In retrospect, the Canadair CL-84 V/STOL program might have been the first Tilt-Wing concept vehicle to demonstrate considerable success with the technique and generate interest worldwide. The program also generated a great interest from the Canadian military, where the model was given the CX-84 designation.
Interesting is the fact that with this program there was a research and development period (1958-1963) of a considerable length before the first prototype was constructed. The R&D would produce a design that would incorporate a number of significant innovations. Included were the following features:
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A large chord wing which was to be immersed in the propeller slipstream
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Huge propellers on a pair of powerplants
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All engines, rotors, and the tail rotor which were connected together by intricate shafts and gear boxes
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Conventional-style cockpit controls
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Stability augmentation system for reduction of pilot workload in low-speed flight conditions
Since there was a complete lack of experience with this type of concept, there was considerable scale model testing in Canadian wind tunnels to ensure the design.
Canadair also used its analog computer facility and a cockpit mockup to create a realistic CL-84 flight simulator. The simulator proved to be an effective development tool and was instrumental in the design of the cockpit controls.
The CL-84 was a sizable machine with a maximum height of 14 feet, seven inches, and a rotor tip-to-tip length of over 4.2m. The wings had a total area of 21.6m2 with the trailing and leading edge flaps having a sizable 4.7m2. The empty weight of the plane was 3380kg. In a maximum payload configuration, the plane could be loaded to 675kg for a pure VTOL mission, while 1620kg of payload was accomplishable in a STOL or conventional mission.
Following completion of R&D, the first of four prototypes would be built in two years, with the first vertical flight coming in May 1965, followed by its first conventional flight seven months later.
The first total transition flight was accomplished on January 17, 1966, at the company's Montreal facility. Transition was made from hover to forward flight and back. Flights were made in light snow with wind gusting to 25 miles per hour. The flight, with company pilot W.S.Longhurst at the controls, came seven months ahead of schedule.
The building of that first prototype could best be described as a Lockheed "Skunk Works" operation with word-of-mouth communications being used to supplement the very-sparse drawings during its construction.
The prototypes by normal standards were very small craft weighing only about 3600kg, about a quarter of the weight of the XC-142 that would follow. The wings were only 10m in length, and mounted on the underside were a pair of Lycoming T53-LTCIK-4A turboprop engines turning 4.3m-diameter propellers.
The horizontal tail was placed relatively low so that it was below the wing wake during cruising flight and always within the slipstream of the wing-tilt angle. The placement within the slipstream was important in order to prevent abrupt changes in pitching moment as a function of wing-tilt angle. Therefore, it can be seen that in addition to using the Tilt-Wing concept, the CL-84 also benefited to a lesser degree from the Deflected Slipstream technique.
Pitch control was provided by a pair of horizontally-mounted two-bladed propellers mounted on the rear of the aircraft. When in conventional flight, the props were stopped to minimize drag.
Roll control was maintained by differential thrust from the main engines while ailerons accomplished yaw control.
Capabilities of the CL-84 were impressive, being able to lift 2930kg of fuel and payload in a STOL take-off, or 1850kg of fuel and payload in a VTOL mode. Amazingly, a 65km/h wind could double the VTOL payload capability.
An advantage of the CL-84 came from the pilot's seat where it was piloted pretty much like a conventional aircraft. The pilot sat in the left seat, but a dual set of controls were in place. Even with its complex control mechanisms, the control stick and rudder pedals produced the desired control functions. It really wasn't necessary to learn a completely-new flying technique with the CL-84.
Amazingly, the pilot could fly the plane without actually knowing the wing angle. The innovative engineering of the CL-84 was really brought out by the plane's control system.
A number of significant flying maneuvers were accomplished by this program, including forward flight from hover (wing tilt 88 degrees) to 60km/h (wing tilt 48 degrees) and return to hover mode, demonstration of adequate control in winds gusting to 45km/h, rearward, sideward, and turning flight in and out of ground effect, and sustained flight with hands free of the controls.
A so-called mixing box brought all the control forces together to act as one as it linked the elevators, rudder, ailerons, and propeller blade angles together.
By April 1966, the CL-84 had completed 70 test flights consisting of hover, conventional flight, STOL, and transition flights. Investigation into the high-speed regime followed undercarriage retraction tests, with speeds up to 370km/h in 60-degree banked turns being achieved. Other significant test accomplishments occurred during low-speed maneuvers. For example, the CL-84 easily completed 2G turns at only 165km/h, followed by 60m radius turns at 90km/h.
The plane also demonstrated exceptionally stable hovering flights, some with pilot hands and feet free of the controls. The CL-84 was, in fact, hovered and even landed vertically with the stability augmentation system not operating.
The first prototype was flown for two years by 16 pilots for a total of 145 flying hours. A number of military applications were tested, including dropping of external stores, mini-gun firing, simulated rescues from hover, use of a cargo sling, joint operations with a helicopter at seas, and hover downwash tests. A number of United States teams also evaluated the plane.
Unfortunately, the first prototype was lost in a reliability test accident in September 1967. Fortunately, both pilots ejected safely from the plane. The aircraft was flying at 980m at 280km/h in a forward velocity mode when the plane yawed to the left and quickly pitched downward. The investigation that followed identified the probable cause as a propeller failure. The plane was on its 306th test flight when the incident occurred. The program, though, went on with the construction of the three additional versions which incorporated a number of design changes from the original. Only two of them would actually fly. Two of the planes would be involved in non-fatal accidents due to mechanical problems.
The additional prototypes were built between February 1968 and February 1970. The testing of them would continue until 1974, over 20 years since the design work had started.
Two years before the end of the program, the CL-84 was demonstrated to the US Navy. Most significant was a flying demonstration that took place off a hundred-square-foot pad at the Pentagon. The USS "Guam" also hosted a number of both CL-84 STOL and VTOL flights from its deck.
Even though the model had achieved considerable success, neither the United States or the Canadian government showed enough interest to bring the program into production.
Even though two of the prototypes would have non-fatal crashes, the CL-84 was overall considered a success.
And today, one of those planes has survived, the second of the three prototypes produced, which is on permanent display at the National Aviation Museum of Canada in Ottawa. This particular plane made 196 flights with almost 170 flight hours. The plane was donated to the museum by Canadair in 1984.
S.Markman & B.Holder "Straight Up: A History of Vertical Flight", 2000
