Canadian American Strategic Review





DND 101


Background  —  Canadian Origins of the MOSV?  —  CL-91 Dynatrac

The Canadair CL-91 / XM-571 Dynatrac Articulated  Amphibious  Carrier
Canadair's CL-70  RAT could hardly be described as a commercial success – which was the whole point of diversifying this aircraft-builders product line. With lessons learned from the RAT, Canadair enlarged its articulated vehicle as a more capable all-terrain carrier for the US Army. The CL-70s were to fulfil multiple roles with all  specialist gear fitted  in the rear section. [1]

In scaling up their concept, Canadair also switched  from VW to a larger air-cooled Corvair engine.[2] At the same time, the original articulated joint (devised by Andy Nowakowski) was revised by Ian A. Thomas.[3] The RAT's transfer joint  had proven  fragile  under  certain conditions  while its 1.1L VW flat-4 engine tended  to over- heat under load.  The CL-91 drivetrain was to overcome such woes.

As would be expected with a complicated component  like the new articulating transfer joint, parts were made to fine tolerances. This was nothing new to an aircraft maker like Canadair. The designers also chose to abandon the RAT- style band tracks in favour of individual track links forged from aluminum. The cab bodies also employed aluminum - in double layers sandwiching a balsa wood core.  Weight control was the goal in order that the CL-91 be readily air- portable in US Army CH-47 Chinooks or CV-2 Caribous as well as maintaining a high 'floatation' over soft terrain.

A less obvious change between the RAT and CL-91 was that, in the latter, the engine was moved forward in the front cab freeing space for  troops. Compared with the RAT,  the Dynatrac's drive sprocket positions were also reversed. This placed  the sprocket close to the articulated joint allowing both quick disconnect and an optional third powered cab – in place of an unpowered trailer.

The CL-91's drivetrain arrangement was meant to maximize usage flexibility but also made the vehicles more complex (with the added disadvantage of a shallower floor in the rear components ). Nonetheless, the US Army was sufficiently impressed by trials at Aberdeen Proving Ground in Aug. 1962 to place orders for an initial  batch as the XM-571 Carrier, Articulated.  [4] Further tests with the first 7 XM-571s resulted in minor changes mainly to improve cooling  –  the US Army was looking for vehicles suited to use in Vietnam, necessitating a larger cooling fan for the XM-571. The Dynatrac body construction made the XM-571 naturally buoyant, tracks propelling the vehicle in water. But, it turned out, XM-571 were little used in Vietnam.

 Canadair Dynatrac  Specifications
 Crew:   up to 10 (tow 12 skiers)
  Length: 6.09 m, Width:
  1.58 m, Height: 1.52 m
  65hp 2.3L GM Corvair
  air-cooled 'boxer' 6-cyl
  Dana 4-speed trans.[5]
  Over snow: 50 km/h, in
  water: 3 km/h (loaded)
 Weight:   GVW: 3300 kg [6]

Their use of  aircraft-quality materials came back to haunt Canadair. The US Army did buy 57 trial XM-571s [7]  but  further CL-91 sales were restricted to two each for the Canadian and British armies. No one, including Canadair brass regarded that as a success. Vietnam- based XM-571s were used as utility carriers with some success but never in their intended role as ammuntion carriers with the frontline artillery units. The CL-91 was simply too expensive. The Australian Army wanted 384 CL-91s [8]  but  this 'small order' was rejected by the Canadian government. Canadair pressed on with a Dynatrac II but, by then, Canadair brass had lost patience [9] with its vehicle division which closed down in 1976.

[1] Other anticipated roles were Electronic Countermeasures and reconnaissance (rear-mounted M40 106mm recoilless rifles being trialled).
[2] This followed the RAT approach but, for the CL-91, GM specially adapted an industrial model of  Chevrolet's Corvair flat-6 engine for Canadair.  As expected in an industrial powerplant, these engines have heavy-duty features  –  including  much enlarge oil pans, fans, etc.
[3] For the Canadian patent  Articulated Joint Assembly  (CA 713891, issued 1965-07-20), Thomas is listed as inventor, Canadair as owner.
[4] The XM-571 is also referred to as an Articulated, Utility Carrier or  even as the XM-571 Carrier, Utility, F-T [Fully Tracked], Articulated.
[5] According to Corvair specialist Dave Newell, the first 10 CL-91s had 4-spd Corvair van transmissions (GM also provided the two-spd transfer case). Other changes to the Dynatrac's Corvair engine included type-specific cooling shrouds, exhaust manifolds, oil filter, deep- sump oil pan, and 24-volt military generator. Engines for the CL-91 Dynatracs were all built at the GM Powertrain Plant in Tonawanda, NY.
[6] Curb weight for the CL-91 Dynatrac was 2215 kg (4870 lb). Payload was 900 kg (2000 lb) –  or 1365 kg (3000 lb) in overloaded condition.
[7] At Aberdeen, the CL-91 tested successfully against 22 competitors. There followed mobility tests at Fort Bragg NC, then cold-weather trials in Alaska. Overseas trials were conducted first in Thailand in 1966, then Vietnam in 1969  (with elements of the 1st and 25th Inf Divs).
[8] Australia wanted 500 CL-91s but the Army could only afford 384 vehicles at first. Canadian trade representatives decided that an order for 384 was insufficient to warrant full-scale production.  This was rich considering that DND had only ordered two Dynatracs!  However, at that time, such decisions were made by the Ministry of Defence Production  – other federal government involvement being the Defence Research Board, and the US/Canada Defence Production Sharing Agreement (1956) and Defence Development Sharing Agreement (1963).
[9] The Dynatrac II was a product of Calgary-based Canadair Flextrac Ltd  (formed with personnel from Flex-Track Nodwell ). Three proto- types of this larger Dynatrac IIs were built but they showed no improvement over existing FN designs. Canadair Flextrac was sold in 1976.