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Oct 26, 2001, 10:41 PM
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Thread OP

A little Cobalt Motor Advice Needed


I am trying to set up a cobalt Twin Jet. I want to use 8 cell packs I have. Several people are using 4014 cobalts with 10 cells and happy. What would be the negative side of using a pair of 11, 12 or 13 turn motors. As I see the specs more RPM with a lower cell count. Am I missing something, I plan to use CP1700's and or RC2400 packs.
Thanks
Crash
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Oct 27, 2001, 12:08 AM
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stone_axe's Avatar
I've often wondered that myself.

I prefer a lower cell count and a hotter wind. It may be a little less effecient, but it is a whole lot lighter too!

I think people fall into the "10 cell trap" too easily.


David
Oct 27, 2001, 08:30 AM
Flying Welder Pilot
Plane Crazy's Avatar
The 11 turn would be way too hot (2900kv) for a direct drive prop with a 7 cell pack you would be over the 20000 rpm limit of the props and drawing 25 amps per motor a total of 50 amps which would give you pretty short durations. The 13 turn (2430kv) with 8 cells should work but you would still be drawing pretty high amps in the 40s which is pretty close to the 14 wind motors on 10 cells.

Gordon
Oct 27, 2001, 10:51 AM
Have a look here.
http://www.maxxprod.com/mpi-26.htm
They reckon on 20A draw per motor at max throttle. There's no reason you can't throttle back some of the time.
I don't know what props you'd use on the Twinjet but a swop to a higher pitch would allow high speed reduced throttle flight.
Using the Gunther props would give about 25A and 85mph pitchspeed- not the most efficient but it would really go. 6*4 would be a little more efficient but wouldn't howl as well in a dive.

Either way, when your Twinjet is just a memory, the hot motors will work well with a gearbox or direct drive in a new plane. The cooler winds are useless with gearboxes as they're too slow to pass much power through.

Two 11 turn motors with 2.5:1 gearboxes , 9*7 props and 10 cell pack will produce about 48 ounces thrust at 57 mph and draw 35A. That'll almost hover a three pound plane vertically or fly a 40 size trainer or sports plane. A typical plain bearing 40 like the OS 40FP produces about 60 ounces of thrust so we ain't far behind that.

Go with the hot motors every time!

Andrew
Oct 27, 2001, 01:28 PM
Registered User
Sabrejock's Avatar

Hot Motors


A settup using a hot motor and fewer cells vs a "sport" motor and more cells gives less duration for the same power output. Lower voltage and the same wattage demand equals higher amps(less time). So, it all boils down to what you need for performance. If you get desired performance from a higher wind motor and more cells, That's the way to go. Only if weight is a real problem and you can endure the loss of flight time, should you opt for the hot motor/fewer cells option, IMHO. Tex.
Last edited by Sabrejock; Oct 28, 2001 at 04:28 PM.
Oct 28, 2001, 02:54 PM

Hot Wind Motors


I agree you will get shorter flights using the same cell size. But you can end up with more power or longer duration for the same weight using a hot wind motor with fewer larger cells. As an example:

Lets compare a typical Speed 480 using a 8 cell 800 AR pack to a hot wind old Astro cobalt 02 using a 5 cell RC2400 pack. Both motors weigh 3.5 0zs without prop adapters and both packs weigh approximatly 10 oz. If we assume a equal flight times of say 6 min.

SP480
.800 amp-hr X 60 min=48ma-min
48ma-min/6 min=8amp draw
8 volts X 8 amps= 64 watts

Cobalt 02
2.400amp-hr X 60 min=144ma-min
144ma-min/6 min=24 amps draw
5 volts X 24 amps=120 watts

Or if we wanted equal power but longer flight times on the Cobalt set up.
64 watts/5 volts =12.8 amps
144ma-min/12.8amps=11.25 min running time

So you can have almost double the flight times or double the power from the hotwind setup for approximatly the same weight. I know the above comparison is overly simplifed. I havn't taken into account the heavier wire and ESC required in the cobalt setup. I've also assumed 1 volt/cell for symplicity. Even if you used the new CP1300 cells for the 480 setup you would only get up to about 100 watts compared to 120 for the cobalt. This also doesn't take into account the difference in efficiency between the two motors. About 75% for the cobalt vs 60% for the 480.

By the way I used the Cobalt 02 because I have 2. And they work great on 4-6 cells dirrect drive and even better on 6 cells geared. They have a Kv of 3830rpm/v.

My point is don't be afraid of high Kv motors, they can work great if used appropriatly.
Oct 28, 2001, 09:33 PM
Registered User
Thread OP

Cobalt Motors


I am going to try the 13 turn versions, two proped @ about 6x4 would probably provide enough power to do anything I want and still have enough speed to keep the blood moving.
TIA
Crash


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