RE: "X" NiMH = "Y" LiPo ???
It might be easier to figure out what Lipo you need for what your trying to fly.
A 3s lipo will give you about the same voltage as 10 NiMh when they are fully charged.
There are a lot of factors to consider when matching up your Lipo to what your flying. The things you need to consider are the weight of the plane, motor size, prop size and if the motor is geared or not. Then you need to consider if the motor, ESC and battery can handle the amp draw being put on them.
I don't think I'm giving the help you need but 1 cell lipo is about the equivalence of 4 NiMh cells.