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Dec 16, 2019, 06:59 PM
Still circling in sink...
There's a good reason why you can get more power from the same motor by using higher voltage (such as switching from 3s to 4s). The motor's power handling is limited by how much it heats up. The heat generated by running power through a resistor is not really proportional to the power (watts), it's proportional to the square of the current. The formula is P = I^2 x R. Now our motors aren't really resistors, but the principle holds. Increase the voltage but keep the current the same, and the motor will not heat up more, but will still produce more power. Again, since our motors aren't resistors, this is not exactly true, but it's close.

But, since increasing the voltage increases the motor's rpm, you do have to drop the prop size to compensate and keep the amps down.

And if the system is limited by the battery and not the motor/prop combination, then all these other calcs become less relevant.
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Dec 16, 2019, 08:09 PM
Registered User
scirocco's Avatar
Quote:
Originally Posted by tom1968
There's a good reason why you can get more power from the same motor by using higher voltage (such as switching from 3s to 4s). The motor's power handling is limited by how much it heats up. The heat generated by running power through a resistor is not really proportional to the power (watts), it's proportional to the square of the current. The formula is P = I^2 x R. Now our motors aren't really resistors, but the principle holds. Increase the voltage but keep the current the same, and the motor will not heat up more, but will still produce more power. Again, since our motors aren't resistors, this is not exactly true, but it's close.

But, since increasing the voltage increases the motor's rpm, you do have to drop the prop size to compensate and keep the amps down.

And if the system is limited by the battery and not the motor/prop combination, then all these other calcs become less relevant.
Sorry, but that's not at all accurate. There are 2 sources of heat in an electric motor. the resistive or copper losses you've just described and the magnetic or iron losses. Iron losses are voltage x no load current. As you increase voltage, the iron losses increase. The peak efficiency point of an electric motor is when the iron and copper losses are equal. And that peak efficiency current will be higher at higher applied voltage.

For the same power throughput, motor efficiency - which basically defines the amount of heat generated that will kill the motor - is basically the same whether you use low voltage high current or vice versa. Where a higher voltage setup can have significant advantage is in being able to use smaller ESCs and thinner wires, and achieve greater flexibility in pack configuration, eg 2x 2S, 2S+3S for narrow fuselages.

It doesn't matter that this was written for brushed motors as the principles are the same (see Chapter 1): http://astrobobb.com/electric_motor_handbook.pdf
Dec 16, 2019, 10:15 PM
Tragic case
davidleitch's Avatar
Just going back to props for a moment and observing that a 16x6 is kind of like a diesel it will pull vertically really well assuming the battery can drive the prop properly but it’s a limited top speed. A higher pitch prop with a shorter length will give a higher top speed but take longer to get to top speed. I prefer the higher pitch particularly in wind. The models are light and so accelerate quite well despite the initial drag. I suspect the smaller length props unload more after the model has sped up but that could be wrong. 4s has become the norm so there must be a reason. 4s 650 has same energy content as 3s 850 or near enough but everything seems a bit better in my experience. Perhaps it is that 4s is just better suited to the motor prop combo I like.
Dec 16, 2019, 10:20 PM
Time for me to Fly...
Mr. Wiz's Avatar
I suppose choosing a 3s based power system or choosing a 4s based one is a bit of a personal thing. Back when I flew pattern, I always preferred the higher voltage power systems. I just didn’t care for the higher current draw of the lower voltage systems, nor did I like spinning those huge props.
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Dec 16, 2019, 10:57 PM
Registered User
scirocco's Avatar
Quote:
Originally Posted by davidleitch
Just going back to props for a moment and observing that a 16x6 is kind of like a diesel it will pull vertically really well assuming the battery can drive the prop properly but itís a limited top speed. A higher pitch prop with a shorter length will give a higher top speed but take longer to get to top speed. I prefer the higher pitch particularly in wind. The models are light and so accelerate quite well despite the initial drag. I suspect the smaller length props unload more after the model has sped up but that could be wrong. 4s has become the norm so there must be a reason. 4s 650 has same energy content as 3s 850 or near enough but everything seems a bit better in my experience. Perhaps it is that 4s is just better suited to the motor prop combo I like.
I see it a little differently. Thrust is not a matter of diameter or pitch, it is a function of both, so the high gear low gear analogy doesn't really work for me. It is a case of understanding your motor prop combination's performance from static to top speed and that is easy enough to model.
If a lower diameter higher pitched prop develops more thrust at full throttle at a given airspeed then the model will accelerate faster, and get to its higher top speed faster,and I don't think (at least for comparable loads) there is any appreciable difference in prop acceleration to max rpm. The motor doesn't know whether the torque on the shaft is high diameter / low pitch or vice versa, it just draws current to deliver the torque. There is a limit of course - if a very high pitch:diameter prop is statically stalled initial acceleration out of the hand will be slower, but unless using square props the prop is unstalled in barely a second.

Where the higher pitch speed setups become really important is when you want a very high rate of climb, eg low level lift didn't eventuate. . At our thrust to weight levels, that is near vertical and is limited by the forward speed the model can attain. If you need to gain 150m in the last 7-8 seconds, say 20m/s, the model needs to be able to fly at 72km/h vertically, or even faster at an angle less than vertical. It doesn't matter how much static thrust a low pitch:diameter setup has if it can't fly fast enough to get the desired climb rate.

Now if only I could fly well enough to take advantage of all that performance.
Dec 18, 2019, 03:22 AM
Registered User
Tuomo's Avatar
Quote:
Originally Posted by scirocco
...Where the higher pitch speed setups become really important is when you want a very high rate of climb, eg low level lift didn't eventuate. . At our thrust to weight levels, that is near vertical and is limited by the forward speed the model can attain. If you need to gain 150m in the last 7-8 seconds, say 20m/s, the model needs to be able to fly at 72km/h vertically, or even faster at an angle less than vertical. It doesn't matter how much static thrust a low pitch:diameter setup has if it can't fly fast enough to get the desired climb rate.
I posted this earlier, setups having about 100km/h or slightly higher pitch speed (in ecacl) feel right for F5J type flying. At least I feel so... That good old 16x8 3s setup is frustratingly slow and static thrust has relevance only at the brief moment of hand launch.

Quote:
Originally Posted by scirocco
Now if only I could fly well enough to take advantage of all that performance.
We must remember that F5J is not about climbing high... It is also more difficult to fly efficiently a distant thermal compared to a thermal close by. But having good motor performance giver more tactical freedom during 30 sec motor window.
Dec 18, 2019, 06:26 AM
Wake up, feel pulse, be happy!
Piece's Avatar
Quote:
Originally Posted by scirocco
Sorry, but that's not at all accurate. There are 2 sources of heat in an electric motor. the resistive or copper losses you've just described and the magnetic or iron losses. Iron losses are voltage x no load current. As you increase voltage, the iron losses increase. The peak efficiency point of an electric motor is when the iron and copper losses are equal. And that peak efficiency current will be higher at higher applied voltage.

For the same power throughput, motor efficiency - which basically defines the amount of heat generated that will kill the motor - is basically the same whether you use low voltage high current or vice versa. Where a higher voltage setup can have significant advantage is in being able to use smaller ESCs and thinner wires, and achieve greater flexibility in pack configuration, eg 2x 2S, 2S+3S for narrow fuselages.

It doesn't matter that this was written for brushed motors as the principles are the same (see Chapter 1): http://astrobobb.com/electric_motor_handbook.pdf
I agree with everything in this quote and I don't want to seem like I don't, so let me choose my points carefully.

Although it's true that the motor's maximum total power throughput will remain constant regardless of the voltage and current used to achieve it, that's only applicable when the final RPM and power are held constant. Referencing the post by Tom1968, although I do have my own stern words for his resistor comparison, the core concept is not strictly wrong. The difference in this case, unlike the typical voltage-vs-current equivalence, is that motor RPM has increased.

Iron losses will increase with added voltage, and in a system where max motor efficiency is the goal, that's not necessarily desirable. When power density is at a premium, though, running the motor at its peak efficiency point isn't feasible. In many cases, efficiency is just a curve that suggests which motor might survive the highest power throughput.

Torque is inherently limited by whatever serves to limit current (usually lack of cooling), but that limit doesn't strictly depend on voltage. The faster a motor spins, the more power it transmits at any given torque. If you feed it more voltage, it will burn a little more energy in the stator, spin a lot faster, and transfer a lot more power, provided the load is adjusted to maintain (or slightly decrease) shaft torque.

This is unhelpful info in the world of conservative direct-drive setups where the motor is bolted to an optimal prop and RPM tends to be tragically low, but this thread doesn't strike me as a crowd of Sunday cruisers, and the "optimal" prop looks like a matter of user preference. Adding a cell and reducing prop diameter does grant a significant and safe increase in overall power, and for all anyone knows, that smaller prop may be better suited to task.

The same concept explains why a high Kv motor with a gearbox radically outperforms the same motor wound for an equivalent low Kv, and why everything with a small fast prop seems to have outlandishly high power numbers, and why you can't double the motor's current rating just by halving voltage.
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Dec 18, 2019, 06:33 AM
Wake up, feel pulse, be happy!
Piece's Avatar
Also, I didn't miss the battery suggestions. I've already been looking at a lot of CNHL packs for the variety of form factors. If they perform well, that's good to know, although I realize different cells from the same label may not be consistent. At some point you just roll the bones.
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Dec 19, 2019, 12:58 PM
Still circling in sink...
Thanks scirocco and Piece for the corrections.
Last edited by tom1968; Dec 19, 2019 at 01:07 PM.
Dec 20, 2019, 06:59 AM
Registered User
Tuomo's Avatar
Quote:
Originally Posted by Piece
Also, I didn't miss the battery suggestions. I've already been looking at a lot of CNHL packs for the variety of form factors. If they perform well, that's good to know, although I realize different cells from the same label may not be consistent. At some point you just roll the bones.
Lipos are the cheap part of our hobby. Just buy 3-4 packs for testing. Then stick with them for a season if they are good. If not, give them to somebody who is less critical about performance.
Feb 25, 2020, 02:33 AM
Team Futaba
Silent-AV8R's Avatar
Some FWIW data. At the recent SWC in Arizona I was flying my 49 ounce Optimus. Tenshock 1515-15T/Castle Edge-Lite 50 amp ESC. GM 13x8 prop on 4S (800 mAh 65C Dinogy Lipos).

For the 6 rounds I averaged 45.3 amps (43.3-46.2).
Watts had an average peak (usually less than 2 seconds) of 624 watts (592-647).
The average for each launch was 326 watts.
Launches were ballistic and I had no problems penetrating into the high winds during the last 2 rounds.
Minimum voltage was 13.1 volts. Again, fairly short-lived drops.

This is much more power than I used to have in this plane and I need to work on managing it. But I know I can run and climb at my discretion no matter the conditions or ballast.
Feb 25, 2020, 02:40 AM
Tragic case
davidleitch's Avatar
I've spent the past couple of months trying to manage all the extra power I put in moving from 3S to 4S and bigger motors. Its getting the the thermal sniffer level right and learning how to use it. Looking at my recent results clearly I haven't got there yet.


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