| The KV rating says 3900rpm/V I assume this is the no load rating, I | multiply the battery voltage times 3900?

Yes, that is a no-load rating, and your math is correct.

By itself, the kV rating assumes that your motor is perfect -- no resistance, no drag, etc. Such a motor does not exist, of course, but your math is probably pretty close. If you include specifications like idle current (Io) and internal resistance you can pretty much model exactly what the real-world motor will do under certain conditions.

In any event, the kV rating is perhaps the most important one, as it tells how `fast' the motor wants to go. If the kV rating is too high, you can reduce it by using a gearbox -- a 2000 rpm/V motor with a 2:1 gearbox is essentially the same as a 1000 rpm/V motor without the gearbox, all else being equal.

If you want to know all about electric motors, get this book --

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It's a bit dry, and often too technical, and somewhat out of date -- (but the physics involved has not changed, of course -- it just barely mentions brushless motors, for example) -- but it's a good read.

| The rating for the ESC is 20v w/o bec what does this mean?

It means if you don't use the BEC, the ESC can handle up to 20 volts. If you do use the BEC, the voltage it can handle will go down, depending on just how much current you draw through the BEC.

(The BEC is probably just a 5v linear regulator on the ESC, designed to provide 5v of power for your servos and receiver. I'll assume that you understand what this means and the implications of how inefficient a linear regulator is ...)