The vertical takeoff and landing (VTOL) aircraft has been an elusive goal, but by the end of the 20th century, a British and an American model were in military service. The helicopter can achieve vertical takeoff and landing but is limited in its forward speed because at more than 200 miles per hour it will stall. The reason is that the helicopter blade is shaped like an airfoil, and as the speed of the aircraft increases, the blade on the retreating half of its rotation is carried forward sufficiently fast to cause the blade to create a downward thrust rather than an upward thrust. Because of this feature, the helicopter is limited in its missions and range, not being capable of engaging jet fighters in air combat and being highly vulnerable to ground fire from shoulder-held missiles when flying in a combat-support role.
The search for a VTOL included the requirement that the aircraft achieve high speed and long range and at the same time be capable of vertical takeoff and landing, like a helicopter. The British solution is the Harrier (AV-8), first known as a jump jet. Developed by British Aerospace, the Harrier first successfully flew on August 31, 1966, and entered into service in 1969. It was powered by a Rolls-Royce jet engine and saw successful combat service in the Falklands War in 1982 and in the Persian Gulf War in 1991. The engines themselves remain stationary, but their thrust is directed downward or rearward by pivoting vectored-thrust nozzles.
The American design was the tilt-rotor V-22 Osprey, developed by Bell and Boeing. Delivery of the V-22 began in 1999, but the aircraft was troubled by accidents and poor performance. The aircraft is lifted by two 38-foot propellers known as proprotors, which serve much as helicopter blades for direct lift and descent. The proprotors, interconnecting shafting, and their engines are mounted on a wing, which is tilted vertically during liftoff and descent. The wing and its rotors are tilted horizontally for forward flight. The limit to forward speed is reported at just over half that of the Harrier, under 400 miles per hour, considerably faster than a helicopter but far lower than a jet-propelled
aircraft. The Osprey remained extremely controversial because of a series of accidents and because of excessive cost overruns in its manufacture. Originally planned to cost about $24 million each, the cost escalated to more than $80 million apiece. An Osprey accident in 1992 killed 7 troops; another, in 2000, killed 19 marines.
Alternative VTOLs have been considered, with experiments on a tailsitting jet and an X-wing aircraft. In the latter, the aircraft rises like a conventional helicopter and proceeds to near-stall speed in forward flight. Then the trailing blades are converted to airfoils by the ejection of airjets from within, and freezing the rotation to convert the total X-shaped blade assembly into a set of wings that create lift, and the engine power is shifted from rotating the rotors to forward jet propulsion. The mechanical difficulties and funding decisions with that design have prevented it from reaching a production stage.