what are your aspiration for the future. It really is rather easy and does follow some simple math.

A good pamphlet I liked when I was starting out was "The Electric Motor Handbook" by Robert Boucher. It doesn't go into brushless motors much and it is heavy on the power charts of the Astro motors, but the background knowledge is very helpful and easy to follow.

Title: Re: Frustrated - Moi!
Post by: daveh on October 03, 2015, 05:20:51 PM

What size and type of model are you looking at?


Title: Re: Frustrated - Moi!
Post by: mjmccarron on October 03, 2015, 05:51:16 PM
This might help.


This is a widely debated topic and there are no easy answers. Generally, if you can get the specs on the motors you are looking at, a scale model flies well on 75 - 100 watts per pound of aircraft, Sport models 100 - 150 watts per pound, and high performance 200 +. Watts are voltage x Amps. Voltage is easy, it's determined by the battery pack (2S = 7.4v 3S = 11.1v etc. for LiPo's) Amps can be varied by the size and pitch of the prop. I found eCalc helpful in that it can get you very close to the desired results without spending a fortune on motors and props to experiment with. Experience will quickly allow you to guess a good starting point but until one gains experience that role is filled by frustration. If you still need help and are frustrated, post some specifics and I'm sure you'll get some sound recommendations.

Best of Luck!

Title: Re: Frustrated - Moi!
Post by: nitrowing on October 12, 2015, 12:56:34 PM
I would skip the watts  per pound, it doesn't really work, as watts change with props and can be hugely misleading.
Look for thrust with a suitable prop.
weigh your model empty and calculate how much more weight is suitable for the flights intended, then you can pick a motor and esc and battery that can supply that thrust. many like a 1:1 power ratio or greater, but less will work on a lightly loaded wing. most parkfliers use a 2 cell lip.

Can you post your model that you have in mind?

Title: Re: Frustrated - Moi!
Post by: Konrad on October 12, 2015, 05:31:02 PM
Now I take the opposite  position static thrust in meaningless as the plane needs to be moving to produce lift. This is true wether the model is powered by an IC engine or electrons. Few models actually have more that a 0.5 static thrust to weight ratio.

Now watts per pound is a very good gauge  as to the potential performance of the model. But it is not the whole picture. What it does not address is the pitch speed. As you may know the lift equation uses velocity to derive the lift . It is the pitch of the propeller as a function of RPM that gives us pitch speed.

I like to spec my props for about  2.5 to 3 times the stall speed of the model.
This is an approximation  by Keith Shaw I use to find stall speed:
Stall speed = 3.7 x square root of wing loading
speed in mph
wing loading in oz/sqft

All the best,

Title: Re: Frustrated - Moi!
Post by: TimWescott on October 12, 2015, 05:39:17 PM
One way or another you have to choose both prop details and motor size.  Watts per pound will get you the right motor, then you just need to pick a prop that won't burn it up and will make the plane fly at a good speed.  The choice of prop is usually in the motor specifications -- they give a range of props to use; generally the big side of the range is for the motor with the fewest cells that they call out, while the small side of the range is for the motor with the most cells that they call out.

It's a really good idea to get yourself a wattmeter, or at least a multimeter with the correct current range, and measure the current your chosen motor/prop combination pulls on the ground -- that's pretty much the most current it'll pull anywhere; if that current is within your motor's ratings then you're good to go.

Going the other way, you can choose a prop, then choose a motor that'll spin it fast enough, then ignore watts and burn the motor up.  Or you can choose a prop, choose a motor that'll spin it fast enough, then ignore watts and choose a motor that's way heavier than you really need.

I don't feel that choosing an electric motor for a given model is more complicated than choosing a good IC engine.  It is, however, totally different (except that, in general, bigger model = bigger motor), so it can be incredibly frustrating if you're used to choosing IC engines for planes.

Title: Re: Frustrated - Moi!
Post by: Konrad on October 12, 2015, 06:38:08 PM
I don't feel that choosing an electric motor for a given model is more complicated than choosing a good IC engine.  It is, however, totally different (except that, in general, bigger model = bigger motor), so it can be incredibly frustrating if you're used to choosing IC engines for planes.
I agree with what you say, but don't understand the why of the last part. Why would it be more frustrating than an IC engine? With simple tools one can find what works best for the motor. Few really have any idea if the IC engine is working at peak efficiency. 

Folks seam to choose IC  engines based on what they perceive, there is little objectivity. That is folks with the IC engine have no way of knowing if the engine prop and air frame are properly sized.  Most wouldn't know if the 40 (6.5) sized aircraft would fly best on an OS 40 LA and 11 x 6 prop or a ST 40X on a 8x6 prop.

Now the issue with electric motors, is that since they have such a flat efficiency curve  they can fit a wide range of applications, and operate very well.  One can get a feel for this looking at the site Mike linked to, E-Calc.

So unlike a glow or IC engine which has a rather narrow operating band (I tried to show an extreme example of this)  the electric motor if much more flexible as to the variables (prop, voltage and power) one might want to use with it. It is this wide range of possible solutions that confuses the beginner (more so the novice coming from the world of IC) when looking for the "one proper set up". As it has been said before it depends, the very flexibility of the electric motor is often what freaks out new comers from the world of the IC engine.

I whole heartedly agree that like an IC engine needs a tach to set up the engine. An electric motor flier needs an AMP meter! Otherwise you are just playing Russian roulette and are bound to smoke something! Unlike an IC engine the electric motor's suitability is easy to see (find) with a few tools (a volt, Amp and ohm meter) and some simple math. An IC engine would take a complicated dynamometer to plot the power curve, few of us have bothered to use one of these.

All the best,

Title: Re: Frustrated - Moi!
Post by: Faither on February 01, 2016, 05:12:23 PM
I'm sorry for the late reply on this subject but I've not been on the forum for many months.

Like you, I have little understanding of electric power but, even so, I have built a number of successful electric models, including a quarter scale Bristol M1C monoplane and an 84" span EDF Avro Vulcan.  (Plans coming soon on the builders forum.)

The secret is a software package called Motocalc.   This is a performance prediction software package specifically for electric model aircraft.  It works for any model - you simply enter the model details and the software comes up with dozens of viable power packs - different motor and propeller combinations, battery details etc.   You can then experiment with further combinations to see what effect  any change would have.   
The software predicts flying speeds, stall speed, flight times (which you can tweak by using a slider control to reduce throttle output) and will predict likely power or aerodynamic issues.

The software can be downloaded directly from motocalc's website and you get 30 days free trial which is plenty time to decide whether or not you would be interested in paying for it.   

I did.

And I'm a Scotsman!


Title: Re: Frustrated - Moi!
Post by: sprogs on February 03, 2016, 06:15:23 PM
Thanks Ian, I'll look it up asap, and I hope to give thanks to you for the pointer.