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# CHOOSING A MOTOR, SPEED CONTROL & BATTERY PACK: TAKING OUT THE MYSTERY

A power system’s key components: brushless outrunner motor, ESC and battery pack. When matched and running efficiently, they produce a lot of power.

For many, E-power is a sort of mystery, but we all know that it’s a clean, very quiet power source- attributes that appeal to many flying clubs whose members worry about losing their field because neighbors frown on their noise. How should you pick right power system for your park flyer? How can you tell whether you’ll be happy with its performance? These are my tips to pick power for your plane.

HOW TO PICK A POWER SYSTEM

1: MOTOR SELECTION

Everything starts with your motor selection.

2: KNOW WHAT YOU WANT

Next, how much performance do you want? A trainer needs less power than a 3D aerobatic flyer. You can start with the standard “input watts per pound” guidelines that have been around for quite a while (see the sidebar “Watts per pound/ounce”). When you have some idea of how many watts per pound (or per ounce) you need, the next step is to check your model’s weight with batteries. This can just be an educated guess; it doesn’t have to be an exact weight. Now start looking at brushless motors to check the range of watts output.

The instructions with many motors don’t show how much power in watts you can expect from them, so you have to do a little math. Most motor stat sheets (they’re often online) give the motor’s battery-cell count-voltage. It’s also important to know the continuous and maximum current, in amps, a motor can safely draw. Multiplying the amps by the voltage gives you the motor’s wattage.

Example: let’s say that your motor will run on a 2- or 3-cell LiPo-a voltage of 7.4 or 11.1 volts, respectively. The motor specifications suggest a continuous current of 10 amps. Multiply the volts by the amps and you get 74 and 111 watts, depending on the voltage used. If you power a 10-ounce model with this motor and a 2-cell battery, it would have enough power to do advanced aerobatics. With a 3-cell pack, that same motor would be powerful enough for unlimited 3D performance. That’s all there is to selecting the best motor for your aircraft; now you have to match the ESC and battery to the motor’s current draw.

WATTS PER POUND/OUNCE

Use these standards to determine the type of performance you can expect from a new power system. For light park flyers, use the watts/oz. column.

 Watts/lb. Watts/oz. Type of aircraft 50 – 70………. 4.35 – 5.65……… Trainer& slow-flying scale models 50 – 70……… 3.125 – 4.35…….. Minimum power level for decent performance; good for lightly loaded park flyers 70 – 90………. 4.35 – 5.65…….. Trainer& slow flying scale models 90 – 110……… 5.65 – 6.87……… Sport aerobatic & fast scale flyers 110 – 130…….. 6.87 – 8.15……… Advanced aerobatic & high-speed models 130 – 150…….. 8.15 – 9.35……… Lightly loaded 3D models & ducted fans 150-200+…….. 9.35 – 12.5+…….. Unlimited performance 3D models

It’s easy to find the right battery pack for your power system among the many available sizes.

3: BATTERY SELECTION

Now that you’ve chosen a motor, selecting the battery and ESC is a snap. First, consider the battery. From my example, we know that the motor needs to pull at least 10 amps continuously from the battery and that it can run on a 2- or 3-cell pack. The performance you want will determine how many cells you should use. But you have to pick a pack that can supply the amps, or current, you want. You do this by checking the pack’s “C” rating. Multiply this rating by the pack’s capacity in mAh to determine how many amps can safely be pulled from the pack.

An 800mAh pack with a 10C rating can provide only 8 amps continually (10*0.800 = 8 amps). If your motor draws 10 amps, the pack will get hot and most likely become bloated and unusable. But if you use an 800mAh battery that has a 20C rating (20*0.800 = 16 amps), it will safely provide 16 amps continually and is safe to use. You can also use a pack with a larger capacity of, for example, 1500mAh with the same 10C rating. It will safely provide 15 amps of continuous power and will also work well with a 10A motor setup. The advantage of a pack with a higher C 4rating is that it’s lighter and smaller, but there’s a downside-shorter flight times.

4: CHOOSE AN ESC

Once you know the motor’s requirements, deciding which ESC to use is easy. For the ESC to work with your motor, it must be rated for voltage of your LiPo pack, and it must be able to handle the motor’s amp draw. Using my example, the correct ESC for your motor would have to be able to handle at least 10 amps continuously and work with 7.4 to 11.1 volts (2- or 3-cell LiPos). It is always better to have an ESC that is rated slightly higher than the amps you really need. But don’t exceed three times the required amperage, and remember that larger ESCs weigh more.

Keep in mind that these are guidelines, and when you have your motor system in place, you’ll be able to measure the current going through it. Variables such as prop size can push a motor’s amp draw above the recommend limit, and that subjects the entire power system to unnecessary stress and wear. The only precise way to test your system is with a watt meter.

USE A WATT METER

To know how your motor’s operation meets its manufacturer’s specifications, you must know several important numbers: current (amps), voltage (volts) and power (watts). One of the best ways to do this is with a Super Watt-Meter from AstroFlight. Plug this meter between the battery and the ESC, and you’ll be able to monitor what your power system is doing.

This is important for two reasons: first, every motor, ESC and battery operates at a specific maximum allowable current measured in amps. Exceeding their limits could soon the burn out motor, the ESC, or the battery. Even running the system at slightly above the maximum specified current could cause any or all three of your electrical components to fail prematurely. Prop size is among the variables that affect the flow of current through a system. A larger prop will pull a higher current from the battery through the ESC and make the motor run hot. Second, use the meter on your flight system to determine the best prop size to obtain the maximum recommended the motor wattage without overworking all of the electrical components. It allows you to maximize your plane’s power, at least, with that electrical system. The Watt-Meter is a worthwhile investment.

FAQ

Q I bought an ESC and noticed that its plug is different from the one on my battery. Did I buy the wrong ESC?

A Not at all; manufacturer plugs vary. You have two options: cut the plug off and replace it with one like the one on your battery, or make a jumper plug out of a short wire attached to the correct ESC plug at one end and to the plug that matches the battery at the other end. Either way, you must keep the correct polarity between the battery and the ESC.

My plane feels sluggish-almost as if it doesn’t have enough power. Should I replace the motor, the ESC and the battery to increase power?

A That would solve your problem, but let’s try to save money. Using a watt-meter, make sure that your system is propped correctly and that you’re getting maximum power. If your system can allow it, use a pack with more cells. This will give a big boost to performance. If this fails, it’s time to replace the electrical components.

If you follow these guidelines, you’ll be able to select a safe and suitable power system for your park flyer. You won’t have to rely on the electrics gurus at the field for help. You’ll do it yourself.

Updated: July 16, 2015 — 10:15 AM

1. Thanks, nice post! But if you calculate the watts/oz. won’t you suppose that the plane need to fly at full power continiously to fly well??? (Srry for my bad English, it isn’t my native language.)

2. If you cut off the battery plug be careful you don’t short out the battery when adding a new plug!

3. Correctly sizing the prop for the plane and power system
is just as important as everything else.

4. What about static thrust? I find this a very useful parameter to know and gives you the information needed to optomize performace of the motor/prop/battery combination.

5. A good article, John, but you erred when you said, “Multiply this rating by the pack’s capacity in mAh to determine how many amps can safely be pulled from the pack.” You should have said “amps” there, instead of “mAh”. Your examples use amps. Many who are new to e-flight are confused about amps versus milliamps, especially when trying to figure out how many amps to set on their chargers. Chargers are rated in amps, ESCs are rated in amps, and motors are rated in amps. Why are batteries still rated in milliamps?

I’m glad that many battery vendors are doing the math for us and printing the continuous and burst/peak discharge ratings on the battery label, but that’s in small print below the big print that proclaims the battery’s capacity in mAh. It no longer makes sense to label the battery with mAh.

Please use your influence in the industry to get the vendors to start labeling their LiPo batteries in amps. It’s time to make the change.

6. Sorry for my tardiness in replaying guys, I was traveling, covering the XFC.

I agree with you Eric Static thrust is a good parameter to know. Is there a way that you use to measure the static thrust? Or anyone else who has a suggestion, I would like to know how you are accomplishing that.

Joel,
That is a good point; although I believe my math is correct in the examples, having to convert mAh into Amps is a bit of a pain. But the simple way to remember is that a 1000 mAh=1 amp (hour). For Example an 800mAh pack with a 10C rating can provide only 8 amps continually (10 x 800 mAh = 8,000 mAh or 8 amps).
I always like to think of it this way a 4500 mAh pack is a 4.5 amp pack.

Unfortunately Joel, I think we will be dealing with mAh packs for some time to come. It has become the standard in the industry and most likely not to change anytime soon. But I do agree with you I like how they are now letting up know the amount of Amps you can pull out of the battery, so we don’t have to do the math. I hate math.
🙂

7. Trust me Elmardus your second language is way better than I would be if I had a second language. I have enough trouble with just speaking English, and I mess that one up all the time.

You are correct in assuming that, you would have to fly the plane at full power. Just a little history here on that chart, it came out some years ago when the batteries and motors were nowhere near as strong as they are today. So we did have to run everything at full power just to get it to fly.

But I think you could multiply the watts in that chart by 125%, that way you would be flying at 75% full power and not be so hard on the power system with a little in reserve if needed.

And John (another John, not me) is absolutely right, matching up the correct prop to the system makes all the difference in the world to the performance of the plane. But that can be a whole another article in itself.

8. im trying to make a plane it come to around 320kg weight i want to make it travell ar 200 km/hour for 90 to 120 minutes hours its wing span would be around 3 to 3.2 foot what should be the power of the motor and is there any other factors that would have any effect on the plane

9. Excellent article, it’s taken some of the mystery out of setting a plane’s electronics up but i’m still learning. Your explaining isn’t the trouble, it’s more like i’m a bit stupid ! I’ll get there in the end,,,,,I hope LOL.

10. Can a receiver blow if you have a 3 cell battery 11volts and the receiver requires 6volts. And I made a scale f-15 jet and I want to know what motor to put on it (A EDF Motor or Just a nice performance motor 2200kv to 3000kv outrunner motor) and a 5 or 6ch transmitter or receiver. I want to know what’s the best combo of electronics to put on this jet.

11. Good article – thank you

12. Hi,,

I have a Extreme Extra 300 E It weights in at 2KG made of wood I have bought :-
ESC = 92amp(Max) 3s (Max) 13v (Max)
Batt = 3s 11.1v 3200 A spec lipo 65c
Motor = 550 3800KV Sensorless Brushless Motor 4 Pole
Wood Prop = either 15×8 or 15×6
HAVE I MADE ANY ERRORS ?????????????

13. hallo …in youtube i saw that one plane used two 5.0 amp batteries in parallel = 10000mah/10 amp , idont know the C rating .. say it’s 20 amp battery so battery amp is 10*20=200 amp ..but they used 60 amp ESC … BUT as we know esc amp should be higher than battery and motor if its not, then it will get hot and as travelling 60 minutes it will caught fire .. so how it’s possible to use 200 amp battery eith 60 amp eac …please tell me. describe please …

best regards
sakib

1. The size of the ESC is based on the amount of current use by the motor, not the size of the battery. The size of the battery, is the amount of amps it has stored up inside its cells, not all of this will go to the system at one time. its like your car, you have a 16 gallon tank, but the engine does not suck up 16 gallons at one time, it take in a small amount so that eventually it uses up the entire tank.

14. I have a 1943 Berkeley Custom Cavalier. It’s total weight without any power, radio, servo, prop, battery, ESC components
Is about 12 pounds. I know little about electrics. I need assistance to equip the plane and start a period of taxiing and ground control practice. Am open for business. I invite comments.

15. This article was extremely helpful, but your link to the “electrifly” website was unspecific. I was unable to find the “motor selection assistance” page or graph or program, what ever it was. But with the valuable info you gave in this article, I should be fine. Thank you.

16. Great article and very helpful. However the section on motor selection was, IMO, a bit oversimplified. No mention was made of the Kv rating of the motor and this is extremely important. A plane designed for speed or EDF wants a higher Kv rated motor while a trainer, STOL or other performance requiring low-speed high and power will require the higher torque of a lower Kv rated motor. These factors must be taken into account before settling on a motor and then choosing a prop, ESC, etc. etc. Otherwise, good stuff here.

17. I am making a bwb rc plane using foam, empty weight of aircraft is around 400g, to my estimation, batteries and servos will add up to make the total weight of around 2 kg. I wish to use 2 ducted fan to propel the plane, can you please guide me about the size of ducted fan required, the battery required, the speed controllers ?

18. I still don’t know what battery to use for a 20 size motor

19. Trying to match a 20 size combo motor to tha correct battery

20. i have purchased a 1000kv brush less motor in specification write max(p)=190/3 and biggest thrust(g/s)=1200/4 sir i want to know what means of specification

21. May peace and prosperity be with you!
Hi, i am new to custom rc models. I build a F-16 weight (640g including battery) and its is hovering or stalling here is the video link
Specs:
Plane material :5mm foam board,
Size: (L*W) 1025mm (41in)* 787.5mm (31.5in)
Motor: Racerstar BR2212 1400KV (40g),
ESC: 30A,
servos: 9g*3,
Tx,Rx: FlySky fs-i6,
Battery: 1800 Mah 11.1v, any suggestion to stop the stalling

1. Looks like you need to find the center of gravity and balance your plane.

22. I am using 2200kv brushless motor for rc plane …what ampere of ESC and how much MAH. Battry should i have to choose ?

1. hi use a 40amp esc if using a 2cell 6×4 prop 3cell batt use 5×5 prop the size of mah is up to you and the size of plane ballance and what fits in duration of flight req etc

1. sorry correction if using a 2 cell batt 7×4 prop not 6×4 as i previously stated Akash

23. can i use 1500mAh 20C lipo for a2212/t13 battery with 10A?
will it fry my motor ?
u said dont use ESC with 3 times the ampere u need. so it i use 30A ESC will it affect my motor ?

24. Correction:

3 cell not 30c

25. Hi !I am using a 2200pmah battery 10c ,motor of 1200 kv motor ,and an esc of 30amps.Does this combination work perfectly.The same combination has crashed for 2 times