|Feb 13, 2014, 09:38 PM|
Changing KV on Suppo 7035-8 (190kv)
Hoping to get responses from Manuel V or Jackerbes, but all would be appreciated.
Big, 50cc type electric motor, 1250 gr, 190kv, 12s. I noticed it is well wound, but with quite fine wire for good fill, works well, ran cool.
My original install was in an Aeroworks 300Extra, 23/10 2 blade prop, for 4200watts @ 99a showing 42.4v. That 5.6hp! great performance on 6s/5000/30c Zippy's. Flight times a little restricted at 6 minutes with average acrobatics and no 3D.
I now have the motor installed in my new 132", 28lb Turbo Beaver. I expected the 21/10/ 3 blade PMT Cf prop to show around 3500watts (my guess), around 80a. I was hoping to get 8 minute scale flights with maybe a minute reserve.
Got a real surprise, amps climbed to 117, watts to 5238 at 44.6v!!! That's 7.02HP!! I'm using a CC HV110 Esc, and was hoping for 3500w and much longer flight times. While the power is fantastic, its not needed on this plane and will really restrict flight times.
The 21" 3 blade is scale, and going down to a 18/19" prop is not what I want to do. CC said I could use the throttle endpoints to limit power, but most prefer to install smaller prop.
The alternative is to rewind the motor for a lower KV! I know absolutely nothing about this and hope someone hear can provide some answers.
Is anyone out there willing to work out the new KV to give me 3500-3600watts? Is anyone willing to do the job? I'd be more than happy to pay a reasonable charge for the job. Is there any disadvantage to lowering the KV? Thanks in advance. Doug B
|Feb 13, 2014, 10:03 PM|
USA, ME, Ellsworth
Joined May 2008
I don't have any experience with motors or props of that size. But I would guess that the change from a 23x10 two blade to a 21x10 3 blade increased the prop blade area by quite a bit and that brought the load up quite a bit.
The input power increase from 4200W to 5238W would be a 25% increase in input power and that is probably easily explained by the 3rd blade on the prop. Those three blades would have an exposed length that is like 1/3rd again more than the 2 blade prop.
What brand were the props? Maybe there is some test data available for them? Are there any recommended RPM limits? I always enjoy seeing some recommended RPM limits and test data if any is available. I would really like to see some RPM limit numbers and test data before I started trying to figure out what Kv would be would be good for 10S or 12S.
Just out of curiosity, is there a 21x10 two blade available? That would probably run at a little higher RPM than the 23x10 and might give you similar and acceptable performance with a two blade prop that is also the scale size. Or is the three blade prop desired for appearance reasons?
As far as the Kv, the motor can be probably rewound for a lower Kv. If it is an 8 turn motor now (as suggested by the model name). That would involve adding a turn or two and it is usually feasible to at least match, if not improve, the cross section area of the existing fine strand bundles with one to three larger strands in the course of a rewind like that.
I think the strand size for a single strand, matching or bettering the present surface area, might not be practical because of the difficulty in winding with strands that large.. I would probably look at is as a wind needed to be wind that was done with two larger strands. winding.
I would be exploring very new ground with a rewind on a motor that size. And I am not really set up or equipped to for static testing at voltages and current levels that high. Nor do I have a static test stand that is up to these power levels.
|Feb 13, 2014, 10:59 PM|
Hi Jack Boy that was one fast response.
The props used where a Hawk 23/10 2 blade wood prop from SDS Hobbies, weight, just under 200 grs. The 21/10/ 3 blade prop is a PMT carbon fibre from Graves RC. Weight on the 3 blade is 169grs.
The 23/10 has wider blades than the CF 21/10. Neither product shows any rpm restrictions mentioned, both are gas props. Both have amazing thrust.
The 23/10 spun 7100 rpm, the 21/10/3 spun up to 7500 rpm.
I must mention here that I used 2/6s/5000/30c Zippys when I ran the 23/10and 2/6s/5000/40c Zippys on the 21/10/3. Voltage on the 23/10 was 42.4 with the 30c, while it was up at 44.6 on the 40c with the 21/10/3.
I am a stickler for running scale blade counts and sizes if it at all possible. I do not wish to use a 2 blade prop on the Turbo Beaver.
The 7035-8 is a 14 magnet, 12 pole motor, the -8 designates the kv of 190. they used to make -9 too with a kv of 179, but no longer available as near as I can tell. I could not test them on my test stand either, had to wait until everything in the plane and a helper was available!! hehe. Thanks for responding so quickly with knowledgable info. Doug B
|Feb 14, 2014, 01:06 AM|
I think no need to rewind, unless the motor is not efficient, which does not appear. but otherwise the motor is quite efficient.
To 7500 rpm to 44.6V,
With 11slipo 6700 rpm to give a 40-41 Volts and will consume around 95-100A. to about 4000 watts.
If You want to rewind to low Kv, then I can do it.
But again, 11slipo is the solution.
|Feb 14, 2014, 03:26 AM|
If your esc is up to it, you can throttle back to the power you need while cruising around.
And when you need it (towing gliders), you will have the power on tap.
That will give you the flight time you need.
Test it at ~50% power, if your esc is not running hot, it will work.
I rarely fly my scale jobs at WOT, even my learjet flew 8 min with ROG and twin EDF on NiCds.
I cruised along at ~30% throttle.
|Feb 14, 2014, 05:00 AM|
I dont think the 3 bladed prop is the culprit, since it is turning faster than the 2 bladed.
2 bladed "loaded" Kv=167,5
3 bladed "loaded" Kv 168.
The increase in power is because of the 40C battery capability of putting out more A with higher V. Run it with the 30C battery and you will have similar numbers for both props.
If you are worried, do as Manuel suggests, go down to 11 cells.
|Feb 14, 2014, 09:37 AM|
First, thank you manuelv, MagnusEI, and Espeedbuff for responding!!
All your inputs are very much appreciated.
I agree with all your comments and will look into the possibilities of changing to 11S, testing the system as is for Esc capability to run at lower settings.
Las summer, I sold almost every saleable plane I had to raise the $$ for this build. I am on a very restricted disability pension, and funds to do some things just don't come easy!
I agree that the 40C batteries are in a big way responsible for the upgraded performance. The 12S system as it stands is very efficient, I estimate around 88%. I had the kv under load pegged at 169, so pretty close.
If the plane can fly at 1/3 throttle, and the esc can handle that, I guess there's no real reason to get excited. The 5200+ watts available will give it some real short field TO abilities!!
I've stripped the cowl off, will remount the prop and run some more tests to check esc and motor under loads. I double checked every connection to the batteries and esc connections to Rx, all appears good. Doug B
|Feb 15, 2014, 12:47 PM|
+1 with Manuel
Search the right voltage to match desired rpm, thrust (and enough pitch speed)
Try 10S .. if not enough pleasant fly .. then 11S ..
|Feb 25, 2014, 09:17 AM|
I'm with you, Louis!
Search the right voltage to match desired rpm, thrust (and enough pitch speed)
Try 10S ..
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