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Author Topic: Cobra motor sale  (Read 1300 times)

Offline John Rist

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Cobra motor sale
« on: November 22, 2018, 07:08:21 AM »
 Innov8tive Designs is having a Black Friday sale.

innov8tivedesigns.com

Save 15% On All Cobra Products
Use Coupon Code: COBRA15
Valid from today to November 30, 2018

Save 25% On All Scorpion Products
Use Coupon Code: SCORPION25
Valid from today to December 31, 2018

I just bought a Cobra 3520/10.  The 15% off covered the shipping and then some.  Another thing to be thankful for on Thanksgiving day.   #^  By the way they have lost their Scorpion import contract.  They are selling off all of their Scorion products.
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Offline John Rist

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Re: Cobra motor sale
« Reply #1 on: November 22, 2018, 07:39:05 PM »
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Offline Vitalis Pilkionis

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Re: Cobra motor sale
« Reply #2 on: November 23, 2018, 03:09:47 AM »
Quote from: Motorman on November 22, 2018, 09:04:15 PM
I don't get the whole matching motors to cell count thing. Why wouldn't that motor work with my 6S if I ran the same prop and rpm as a 5S.

I am sure it would work with 13x5(6) 3-blade prop on 6S with adequate cooling. This motor is quite bigger than Axi 2826/13 which turns 12" 3-blade props with ease.


Vitalis
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Offline John Rist

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Re: Cobra motor sale
« Reply #3 on: November 23, 2018, 04:09:36 AM »
Quote from: Motorman on November 22, 2018, 09:04:15 PM
I don't get the whole matching motors to cell count thing. Why wouldn't that motor work with my 6S if I ran the same prop and rpm as a 5S.

It's all about max RPM vs governed RPM.  Per the Scorpion prop chart, as listed on Innov8tive Design, the working voltage of a 5 cell battery under load is 18.5 volts.  The KV  of this motor is 710.  So 710 x 18.5 volts = 13135 RPM.  Divide 18.5 V by 5 and multiply by 6 gives a working voltage for a 6 cell battery of 22.2 volts.  So 719 x 22.2 volts = 15975.  If you are governing the RPMs at 9000 rpm the 5 cell has an overhead of 4135 RPMs.  The overhead for the 6 cell would be 6975 rpm.   Here is where it gets tricky.  The question is witch setup has the motor running at it's most efficient?   I don't know the answer to this one.  You motor experts out there can answer this one.  The other problem is governing RPMs with a KR timer.  The KR timer starts up a motor at full RPMs and then throttles it back to the set RPMs.  It the initial full speed RPM's are to high it creates a fault and the KR timer shuts off.  I am not sure at what over RPMs this occurs.  Perhaps Keith Renecle will chime in here.   Anyway for optimum performance one needs to match the KV of a motor the the cell count one is using.   The other point one can make is that the higher the cell count for a given power level will give lower current.  This will result in a lower IR loss (heating) making for a more efficient system.
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Offline Vitalis Pilkionis

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Re: Cobra motor sale
« Reply #4 on: November 23, 2018, 05:24:11 AM »
Although discharge curve of Lipo is quite flat, but in CL we most often use the smallest packs to save weight, therefore at the end of a flight voltage can drop down to ~3.2V (or even lower) per cell.  That's only 16V on 5S pack!
Also one must keep in mind that efficiency of a brushless motor is not 100%. It depends of a bunch of factors, but it generally tends to drop down with heavy load. Usually manufacturers provide data of an efficiency with various props and voltages, but not always. If there is no such data I usually take a pessimistic value for rpm calculation, e.g. 80%. So 16 x 710 x .8 = 9088 max rpm at the end of a flight.


Regards,
Vitalis
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Offline Joseph Daly

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Re: Cobra motor sale
« Reply #5 on: November 23, 2018, 07:01:31 AM »
Quote from: Motorman on November 22, 2018, 09:04:15 PM
I don't get the whole matching motors to cell count thing. Why wouldn't that motor work with my 6S if I ran the same prop and rpm as a 5S.

MM,
It will work with a 6s battery. It is good motor, I have about 350 flights on one using a 6s battery with a Igor 12x5N prop.
Joe
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Offline Vitalis Pilkionis

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Re: Cobra motor sale
« Reply #6 on: November 23, 2018, 07:59:24 AM »
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Offline John Rist

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Re: Cobra motor sale
« Reply #7 on: November 26, 2018, 07:56:37 AM »
Got another eMail from Innov8tive Designs on a Cobra motor sale

Save 25% On All Cobra CP & CPL Multirotor Motors & Multirotor ESCs
(25% off is in addition to some items already at reduced sale prices)
Use Coupon Code: COBRACP25
Valid from today to November 30, 2018

or

Save 15% On All Cobra Products
(15% off is in addition to some items already at reduced sale prices)
Use Coupon Code: COBRA15
Valid from today to November 30, 2018

or

Save 25% On All Scorpion Products
(25% off is in addition to some items already at reduced sale prices)
Use Coupon Code: SCORPION25
Valid from today to December 31, 2018
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