4S or 5S LiPo? Is eCalc lying?

[MombasaFlash]

Following recommendations a few weeks ago to switch to 5S packs on a fairly bog standard AD-8 HLE with AXI 2814/22's to reduce temps and increase flight times, and on the very point of ordering some 5S packs, I decided I had better do a quick eCalc check. First I entered all the details with the 4S cells that I normally use, just to see the resulting calculated flight time in order to verify I had entered everything more or less correctly, and then simply change 4S to 5S.

Big warning in red saying over the limit ... motors will overheat ... blah, blah

So much for the 5S recommendation for the AD-8 methinks.:dejection:

Then, just for kicks, I enter in all the stuff for the bog standard MK Octo with its ROXXY 2827-35 motors and the same flat 4S cells it has always used without complaining and there is a big red warning saying 'max power over the limit of the motor'.

Huh?

According to eCalc it has to have 3S LiPo's ! ?

So I am back to square one. I have no idea whether it is safe to use 5S packs with a standard AD-8HLE or not.

DW AD-8 - 4S

DW AD-8 - 5S

MK Octo - 4S

 

[mbsteed]

I have to say my experience with eCalc has been similar, maybe I am missing something but every time I have used it, it didn't seem to provide useful information - maybe is it not setup well for multicopters.

 

[holco]

eCalc is revering to the max power, but with multirotors you never use that

Take a look at my motor database, the Axi is also in it.

http://www.rcgroups.com/forums/showthread.php?t=1388583

 

[jes1111]

Couple of problems:

- you're using "incl. Drive" in the top line, which means that when you change the battery the AUW is not changing. I subtracted the reported "Drive" weight from your first sheet, yielding a "without Drive" weight of 2614g and entered that in the top line (confirming that the reported AUW in the bottom now comes back to 5062g. Once you've done that you can now change any of the drive components (motors, batteries and ESCs) and the change in AUW will be properly accounted for. On that basis, switching to 5S (with everything else the same) ups your flight time to 11.62min (an increase of 1.18min). The max current per motor hits 25.02A, 0.02A over the limit - I wouldn't worry about that (for an AP machine): well inside the margin of error inherent in such calculations.
- the motor temperature warning lights up because you haven't changed the motor cooling drop-down to "excellent" (which it is on an MR).
- changing to 5S also lowers the hover throttle from 55% to 39%. On an AP ship I'd be a little concerned about this - you're subtracting from the "negative headroom" available for correcting/manoeuvring. 39% might not be critically low, but with a lighter camera it would possibly become so.
- even on 4S, the APC SF props is way beyond its RPM limit (which is 5417rpm). At max throttle it's 8484rpm, so 55% of that (hover throttle - not an exact calculation but close enough) is 4666rpm (uncomfortably close to the specified limit). Changing to 5S just makes the problem worse: 9910rpm at max throttle. I'd be looking for some better low-pitch 12" props!
- you've altered the air pressure to 1018hPa - if you return it to 1013mPa the red disappears, as it does if you go to a higher altitude or hotter ambient temperature.
- the usual reason to up the voltage is to allow chunkier, lower kV motors spinning big-*** props or smaller props with the same motor. Just changing the voltage and nothing else ain't gonna make a whole heap of difference.

 

[MombasaFlash]

... Take a look at my motor database ...

Phew! Impressive bit of work there.

Couple of problems: ...

Good. A useful reply. Thank you.

Point taken about the With/Without Drive option. I plumped for 'With' because there are variables that I have no info on and entering the true 'without Drive' weight never ends up with the actual AUW. I didn't worry about the AUW not changing with the heavier battery because, even with the lighter battery AUW weight the "...current was over the limit". Also, for example, I have no idea which 'Controller' to select and this too affects everything.

Anyway, your response tells me that whacking in a couple of 5S packs will likely introduce more problems than it would debatably solve and that the 12"/3.8 APC props are not ideal. So much for the advice given on these pages to use 5S packs. The main concern was runaway BL Ctrl temps but it looks like the 5S option will not help there either.

Not sure what to do about the props though. These APC SF 12x3.8 props are SPECIFICALLY recommended for the AXI 2814/22 equipped AD-8 HLE on the published Droidworx spec sheet and I bought stacks of them to keep as spares. The prop speeds are presumably calculated and regulated by the FC so the Tx throttle cannot be messed with and limited as one would with Throttle Curves for a single rotor heli.

I do not find many prop options available in the 12" size and increasing the prop size is not an option without a major change of boom length. About the only option is going DOWN to the Graupner 11x5's - which someone here did suggest.

Having said all that I am still not entirely convinced because, according to eCalc, the really bog, bog, BOG standard MK Octo - a 100% MK 'kit' - is not supposed to use the MK recommended 4S LiPo's. Changing to a 3S makes eCalc happy but the Octo doesn't get off the ground !! So where does that leave us with trusting eCalc as a reliable tool?

NB. I do not really know which 'Controller' to specify for the v1.2 BL Ctrls so have selected 20A and Robbe ROXXY B2827-35 motors are not available on eCalc, only C2827-35. Otherwise the AUW, including Optio S12 is correct.


MK Octo with Highsight lll and Pentax Optio S12

 

[jes1111]

re: MK - well... just about everything is "marginal" with them (and their products), I reckon!

I regard eCalc as an extremely useful tool for a] getting into the right ball-park and b] watching how different components alter a given scenario. Absolutely reliable? Of course not - too many variables for which it cannot account (such as wind, flying style, poor connections, undersized wiring, etc.)

re: props - yeah! it's a bummer, isn't it? Now we've got a thousand choices for frames, FCs, motors, etc. but b***er-all choice still on props. I wish one of the mfgs would get serious and produce optimised designs in all the common standards. With regards to the ubiquitous SF12x3.8, I'm guessing that its published rpm limit gets overlooked because "I haven't had one break yet" - but IMO "yet" is the operative word there - and I certainly wouldn't send my big-$ gear aloft using them

 

[MombasaFlash]

You see, it would never have even occurred to me that the props would be spinning faster than their design limits (where does one find such published info anyway?). It will be interesting to compare the actual hover rpm's against eCalc results.

I used to have faith in Droidworx being the Good Guys against MK the Bad Guys but it sounds like none of them are angels.

All this bleating about battery capacity, controller temperatures and 6-15 minute flight times etc. with our MK's, DW's or whatever is thrown into sharp and ridiculous relief against the claims from Microdrone with their MD4-1000 and its flight endurance of 88 minutes - WITH video downlink, telemetery and GPS etc. enabled and a camera on board.

Are the rest of us being taken for a ride?