Help! Picking an appropriate motor, propeller, and ESC

[TheRealSeal]

This may not exactly be the right place for this, but since you guys hopefully know a lot more about electric motors than me maybe I can get some answers. This is kind of a loaded question, but if I were to go about scratch building a plane (not an FT plane), how can I know what kind of power train to put into it? I’ve looked at several RTF planes with similar size and power that have varying KV rating for the motors yet similar ESCs and propellers but completely different battery recommendations. Can someone lay down some general calculations I should use because I am totally lost.

 

[Merv]

The amount of thrust needed from the motor & prop depends on the weight of the plane and how you want to fly. A slow flying trainer, the thrust will need to be 60-80% of the weight of the plane.
A typical plane, 80-100%. A war bird or one with a little zip will be 100-150%. A 3D plane will need 150-200% thrust to plane weight.

Here is a FT thread with more information.

Thrust vs Power to Weight Ratios for Prop Planes

I'm scratch building the FT Cruiser and would appreciate all the help that anyone can provide in designing the power system from the ground up. I know that I can use the parts specified by Flite Test but I really want to learn the fundamentals for future builds. I'm not into 3D flying yet so I...

forum.flitetest.com

Watts per pound is another useful guideline. You'll need to weigh the plane and estimate the weight of the motor and battery.

• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.

When it comes to Kv, think transmission. High Kv generally use smaller diameter higher pitch props. Resulting in higher top end speed and lower thrust at low air speed. Just like a high gear in a transmission. Lower Kv generally use larger diameter lower pitch props. Resulting in lower top end speed but greater thrust at low air speed. Just like a low gear in a transmission.

Two motors can output the same power (watts) but at different air speeds. A race car and a tractor may both have the same horsepower but at different speeds.

 

[TheRealSeal]

The amount of thrust needed depends on how you want to fly.
A slow flying trainer, the thrust will need to be 60-80% of the weight of the plane.
A typical plane, 80-100%. A war bird or one with a little zip will be 100-150%. A 3D plane will need 150-200% thrust to plane weight.

Here is a FT thread with more information.

Thrust vs Power to Weight Ratios for Prop Planes

I'm scratch building the FT Cruiser and would appreciate all the help that anyone can provide in designing the power system from the ground up. I know that I can use the parts specified by Flite Test but I really want to learn the fundamentals for future builds. I'm not into 3D flying yet so I...

forum.flitetest.com

Watts per pound is another useful guideline. You'll need to weigh the plane and estimate the weight of the motor and battery.

• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.

When it comes to Kv, think transmission. High Kv generally use smaller diameter higher pitch props. Resulting in higher top end speed and lower thrust at low air speed. Just like a high gear in a transmission. Lower Kv generally use larger diameter lower pitch props. Resulting in lower top end speed but greater thrust at low air speed. Just like a low gear in a transmission.

Two motors can output the same power (watts) but at different air speeds. A race car and a tractor may both have the same horsepower but at different speeds.

How or where can the motor wattage be found? All I ever see on the electric motors are the barrel dimensions and kv rating. Do I have to calculate that based on the battery and ESC as they’re supplying the power?

 

[Merv]

How or where can the motor wattage be found?...

It’s in the description or specs of the motor, the exact location will vary from vendor to vendor. Sometimes it in the text, sometimes in a table.

Here is one example from FT

 

[TheRealSeal]

It’s in the description or specs of the motor, the exact location will vary from vendor to vendor. Sometimes it in the text, sometimes in a table.

Here is one example from FT

Okay, so let’s use this as an example. This is the motor that comes stock in the Dynam Waco, which weighs roughly 3.5lbs. This states it has an output of up to 630 watts (not sure if that electrical or mechanics though). So would that mean this would have a ratio of 180? How does the battery affect this with different cell number, mAhs, and discharge ratings?

 

[Merv]

...So would that mean this would have a ratio of 180?...

Yes, this setup would be about 180 the watts per pound.

...How does the battery affect this with different cell number, mAhs, and discharge ratings?

Im guessing the mfg of this motor would recommend a 2-4 cell battery. On the 2S use the 12 inch prop. On the 4S use the 9 inch prop. The 4S setup will be the fastest.

The battery capacity (mAh) will determine how long you can fly. The more mAh the longer the flight, up to a point. At some point the flight time will max out due to the additional weight of the battery. Example: If a 1000 mAh battery will give you 6 minutes of flight then a 2000 battery may give you 10 minutes and a 3000 maybe 12 minutes.

The amps a battery can supply are calculated, the formula is mAh/1000 x C = Amps. Example a 2,000 mAh 20C battery can supply 40 amps (2000/1000 x 20 = 40). The problem, in the past few years C ratings have become highly exaggerated.

Here is some more information

 

[TheRealSeal]

Yes, this setup would be about 180 the watts per pound.

Im guessing the mfg of this motor would recommend a 2-4 cell battery. On the 2S use the 12 inch prop. On the 4S use the 9 inch prop. The 4S setup will be the fastest.

The battery capacity (mAh) will determine how long you can fly. The more mAh the longer the flight, up to a point. At some point the flight time will max out due to the additional weight of the battery. Example: If a 1000 mAh battery will give you 6 minutes of flight then a 2000 battery may give you 10 minutes and a 3000 maybe 12 minutes.

The amps a battery can supply are calculated, the formula is mAh/1000 x C = Amps. Example a 2,000 mAh 20C battery can supply 40 amps (2000/1000 x 20 = 40). The problem, in the past few years C ratings have become highly exaggerated.

Here is some more information

So I’m revisiting this thread because I’ve not really gotten any clarity, in fact the more I learn the more I’m confused lol. What I do understand is that the kv rating dictate the size and pitch of the prop and thus the speed envelope. I also understand that wattage and weight are the most important factors when choosing a motor. Where I am still confused is where these wattage numbers come from. Why do motors with the same canister dimensions have higher wattage at higher KV? Or what determines why two motors with the same kv have vastly different max watt output. I also can’t wrap my brain around how wattage, current, prop choice, etc are effected by the battery choice (cell number) and how that affects max draw and ESC choice. The more I dig the more I reel.

 

[Merv]

Here is another post that may help

spoon fed

Good day to all. I have been flying for several years but stopped for a while. When I came back on the scene, everything I owned was antiquated. Mostly rotor, Ive had a couple of 3 channel planes, couple of inexpensive drones. I now want to start building some foamy planes. I started by trying...

forum.flitetest.com

... What I do understand is that the kv rating dictate the size and pitch of the prop and thus the speed envelope...

Correct, you also need to consider the cell count of your battery, that is the voltage. The higher the voltage the faster the motor will spin. If the motor spins faster, it will need a smaller prop to prevent overheating.

...I also understand that wattage and weight are the most important factors when choosing a motor...

Correct, just add flying style to the list. Flying lazy circles is one thing, flying fast or 3D just takes more power.

... Why do motors with the same canister dimensions have higher wattage at higher KV? Or what determines why two motors with the same kv have vastly different max watt output...

The Kv is changed by the construction of the motor, how many winds and poles a motor has. The amps is determined by the size of wire used in the motor. There is a physical limit of how much wire can fit inside a motor. You can have a lot of winds of a thin wire or fewer winds of a thicker wire.

When comparing canister size, some manufacturers cheat. They report the size of the rotor, not the size of the stator. It's the size of the stator that matters.

...I also can’t wrap my brain around how wattage, current, prop choice, etc are effected by the battery choice (cell number) and how that affects max draw and ESC choice. The more I dig the more I reel.

Increasing the Kv or the voltage will make a motor spin faster. Spinning a motor faster will move the air faster. Moving air faster, takes more watts. For a given voltage, more watts takes more amps. If the wattage or amperage exceeds the rating of the motor or ESC, you will burn something out.

The solution, lower the voltage, lower the Kv or use a smaller prop.

 

[TheRealSeal]

Here is another post that may help

spoon fed

Good day to all. I have been flying for several years but stopped for a while. When I came back on the scene, everything I owned was antiquated. Mostly rotor, Ive had a couple of 3 channel planes, couple of inexpensive drones. I now want to start building some foamy planes. I started by trying...

forum.flitetest.com

Correct, you also need to consider the cell count of your battery, that is the voltage. The higher the voltage the faster the motor will spin. If the motor spins faster, it will need a smaller prop to prevent overheating.



Correct, just add flying style to the list. Flying lazy circles is one thing, flying fast or 3D just takes more power.


The Kv is changed by the construction of the motor, how many winds and poles a motor has. The amps is determined by the size of wire used in the motor. There is a physical limit of how much wire can fit inside a motor. You can have a lot of winds of a thin wire or fewer winds of a thicker wire.

When comparing canister size, some manufacturers cheat. They report the size of the rotor, not the size of the stator. It's the size of the stator that matters.

View attachment 238305




Increasing the Kv or the voltage will make a motor spin faster. Spinning a motor faster will move the air faster. Moving air faster, takes more watts. For a given voltage, more watts takes more amps. If the wattage or amperage exceeds the rating of the motor or ESC, you will burn something out.

The solution, lower the voltage, lower the Kv or use a smaller prop.

On the subject of batteries and voltage, is it assumed 3.7V per cell as an average or working voltage even though the max charge voltage is 4.2V? I think I saw sometime back that if you go from a 2 blade to 3 blade prop you drop the diameter by 1 and increase the pitch by 1. How should the prop change when increasing cell count? Just drop the diameter or should the pitch change as well? I know the voltage increases the wattage but why does the current change? How is current calculated or found to know what ESC is appropriate? I’ve seen that’s it’s recommended to go bigger on the ESC to avoid the possibility of burning it up and losing radio control. So why have some said that using an ESC with too high of an amp rating can dump extra heat into the motor? Is that because typically the ESC would give out before the motor but if the ESC has a higher ceiling then the motor can run without limit and get hotter?

I know this is a TON of questions, and I’m very grateful for all the answers!

 

[Merv]

...I think I saw sometime back that if you go from a 2 blade to 3 blade prop you drop the diameter by 1 and increase the pitch by 1.

I definitely agree with decreasing the diameter & may want to decrease it by more. Changing the pitch, I not sure it is necessary.

Decreasing the diameter or the pitch will lower the load of a prop (a smaller prop). Adding a blade or using a bullnose prop increases the load (thrust) and decrease the efficiency (watts per gram of thrust). I would only use them when you need more thrust and a larger diameter 2 blade prop will not fit. That is you don't have the room to swing the prop

How should the prop change when increasing cell count? Just drop the diameter or should the pitch change as well?

It doesn't matter which you change. With more cells, you simply need a smaller prop. It's doubtful changing only the pitch will be enough.

Those kind of rules of thumb are just that, intended to give you a clue of how things work. They are not absolute rules.

The only way to know is do a prop test any time you change to your setup, that is a new prop size or cell count. On the ground, run the setup at full throttle for 10 seconds or so. Then stop, check the temperature of the motor, ESC & battery by touching them. Warm is OK, but if it’s so hot you don’t want to hold onto it, STOP. The prop is too big, you need a smaller prop. If it passes, try a 30 second run, then check the temps. If it passes, try a 60 second run. If it passes, go for a fly. It’s always a good idea to check the temperatures after a flight. Keep in mind that what works when it 70 may overheat when it 95+.

Watt meters are very useful in testing a new setup, they measure watts, volts and amps. Letting you know of the potential of a problem.

I know the voltage increases the wattage but why does the current change? How is current calculated or found to know what ESC is appropriate?

The formula is: Volts x Amps = Watts
If Watts increase either Volts or Amps must change.

...some said that using an ESC with too high of an amp rating can dump extra heat into the motor?...

I would disagree with that statement. It's the prop causing the motor to overheat, not the ESC.

...Is that because typically the ESC would give out before the motor but if the ESC has a higher ceiling then the motor can run without limit and get hotter?...

I agree with this, it's too much prop causing the problem.