So much effort has been expended upon short and vertical take-off aircraft by the Canadian aircraft industry that the acronym V/STOL has become almost synonymous with the names Canadair and de Havilland. With some financial assistance from the Canadian government and other agencies, both companies have been enabled to undertake research programmes into various forms of V/STOL aircraft, and some work remains to be done under existing contracts. The extent to which any of this work may provide a basis for future developments is not as yet clear, however, for in Canada, as elsewhere, there is a severe restriction on the finance available for new aircraft development.

Despite world-wide interest in the potentialities of V/STOL or STOL civil airliners, the precise form that such aircraft should take remains uncertain, and in such circumstances it is understandable that governments should be no more eager to spend taxpayers’ money than boards of directors are to risk their shareholders’ profits.

Canadair’s stake in the future market for V/STOL aircraft depends totally on the tilt-wing concept. Although project studies have covered a range of possible configurations, Canadair’s only practical experience in this field has been obtained with a series of small tilt-wing prototypes, designated CL-84 and CX-84, and the 70-passenger CL-246 STOL transport that the company projected during 1970 embodies the technology developed and demonstrated with these earlier vehicles. This being so, the CL-246 could be built, according to its designers, with the minimum of new R & D expenditure - exact sums are not being quoted, however. Advantages are claimed for the CL-246 configuration in respect of performance, noise levels, operating costs, flight characteristics, passenger comfort and ride qualities. Disadvantages, which are not listed by the designers, appear to include the mechanical complexity of a tilting-wing mechanism and some possible-airline/passenger resistance, both to the idea of seeing the entire wing moving up and down, and to the use of "old-fashioned" propellers on an aeroplane intended for introduction towards the end of the ‘seventies and service throughout the ‘eighties.

The CL-246 was designed to operate from the 1,800 ft (548 m) run ways proposed as a STOL-port standard by the FAA. It has four 1,900 shp Lycoming T53-19A turboshaft engines each driving a 16-ft (4.87-m) diameter Hamilton Standard propeller to provide virtually uninterrupted airflow over the 76 ft. 5 in. (23.29 m) span wing. The entire wing trailing edge is taken up by ailerons and flap-ailerons that enhance the lift by deflecting the slipstream; tilting the wing/ power plant component serves to vector the thrust, so that short take-offs are achieved with the fuselage maintaining a near-constant and conventional attitude.

Whereas the CL-84 research aircraft feature a fully-tilting wing, allowing the propellers to provide a vertical thrust component, wing movement on the CL-246 is limited to a maximum incidence of 30 degrees, with a figure of about 18 degrees regarded as being normal for take-off and landing. This places the aircraft firmly in the STOL category; the configuration is in fact called "variable-incidence STOL" by Canadair, rather than tilt-wing. Apart from the limitation on wing tilt-angle, however, the system is similar to that used in the CL-84.

Interconnecting shafts between the four propellers ensure that all maintain equal power in the event of failure of an engine during the initial take-off and landing phases, when asymmetric thrust would be hard to control. The propellers can, however, be individually disengaged and feathered in case of gearbox malfunction. Mechanical interconnection between the wing and variable incidence tailplane ensure that the latter is actuated as the wing moves to compensate for trim changes. Outrigged fins in the propeller slipstream augment the effectiveness of a large central fin in reducing the aircraft’s responsiveness to gusts at low speeds, this being one of the areas in which special attention is necessary in V/STOL aircraft design. Only the central fin has a rudder. For lateral control at low speeds, differential propeller pitch is available.

The basic layout proposed for the CL-246 provides 14 rows of seats, each seat row having a pair to starboard of the aisle and a triple unit to port. Each seat row is adjacent to a window. All baggage compartments are above-floor, forward and aft of the cabin and in the nose. To avoid interrupting the cabin height and to simplify the wing hinge and actuating mechanism, the wing is carried above the fuselage (which is of flattened-circular cross-section), with a fairing over the centre section. Each twin-wheel main undercarriage unit retracts into a fuselage-side sponson which provides convenient stowage space for the air conditioning system and other equipment. Two-pilot operation is planned, with a supernumerary seat on the flight deck.

The background

Canadair began investigation of V/STOL designs in 1956, one of the earliest proposals being a transport for the NATO-inspired NBMR-4 specification for a military VTOL transport. In 1963, the Canadian Department of Defence Production agreed to share in funding the design, manufacture and testing of a prototype tilt-wing, deflected slipstream aircraft and this appeared in due course as the CL-84.

First flown on 7 May 1965 in the hovering mode, the CL-84 was designed to prove in flight the Canadair idea for combining vertical landing and take-off capability with a relatively high top speed in level flight. The vertical capability was obtained by providing a fully-tilting wing actually the maximum tilt angle was 100 degrees, allowing the aircraft to fly backwards at 30 knots (55 km/h) or, more importantly, to hover in a 30-knot (55-km/h) tailwind. Power was supplied by two 1,400 shp Lycoming LTC1K-4A turboprop engines (variants of the military T53) driving 14-ft (4.27-m) diameter propellers and the wing had fullspan slotted trailing edge flaps that also operated differentially to function as ailerons, and Krueger leading edge flaps. The tailplane was interconnected with the wing, in the same way as on the CL-246 described above, for wing angles up to 30 deg, but it then returned to zero incidence and pitch control was obtained in the hover and low forward speeds through small two-blade counter-rotating tail rotors at the end of the fuselage. For forward flight, the tail rotors were stopped and braked, with all blades fore-and-aft.

Although the CL-84 (which carried the civil registration CF-VTO-X) was regarded primarily as a research vehicle, normally carrying a pilot and test observer (also a pilot) side-by-side in the cockpit, it was designed with various military roles in mind, and the fuselage was large enough to seat 12 troops or carry freight or stretchers. The ability of the craft to hover, coupled with high speed to and from search areas, made it of special interest in the air-sea rescue role, and live (simulated) rescue hoists were demonstrated with the CL-84, from both land and water, in September 1966. In the following month, the first military pilots were checked out on the CL-84 and a US tri-service 20-hr flight evaluation completed in August 1967 brought the total number of pilots to fly the type to 14. These included, in addition to Canadair crews, pilots representing the Canadian Defence Forces, NASA, National Aeronautical Establishment, RAF, US Army, US Navy and US Marine Corps.

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