Dean,

I already have the 2820/8 so I think I'll give that a go first. The A123 3S 2P battery pack from MEC is a nice  package.  Since the 2820 likes higher RPM what do you think about going to a little flatter pitch say 4 (with a 12 to 13" diameter) and let the rpm float up to the 11K range? What should the stationary amp draw be?

Best,           Dennis

Dean,

Thanks for the feed back. The pack I am planning on is the 3S2P 4600mah A123, I will likely need to pull more than 18 amps in the air to get a useable prop for a 600sq in ship, think it will be more like 25amps but the 4600 pack should do this with no problem don't you think? Is motor temperature the limiting factor on how many amps the particular setup can take? I assume the larger the prop diameter the higher the motor load, the more amps and the more heat i.e. moter temperature. Correct?
From what others have posted if you can't touch it, its to hot. What is a reasonable priced clamp on brand of Amp meter to get to measure motor amps with and how do you use it?

Thanks,        Dennis

Thanks Igor,
After all, you are the one who has actually flown the A123s! Never argue with the experiment, right?

Dennis, if you're going to run a 4400 mAH mAH A123 pack, then your average current could be as high as 36 Amps! That would only use a little less than 90% of the capacity for 6 minutes of flight. When the cells drop to 2.8V as Igor says, a 3S setup will provide a maximum of 302Watts. That will conservatively fly a 40 ounce airplane, and will probably fly a 48 ouncer in all but the ugliest air. Push the batteries to their limits, and fly the schedule in 5:30 and maybe you can manage the mid 50's with really good performance.

Dean

Here are again in-flight data of 6s1p A123 ... now I see end of flight voltage is only 16V!!!!! it means 3s pack will be only 8V.

http://www.olive.cz/pub/igor/1.jpg
http://www.olive.cz/pub/igor/2.jpg

Hi Dennis, Ground testing is good ... except that typical ground currents are maybe 130 % of the flying current, so running for the full 5:45 on the ground will not work.

Igor's warning is a good one to heed, though hopefully you will be using more like 75% of your battery capacity, so that the voltge is not so low at the end of the flight. The ESC low-cutoff voltage may get in the way, too.

First, calculate the RPM that will make 4" pitch fly the lap time you want, on the line length you plan to run. If it works out to more than 8 * Kv, then go up in pitch. Once you decide on a pitch, and an RPM, try running a small diameter prop with the correct pitch at the desired RPM for the full 5:45. Measure the current. In real life, your in-flight current will be maybe 26 ~ 29 Amps. The same prop on the ground will consume maybe 35A. So, work your way up in prop until you get maybe 27A, and make sure that it runs for a full 5:45 on the bench. Monitor the voltage as you run. Once you are happy that this will work, add an inch of diameter, verify that you are somewhere near 35A of current and shut down the timer. Recharge and install that into the airplane for te first flight.

Make sense?
I hop it helps.
Dean
 

Dean,

The question is given these choices is it better to take a higher Kv rated motor (AXI 2820/8 @ 1500) and set it up at a low throttle to run at the desired lower rpm  or take the lower Kv rating (AXI 2826/8 @ 1150) and run at a higher throttle setting to run at the same rpm for both motors? Which one pulls the lower amps (the mohm for the 2820-8 and 2826-8 are very close 29 vs 30)for the same end rpm?

On the test ship the 2820/8 ran great at 8800 rpm for 6mins on an 11 3/4 x 4 1/2 APC E prop with a lap time of 5.25sec on 63.4ft lines for the whole flight. I am going to increase the pitch to 4 3/4 to get the lap time to 5.0 and I will be a happy E flyer. Am starting the new ship so would interested in thoughts on the above for the next motor selection.

Best,      Dennis

Dean,
I think you meant to say the power will be the same, not the amps for two different kV motors but running the same prop at the same rpm, but using the same cell count battery.

oops, I just realized that I was wrong---- if you are using the same amount of power with the same battery, the average current from the battery has to be the same for both motors. Of course the current in the motors will be different---as will the average applied voltage [as seen by the motor from the ESC].

I think, however, it is an open question whether the power really is exactly the same. It would be if the ESC can make the transform from high volts/low amps to low volts/higher amps that it needs to do for the higher kV motor. My "guess" is that it is better to go for the lower kV as long as you are running it at ~70-75% of WOT at cruise, so that you have the reserve on tap when you need it in the high power maneuvers to keep the rpm constant. In my Super Clown with the 1500kV motor running at ~9000rpm on a 3s, I was exactly doing this conversion, and it was doing fairly ok.

Anyway as soon as I get my ENobler back to where I think it was last year, I plan to go to a lower kV motor (both same  manufacturer and model but different windings), so I'll see how it works.