In 1954, Bell Aircraft Corporation expanded its Vertical Take Off and Landing (VTOL) research program with an examination of tilt-jets as an alternative to its tilt-rotor system, pioneered on the company's XV-3. Although this privately funded project explored a new avenue of VTOL technology that ultimately proved unsuccessful, it provided essential data on the challenges inherent to this type of aircraft.
Lawrence Bell's company survived the post-World War Two slump in the aircraft industry by embracing new technology, including helicopters and research aircraft such as the record-breaking X-1 (see NASM collection). By the early 1950s, the military had become interested in VTOL aircraft that combined the vertical flight characteristics of a helicopter with airplane-like speeds. In 1952, a Bell feasibility study established that VTOL jet fighter designs were possible. The company decided that the potential benefits of the new technology were worth pursuing, even without the government support that was usually necessary for such projects.
Bell's Niagara Frontier Division managed the construction of an Air Test Vehicle (ATV), designated as the Model 65, to investigate the use of tilting jet pods to provide lift in vertical flight and forward thrust in horizontal flight. During cruise flight, a conventional wing provided all of the lift. To complete the aircraft at minimum cost, the design team relied on off-the-shelf components, including a T-tail Schweizer sailplane fuselage and a Cessna 170 wing. A set of Model 47 helicopter skids supported the ATV on the ground, which gave the aircraft an ungainly grasshopper-like appearance. The ATV used two Fairchild J44 turbojets, borrowed from the Air Force, for vertical lift and forward propulsion. These attached to either side of a swivel-mount positioned behind the open cockpit. A separate Continental-Turbomeca Palouste gas turbine provided 1.1 kg/sec. (2.5 lb/sec.) of thrust for the reaction-control system used in the vertical flight mode.
As in other VTOL aircraft, the ATV required two sets of control mechanisms - one for flight in the helicopter-like vertical mode, and the other for conventional horizontal cruise flight, which relied on standard airplane controls. When the aircraft was hovering or in slow flight, exhaust ducts on the wingtips provided roll control, while an assembly of valves and exhaust pipes mounted on the tail provided pitch and yaw control.
Bell completed the ATV in December 1953. In January 1954, test pilot David Howe made the first flight while tethered to the ground to prevent an inadvertent loss of control while testing the highly experimental control system. During a flight in February, a compressor disc failed while the ATV was hovering, resulting in a fire and significant damage to the aircraft. Fortunately, Howe escaped injury. Bell engineers quickly made repairs, and the test flights resumed. The first free flight occurred on November 16, 1954. By early 1955, the performance limitations of the aircraft led to the termination of the program after only 4.5 hours of operation with the ATV.
The slow response of the early gas turbine engines was one of the greatest inherent flaws in the ATV and other VTOL designs of the time. These engines were necessary for their power-to-weight ratio, but they were unsuited for the numerous power adjustments required for hovering. The ATV's J44s also produced a meager 998 kg (2,200 lb) total thrust, which was nearly equivalent to the aircraft's gross weight. The lack of thrust prevented a full transition from the vertical to horizontal flight modes, which resulted in the temporary installation of conventional wheeled landing gear to allow tests of the horizontal flight characteristics of the aircraft.
The ATV project's greatest contribution was the generation of data, which formed the basis of Bell's next jet VTOL project, the X-14 (known inside Bell as the Model 68). The design for this aircraft emerged several months after the termination of the ATV program. The X-14 differed greatly from the ATV as it used moveable exhaust ducts to vector thrust from powerful fuselage-mounted engines, but the vertical reaction-control system was similar.
The X-14's vectored thrust approach to vertical flight eventually found widespread acceptance when it was adopted for use in the Hawker-Siddeley Kestrel (see NASM collection) and its later developments. The X-14 provided a vast amount of VTOL data for the National Aeronautics and Space Administration (NASA) before incurring damage in a 1981 crash. Neil Armstrong once flew it as a Lunar-lander trainer.
The ATV was not a successful test aircraft, but it provided considerable insight to the challenges of producing jet VTOL designs. With the data it provided, Bell was able to improve upon the concept and make a substantial contribution to the VTOL field.