The McDonnell XV-1 (see NASM collection) and Bell XV-3 demonstrated that Vertical Takeoff and Landing (VTOL) aircraft could hover like a helicopter, but with a considerably higher top speed than conventional rotary wing aircraft. The U.S. military then began to examine the most effective approach to VTOL propulsion. A number of experimental projects appeared throughout the late 1950s and early 1960s with a variety of propulsion and lift devices to achieve vertical flight, including tilt rotors, tilt propellers, tilt jets, tilt wings, tilt ducts, lift fans and deflected thrust systems. The tilt wing emerged as a successful VTOL system, but other approaches proved more viable under operational conditions. The Vertol VZ-2 was the first tilt wing aircraft to successfully transition from vertical to horizontal flight.
After Frank Piasecki left the Piasecki Helicopter Corporation in 1956, it became the Vertol Aircraft Corporation and aggressively pursued government helicopter and research contracts. On April 15, 1956, the Office of Naval Research issued a contract to Vertol, for a tilt wing VTOL design, funded by the Army, and designated as the VZ-2, though inside the company many knew it as the Model 76. The justification for the Army absorption of the Navy research costs most likely arose from Air Force apathy towards Army aviation research programs. The Air Force normally handled procurement of Army aircraft, but did not demonstrate much interest in the Army's need for specialized aircraft, and made little effort to support its VTOL projects. Vertol quickly completed the VZ-2 for its April 1, 1957 rollout.
The VZ-2 could take off and land vertically by pivoting its entire wing upwards, along with its lifting propellers. The aircraft could then transition to conventional airplane-like horizontal flight by lowering the attitude of the wing. This configuration appeared to offer a greater degree of simplicity than other tilt engine and tilt rotor designs, as only one major component had to pivot. However, the VZ-2 required additional control systems for helicopter flight, which greatly increased control complexity. In hover and slow flight, pitch and yaw movements required the use of auxiliary-thrust devices because the high loading and twist of the propellers did not permit cyclic control as on a helicopter. Tilt wings did rely on differential collective pitch to maintain roll control.
The ungainly VZ-2 gave precedence to function over form. To keep costs down, Vertol engineers utilized a variety of off-the-shelf components. A Bell Model 47 helicopter made up the VZ-2's narrow slab-sided fuselage and large bulbous two-seat cockpit. The exposed Lycoming T-53 gas-turbine engine, perched on the top of the fuselage behind the cockpit, gave the aircraft a decidedly experimental appearance. Exhaust gases exited through an unusual forked duct that prevented damage to the vertical stabilizer. A complex cross-shaft drive system powered the rotors, regardless of wing tilt. A spindly fixed tricycle landing gear with a castering tailwheel supported the VZ-2 on the ground and a small fourth wheel, mounted on the underside of the cockpit, prevented damage from nose-strikes.
One of the biggest technical challenges in the development of VTOL aircraft was the integration of two sets of control mechanisms - one for flight in the helicopter-like vertical mode, and the other for airplane-like horizontal flight. In cruise, the T-tail control surfaces, which consisted of a vertical stabilizer/rudder and a stabilator (an all-moving horizontal surface), maintained pitch and yaw control, while conventional wing-mounted ailerons provided roll control. Just below the stabilator, a horizontally mounted four-bladed rotor, 61 cm (2 ft) in diameter, provided pitch control in the vertical flight mode. A hole in the stabilator allowed air to flow freely through the rotor. Another similarly sized rotor, mounted vertically on the lower right side of the vertical stabilizer, provided yaw control during slow speed flight and hovering. During the transitions from one flight mode to the other, the VZ-2's control system phased in control of the horizontal and vertical modes proportionally with the degree of wing tilt.
On August 13, 1957, test pilot Leonard LaVassar made the first flight, conducted entirely in vertical flight mode. On January 7, 1958, he made the first horizontal flight, but he did not complete a vertical-to-horizontal transition until July 15, 1958. For the next nine months, the VZ-2 continued to undergo tests at Vertol's facilities in Morton, Pennsylvania. During its initial flights, tufts of string covered much of the VZ-2's external surfaces to aid in the determination of airflow at various wing angles. Subsequently, the aircraft underwent testing at Edwards Air Force Base for five months. On October 9, 1959, Vertol turned the aircraft over to the National Aeronautics and Space Administration's (NASA) Langley Research Center. NASA conducted the Army's share of the flight tests because the service did not have any pilots qualified to fly VTOL aircraft.
The aircraft handled better than many of its VTOL cousins, but it had to overcome several hurdles. The most serious of which was the wing stall that occurred during the transition phase, which resulted in heavy buffeting and loss of roll stability in descents. In November 1961, Vertol replaced the wing with a new design that eliminated most of the buffet by incorporating flaps along the full length of the wing, along with a drooped leading edge. Split ailerons, which replaced the earlier conventional installation, provided roll control in horizontal flight. Pilots liked the VZ-2's maneuverability, but found that it handled poorly in vertical flight mode while flying in gusty conditions - a problem attributable to the sail-like qualities of the wing. Other modifications included revised exhaust ducting, a simplified control system, some limited fuselage covering, and the removal of the right seat in favor of extra test equipment.
The VZ-2 continued to fly through the early 1960s and accumulated an outstanding flight record as a technology demonstrator. By the time of its last flight on April 16, 1964, it had logged 454.5 hours, of which 73.2 were in free flight. The VZ-2 made at least 34 complete conversions between vertical and horizontal flight modes, along with 240 partial transitions. During the VZ-2 test program, the military services issued contracts for the development of larger, transport-sized tilt-wing aircraft. These included the Hiller X-18 and the LTV-Hiller-Ryan XC-142A. In its final months of service, the VZ-2 served as a trainer for the large four-engine XC-142A. In 1965, after completion of its flight test duties, Boeing, which had bought out Vertol, donated the VZ-2 to the Smithsonian Institution.
The tilt-wing and other VTOL programs that had prospered in the early 1960s soon fell victim to Secretary of Defense McNamara's abhorrence of military research programs. While VTOL aircraft have never been commercially viable, their speed gave them an advantage over conventional helicopters in military operations. When research into development of VTOL transports resumed nearly a decade later, engineers determined that the tilt-rotor configuration offered the most advantages and the tilt-wing became a technological dead-end. However, the data gathered during Vertol's tilt-wing program contributed significantly to this evaluation, and to a considerable extent, justified the value of the VZ-2 as a technology demonstrator.
Wingspan:7.59 m (24 ft 11 in)
Length:8.05 m (26 ft 5 in)
Height:3.04 m (10 ft)
Weight:Empty, 1,134kg (2,500 lb)
Gross, 1,452 kg (3,200 lb)
Engine:Lycoming YT-53-L-1 gas turbine, de-rated to 600 shp
References and Further Reading:
Markham, Steve, and Bill Holder. "Straight Up: A History of Vertical Flight." Atglen,
PA: Schiffer Publishing, Ltd., 2000.
Rogers, Mike. "VTOL Military Research Aircraft." Sparkord, United Kingdom: Haynes
Publishing Group, 1989.
VZ-2 curatorial file, Aeronautics Division, National Air and Space Museum
R. D. Connor