I tried searching for this information and the more I read the more confused I was.

1. For example, I have a 2212 1100KV brushless motor. What do the numbers mean. I know the KV is rpm per volt but wouldn't that change with propeller size? For example what is the 2212 meaning. and what does the number of turns change things.

2. I want to replace an OS MAX .046 in my sport aerobatic airplane with a electric power system. Where do I start?

3. I see brushless motors rated 3S or 3S or 4S. Am confused. wouldn't you pick the motor by capable current and let me figure out was voltage to send it?

4. On the same subject only with ESC. If the esc is rated for 40AMP 3S LIPO. Couldn't I use 4S as long as I keep it under 40 AMPS?

5. Is there some magic way to determine what propeller size to use for any given motor?

Thanks,
BTW I looked for a forum for Stupid persons but didn't see one. LOL

Possibly an easier place to start would be the --- Glow to Electric Conversions forum

Just search in there for the model you wish to convert.

Electrics, (electrickery), can get quite complicated, especially if the high level tech guys decide to answer.

Ray.

Originally Posted by Biker66

I tried searching for this information and the more I read the more confused I was.

I remember that feeling. No worries, help is on the way!

I'm crazy-busy at work for the next hour or so, but I promise I will be back, and will do my best to answer your questions. (They are good questions, by the way!)

-Flieslikeabeagle

Biker66
kV is the speed the motor tries to run at. Remember brushless motor are basically a constant speed type so will draw as many amps as required to make the prop turn at the kV times the battery voltage.
The problem is that you have to use a prop that will keep the amps within the capability of the motor or the ESC or the battery but will generate enough thrust to fly the plane.
At least initially by far the simplest route is to use recommended components that others have already established will work together.

I'm back, as promised, though it took an hour longer than I thought it would.

Okay, here are answers to your questions:

Originally Posted by Biker66

I tried searching for this information and the more I read the more confused I was.

I remember that feeling. No worries, help is on the way!

Originally Posted by Biker66

...2212 1100KV...I know the KV is rpm per volt but wouldn't that change with propeller size?

Kv is rpm per volt with no load on the motor (no propeller).

Your question about motor rpm changing with propeller size is a very good one. We are all used to the idea that internal combustion engines (whether in our cars or our model aeroplanes) will rev themselves up to such a high rpm that they will self-destruct if we run them without a load. And if we put a heavy load on them, they will slow right down.

Electric motors are different. If you had a magic 100% efficient electric motor, fed from a magic battery which kept its voltage perfectly constant, the motor would turn at exactly the same rpm, no matter what load you put on it..

Real motors are not 100% efficient, of course. Because it's not 100% efficient, the rpm will drop when you load it. But it won't drop very much - nothing like what you're used to with a small glow engine.

In fact, there is a somewhat reasonable rule-of-thumb for the cheap brushless motors we all use these days: load it down until the full-throttle rpm drops by 30% from the (also full throttle) no-load rpm. No more. (If it runs at 10,000 rpm with no propeller, don't load it any lower than 7000 rpm.)

If you want to monitor motor rpm, I suggest a laser tachometer. You can find them on Amazon, etc. I put a strip of masking tape on the outside of the motor (the rotating part, for an outrunner) to give the tach something to read.

Originally Posted by Biker66

For example what is the 2212 meaning. and what does the number of turns change things.

This is semi-random stupidity that somehow became commonly used. Usually (but not always) the "22" is the diameter, in millimetres, of the stator inside the motor. The "12" is usually the thickness of the stack of lamination in the stator.

This is a ridiculous way to specify a motor size. It's like specifying a car engine by the bore and stroke of each cylinder.

The number of turns of wire inside the motor changes the Kv. But only the motor winder really needs to know this. Another ridiculous way to specify a motor to the user.

What is more useful:

1) Look at motor Kv (it needs to match your model and your type of flying, more on that later).

2) Look at motor weight. Your model has to carry this, and it is also a clue about how much power the motor can handle. A crude rule-of-thumb for cheap motors is that they can handle about three watts per gram of motor weight. So a typical budget-priced 1-ounce brushless motor (28 gm) can most likely handle roughly 85-100 watts maximum, even if the manufacturer claims it's good for 575 watts.

3) Look for "motor winding resistance", or "Rm". Most cheap motor manufacturers won't provide this. But if it is provided (and if it is honest), the lower it is, the better. Low Rm means a more efficient motor, all else being the same.

Originally Posted by Biker66

2. I want to replace an OS MAX .046 in my sport aerobatic airplane with a electric power system. Where do I start?

I wrote my own software (WebOCalc) to make it easy to do exactly this sort of thing. It makes the process much simpler. I can help you with this if you'll provide the following pieces of information:

1) The estimated weight of the model (dry weight of the airframe + add 25% more for electronics is an okay starting point.)

2) Wing span of the model.

3) Either wing area (usually in the specifications if it's an ARF), or measure the wing chord, or measure tip and root chords if it's a tapered wing.

4) The biggest propeller diameter that will fit on the model comfortably, allowing an inch or two ground clearance so you don't smack the prop into the runway/grass on every landing. (For electric power, you almost always should use the largest propeller that you possibly can; it will fly your plane much better than the usual under-sized glow prop will.)

5) The sort of flying you want to do with this model. Is it an aerobatic model? A scale warbird? A trainer? Do you want to just fly in slow lazy circles? Do you want enough power to fly vertical up-lines for precision aerobatics? Or somewhere in between, you want to fly sport aerobatics, but no need for unlimited vertical or hovering?

If you get back to me with answers to these five questions, I can help you pick a motor, battery, and propeller that will pretty much do exactly what you want them to do.

Originally Posted by Biker66

...I see brushless motors rated 3S or 3S or 4S. Am confused. wouldn't you pick the motor by capable current and let me figure out was voltage to send it?

You are exactly right. Most motors capable of running on 3S are also capable of running on 4S. (Very high-rpm motors might fly apart if you feed them too much voltage.)

Originally Posted by Biker66

4. On the same subject only with ESC. If the esc is rated for 40AMP 3S LIPO. Couldn't I use 4S as long as I keep it under 40 AMPS?

Not always. This depends on the maximum voltage of the transistors (MOSFETS) used in the ESC. If the manufacturer doesn't specify that the ESC can handle the higher voltage from a 4S pack, don't take the risk - it might fry your ESC.

Originally Posted by Biker66

5. Is there some magic way to determine what propeller size to use for any given motor?

Yes. It's too mathematical to easily do by hand. I made it easy because it's built into WebOCalc. Give me anwers to the 5 things I asked about your model above, and I will run it through WebOCalc for you, and explain anything that needs explaining.

Originally Posted by Biker66

BTW I looked for a forum for Stupid persons but didn't see one. LOL

That forum is magical - you can only find it if you actually are a stupid person. You're not, so it's invisible to you.


-Flieslikeabeagle

Originally Posted by Biker66

I tried searching for this information and the more I read the more confused I was.

1. For example, I have a 2212 1100KV brushless motor. What do the numbers mean. I know the KV is rpm per volt but wouldn't that change with propeller size? For example what is the 2212 meaning. and what does the number of turns change things.

2. I want to replace an OS MAX .046 in my sport aerobatic airplane with a electric power system. Where do I start?

3. I see brushless motors rated 3S or 3S or 4S. Am confused. wouldn't you pick the motor by capable current and let me figure out was voltage to send it?

4. On the same subject only with ESC. If the esc is rated for 40AMP 3S LIPO. Couldn't I use 4S as long as I keep it under 40 AMPS?

5. Is there some magic way to determine what propeller size to use for any given motor?

Thanks,
BTW I looked for a forum for Stupid persons but didn't see one. LOL

You are getting advice as good as it gets from flyslikeabeagle.

I would recommend a similar approach, using either the WebOCalc tool or another motor modelling tool ECalc. Rather than muddy the water here, I'll just add links to the methodology I use.
https://www.rcgroups.com/forums/show...odology-Part-1
https://www.rcgroups.com/forums/show...odology-Part-2

Thanks for the vote of confidence, Scirocco!

Biker66, many on this forum (including scirocco) know how to pick a good power system for a model. It doesn't matter who helps you, as long as the help comes from someone who knows what they're doing.

If we haven't already lost you, come back with answers to those 5 questions about your model and how you want it to fly, and we'll soon get you pointed at suitable components (motor, prop, battery).

-Flieslikeabeagle

I go about it quite differently, since nothing about shopping for a motor frame / KV / ESC? cell count does anything to produce the needed energy , I start out like this:

750 watt is 1 Horsepower...

So an OS manual states it is 1.75 HP,,,,,,

Next up, is my tried and true formula ,,,, 10 grams of battery will produce 25 watts for aprox 3 1/2 minutes flight time,

So I calculated I need roughly 1300 watts to produce a needed 1.75HP into a propeller.

So then simply 1300/25=52X10= ahaa 520 grams of battery energy density is needed to produce 1300w to replicate a 1.75HP .

This is very KEY to an optimal wing loading/ thrust to weight/ and where will it be placed to make CG. Its EAASSSY to add more battery for a very poor flight time return for the losses in other categories.

Consider this, if its a 1 cell or 10 cell weighing in @ 520grams it will output 1300W in either case. Using Ohms law the 1s @4.2VDC will be doing it @ 300 amps, while the 10s @4.2VDC per cell will be doing it @30 amps. ahhha ESC is rated @ what??? AMPS

So the frame numbers (2212) of a motor will have a stated watt handling value as well as cell count/voltage range, the more iron ore( ie gram weight) = more watts capable, typically the first two digits are the diameter the second two are the length. Just insure it can handle your 1300 watt needs.

Next up is a bit more convaluted since KV/Cell count are the next choices I liken to the gear selection in a transmission to insure the PROP, which is the load, is using 1300 watts.

So do you like a 10X6 or 11X7 higher revs of old 2cycle or more like a 12X6 or 14 X4 of a 4 cycle lo end grunt?

So if I preffer a 10X6 because of ground clearance of my sport model circle flyer. I will again use the OS manual and see this is in the 17K rpm range. So
Every 1000 of KV is 10K rpms no load @ 10 volts hmm this is when toying with online calculators make it easy because we already have a good idea of the motor gram weight and choices in frame sizes needed to handle a 1300w prop load.

I'll just post two examples with a quick toying in a motor calculator in which I picked a frame size of 3020. you can see just changing the cell count changes rpm range , KV changes could also as well. You can hopefully start to make some sense of how energy density of liquid fuels corralate into electron fuel speaking very loosely.............

FWIW, I was the one who was asked what in electrics was needed to create the EF1 electric class of pylon racing. I used the exact same formula to come up with a 314gram battery weight limit out of the gate to replicate the HP range of .32-36 nitro motors. The pylon guys had free range to find prop/KV/cell count/motor size to accomplish 10 laps of racing in which fast time of each lap could be achieved. I heard reports of 4s-8s but they all started to appreciate the fact that the gram weight limit was a wall to 10 laps of equal perfomance. After first season testing they only agreed to a one prop prefference, I then told them to establish a KV and cell count, to not crush batteries as an added expense to an entry level class.

Hi C/F
Scorpion Calc freeware is now (2020) at version 3.85
v 3.67 was deleted in 2017
http://www.rcgroups.com/forums/showthread.php?t=736782
Louis

Originally Posted by c/f

...nothing about shopping for a motor frame / KV / ESC? cell count does anything to produce the needed energy...

That's not really true. WebOCalc starts by finding out what the airframe needs (thrust, pitch speed, prop size), and from that, how much power is needed from the motor, and from that, how much stored energy is needed in the battery for "x" minutes flying time.

Originally Posted by c/f

...an OS manual states it is 1.75 HP...
<snip>
...produce 1300w to replicate a 1.75HP...

This approach is workable, but very inefficient. We can do much better when we convert from glow to electric!

Glow engines only deliver their full power at very high rpm. So you are forced to use a tiny and inefficient propeller. This can waste an enormous fraction of the available power - it is quite easy for propeller efficiency to fall below 40%, meaning 60% of the power from the glow engine is being completely wasted. (All that power goes into wildly churning up air, rather than actually propelling the model forwards.)

A properly chosen electric motor can deliver its power at a much lower rpm. You can use a bigger propeller, and if you pick one that is properly matched to the airframe, propeller efficiency can climb to maybe 70% (instead of only 40%).

And what this means is that 1 watt from the electric motor propels the model as well as 1.75 watts watts from the glow engine. (1.75 comes from the ratio 70%/40%).

Put in other words, 0.57 watts from the electric motor (spinning the 70% efficient prop) will do as good a job as 1 watt from the glow engine (spinning the too-small 40% efficient prop at too-high rpm).

So, if the glow engine puts out 1300 watts, you only need 742 watts from the electric motor to propel the model just as well as the glow engine. That in turn means you can use a smaller and lighter battery, and a smaller and lighter motor.

The lighter weight means the model will fly better. The smaller motor and battery will save you money, too.

In practice, you'll probably need less than 740 watts to match the performance of that OS engine. This is because that 1.75 hp number is almost certainly only achievable on the dyno, at very high rpm. Once the engine is loaded down with a propeller, it's probably putting out far less power.

The short version of the above: you can get better performance with fewer watts from electric power, as long as you pick the power system properly.


-Flieslikeabeagle

Thank you all very much, I think i Have the information I need now.

Originally Posted by Biker66

... and what does the number of turns change things ...

You don't need to know it, number of turns is already incorporated in Kv, thus making current and power calculations much clearer and simpler. It also makes understanding motors easier. Why the car guys still use turns instead of Kv I don't understand.
Another thing to forget, only relevant for diy motorbuilders, Kv is inversely proportional to number of turns.

Originally Posted by Biker66

... Kv is rpm per volt but wouldn't that change with propeller size? ...

Like mass, Kv does not change with speed, both are physical properties.

Originally Posted by Biker66

... I see brushless motors rated 3S or 3S or 4S. ....

Motors can handle any voltage in RC land, as long as rpm and current don't get too high. Insulation lacquer/resin can handle several hundred volt.
Highest voltage goes with lightest prop and vice versa, to keep current in bounds. Don't combine highest voltage and biggest load, excessive current.

Originally Posted by Biker66

... If the esc is rated for 40AMP 3S LIPO. Couldn't I use 4S as long as I keep it under 40 AMPS? ...

Both ratings are limits that must be adhered too.

It is a good practice to have some headroom built in.
About derating motors, controllers, batteries, electronics in general:

• Derating, starting at ... Then there is the 75% rule of thumb ...
  Questions and Answers.... - Page 928 - RCG
  
• Motorcurrent is proportional to voltage squared, and proportional to Kv cubed.
  A 10% change in battery voltage will give a 20% change in current, a 10% change in Kv will give a 30% increase. Not quite, because battery voltage will sag more due to higher current, but you'll get the idea.
  
• extra current with one or two cells added, simple table
  A.k.a. "Why did my motor and/or controller and/or battery go up in smoke ??? ¨.
  In depth discussion
  www.theampeer.org/ampeer/ampnov15/ampnov15.htm#ADD
  
• About de-rating controllers and motors, starting at for everyone else:
  Questions and Answers.... - Page 290 - RCG
  

Vriendelijke groeten en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
• E-flight calculators • watt-meters • diy motor tips&tricks • Cumulus MFC •

Note that Kv says very little about max.power or max. current a motor can handle, or about max. torque, or propsize. A 1:1 train motor and the motor in your toothbrush or in a bedroom appliance can have the same Kv = 2000pm/volt.

Kv is the velocity Konstant, a physical property (like mass, length and volume), expressed in the unit rpm/volt (like kg, meter and m³).

Kv is not a rating, not a figure of merit, not something the motor delivers (explanation).
A Kv=2000rpm/volt motor on 10volt and a Kv=4000rpm/volt on 5volt will give same (no_load)speed of 10,000rpm, provided the motors can handle the current and power.


E-motor versus ic-engine behaviour
They have a fundamentaly different behaviour.
An engine tries to keep a constant torque: as load goes up, rpm goes down.
An e-motor tries to keep a constant rpm, no matter prop-pitch and -diameter: as load goes up, torque must go up, and therefore current has to go up.


It's all about what the motor wants to do versus what the motor can do.
Kv matches desired rpm and battery voltage, there's nothing more to it.

1. Want, try: Kv
   Kv and voltage determine how fast motor wants/tries to run
   (rpm_noload = voltage × Kv, or, in other words, Kv = rpm_noload / voltage.)
2. Current and torque
   RPM and prop determine torque needed, which in turn determines current drawn
   (current = torque × Kv, in SI units!, or proportional to Kv³ ).
3. Can: max.current and max.power
   Max.current and max.power determine whether that battery/motor/rpm/prop combo can run without going up in smoke.

As another old nitro guy who is exploring electric RC, I have to say that this has been a very useful thread.

--ElJay

Originally Posted by Ron van Sommeren

... Like mass Kv does not change with speed, both are physical properties. ...

Another way of looking at it: even when 'motoring' our motors act as generators. Drive the motor with a powerdrill, measure generated voltage (AC) setting.

1/Kv = voltage/rpm

This is how Kv is determined. Load, current and power do not come into play, only voltage and rpm.



Four simple straightforward methods for determining Kv, in case sticker/printing has faded:
www.bavaria-direct.co.za (RCG user skylar)
→ Motor constants
→ The speed constant or velocity constant (Kv)


Prettig weekend en wees voorzichtig, Ron