Ecalc assistance 4s or 3s?

[Orracle]

Hey guys

I'm looking for some help with some calculations for a quadcopter I plan on building. I have already ordered the motors and props, Rctimer BC2836-9 880kv and 12x4.5 props. Now I have spent alot of time on ecalc figuring out what I want to order and although I thought I had everything figured out I am now second guessing myself now that I am beginning to order components.

For the calculations I am using the following figures:

AUW: 2000g
Battery: 5000mah 4s
Motors: Rctimer BC2836-9 880kv
ESC: 40amp (recommend for the motor?)
Prop: 12x4x5

The max power rated for these motors on ecalc is only 160w yet according to the manufacturer it is 243w. So there is problem No.1 Problem No.2 is battery choice, I have searched previous threads and I see 4s batteries being preferred over 3s, especially for lifting a payload. At the moment I am not worried about a payload, need to get a handle of flying it first. However when using a 4s battery in ecalc the input power to the motor is way to high? Should I use 3s batteries and when I eventually/hopefully start adding a payload run 2 in parallel? I have also taken the time in copying other people's Setups in ecalc that are using 4s batteries with a similar setup as to what I am trying to achieve, yet I get the same result of the input power being too high for the motors being used.

Any advice would be much appreciated so that I can finally finish ordering all the components.

Cheers Dan

 

[jes1111]

Hi Dan,

You can't really play "what if" on batteries, motors, etc. unless you choose the "without Drive" option for the "xCopter Weight" and enter the figure for only your frame, landing gear, etc., i.e. exclude the motors, ESCs, batteries. Is that the way you're doing it? If not, that could be skewing your numbers. Perhaps post a screen grab so I can duplicate your setup and hopefully advise?

Jeremy

 

[Orracle]

*EDIT* Sorry messed up the screenshots

Hi Jeremey

Thanks for the reply, I have already learnt a lot of valuable information from your previous posts on these forums. To answer your question I have tried both ways and when I do put in the AUW including drive I am a bit conservative and add a little extra weight as I have a good idea of what the frame will weigh.

Here are the screenshots of ecalc including drive and excluding drive using the same specs as above:



And here is a screenshot using 3s LiPos:



Motors - http://www.rctimer.com/index.php?gOo=goods_details.dwt&goodsid=522&productname=

My goal as mentioned in my intro thread (http://www.multirotorforums.com/showthread.php?1081-Hello-from-South-Africa-(Go-bokke!)) is to get a MRC up in the air and start flying/crashing . I originally planned on building a very small quad but I decided on buying bigger motors which are more practical for future use. So at the moment I have shelved the small quadcopter.

For testing purposes and for learning how to fly a MRC I will build a frame based off the Arducopter (made a bit stronger to handle more abuse) as I will be using ardupilot FC, I have a friend who is a arduino expert so he has kind of converted me to. It's also not expensive. I want to get the most of these motors and have them running at their most effecient so I need to decide on what batteries to get that will help accomplish this.

 

[jes1111]

It does appear that the eCalc rating for that motor is wrong - the RCTimer site states 243W, so we'll assume that figure is correct. You can fudge this in eCalc by first select the motor from the list, then changing to "Custom" and entering the higher figure for "Limit(up to 20s). Remeber too to set Motor Cooling to "excellent".

What eCalc then shows is the motor is happy with 3S but only likes 4S with much smaller props (which makes sense - higher voltage generally requires smaller props or lower Kv). So definitely 3S 5000mAh 20 or 25C is the best choice. Btw - if can find 20C rather than 25C it saves a little weight.

Interestingly, Google pointed me back to this thread: http://www.multirotorforums.com/sho...C-2836-9-motors-and-different-prop-experience - which seems to confirm the same: @ovdt experienced very hot motors with 11" and 12" props.

With 3S you're in good shape - you could add a light camera and/or FPV gear (say an extra 700g) and still be hovering at 65% throttle.

View attachment 1267
View attachment 1268

 

[Orracle]

Thanks for that thread, that is very helpful.

So I will go with a 3s battery probably from Hobbyking
http://www.hobbyking.com/hobbyking/store/__11935__Turnigy_nano_tech_5000mah_3S_25_50C_Lipo_Pack.html

I will just have check the motors for the same heat issue as mentioned by ovdt and possibly switch to smaller props as well.

Hopefully I dont have to wait weeks for everything to arrive

 

[ovdt]

Sorry, I missed this topic.

Jess is the master of ecalc analysis, so nothing to add to his words

I have several BC2836-9 motors. I only have 4s batteries so, the biggest prop that I could use safely was 11x5. With 3s, you'll be more comfortable than I am, but be sure to check out the heat if you add payload to your bird.

 

[Orracle]

Ye I will be careful to monitor the heat with the 12" props and change to smaller ones if needed.

Ovdt did you order your motors directly from RCtimer? Roughly how long did you wait for your order? I paid for "express shipping" we shall see how really "express" it is Also I would like to know what ESC's are you using with your BC2836-9's?

 

[ovdt]

I live in Turkey and it takes about 7 days until I get my hands on the shipping package. And yes, I ordered directly from RC Timer. Just send them an email and request a tracking number. Sometimes they use DHL, sometimes EMS.

I used 2836-9 motors with MK BL Ctrl 2.0. It's rated at 30A continious, 40A burst. After seeing that these motors max thrust is not enough for me, I also ordered BC3536-11, which has around 2kg thrust and can spin 13" props. But, BC3536-11 doesn't have long wires like 2836-9. That's OK if you're using standart ESCs.

 

[Orracle]

Hmmm RCtimer recommend using 40A ESC's yet I see on eCalc that the max current will be 18A so are 40A ESC's not overkill?

I emailed RCtimer like you suggested... they havent even sent the order yet and I ordered them almost a week ago. They said they will send them today. Suppose I can't do much with them anyway till the rest of my parts arrive.