Granberg, look up NESTED TRANSFORMATION.

This is what you need.
The concept being, you have a world, with its coordinate space (positions relative to a zero point and arbitrary orientations of Up, Right and Forward), when you send stuff to be rendered, the graphics system uses your models transformations as well as the point of view (or camera transformation) to take that world to the coordinate space of the screen,

When the position of one model is relative to another, in this case the rotor is relative to the chopper, for this to be represented the rotor must not be in the world's coordinate space, but in that of the chopper, so the transformation of the rotor needs to first be applied that of the chopper before you send it to the graphics system.
Seriously look up nested transformations, once you understand it, it will make your life easy.