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Author Topic: Prop vs RPM vs # of Cells  (Read 897 times)

Bruce Shipp

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Prop vs RPM vs # of Cells
« on: January 12, 2013, 10:46:17 PM »
I am in the process of concerting a piped SV 11 to electric.  The plane will be in the 64 oz range.  I have decided on the eFlight 32 - 770Kv.

Props I have seen used with this motor and this sized airplane to be 12-6, 13-5.5 or 13-4.4. 

I have seen setups turning the 12-6 with 4 cells.  The question is:  How many cells will be required to turn the 13-5.5 or 13-4.5.  wil I need to step up to 5 cells to have the RPM margin at the top end?
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Offline Keith Renecle

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Re: Prop vs RPM vs # of Cells
« Reply #1 on: January 12, 2013, 11:01:07 PM »
Hi, that motor with the low kv of 770 will require a 5-cell lipo of around 2700 mAh. This will work for the 12 and 13" props you mention. I fly mainly 12 x 6 or the wide blade 12 x 6.5 APC from Dennis Adamisin and I use 4-cell 3300 mAh batteries, but then my motors have a kv of around 900. The low kv (rpm per volt) motors of 650~770 will not get enough rpm on a 4-cell setup. The advantage of the higher voltage is that the overall current comes down. The weight will be similar to the 4-cell because you can come down in capacity. The disadvantage of the 5-cell setup is usually the thicker width and maybe the price. The fuselage on my Newtron stunter is on the thin/narrow side, and the weigh is only 61 oz. so I prefer the 4-cell option with the 900 kv motors. Good luck with the SV-11.........it's a good machine!

Keith R
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Keith R

Online Dennis Adamisin

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Re: Prop vs RPM vs # of Cells
« Reply #2 on: January 13, 2013, 09:28:53 AM »
One of the best SV-11's I have seen is Bob Branch's using a AXI 2826-12 (kV=760) on 4 cells turning an APC 12x6EP about 8400 RPM.  If he was to substitue a Power 32; the motor has about the same kV, is physically larger & more powerful.  It would probably use a little less battery because it woud never be lugging.  That set-up could also easily make the jump to the WIDE 12x6.5 WEP, reducing the RPM to around 8200.  The Power 32 set-up would also work well with a 13x6.5EP, at around 8100RPM - all on 4S.

The 13x4.5EP and 13x5.5EP like turning somewherein the 9300-9600 RPM range.  The 770kV on 4S comes up a little short on RPM capability - 5S is the answer!  Also, with higher voltage the current draw goes down - as does the heat.

The 5S system with 770kV is too fast for the 6 and 6.5 pitches, if you decide that you really want to limit the prop diameter to 12" then you would be best served by clipping the 13x4.5EP or 13x5.5EP to 12".

With the Power 32 I would be inclined to use the 13x5.5EP & 5Sx2700.

Of course left to my own devices  I would use an Arrowind 3526 (685kV) and 6Sx2200.   8)

« Last Edit: January 13, 2013, 11:09:29 AM by Dennis Adamisin »
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Denny Adamisin
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Bruce Shipp

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Re: Prop vs RPM vs # of Cells
« Reply #3 on: January 13, 2013, 11:16:15 AM »
Dennis and Kieth, thanks for the replies.  They confirm what I had come up with but I wanted to be sure before I ordered batteries.  You both gave great answers to the questions I had in my mind but was not able to get in my post.  Thanks!

Dennis, this is the continuation of the project we discussed back in June.  Life kind of got in the way this summer and fall so now we are back on track.  I need to take a good look at the plane and determine available prop clearance and battery compartment dimensions. 

Thanks again for the info.

Bruce
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Offline Robert Redmon

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Re: Prop vs RPM vs # of Cells
« Reply #4 on: January 14, 2013, 01:15:27 PM »
Dennis and Keith,

I have already communicated this information to Bruce, but thought my direct experience might be useful to others as well. As has already been suggested, the 770kv E-flite Power 32 motor does work (well) with a 4s 3300 mah pack and a 6 or 6.5 pitch prop turning below 9000 rpm. I have about 50 flights on a 65 oz. 690 sq. inch Legacy variant running APC 12-6  and 12-6.5 props turning from 8500-8700 rpm on a 4s pack without issue (2100-2200 mah per 5:30 flight). I have had no problem with a lack "headroom." I should be able to turn 8700 rpm down to 11.3 v under load.I would like to try a 5s pack, but one would not fit given my current setup. Oh, and I am currently using Keith's KR2 timer/governor currently (gain =5).

I was flying the same setup last year with a E-Flite Power 25 (870kv), and though I was pleased with the performance, I was never able to get it to run as cool as I wanted. The P32 runs much cooler. I put the P 25 in a somewhat smaller/lighter (650sq/57oz) plane, and it now runs reasonably cool as well. In IC lingo, I guess there is no replacement for displacement.
 

Bob
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Robert Redmon
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