Gonna wade in here (sigh).
So, before I start -- I've designed switching motor drives before. That's basically what an ESC is, except that the ones I've designed have been fancier and much larger for the amount of power handled. Which is the long way of saying -- yes, I know what I'm talking about.
John, Fred, and Mark seem to be on track here.
I am trying to get a rough idea of where the amps will move (assuming same motor, prop and rpm setting) if I go from a 4S pack to a 5S pack. Is there a straight forward formula to calculate how many amps one needs with the 5cell vs. the 4cell?
To a first-order approximation, yes. If you know the current for four cells, multiply it by 4/5 -- that's close to the current for five cells if you don't change anything else.
Also how does the total mah capacity change?
The same factor of 4/5.
About all the battery voltage, Kv, RMS voltage, current at the motor, change your prop, freak out, tear off all your clothes and go dancing through the streets, etc., etc. --
An ESC is a
switching amplifier, and a motor is a honkin' big inductor.
To the extent that the efficiency remains the same (this is important), if all you do is increase the cell count by one you'll take the same
power out of the batteries to drive the motor at the same
speed.
In general, if the system works with four cells, it'll work with slightly diminished efficiency with five cells -- you'll lose a few percentage points efficiency, your motor and ESC will run marginally hotter, the current reduction won't be
quite a factor of 4/5 -- but unless something is really wrong, it'll be within 5%.
I'm even sure that it'd be worth the experiment to try slapping six cells into a "4 cell" system. I suspect that at that point the motor would start getting perceptibly warmer, and you'd have to go to a more expensive ESC, which might also run warmer. All things being equal, efficiency would be down even more than with the four to five cell jump. I'd expect that if you tried it, you'd say "yes! a win for science!", then you'd want to go shopping for a motor with a lower Kv rating.
... Depending on the Kv of the motor chances are that you will have to switch to a smaller prop if you up the voltage. ...
Nope. Not for CL stunt. Yes, that's absolutely solid advice for someone flying RC without a speed-regulated ESC, but it's not how electric CL stunt works. Control line stunt is just about the opposite of that. With "normal" RC, the voltage to the motor is
always limited by the voltage at the battery, and any time you nail the throttle that's what the motor sees. With control line stunt you
never ever want the ESC to be at "full throttle" -- when that happens, it means that the motor speed has fallen out of regulation, and unpredictable things (like sagging in the overhead maneuvers) will happen.
What
does happen is that the system regulates the speed of the motor. The motor, being pretty efficient, needs the power it supplies to the prop plus 10 to 20 percent more. The ESC, also being pretty efficient, needs the power it supplies to the motor plus 5 to 10 percent more. Matching the motor Kv to the "full throttle" output of the ESC does make everything more efficient, but not terribly so -- as long as you're not overheating anything, running out of voltage overhead when the plane's overhead, or drawing the batteries down too much each flight, you're fine.