Transform

Hi guys,could you give me a hand ,now I trapped in the XNA transform problem,I write a simply demo about a helicopter fly with a propeller and rotor,now the problem is I'm going to change the Y value of the helicopter also change the propeller and rotor,after I done it,the strage thing appear rotor and propeller didn't fowllow the direction
of helicoper,even the rotor is far away from helicopter!How could I do?