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More power... Is 6s the solution with my setup?

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Hi,

I own a Oktokopter XL with Axi 2814/22 motors, 4s, and Gemfan 11x4,7 props, Okto XL power distribution board (v8b) and V2 bl-ctrl.

Problem is my okto weight a lot, 6,5kg AUW and power is not in excess. It flyes perfectly, but as you probably know more power is always welcome on a multi as that´s what it need for climbing, maneouvering, flying fast...


Now the question, my setup draw a current of around 60-70A hovering and 120A WOT. If my memory serve me well, that´s the limit for the power distribution board. Also the motors are on the limit, in summer they get extremely hot, to the point over 25-30ºC I always fly each battery in two parts to let the system cool down a bit in between.

ESCs are also on the limit, I´ve seen over 80ºC on some of them in summer (log file).


So the question is, would 6s really improve power? If so, would I need new motors? My motors cost me over 700 euros and that was a great price, they´re more expensive, so it would hurt me replacing them



My main doubt is if increasing voltage you can keep same current or not. I know to increase power it´s better increasing voltage than increasing current, but not sure if you still need to decrease current a bit.

Could V8b power distribution board handle 6s and 120A?

Same for motors and escs.... Could they handle same current with 50% higher voltage?


I know I obviously would need to reduce prop size, wich brings in a new doubt.... Would this setup really be more efficient? Because prop size is one of the main parameters affecting efficiency, so not sure if increasing voltage and reducing prop size is really worth...


I´ve used Graupner 11 inches props, but with 4s the setup was weak. Now I use Gemfan props wich are 11 inches diameter too, but they´re wide blade style and provide more power. I´ve also tested APC 13x4 props but they weight too much and couldn´t adjust the okto. Powerwise they were very very similar to Gemfan props. I think with 6s I´d need smaller props, even smaller than Graupner wich are 11 inches diameter and thin blades, and I´m not sure if using 10 inches props would reduce efficiency to a point the new setup is not worth....



My batteries are old now and need replacement, so I´m considering if switching to 6s or not


Any comment will be welcome :D
« Bearbeitet von AndresMtnez am 22.01.2016 13:22. »
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Hello,

you need the BL-V3 with props 12...14" ;),

regards Stephan
« Bearbeitet von Stephan Lukas am 22.01.2016 14:44. »
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If you will have to buy new batteries anyway, I would definitely go up to 5S at least. For 5S an upgrade of the flightcontrol is much easier than upgrade it to 6S. If you need 120 A at 14,8 V, it's 1800 W of power. At 18,5 V the same power requires only 97 A. With a 6S battery only 81 A.
You should also use high-quality CFK props of the maximum possible diameter (I recommend T-Motor, these props decreased current and BL temperatures of my copter dramatically).

Using BL-V3 would be a good idea as well. And reduce the copter's weight, whereever possible. This should be the first step.
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AndresMtnez meinte
Problem is my okto weight a lot, 6,5kg AUW and power is not in excess.

That is heavy for an octo-copter with BL-Motor-Ctrl V2. Think about how to reduce weight.

AndresMtnez meinte
ESCs are also on the limit, I´ve seen over 80ºC on some of them in summer (log file).

As long as they are below 90 C it is Ok. Just make sure there is good ventilation for the FETs, both top side and underside. It is when they are at >90 C that you should be concerned.

AndresMtnez meinte
I know I obviously would need to reduce prop size,

Why?

In your case I would use 30 cm props, e.g. Graupner E-Prop 30/15 cm. Your motors would run slower for the same lift. I think that makes them run cooler.

If your motors are strong like MK3638, and the frame has the space (not standard Okto XL layout), you can also use 33 cm props, e.g. Graupner E-Prop 33/20 cm.

AndresMtnez meinte
I´ve also tested APC 13x4 props but they weight too much and couldn´t adjust the okto.

The BL-Motor-Ctrl V2 does not have active breaking so I can see why that could be a problem for so flat props. However props with higher "climbing" will slow down faster in the air, e.g. E-Prop 33/20 cm.
« Bearbeitet von Foersom am 23.01.2016 14:41. »
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Thanks for the replies! :D


Yssupotko meinte
If you will have to buy new batteries anyway, I would definitely go up to 5S at least. For 5S an upgrade of the flightcontrol is much easier than upgrade it to 6S.

Good point, didn´t remind 6s require esc´s modification too.... actually I can´t reming what were the differences between 5s and 6s, I know v2 escs handle up to 5s but.... what mods would require 6s? My esc´s have 25v 330uf capacitors, but if my memory serves me well, they need some other mod for 6s?



Yssupotko meinte
You should also use high-quality CFK props of the maximum possible diameter (I recommend T-Motor, these props decreased current and BL temperatures of my copter dramatically).

Problem with bigger props is max current. I´ve tested many props (bench tested) and those gemfan 11x4,7 and apc 13x4 are the best. There are bigger props wich would improve efficiency, but also WOT current, and I´m on the limit there

I´m on the limit there, am I?

v2 escs specs says they can handle up to 35-45A, but mine, at 15A WOT get quite hot... and they all have aluminium disipators both up and down... :? I´m missing something or they´re extremely overrated?


What about power distribution board? I know I read somewhere it can only handle 120A, but can´t find it now.... Is that correct or it can take higher current?


Foersom meinte
AndresMtnez meinte
I know I obviously would need to reduce prop size,

Why?

Because I´m on the limit current wise, so if I increase voltage (and motor rpm) current would go up dramatically with same props

Foersom meinte
AndresMtnez meinte
I´ve also tested APC 13x4 props but they weight too much and couldn´t adjust the okto.

The BL-Motor-Ctrl V2 does not have active breaking so I can see why that could be a problem for so flat props. However props with higher "climbing" will slow down faster in the air, e.g. E-Prop 33/20 cm.

Interesting, didn´t know it but makes sense. Thank you!
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Searching for info about the axi motors at higher voltage I´ve found this thread at RCG with some tests with different props at 4 and 5s

http://www.rcgroups.com/forums/showthread.php?t=1388583&pp=10&highlight=2814+22

He tested graupner 11x5 props, his results at 4s are very similar to my tests, and we both tested apc 12x3,8SF props too with same results, so they´re comparable.

With this props and 5s max current is around 29A providing 2400g thrust, when my current setup draws 28A at 4s for 1800g thrust.
externer Link:
User image

Note when bench tested these numbers are noticeably higher than when in the copter, my current setup draws 28A on the bench test, what should be 224A for the eight motors, but I´ve measured 120A max with an amperimeter. Anycase if both setups draw same current (different voltage) at the bench should draw similar in the copter


I like the idea, if the motors, escs, fc and powerboard can handle same current at higher voltage (5s) I´d get around 30% more thrust :D
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AndresMtnez meinte
...if the motors, escs, fc and powerboard can handle same current at higher voltage (5s)...

Can someone tell me if this is true?

A friend of mine have some 5s 4000 30C batteries I could use to test the setup (using two in parallel, as I do currently with my 4s 5000 20C batteries) before investing on my own batteries, but that will require some mods on the copter, and I need to be sure I´ll not work on the mods only to burn something on the test.

My current setup at 4s draws 28A per motor (bench test of a single motor) and 120A total (measured on the copter) can I upgrade to 5s and same current (smaller props) without burning the powerboard or escs?
« Bearbeitet von AndresMtnez am 29.01.2016 14:18. »
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Up

Still considering this, but would really love to read some formed opinion at the respect.

Will my setup resist same current at higher voltage? Ok I´m not asking about axi motors, but I guess Holger, Ingo or any other person from MK should be able to reply this.... Max power or current depending on voltage is a pretty basic spec manufacturers should perfectly know
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AndresMtnez meinte
Still considering this, but would really love to read some formed opinion at the respect.

You fly with a heavy micro-copter. I would recommend to change to 6S board, either OktoXL V3 or Double Quadro V3. These board are better at handling high heat due to high current. You also get the benefit of lower current due to higher voltage, so less loss in battery and cables. Use MK3644 motors.
« Bearbeitet von Foersom am 25.05.2016 07:41. »
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Thanks for the input :)

I agree that´s the best option, but it would be similar to building a whole new MK. New batteries, new ESCs, new power board, new motors.... that´s a lot of money.

I was hoping for a more affordable option without changing motors or ESCs, that´s the reason I was considering 5s

Also, keep in mind my MK works great with 4s, I don´t need any dramatic change, I only would like to improve perfomance a bit more so I can make the most of my design, wich is similar to a mini-racer, but on a 6,5kg MK :mrgreen:
« Bearbeitet von AndresMtnez am 25.05.2016 18:47. »
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Stephan Lukas meinte
Hello,

you need the BL-V3 with props 12...14" ;),

regards Stephan


But the BL-V2 work with 6s, no?
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No...
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Stephan Lukas meinte
No...


Crap!
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Not even changing the capacitors?

Or that was to make them work at 5s?


I´m confused about what modification do I need to test 5s. My V2 MKESCs use 25v capacitors, so no problem there, but I have some doubts about the rest, as I´ve read so many things about 5s and 6s upgrades that now I´m confused about what´s needed for each mod... :oops:
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Any progress since end of may ?

What fC do you have ? 2.1 2.2 or 2.5 ? 25V caps are absolutely not enough for 6S, you need 35V capacitors for that.

I am also surprised with the weight of your copter, 6,5Kg is really a lot, can't you put him on a diet ?
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papymouzot meinte
Any progress since end of may ?

What fC do you have ? 2.1 2.2 or 2.5 ? 25V caps are absolutely not enough for 6S, you need 35V capacitors for that.

I am also surprised with the weight of your copter, 6,5Kg is really a lot, can't you put him on a diet ?

Hi, sorry for the delay, didn´t see the reply :oops:

I´ve not done any mod yet, but will do it now for 5s.


No I can´t put my copter on a diet, that weight is including the gimbal and dslr camera and 10Ah batteries. It is as light as it can be. The frame is my own design with several advantages but is heavier than the standard mk frame. I can live with it, as I´ve done for the past 4-5 years I´m flying with it, but a bit more power would make my frame design shine even more, so I´ll do the mod now that all my batteries are at the end of their lifespan


I´ve been reading all wiki pages and manuals of all my components to be sure what do I need for the mod as I assembled my okto some years back, and this is my summary. Please anyone correct me if I´m wrong!!


1- Navi-Ctrl 2.0:
no mod needed

2- MK-GPS 2.1:
no mod needed

3- External Compass:
no mod needed

4- Okto Distribution Board v8b:
no mod needed

5- BL-Ctrl v2.0:
No mod needed (they use 25v capacitors)

6- FC 2.1 upgraded to 2.2:
6.1: Replace the two REDCOM with TRACO TSR1 2450 regulator. I guess they´re the two "big" square black components under the FC. I don´t use any servo but I think it´s a good idea to replace both of them
6.2: Replace 16v rated capacitor (C55) with a 25v one
6.3: There´s a point in the wiki I don´t understand, please help!

Zitat
All SMD capacitors in the input area should be rated to 25V. However, the Kerko - subject to availability upon placer - also only 16V. (yes, the FC is also sold as a max. 4s in the shop) If in doubt, replace.


What SMD capacitors are they talking about?
What´s the Kerko?


7- Extension PCB:
Remove the bridge soldered at i2c and solder a TRACO TSR1-24120




Is my summary correct?

Please help me with point 6.3
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Hello,

I am not an expert at all but your points seem OK to me for 5S according to what I know. The C55 capacitor in the input area is normally already a 25V. (same as FC 2.5). The "Kerko" is a KERamik KOndensatoren, I don't know however which one it is as the ceramic capacitors are not labeled and the labels on the pcbs are at times under the components.

I heard that the only thing to check for 5S was the two Recom DC-DC and the 330uF capacitor, but I never tried it.

Have you tried (can you ?) 13" propellers ? I used APC propellers to test as they are very cheap and they seem well done. I really would test 13" props if I were you, 13x6.5 or 13x7 maybe. Weight, I just checked with 13x6.5 they are approx 25g each, the same as Xoar 13x6.5 props. I believe this will also make a difference improving your flying time.

I just put a Hexa on the scale, it is a Cinestar 6, full carbon, MK electronics (FC 2.5, NC 2.0, GPS 3 and BL V2), 6x MK3638 with 13x6.5 props with a small gimbal and a go-pro total 500 grams) and with a 4S 6.6A liPo it is 3.2 Kilos, ( and i find it heavy), ready to fly. It is very far from the weight of your Copter. Making my copter a Octo would mean adding 400 grams with the same configuration (arm, motor, prop, bl), still, at 3.6 kilos it is nearly 2 kilos away from your setup. Do you have a 2.5 kilos gimbal+camera attached to your machine ? (my heaviest gimbal + camera today is 1,6 kilo, including video transmission.)
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Thanks papymouzot

To compare your copter weight with mine you should add

2x: escs, wiring, frame arms, motor supports, motors, prop adaptors and props
Almost 1kg for gimbal and camera (800g gimbal plus panasonic gh2 (460g) camera and lens (175g)
Around 300-400g more due to 10Ah battery
The rest should come from my home made frame I think


About APC props, I tested 13x4 but couldn´t dial the copter correctly. Foersom told me that probably is due to the low pitch instead of the high weight as I was assuming, but didn´t test it. Also, 13x4 props were on the limit (30A) so I´m not sure my motors (axi 2814/22) could handle a 13x6,5. Maybe with a lower average current they can, but with my heavy setup I wouldn´t be confident
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Good evening,

I used to compare the weight of the spare parts I have for my copter, and I included everything, arm, motors (with wires of course), escs, motor support.. I omitted nothing I believe. The parts for the Cinestar frame are quite simple and quite light, it is mainly pure carbon fiber and plastic. It isn't as light as my previous MK Hexa XL frame but on the other side, it is much bigger and rigid. I did not add more weight for the battery however, my mistake.

For the propeller size I would not be able to say things for sure but I was surprised during my tests that at times a propeller with a higher pitch can give much better results than one with a lower pitch, the motors need to turn at a much lower speed for the same thrust and you save some Amps this way, here is an image taken from a post from MK about motors tested by them, a post which you most probably saw a long time before, the graph is quite interesting, it says to me that to be sure, we must test it :)

User image

[url=http://wiki.mikrokopter.de/en/MotorCurves
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Main problem is I´m concerned about max current. My current setup, when bench tested, draw less amps than any 13 inches prop (except apc 13x4), but when on the copter my total current is 120A or even a bit more. According to some specs, even my power distribution board (v8b) is on the limit, so using any bigger prop, diameter or pitch, would increase total current over specs.


I´ve asked several times about real limits about different components, like distribution board or V2 escs as they´re rated for 30A but get pretty hot at 15A. Never got any reply, I don´t know if MK staff does not know the real limits of their own components, or if they have overrated some of them so much they do prefer to ignore this sort of questions.... :roll:
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I think I understood your concerns. :) MK BLs are all rated for 30A continuous and 120A max for a board regardless if it is Hexo or Okto. In your case it will limit it to 120A which is a lot of current and your desire to move from 4S to 5S is justified and a good option to reduce current drawn by your motors.

Look again at the graph above, you always need less Amps for a 13x6,5 rather than a 12x3,8 for the same thrust. Not a lot, but less anyway.

Have you installed already the heatsinks on your BL's ?
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Yes my esc´s do have heatsinks.

I know a 13x6,5 draw less amps for same current but, what does happen when I do a WOT climb or need to do fast forward flying? Max current will go over 120A... :(
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Holger, Ingo, no help about point 6.3?
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Is it really that difficult for MK crew to confirm or correct my summary?

Is this the support we get on the official MK forum? :(
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It is so frustrating when you need support from a manufacturer and you´re sistematically ignored....



I asked in September, have patiently waited for an official reply, and in November I still didn´t get any reply :(

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