Through the years many concepts have been proposed to overcome the limitations of the conventional helicopter. Harrier, none have gone much farther than the prototype stage. The tilt rotor, which tilts the lifting rotors to become propellers, was initially the most commonly tried approach. These aircraft all flew, but none progressed beyond the exploratory development stage. TiltRotor is a Bell Helicopter trademark label.
Tilt rotor is also an accurately descriptive term.
Again, these developments are not specifically aimed at designs that would go directly into production. XV-15, TiltRotor . One reason for the delay is the program's low-level of funding. Starting with hovering flights in 1977, the XV-15 has now been flown successfully throughout its entire flight envelope. One way to eliminate the problem of stalling and reverse rotor blade flow that limits the forward speed of a helicopter is to use two counterrotating rotors.