An outstanding example of advanced technical ingenuity by the Canadian aircraft industry was the Canadair CL-84 tilt-wing V/STOL aircraft. This, studied in detail for seven years, built as a prototype in a little over a year and flown in two versions on test and demonstration work during nine years, was shown to be technically sound and successful. It was not, in the event, wanted by the US authorities, which were probably the only prospective buyers of a developed version.
The CL-84 was-to summarize its features briefly before a fuller technical description later-a twin propeller-turbine monoplane in which the wing and the powerplants could be tilted hydraulically so that the wing incidence changed through 100 degrees to the vertical from a normal flight angle to those for STOL and VTOL regimes. The incidence of the normally fixed 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 (i.e. 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 control reactions for the pilot between the different regimes of flight.
Originally a private venture to meet a NATO specification, the initial studies were later financed also by the Canadian Defense Research Board and Department of Defense Production. The decision to go ahead with a flying prototype with two 1,400 shp Lycoming T53 shaft-turbines, an airframe for static tests, an additional set of engines, transmission systems and propellers, and engineering work was announced in February 1963 and the construction of the prototype began in November with Canadair putting up a quarter of the cost.
The roll-out of the prototype, registered CF-VTO-X, was on December 9, 1964, and the first flight, in hover mode, was made by W. Longhurst five months later, on May 7, 1965. It was flown as a conventional aircraft for the first time on December 6 by Longhurst, who made the first transitions between forward flight and hover, and vice versa, on January 17, 1966.
It was later reported that trim changes were relatively small and that the transition was smooth and rapid, with only a 12-seconds delay between hover and normal flight at 115 mph (185 kmh). A score of transitions, in varying wind conditions, were made during the following days, when STOL experience was gained with the wing at a 30 to 40 degree tilt angle. The wing was locked down in flight for the first time on March 18, when the prototype was climbed, as a normal aircraft, to 6,000 ft (1,829 m) .
For zero-wind conditions the VTOL wing-setting was 85 degrees, but this was increased to 102 degrees to allow backward flight at some 35 mph (56 kmh), or hovering in a tail-wind of equivalent speed. With intermediate settings of the wing up to about 45 degrees, the CL-84 became a STOL aircraft, with greatly increased payloads.
In this configuration it could clear a 50 ft (15 m) obstacle in a distance of 500 ft (152 m) with twice the payload possible when operating in the VTOL mode. Low-speed maneuverability in the STOL mode was good. With the wing set at 45 degrees, a 2g level turn at 63 mph (101 kmh) could be completed within a radius of less than 200 ft (61 m) .
In addition to routine development tests, the prototype CL-84 had, since demonstrating transitions and other V/STOL capabilities, completed a 20 hr tri-Service evaluation by the USN and Marine Corps, US Army and USAF. It had also been flown by pilots from US, UK and Canadian military and civil agencies, had demonstrated live rescues from land and water, and given other examples of a capacity to perform in the roles envisaged.
On September 5, 1967, however, the prototype was written off in a crash. After making a series of maneuvers while flying in the conventional mode, the CL-84 entered an uncontrollable turn to the left and the nose dropped at a speed of about 170 mph (274 km/h) and a height of 3,000 ft (914 m). The pilot, Longhurst, and flight engineer, the only occupants, ejected successfully at 2,000 ft (610 m). The loss of control was probably caused by a failure in the differential propeller control system. Before this accident the prototype had, in addition to 260 hr of ground running, completed 145 flying hours in 305 flights, including 346 VTOL sorties, 151 transitions and 109 STOL sorties.
The generally promising performance of the prototype led in February 1968 to an order for three developed CL-84-1s for the RCAF as evaluation aircraft to determine the operational potential of such a V/STOL type in various military roles. These were given the RCAF designation CX-84 with the serials 8401, 8402 and 8403. By the time the first was completed the RCAF had been integrated into the CAF and the type was allotted the CAF designation CX-131 and the serials 13101 to 13103. Although the aircraft were delivered in CAF markings, the CAF designation and serials were never applied, instead the RCAF designation and serials were shown and were continued to be used.
The first was flown, initially in the hover mode, on February 19, 1970, by Longhurst and the second by F. D. Adkins and C. Harcourt on September 15, 1972. The third was completed, and thereafter maintained to current modification standards, but was stored and not flown. The developed versions were generally similar to the prototype, though fitted with more powerful Lycoming T53 propeller-turbines of 1,500 shp, but the control system differed considerably in detail following experience with the prototype; transmission and hydraulic system changes were also made. Further modifications to 8401 followed during the winter of 1970-1971, after the early flight tests. Another version, the CL-84-1C, was proposed as a production version, but this was later superseded as a project by the 16-passenger CL-84-1D version with 1,900shp T53-19As.
There were several other studies of 'growth' versions-such as one to be fitted with General Electric T64 engines and with increased span for operations in hot/high conditions, and a 70-passenger civil transport proposed in 1970. This, designated the CL-246, was to be powered by four Lycoming T53-19As and was intended for STOL operations, with the wing tilted only to 18 degrees for normal take-offs and landings and with a maximum tilt of 30 degrees. The CL-246 would have been able to operate from the 1,800 ft (549 m) runways proposed as a STOL port standard by the US FAA.
Testing with the first of the developed CL-84s during 1971 included, in the 150 hr contract schedule, stores-dropping (using 100-gal tanks) and target shooting with a pod-mounted 0.30 in (7.62 mm) multi-barrel mini-gun at the Canadian National Defense range at the Nicolet test establishment. Good results were obtained in all flight regimes and the CL-84 could be yawed accurately, while hovering, to attack two targets.
Tests early in 1972 included demonstrations from the Pentagon helicopter pad in Washington and landings and take-offs to and from the assault carrier USS Guam off the coast of Virginia. These and other demonstrations were made at the invitation of the USN. On February 16, after flying to the carrier from Oceana Naval Air Station, Canadair's pilot gave a convincing demonstration, with wave-offs, of STOL touch-and-go landings, vertical landings and take-offs, and other maneuvers, though he had had no previous experience of deck-landings and was accompanied by a USN pilot who, at that time, had only brief experience of familiarization flying of the CL-84.
Following routine test work, 8401 was used between October 1972 and January 1973 for training of six pilots from Canada, the USN and the RAE in preparation for a tripartite electronic pilot-display test program described later. On July 12 it was flown to the US Naval Air Test Center (NATC) at Patuxent River, Maryland, for training and evaluation flights by USN and USMC pilots. After only a few days of evaluation work 8401 was lost in an accident on August 8 in Chesapeake Bay. Control was lost following failure of the port propeller gearbox. Both pilots, one USN and one USMC, ejected successfully and suffered only minor injuries.
The second aircraft, 8402, had earlier been assigned and equipped for work on the tripartite test program. This involved a series of trials to investigate head-up and head-down instrument-display requirements for V/STOL guidance in busy terminal areas, and to investigate also the characteristics of the CL-84 as applicable to instrument flying and shipboard operations. The United Kingdom provided the principal instrumental display systems, the Canadian Government supplied the aircraft and the US Navy assumed responsibility for the tests at the NATC, with each nation contributing two pilots to the team which was formed in September 1972.
Check flights were started with 8402 during that month and the specially instrumented aircraft was delivered to the NATC on December 7. The trials were divided into three phases, handled in sequence by the RAF, the USN and the CAF. Phase 1 had the objective of evaluating the displays and also checking their applicability for use with the Harrier V/STOL strike fighter. During these trials an RAF pilot from the RAE, with a Canadian as safety pilot, made what was probably the first transition on instruments, under the hood, from conventional to hovering flight with a convertible-type aircraft.
Phase 2, which started on May 30, 1973, was designed to develop an instrument-flying display which could be used for Naval aircraft and for V/STOL aircraft in particular, with the emphasis on pictorial rather than plan-position representations. This phase took longer than expected because of the accident to 8401, which involved a delay of nearly two months while an investigation was completed and modifications applied, and because there was now only one CL-84 available for this and other programs. The result was that Phase 3 had to be curtailed.
This part of the trials, which apart from some exploratory flying, did not start until April 8, 1974, was designed to examine the systems in relation specifically to the CL-84 and its flying characteristics. After the completion of the program on May 2, two additional sorties were made to check the displays in the CL-84 when being flown at night for the first time.
Meanwhile, 8402 had, before the completion of these tests, been prepared to go to sea aboard the USN carrier USS Guadalcanal for a ten-day exercise related to the possible development of the CL-84 as a V/STOL aircraft for the Sea Control Ship (SCS) project then being considered. For this it was proposed that relatively small aircraft carriers in larger numbers might be deployed in limited combat situations. The aircraft for such operations would need to be V/STOL, but with a much better performance and fighting capability than any helicopter. An SCS version of the CL-84 was projected.
After in-port exercises to check mobility on board, the USS Guadalcanal put to sea with the CL-84 on March 18, 1974. Two USN pilots, one USMC and one CAF pilot made 16 flights during the ten days, logging ten hours of flight time, excluding the ferry flights. After returning to the NATC for the completion of the tripartite tests, 8402 was ferried to Montreal on June 11.
The total flight times logged by the CL-84-1s were 161 hr for 8401 and 169 hr for 8402; including the times already quoted for the prototype, the CL-84s had flown during their active lives a total of 476 hr in 709 flights. All three were basically similar in design, but several changes were made in the design and equipment of the CL-84-1s.
In addition to the increased power, and the transmission and flight-control system modifications already mentioned, they had more complete duplication of hydraulic systems, provision for carrying external stores and a strengthened cabin floor, in addition to redesigned instrument layouts and avionics, and an auxiliary fuel system. For the USN evaluations, the engines were replaced with an uprated-for-hot-conditions version, the instrument panels were revised for nighttime operations and the flap systems were modified.
The operational and technical features of the CL-84 have already been described in outline, but it may be useful to add some more detail before summarizing the performance and other data. The wing was tilted, by an hydraulically driven ball-screw actuator, between incidences of 2 and 102 degrees around a fuselage hinge-point at about mid-chord. The wing had full-span slotted flaps which operated also as ailerons. There were full-span Kruger flaps for the leading edge. These were hydraulically extended while the wing was between incidences of approximately 10 and 70 degrees; a central flap above the fuselage extended at all wing-angles beyond 23 degrees. When in the fully-down position, the wing was locked.
The incidence of the horizontal tail surfaces was programmed to vary automatically under hydraulic power so as to move with the wing to an incidence of 30 to 45 degrees while the wing was moving through its STOL incidences, but returned to the horizontal as the wing-tilt increased towards the VTOL setting. Elevator and rudder were, like the flap/ailerons, hydraulically operated, but with manual reversion.
The two Lycoming T53 free-turbines drove 14 ft (4.3 m) diameter four-bladed speed-governed propellers, the slipstream from which immersed the whole wingspan. The engines were interconnected through cross-shafts, via a gearbox in the fuselage. From this another shaft drove the two contra-rotating propellers at the rear of the fuselage which provided pitch control and attitude stabilization in the hovering or transitional modes. This propeller was declutched and aligned fore-and-aft during conventional flight. A dead, or deliberately shut-down, engine was automatically declutched from its propeller drive and both propellers were then driven by the active engine.
The controls of the CL-84 were conventional, with a column for pitch and roll, and pedals for rudder control. The mechanical 'mixing' unit adjusted the control effects between VTOL, STOL and normal flight according to the wing-tilt. The feel and effect of the control movements were thus relatively normal in all regimes.
In hovering flight the fore-and-aft stick movement still provided pitch-attitude control, lateral movement induced roll, and rudder application provided yaw effects. Fore-and-aft control was then, however, being provided by the varying pitch of the tail-rotor blades and not by the elevator; roll control by blade-angle differential of the main propellers; and yaw induced by differential movements of the flap/ailerons. The mixing unit 'chose' the form and balance of the controls during transition, with normal surfaces progressively taking over or relegating their normal duties. There was artificial 'feel' by spring-loading, and a three-axis stability augmentation system, based on rate-gyro signals, was used to improve the handling qualities during transition and in the hover mode.
The normal fuel supply for the CL-84 was of 247 gal (935 l) in the wing, with provision for two 120 gal (454 l) drop tanks. The two pilots had ejector seats which could be used at any angle of wing-tilt.