Some aviation concepts have extremely long gestations, and this can occur for a variety of reasons such as technical difficulties, or a shortage of funds.
The tiltrotor or ‘convertible’ aircraft experienced such a convoluted development path with the first production aircraft entering front-line service only fairly recently, after a long design and prototyping process that began in the 1950s.
This era of aviation history saw an explosion of conceptual thinking on experimental aviation theories such as the ‘convertible’ aircraft, and the first successful technology demonstrator was built during the 1950s by Bell, as the XV-3 tiltrotor aircraft.
The XV-3 was small, underpowered and relatively slow, but successfully demonstrated the difficult transition from hovering flight (helicopter mode) to horizontal flight (conventional aircraft mode) and back again, over a long series of test flights.
The entire test program was fraught with difficulties, as might be expected of a test project for an entirely new flight concept, but the XV-3 was a successful technology demonstrator that eventually spawned the modern tiltrotor aircraft serving in many national air arms today.
Genesis and Development
The Second World War saw aviation innovation and design advance in quantum leaps, including the first small helicopters seeing active service in the dying days of the conflict.
After the war much development work on rotary-wing aircraft took place in many places around the world, but no more than in the United States, where all branches of the American military enthusiastically procured helicopters for employment in a large variety of combat roles.
The concept of a ‘convertible’ airframe had been visualised for some time by the US military and aircraft manufacturers, but the technological imperatives of the immediate post-war era placed emphasis on the development of jet aircraft for combat and commercial use.
Much development took place on rotary-wing aircraft as well but plans for a ‘convertible’ or tiltrotor design project languished until the start of the next decade.
Both the United States Army (USA) and the United States Air Force (USAF) saw the possibilities and advantages of a tiltrotor aircraft design. Much-combined staff work led to the establishment of the Convertible Aircraft Project in early 1951, and a Request for Proposals (RFP) was released to encourage aircraft manufacturers to submit designs for consideration.
Both services realised that the project would have several formidable technological hurdles to be overcome and that this could be time-intensive, so the tender was for a technology demonstrator, not a viable design for serial production.
In October 1953 Bell Helicopters was awarded a contract to produce two prototype tiltrotor airframes for testing purposes under the Convertible Aircraft Project.
The original designation for the prototypes was XH-33, which classified them as helicopters but this was changed to XV-3 under the international ‘convertiplane’ label. This was changed again in 1962 to its final designation XV-3 when the ‘V’ prefix was changed to denote Vertical Take-Off and Landing (VTOL) platforms.
Aviation theorists had foreseen the possible advantages of tiltrotor aircraft design for some time, with the ability to hover for take-off and landing combined with the speed, range and economy of horizontal flight once off the ground.
The technological challenges were formidable, however, and the difficult ‘transition’ stage between the two flight modes was especially troublesome until a safe flight profile for this could be formulated.
In August 1955 the maiden flight of the first prototype took place, with the aircraft in helicopter mode only. Two hundred and fifty flights in total were undertaken until the program’s end, and one hundred and ten of these flights were utilised to explore the ‘transitional’ stage of flight between the two flight modes.
With the concept of a ‘convertible’ airframe validated by the test sequence, the program was formally terminated in June 1966.
The Bell XV-3 was always a test airframe and a technology demonstrator, and as such had relatively modest dimensions adequate enough for the purposes of the testing regime.
Accordingly, the weight of the airframe was kept as light as possible, and the avionic fit was restricted to that necessary for safe flight operations, and to record any test data generated by the flights.
The power plant was chosen for its combination of adequate power and relatively lightweight, as well as being able to be modified to power the innovative flight systems required in an experimental tiltrotor aircraft.
The cabin had seating for the test pilot, but the fuselage design was far from conventional, with a mid-mounted wing equipped with helicopter-sized rotors at each wingtip. These rotors were mounted in a swivelling nacelle that rotated up to 90 degrees between vertical and horizontal, according to the flight mode desired.
The engine was mounted in the fuselage behind the pilot, and drive shafts transferred engine power along the wing to the gearboxes in the nacelles. The fuselage was equipped with skids instead of wheels, as this was perfectly adequate for vertical take-offs and landings.
The height of the XV-3 was 13 feet 3 inches (4.04 metres) and the total length of the airframe was 30 feet 4 inches (9.25 metres) and the wingspan is 31 feet 4 inches (9.55 metres).
The XV-3 was powered by a single Pratt & Whitney R-985-AN-1 Wasp Junior 9-cylinder air-cooled radial piston engine, which developed 450 horsepower.
This was able to give the following performance figures: a maximum speed (in horizontal flight mode) of 184 mph (296 km/h) and a best economical cruise speed of 167 mph (269 km/h). The maximum range (again, in aircraft mode) was 255 miles (410 kilometres), and the service ceiling of the aircraft was 15,000 feet (4,600 metres).
After the first flight of the airframe in 1955, an extensive test program was initiated. Almost immediately a series of problems arose relating to the ‘transition’ stage of flight.
Any attempt by the test pilot to rotate the rotor nacelles into horizontal flight caused severe vibration, and this was regularly occurring just in hover mode as well. Bell did some re-design work to attempt to rectify this issue, but the problem kept re-occurring despite these modifications.
In October 1956 the first prototype again attempted to transition to horizontal flight, but as soon as the nacelles had tilted 17 degrees past vertical disaster struck. The airframe experienced vibrations so severe the pilot blacked out, and the XV-3 crashed injuring the pilot, and the airframe was a total write-off.
Work commenced on a re-design of the rotors in the second prototype, and the original three-blade design was modified into a two-blade configuration.
Despite these major modifications the airframe continued to experience vibration occasionally when attempting to change flight mode. The second prototype spent some considerable time at a National Advisory Committee for Aeronautics (NACA) facility for wind tunnel tests, and the resultant test data led to a re-design of the wing structure and the rotor blade size.
These modifications fixed the problems and the aircraft returned to flight test duties in late 1958.
On the 18th of December 1958, the first successful transitions between flight modes were flown by a Bell test pilot, and military pilots started accomplishing this feat regularly soon after this date.
110 flights were undertaken with successful transitions between both modes of flight, and the first flight manuals for tiltrotor aircraft operations were starting to be compiled and published.
The section of the flight test program exploring the transition flight procedures ran from 1958 to 1962, and the flight program ran for a total of 250 flights for a total of 125 flight hours.
In 1966 a further stint at a now-National Aeronautics and Space Administration (NASA) wind tunnel to explore potential problems with some of the airframe’s flight envelope ran smoothly until the last scheduled test – a wingtip malfunction caused both rotors to fail at a high simulated speed.
The resulting incident caused major damage to the aircraft, and severe damage was also sustained by the wind tunnel test chamber.
As all test data needed had been provided by this date NASA declared the test program for the XV-3 to be complete. The sole surviving airframe was stored for a while in Arizona, but was eventually restored and donated to the National Museum of the United States Air Force in 2007.
All technology demonstrators have hard, eventful and often short and unglamorous careers. As an entirely new aviation concept, the tiltrotor XV-3 experienced this phenomenon first-hand, despite proving the concept of ‘convertible’ aircraft to be a viable proposition.
Despite this test success, the concept of tiltrotor design languished until the 1970s, when the XV-15 tiltrotor demonstrator revived interest in this interesting and potentially important aviation concept.
The XV-3 is little known today, but deserves an important place in the history of aircraft innovation, since it started the process that would eventually result in the service introduction of tiltrotor combat aircraft.
Today, variants of the V-22 Osprey are operating with the USN, USMC and the USAF, and the US Army is about to introduce a new tiltrotor platform for battlefield lift of troops and equipment into the battle zone.
Undoubtedly more designs of tiltrotor aircraft will appear in the future, and further cement the legacy of the XV-3 as an important step in the development of these exciting new aircraft.
- Crew: 1
- Length: 30 ft 4 in (9.25 m)
- Wingspan: 31 ft 4 in (9.55 m)
- Height: 13 ft 3 in (4.04 m)
- Empty weight: 1,907 lb (865 kg)
- Gross weight: 2,218 lb (1,006 kg)
- Powerplant: 1 × Pratt & Whitney R-985-AN-1 Wasp Junior 9-cylinder air-cooled radial piston engine, 450 hp (340 kW)
- Main rotor diameter: 2 × 25 ft 0 in (7.62 m)
- Maximum speed: 184 mph (296 km/h, 160 kn)
- Cruise speed: 167 mph (269 km/h, 145 kn)
- Range: 255 mi (410 km, 222 nmi)
- Service ceiling: 15,000 ft (4,600 m)
- Rate of climb: 1,260 ft/min (6.4 m/s)