I hooked 21 3700MaH cells to an 18v Dewalt drill motor last night, and it near jumped out of my wifes hands. Forget the chuck, and while the motor is turning in a vice on 6-7 cells, use a mill (*******) file to grind away the shaft until it fits into a 5mm coupler. Then make a motor mount to fit the bolt pattern. Also will need to put diode or capacitors to reduce motor noise and keep servos from glitching. But I bet that it would power a good sized prop without burning up.
Duck