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.
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.
Q 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.