I'm trying to wrap my head around how to determine which motor/ESC or prop to use. If I have a 3200kv motor, how do you know which ESC or prop I should use it with? What's the math involved? Thanks.

You're kinda working backwards. Here is a breakdown of how I size a motor/esc/prop combo (doing a glow conversion mostly)

Look at the size prop you can put on the plane and choose one (let's say it is a .46 conversion). 12x6 prop (just a swag)
Look at the weight of the plane and the type of flying. 75W/lb for trainers up to about 150-200W/lb for 3D.
A decent outrunner motor should be limited to about 3 watts per gram of weight of the motor.

Let's use a 7 pound trainer now as an example. 7lbs at 75W/lb is 525W. At 3 grams per watt we're looking at a motor of at least 175 grams. Ok, this will get us in the ballpark.

Now, this is where the iterative process starts. Looking into the different motors available (I shop Hobbycity.com a lot) You kinda have to fit the motor to the prop and back and forth. It helps to have a good database to start with too. I use Drivecalc, which is freeware and is powerful. Here is what I do:

knowing that I have a 12x6 prop and I want to get about 525 watts of power into the prop I need to know how fast that prop will need to spin. I have a program that has the prop constants in it called ThrustHP. Using this and knowing I have the 12x6 and 525 watts, I can convert for HP and get about .6962 HP. Knowing this I can play with the RPM number to get the same power. I see that 9250 RPM will give me about the power I want.

OK, now that I know that I need about 9250 RPM at load, I can find out what my no load speed should be. At about 75-80% of no-load most brushless outrunners make the most efficiency, so if I take 9250/.8 I get about 11,500 RPM.

I would use this information and the motors that I typically shop to find out what the range of Kv is for the motor s in the 175-200g size range. Knowing that I shop HC, most of these motors are between 900 and 1100 kv (looking at the Turnigy SK35-48). So if I use one (1100 kv) and take 11,500/1100 I get 10.45 Volts, and if I take 11500/900 I get 12.7V. So a good nominal battery might be a 3S lipo. and because it is nominally 11.1 v, I would want to choose the 1100kv motor so I can get at least 11,500 RPM nominally on the prop.

OK, now we have a prop, motor, and battery type, what size do we need? Well, at %80 efficiency getting 525 Watts out of the motor, we would be putting about 650 Watts into the motor. If we have a battery that is 11.1v, then we would be pulling about 60 amps peak. This tells us that we need a battery that at peak can provide 60A, and also an ESC that can handle 60A peak.

How big of a battery do I need? Well, common ratings are 20C continuous and 30C peak, so if you had a 2200 mah battery (which is very common), then at continuous it should be able to supply 20 * 2200 or 44A cont. and 66A peak. I always urge you to add about 15-20 percent overhead. 2600 mah batteries and 3300 mah batteries are getting more common and they will allow for higher current drains and will give you better battery life if you are running that high of current.

This is a pretty comprehensive overview, but these are the things I run through when doing an electric conversion. If you want help on your specific project, feel free to PM me.
Best Regards,
Curtis

Basically I should pick the biggest prop this plane can handle and then find a motor, then get an ESC that can deliver the right amount of power? I was think something like 20C, 1500mah to 2200mah, 3S. But that might be too heavy. For props I have 5x4, 5x4.5, 6x4, 4.75x4.75. Motors I have a selection of 1800kv, 2200kv (inrunner), 3200kv (inrunner), 3500kv, 3800kv, and 5000kv. They're mostly small outrunners and weight about 100g.

Thanks! Here's the specs for my plane. It's a Funjet clone.

Wingspan: 875mm
Length: 700mm
Height: 170mm
Flying Weight: 550g

If you can tell me what model the motors are, I could give you a good ballpark for what the 3S battery will do with various props using Drivecalc (assuming their data is in the database).

BTW, yeah, that is typically how I would do it. I wouldn't necessarily pick the largest prop, but one of nominal size. You wouldn't put a 4-6 inch prop on a 40 sized trainer, and you wouldn't put a 12 inch prop on your funjet. I figure this is the easiest way to start looking at combos.
Curtis

Curtis, I must say that you put it in words very well. Your post should go in the hall-of-fame for concise and detailed, yet understandable information on electric power that I've seen in a while.

If you don't mind, I am going to copy and paste that into a word document, giving you credit of course, and put it in the club's newsletter.

Great job!!

CGr.

The only thing I might add is that when calculating the weight of the plane, you really need to include everything: airframe, motor, esc, batteries, servos, receiver.. everything. Obvious, you don't know some of this stuff because you don't have them in your hot-little-fist yet, but that can also be a swag, but usually, you can get pretty close to do the calcs.

CGr.

My motors are all several brands. Turnigy mostly and some unheard of ones like Hobbymate, Icecold.

I actually found a site that does the calc but how accurate is this? It doesn't look like it takes into account the weight of the plane.

[link=http://adamone.rchomepage.com/calc_motor.htm]ESC/Motor/Prop Calculator[/link]

I played around with some numbers. Basically a 25A ESC, 2200mhz 3S batt, 4.75x4.75 prop, 2200kv motor. I get about 79mph. Yet when I increase the motor to 3200kv, the speed drops down to 77mph. Should it go up? When I increase the prop diameter, it slows down, and the only thing that makes it go faster it seems is the blades with higher pitch. 4.75x5.5 gives me, 84mph.

I guess you have to get the right balance of prop/motor, cause if I increase motor rpm, it goes slower, but then I have to increase the pitch of the blades, they go faster, and if I increase diameter, it slows down the plane.

Thanks CG,
By all means, feel free to share. I've learned a lot of things from this site and RCGroups.com as well and hopefully I can give back something.

Curtis

Hey K3nv,
Sorry, I was out all day and am just getting back to it.

The site you showed is very similar to DriveCalc, which is found at DriveCalc.de (I think). DC is very similar to MotoCalc which a paid for program. What these programs will tell you is what your combo will do. It is hard to imagine what values your airplane will need, and there are just so many factors in designing for them, that we just need to start somehwere. Like you saw, when you increase the kv the speed goes down. Weird huh? Well, it may be that you are loading up the motor more and it is getting "bogged" down. That is why we shoot for the %80 efficiency where most motors run best.

Seriously, if you give me the exact Turnigy or Hobbymate or Icecold motors I could give you a few screenshots and combos real quick like. Just pick a few motors to try to get the hang of it.

The important numbers that the programs will give back are the speed and the thrust. Generally we want to fly at 3x the stall speed. Trainers fly slow (maybe stall at 15-25 mph), where a jet might stall at 50-75 mph. So if you want to cruise then pic the appropriate number. One of the funny things you will see is that as your stall speed increases, generally your thrust to weight ratio drops. On a slow stall trainer or even a 3D machine, the thrust is high, where a jet might just have the thrust to make speed(but not accelerate very fast).

Hope this helps. Feel free to send me a PM or post here and we can continue the discussion. Real numbers will help a lot.
Curtis

I prefer to work from an estimate of the total weight of the plane plus a factor for bigger batteries, then use the resulting weight to find my watts/lb.

Then take watts/lb to determine the prop and motor given what beau0090_99 posted.

You are advised to ALWAYS leave a 20% "overhead" available... in other words never run your electronics, ESC, Motor, etc. to 100% of their capacity.

Your batteries will thank you for it too.

Here are my motors:

HXT 2835 (380S) 2200kv inrunner
Walkera HM-22E-Z-36 3200kv inrunner
ICECold AKE623500FB 3500KV outrunner
Turnigy 2627 Brushless 3800kv outrunner
Hobbymate HB2622 5000KV outrunner

if you can show me how I can find the most efficient motor+prop combo, I could figure it out myself one day

Unfortunately all I could find for you was the HXT motor in the database. It is pretty large, but there are a ton of motors out there to choose from, so it is tough.

Anyway, here are the two charts that I produced with two of the several props that you had. The two props are kind of at the opposite ends of their respective ranges, so one is speed and the other is thrust. Take a look...

As you can see in the first picture, the prop is a 4.74x4.75 and it will pull about 13.7 Amps at 10.69 volts pulling in 146Watts. It will make about 328 g thrust, and do about 112 km/h which is clicking along pretty well (actually very good for a flying wing or funjet).

The next chart shows the 6x4 prop pulling in about 16.3A at 10.61v and 173 Watts. It will make about 640g and do about 85k/h which is significantly slower and might not be very fun. Both props are running about 70-74% efficient which isn't great for an inrunner, but considering peak eff for this one is about 78, you aren't too bad off.

At this point I would say try all props and see what you like. Make sure you have an ESC that can handle 16A * 120% or about 20A and you should have some great fun.

I wish I could check out the other motors for you, but I don't have any data. If you dig it up yourself (motor electrical data, no load Current, kv, etc.) you can make your own motor file in DriveCalc and estimate them. The charts we see were generated using data obtained from actually running the motor with various props, battery sizes and using a data logger, so the data is actually very accurate.

I hope this helps again.
Best regards,
Curtis

Wow!! thanks for your help! I'm gonna use the 2200kv first then. I bought it with the funjet, the stock motor is 1800kv. I also got a 25-30A ESC so this should be a good setup. Your results confirmed that. I'll try out this drive calc.

With the second setup with 6x4 prop, it has more thrust.... shouldn't it mean it'll be faster than the 4.75x4.75? Somehow it's not.

So basically, when i find a combo that works, I first off have to know the peak effeciency of the motor and then come up with a drive efficiency that is close to it? Again, that's for all your help!

No problem at all. Check out the tool and let me know if you have questions. It is pretty powerful, especially if you decide to invest in a Datalogger, or even a wattmeter (every electric pilot's best friend). The datalogger will show you RPM, Current, Voltage and other things like temps and stuff. I have one and have built a few motor databases and it is very useful.

Remember that the 4.75x4.75 has a pitch that is about 19% higher than the 6x4, and because the diameter is 1.25 inch smaller, it can use it's power into providing thrust at speed, rather than thrust at stall. This allows the prop to propel the plane at a rate actually higher than 19% greater than the 6x4 prop. It's kind enigmatic if you think about it.

Peak efficiency is more of a guesstimate to see if we are applying the motor properly. If the motor's peak was say 90%, but we propped it for 60%, then we are probably misapplying the motor. If it is 78% and we prop for 73%, we are doing pretty well with the given motor. I think you're on to it. Keep playing around with things. I would always recommend a wattmeter or a datalogger so you can see what your actual setup is drawing to keep your ESC, motor, and battery happy.
Good luck,
C

I'll check out the datalogger and wattmeter. Does the datalooger attach to the plane and record telemetry?

I don't know if it was a good idea for me to buy a funjet for my first plane.... but I accidentally loaded it up in one of my helicopter sims and i had a blast flying it... now I'm hooked on to planes too. what on earth did I get myself into. I use to be a rc car guy. then helis and now planes! I find planes much easier to fly than helicopters actually. When I first controlled the funjet, it was flying it like I've flown it in another life. We'll wait and see if I can fly it in person hehe.

I was playing around with Drive Calculator. And used a 3900kv motor instead of the 2200kv. Now my speed shot up wot 139km/h. But efficiency was 45%. What does this actually do to the plane? I go faster but what am I losing? So I'm losing 30% eff to what? Also when I put in a smaller prop, 3x3, it goes faster! So if I wanna use a faster motor, I have to use a smaller prop? It's tough to figure out how a prop and motor affect each other. I still don't get it.

What is happening is you are loosing that energy you put in to heat. The 3900 kv motor is used for a couple of things:1. A heli motor where it is geared way down before going to the rotor. 2. A motor for use in a ducted fan, where the impeller diameter is very small (maybe 55mm ~ 2 inches) or 3. a plane like you have (a funjet) but using a 2S battery instead of a 3S. The higher kv motor in the same sized motor will generally handle more current, because there are fewer turns of wire, so the wire in the same stator can be of larger guage and handle the increased current.

You can think of the number of turns kind of like the gear ratio in a transmission. The more turns, the higher the gear ratio, and the lower the output speed of the motor given the same voltage. Note: this doesn't perfectly equate across product lines, but for the same design motor with the same number of poles, it is generally true.

Confused yet? Finding that "perfect" motor is impossible, but I think with a little tinkering you can find one that will work well for you.
Curtis
P.S. On that first chart you posted, did you see what happened to the current? Try putting your 3900 motor back in there with the 4.75x4.75, but change the number of cells in your battery to 2 (like in bullet #3 above) and see what you get.

From the rc car world and BL motors, more turns means more torque but less speed. So for big trucks it's good to have a higher turn motor. For speed, I go will a lower turn motor.

I'm trying to find some sort of pattern here. My goal is to get decent speed while maximizing efficiency for the parts I use. I guess I should set some requirements for what I want the plane to do and what motors/batteries/esc/props I have. Then find a combo that meets those goals? "I want my jet to go about 100kmh with a 2S/3S/4S battery and a 30-40A ESC with about 80% drive eff", if I had this as a goal to strive for would that work?

I guess you can have lots of combos that would meet the exact requirement so one guy may have smaller prop and higher rpm motor vs another guy with a bigger prop etc. And they both perform the same. that's what's kinda confusing to me right now. I can't seem to find a pattern.

I noticed DriveCal has a "Match Prop" button does that mean it'll magically figure out what the best motor to use for my prop? I tried that and it found motors that had drive eff as high 90%.

That's pretty much how I do it, then apply it to what beau0090_99 posted above.

I find it better than starting with a prop, as in cases such as your working from a prop size is either difficult or impossible.

BTW: don't start out by limiting the ESC size you will use, let the numbers determine that.

-

First question:

How much does your plane weight, and what type of plane is it?

These are the specs for my jet:

Wingspan: 875mm
Length: 700mm
Height: 170mm
Flying Weight: 550g

Okay so find a prop, find a motor that can drive the prop efficiently, and then find an ESC that can deliver the amps needed? How do you know what size prop is approriate for a certain plane. Looking at my jet, if we start with a prop then look for a motor.

Before continuing, could you post a link for the plane? Someplace it is being sold?

That helps for reference...

Replying to both posts of your K3 - Yes.

For a fun jet or flying wing, the prop can actually limit you more than a trainer plane would. Most funjets are in the 4-6" range, whereas some trainers could fit a 9-14" prop on the front. A lot of people choose to put the largest prop on an electric conversion as possible, but I prefer to use what is "normal" OpJose is right though too, you don't need to start this way, but when you get past the W/lb estimation, there are still a lot of factors to narrow down and I like to choose a prop size and go from there. You can always dither back and forth from your starting prop size to find what you prefer.

Going back to the second to the last post you put up, that is a good way of doing it, but like Opjose said, let the numbers show you what ESC size you need. I'm glad you had some motors to pick from, so I could find one that might be in a good range.

Like most people will say here even regarding nitro planes, buy a bunch of props and see which one makes the plane fly the way you like it to.

It is near impossible to say how the plane will act once in the air, because, depending on the actuall aerodynamic effects of the airframe, it may unload differently in the air than on the ground. Try Try Try and make sure to get yourself some kind of Wattmeter.
Regards,
Curtis

As he said!

This is ESSENTIAL for electrics.

It is the only way to know what your power system/configuration is actually doing.

Heck I'd love a recording Watt meter to throw into my planes during flight.... to take this even further.

Here's the link to the plane.

[link]http://hobbyking.com/hobbycity/store/uh_viewItem.asp?idProduct=9614[/link]

Assuming that the flying weight estimate is WRONG, assume an all up flying weight of 680 grams, or 1.5lbs by the time you are through upgrading the electronics.

EDF's need WELL over 200watts/lb for decent flying and many pilots strive for 300 watts /lb if possible for greater than 1:1 thrust ratios.

So let's take 250 watt/lb as a potential target.

250 x 1.5 = 375 watts in an ideal situation x 20% gives you around 450 watts to shoot for, to turn this plane into a fast flying missle.

450 watts / 11.1v = 40.5A, so 40 Amps would be the maximum draw with this setup, requiring at least a 2000mAh or 2200mAh 20C battery pack.

So the first question is will such a pack FIT!

The plane calls for a 1300mAh pack so it may be possible to get a 2200mAh pack in there, but you may need to cut some foam out to make things fit.

You could also back down from my numbers a bit and opt for reduced performance... so bear this in mind.

Ok, so we THINK we can use a 2200mAh 11.1v 20C pack, and a 45A to 50A ESC ( there's that margin for safety, the more the better! ).

So next you need to look at a 450 watt edf and motor combo.

This unit came to mind ( as I picked up one of these some time ago myself and they work fine... )

[link=http://hobbyking.com/hobbycity/store/uh_viewItem.asp?idProduct=5484]EDF Unit[/link]

But notice with only a 3S pack, the unit only produces 340 watts as reported by one user. With a 4S pack people are reporting the performance I started out looking for.

So can a 4S 20C pack fit?

If you already have the plane you'll need to measure the area available for a battery.

Or you can run the plane at 340 watts knowing you can upgrade later.

Now I arrived at all of this in a VERY loose manner, and typically I mull over my power configurations for days before deciding which way to go.

You may want to do likewise.

Don't forget you'll need a mounting mechanism, and you'll need to keep the C.G. where they recommend by moving the battery / motor position.

Frankly you are adding a lot of weight to get the EDF in there, why not configure the plane as a pusher prop plane instead?

You don't have to go so high on the wattage to get decent speed, which means you can use smaller motors, ESC and batteries to get good results.