Power 15 is roughly a 2820,3020 class motor then just match the kv closely. Cobra C-2820/12 at 970 kv is close enough. I have found that if they weigh almost the same that's another indicator that it will work. For example the  Cobra C-3510/20 at 1000kv will also work but its shorter and fatter so while power wise it will do the job it may not fit in many applications size wise therefor not a good drop in replacement in most cases.

All my batteries are 4 cell. Can you match them by watts? power 15 is 34a x 14.4v for 498watts and weights 152g but the Cobra is 590 watts at 138g. So that means the Cobra is more powerful and lighter?

Bumping this old thread.

Someone told me I need to use more rpm to better match my ESC. So that means if I'm changing motors anyway I should get a motor with less Kv right?

If I get less rpm per volt I'll have to program in more rpm?


MM

Thrust (watts) being the constant, a lower kv motor will swing a bigger prop at lower rpm.
A higher kv motor will swing a smaller prop at higher rpm.

If you're looking for more rpm, get a motor with higher kv and go to a smaller prop.

My fleet of electrics turn between 9k and 11k rpm. Not sure of the rpm you're trying to achieve, but I don't focus on that. I decide on the prop diameter I want based on plane size, then use a motor that will swing it. I think the Cobra 2820/12 would be a good choice for 600 sq in with an 11x6 or 12x6. Check amp draw with a watt meter to keep it within ESC specs, get a lap time, and adjust.

I don't claim to be an expert on this stuff, and this is free advice, therefore worth the price.

The way I approach selecting a Cobra motor is to go to Innov8tive desing.  Under products, airplane motors is listed most of the airplane motors.  I am using there 2826/10 motor.  Once you pull up a motor on their web sight they have prop charts.  Props are rated for suitability.  Looking at this data, for a given motor, is how I determine if the motor will turn the size prop I want to use at the governor's RPM set point. The prop chart assumes no governing.  So a prop rated as marginal may work because at the lower RPM it requires less power. 

 
http://innov8tivedesigns.com/

Hmmm, I think that I may be the somebody though I don't recall being asked this specific topic, and I do answer emails.

I seem to recall part of this conversation, based on the Castle ESC and my emails to Castle.  Using the Demo mode of the Castle, for control line and high governor, the optional desired head speed boxes will give a head speed/rpm corresponding to a given pulse width.  While the Head speed is not set in governor high, the rpm for a given pulse width is displayed.  That pulse width can be for timers allowing that method, and the rpm will be quite close.  A given rpm will also be noted to be too fast for governing or on the other end too slow for proper governing.  The best set RPM range for the ESC, according to my correspondence, is in the governing range as shown by the program.  If the timer system is active, the rpm may go above that range if the set rpm is close to limit.  Castle stated that when out of range, the  ESC would "attempt to run at the speed it is being told to, and if not possible, it would run at the closest range."  It was not entirely clear that for a brief bump up or down out of range, how much "governing" matters, as opposed to having the set rpm in range.

Playing with the demo mode and varying the KV didn't change the rpm to pulse width, but did change the governing range.

My take was that one could pick a prop to stay in the governed rpm range and that meant a slight tweak of pitch, seemingly possible with the custom self made prop.

This "somebody" hopefully doesn't fall in in the idiot category, and clearly not in genius



That discussion of Demo mode on the Castle ESC started here in reply 5

http://stunthanger.com/smf/gettin-all-amp'ed-up!/another-dog-another-'tric-fiorotti-v4-4-timer-with-accelerometer/msg482345/#new

I have had very good results with the KR timer set up.  It may not be the ultimate for top level flyer but it is easy to set up and so far it has worked.  It works with most any ESC (cheep).  It doesn't require a computer or computer genius to set up.  The auto shut off works!  I have never burnt up anything due to a prop strike.  My personal opinion that for folks starting out, sport flying, and most competition it is the way to go.