The Helicopter Rotor is a vehicle part that is the third to be introduced as one of the items to purposely allow to make vehicles fly (the other two being Airplane Wings and Balloons, and can be considered as the normal Propeller's bigger counterpart, but with the added feature of being self-stabilizing to a certain point.
To activate, the player must press its respective button (a blue box with the helicopter rotors in it), located at the bottom of the screen, if one is to activate all helicopter rotors still available at once, or the part itself, if one is to activate them individually.
Interestingly, Helicopter rotors will always attempt to face themselves up, even if they are mounted forward or backward. This may cause unwanted flipping of your vehicle. Wings can counteract this. They seem to have some kind of self-righting mechanism to keep themselves pointed up, though they can still be flipped.
An alien version of this part exists. The alien version goes in reverse.