 Vertical motion is controlled with conventional rotor blade pitch and either a conventional helicopter collective control lever (as in the Bell/Agusta BA609) or a unique control similar to a fixed-wing engine control called a thrust control lever (TCL) (as in the Bell-Boeing V-22 Osprey).The tiltrotor's advantage is significantly greater speed than a helicopter.In a helicopter the maximum forward speed is defined by the turn speed of the rotor; at some point the helicopter will be moving forward at the same speed as the spinning of the backwards-moving side of the rotor, so that side of the rotor sees zero or negative airspeed, and begins to stall.