Canadair CL-84
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The CL-84 was a Canadian V/STOL turbine tilt-wing monoplane designed and manufactured by Canadair between 1964 and 1972. Only four of these experimental aircraft were built with three entering flight testing. Despite the fact that the CL-84 was very successful in the experimental and operational trials carried out between 1972 and 1974, no production contracts resulted.
Two of the CL-84s crashed due to mechanical failures, but no loss of life occurred as a result of these accidents.
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[edit] Design and development
The wing and the powerplants of the aircraft could be tilted hydraulically so that the wing incidence changed through 100 degrees from a normal flight angle to those for STOL and VTOL. The incidence of the tailplane (or stabilizer) was automatically altered to deal with trim changes as the wing-incidence varied.
Contra-rotating rotors on a vertical axis in the tail provided fore-and-aft control during hovering and transitional flight. The propulsion and lifting propellers were handed (ie revolved in opposite directions) and were interconnected by shafts through a central gearbox from which the tail rotors and accessories were also driven. The thrust from the propellers was matched automatically except when over-ridden by the pilot for lateral control in slow or hovering flight. A mechanical "mixing" unit was used to adjust the functions of the various controls in the different modes of flight.
Two 1,500 shp Lycoming T53 shaft-turbines were used to drive 14 ft four-bladed propellers. The engines were connected by cross shafts, so that in the event of the failure of one engine, it could disconnect and both propellers be driven by the remaining engine.
There were two main reasons for the technical success of the CL-84 design. Aerodynamic considerations were given a very high priority, and the controlling of power was kept as simple and direct as possible.
The propeller disks extended slightly beyond the wingtips, so the whole of the wing (except for the portion above the fuselage) was immersed in the propeller slipstream. This, together with full-span leading edge and trailing edge flaps which were programmed with wing tilt angle, ensured that the wing was never stalled. Trim changes were minimized by programmed tilting of the tailplane. All programming was based on extensive testing in the wind tunnel and on an outdoor mobile test rig.
The power of both engines was controlled by a single "power lever" in all flight regimes. To provide crisp thrust control during hover, movement of the power lever caused a direct adjustment of blade angle, analogous to the collective pitch control of a helicopter, with the propeller cpu governor making a follow-up adjustment of blade angle to maintain the selected rpm. The direct adjustment of blade angle was faded out automatically as the blade angle increased with increasing forward speed.
The only unfamiliar control function the pilot had to deal with was the wing tilt control, which was a switch on the power lever (and took the place of controlling the flaps). The combination of smooth aerodynamics and simple power control made it easy for fixed-wing pilots to perform transitions between hover and wing-down modes on their first flight in the CL-84.
[edit] Testing
"CF-VTO-X," the CL-84 prototype first flew in hover on 7 May 1965, flown by Canadair Chief Pilot Bill Longhurst. On 12 September 1967, after 305 relatively uneventful flights, CF-VTO-X was at 3,000 ft when a bearing in the propeller control system failed. Both pilot and observer successfully ejected but the prototype was lost. Canadair redesigned its replacement, the CL-84-1 incorporating over 150 engineering changes including the addition of dual controls, upgraded avionics, an airframe stretch (1.6 m., 5 ft. 3 in. longer) and more powerful engines (boosted by 100 hp.).
The first newly designed CL-84-1 (CX8401) flew on 19 February 1970 with Bill Longhurst again at the controls. He continued with the CL-84 program until his retirement from active flying in January 1971. Doug Atkins then assumed the role of chief test pilot. At about the same time, at the height of the Vietnam War, the US Navy expressed interest in the concept. Atkins was dispatched on a cross-country tour that took a CL-84-1 to Washington, DC, Norfolk, Virginia, Edwards Air Force Base and eventually full-blown trials on the USS Guam. The CL-84-1 performed flawlessly, demonstrating versatility in a wide range of on-board roles including troop deployment, radar surveillance and anti-submarine warfare.
The potency of the CL-84-1 as a gun platform was dramatically accentuated in a Canadair promotional film. Fitted with a General Electric SUU 11A/A pod with a 7.62 mm mini-gun, Adkins maintained a rock-steady position as he sprayed a ground target. The rotating six-barrel “Gatling” gun delivered a devastating 3000 rounds per minute, ripping up the target.
Continuing Tripartite trials by Canadian, US (Navy/Marine) and RAF evaluation pilots at the US Navy’s Patuxent River Experimental Test Center showed that the CL-84-1 was a suitable multi-mission aircraft. RAF Flight Lieutenant Ron Ledwidge became the first to make a descending transition from hovering to conventional flight and back to hovering while on instruments.
On 8 August 1973, disaster struck as the first CL-84-1 was lost. A catastrophic failure occurred in the left propeller gearbox in a maximum power climb. The US Navy and US Marine pilots aboard ejected safely. Canadair representatives were sure something was wrong – the entire propeller and supporting structure of the gearbox had broken away during the climb. It was rumoured that the pilots had attempted to set an unauthorized climb record to 10,000 ft. to take that distinction away from the F-4 Phantom that had held it. The second CL-84-1 (CX8402) was rushed stateside to complete the Phase 2 trials onboard the USS Guadacanal. In the face of gale storm conditions, the "84" performed magnificently in tasks such as ferrying troops and “blind-flight.” Phase 3 and 4 trials proceeded immediately after, but, despite rave reviews from over 40 pilots, the CL-84-1 did not land any production contracts.
[edit] Cancellation
The end of the Vietnam War meant a scaling back on military requirements, but Canadair designer Fred Phillips had been cognizant of other factors gravitating against the "84." The first and most crucial was the “NBH” (not built here) factor; Canada had overcome it with other sales to the U.S. military but the de Havilland Canada Beaver, Otter and Caribou loomed as exceptions to the rule. It was also “a prop job in the age of jets” and lastly, the CL-84 “tilt-wing” concept did not have a “grand champion” who would fight for it in boardrooms and military procurement offices. Canadair had tried to sell the Dynavert to others – Germany, Holland, Italy, Scandinavia and the United Kingdom were all courted, but, in the end, the Canadair CL-84 died in 1974 for lack of interest, not even in Canada.
A prototype and three evaluation aircraft had been built. The three CL-84s that flew made a total of over 700 flights and were flown, besides Canadair test pilots, by 36 pilots from Canadian, UK and U.S. military and civil agencies. The two surviving CL-84s ended up in museums, CX8402 resides in the Canada Aviation Museum in Ottawa alongside another faded dream of technological greatness in Canada – the Avro Arrow. CX8403 was never flown; it was donated to the Western Canada Aviation Museum. Shipped as two main sections, fuselage and wings, the last CL-84 has never been restored and only the fuselage sits forlornly in the main display gallery. Visitors sometimes take time to read the display that tells the story of one of Canada’s greatest achievements in V/STOL development and wonder “what if?”
[edit] References
- Airliners.net photo 0108113 [1]
- Canada Aviation Museum CL-84 pageCanada Aviation Museum CL-84 page accessed 09 October 2006.
- "CL-84 Aircraft Operating Instructions." available on CD from http://www.flight-manuals-on-cd.com
- F.C. Phillips. The Canadair CL-84 Experimental Aircraft - Lessons Learned. "AIAA-1990-3205," AHS, and ASEE, Aircraft Design, Systems and Operations Conference, Dayton, OH, 17-19 Sept., 1990.
- F.C. Phillips. The Canadair CL-84 Tilt-Wing V/STOL Programme. "The Aeronautical Journal of the Royal Aeronautical Society, Vol. 73, No. 704, August 1969."
- Pickler, Ron and Milberry, Larry. Canadair: The First 50 Years. Toronto: CANAV Books, 1995. ISBN 0-921022-07-7.
[edit] Specifications
- Wing Span: 33ft 4in (10.16 m)
- Maximum width over propeller tips: 34.66 ft (10.56 m)
- Length: (tip of instrumented nose boom to tip of tail prop) 53.62 ft (16.34 m)
- Maximum height over propellers during wing tilt: 17ft 1.5in (5.36 m)
- Diameter - main propellers: 14.0 ft (4.3 m)
- Diameter - tail propeller: 7.1 ft (2.2 m)
- Weight empty (equipped, less crew): 9023 lb (4093 kg)
- Max T/O Wt (VTOL): 12600 lb (5710 kg)
- Max T/O Wt (STOL): 14500 lb (6580 kg)
- Max Speed: 321 mph (518 km/h)
- Max Range: (with max wing fuel, VTOL, & 10% reserves) 420 miles (680 km)
[edit] Trivia
At the time of the CL-84 project, Canadair was a subsidiary of General Dynamics. Someone at General Dynamics thought it would be neat for all their products to have names starting with "Dyna-", and as the CL-84 was a vertical take-off aircraft, it was christened "Dynavert." This moniker was received at Canadair with a mixture of amusement and chagrin - mostly the latter. The name was never used within the project - the aircraft was always referred to as the "84."
It is interesting that the present day V-22 Osprey VTOL aircraft bears a similar appearance some 30 years later although it utliizes a tilting engine concept.
