The difference being that the engines rotate on the end of the wings and move into a twin-prop plane configuration, so it can't really be classed as a helicopter.Not to mention that helicopters aren't really VTOL's as they use vertical force for everything, not just takeoff and landing
A vertical take-off and landing (VTOL) aircraft is one that can hover, take off and land vertically. This classification includes fixed-wing aircraft as well as helicopters and other aircraft with powered rotors, such as cyclogyros/cyclocopters and tiltrotors.
