Disclaimer:  I don't fly CL electric, but I follow this stuff, and for a good long time my job was to design electronics and software that makes motors turn.  So I know the basics, but don't know some of the details off the top of my head.

1.  I have a Cobra 2820/12 970kv motor.  What is the IC equivalent?  Do I have a Baby Bee .049 or a ST60?

Equivalents aren't so nice and tidy (for that matter, a Brown Jr. 60 isn't the same as a brand new OS 60AX).  For stunt you generally size the stuff by the weight of the plane.  You want a motor that can handle a peak power of about 11 watts per ounce.  If I'm not mistaken that motor should be good for a plane up to 60 or 64 ounces, at least.

Don't forget the ESC -- you need one that can handle that same 11W/ounce, for whatever battery voltage you've chosen.

2. What size battery do I need?

That depends on airplane weight and your system.  A governing system like a Hubin timer + helicopter ESC, or a KR timer needs about 7W/ounce on average.  A Fioretti or Burger system needs more, but I'm not sure exactly how much more.  You probably want a 4 or 5 cell pack, 3300mAh (4-cell) or 2700mAh (5-cell) BUT it depends on the airplane weight, and I just made a guess based on my recollections of what folks have said -- you need to give us a target weight.

3. How big is the battery bay supposed to be?

Big enough!  Don't make it barely big enough to fit the smallest Zippy pack you can find -- the available stock of cells change with market forces.  The weight and volume stay the same, but sometimes long skinny cells are replaced with short fat ones.  And, at any rate, you'll want to be able to swap out different batteries, and move your batteries around to adjust CG.  Hopefully someone with Real Live Experience will answer here.

Ken hi there.
First there is no simple answer to your question. In general most will use about 150W per pound which works but it places stress on the ESC and battery and tome is a little on the edge. I have a motor capable of 1150W in a 58oz model which is over 300W per pound. This might be one example of plain overkill and on the the upper side.

The thing is I don't come even remotely close to that power usage. The initial reason for the oversize motor was to get the CG in check. Consequently I got a setup that is very easy on the ESC and battery. Average power is in the 17A during the flight perhaps i more at 14-20 range and you can imagine a 66A ESC is not even braking a sweat, plus the battery doesn't have to release the juice at high rate. Now I find out that I can potentially go to a 33A ESC and a much smaller battery saving some extra weight. Spending 1OZ in the motor might have saved me 2-3 oz down the line. Keep this in mind as you select something. Also plan to leave room for a bigger motor in the future you have nothing to loose, but the weight upstream.
Most 60 size ships will  draw about 2000-2200mAh per flight on 6S and a 2800 is a good start for something in 65oz model. Get or borrow one battery and see what you actually use and adjust from there.

Look at list your setup and see what is the accepted setups for the size model you plan to build ( i see you started researching on designing the model of the century  )

To answer the Cobra 2820 motor for my liking you might have a 25 IC equivalent 40 IC max. but check this out and you will be amazed at what would be possible. Expect low voltage to go hand in hand with big props and high voltage to go with small props.

Here is a little story. I went to KOI this year and I had a couple of batteries unused. So I decided to discharge them and run the motor on the ground. After 5 min of flat out running (85-90F min outside) I stopped the motor and immediately checked to see how hot it got with my bare hands. It was barely worm for sure very comfortable to the little warm.  Now I am sure you are even more confused.

Traian

My first victim will be a Fancherized Twister made to look a bit like an F-16 only because I still can't "Just Build" a kit no matter how I try.  I don't think I am psychologically capable of not modifying things.  Building for classic is like being waterboarded (and I have been so I know how bad that is!).

I think the power range you gave me puts this engine on the right plane.  I still need that "Dummies" course. For example, I know from what you and others have said that I shouldn't put stress on the ESC for example.  What is an ESC?  I can guess that it might be the speed controller since I have one of those but it might just as well be an Electronic Swamp Cooler (I have one of those too).  It is like telling someone that their NVA will require reaming and getting slapped!

Thanks though, I sort of under stand most of what you said - Ken

ps - I am seriously jealous that you were able to go to KOI.  That was my "what do you want to be when you grow up" dream when I was 12.  I never made it there.

Ken, I don't know if you've seen my post in classic's but I'm building a Hi Johnson Stuka and that motor you have is what I bought for it. So I guess for us dummies that gives you in the 30-40 size range.

Ken, another thing you can do is to look at the "List Your Setup" thread for similar planes to yours (it's stunt -- there's always a similar plane).  Copy the ESC ratings, battery size, and the motor size and Kv.

A good hint on comparing motors: if you have two outrunners that are about the same weight and Kv rating, then chances are you can swap one for another.  That's going to be true until someone invents an even more awesomely powerful magnet material than neodymium-iron, or until we run out of rare earth elements to make magnets out of.

I agree that motor is equivalent to a .25 to .40 glow engine.
I’ve used KR timers and Hubin timers. I like both, it’s hard to choose a favorite. One feature of the KR timer that can come in handy is the ‘prop strike’ feature. If the timer senses a prop strike it shuts down, preventing the high amps of a locked rotor and the resulting smoke show.
As for ESCs it’s hard to beat a Castle Creations Edge Lite 50. I know they’re not the lightest or the least expensive, but they can log data that you can look at, such as amp draw, to help you determine what your setup is doing. For instance, if the amp draw never exceeds 30A you can swap a Talon 35 ESC in there and save a little weight.
My Primary Force and Vector 40 both use 4S 2800 batteries. The Skyray 35 and Ringmaster use 4S 2200 batteries.
One important thing not touched upon yet is your charger. You want to consider your charger to be an investment in the health of your batteries. Spend a little money, get a quality charger, and do a bunch of reading on how to keep lipos happy. Do not exceed 80% depth of discharge (20% capacity remaining) and store them around 50% and you should enjoy good results.
I’ve settled into a comfort zone with Revolectrix chargers, Cobra motors, Castle ESCs, and ThunderPower batteries. It’s hard to choose between KR or Hubin timers, I’m still using both. I’m sure there are other brands that work well, but I have found these to all work well.

The KR timer uses a programming stick with three buttons and a flashing light. It’s easy to change settings, but as far as I know there is no way to read the current settings, so a notebook would be handy.

The KR timer has no clue of what current the ESC is pulling, so it can't help you there.  Reading current is a function of the ESC -- if you want that, you need to get one with that feature.

I’m talking about the timer settings. If you want to increase, for instance, the gain setting you can not retrieve the current (existing, not amps) setting, all you can do is set the new gain. You have to know the current (existing) setting as a reference point, hence the need for a notebook.

Oh, that current!  Well, that's different.  Never mind.

Here is a Quick and dirty for you.
Get one of these https://www.newfrog.com/product/rc-150a-60v-dc-watt-meter--power-analyzer-with-backlight-lcd-display-104628?currency=USD&gclid=Cj0KCQjwlqLdBRCKARIsAPxTGaUFe6bgjMN167ibGVHA_tJG9ApA0b8IKqWHzNwDfo1-o4WVY8yCapAaAvCbEALw_wcB
And one of these https://www.motionrc.com/products/gt-power-servo-and-esc-tester?variant=19057582982&utm_medium=cpc&utm_source=google&utm_campaign=Google%20Shopping&gclid=Cj0KCQjwlqLdBRCKARIsAPxTGaX4g5AwKnjQQN8G8MIkKzFN-bxuAsJcABRS5CtnpwQLZt4oPiOU0jwaAlgREALw_wcB

Get a big ESC 50-60A and a big bateery capable of releasing the 50-60A.
What is an ESC? Electronic speed controller. Our brush less motors need that to run because the way they are built. Brushless motors work by sequentially energizing the coils inside the motor which in turn attract or repel the magnets in the motor. So depending on how fast this change occurs in the motor presto you have the motor turning at a certain RPM. How does the ESC know how fast to turn the motor? In the past RC dudes where connecting the throttle to a servo which received the signal from a receiver. With the introduction of electric motors the logical conclusion was to insert the RPM control in the ESC (that is why all ESC have a servo plug built in to them) so the ESC acts like a throttle servo. All brushless ESC have things in common 3 wires set to connect to the motor (these are connected to the motor, if motor goes in wrong direction swap any 2 wires and you have reversed the direction) also there is 2 wire set that connects to battery one red one black (red is positive black is negative same as on the battery make sure they match) and a servo plug (this is where the impute for throttle comes from. Impute from the throttle can come from any kind of RC servo pulse signal. This can be a RC receiver a servo tester the timers we use etc. If you look in what signal does an RC servo receives you find out that it is a PWM (pulse width modulated, that is on for a period of time then off (ranging from 1milisecond to 2 milisecond) read here https://www.servocity.com/how-does-a-servo-work

About motors.
They have size a diameter and can length and most motors are labeled as such for example a 4250 which means 42mm outside diameter and 50mm length. A lot of manufacturers are labeling motors as such. All motors have a Kv rating. Kv is kilo voltage in another words for every volt you supply to the motor the motor will respond with a certain RPM, for example if the motor is rated 1000Kv it means if you put one volt in that motor it will turn at 1000rpm, if you put 5V in that motor it will turn at 5000rpm. All motors are rated for a Max power and Max Amp that should not be exceeded. Electrics motors like to commit suicide they might be a little motor trying to swing a 2 foot prop they do not know that they are little they start to draw the power out of the battery and the ESC is pumping all that power in to them till they burn out.

Batteries
Batteries have 3 major things that qualifies them. First is voltage 3.7Volts per cell (1cell battery AKA 1S has 3.7V,  2 cell  2S battery is 2x3.7=7.4V etc.). Second is capacity mAh (how much juice is stored in the battery). Third is C rating and this deals with the ability to release the juice out of the battery (low c rating is like having water coming out of a straw where high C rating is like releasing water from a fire hydrant. 

So how do you know you have the right setup? The answer is that you either copy something someone else likes or you find out yourself. Here is the DIY version. Say you want to power your twister and you know that you used a OS40 with a 11x5 at around 10000 rpm. Now you need to find something that will give you similar results. First look at motor specks they usually recommend prop sizes from to they also recommend a voltage range too (big props will work for the lower voltage while small props will work at the high end of voltage).  Check the Kv on the motor and figure out what battery voltage you need to turn the prop at 10000 (actually go to 12000 to have some head room). So look at your Cobra 970Kv  12000 divided by 970 is 12.4V so you need about 12.4V to make this work. A 3S battery will give you 11.1V (not enough) and a 4S battery will give you 14.8V (above what you need so that is OK). Redo calculations and you have 970x14.8 is theoretically  14356rpm and since you need 10000 then you can expect 70% throttle. Look at the above items I mentioned and connect the battery to the analyzer to the ESC and plug in the servo tester in to the ESC and add the prop to the motor. As you ramp up the rpm slowly watch the power levels rising and do not exceed the Motor or ESC ratings or else you will burn something. Assuming the motor can do it you now know exactly what Amps you need to run the motor (this will tell you what ESC you need to match and what Power you are asking from the battery (Watts)). What should you do with the Watts? Watts = VoltagexAmps. Watch this and figure out what battery you need and that will give you a good solid start place.

Once you grasp this all the rest is the same.

Good luck

Brushless motors work by sequentially energizing the coils inside the motor which in turn attract or repel the magnets in the motor.

Dunno if this'll help or hurt: All permanent magnet motors work that way.  The difference is that brushed motors do that internally and mechanically, using brushes and commutators, while brushless motors depend on an external ESC.

Look here http://innov8tivedesigns.com/images/specs/Cobra_2820-12_Specs.htm
Say you want a 11x5 at 9800rpm  9800/970=10.1V  So 3S will do maybe? 3S is 3x3.7 11.1. There is a little head room but iffy. If you look at chart APC 11x5.5 3S will not produce the rpm you need and if you go to 4S you are in the red. Maybe a lighter prop Xoar 11x6 or 11x5 which is lighter than the APC will work but still you are dancing on the edge.

Next size up Cobra 3510/20 similar but look they did not even consider 11x5.5 in the 4S. http://innov8tivedesigns.com/images/specs/Cobra_3510-20_Specs.htm

Notice in the charts as the voltage is going up the big props start to disappear.

I am trying to show how to think setup.

FYI i use a 4250 with a 11x5 3B prop. Yes the motor is way overkill and i selected it to balance the model but it was the right decision. My system runs very cool with barely any heat and when the active timer commands rpm changes the repose is instant. Also I can go up or down in motor size prop size I have lots of freedom.

Hi Ken, did you read the pdf file I sent? There is a drawing in there showing how the motor, esc, battery and timer is wired. I added just the drawing here. The timers are all basically the same except for mine which needs one extra wire from one of the motor wires to the timer.

Basically the timer replaces the R/C receiver that the esc needs to run the motor. It send out a signal just like a standard R/C servo pulse which varies in width from 1 to 2 milli seconds. All of these timers have a small processor chip in them that allows the user to set the start-up delay, the in-flight rpm, the time of the flight etc.

Keith R

Bigger motor requires less energy to turn the same prop to same rpm that means lower Amps (smaller ESC) lower power draw from battery (lower C rating = lighter battery for same capacity) and usually less power consumption (less mAh capacity which is lighter battery yet).

Thats is true only to some extent. Bigger motor has higher iron loses and lower copper loses, so there is always some balance called optimal efficiency. It is where both loses are equal. Excessive motor will waste energy in iron the same way as too small motor in windung resistance.

Larger motor will run certainly colder, but it does not mean it has lower loses, larger motor can radiate better and thus to run colder at the same amount of loses or even more loses than that smaller, so its temperature is not sign of lower loses.

And the reason why many pilots use rather smaller (thus overloaded) motors is clear - if I have to choose between little larger than optimal motor and little smaller than optimal, I will certainly choose that smaller, it will run hot, but allow better corners (because of lower weight and moment of inertia - even with heavier battery which is close to CG). 

Hi Ken, did you read the pdf file I sent? There is a drawing in there showing how the motor, esc, battery and timer is wired. I added just the drawing here. The timers are all basically the same except for mine which needs one extra wire from one of the motor wires to the timer.

Basically the timer replaces the R/C receiver that the esc needs to run the motor. It send out a signal just like a standard R/C servo pulse which varies in width from 1 to 2 milli seconds. All of these timers have a small processor chip in them that allows the user to set the start-up delay, the in-flight rpm, the time of the flight etc.

Keith R

Thanks - sorry I missed the graphic.  It is amazing how much of what we learn becomes background and we forget that it is all new to someone else! 

Ken

eFlight is probably middling, if they sell batteries at all.

I buy thunder power only. But, being local, they love us las vegas guys. Eflite makes batteries but are unjustly expensive. But they have a new line called Kinexsis that are a good price and hold up to my 3d helicopters very well.

If it is One Flight per Charge the what benefit is there to buying batteries that have more capacity than we will use in a pattern?  Do I need a 3300 or would a 2200 be enough?  I am trying to keep the battery as small (physical size) as possible.

Ken

You want the flight to use about 75-80% of the capacity of the battery -- more than that and you significantly shorten the life of the battery.

One of the things you need to get used to is to think in terms of the energy capacity of the pack, not just the mAh -- a 5-cell 2800mAh pack has the same energy in it as a 4-cell 3500mAh pack.  Assuming that either one will suit (and for most configurations you can bump up a cell and be OK), you'll use about proportion of the charge either way.

If you want to get into the math (and please don't be scared off if you're not a mathophile -- just stop reading), just calculate the total energy as the capacity times the nominal voltage.  For the energy in watt-hours, take the capacity in mAh times the cell count times 3.7V, then divide by 1000 to get watt-hours instead of milliwatt-hours.  Figure that a normal flight with a regulating charger will take about 0.7 watt-hours per ounce of airplane weight -- it's more for an Igor or Fioretti timer, but I don't know by how much.

A quick and dirty is to run the motor on the bench for a minute at the Rpm you need in flight. Charge the battery and see what was put back in the pack. Since 5.5min flight is enough to finish a flight, add 25% to the 5.5
and multiply to whatever you put back in the pack and you have your minimum battery capacity. You can do this on the bench and you can check the current to make sure you don't kill the ESC or the motor or the battery. You need to realize that every plane will have different requirements different power levels rpm, change a prop and it all changes again, so you have to do some testing. Start conservative check how much you put back in the pack (battery pack) do some simple math and predict what you use during a full flight and make sure you stay below the 80% battery usage (70-75% is even better).

Where I am at now is getting the first batteries.  I have concluded that a 4s 2200 30c is as good a starting point as any for my engine, but all of my research says I should start with the prop. and work backwards from there.  Since this is totally unworkable in real life I can only assume that there is a "power envelope" for a given engine/ESC combination and you need to only run props that stay in this envelope. So, what I need next is to test the electronics so I can finish the plane and to do that I need the battery.   

Ken

What's the projected weight of the model?

Where I am at now is getting the first batteries.  I have concluded that a 4s 2200 30c is as good a starting point as any for my engine, but all of my research says I should start with the prop. and work backwards from there.  Since this is totally unworkable in real life I can only assume that there is a "power envelope" for a given engine/ESC combination and you need to only run props that stay in this envelope. So, what I need next is to test the electronics so I can finish the plane and to do that I need the battery.

I'm not sure if the "choose a prop and everything will follow" is best, or if "take the weight and calculate from there" is best.

But as of five years ago or so, the mantra was to figure out the finished weight of the plane and use that to calculate power (both average and peak), then use the average power to calculate battery size, and the peak power for motor and ESC size.  Then you use all of that to figure out a prop size (which, if you did it right, should get you about the same result as starting with the prop).

Because a motor's weight is proportional to it's power-handling ability, and ditto for batteries and ESCs, there should be a rule of thumb for figuring out the final weight from the empty weight -- I just don't know it.

Question, as I'm probably in the same boat as Ken..... How do you build a fuselage for an unknown size of motor and battery?

• Set the wing size, span and area
• Calculate (or steal from successful designs) a target weight
• Use that target weight to figure out the required motor size & battery capacity
• Look at available batteries; make a space a bit longer than the longest one, deeper than the deepest one, and wider than the widest one
• Find the next size up motor from the one you chose above
• Design a fuselage around that

Note that the trend among the Top Dawgs is to build a really big fuselage with thin molded balsa, because the plane can be lighter for the amount of rigidity (but more fragile to knocking around).

Note, too, that it seems to work well to just convert a slimer to 'lectric by leaving out the motor mounts and fitting motor and battery into the available space.  There's too many successful conversions out there for this to be a fluke -- a right-sized motor is a bit smaller than a right-sized engine, a right-sized battery is a bit bigger than a right-sized fuel tank, and everything seems to fit.

Got it. I'll not be designing my own at all. My goal is to first electrify my Stuka. Second would be building and electrifying my Stylus. A design by Tom Dixon, inspired by the Chinese world champ plane the "Skywriter". Both are already proven designs. Just wondering out loud if I need any fuse mods. I guess that will not be necessary.

It's not a bad idea to check -- if you have an especially narrow or shallow fuselage getting a battery to fit may be a challenge.  I've also seen stories of folks needing to notch the leading edge of the wing inside the fuselage -- you'l like to know this in advance, particularly if it means the bellcrank bumping into the battery.

I need to know total weight.  Model with battery ready to be launched.

Weight vs. power for a motor is known, or knowable.  Weight vs. total energy for a battery, ditto.  8th grade algebra exists.  Lunch hour approacheth -- I shall thumb some rules.

I need to know total weight.  Model with battery ready to be launched.

We all have to walk before we can run.  All I need to walk is an appropriate battery and prop for a 2820/12 and a Castle 50 ESC to go on a Twister.  Friends recommend an E-Flite 2200 4s 30C.  You can't get these so what now.  My next project will be to put a new nose on my Nobler then the real goal which is my PA ship under construction for next year.

ken

Just about any 2200 4s 30C.  I trust Zippy, Turnigy (both HobbyKing products), Thunderpower ($$$!!), whatever Brodak sells, and whatever Tower Hobbies sells.  If your motor comes with recommended props, go with what they recommend for 3D flying with 4 cells.

Are there any Twisters, Banshees, Orientals or the like in the "List your setup" thread?  If so, look what they're using and use that for a prop.

C'mon.  Dive in.  The water's warm.  Don't mind those things that look like ice cubes.

Just ordered 2 2200 4s 30c's and a charger from HobbyKing.  I did it at work so that my OS46 doesn't know what I just did.  I plan to fly it Saturday AM and I don't want any jealousy.  As soon as I get the Twister through a pattern I will know enough to make the next conversion better.  You guys have been great!

Ken

Just ordered 2 2200 4s 30c's and a charger from HobbyKing.  I did it at work so that my OS46 doesn't know what I just did.  I plan to fly it Saturday AM and I don't want any jealousy.  As soon as I get the Twister through a pattern I will know enough to make the next conversion better.  You guys have been great!

Ken

If you don't have a way to measure current into the ESC (Wattmeter or ammeter), start with a short flight (one or two minutes), then check the motor, ESC & battery temperature immediately.  If the motor or ESC is hot, don't do a longer flight.  If it's just warm, go for longer flights.

The power electronics engineers guide to temperature:

• Look at it.  If it's glowing, it's really hot
• If it's smoking, it's really hot (but not as hot as if it's glowing)
• Tap it briefly with your fingertip.  If you can feel heat, or hear a sizzle, it's really hot.  If it sizzles, it's over 100 degrees C.  Cowards will lick their finger first
• Tap it for longer and longer times.  If you can hold your finger to it without saying "ouch", then for most people that means its below 50 degrees C -- you've narrowed it down!
• Below 50 degrees C you don't care -- it's cool enough

Note that some power electronics engineers go around with burnt fingertips.  It's an occupational hazard...

We all have to walk before we can run.  All I need to walk is an appropriate battery and prop for a 2820/12 and a Castle 50 ESC to go on a Twister.  Friends recommend an E-Flite 2200 4s 30C.  You can't get these so what now.  My next project will be to put a new nose on my Nobler then the real goal which is my PA ship under construction for next year.

ken

I have that same motor on my Primary Force (47 oz) and it does well with an APC 10x5.8 (F2B) pusher prop. I use a ThunderPower 4S 2800 battery and land with 21% remaining.
I think the 2200 mAh battery might be too small for a Twister.
Generally I set my timer for a 5:40 flight duration. With a new plane I set the timer for 2:50, do some trim testing, and check battery usage. Since the 2:50 flight is half of what I want, it’s easy to get an accurate guess if the battery can handle 5:40.