Just wondering if the 6 cells setup will give more rpm range on the top end vs 5 cells.  I have only heard that it has more capacity.. but no one has ever mentioned that there are any performance advantages.   

Joe,
rpm capability is based upon voltage,
batteries loose voltage as they run down the charge and as load is increased

what can happen with a 5S setup versus 6S, if you are close on capacity the 5S while under load will temporarily drop rpm enough that your desired rpm is not available because of the lower voltage, so having more available voltage, ( higher voltage) gives you more head room to drive the prop..

Thats why if your system is marginal, when you go into overheads or the clover you can hear the motor labor for rpm, its loaded the battery with current draw and the voltage has tailed off below that needed to get the rpm, but then you notice when you go to level flight, it picks back up and flies out the timing,, as notmal
or so is my understanding

well yeah kinda, ,except that ignores the cell internal efficiency,,

on a pack with higher internal resistance it gets a bit cloudier,,
( and sometimes, smoky,, lol,, that's an insider electrical joke son)
I know on my packs I have to watch the whole capacity thing, using less expensive packs, I really try to stay well under 80%

mostly because as Fred has discovered, the lesser expensive packs tend to act like a smaller capacity higher dollar pack, IOW, they aint what they say they is

Thanks all for the replies..  

The capacity always seems to be around 80% +/- 5.   even though I was running it down to 85% for a while.   but it is back around 80% now.   I was thinking of running the 6 cells to get more rpm with the same setting, so I could back off the rpm on the timer.   and not sure how much rpm increase I'll get with the 6 cell.  I think in the ballpark of 500-800 should be good.

Dunno what equipment you're using (nor would it help me if you said), but a governor (whether it's in the timer or the ESC) is going to have a specific mapping from throttle setting to RPM.  Without changing that programming, changing the cell count isn't going to change the timer setting.

And -- why would it make a difference?  As long as you can set the RPM fine enough, you shouldn't care how it's being achieved, just that it's being achieved.

Well, of course you have to change the setting on the ESC to 6 cell.   but is it going to give me more rpm?  or its regulated by the governor to the set rpm for the 5 cell.. so results is no diff in rpm other then capacity.

rpm of the motor is defined by two things,, one is the Throttle setting as seen by the ESC, this signal is provided by the timer or the reciever which plugs into the ESC
the other thing that sets the limit is the voltage available to the ESC to provide the requested rpm...
in MOST ESC that I have used the throttle "throw" is established by the Transmitter potentiometers when you cycle the stick from low to high and back to low...

otherwise the upper limit is set by the voltage not a setting,,,

Depends ...

If you fly constant RPM and you use the same motor and prop with 1 cell more (+20%) you will have more capacity (Wh) but the ESC will go down with throttle to keep the same RPM and that will cost you efficiency (I just wrote it in another thread here). So you will not gain full 20%, better is to have larger capacity.

You will have for sure better behavior at discharged battery, so it all depends on head room. You have to use battery which is JUST able to hadle prop at discharged battery uphill. That is all.

However IF you use active timer with autothrottle, you must count that it must be able to add power uphill, means say + 1000RPM on top, so therefore we use 6cells and that is where you can see difference in 8 overhead or 4leaf especially in wid.

The current with higher cell count is really lower (the same power at higher voltage) on battery leads, but it does not mean that also motor current is lower. The same prop needs the same torque and the same torque needs the same average current on the same motor, so it means battery current is converted to lover voltage and higher current at motor compared to battery current and voltage, that conversion costs something and that price is heat because of recuperartion currents in recuperation diods (or FETs - depends on ESC construction), so that is that heat which you will explore when you go to higher cell count without changing motor or prop. Mean higher voltage does not automatically means lover heating in leads.
 

Brushless ESC's use FETs instead of catch diodes.  There is a diode there to catch the flyback voltage right at switching, but then the FET turns on.

I did a motor-drive project professionally a couple of years ago, and since I had never answered the question to my satisfaction I asked a forum of experts that I'm part of what the loss characteristics of a typical DC motor are (or, for that matter, the loss characteristics of any ONE motor).  The answer I got back was a unanimous "we dunno".

I don't think the loss from using six cells instead of five will be significant, but it depends on both the motor and the ESC, and it could depend heavily on the ESC's code version.  Any given ESC that's super-efficient on one motor should tend to be efficient on others, and ditto for a really efficient motor, but you'd have to test things in combination if you really needed to know.

My understanding is that when you drive the motor at less rpm than full throttle, the ESC is where the inefficiency lives, it is still switching but diverting the power away from the motor?,, ( laymans terms and comprehension, and I use the term comprehension loosly )

Its set by the timer,, and it doesnt care about limits as such, it just wants to know that a given pulse width is a given rpm, dont care about upper and lower limits,, and what happens is when the battery voltage ( especially  under load like towards the end of the flight when voltage is tailing off, and current drain is high like overheads or the clover) is to low, the timer and esc are still asking for the rpm but the motor just cannot respond without the voltage, I have seen this "brown out" condition in the clover several times, especially with older less freindly battery packs,, they get marked for use at higher density altitude locations where it draws less power.. NOT at contests where its 65 degrees and 400 foot elevation

You were saying that high battery voltage = more speed.  I'm pretty sure that for a given governor setup, as long as you've got voltage overhead, high battery voltage = same speed.  Trying to do something difficult would make the ESC programming bizarre for the poor helicopter pilot who's trying to sort out his system.

Yes, of course, when the system runs out of oomph then it can't maintain speed.

I look at it this way.  If it is a IC set-up AKA slimer, a six cell set-up gives you 20% more in the tank, assuming same total MaH.  I have considered this myself but have a hard time justifying the extra weight.  My airplanes are already portly.  Thuderpower batteries definitely stand up better to a more severe draw down.  I've been keeping an XL spread sheet tracking % left after flights.  The eight TP 2700 V8 batteries are averaging right at 20% after 160 flights total.  To be fair, these batteries are last year's and they have at least twice that may cycles on them.  I have some new ones to take to NATS.

A six cell system won't run any more RPM's unless you tell them to.  Honestly, I couldn't care less about "a poor helicopter pilot"

You were saying that high battery voltage = more speed.  I'm pretty sure that for a given governor setup, as long as you've got voltage overhead, high battery voltage = same speed.  Trying to do something difficult would make the ESC programming bizarre for the poor helicopter pilot who's trying to sort out his system.

Yes, of course, when the system runs out of oomph then it can't maintain speed.

Tim its also important to remember, it can run out of ooomph for a short brief moment,, like in the middle of a loop of the clover,, but as soon as you reduce the load, the pack will be able to regain rpm.... until the next load spikes.. this is the problem I have been having with several of my packs,, at least in ROseburg,, I get a brown out during the clover,,
these are last years packs which were not treated especially well because I have learned some things since then,,

Brushless ESC's use FETs instead of catch diodes.  There is a diode there to catch the flyback voltage right at switching, but then the FET turns on.

I know, I wrote it, but it does not change the effect which I described :- )) .. or at least not too much

I don't think the loss from using six cells instead of five will be significant, but it depends on both the motor and the ESC, and it could depend heavily on the ESC's code version.  Any given ESC that's super-efficient on one motor should tend to be efficient on others, and ditto for a really efficient motor, but you'd have to test things in combination if you really needed to know.

Well ... it is known fact that if BLDC motor runs too much limited by PWM, it 1/ runs warmer and 2/ in some cases ESC even overheats. Using prop with lower pitch at the same diameter usually leads to colder run, if not on motor, then certainly on ESC - while the logic says that if prop has lower pitch, it will need more watts to pull at the same model speed. But reality is opposite, so that answers why :- ))

The background is clear, BLDC (unlike classic DC) in PWM regime has very variable current, so at the same RPM we have iron loses equivalent, the copper loses should be also (average current is the same) but the current is not almost constant (like on classic DC) so since copper loses are = IIR then average loses are higher, compared to the same motor running at full power or at least lover battery voltage.

Yup.  You should be nicer to your packs.

Or go back to slime, the Real Man's^TM power source.

No I prefer to emulate the approach of the Real Champion ^TM
I dont need to be slimey to prove my manhood,, do you ?

Now boys, calm down. To each his own.

Hey Howard said he missed my interaction with Tim so I was trying to let him have his fun this way,
besides TIM WANTS to fly electric, I know he does

Fuel smells better when it's burning than electrics do.

Eventually I'll break down and fly electric.  I'm working on an ESC specifically for old-time, but there's some size & weight issues that need to be resolved.

well played Tim, well played

I just received a video from Rogerio Fiorotti, and it does shows rpm increase with higher voltages from 4 cells to 5 cells.

Just wondering if the 6 cells setup will give more rpm range on the top end vs 5 cells.  I have only heard that it has more capacity.. but no one has ever mentioned that there are any performance advantages.   

I am not convinced that
1- either you are not asking the question you mean to
or 2- you are not getting answers to your question

bottom line, more voltage more rpm,, the theoretical rpm of the motor is the Motor KV * pack voltage,, HOWEVER if you are using a timer it sets the rpm regardless of the input voltage as LONG as the input voltage is hi enough to get the requested rpm....
so using a 6 cell versus a 5 cell with the same prop will NOT give you more rpm because the timer sets the rpm....

Thanks for the clarifying that.    I was just going to ask what mode it was on..  so, on the Simple mode, the rpm does increase with higher voltage,  but not in the Set rpm mode.

Exactly.


Rogerio

Mark,  I’m not sure what you’re getting at..  but many has provided the answers and very good explanation on the subject above including yours that I do appreciate very much.   This is something new that I just received from Rogerio..  that shows something I wasn’t aware of with Castles ESC.

Not really getting at anything as such,,
just trying to make sure you were getting the info desired, I am not the expert, just another user,,

I am really confused here. Given, I need a certain prop power Wattage output for a particular model, then I switch from a 3 S to 4 S pack, but drop the prop size to give me the same Watt drain, and change the mAh pack size appropriately to the total Watt-hour value, will I gain efficiency?

NO ... motor efficiency depends on ratio of current to voltage ... so if you want good efficiency, you have to find proper load. The same efficiency you can reach also at high or low voltage. - But at different RPM - so you have to properly match Voltage, prop and motor to reach best efficiency.

Where it differs, is prop slippage. So if you use the same motor but 4 cells instead of 3 cells, it will work at its best efficiency only at higher RPM, so you have to choose prop with smaller pitch and little smaller diameter - means better P/D ratio. That will give you better speed stability. And that is what we need on our stunters. It will also probably cost you more power.

However we tend to overload our motors, so likely if you go to 4cells and lower current (to have the same power) you will PROBABLY (depends on you current configuration) go really to better efficiency.

Also, in a totally side issue, do expensive motors use less power per input Watt than cheap ones, or do they just last longer. (new can of worms!  )

there are many differences between cheap and expensive (expensive for a reason, not just overpayd trash) motors. But most important is this:

1/ better magnets - either stronger flux making that the same KV needs less turns in coil and therefore lower internal resistance
or - magnets which will last higher temperature

both lead to more watts from the same weight and better efficiency

2/ better material for iron - either alloy or lamination - making less heat in iron  - the same like 1/

3/ real bearing instead of trash - real motors will last years before you have to change bearings, while motor which costs less then apropriate bearing used inside clearly cannot last so long :-P

4/ better mechanical solutions (rotor vibrations, bearing sizes, magnets fixing, magnetic circuit solution etc) ... until not stolen by those "chip" producers :-P

OK,

I switched to 5 cells and the power and capacity are there, but I am noticing more heat buildup in the motor.....speed control and battery are cool.....motor is most likely too hot.

Info on power package: e-flite 25, 870 kv, castle edge 50, prop 11-6 carbon 3 blade. Currently using 5S 2800 mah.

Thanks

Tim

Tim

I am assuming you are in governor mode.

You need to measure the motor temp. Can you put your hand on it after a flight?
Eflite lists max operating temp as 220 degrees F.

If you are high, look at supplying more cooling air to the motor and make sure nothing is rubbing on the can.

Otherwise the setup looks fine.

In retrospect though I am a bit puzzled why the 4S setup is running out of juice.

Kim

Info on power package: e-flite 25, 870 kv, castle edge 50, prop 11-6 carbon 3 blade. Currently using 5S 2800 mah.

I am willing to change the motor if I have too....but I really want to use the custom prop  I am using.... Any suggestions?? or am I screwed??

Thanks

Tim

I would switch to either E-flite 32, Cobra 3520-12, or AXI 2826-12 with 5 cell batt.

i'd prefer a 3525/10 (780kv) with a 5 cell. But that's me.

It's a bit heavy (8.92oz)  vs the 3520-12 is 7.62 oz but if its tail heavy.. why not.