Correct, sort of
Remember the motor has to be of the permenent magnet type.
Divide the rated rpm by its rated voltage
e.g. 4000 rpm/ 120v = 33 rpm per volt.
Multiply this by maybe 14v for a cut in voltage to charge a battery, 14x 33 = 462 rpm
allow maybe 20% more for losses, so around 550 rpm to reach a charging voltage.
This is quite high, any decent power would require considerably more rpms, maybe 800 rpm, dependent on the prop, speeds in this area would be less than 4' in diameter, and a 2 blader - there's lots more to it than that, but in general.
The DC motors on eBay and the like are o.k for starters/ learning, but by no means the best solution, treadmill motors from experience are only any good for 6v battery charging.
Hope this helps.