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XV-2 / S-57

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.

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.

A stowed- or stopped-rotor aircraft has never flown. Full-scale models of both stowed-and stopped-rotor systems have been tested in the National Aeronautics and Space Administration (NASA) wind tunnels at Ames Research Center.

The stoppable-rotor design can hover and fly at low-speeds like a conventional helicopter, whereas in its stoppedrotor mode it can fly at high speeds comparable to those of fixed-wing aircraft. A two-bladed rotor is used to generate the required lift for hover and low-speed forward flight. Once the rotorcraft is at a sufficient forward velocity, the required lift generation is transferred from the rotor to delta wing.

Compound helicopters capable of stopping rotors in flight and folding them up or converting the rotors into fixed wings have the highest speed potential of any rotorcraft configuration. One design strategy is to fly the rotorcraft fast enough to transfer lift to separate wings and then stop and fold the rotors. From this point in cruise flight, the aircraft is basically an airplane, and maximum speed is a function of aerodynamic design and engine power. Rotor blades, which are generally flexible, are difficult to stop or start during forward flight because of the severe stresses and forces resulting from the blades' flapping in the wind.

A stowed- or stopped-rotor aircraft has never flown. Full-scale models of both stowed-and stopped-rotor systems have been tested in the National Aeronautics and Space Administration (NASA) wind tunnels at Ames Research Center.

NASA and the Defense Advanced Research Projects Agency developed and tested technologies for a different concept with very high-speed potential during the 1980s called an X-wing, but the program has been scaled back to a low-level research effort. A four-bladed rotor provides vertical thrust like a helicopter rotor for takeoff and low-speed flight and is stopped and locked into an "X" position relative to the fuselage and serves as a wing for high-speed cruise. The X-wing design, if lightweight enough, could hover as efficiently as a helicopter and fly as fast as a jet. The RSRA/X-Wing vehicle was a proof-of-concept stopped rotor aircraft configuration which used rotor blades primarily constructed of laminated carbon fiber. Delamination of the main spar during ground testing demonstrated that significant interlaminar stresses were produced. Analysis confirmed the presence of out-of-plane load components.



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