My answer to this question is not great, but it might get you somewhere. A unit is not met to be upside down. Apparently, the only thing that can flip an object over is a shot from a big gun, such as a tank or a missile. Even sometimes it does not flip over. A chopper can flip if it collides in a certain place on a tree or building, in that case I assume that it is a glitch in the game, so in that case, I don't think it is possible.