I know, I covered this back in 1st year university, or maybe I knew but forgot.
The thing is, the NiMh that came with the heli is 7.2V, 650 mAh, and the Lipo 2 cel I was looking at is 7.4V 850 mAh so I thought voltage and capacity(I don't mean current here) would be good, the weight would be just over half, and the only question would be whether the peak current supported by the LiPo cels was high enough. (ie: maybe at high torques the motor's drawing 3 amps but the Lipos can only support 2 amps without overheating). I had heard that different LiPos of similar rating (V and mAh) might support different peak currents.
mAh is millam-hours, right?, so doesn't it roughly translate into endurance if you assume voltage and current draw are kept constant ignoring that the drop-off when the battery discharges completely is not ideally sharp.