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

May 29, 2011 9 Comments by

 

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.

You start with the motor because it’s where all the performance comes from, and it dictates which power setup you’ll need. In this article, I refer only to brushless outrunner motors. You have to determine your model’s power requirements. Some manufacturers, e.g., electrifly.com, now make this easy because they have online programs to help you select the right motor. Just follow the steps to input information about your aircraft, and you’ll see three motor setups for your plane. But with a little knowledge you will be able to select the correct motor for your model without a program or help from somebody else.

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.

Featured News, John Reid

About the author

West Coast senior editor About me: I’ve been involved with RC aircraft since high school and have flown just about everything. I started my RC career with scratch-building, but now like many pilots I rely on ARFs to get me in the air. My main focus is on pylon racing, aerobats, combat and scale warbirds.

9 Responses to “CHOOSING A MOTOR, SPEED CONTROL & BATTERY PACK: TAKING OUT THE MYSTERY”

  1. Elmardus says:

    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. John R Anderson says:

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

  3. John says:

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

  4. Eric says:

    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. Joel says:

    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. John Reid says:

    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. John Reid says:

    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. atul DEmon says:

    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. Rusty. says:

    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.

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