Cobra C-2820/12 Brushless Motor, Kv=970

[Mike_Ostella]

Has anyone given this cobra motor a try?  http://innov8tivedesigns.com/parts/brushless-motors/cobra-c-2820-12-brushless-motor-kv-970

If so what prop and size/weight plane?

I have 55 oz Vector 40 ARC that has an e flite 15 in it. Prop is 11 x 5.5 tractor, 59ft lines @ 5 sec lap at 9,695RPM. Seems to need a bit more power. Battery is a thunder Power 2700 4s.

Eric Viglione helped me with maiden flights last weekend. Wind has been relentless this season in Florida.
First full pattern today was today in mostly mild wind, but with an occasional puff that got my attention. I'm going to try a 12 x 6 APC as a next step.
Graphics and final colors have not yet been applied. Plane was highly modified, so it doesn't look like a Vector anymore. More like a Satona/Katana.

[Randy Powell]

I'm getting ready to use a Cobra 2826-12 KV-760

[Joseph Daly]

Mike,
I use one on a Forerunner. It weights about 58oz. and I fly it on 62ft lines at about 9500 rpm's with a 11 x 5.5 apc prop. The motor has no issue pulling it through the pattern.

[Mike_Ostella]

Thanks Joe
sounds great
I'll give it a try.
Mike

[Mike_Ostella]

I thought about that 2826 Randy. I have one slated to go into my Aurora. I even made sure the Vector nose had room for it , thinking that i might need more power , but it's heavier and will use more battery??,, so I'm going to try the 2820/12. The cobra is @ 1/2 oz lighter than the e flite 15 as well. That's a big plus.

Cheers
Mike

[Avaiojet]

Mike,

I have a model that's almost the same weight.

I was told to use the Cobra 2820/14.

I know little about electrics, so I didn't ask why that motor choice?

I just bought what I was told to by from a SH modeler who does electrics.

It's simpler that way. 

Good luck.

[Mike_Ostella]

Yes, the 2820/14 is another strong possibility. I have two more weeks in Florida, so I'll Fiddle around with props and try to shed some weight on the plane. Also, I'm not so sure my ESC settings are optimized. Plane tracks nice in round maneuvers. By the time I get back to NJ I will hopefully worked out my motor choice, which may be to just leave the e-flite 15 in the plane.
Mike

[Target]

I was told by a reliable source that that most C/L motors will have a Kv of 850-950, and I should plan to run them on 4S packs.
So my question is, what pitch speed are you all looking for when you look at the "Propeller charts" for these motors?
Based on what I have seen on some of the "List your setup" threads, I am guessing the pitch speed in the charts to be about 55 mph. I realize that the chart pitch speed is 100% throttle, and that the timer is being used to reduce the setting a bit to achieve the desired rpm, which must be lower.

I'm just curious is there is a general pitch speed and thrust (ratio to weight, I realize) I should look for in a motor with the popular 11x5.5 prop.

Thanks,
Chris

[William DeMauro]

Most of those charts are made with RC flying in mind. For most of us that motor works best with the APC 11x5.5 thin electric prop. Just copy a known set up to get yourself started.

[Mike_Ostella]

ok, so I upgraded all my castle software and re programmed my ESC. That helped the power train a lot.

Plane was flying like it was very wing loading challenged.

I had some expert help from Jim and Wes Smith this past weekend. In 6 two  minute trim flights, we improved the stalling in the corners by adding tail weight,  moving lead outs forward and speeding up the plane.

Plane is set up as follows

motor - E-Flite 15
APC 11x5.5E tractor prop
Castle ice lite 50 esc upgraded to 4.22 or .21 or whatever it was last week in governor mode, governor high (35)
Hubin F-9 timer in castle new high.  RPM's just under 10,000
TP 4s 2700 battery
Plane weight was 54 when we started. Now up to 55 because we added 1 oz of tail weight (thanks to Eric Viglione for thinking of me and bringing his scale along)
Line length 59' eyelet to eyelet

Lap time was 5.1
don't know what a full pattern of about 5 minutes and 40 seconds would take out of the battery as yet.

next flight goal is to get the plane down to a 5 second lap. With the wing loading the way it is, it's going to need to fly faster than 5.1. I'm still able to keep up with it so I'll give it a try. the elevator throw is adjustable so I might play with that too. I built the plane with the flaps cut back from full span so that might be working against me. This is a situation where more flap than elevator could help? worth a try if the other changes don't get me all the way to "practice ready".

[Tim Wescott]

What was leading you to conclude that it needed more power?

The size of the motor doesn't dictate the power you can get out of it, at least not directly.  The size of the motor just dictates whether or not your plane will leave a trail of nasty-smelling smoke when you are getting adequate power.  What dictates getting adequate power is the battery pack (number of cells and capacity), the motor kV, and the prop used.  Within that, assuming you could get a bunch of motors with the same kV, you can go all the way from a humongous motor that won't let the plane get off the ground to a teeny one that'll burn up in one flight, and still get roughly the same power to the prop (and, hence, airframe).

I just ran the numbers for someone else, and I think your 2700mAh pack may be marginal.  I was getting 3000mAh for a 48 ounce plane, and you're over 10% heavier.  I think you really want to know how much is being put back into the pack, because you may need to pork that plane up with even more battery.  I'm just guessing, so I'd be interested in hearing just what your power consumption is.

Gloom and doom aside, you may want to try longer lines.  Longer lines will let the plane fly faster for the same lap time.  You'll get more lift and the same diameter corner will look sharper in comparison to the maneuver.  With longer lines on the thing you may be able to slow down the lap speed and still get the flying qualities you want (at the expense of needing even more battery, of course -- nothing in life is free).  I tend to fly longer lines than conventional wisdom would dictate, but I tend to fly porky planes and get pretty good scores, so I don't think I'm entirely wrong in my thinking.

[Mike_Ostella]

Hi Tim

What led to my thinking that it needed more power was that the lines were going slack at the top of the wing over. Why I thought it needed a bigger motor was my glow mentality. All the electric motor sizing charts are for R/C so it's frustrating. 

The problem was the ESC programming. Power is adequate now and post flite temperatures are OK. The cobra 2820/12 will give me a few more KV and lighten up the nose a bit.

We fly on Short lines at GSCB in New Jersey because of the turbulence there, but I'll try longer lines at another site. Sounds like it might help. I'll know more about the battery drain the next time I go out. If I have to increase the 4s battery capacity, I'll have to cut away F-2 and cut a pocket in the leading edge and design a new battery mount. going to a 5s might be easier.

When I recharged after the one full pattern I flew, I put 1850 back in the battery. Everything is different now, so I'll have to try again to see if that changed as well.

Thanks for the feedback
Comments are welcome, I'm learning a lot

[Keith Renecle]

Hi Mike,

The Vector 30 is a nice airplane, but the ARC/ARF ones come out heavy and 55 oz is really pushing things for that wing area. Adding more power, bigger batteries etc. just make things worse. I can remember the Top Flite Score looking like a  nice stunt plane but it weighed a ton. A friend of mine bought the one I had from me and put in a PA .61 on a tuned pipe and it had more than enough power, but still flew like a brick! For rough weather on that size model I make my lines 60 ft. eyelet to eyelet and that gives me what I feel is the optimum length. I have made up a lot of lines for newcomers to stunt as well and it seems to be an ideal length even for gusty conditions. The lap times of 5.0 to 5.1 or even slower in nice weather, work just fine.

The Cobra 2820/12 is an ideal motor on the APC 11 x 5.5 E prop as well. If you can get hold of a 2200 mAh 4 cell pack to try, it would be most interesting. Just set the flight time to 5:15 and it should have enough reserve for a good flight. That will drop the overall weight of the model as well. If you're using 1850 mAh with your present system, this will not work the 2200 pack too much for a few flights because you should use a bit less with the shorter time and lighter weight. What is fun with e-power is that everything is measureable and you can get the necessary data easily. If the Ice Light has the CC data recorder you can have a look at all of the parameters on a pc later.

Once you are more into this e-stunt thing, then build a lighter version of the Vector from the plans or one of Randy's kits. RSM also has some nice kits, and a 48 oz version would really work well with that Cobra and a 2700 mAh pack.

Keith R

[Mike_Ostella]

Hi Keith
I have a spyder 4s 2200Mah pack , but it is identical in size and weight as my thunder power 2700 packs. I did some searching for a 2200Mah pack but couldn't find one that is lighter, or that will fit in the same approximate space as the thunder power. I love that thunder power battery because it is smaller and lighter than any other I have been able to find. Expensive at @80.00 but I buy 'em on sale and when they offer free shipping. Helps the toy budget a bit.

I use the spyder 2200Mah pack in my 48 oz top flite E converted ARF. That plane gets whipped around like a rag doll with the eflite 15 and the tp 2700 or the spider 2200. The problem with that plane is that it hunts. The vector doesn't hunt at all, even when we added tail weight. The vector has some positive incidence in the stab and a tractor prop. Draw your own conclusions. 

I'll scratch build a "designed for electric" something or other for the next airplane. Till then I'll optimize this bird and learn how to nurse it around. There are several things I can do to lighten up the nose a bit but all that will do is help reduce some of the stick on weight we put in the tail, so it might get down to 53 or 52.5, but that's it. I'll attack that project scientifically, with a good scale so I can document the changes, and hopefully, the amount of improvement.

Still very worthwhile as a learning project.

Thanks for your suggestions and thoughts.

[Keith Renecle]

Hi Mike,
O.K. that TP 2700 is really light, but a bit above my pay grade. Out here in Darkest Africa, we pay a small fortune for them. One more gadget to get if you don't already have one is one of those battery checkers. I have the Hyperion one but you get others that are identical at around half the price. They also work well for small balancing differences without using the charger. It tells you how much is left over after the flight and you can go down to 12~14% in the pack quite safely. There is a recent thread on this somewhere.

On the Nobler just try a normal tractor prop and not the pusher, and it may just get rid of the hunting. The motor thrust line is quite high above the wing and that makes for more trimming problems with a pusher prop.

Keith R

[Mike_Ostella]

So I flew a full pattern with the Vector.

to review, the setup was as follows:

tp 4s 2700 battery
airplane weight 54.5
rpm 10,011
apc 11 x 5.5 prop
59' lines eylet to eyelet
Flight time 5:45
 
There were 13 laps after the pattern.

When I recharged the battery took 2200

That's around 200 more than I'm comfortable with for the 2700 pack.

New Plan....Reduce weight everywhere possible
 lighter spinner
 lighter esc (moving from ice lite 50 to Phoenix 45, Will DeMauro's suggestion)
 moving from E-Flite 15 to Cobra 2820/12
 Changing the aluminum LG to Carbon Fiber
 Shortening all the wires to exact length
 exchanging one oz of tail weight for some trim colors (plane looks Blah in white primer)
 I'll shorten the flight time by around 25 seconds
 We measured the Vector against the Nobler, The although the Vector appears larger, The difference is fractional. with that in mind, trying 58' lines might be    worth a try, especially since I fly in the bowl of trees in Lincoln Park most of the time. The plane won't look cramped in that hemisphere IMO.

I should get the ready to "push the button" weight down to 52.5 oz
Parts are on order. I'm optimistic.

Thanks to Will DeMauro, Jim and Wayne Smith, Jim Damerell, Bob Hunt, Tom Hampshire and Eric Viglione for helping me get to this plateau.

Mike Ostella

[Target]

Would trying just a higher pitch prop at slightly less RPM and the shaving of the extra laps save you some MA, even at the same weight?
I'm a newbie, just a suggestion based on some of what I have read here.
The shorter lines makes a shorter flight too, so that should save some juice alone, right?

Regards,
Chris

[William DeMauro]

Hi Mike,
 I enjoyed our chat yesterday. I think you should shorten your flight time to 5:30 to start and work from there. I checked the line length of Kevin's UHP Gieskie Nobler and they are 58.5'. After that everything pretty much changes as I checked the weight and I it is actually at 41oz and uses about 1650 mah out of a zippy compact 4s 2700 25c battery. He is using the smaller 2814/20 cobra and the 10x5.8 APC thin pusher prop. He has the FM9 timer and is using a PHX 35 for a speed control. Being that you will not be able to get the weight even close to the Gieskie Nobler, but you should be able to to get it close to 50oz, I would think that you can get your battery consumption down to about 2000mah or a bit less, which will be fine for your 2700's. Even as  it is 80% of 2700 is 2160 so you are ok on consumption. I think the biggest gain that you will see is that it will fly much better. If I can work it out for you I will let you have a flight or 2 on the Gieske Nobler next time we are flying at the same location.
William

[Keith Renecle]

Hi William,
Thanks for those details on that Gieske Nobler. Wow that is a nice weight and I would guess that a 2200 battery will also work fine. To Mike, thanks for posting your test results. The more data we have the better for everyone. Like William says "the biggest gain that you will see is that it will fly much better." Battery technology is developing rapidly now and we will see even better power to weight ratios and also cheaper prices soon. The fact that you can indeed already fly a 41 oz. Nobler with a lot of extra power right now, shows just how much things have developed in the last few years. This is really a great forum!

Keith R

[Mike_Ostella]

Hey Will
that zippy pack sounds interesting, but it's 1 1/2 oz heavier and 35mm longer than the TP. Did you have to cut away some of the nobler wing leading edge to make space for it? Just curious for future projects.
Mike

[Robertc]

On all of my full fuse electrics, I have cut at least an inch out of the L.E. of the wing to allow the battery to sit a bit closer
to the CG.  Need less tail weight.

[Target]

Mike-
What does the TP 2700 weigh, is it lighter than the Hyperion?
The Hyperion 2500 4S is 266g without connector, and 105 x 35 x 35mm by my memory.
I'll be battery shopping soon for my ARF Oriental, and was also wondering if I would need to bore out the fuse bulkhead and wing LE, to get any pack back far enough for proper CG. I guess a lot depends on the weight of the motor prop and spinner though, since they are further forward and have the most lever arm ahead of CG.
My point being that 1/2oz lighter motor may have the same effect as a 1.5oz lighter pack(?)

Regards,
Chris

[William DeMauro]

The Zippy pack fits in the Nobler(barely) without cutting anything out. They are good budget packs that worked real well for Kevin as he was transitioning from an intermediate pilot to an advanced level pilot. Now that he really doesn't fly that plane anymore, and I occasionally only bring it out for others to try I cant really justify the cost of "better" lighter batteries for that plane. It only weighs 41oz anyway.

[Mike_Ostella]

Chris the tp pack is as follows
Capacity/Cell Count/Voltage: 2700mAh 4-Cell/4S 14.8V
Max Charge: 5C
Max Cont Discharge: 25C
Max Burst Discharge: 50C
Max Charge Current: 13.5A
Max Cont Current: 67.5A
Max Burst Current: 135A
Weight (grams): 234
Dimensions H x W x L (mm): 32 x 34 x 102

So the tp has more capacity and it's lighter, a few mm shorter and a bit different in the cross section, but cross section usually isn't an issue.
The way I look at it is, use the lightest components you can afford. Then as you are trimming, if you need nose weight and there is an equivalent capacity motor or battery that's cheaper and heavier, well there is your solution. Or you always have the option of adding ballast. By the way, the cobra costs less than an E-Flite 15, and it comes with the shaft already in the correct position for the Hunt G-10 mount. Also, after the two projects I have completed, all future planes will have the wing upper leading edge sheeting cut back as far back as possible. Right back to the bell crank , to accommodate longer batteries or to allow adjusting cg by shifting the battery forward and back.

Mike

[Target]

Thanks Mike.
I was just going by memory on the Hyperion pack dimensions, but they are close.
The TP is lighter though by an ounce at least, so long as it includes the wires.

I agree, the Cobra 28mm series motors seem like a really good "Bang for Buck" value for ECL.
If the Exceed Rocket that I plan to put in the Oriental isn't ideal, a green motor will likely be going in instead.

I hope your mods make the difference that you want in the plane, and I hope to read of it soon.

Good luck in the change.

R,
Chris

[Mike_Ostella]

My campaign to shed some weight went better than expected. I went from 54.5 oz. down to 50.7 oz. Test flights Wednesday, weather permitting.
Carbon fiber landing gear still to come. that should drop at least another 1/2 oz that lives near the c/g.

[Target]

Excellent!
G/L for the re-maiden.

R,
Chris

[Target]

Stuck on a boat offshore, I found what looks to be a great deal on Mike's favored TP 2700 packs-
 http://www.atlantahobby.com/Store/pc/4-Cell-2700mAh-4S-14-8V-ProLite-+-Power-25C-LiPo-1253p19263.htm
Regards,
Chris

[Mike_Ostella]

So after all the revisions, I got to do some test flights yesterday. three two minute flights and one full pattern.
During the short flights, I switched from tractor to pusher prop and felt no difference in the level flight stability or groove, so I went with the APC 11x5.5 EP pusher.

The plane now weighs 50.5 oz.  The full pattern was flown on 59 ft lines at around 9,788 rpm for 5min 30sec. wind was light but shifting direction, so the pattern was not pretty, but I did all the maneuvers and all the level laps with 9 laps counted after finishing the pattern.

When I recharged this morning I put 1752 back into the 2700 pack, so I'm well within safe territory. By the way, the Cobra 2820/12 worked just fine and was not at all hot after the full flight. Just a little warm as should be expected.

I didn't get a lap time, but I was comfortable with the speed of the plane and it was not ahead of me. No stupid wiggles or stalling unless I really horsed the controls. I had a roll problem that I fixed with a flap tweak, then I reset my wings level with the adjustable flap extension tabs.
To sum it up, it's a completely different airplane, and one of the best I have ever flown. Yeah, I know, bold statement, but I believe it's true.

What's next?
I can get some more weight off by switching to a carbon fiber landing gear (ordered and in transit I hope), Balsa wheel pants, shortening all the wires to custom length and converting to nylon fasteners, but I will also add some color so that weight will be a trade off.
I will say this in conclusion. If you really try, you can find ways to shed weight. A few grams here and there add up.
Now I gotta go practice, no excuses.

Thanks to everyone for the comments and suggestions.

Mike Ostella

[Motorman]

Mike, You can do the pattern in 5 minutes with 65' handle to thrust line at 5.25 sec and have 2 laps left over. Our systems are nearly identical but I use a 2500 mah 4S Hyperion with no problems. If the plane feels sluggish you can go shorter but also consider bigger smoother maneuvers. Have you tried an APC 11x5.5 pusher, all my planes fly better with the pusher. Also, some APC 11x5.5 props are thicker than others, I use the thin ones and the cuff root sometimes has an air brake on the trailing edge you can grind off. My FM-9 is in "Phoenix high RPM" how did you figure to use "New Phoenix" with the old Ice Lite? After you set your rpm do you check it with a tac? 10,000 is a good number if that's what it's actually turning.

MM

[Mike_Ostella]

Just got back from the field.
Update on my prior post.
Laps were timed at 5.2
1/4 oz of tip weight was added which improved line tension.

To Motorman:
Thanks for the suggestions .
Long lines don't work out well at the turbulent GSCB field where I fly. Most of us here fly on shorter lines and with more tip weight than folks that fly in one direction air. Yes, I agree, bigger smoother maneuvers is a no brainer for a wing loading challenged airplane like mine, even at 50 and 3/4 oz.
I prefer to have a few more than two extra laps in case I want to wait an extra lap between maneuvers.
I'm already using the APC thin electric pusher 11x5.5. I'm curious about the mod you mentioned. what is it, and how does it help?
All my ESC's are upgraded to 4.22 (edge I believe) software. On my ESC, I'm in control line, governor high mode, and my FM-9 is set to  phoenix new high. I had a discussion with Will Hubin back in March and he mentioned I should upgrade the chip in my program box for a more accurate RPM , but I'm happy with the 5.2 lap on 59' lines. I'm not concerned with knowing what the exact RPM is? I'll upgrade that chip eventually, but it's not a big concern right now.
Here is my esc file:

#######################################################
# Castle Link Data File
# Created: Wednesday, April 22, 2015
# Do Not Edit This File By Hand
#######################################################
Auto Rotate Enabled: False
Auto-Lipo Volts/Cell: 3.2 Volts/Cell (*)
Brake Delay: .6 sec (Delayed) (*)
Brake Ramp: Fast
Brake Strength: 100% (Hard)
Current Cutoff Type: Soft Cutoff (*)
Current Limiting: Sensitive
Cutoff Voltage: Auto Li-Po (*)
Desired Head Speed 1: 9500
Desired Head Speed 2: 9594
Desired Head Speed 3: 9689
Direction: Forward (*)
Governor Gain: High (35)
Governor Mode Type: High
Head Speed Change Rate: 6
Hex55: 85
Initial Spool-Up Rate: 6
Motor Start Power: Medium (59) (*)
Motor Timing: 4
Power-On Beep: Enabled (*)
PWM Rate: 12 Khz (*)
Throttle Response: Medium (5) (*)
Throttle Type: Governor Mode
Vehicle Setup - Battery Type: LiPo
Vehicle Setup - Battery Voltage: 14.8000
Vehicle Setup - Gear Ratio: 1.000
Vehicle Setup - Motor Kv: 970
Vehicle Setup - Motor Number of Poles: 14
Vehicle Setup - Number of Cells: 4
Vehicle Type: Control Line
Voltage Cutoff Type: Hard Cutoff


More flights on Monday.

Mike O

[Motorman]

I hear ya on the extra laps, my field has crazy air like that but it's behind my house so I can pick good days to fly. 20 min and my batteries are charged.

What I'm saying is, the thin electric APC 11x5.5 like the one you are using, has more than one mold or something and sometimes you find a thicker one. Also the way the plastic shrinks when cooling or something gives some props more under camber at trailing edge especially at the root. It only makes more load. You can take 100 grit paper and a sanding block and take it down flat. Also, sand the whole prop with 220 grit in your bare hand to give it a matte finish and it will be less load.

I keep my motor bearings oiled good. If it's binding you'll get the same rpm but use more battery to do it. I took the bearings out of my Power15 and filled them with grease, they were pretty dry. Cut an angle on the end of the snap ring and it will be easier to get out next time. The trick is to get the motor back together with just a tiny bit of end play.

My other Power 15 I never had apart just dripped thin oil on the shield and it's working just fine. More than one way to get it done I guess.

Thanks for the advise on the timer and stuff, I'll check that out.


MM

[Target]

Stuck on a boat offshore, I found what looks to be a great deal on Mike's favored TP 2700 packs-
 http://www.atlantahobby.com/Store/pc/4-Cell-2700mAh-4S-14-8V-ProLite-+-Power-25C-LiPo-1253p19263.htm
Regards,
Chris

I ordered two packs from Atlanta hobby at 0600 this morning. At 0800, I received a shipping notice.
This appears to spectacular service.
Will report on receipt.
Regards,
Chris