ConvertiplaneThe separation of lift from thrust (that is, using an airfoil and an engine instead of flapping wings) was the insight that made powered flight possible. Reuniting lift and thrust into propulsive lift, with the new technology earned over a half century of flight, promised a revolution in the relationship between aircraft and the populations they serve. Wing-tip rotors lift the aircraft like a helicopter, then the rotors tilt forward like propellers and transfer the lift from the rotors to the airfoil until the aircraft flies like an airplane. Helicopters do not fly forward efficiently. Fixed-wing aircraft find forward efficiency in higher wing loading, which requires longer runways, which then mandate bigger and more congested airports, farther from population centers. Tilt rotors can fly longer distances than helicopters.
After WW II, interest in what came to be called a convertiplane (aka, an autogyro with wings or a compound helicopter or an airplane with a rotor) was reawakened. The high speed limitations of helicopters were becoming evident and the gap in maximum speed between this rotary wing aircraft and the fixed wing airplane was widening. And it became necessary for rotorcraft engineers to apply airplane technology. That is, they added engineering of streamlined fuselages, efficient wings, variable pitch propellers and retractable landing gear to their growing knowledge of high advance ratio, high advancing tip Mach number rotors.
For one group of military planners in the late 1940s and early 1950s, mission requirements included significant hover duration, low speed maneuvering and agility, and a speed and range greater than current helicopter capabilities. This, and additional mission factors such as the need for moderate downwash velocities below the hovering aircraft to enable safe rescue operations, led the planners to specify low disc loading for the new VTOL vehicle. These considerations resulted in the August 1950 initiation of the joint US Army and US Air Force Convertiplane Program. This program formulated the Convertible Aircraft Program Request For Proposal (RFP) to provide demonstrations of different approaches to meeting the convertiplane requirements.
Negotiations began with three contractors (McDonnell [the V-1 or XL-25], Bell [the V-3 or XH-33], and Sikorsky [the V-2]) over convertiplane designs. The aircraft selected from the design competition were the XV-1 compound helicopter (proposed by the McDonnell Aircraft Co.), the XV-2 stoppable rotor aircraft 5 (proposed by Sikorsky Aircraft), and the XV-3 tilt rotor aircraft (submitted by the Bell Helicopter Company). Two designs, the XV-1 and the XV-3, survived the initial evaluation phase and were developed as test aircraft for limited flight evaluations.
The convertiplane was to be an aircraft that could takeoff and land vertically, like a helicopter, and perform like a fixed wing airplane once in flight. The requirements called for an aircraft capable of landing in small areas and which would have the capacity to transport tanks, equipment, or troops. Although none of the three contractor's designs would enter production, the initial convertiplane program was the forerunner of the VTOL/STOL (vertical takeoff and landing/short takeoff and landing) program, which produced the Lockheed V-4 Hummingbird and the Ling/Temco/Vought XC-142 transport of the 1960s and later the Bell V-22 Osprey of the late 1980s.
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