Initially designated XL-25 in the liaison aircraft category, then XH-35 in the helicopter class, and finally XV-1 as the first type in the new vertical lift category of aircraft designations, this machine worked on the unloaded rotor principle. Designed by Friedrich von Doblhoff, the Austrian helicopter pioneer responsible for the wartime WNF 342, the XV-1 was the result of an experimental programme undertaken jointly by McDonnell, the US Army Transportation Corps, and the USAF Air Research and Development Command.
Initiated by a Letter of Intent dated 20 June, 1951, the XV-1 project proceeded through mock-up inspection in November 1951, and the first aircraft (53-4016) was completed some 22 months later.
Combining the features of a twin-boom, twin-tail, fixed-wing aircraft with those of a single-rotor helicopter, the XV-1 was powered by a 525hp / 391kW Continental R-975-19 seven-cylinder radial. For vertical flight this engine drove two compressors which fed air through tubes to small pressure jets at the tips of the three-blade rotor, with the pressure jets operating on the principle of ignition and expansion of the fuel gases. For forward flight, the engine drove a two-blade pusher propeller mounted aft of the fuselage between the tail booms while the rotor autorotated. The XV-1 was intended to take-off and land as a helicopter, with transition from helicopter flight mode to conventional flight being made by transferring power from rotor to propeller as soon as the forward speed exceeded the stalled speed of the wing. Side-by-side accommodation was provided for a pilot and a co-pilot with room behind them for test instrumentation. Alternatively, accommodation could have been provided for a pilot and three passengers or a pilot and two stretchers.
The 9m long fuselage, mounted on skid landing gear, and mid/high-set wings mounted twin tailbooms with twin vertical surfaces (about 3m in height), inter-connected by tailplane and elevator.
The 7.9m span fixed wing featured a slight sweep on both the leading and trailing edges, although the angle of sweep was greater on the leading edge. It was also designed with a high aspect ratio and joined the fuselage at the same level as the top of the cockpit, directly below the mounting for the upper rotor. The wing terminated in the fuselage into a large bulbous housing on either side of the fuselage.
Project test pilot John R. Noll began tethered hover flights on 11 February, 1954, but difficulties with the pressure jet system delayed initial free flight until 14 July, 1954. The first successful transition from helicopter flight to conventional flight was made on 29 April, 1955. The flight test programme revealed several design deficiencies which were progressively corrected through the introduction of such modifications as a cut-down rotor pylon, small anti-torque rotors fixed to the end of each taitboom, redesigned landing skids, and other minor detail changes. During the preceding months, the second XV-1 (53-4017) had joined the flight trials programme. This machine differed from the first prototype in having a bulkier but streamlined undercarriage and cut-down rotor pylon to reduce interference drag. Numerous detail improvements, including the addition of a small steering rotor behind each boom, were progressively incorporated.
In evaluating the program, the consensus was that the basic concept was sound, but the piston engine powerplant could not provide the needed performance to optimize the design. It was felt that use of a gas turbine engine in this application would solve the problem.
Although on 10 October, 1956, the second prototype XV-1 had become the world's first rotary wing vehicle to reach a speed of 322km/h, the gain in performance over conventional helicopters did not warrant the added complexity of the convertiplane configuration. Furthermore, the potential of the McDonnell XV-1 was seriously limited by its use of a piston engine instead of gas turbines as adopted to power European convertiplanes which preceded or followed it.
The programme was terminated in 1957 after the two prototypes had been flown for a total exceeding 600 hours. The first XV-1, 53-4016, then went to the Army Aviation Center Museum at Fort Rucker, Alabama, and the second, 53-4017, was donated to the Smithsonian Institution's National Air and Space Museum, in Washington, DC.
Engine: 1 x Continental R-975-19 radial, 410kW / 550 hp
Wingspan: 7.92m / 26 ft
Rotors: 3-blade rotor; 2 blade pusher propeller
Rotor diameter: 9.45m
Empty weight: 1940kg
Loaded weight: 2497kg
Gross weight: 2160kg
Fuel capacity: 315 litres / 225kg
Rotor loading: 35.6kg/ sq.m
Power loading: 4.8kg/hp
Maximum speed: 327km/h
Cruising speed: 190 km/h
Maximum rate of climb: 6.6m/s in 3.2 minutes
Vertical rate of climb: 1.6m/s
Accommodation: Pilot and 3 passenger