Mini-HowTo How to choose a power system

[aeajr]

scott,

Once you get the first output from WebOCalc, play with it. Try a slightly different KV, or different pack. How would it work with a 2S pack? How about a 4S pack?

Change the amp target and see what happens.

Once you have a combination that you like, here is what you do. Note the kV number, the prop size, the amps and watts. Now look for a motor that fits these numbers as closely as possible.

WebOcalc doesn't say "buy this Rimfire outrunner". WebOCalc says that you want a motor with a 500 kV rating that can handle 35 amps using a 10X6 prop. You can go look at Rimfire, Turnigy, .... any brand you like and see if they have a motor that is close to this.

What you will find is something like this:

This is the Great Planes ElectriFly RimFire
42-50-800 Brushless Outrunner Electric Motor.


SPECS: Aircraft Application:
75-100 Watts Per Pound
Trainer & Sport Performance Weight Range: 9.2-12.3lb
100-150 Watts Per Pound
Aerobatic & High Speed Performance Weight Range: 6.2-9.2lb
Over 150 Watts Per Pound
3D & Extreme Performance Weight Under 6.2lb

Motor Diameter: 1.65" (42mm)
Motor Length: 1.97" (50mm)
Shaft Diameter: .20" (5mm)
Shaft Length: .67" (17mm)
Lead Length: 3" (76mm)

Will this motor fit in my plane? Is it too wide, too long?



Connectors: Bullet type, 4.0mm diameter
Max. Constant Current: 50A

Is 50 amps enough for my application? WebOCalc says I need a motor that can handle 45 amps, so this seems OK.

Max. Surge Current: 80A
Max. Constant Watts: 925W
Max. Surge Watts: 1480W
No Load Current: 2.6A
Input Voltage: 11.1-18.5V


This motor can be be used on a range of voltages which should I use?



RPM/V (kV Rating): 800
Weight: 7oz (198g)
Suggested Propeller Size: 10x5E-14x7E

It can use a range of props? Which should I use?


If you were looking for a motor of about 800 kV that could handle 45 amps, this could be a contender. But which prop should you use? Well that depends on which battery pack you are going to use. Typically when you get a range like this, the big prop goes with the lower voltage battery and the smaller prop goes with the higher voltage battery.

But why guess? Put the specs into WebOcalc and see which prop fits your needs. Try it with different battery packs and see what works.


A thought on voltage.

What we are shooting for here is power. Power is measured in Watts.

Watts is calculated by Volts X Amps.

That means if I want 600 watts,

I can use 10V and 60 amps = 600 watts ( using round numbers)

Or I can use 12V and 50 amps

Or I can use 20 V and 30 amps.

So you have a choice of voltages you can use with this or other motors. Which should you use?

That will depend on space, and on cost and on duration as well as the propsize you want.

You could use a 3S 3000 mah pack that has to provide 60 amps, then you will get about 3 minutes duration out of it and you are probably pushing the pack hard.

You could use a 6S 3000 mah pack that only has to provide 30 amps, then you will get 6 minutes duration and the pack is not working hard at all.

But can you fit a 6S pack? How much does it cost? How much duration do you want?

Just giving you some ideas on how these things work. That is why WebOCalc gives you a choice of voltages or pack configurations to try. Changing the voltage by changing the pack size will change the recommended prop, or with a change in voltage you may find you want to change the kV of the motor.

If you run on a 3S pack, then you might want a 1000 kV motor
If you run on a 6S pack then you might switch to a 500 kv motor.

Each combo will give you about the same RPMs and can probably turn the same prop.

Complicated? Well sure, till you start to get the hang of it. But tools like WebOCalc let you do the "what ifs" to see what combo works best for you.

Driving a car is wildly complicated.
Manipulating two foot pedals
Maybe add clutch and shift points
determine steering wheel position
watch your speed
estimate braking distance
what if it is raining or snowing
Keeping track of the cars around you
anticipating turns
Watching road signs
taking exits
Lights! dogs! Kids in the back seat!!!!! Cops!

But people do it every day and most times they make it home without a scratch.

If you want to keep things simple, buy RTFs or stick with the recommended power systems as provided by the MFG of the plane.

If you want to design your own power systems then you need to learn about this stuff. WebOCalc can help you learn and understand the relationship between all these various factors.

Isn't this fun?

[flieslikeabeagle]

You're welcome, Scott!

If you haven't discovered them already, I recommend the Turnigy Plush ESC's from Hobby City/Hobby King ( http://www.hobbycity.com ). I have several of these in various models, and in my experience they are butter-smooth and utterly reliable. One of their 30 A models would be great in your Accord - that would leave you a nice safety margin, or the option to hop-up the Accord to insane power levels later if you prefer.

I don't recommend the Turnigy Plush ESC's lightly - over the past several years I have used a number of brushless ESC brands including Align, Astroflight, BP Hobbies, Castle Creations, Cool Running, GWS, Hacker, Hobby Wing, Hyperion, Jeti, Suppo, Welgard, and probably a few more I've forgotten. In my opinion, and for the sort of use I put them to (sport and precision aerobatic flying with fixed-wing aircraft), the Turnigy Plush ESC's perform just as well, or in several cases, better than any of these brands. The icing on the cake is that the Turnigy ESC's are also about half to one-third the cost of many of these other brands.

Another product from Hobby City I give a thumbs-up to is their Flightmax and Rhino lipo packs. One-third the cost of some of the heavily marketed brands, and, as far as I can tell, zero performance difference for my kind of flying. Perhaps competition pilots would notice a difference, I can't say. I certainly cannot, though.

What about a motor? Your Accord is a superbly light aircraft and highly capable of aerobatics. While many of the inexpensive motors from Hobby City would fly it, in my opinion this is such a nice airframe that it would benefit from a higher-quality motor.

One other thing: Most ESC's on the market now have a built-in BEC or "Battery Eliminator Circuit". The BEC is a circuit that supplies 5V to the radio and servos, borrowing power from your higher-voltage 3S lipo pack to do so. Unfortunately, even though many ESC's on the market claim to have "2 A" or "3 A" BEC's, the fact is that the vast majority of BEC circuits are not really entirely adequate for use in a model with four servos, a modern RC radio receiver, and a 3S lipo pack - it all adds up to too much load on the BEC. If the BEC overheats and shuts down in flight, kiss bye-bye to your model.

A few ESC's on the market include something called a "switching BEC" onboard. This is a better and more recent BEC design that does not suffer from the heating problems of the older BEC designs (called linear, or analogue, or analog in American English). However, these ESC's tend to be both expensive, and somewhat rare.

The easy way around this concern is to use a separate "UBEC" or "switching BEC" to supply power to your receiver and servos. It works alongside your normal ESC, offloading the BEC's job from the ESC. Hobby City has nice UBEC's for under $8, so the added expense is minimal.

I use one of these UBEC's in all but my very smallest and lightest aircraft now. The UBEC weighs a few grams - your Accord wouldn't even notice the added weight. Given how nice an aircraft the Accord is, I think it deserves the added safety insurance of a UBEC. (I have a couple of NES models also, a Vermont Belle 1300 and a Samba. The VB1300 recently had a mishap when a loose ceiling tile fell on it , breaking the vertical stab and some fuselage stringers.)

If you need any help with how to use a UBEC along with your ESC, let us know, I or someone else here will help.

-Flieslikeabeagle

[flieslikeabeagle]

Originally Posted by aeajr

WebOcalc doesn't say "buy this Rimfire outrunner". WebOCalc says that you want a motor with a 500 kV rating that can handle 35 amps using a 10X6 prop. You can go look at Rimfire, Turnigy, .... any brand you like and see if they have a motor that is close to this.

Very true, and this is something that is important to me, so it will stay this way. In our commercial, business-oriented, profit-driven culture, sometimes it seems as though everything and everybody has a secret agenda - steering your heart, mind, and the contents of your wallet towards some product or the other to make someone else richer at your expense.

WebOCalc is not created by a business interest - it's created by me, a hobbyist like you. I value my own freedom of choice, so I assume you feel similarly. Because of this, WebOCalc will never steer you towards a specific product or brand - even if I honestly believed that product was the best one for your needs, there would be the faint taint of a conflict of interests lurking in the background, and you might (with reason) wonder if I'm being paid to shill somebody's brand-name parts.

So WebOCalc helps you find out what your airframe needs, based on simple physics and aerodynamics . After that, WebOCalc leaves it entirely up to you to find a product that meets those requirements. Your money, your opinions, your decision. Like Sinatra, you can justifiably say "I did it my way."

Originally Posted by aeajr

It can use a range of props? Which should I use?

This is a crucially important question, because it is the propeller that converts the power from the motor into forward motion of your model. Pick the wrong propeller, and you can throw away a huge amount of performance without even knowing it. And I do mean huge - it is quite easy to give away 15% or 20% or even more of the performance, simply by choosing the wrong prop!

So what's the "right" propeller? One of the not-so-obvious things about WebOCalc is that it really helps you find a good, objective, answer to this question, one driven by science and not opinion.

WebOCalc finds an array of props, all of which will use the amount of power you specified, and all of which will generate about the same pitch speed you specified. Some, however will make more thrust than others. Clearly, if one prop makes more thrust at the same pitch speed from the same amount of power, it is doing a better job of moving air. In other words, the props with the most thrust are the more efficient propellers for your application.

To make it even easier for you to home in on the best propeller(s), WebOCalc tells you how much thrust (read: efficiency) you lose by choosing any other propeller in the list instead of the best one.

When I'm picking a power system for a model, I prefer to think in terms of finding the optimal propeller rather than the optimal motor. Once WebOCalc helps me find the propeller that will give the model the best performance, I then start hunting for a motor that will work with that prop. The motor is just a torque generator to spin the propeller; it's the propeller that, well, propels the model through the air!

Originally Posted by aeajr

You could use a 3S 3000 mah pack that has to provide 60 amps, then you will get about 3 minutes duration out of it and you are probably pushing the pack hard.

The "C-rate" selection in WebOCalc helps you decide how hard you want to push your packs. If you pick "10 C", WebOCalc will suggest larger lipo packs that won't be pushed as hard. Pick "30 C" and WebOCalc will suggest small lipo packs that will be flogged within an inch of their lives - expensive, short-lived, but lighter for extreme performance. Once again, you get to decide, and WebOCalc will help you meet your goals.

Originally Posted by aeajr

That is why WebOCalc gives you a choice of voltages or pack configurations to try.

True, also someone may want to know if they can use a pack they already have. By providing options, WebOCalc gives you more flexibility to use what you already have, or what you prefer to buy.

Originally Posted by aeajr

Driving a car is wildly complicated.

Excellent example.

I clearly remember how hard it was, learning to drive. Trying to remember which pedal did what, making sure I stayed centered in my lane and under the speed limit, trying to read all the street signs, keeping an eye on pedestrians and parked cars...geez, so many things to do! Brain overload!

Originally Posted by aeajr

Isn't this fun?

I still think so.

I'm trying to make power decisions about a little Hyperion Helios 10e at the moment. Shall I use the factory recommended Hyperion Z2213 motor (I happen to already have one)? Or do I want a bigger motor I can push to higher power levels, since I want effortless unlimited vertical on the Helios? Will I need a bigger and heavier motor to get enough nose-weight to balance the model? Will the motor I end up with work with the two 1300 mAh 3S lipo packs I already have?

It never ends. Good thing, I guess, it helps keep our brains from turning to mush!

-Flieslikeabeagle

[wolw]

Originally Posted by flieslikeabeagle

I clearly remember how hard it was, learning to drive. Trying to remember which pedal did what, making sure I stayed centered in my lane and under the speed limit, trying to read all the street signs, keeping an eye on pedestrians and parked cars...geez, so many things to do! Brain overload!

Excellent example, after you've built, tested and used a couple of setups you'll probably end up with a few "basic" ones that you like and use in different models. By then you can make an educated guess of a suitable setup just by looking at the data on the model you are about to buy.

I played around with WebOCalc, used models I've built with measured setup's and found it to be as spot as a calculator gets, nice work

Peter

[flieslikeabeagle]

Originally Posted by wolw

after you've built, tested and used a couple of setups you'll probably end up with a few "basic" ones that you like and use in different models.

When I started out in the hobby my first few planes were all somewhat similar - foamies around 16 oz with top speeds around 45 mph. So the same power system would usually work fairly well for all of them, just as you say.

Oddly enough, as I got deeper into the hobby, my models started to all be much more different from each other. Take three examples - a little 48" Spitfire built for scale-like hot-and-heavy flight, a 72" Rascal 40 built with an emphasis on cheap components and A123 power, a 54" Extra 260 built with the goal of as much weight reduction as I could reasonably get combined with enough power for easy precision aerobatics, and a night-flying Slow Stick with the all-encompassing goal of building lighter, lighter, and even lighter.

Because all of these were built with pretty different goals in mind, and size and weight also vary a lot from one to the next, each one has a custom power setup fine-tuned for it. Some of these power systems are pretty different from what most people would use on a similar model.

For instance, the Slow Stick has a 15-gram Medusa 12 mm inrunner in a GWS 11.8:1 gearbox spinning a huge 13x9 GWS RS prop at about 5.5 amps current draw from a small 3S lipo pack. It's a rather expensive power system (the little motor is $40 or so), but it generates something like 20 - 25 oz thrust from only 60 W, at a pitch speed around 25 mph or so.

The Slow Stick itself weighs only around 12 oz ready to fly, compared to 18 oz - 20 oz for most of the ones I see at the park. At this 12 oz weight and with over 20 oz thrust, it has unlimited vertical...from a 15 gram motor!

WebOCalc guided the development of this model, because it predicted I could get this sort of excellent performance if I could find a way to use that huge and very efficient 13" prop. A 14 or 15 inch prop would be even better, but I could not find a gearbox with a deep enough gear ratio!

By the way, unless you've experienced it, you would be shocked at how much air these big 13" and larger props will move even with only 60 watts of power driving them. It's like a small storm inside the house - loose bits of paper get sucked off the tables, your clothes flap in the breeze, and if you're standing behind the prop, it becomes hard to breathe because of the draft hitting your face!

The Hyperion Extra 260 is another somewhat atypical model. Most of these fly around at 5 - 5.5 lbs weight (2,27 kg - 2,50 kg) with 750 W - 800 W power systems. The one I built weighs a little over 4 lbs (around 1.85 kg) and packs 570 W - and that's with a slightly heavier 5S A123 pack in it. Because it weighs a pound and a half less than most, it flies like a dream with no vices. The light weight also ensures it has effortless unlimited vertical even with "only" 570 W. The heavier versions of this model are rather "snappy" and will bite you if you're not careful - no such worries at 4 lbs, though.

The secret to the Extra's performance? A Hyperion Z3025-12 motor, which has an unusually low Kv of 665 rpm/V. That let's me spin a relatively big 13" propeller while drawing only 40 A from a 5S A123 pack.

Notice the common theme: bigger propeller = better performance on less power!

Then there's the Spitfire; it uses a cheap $25 Turnigy motor and 3S A123 pack. Weighing around 36 oz, it flies very well despite the cheap and somewhat heavy power system. Again, it uses a bigger-than-most 11" prop, versus the more common 9" and 10" props most people use on the model. It has more than enough power to fly like a warbird - no unlimited vertical, but huge loops, easy Immelman's and Split Esses, big Cuban Eights, and so on, are well within its repertoire.

Originally Posted by wolw

I played around with WebOCalc, used models I've built with measured setup's and found it to be as spot as a calculator gets, nice work

Peter

Thanks, Peter! Appreciate the feedback!

-Flieslikeabeagle

[flieslikeabeagle]

Update: the newest version of WebOCalc, version 1.5, just went live on my website.

Lots of small changes, for instance it will suggest what size (amp rating) your ESC should have, and suggest a lipo pack capacity (mAh) based on the "C" rating you specify for your battery.

You will find the newest version of WebOCalc in the usual place: http://flbeagle.rchomepage.com/ , Software, WebOCalc.

-Flieslikeabeagle

[michaehay]

Hello, great tutorial. THANKS.
Can someone also explain what 2S, 3S, 4S mean?
Thanks.

Michael (the newbie kind )

[Chris F]

The S means the cells are wired in Series. This means each cells voltage (3.7 Volts) is added and the capacity remains the same. For example a 3S pack has 3 cells wired in series giving a nominal voltage of 11.1 Volts. A 6S pack can be made from two 3S packs wired in series giving 22.2 Volts.

P stands for Parallel. When cells are wired in parallel the votage is the same and the capacity (mAh) of each cell is added. For example a 3S 2P 2200 mAh pack is 2 3S packs wired in parallel to make a pack that is 11.1 Volts and 4400 mAh.

[aeajr]

One note on S and P. These things apply to NiCd and NiMh packs too but you never see them quoted that way because very few NiXX packs are ever parallel wired. So if someone says 7 cells for a NiXX pack, they mean 7S.

In the early days of Lipos, the packs could not put out a lot of amps. Today we have 15C, 20C, 30C even 50C packs. But in the early days we had 2C 4C 5C cells. So, to get enough amperage to run our motors we had to parallel wire the packs. So a 3S2P or even 3S3P packs were common 6 years ago.

Now you only see that in BIG capacity packs like 6S 2P 22.2V 8000 mah packs or larger. That would have 12 lipo cells. Two groups of 6 4000 mah cells wired in series giving 22.2 Volts and 4000 MAh capacity. Then those two groups are wired in parallel to give 22.2V and 8000 MAh capacity.

YOu could fly a 35 pound plane with a pack that big. But in the early days of lipos, a 10 pound plane would have been a challenge.

Hope that helps.

[Pit-Viper]

Hey guys, I am fairly new to electrics. I have a couple planes that i've been flying for a few months now. I picked up a twin eingine plane that was setup up with 2 OS.20's on it. I am going to convewrt it to electric but needed to know how to determine what size motors I would need for this project. I don't know the name of the plane, it is about 15yrs old. I bought it from a guy selling it for a friend who had passed. It does weight 3lbs with nothing in it and am guessing it will be closer to 5-6lbs RTF. I will try to get a pic of it on here and maybe someone knows what it is. It looks kind of like a Pica Duelist.

Thanks shane

[Pit-Viper]

Ok here are a few pics of it. 53" WS and 43" length.

Shane

[aeajr]

Originally Posted by Pit-Viper

Hey guys, I am fairly new to electrics. I have a couple planes that i've been flying for a few months now. I picked up a twin eingine plane that was setup up with 2 OS.20's on it. I am going to convewrt it to electric but needed to know how to determine what size motors I would need for this project. I don't know the name of the plane, it is about 15yrs old. I bought it from a guy selling it for a friend who had passed. It does weight 3lbs with nothing in it and am guessing it will be closer to 5-6lbs RTF. I will try to get a pic of it on here and maybe someone knows what it is. It looks kind of like a Pica Duelist.

Thanks shane

I am sure we can help you.

Have you read the first page of this thread. Knowing how to select engines is what this thread is all about. The bulk of the information is in the first few posts.

Also look at the last two pages. Between those you will be prepared to ask questions and understand answers.

Take a look, then ask your questions.

[Pit-Viper]

I understand how to decide watts vs. weight and the proper esc with the proper lipo's along with Kv rating and how they apply. I've read all that, I guess what I'm asking is on twins, how do you divide the power? I am only new to electric twins, been flying gas for a few years and have shared a wealth of information with others in those areas. Just looking for a little insight. If I was going to run one motor on this plane I would be looking around the 500-700w range and probably in the neighbor of 1200-1800kv based on the way I like to fly. From what I understand I would be looking in the 200w range for each motor and around 2500kv. Will only have clearance for a 8-9" prop. I like sport flying a lot when not flying my warbirds, so with this plane I am thinking fairly fast with moderate aerobatics.

Shane

[NoFlyZone]

Hi Shane,

List the wingspan, AUW, wing area, and the largest diameter prop that can fit on the plane and still have ground clearance.

Chuck

[aeajr]

Originally Posted by Pit-Viper

Hey guys, I am fairly new to electrics. I have a couple planes that i've been flying for a few months now. I picked up a twin eingine plane that was setup up with 2 OS.20's on it. I am going to convewrt it to electric but needed to know how to determine what size motors I would need for this project. I don't know the name of the plane, it is about 15yrs old. I bought it from a guy selling it for a friend who had passed. It does weight 3lbs with nothing in it and am guessing it will be closer to 5-6lbs RTF. I will try to get a pic of it on here and maybe someone knows what it is. It looks kind of like a Pica Duelist.

Thanks shane

Originally Posted by Pit-Viper

I understand how to decide watts vs. weight and the proper esc with the proper lipo's along with Kv rating and how they apply. I've read all that, I guess what I'm asking is on twins, how do you divide the power? I am only new to electric twins, been flying gas for a few years and have shared a wealth of information with others in those areas. Just looking for a little insight. If I was going to run one motor on this plane I would be looking around the 500-700w range and probably in the neighbor of 1200-1800kv based on the way I like to fly. From what I understand I would be looking in the 200w range for each motor and around 2500kv. Will only have clearance for a 8-9" prop. I like sport flying a lot when not flying my warbirds, so with this plane I am thinking fairly fast with moderate aerobatics.
Shane

OK, so based on your two posts, I would target 100 W/pound. If the plane will weight 5 pounds, that would be 500 watts, or about 250 per motor. Even if you lost one motor, 250 would give you 50 W/lb which should be enough to get it home.

I will use WebOCalc to model the power system
http://flbeagle.rchomepage.com/softw..._imperial.html

You said you wanted it be somewhat fast. I don't know what you consider fast. I chose a target of 90 mph.

And I am looking for a total thrust of about 70 oz, or about 35 per motor.

Since there are two motors I will put in a target of 50 W/pound and a max of a 9" prop. Using a 3S pack and a target of 35 amps I get

APC-TE 8.0 X 8 prop 11,224 RPM, 85.5 mph and 38.0 oz thrust per motor using a 1400 kV direct drive outrunner. That is a 380 watt motor and 75 watts/pound for ths motor or 150 W/pound overall.

Put in two of those and you have 76 oz of thrust against a projected 80 oz plane.


How does that work for you? Do you have room for outrunners?

If you want to tone it back a bit:

Use a 2400 kV motor on 3S, 26 amps, 280 watts for a total of 112 W/lb
Use a 5X4.5 prop, 88.7 mph, 29 oz thrust per motor.

58 oz on an 80 pound plane will need a bit of a roll out on the runway but it should fly very nicely and hit 88 mph.

How we doing?