You might want to do some reading on these two links. One has to do with actual battery measurements, and the other has to do with the fine-line balance between power and weight of a battery. I know it is all discussed with helicopters in mind, but it is applicable to all flying machines.
http://rc.runryder.com/helicopter/t723001p1/
http://rc.runryder.com/helicopter/t723196p1/
Rafael