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Cobra plans underway

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..

Old 08-25-2019, 04:07 PM
  #102  
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Cool

Looks good! Yep, 5S or 6S will really wake the boat up. Propcalc estimates that on 6S you’ll 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
  #103  
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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.
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!!
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, don’t 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
  #106  
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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.
Old 09-03-2019, 04:29 PM
  #107  
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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
  #108  
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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.
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.
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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Old 10-08-2019, 11:39 AM
  #111  
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Hi Vintage,

Cool thread and builds!

I wanted to let you know about a couple things with the props you are running on both boats. First the APC (grey) prop you have is a pusher, so it turns the wrong direction from normal. This means that the right thrust you built in is actually having the opposite effect it was intended for. Put a regular tractor prop on of the same size and the handling issues you mentioned may go away.

Second the black prop on the Cox motor in the pictures above is installed backwards on the motor. Flip it over before you run it.

Cheers
Red
Old 10-29-2019, 10:30 AM
  #112  
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It's funny, you look at something for so long and wonder why it's not working and you completely overlook the obvious! Thanks for pointing that out! I have some proper props now, hopefully I'll get a chance to try it again before everything freezes up here! Just waiting on some new spinners.. I may of lost my last one in the water.... Luckily the wind was blowing towards shore! Which brings me to my next (very important) project: a recovery boat!!
Old 03-05-2020, 04:03 AM
  #113  
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C'mon warm weather!!! I didn't get a run before everything froze up sadly, but as soon as there is some open water I'll have both boats on the river!! I live in Newfoundland so chances are it'll be the end of April before that happens....
Old 07-26-2020, 06:02 PM
  #114  
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Well, its the middle of July and I haven't touched the boat yet! With all that's going on, I guess its the least of my worries (I still travel for work).. Anyways, I do plan on getting my boat out on the river for a few runs when I get home in a few days! I did order a 10x7 inch prop as well so I might try and see if it'll fit. Updates and a video to follow! Hopefully.. While we wait, here is a size comparison of my two boats:
Old 08-15-2020, 04:45 PM
  #115  
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I HAVE A NEW VIDEO FINALLY!!! Now, its not the best of quality since most of the filming is done by my 10 yr old son, and the video editing app I used put a blurry border around it.. BUT you can get a look of how it runs on calm water. The first part I am running a 9x6 prop and 20 v (still haven't sprung for proper lipos yet..), then I put a 10x7 on and run it on 12 v for a bit, so the kids can try it out. Finally I put the 20v in at the end. As you can see, with the 20v I can't go full throttle without the esc kicking off, and it porpoises at speed. I think if I actually get a proper lipo, that might solve the first problem, and moving my center of gravity up a little bit will solve the porpoising.. Anyhoo, here's the video, I hope you enjoy!
Old 08-15-2020, 05:39 PM
  #116  
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Were you ever running WOT? Seems awfully slow. Try moving the CG back to free up the hull, it should run on the back half of the sponsons.





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Last edited by Got RPM; 08-15-2020 at 05:43 PM.
Old 08-15-2020, 06:27 PM
  #117  
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It could be a weight issue in general, depending on the weight of the ESC and electronics. You have to remember, it was designed for a .25-.30 nitro motor and 4-6 ounce fuel tank. Your electric motor may be about the same weight as the nitro motor but the batteries probably weigh more than a 6 ounce tank of fuel. In another thread, they had a Cobra, powered by a nitro motor with tuned pipe, that was running on the back half of the sponsons but not significantly faster
Old 08-15-2020, 08:11 PM
  #118  
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Yeah, I was never at full throttle.. The ESC would cut out at less than half throttle, so I think if I had proper batteries (2 3s or a 6s) it would be much faster. It might even be a setting in the esc.. I haven't had a chance to research it too much. I will try moving the c of g back in the next run just to see if that helps! I think it has the potential to be much quicker, I just have to spend some more time (and money if my wife lets me!) getting it dialed in..Oh, and I have to make a new fin because I may of damaged it when I ran aground.... I'm pretty sure the 10 inch prop is the max I can fit in that location. I saw a video of one guy who just mounted the motor up higher and put on a bigger prop, but I like the look of the boat, and I'd rather sacrifice a bit of speed than the aesthetics of it.

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