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Cobra plans underway

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Old 08-24-2019, 12:27 PM
  #101  
vintageshack
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It's running and floating FINALLY! And it only took 5 years! Here it is in the water for the first time! It was a pretty windy day but it seemed to handle it pretty well.. I will need to adjust the motor angle a little bit because it doesn't turn quite as well in one direction.. And I will definitely have to go with more voltage soon! I may experiment with a couple different props as well..

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Old 08-25-2019, 04:07 PM
  #102  
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Looks good! Yep, 5S or 6S will really wake the boat up. Propcalc estimates that on 6S youíll be pulling over 50 amps using the 9x6 prop with twice the static thrust. Limit your run time to two minutes the first run (use a stopwatch) then check the heat of the motor and ESC. Without a cooling air exit the motor will get hot.


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Old 09-02-2019, 05:10 PM
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I went out for another run with 2 lithium ion 12 v batteries in series, and it makes quite a bit of difference! Unfortunately it was yet another windy day, so I couldn't get it out in the open.. Here's a quick video of the run:


I checked the temps of the motor and Esc after about a 4 min run and they weren't too bad.. The motor was at around 42C and the esc was at 38 C.
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Old 09-03-2019, 04:50 AM
  #104  
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Looking good for part throttle and low speed operations. If that is all the hot it gets should be fine wound up at speed. Can't wait to see it in open water!!
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Old 09-03-2019, 04:00 PM
  #105  
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There is a major problem using 8S with that motor and propeller. APC states that the maximum safe rpm for their E props is 150,000/prop diameter. For their nitro props it is 190,000/prop diameter. You are well over that limit, donít do it for safety sakes!

https://www.apcprop.com/technical-in...on/rpm-limits/


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Old 09-03-2019, 04:22 PM
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I don't think I mentioned anywhere that I was going 8s, 6s would definitely be my limit.. I will be experimenting with different props after that point.
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Old 09-03-2019, 04:29 PM
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You stated you were using two 12 volt batteries in series. That is close to 8S. You are still over the recommended safe rpm with 24 volts and the E prop:

825 x 9 = 19,000 rpm

150,000/9 = 16,660 safe rpm

Do whatever you want, it’s your face (and your kid’s). I’m outahere.


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Old 09-03-2019, 04:47 PM
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I appreciate your concern, but I should clarify.. These temporary batteries are well worn out dewalt drill batteries. I cannot even go anywhere near full throttle before the esc senses the cells have dropped too far. When I do get some sensible batteries(6s at the most) , rest assured I won't be pinning the throttle anywhere near me or my children.
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Old 09-05-2019, 02:51 AM
  #109  
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Originally Posted by Got RPM View Post
You stated you were using two 12 volt batteries in series. That is close to 8S. You are still over the recommended safe rpm with 24 volts and the E prop:

825 x 9 = 19,000 rpm

150,000/9 = 16,660 safe rpm

Do whatever you want, it’s your face (and your kid’s). I’m outahere.


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How in heavens name do you get 8S out of 24 volts? Totally dead 8S is 24 volts. Fully charged 6S is only 25.2. Nominal 8S is 30.4. With throttle pinned the 6S will drop to something around 22 volts. You cannot base prop RPM on fully charged voltage.

So that's a Red Herring.

Last edited by davidmc36; 09-05-2019 at 02:56 AM.
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Old 09-13-2019, 05:52 PM
  #110  
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Well, no updates on the cobra since I am up north for a bit, but here's an update on another build! I dub this one "Spare Parts" because I built it from leftover wood from the cobra build, stolen electronics from my sons rc car and a prop from an old control line plane! I've tried it once so far in a little brook, but the C of G was off and I think the current was too strong (oh and a dead battery). I originally started this build with the intention of using the. 049 glow engine from said control line plane, but I've since given up on that engine.. Not sure how it'll perform but it was fun to build!
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