SteveRobey
Member
Hi all,
I'm posting this in the sub-forum I am because I am new to the hobby and am learning everything as I go. If you are an experienced builder and you find some information here you feel is incorrect, please feel free to chime in, but please also include your sources and any mathematical expressions that back up your statements. I intend this to serve as a guide for other beginners looking for the proper resources.
So let's begin by posing a scenario: You've built and flown your first electric plane with all of your parts purchased from Flite Test and now you want to take a stab at souping it up on your own to give the plane more speed/power. How do you know what parts you need to use to give your build optimal performance? Well the first three things to look at are your motor, your battery and your ESC. These three things must be properly matched to ensure efficiency, and to ensure you don't kill your ESC. These small ESCs may be fairly inexpensive, but who wants to fry one and have to wait for another to ship? I know I don't. Flite Test has done an excellent job of pairing up motors and ESCs in their power pack kits and the recommendations with each of their models are spot on. But if you decide you want to design your own power system there are some things you have to look at. The first two being the motor you want to use and the battery. I'll use the power pack A for this example.
The pack A comes with an Emax MT1806 which is a 2280KV motor. I looked up the specs for this motor and found that the rated wattage is 117.7 watts. So with that information, we have to calculate the motor's maximum amperage with a given battery. I'll use the larger recommended pack for the FT Tiny Trainer for my example because that's what I have. This is an 800mAh 3S 20C pack. We can calculate it's voltage by the number of cells in the pack. This one is 3 cells and each cell is 4.2 volts when fully charged, for a total voltage fully charged of 12.6 volts.
Now back to motor amperage. To calculate this we devide motor wattage by battery voltage (W / V = A). So for this example: 117.7 / 12.6 = 9.34 amps. So for this motor - battery combination we know we need an ESC that will handle at least 9.34 amps. The power pack A comes with a 12 amp ESC so that's perfect.
The next thing to look at is the power output of the battery pack you're planning to use in amps. To do this, we multiply the pack's mAh rating by it's C rating divided by 1,000.
The mAh rating is the capacity of the cell. For example, an 800 mAh cell can deliver 800 milliamps or .8 amps for one hour. The C rating is an indication of the continuous current draw in amps that the cell can deliver.
So now to calculate battery amperage with a known pack of 800mAh 3S 20C we multiply mAh * C / 1,000.
800 x 20 = 1,600 divided by 1,000 = 16 amps. So we know our battery is capable of delivering more than enough current through the 12 amp speed controller to drive the motor at maximum efficiency. This is a good time to note that the motor will not draw more power than it can use. It's only ever going to draw 10.6 amps and will never try to draw more than that.
Calculations:
So moving forward from here, and readdressing the posited scenario, you want to give your plane more power. You can refer to the datasheet for a given motor you want to use, or that has been recommended to you and from here you can calculate the minimum values your ESC and battery need to meet to keep up with the demands of that motor. The other very obvious thing you want to look at is your propeller size. For this you can start with looking at what the manufacturer of the motor recommends in their datasheet. Outside of that we need to look at the KV rating of the motor.
This was a little misleading to me at first because I thought KV was for kilo-volts and that just couldn't be right. So the KV rating of a motor is the number of revolutions per minute running at 1 volt the motor will turn with no load, meaning without a propeller attached. Don't run your brushless motor without a prop. They dislike that. So the higher the KV rating of the motor, the faster it will spin and the less torque it will have. You need more torque to turn a larger propeller. More torque also means more current draw, which will mean you may need a higher rated ESC and the appropriate battery to deliver the power the motor will potentially draw when you push the throttle up.
You will often find several motors of the same dimensions exactly with very different ratings, this is because the RPM of the motor is a product of it's windings. Fewer thicker windings in the motor mean it will carry more amperage at lower voltage. More thinner windings mean the motor will carry more voltage at fewer amps. To simplify, a lower KV rating means more torque but less revolutions per minute. Higher KV ratings mean less torque but higher revolutions per minute. So with this knowledge it's very important to consider what type of aircraft you are working with and what type of flying you might want to do.
Back to propeller selection. There are two numbers that typically define a propeller (Excluding variable pitch). The first is the propeller diameter. The diameter of a two blade propeller is the tip to tip distance of the blades. For a multi-blade propeller (3 blade, 4 blade etc.) the diameter is 2x the length of one propeller blade. The second number is the pitch of the blade, sometimes referred to as twist. The pitch measurement of a prop indicates how far, in inches, that propeller will move through the air per single revolution of the engine (i.e. every single complete turn of the prop). However, the pitch measurement of your prop must only be taken as a guideline because real-life factors come in to play to influence this distance Eg. the material of the prop, its condition, efficiency, air density on the day etc. So, pitch measurement is really only a theoretical value but it is good enough to help you choose the right size propeller for your airplane and your needs. Essentially, the higher the pitch value, the faster your plane will go.
Here's how these two factors affect flight: An increase in diameter of the propeller will produce more thrust and an increase in pitch will move air faster. So this is a ratio of torque and speed, which you can relate back to the motor you have chosen. As always though, use the recommended prop sizes listed by the manufacturer as your guideline. With that guideline, changing the propeller size, you generally will move up in diameter and down in pitch at the same time. This will ensure you maintain the same load characteristics on the motor. So for example if your motor is recommended to turn a 6x3 prop, a 7x2 or a 5x4 would have the same load on the motor. Increasing just pitch or just diameter is going to mean more load on your motor. So if you want to be able to turn a larger propeller, reconsider your motor, esc and battery. A smaller diameter propeller with a high pitch value will push a small air column very fast, like the prop on an EDF. A larger diameter propeller with a low pitch value will push a wider column of air at a slower rate.
Lastly, consider the aircraft you plan to put all of these parts into, it's flight characteristics and the type of flying you want to do with that aircraft. They're not all designed to haul ass. For example, the FT tiny trainer, as the name would imply, is a slower plane with a bit more drag. So you would want to consider a propeller with more thrust at a slower speed. This would indicate a large diameter propeller with a relatively low pitch, like the 6x3 that comes with the power pack A. When you are at the point where you want to swap out the glider wing for the sport wing with ailerons, you might also be ready for a little more speed. At that point you could consider a smaller diameter propeller with more pitch.
I didn't come up with any of this information on my own and referenced several online sources as an attempt to compile some information on this aspect of designing and building RC model airplanes. Again I am very new to this and all of the information I have provided here in my own words is also hand written on a notebook. Again, I learn as I go so please feel free to provide any corrections or clear up any misconceptions but provide data and references if you do. Let's keep this scientific rather than anecdotal please. Below are links to the online resources that I referenced, the credit belongs to these authors unless otherwise stated.
Sources:
Community edits:
Special thanks for forum members who have helped me make corrections to this guide!
I'm posting this in the sub-forum I am because I am new to the hobby and am learning everything as I go. If you are an experienced builder and you find some information here you feel is incorrect, please feel free to chime in, but please also include your sources and any mathematical expressions that back up your statements. I intend this to serve as a guide for other beginners looking for the proper resources.
So let's begin by posing a scenario: You've built and flown your first electric plane with all of your parts purchased from Flite Test and now you want to take a stab at souping it up on your own to give the plane more speed/power. How do you know what parts you need to use to give your build optimal performance? Well the first three things to look at are your motor, your battery and your ESC. These three things must be properly matched to ensure efficiency, and to ensure you don't kill your ESC. These small ESCs may be fairly inexpensive, but who wants to fry one and have to wait for another to ship? I know I don't. Flite Test has done an excellent job of pairing up motors and ESCs in their power pack kits and the recommendations with each of their models are spot on. But if you decide you want to design your own power system there are some things you have to look at. The first two being the motor you want to use and the battery. I'll use the power pack A for this example.
The pack A comes with an Emax MT1806 which is a 2280KV motor. I looked up the specs for this motor and found that the rated wattage is 117.7 watts. So with that information, we have to calculate the motor's maximum amperage with a given battery. I'll use the larger recommended pack for the FT Tiny Trainer for my example because that's what I have. This is an 800mAh 3S 20C pack. We can calculate it's voltage by the number of cells in the pack. This one is 3 cells and each cell is 4.2 volts when fully charged, for a total voltage fully charged of 12.6 volts.
Now back to motor amperage. To calculate this we devide motor wattage by battery voltage (W / V = A). So for this example: 117.7 / 12.6 = 9.34 amps. So for this motor - battery combination we know we need an ESC that will handle at least 9.34 amps. The power pack A comes with a 12 amp ESC so that's perfect.
The next thing to look at is the power output of the battery pack you're planning to use in amps. To do this, we multiply the pack's mAh rating by it's C rating divided by 1,000.
The mAh rating is the capacity of the cell. For example, an 800 mAh cell can deliver 800 milliamps or .8 amps for one hour. The C rating is an indication of the continuous current draw in amps that the cell can deliver.
So now to calculate battery amperage with a known pack of 800mAh 3S 20C we multiply mAh * C / 1,000.
800 x 20 = 1,600 divided by 1,000 = 16 amps. So we know our battery is capable of delivering more than enough current through the 12 amp speed controller to drive the motor at maximum efficiency. This is a good time to note that the motor will not draw more power than it can use. It's only ever going to draw 10.6 amps and will never try to draw more than that.
Calculations:
- Calculating motor amperage:
Motor wattage divided by battery voltage (W / V = A) - Calculating battery amperage:
mAh multiplied by C divided by 1,000 (mAh * C / 1,000) - Calculating battery voltage:
3.7 volts times the number of cells.
So moving forward from here, and readdressing the posited scenario, you want to give your plane more power. You can refer to the datasheet for a given motor you want to use, or that has been recommended to you and from here you can calculate the minimum values your ESC and battery need to meet to keep up with the demands of that motor. The other very obvious thing you want to look at is your propeller size. For this you can start with looking at what the manufacturer of the motor recommends in their datasheet. Outside of that we need to look at the KV rating of the motor.
This was a little misleading to me at first because I thought KV was for kilo-volts and that just couldn't be right. So the KV rating of a motor is the number of revolutions per minute running at 1 volt the motor will turn with no load, meaning without a propeller attached. Don't run your brushless motor without a prop. They dislike that. So the higher the KV rating of the motor, the faster it will spin and the less torque it will have. You need more torque to turn a larger propeller. More torque also means more current draw, which will mean you may need a higher rated ESC and the appropriate battery to deliver the power the motor will potentially draw when you push the throttle up.
You will often find several motors of the same dimensions exactly with very different ratings, this is because the RPM of the motor is a product of it's windings. Fewer thicker windings in the motor mean it will carry more amperage at lower voltage. More thinner windings mean the motor will carry more voltage at fewer amps. To simplify, a lower KV rating means more torque but less revolutions per minute. Higher KV ratings mean less torque but higher revolutions per minute. So with this knowledge it's very important to consider what type of aircraft you are working with and what type of flying you might want to do.
Back to propeller selection. There are two numbers that typically define a propeller (Excluding variable pitch). The first is the propeller diameter. The diameter of a two blade propeller is the tip to tip distance of the blades. For a multi-blade propeller (3 blade, 4 blade etc.) the diameter is 2x the length of one propeller blade. The second number is the pitch of the blade, sometimes referred to as twist. The pitch measurement of a prop indicates how far, in inches, that propeller will move through the air per single revolution of the engine (i.e. every single complete turn of the prop). However, the pitch measurement of your prop must only be taken as a guideline because real-life factors come in to play to influence this distance Eg. the material of the prop, its condition, efficiency, air density on the day etc. So, pitch measurement is really only a theoretical value but it is good enough to help you choose the right size propeller for your airplane and your needs. Essentially, the higher the pitch value, the faster your plane will go.
Here's how these two factors affect flight: An increase in diameter of the propeller will produce more thrust and an increase in pitch will move air faster. So this is a ratio of torque and speed, which you can relate back to the motor you have chosen. As always though, use the recommended prop sizes listed by the manufacturer as your guideline. With that guideline, changing the propeller size, you generally will move up in diameter and down in pitch at the same time. This will ensure you maintain the same load characteristics on the motor. So for example if your motor is recommended to turn a 6x3 prop, a 7x2 or a 5x4 would have the same load on the motor. Increasing just pitch or just diameter is going to mean more load on your motor. So if you want to be able to turn a larger propeller, reconsider your motor, esc and battery. A smaller diameter propeller with a high pitch value will push a small air column very fast, like the prop on an EDF. A larger diameter propeller with a low pitch value will push a wider column of air at a slower rate.
Lastly, consider the aircraft you plan to put all of these parts into, it's flight characteristics and the type of flying you want to do with that aircraft. They're not all designed to haul ass. For example, the FT tiny trainer, as the name would imply, is a slower plane with a bit more drag. So you would want to consider a propeller with more thrust at a slower speed. This would indicate a large diameter propeller with a relatively low pitch, like the 6x3 that comes with the power pack A. When you are at the point where you want to swap out the glider wing for the sport wing with ailerons, you might also be ready for a little more speed. At that point you could consider a smaller diameter propeller with more pitch.
I didn't come up with any of this information on my own and referenced several online sources as an attempt to compile some information on this aspect of designing and building RC model airplanes. Again I am very new to this and all of the information I have provided here in my own words is also hand written on a notebook. Again, I learn as I go so please feel free to provide any corrections or clear up any misconceptions but provide data and references if you do. Let's keep this scientific rather than anecdotal please. Below are links to the online resources that I referenced, the credit belongs to these authors unless otherwise stated.
Sources:
- Meaning of KV rating:
http://www.flyelectric.com/ans.kv.html - Sizing RC airplane propellers:
http://www.hooked-on-rc-airplanes.com/sizing-rc-airplane-propellers.html - Power Pack A contents:
https://store.flitetest.com/power-pack-a-minis/ - Electronics matching process:
https://www.hobbywarehouse.com.au/articles/the-matching-process-continued.html - Emax MT1806 datasheet:
http://www.helipal.com/emax-mt1806-mini-motor-ccw-2280kv.html
Community edits:
Special thanks for forum members who have helped me make corrections to this guide!
- BridgeInspector
for calling out corrections to battery and propeller information / verbiage.
Last edited:
