Power train calculation - a definitive method!

[Geoff Bradley]

I've spent literally hours searching the internet and have come across various methods of calculating 'power trains'. Similar results are often obtained by using these methods but sometimes there can be differences ie. 3S or 4S liopos ??

Also if you look through the different threads - when people ask for help with a particular set up - disagreement is rife !!!!

So isn't it about time the situation was sorted with a definitive method ?

Personally I don't understand why the calculations don't start with the prop ?

Perhaps an article in the mag would be a good idea !

HELP !!!!!!

[Martin Harris - Moderator]

They have. It's called Motocalc

The need for these permutations is what happens when you try to use technology suited to vacuum cleaners to fly aeroplanes though. You don't get these problems with power sources that are happiest when they're emitting smoke!

[Frank Skilbeck]

Or drivecalc, www.drivecalc.de

[Geoff Bradley]

Thanks Martin and Frank - those are two of the many that I have allready come across. Do they give the same results and how do you know they give the correct set ups ??? Also you have to be in front of a computer to use them.

I would rather work things out myself so that I know how the calculations are done.

[PatMc]

How would you calculate the "power train" if using an ic engine ?

[Geoff Bradley]

I'm a beginner Pat but I would ..

Decide on the required Wattage ( ie 150 W per lb for a basic aerobatic)

Convert the Watage to HP ( 1HP = 746 W )

Look at the IC engine specs and pick one with the required HP output.

The prop size for the engine will be given in the engine spec.

No battrey req'd - just a supply of the correct fuel for the engine.

No esc required - just a servo to opperate the throttle.

[kevin b]

Hi Geoff.

I agree with the first 3, but the prop size could also vary. Manufacturers suggest a prop size "range" for "typical" conditions. You don't need a battery, but what type of fuel ? Again it depends on what the engine is to be used for. Lastly, it does just require a servo to opperate the throttle. That is because the engine manufacturer has supplied the "speed/power adjuster" with the engine. My opinion is that electric motors, particularly brushless, should be supplied complete with the esc similarly. I suppose it's a ploy to make the motor look cheaper to buy, but it does contribute to making buying a powertrain more of a minefield for those of us who do not have a great deal of experience.

The only answer I suppose is to buy the complete system from the same manufacturer, which is probably what they are trying to make you do.

Kevinb

[Frank Skilbeck]

With electric motors it's the current that is the limiting factor, so if you increase the volts and prop accordingly you can vary the power output, with an IC engine you can vary the nitro content but it doesn't have anything like as big as an impact as changing the number of cells.

But your analogy with an IC motor isn't quite correct for instance you wouldn't want to use a 4 stroke engine in a pylon racer or a pylon engine in a WW1 biplane even if they were the same HP.

[PatMc]

Geoff & Kevin, have you looked at the electric flight database it's made up from the experiences of forum members with a variety of different types & sizes of models.

I'm a bit pushed for time at the moment but I'll try & get back with a reasonable explanation of how I decide on a power train.

BTW there is no definitive method for deciding electric or ic power train without knowing the requirements of the model it's to power.

[Reno Racer]

I find this is quite good for IC engines

and this is fairy accurate for leccy (obviously confirm with a wattmeter)

For leccy to IC or vice versa another super simple rule of thumb is IC engine size X 2, then add a zero. e.g. 40 (sized IC) x 2 is 80, add a zero is 800. so you need 800 watts to replace a 40 sized engine, which by experience and using other methods ( i have excel calcs for EVERTHING EP), is not that far off the mark. This assumes 2T engine sizes and sport aerobatic flying.

 

Edited By Christian Ackroyd on 26/01/2013 19:40:26

[Martin Harris - Moderator]

I suspect that the point Geoff''s trying to make is that there is no commonality between the majority of electric motor suppliers' designations (and often information in the specifications). In the case of a plan, kit or ARTF, it will normally specify an engine range - you look for a suitable engine in the shed or visit the model shop to buy an engine in the range to suit your choice or pocket. A small amount of experience or research will give you a suitable prop size - bolt it on and try it out without risking burning anything out.

The bottom line is that there are many more variables to consider and unless someone has done it before and you're willing to use their choice, you will need to consider all of them to come up with a workable electric solution. You will also need to develop an understanding of basic electrical calculations (the relationships between power, current and voltage) if you don't already have this knowledge, on top of the aerodynamic considerations.

Edited By Martin Harris on 26/01/2013 19:59:53

[Alwyn Gee]

Geoff

You answered your own question several post back when you stated how you would decide on the power train for an IC model

You decide how many watts you require to for the type of flying you want, then forget about changing it to HP, just go straight to the specs for the motors and find one that gives you the required Watt output. If you are able to calculate HP from Watts then it's just as simple to read the motor specs and multiply the max continuous current (amps) by the nominal voltage of the max number of cells it can use and you get your Watts eg. 45A X 14.8 V (4 cells) = 666Watts. Not forgetting that these figures will always be obtained by using the smallest prop in the range quoted fof the motor. Larger props are for the lower voltages quoted. This puts you in the ball park and then you'll confirm it with your watt meter wont you? Final adjustments can be don with the prop size. Choose a motor that gives you a bit more than the minimum watts you require just as you would an IC engine

ESC is simple. It needs to have a capacity greater than the max burst power amps of the motor

Alwyn

[Geoff Bradley]

Thanks for all the replies - Alwyns method seems to be , from what I've read on the net, to be as I thought but leaves me unsure of a few things ..

Should the motor efficiency be included somewhere in the calculations?

Not all motor manufacturers give recomended prop sizes !

How does the Kv of a motor affect the set up ?

[Reno Racer]

A good read through here might help, Timbo's articles.

Simplistically, Kv dictates prop size for optimum performance and lower Kv equals bigger prop. Motor efficiency is factored into the calculations in ecalc.

[Geoff Bradley]

Thanks Christian - I'll have a read and then return !!!!

Just a thought - if the prop provides our thrust surely our calculations should start with deciding on a prop that will provide what is required and work from there !!!!

[Geoff Bradley]

i've had a good read and followed it OK until he picked 9000rpm out of thin air !!!

Just had a discussion with a friend who says I should calculate the stall speed from the wing loading - then multiply it by 2.5 and 3 to give the speed I should fly at in order to have a stable flight - then pick the biggest prop that will go on my plane which is as close to 1/4 the wingspan - then work out the revs required for different pitch props ..... then work out the kv ...... he didn't get to the end - we will discuss it next week when I see him!

At first I thought - he's pulling my leg - but thinking about it he could be right !!!!!

[PatMc]

Geoff you've said that you would like to be able to calculate what's needed so I've detailed my method for what it's worth. The calculations aren't difficult & once you appreciate where method is going it only takes 10 minutes or so plus some time surfing for suitable motors.

a) Estimate target weight of the RTF model.

b) Decide on the power/weight ratio.
This will vary according to the type of model & required performance. A good way guide for power to weight is to look at the figures used for full size that have the type of performance you want. You can find these figures by looking at the aircraft's specification on Wiki.
A few examples Piper Cub J3 - 40W/lb ; Clipped wing Cub - 53W/lb ; Yak 54 - 143W/lb.
Although we only measure the input power for our model motor it's a reasonably comparable parameter with the quoted full size power as max power is not often applied by full size except in extreme circumstances, even then the quoted power might not be achievable due to atmospheric conditions etc. The caveat is that we choose a suitable size prop & rpm range.

c) Once a) & b) are known it's a simple matter to calculate the target input power in Watts.

d) Calculate the necessary battery weight.
The heaviest piece of the power chain is the battery so in order to ensure that the target AUW is achievable my step is to calculate the necessary battery's weight.
For general sports & sports aerobatic models I find that 5 or 6 minutes of WOT equates to 10 - 15 minutes flying time. Let's go for 6 minutes (0.1 hours). Multiplying the input by 0.1 will give the battery capacity Watt hours.
Dividing this figure by the nominal battery voltage comes up with the size battery in AmpHours. The battery voltage is of course dependant on the number of cells used in series. 2s = 7.2v, 3s = 11.1v, 4s = 14.8v etc. This to be decided in conjunction with the motor.
It might seem that I've jumped the gun here as the voltage hasn't been decided but in fact batteries of the same Watt hours capacity weigh the same.

e) Decide on suitable range of propellors .
The diameter & pitch of the prop will depend on what sort of performance is required from the model. In general terms it's large dia for best climb, small dia for fastest speed. At any given input voltage the prop dia-pitch combination will determine the load which will govern the rpm & thus current (Amps) which will result in the input power (Watts).
I use previous experience & this chart as a starting point (You know how to convert the bhp to Watts)

Although the props listed are for ic it's still a reasonable match for electric props.
Note this only gets the prop to within the ball park of what's needed, some trial and error with different props around the size keeping an eye on a Wattmeter or Ammeter is needed to ensure that the max current isn't exceeded.
Also it's not difficult to estimate where other prop sizes, between the ones shown, would fit in the chart.

f) The Motor.
Most on line retailers give the max current, the Kv & weight of the motor. Sometimes they give a few advised prop sizes with number of cells, this can be useful. They also often give max number of cells & max power but this info is misleading.
In most models I like to install the biggest size motor that will fit for a number of reasons. Most models need some nose weight to get the cg right so it may as well be motor weight as lead. The weight of a motor has a direct bearing on the max current so for a given power a heavier motor will be able to run cooler than a lighter one would. There's generaly a wider range of Kvs available amongst bigger (heavier) motors.

From my own observation whilst bench testing a number of my motors I've noticed that the current is well within limits & power at a useful level when the rpm is aprox equal to 0.7 x nominal voltage x Kv. I use this knowledge to help decide on the Kv & number of cells.
The procedure is simple - on the above chart find the rpm where the desired prop & bhp figure lines cross. Required Kv = rpm divided by 0.7 of nominal voltage. Bear in mind that this is an aproximation. If there isn't any Kv near the result try with a different number of cells and/or a different size prop.

Where possible it's best to use the highest number of cells with a lower Kv in order to keep the current down for the same power. This helps with the next consideration.

g) The ESC.
The ESC need to be able to support the number of cells in the battery & be able to take the current without overheating. Like most people I make sure the ESC chosen is used at a margin below it's max current rating.
Personaly I always use ESCs with built in BEC but I don't use any high current demand servos & where possible switch mode BECs.

If a motor/prop/battery combo turns out less powerful than expected it might be possible to use a bigger prop to increase the power but this needs to be checked against a meter to ensure the current limits of motor &/or ESC aren't exceeded. Another way to increase power is to use a higher cell count but it it will probably be necessary to reduce prop size to keep within current limits.

[Reno Racer]

Electric flight is a funny thing. It all sounds VERY complicated, fiddly and far too technical to start with, after 8 years, I now have a book full of all sorts of graphs, charts and so on i've created on excel and now find it super easy.

That said, easiest option, stick with IC or better still petrol! its a bit more ronseal - it does what it says on the tin. a 46 sized engine swings a 11x6 and flys a plane up to about 7-8lbs and uses about an oz of fuel a minute at full throttle, so normal 12oz tank gives 12 mins of full throttle action. much simpler.

If you tell us (or PM me) what you want, I migt have a chart/database that can help.

Chris

[PatMc]

Christian, my post was deliberately detailed in order that Geoff could calculate own his power train(s) as he requested.
Nothing fiddly or over technical about it. Certainly no more baffling to a beginner than starting from scratch in ic faced with choices of type & size of motor, prop, fuel, tank size, best tank location etc & looking for some method to work out what's required.

After a few months of electric flight experience I'm sure anyone trying the above will only use it for reference, if at all then selecting a power train to suit a new model.

The electric flight database that I linked previously is an excellent reference for anyone not inclined to carry out calculations but happy to make use of others' experience.

 

 

 

Edited By PatMc on 27/01/2013 21:40:21

[Geoff Bradley]

Pat - that is just what I was after - I'll study it carefully and inwardly digest,

I want to understant everything that is happening with my set up and it looks as if this will educate me !!!

If I don't understand I will return !!!!!

Thanks

[Andy48]

Interesting. Nobody has mentioned the single most important factor of the lot. The thrust of the power setup. Measuring static thrust is dead easy, and gives you a pretty good guide. The watts used is only a rough guide and depends on many factors.

[PatMc]

That's because static thrust is useless info unless you want to hover.

[Andy48]

Why?

[PatMc]

Because the thrust reduces as soon as the model is moving forward.

[Andy48]

So what happens to the wattage then, that will similarly change, and therefore by your reasoning is useless info too.