Author Topic: Basic motor selection? (Read 3872 times)

Charles Hofacker · « **on:** July 08, 2015, 11:09:20 AM »

Can any of y'all direct me to some very basic motor selection information? For example high or low kV? I know that voltage constant x voltage = rpm. But Lets assume that you are turning the same prop at the same rpm, i.e. the same power, a high kV will use less voltage but more current where the low kV will use higher voltage and lower current. Is higher voltage and lower current always better. What if you want to turn a smaller diameter prop at higher rpm vs. a larger prop at lower rpm? Is kV an indication of the most efficient rpm? I'm sure that you need to match the engine and prop to the air frame. What engine and battery parameters do you look at to select a motor for an airplane? I'm also sure that these and related basic question have been asked about a million times...

I'm looking for the next level up from the beginner's beginner article so often published in the RC magazines. Plus I'm not to sure that c/l stunt would make the same trade offs as the r/c guys.
Thanks,
CRH
P.S. I like equations and graphs...

John Cralley · « **Reply #1 on:** July 08, 2015, 11:25:51 AM »

Well Charles, You can probably find plenty of equations and graphs to keep you busy but for someone who just wants to get going in electric control line modeling, I suggest you study the very first thread in this group, "List Your Setup!"

If you know which plane you want to use, you can likely find an example of a combination of components that will work for you!

Charles Hofacker · « **Reply #2 on:** July 08, 2015, 11:47:02 AM »

Quote from: John Cralley on July 08, 2015, 11:25:51 AM

Well Charles, You can probably find plenty of equations and graphs to keep you busy but for someone who just wants to get going in electric control line modeling, I suggest you study the very first thread in this group, "List Your Setup!"

If you know which plane you want to use, you can likely find an example of a combination of components that will work for you!

This is great information and I'm really glad to see it. But, why were these choices made? What were the trade offs? What parameters were emphasized and why? I was hoping to find some background information on the choices that were made.

Motorman · « **Reply #3 on:** July 08, 2015, 12:59:42 PM »

It's just allot of guessing and seeing what someone else did after making a few mistakes. You would think by now someone would at least come up with a KV to wing area chart.

MM

John Cralley · « **Reply #4 on:** July 08, 2015, 01:58:13 PM »

Take a look at Norm's Cookbook. He explains how he put together his set of components. This may help you.

Tim Wescott · « **Reply #5 on:** July 08, 2015, 02:57:51 PM »

Quote from: Motorman on July 08, 2015, 12:59:42 PM

It's just allot of guessing and seeing what someone else did after making a few mistakes. You would think by now someone would at least come up with a KV to wing area chart.

kV has practically nothing to do with wing area, aside from the fact that it tends to trend down as motors get bigger because the number of cells in a pack trends up. kV says how fast the motor spins in response to voltage, but there's no physical reason it has to be one number or the other -- it really boils down to the fact that small planes are easier and lighter with a few cells, and big planes are easier and lighter with lots of cells.

Select the kV to get the RPM you want with the prop pitch and number of cells you have. Most people run around 9000 RPM; you need to juggle cells and kV to get that. Select numbers such that

kV > RPM / (0.8 * cells * 3.7V)

The 0.8 in there insures that toward the end of the flight, when the battery is sagging, you still have plenty of overhead to turn the prop. Note that kV and cells are both, at this point, free quantities -- so you can work it in reverse, and calculate

Note also that the 80% figure is for Hubin and Renicle timers. An Igor Burger timer may need more overhead than the 80% figure provides -- if you're using his timer, ask him, I don't know, it's not my job.

cells > RPM / (0.8 * kV * 3.7V)

For a five-cell battery pack and 9000 RPM, this works out to kV > 608 RPM/V. For an 800 RPM/V motor and 9000 RPM, this works out to kV > 3.8 cells (use 4 -- 3.8 cell packs are hard to find). The more "excess" kV you have the less efficient things get, but having one more cell than is "optimal" isn't the worst thing that could happen to you, and then you have more overhead.

Select the motor's top rated power by the ready to fly weight of the plane (including motor & battery). I once totted up all the planes in the "list your setup" and came up with 11 watts per ounce of plane weight:

PMAX = (weight) * (11 Watts/oz)

For instance, a 63 ounce plane would need a motor with a maximum power of 700 watts (give or take). Note that this is for a Hubin timer -- an Igor timer demands more energy, but I don't know how much.

From the same "list your setup" I came up with about 7 watts per ounce of weight, again with a Hubin timer:

Pav = (weight) * (7 watts/oz)

For that same 63 ounce plane, that works out to around 450 watts.

Figure out the average current:

Iav = Pav / (cells * 3.7V)

(Yes, Virginia, this varies by cell count -- live with it). For a five-cell, 63 ounce plane, this works out to 24.3A.

Multiply the average current by your desired flight time to get total charge:

Q = Iav * time

For a six-minute flight (1/10 hour) and a 24.3A average draw, this works out to Q = 2.43 amp-hours.

You want the battery to end the flight with ~~at least 75% charge remaining~~ at least 20-25% charge remaining (unless you like buying lots of batteries). 25% charge remaining means 75% charge used, so divide the total charge by 0.75 or 0.80 to get the battery's rated capacity:

Qrated = Q / 0.75.

For our 2.43 A-h flight and 75% of the charge used, this works out to 3.24 A-h, or 3240 mA-h. So, go shopping for 3300 mA-h, 5-cell packs.

If you have a motor with a kV significantly higher than 610 RPM/V, you may want to go with a 4-cell pack -- so, do all the calculations over again. Ditto if your motor has kV < 610 and you need six or seven cells (although, given what's commonly available that probably means that your motor is bigger than you need for any reasonable stunt plane).

Edited on 7-15-15, thank you Igor for rubbing my nose firmly enough in my error to make me actually look.

Target · « **Reply #6 on:** July 08, 2015, 07:39:58 PM »

Motor weight 1/10th to 1/8th model weight in a 870-950kv on a 4S pack should fly most planes, from what my research says.
Raise the KV if you run a 3S pack, but then the capacity of the packs need to go up with the amp draw to get the duration of run you want. Plus I think that most motors are more efficient and loose less power to heat by running at higher volts and lower amp draws. But I might be nuts....

R,
Chris

Tim Wescott · « **Reply #7 on:** July 08, 2015, 10:34:37 PM »

Quote from: Chris Behm on July 08, 2015, 07:39:58 PM

Plus I think that most motors are more efficient and loose less power to heat by running at higher volts and lower amp draws. But I might be nuts....

It's going to depend on the motor and ESC. How good the motor is at turning electrical energy into mechanical energy is mostly a matter of the geometry of the motor and whether or not the space for wire is actually filled with copper.

I've selected motors for servo applications -- you get a data sheet for a given motor mechanical design with different windings; by and large, all the windings give you roughly the same watts out for watts in, with different motor constants (kV or it's aliases).

Keith Renecle · « **Reply #8 on:** July 08, 2015, 11:30:57 PM »

Hi Charles,

All of the above words of wisdom are dead right. There is however another side to all of this and that it the question of what is readily available. If you build your own motors from scratch then you can get as fussy as you like about Kv and the size and windings etc. If however you are like the vast majority of us, that really want to fly with a good performance the simplest way possible, then there are plenty of shortcuts from all of the info on this forum. What I have found to be invaluable info is the spec charts from the Cobra motor website. Each motor size has so many winding options and all have been tested carefully for so many battery and prop options.

As Chris Behm stated you can choose a motor basically by using the all-up weight divided by a factor of 8 to 10 and then choose the Kv to suit the cell count. Using the highest practical voltage is always a good idea, but keep it practical. To simplify everything, your first priority is to fly the plane through a good stunt pattern, so the size of the motor and battery pack has to be kept within practical limits of size and weight. As Tim mentioned, you need the batteries to last, so you need enough headroom, but if you have allow too much then the size and weight goes up. Just look at the many threads or as John Cralley said, look at the List your Set-up section and this will save you hours of research time. Then when all else fails.......just ask as many questions as you like. This is what makes this forum so great!

Keith R

Tim Wescott · « **Reply #9 on:** July 08, 2015, 11:42:38 PM »

You can play with my stunt calc program now: http://atomiczombieworkshop.com/stuff/stunt_calc/index.html. Its operation is perfectly transparent -- if you're a geek. The basic idea is that instead of leading you by the nose through some predetermined path to nirvana, I provide you with a sandbox to play in, complete with a governess who will tell you if you're over- or under-specifying the problem.

Quote from: Keith Renecle on July 08, 2015, 11:30:57 PM

What I have found to be invaluable info is the spec charts from the Cobra motor website. Each motor size has so many winding options and all have been tested carefully for so many battery and prop options.

Another option is to look at what a motor recommends for "3D" -- ounce per ounce, a control line stunt plane demands roughly the same from a motor as a 3D RC plane. If your plane weighs 50 ounces all up, and you have a motor that's good for a 50 ounce 3D RC plane, then chances are you have a motor that'll work for you.

Quote from: Chris Behm on July 08, 2015, 07:39:58 PM

Motor weight 1/10th to 1/8th model weight in a 870-950kv on a 4S pack should fly most planes, from what my research says.

Paul Walker is currently using 5S, if I'm remembering correctly (it may be 6S). Whichever it is, he's using one more cell than is "appropriate" for his motor's kV, but there's a sweet spot in what cells are available that overcomes the inefficiency of running a bit high voltage for the motor kV. It doesn't hurt to be flexible, as long as you know which numbers are minimums, which are maximums, and which need to be spot on.

So -- the 1/10th to 1/8th the model weight will hold, but if you're significantly bigger or smaller than "normal" don't be afraid to go with different kV values. The "best" kV value is, to some extent, a function of ESC technology, while the motor weight to plane weight is a function of the magnets used. I strongly suspect that ESC technology will continue to change, probably in the direction of allowing for more cells but also in the direction of maintaining efficiency over a higher mismatch between the "right" voltage for your kV and RPM and the actual battery voltage. On the other hand, I strongly suspect that magnets aren't going to change any time soon.

Igor Burger · « **Reply #10 on:** July 09, 2015, 12:14:39 AM »

Quote from: Tim Wescott on July 08, 2015, 02:57:51 PM

Select the kV to get the RPM you want with the prop pitch and number of cells you have. Most people run around 9000 RPM; you need to juggle cells and kV to get that. Select numbers such that

kV > RPM / (0.8 * cells * 3.7V)

The 0.8 in there insures that toward the end of the flight, when the battery is sagging, you still have plenty of overhead to turn the prop. Note that kV and cells are both, at this point, free quantities -- so you can work it in reverse, and calculate

Note also that the 80% figure is for Hubin and Renicle timers. An Igor Burger timer may need more overhead than the 80% figure provides -- if you're using his timer, ask him, I don't know, it's not my job.

that is easy:

kV > (RPM +1000 ) / (0.8 * cells * 3.5V)

:- )))

the background is simple, in worst case when model has to carry mass up (when the motor is loaded and battery cries at 3.5V), motor needs to add aproximately 1000 Rpm on top

and I add - just to explain that KV stuff in human words - the KV is only kind of gear ratio, just like the prop pitch, the task is to match lap time, prop pitch (that is that RPM in formula above) with battery voltage and motor properties (that is that KV * 0.

... KV means no load rpm at given voltage, but our motors are loaded, so having resistanse which lowers effective battery voltage we get aproximately coefficient 0.8 ... if you want it exact, you have to calculate with motor windig resistance in meaning: effective voltage = battery voltage - (current * motor resistance)

KV does not chage efficiency or power of motor

however, lower KV will allow lower current at higher voltage (and propriate ESC and wires) so it is clever (may be not always practical) to use highest number of cells allowed by used ESC. That will lower its current. And not only that, if you have 2 cell setup and you see it is too slow, if you add one cell, you will get too high RPM with low effciency because it will make RPM + 50%, while if you add one cell to 5s setup, you will get good result as you have only RPM + 20%, so higher number of cells allows easier battery voltage matching.

Igor Burger · « **Reply #11 on:** July 09, 2015, 12:20:00 AM »

in short:

Is higher voltage and lower current always better.
- no, theoretically it is the same, there are only small practical differences

What if you want to turn a smaller diameter prop at higher rpm vs. a larger prop at lower rpm?
- it make aerodynamical difference, that smaller prop will be probably less efficient when it has to pull (in high slippage) ... otherwise the speed stability is aproximately depending on input power: more power = better speed stability ... but clearly there are always some limitations, and here you have battery weight :- )))

Is kV an indication of the most efficient rpm?
- no does not mean anything for efficiency

Charles Hofacker · « **Reply #12 on:** July 09, 2015, 07:51:35 AM »

Quote from: Tim Wescott on July 08, 2015, 02:57:51 PM

kV has practically nothing to do with wing area, aside from the fact that it tends to trend down as motors get bigger because the number of cells in a pack trends up. kV says how fast the motor spins in response to voltage, but there's no physical reason it has to be one number or the other -- it really boils down to the fact that small planes are easier and lighter with a few cells, and big planes are easier and lighter with lots of cells.

Select the kV to get the RPM you want with the prop pitch and number of cells you have. Most people run around 9000 RPM; you need to juggle cells and kV to get that. Select numbers such that

kV > RPM / (0.8 * cells * 3.7V)

The 0.8 in there insures that toward the end of the flight, when the battery is sagging, you still have plenty of overhead to turn the prop. Note that kV and cells are both, at this point, free quantities -- so you can work it in reverse, and calculate

Note also that the 80% figure is for Hubin and Renicle timers. An Igor Burger timer may need more overhead than the 80% figure provides -- if you're using his timer, ask him, I don't know, it's not my job.

cells > RPM / (0.8 * kV * 3.7V)

For a five-cell battery pack and 9000 RPM, this works out to kV > 608 RPM/V. For an 800 RPM/V motor and 9000 RPM, this works out to kV > 3.8 cells (use 4 -- 3.8 cell packs are hard to find). The more "excess" kV you have the less efficient things get, but having one more cell than is "optimal" isn't the worst thing that could happen to you, and then you have more overhead.

Select the motor's top rated power by the ready to fly weight of the plane (including motor & battery). I once totted up all the planes in the "list your setup" and came up with 11 watts per ounce of plane weight:

PMAX = (weight) * (11 Watts/oz)

For instance, a 63 ounce plane would need a motor with a maximum power of 700 watts (give or take). Note that this is for a Hubin timer -- an Igor timer demands more energy, but I don't know how much.

From the same "list your setup" I came up with about 7 watts per ounce of weight, again with a Hubin timer:

Pav = (weight) * (7 watts/oz)

For that same 63 ounce plane, that works out to around 450 watts.

Figure out the average current:

Iav = Pav / (cells * 3.7V)

(Yes, Virginia, this varies by cell count -- live with it). For a five-cell, 63 ounce plane, this works out to 24.3A.

Multiply the average current by your desired flight time to get total charge:

Q = Iav * time

For a six-minute flight (1/10 hour) and a 24.3A average draw, this works out to Q = 2.43 amp-hours.

You want the battery to end the flight with at least 75% charge remaining (unless you like buying lots of batteries). So divide the total charge by 0.75 to get the battery's rated capacity:

Qrated = Q / 0.75.

For our 2.43 A-h flight, this works out to 3.24 A-h, or 3240 mA-h. So, go shopping for 3300 mA-h, 5-cell packs.

If you have a motor with a kV significantly higher than 610 RPM/V, you may want to go with a 4-cell pack -- so, do all the calculations over again. Ditto if your motor has kV < 610 and you need six or seven cells (although, given what's commonly available that probably means that your motor is bigger than you need for any reasonable stunt plane).

This is a great summary!

you ought to make it a permanent post ("pinned"? "Sticky"?; I don't know the proper term...) at the top of this forum so it doesn't get lost in all the other info available here...

Tim Wescott · « **Reply #13 on:** July 09, 2015, 12:05:12 PM »

Quote from: Charles Hofacker on July 09, 2015, 07:51:35 AM

This is a great summary! you ought to make it a permanent post ("pinned"? "Sticky"?; I don't know the proper term...) at the top of this forum so it doesn't get lost in all the other info available here...

Hey Charles:

I'm not planning on doing that for several reasons:

It's incomplete: it tells you nothing about prop selection
It's way too technical for most people (which is why I wrote stunt calc)
It depends to some extent on the timer, and I haven't captured the right numbers for Igor's timer
Not everyone agrees with it, I haven't personally tested the numbers, and there always seems to be exceptions to the general rule
It's known to be inaccurate toward the bottom end of the weight scale (which I should do something about) -- smaller planes seem to need more watts/ounce, but I don't have good source data: there needs to be more dead-serious 1/2-A sized stunt flying out there

Igor Burger · « **Reply #14 on:** July 09, 2015, 12:10:33 PM »

Quote from: Tim Wescott on July 09, 2015, 12:05:12 PM

smaller planes seem to need more watts/ounce, but I don't have good source data: there needs to be more dead-serious 1/2-A sized stunt flying out there[/li][/list]

That is true ... so I will give you the other end - we fly 200g indoors with 60W

and one question to you: are you sure with this? I discharge it far more

>> You want the battery to end the flight with at least 75% charge remaining (unless you like buying lots of batteries). <<

Charles Hofacker · « **Reply #15 on:** July 14, 2015, 06:03:50 PM »

Quote from: Tim Wescott on July 09, 2015, 12:05:12 PM

Hey Charles:

I'm not planning on doing that for several reasons:

It's incomplete: it tells you nothing about prop selection

You could always add to it as you collect and integrate more data

It's way too technical for most people (which is why I wrote stunt calc)
Don't under estimate the readers here

It depends to some extent on the timer, and I haven't captured the right numbers for Igor's timer
See number 1 above.

Not everyone agrees with it, I haven't personally tested the numbers, and there always seems to be exceptions to the general rule
everyone never agrees with anything.

It's known to be inaccurate toward the bottom end of the weight scale (which I should do something about) -- smaller planes seem to need more watts/ounce, but I don't have good source data: there needs to be more dead-serious 1/2-A sized stunt flying out there
Add caveats as appropriate

[/list]

i liked it. It's a great start. (p.s. for some reason I can't get "Stunt Calc" to run on my PC... It must be a personal problem) And I'm not really looking for a "cook Book" I'm looking for understanding...

And Thanks for what you have posted...

Tim Wescott · « **Reply #16 on:** July 14, 2015, 07:02:15 PM »

Quote from: Igor Burger on July 09, 2015, 12:10:33 PM

That is true ... so I will give you the other end - we fly 200g indoors with 60W

That's actually a lot closer to 7W/oz than some of the 1/2-A planes on the "List your Setup" thread, which seem to run around 10W/oz average for the smallest ones. It may be a wind issue -- an outdoor 1/2-A plane has to be pretty zippy to not get blown out of the circle, while an indoor plane shouldn't have as much to deal with.

Quote from: Igor Burger on July 09, 2015, 12:10:33 PM

and one question to you: are you sure with this? I discharge it far more

>> You want the battery to end the flight with at least 75% charge remaining (unless you like buying lots of batteries). <<

See, not everyone agrees with my numbers! The 75% value came from a number that either Dean or Dennis A. tossed out a few years ago. There's a definite life vs. % capacity used tradeoff at work, as well as a tradeoff between highly-drained cheap cells vs. life and highly-drained high-class cells vs. life. And -- I don't know exactly what it is.

I suppose the correct thing to do, which sounds like an appalling dodge but is actually correct, is to just kind of list the issues and let people decide how close to the line they want to push things.

Me? I make technology. I don't use it!

Igor Burger · « **Reply #17 on:** July 14, 2015, 11:44:26 PM »

Quote from: Tim Wescott on July 14, 2015, 07:02:15 PM

See, not everyone agrees with my numbers! The 75% value came from a number that either Dean or Dennis A. tossed out a few years ago.

Tim, I agree with the VALUE 75% ... but I think of that word REMAINING in battery after flight

Tim Wescott · « **Reply #18 on:** July 15, 2015, 09:08:53 AM »

Quote from: Igor Burger on July 14, 2015, 11:44:26 PM

Tim, I agree with the VALUE 75% ... but I think of that word REMAINING in battery after flight

D'oh. Original edited for accuracy and clarity. Thank you.

Paul Walker · « **Reply #19 on:** July 21, 2015, 02:34:26 PM »

Quote from: Igor Burger on July 14, 2015, 11:44:26 PM

Tim, I agree with the VALUE 75% ... but I think of that word REMAINING in battery after flight

Igor,
How many flights fo you get from your 6S packs when you take them down past 25% remaining?

I am keeping track of mine...Curious about yours and how it fits with Tim's "recommendation".......

Igor Burger · « **Reply #20 on:** July 21, 2015, 02:49:03 PM »

Complicated ...

I use 6s 2600mAh 25C batteries of variouse types (but mostly cheap types like Turnigy Zippy etc). I charge back aproxomately 2000mAh, from 1900 to 2200 depending on conditions (more in calm and less in wind), Typically I do 200 flights, but I had battery with 400 flights (turningy nanotech) and we had also very bad serie of nanotech which survived only 50 - 60 (all of them at different owners, so it was clearly defective serie).

I must note that I fly only one battery at once, I fly charge and fly again. I change battery when voltage after landing falls to 22V (it usually shows 22.2V), it also does not have enough power to accelerate out of the fourleaf.

Paul Walker · « **Reply #21 on:** July 21, 2015, 06:04:51 PM »

I use 6S2700 batteries. I put between 1900 and 2200 back in depending on conditions. I have 4 over 150 flights now. 3 show no signs of degradation, but one has one cell that discharges 0.10 volts more than the other 5. It still works fine in flight. We'll see how long it goes. Two of those four had deep discharges once or twice, and they still work fine at 150 flights. At this point, they are cheaper than glo fuel on a per flight basis!