Helicopter moddel must be not 3d edited. If you change anything and export - you will lose the animation info.
If you have not touched it, check for that info - open the body file with notepad and scroll to the bottom
It must have something like this:
BOX, -277, -80, -3, 217, 76, 168
N, 0, 0, 0, 333
CBOX, -276.466, -79.420, -2.775, 216.752, 75.807, 167.523
M, still0, 0
M, still1, 1
M, start0, 2999
M, start1, 1
M, fly0, 2000
M, fly1, 1
M, stop0, 5100
M, stop1, 1
And on top there must be lines:
P, 3, 2150, 2151, 2143, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
where P is the moddel animations itself. If they are missing - delete the helicopter (not sarcasming).
If all is fine, you must add a rotor as seperate child (to do that you must add it with seperate prototape in editor).
In the end be shure to check witch animation is looped or single box flagged (open standart helicopters fore refernace) - otherwise your chopper wont take off.