Primarily as a research vehicle, McDonnell designed and built two prototypes of a somewhat complicated convertiplane under the designation McDonnell XV-1. The fuselage, mounted on skid landing gear, had a 391kW Continental R-975 piston engine at the rear to drive a pusher propeller; the mid/high-set wings mounted twin tailbooms with twin vertical surfaces, inter-connected by tailplane and elevator; and above the fuselage was a three-bladed rotor with blade-tip pressure jets. Extended testing as part of a combined US Army/US Air Force programme began with tethered flights, followed by a first free flight on 14 July 1954 and a first conversion from vertical to horizontal flight on 29 April 1955. Although demonstrating a maximum speed of 322km/h, the XV-1 was too complex for the small advantages gained over a conventional helicopter. Subsequently, on 13 November 1957, McDonnell flew the first of two prototypes of a small crane helicopter, designated Model 120, which had been developed as a private venture. This used the rotor developed for the XV-1, but although tested successfully it found no market and further development was abandoned.
D.Donald "The Complete Encyclopedia of World Aircraft", 1997
Looking at the XV-1, it's easy to see the parts and pieces of both a helicopter and a conventional aircraft. The concept carried both a horizontally-mounted helicopter rotor along with a conventional pusher prop for normal flight. It was the first such VTOL developed in the United States. The US Army was very interested in the system for liaison and observation missions, along with tank and artillery spotting. There were also considerations for using it as an airborne ambulance where it could carry two litter patients and an attendant plus the pilot.
The actual development of the XV-1 took place in conjunction with the Air Force's Wright Air Development Center and the Army Transportation Corps. The XV-1 was one of three VTOL concepts under investigation in 1954. Others included the Bell XV-3 Tilt-Rotor and the Sikorsky S-57 Retractable Rotor vehicles.
The 9m long fuselage was interesting, being pure helicopter in the forward portion and a P-38-style boom arrangement with twin vertical tails (which were about 3m in height) on the aft portion of the plane. The horizontal stabilizer was mounted between the booms. Another helicopter similarity was the use of a skid instead of a normal landing gear.
The XV-1 had a gross weight of 2160kg with an empty weight of 1645kg. The fuel capacity was only 315 litres weighing about 225kg. The payload was 185kg.
There was room for a crew of four or five in the rather large cockpit. With its large glass area in the cockpit area, the XV-1 would have been an observer's dream with visibility available in all directions with the exception of directly below
Over the wing was a hump surmounted by the rotor head, and aft of which was the engine cowling ending with a spinner.
The propulsion system was interesting, and complicated, in that the 550 horsepower Continental R-975-42 525 horsepower reciprocating engine provided the power to drive both rotors. The top rotor was NOT driven directly by a hookup with an engine shaft, but by pressure-jet units on the rotor tips which were supplied with compressed air ducted through the hub and rotor blades. The arrangement provided the XV-1 with a ceiling of about 3600m and a cruise speed of about 190km/h. The rotor was used effectively only during the lift and descent phases, while in cruising flight it auto-rotated and produced only about 15 percent of the lift. The cruise flight was then supplied in the conventional manner by the pusher propeller located on the aft of the crew compartment and had the capability to push the plane forward at a maximum, and impressive, 320km/h.
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.
Basically a new concept, the program moved slowly toward actual transition from vertical-to-horizontal flight. The program began in 1949, but it would take until 1955 to get that first goal achieved. The Air Force then entered the program, where its maximum speed capability was accomplished. Following the Air Force evaluation, all research on the program was discontinued.
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.
Other countries, though, would look at the concept and take it to the next level. But much of the early work had already been accomplished by the XV-1 program which proved that it could be done.
S.Markman & B.Holder "Straight Up: A History of Vertical Flight", 2000
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, and the USAF Air Research and Development Command.
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 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.
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. 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. 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.
Although on 10 October, 1956, the 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. Accordingly, the programme was terminated in 1957 after the two prototypes had been flown for a total exceeding 600 hours. The first XV-1 then went to the Army Aviation Center Museum at Fort Rucker, Alabama, and the second was donated to the National Air and Space Museum, in Washington, DC.
Rene J. Francillon "McDonnell Douglas Aircraft since 1920: Volume II", 1997
In June 1951 the Army Transportation Corps, the Air Force Air Research and Development Command, and the McDonnell Aircraft Corporation jointly initiated the development of a single-engined research aircraft incorporating the rotor system of a helicopter with the wings, twin-boom tail surfaces, and pusher propeller of a conventional airplane. McDonnell had already completed the preliminary design studies for just such a hybrid 'convertiplane', the Model M-28, and was therefore able to present a complete mock-up for inspection in November 1951. The mock-up won Army and Air Force approval without the need for major modifications, and the first prototype aircraft (serial 53-4016) was completed in early 1954. The craft had initially been designated the XL-25, though during the course of construction this was changed to XH-35 in the helicopter category and, finally, to XV-1 in the newly created convertiplane category.
The XV-1 was designed to take off and land like a helicopter and undertake forward flight like a conventional fixed-wing aircraft. During vertical flight the single 525hp radial engine drove two air compressors which channelled air through ducts built into the main rotor blades and expelled it through pressure jets at the blade tips; in forward flight the engine's power was applied to the two-bladed pusher propeller mounted at the aft end of the central fuselage pod. The transition from vertical to horizontal fliqht was accomplished by transferring power from the main rotor to the propeller as soon as the XV-1's forward speed exceeded the stalling speed of the main wing. The main rotor was then allowed to autorotate, thereby supplementing the lift generated by the craft's wings.
The XV-1's initial tethered hovering flight occurred on 15 February 1954, and its first free hovering flight took place almost exactly five months later. The second prototype machine, 53-4017, joined the flightiest programme in the spring of 1955, and in April of that year the first prototype made the first transition from vertical to forward flight. 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. These modifications undoubtedly improved the XV-1's performance, for in October 1956 the second prototype became the first rotorcraft to achieve 200 mph in level flight. However, the type's mechanical complexity was ultimately judged to be a disadvantage not counterbalanced by its high speed, and the XV-1 programme was consequently cancelled in 1957. Fortunately, and unusually in the case of an experimental aircraft not ultimately adopted for widespread operational use, both XV-1s were preserved rather than being sold for scrap; 53-4016 was handed over to the Army Aviation Museum at Fort Rucker and 53-4017 was acquired by the Smithsonian Institution's National Air and Space Museum.
S.Harding "U.S.Army Aircraft since 1947", 1990
This experimental aircraft was designed by a pioneer of vertical flight, the Austrian Friedrich von Doblhoff. It was built with the collaboration of the U.S. Army and Air Force. It was one of the first examples of a compound helicopter, with a pusher propeller driven by a piston engine, and a three-blade rotor powered by air from two compressors ducted along the blades to blade-tip nozzles. The aircraft had a small wing supporting twin tail booms. The 550hp Continental R-975 engine was housed in the rear of the fuselage. The cabin seated a pilot and three passengers. The first of two prototypes was test-flown at the beginning of 1954. In October 1956 it became the first rotorcraft to reach a speed of 320km/h. Development was suspended in 1957.
G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984
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