|Bell Model 208 / Model 212 / UH-1N|
At the beginning of the 1960s, the Bell design team considered that the only way to give the UH-1 more power was to install two engines. In 1964, the conversion of a standard UH-1D to a twin-engined configuration was funded by Bell. Work was done in Fort Worth, and by the spring of 1965, the prototype Model 208 Twin Delta was ready to fly. Its powerplant comprised two Continental T72-T-2 Model 217 turbo-shafts mounted side by side and driving, via a combining gearbox, a single output shaft. Designated XT67-T-1, this powerplant offered 1,240shp for take-off. Besides offering more power, this arrangement also increased safety, an important factor in off-shore and rescue operations.
The maiden flight was made on 29 April, 1965. The new helicopter aroused interest on the US civil market but the very first orders came from Canada. On 1 May, 1968, Bell announced that the Canadian Government had approved the development of the twin-engined variant of the UH-1. This variant, known as Model 212, was to be powered by a 1,290shp Pratt & Whitney Canada PT6T-3 Twin Pac powerplant driving a two-blade all-metal semi-rigid rotor. The new helicopter could accommodate a pilot and up to fourteen passengers and, in its military variant, armament could include 7.62mm MAG pod machine-gun systems, 70mm air-to-ground rocket launchers (7 or 19 rounds), side-mounted 12.7mm or 7.62mm machine-guns. The order for a first batch of fifty helicopters was placed by the Canadian Armed Forces (under designation CUH-1N, later modified to CH-135) and the first Model 212 flew in April 1969. More orders soon followed, both civil and military: seventy-nine UH-1Ns for the US Air Force; forty UH-1Ns for the US Navy and twenty-two UH-1Ns for the US Marine Corps; 159 more being ordered later by the US Navy and Marine Corps.
The commercial version (Model 212 Twin Two-Twelve) received its FAA type certification in October 1970 and the FAA Transport Type Category A certification on 30 June, 1971. In June 1977, this Model became the first helicopter to receive FAA certificadon for single-pilot IFR operations with fixed floats. In June 1980, an improved variant of the PT6T-3, the PT6T-3B, was introduced which besides giving more power (1,800shp), also offered improved single-engine performance.
In mid-1988, producdon of the Model 212 was transferred to Bell's Canadian factory and, at the time of writing, more than 1,000 Model 212/UH-1N/CUH-1Ns have been produced. Among the main operators are Helikopter Service A/S (Norway), the Canada Department of Transport (Ottawa), ERA Helicopters (Anchorage, Alaska), the Connecticut National Bank (Hartford) and Grenlandsfly A/S (Greenland).
The Model 212 also entered production with Agusta in Italy under the AB 212 designation. The AB 212 is generally similar to its American counterpart. In addition Agusta developed on its own an ASW naval version, called AB 212AS. The AB 212AS is an extensively modified variant which incorporates a 1,875shp Pratt & Whitney Canada PT6T-6 Turbo Twin Pac powerplant, a strengthened structure protected against corrosion, provision for auxiliary fuel tanks and improved avionics and equipment for ASW missions (such as a Bendix AN/AQS-13B/F low-frequency variable depth sonar). Modifications were also made to the flying controls, fuel and electrical systems. Armament comprises two Motofides 244AS or two Mk.44/46 torpedoes, or two Marte Mk.2 or Sea Skua air-to surface missiles.
A.J.Pelletier "Bell Aircraft since 1935", 1992
The US Forces and Bell were quick to see the advantages of fitting twin engines to their successful Model 205 / UH-1D. It would not only enhance performance and power but would also give increased safety, allowing a single-engine reserve especially for those operating at sea. In 1965 Bell flew a one off prototype Model 208 fitted with twin T72 engines. Canada, already a user of the UH-1D, was interested in developing its own power unit for the Huey. Pratt & Whitney developed their PT6T-3 Twin Pac powerplant and it was fitted to a UH-1D in April 1969. The first orders for this version came from Canada who ordered fifty designated CUH-1N, quickly followed by the US Air Force, Navy and Marine Corps designated the Model 212 UH-1N. Powered by twin Pratt & Whitney PT6T-3 turboshafts developing around 1800shp, positioned side by side and driving, via a combining gearbox, a single output shaft. This allowed for a single-engine operation and combined to sensing torque-meters within the combining gearbox, it could signal to the remaining turbine for additional power in case of failure. The 212 proved successful in both the military and civil roles with over 1200 operating worldwide. Agusta has also produced the AB212 under licence developing an ASW version for the Italian Navy, plus other variants for their overseas customers.
P.Allen "The Helicopter", 1996
The American armed forces, by far the biggest users of the Huey series of helicopters, informed Bell that a twin-engine solution, even with the same power rating, would have advantages in terms of reliability and crew morale, above all on missions over the sea or enemy territory. Accordingly, Bell produced its first twin-turbine powered helicopter at the beginning of 1965, derived from a modified Model 205. The programme was financed by Bell and did not lead to any orders, but when the Texan company decided to fit the helicopter with a PT6T-3 powerplant manufactured by Pratt & Whitney of Canada, the Canadian government announced its support for development of the new helicopter. Shortly afterwards, the Canadian armed forces ordered 50 Bell 212s and the American armed forces ordered 141 (79 for the USAF, 40 for the US Navy and 22 for the US Marine Corps).
The model 212 has an identical fuselage to the Model 205, except for the very compact side-by-side twin engine installation and the nose which is 15cm longer. It has also been produced for the commercial market. In addition, it has been manufactured under license in Italy by Agusta, who built a demonstration prototype in 1971, initially in a civil configuration for VIP transport, but later transformed into a military configuration (corresponding to the UH-1N). The aircraft can fulfil various roles, being suited to hot climate and high altitude as well as civil operation. The Italian Air Force became interested in the Agusta AB-212 and acquired a few for air base rescue services.
Agusta's biggest undertaking was the redesigning of the original airframe to produce the shipboard AB-212 ASW version as a replacement for the old AB-204 employed by the Italian Navy. This version was well-received and has been exported to Greece, Turkey, Peru, and Spain. Distinctive features of the AB-212 antisubmarine helicop-ter include a special anticorrosive treatment for the powerplant, a search radar housed in a dome on the cabin roof, special electronics, sonar, antisubmarine and antiship armament, consisting either of two Mk.44 antisubmarine torpedoes, two AS-12 missiles, or two antiship missiles, depth charges and flares. The crew consists of two pilots and two operators.
More recently, the 212 has also been fitted with a new, four-blade rotor with redesigned blades. This variant, designated Model 412, has been in production in the USA and Italy since 1981. The four-blade rotor can also be fitted to the old 212s.
G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984
On 1 May 1968 the Bell Helicopter Company announced that following negotiations with the Canadian government and Pratt & Whitney Aircraft of Canada, it had been agreed to proceed with the development of a new helicopter based upon the airf rame of the Bell Model 205/ UH-1H Iroquois, of which the first of 10 for the Canadian Armed Forces (CAF) had been delivered on 6 March 1968 under the designation CUH-1H. Power-plant of the UH/CUH-1 consisted of a 1044kW Avco Lycoming T53-L-13 turboshaft engine. The CAF considered that the incorporation of twin turboshaft engines would provide a number of benefits, and this led to development of the initial military Bell Model 212 and the Pratt & Whitney Aircraft of Canada (PWAC) PT6T power-plant for it. This programme was initiated as a joint venture, financed by Bell, the Canadian government and PWAC.
The revolutionary feature of this new helicopter was its powerplant, the PT6T Twin-Pac designed and developed by PWAC, and consisting of two turboshaft engines mounted side-by-side and driving, via a combining gearbox, a single output shaft. This had an output in its initial production form of 4.66kW/kg, compared with 4.19kW/kg for the already-developed Lycoming T53 turboshaft. There was another very considerable advantage: as installed in the Model 212 the PT6T-3 is limited to an output of 962kW for takeoff. In the event of a failure of one of the two turbines, sensing torquemeters in the combining gearbox signal the remaining turbine to develop a power output ranging from 764kW to 596kW, for emergency and continuous operation respectively.
Initial deliveries of military Model 212s were made to the USAF in 1970, under the designation UH-1N, and deliveries of UH-1Ns for the US Navy and Marine Corps began in 1971. The first CUH-1N (later redesignated CH-135) for the CAF was handed over on 3 May 1971. Examples have been supplied also to the air forces of Argentina (eight) and Bangladesh (six). The airframe is generally similar to that of the UH-1H Iroquois, with an all-metal fuselage structure, skid landing gear, and rotor systems comprising a two-blade all-metal semi-rigid main rotor, and a two-blade tail rotor.
A 14-passenger commercial version known as the Twin Two-Twelve was developed more or less simultaneously, this differing from the military model primarily in its cabin furnishing and avionics equipment. The Twin Two-Twelve gained FAA Transport Type Category A certification on 30 June 1971, and the type has since gained certification for IFR operation, requiring a new avionics package, new instrument panel, and stabilisation controls for such use. In June 1977 it became the first helicopter to be certificated by the FAA for single-pilot IFR operation with fixed floats.
The enhanced safety offered by the Twin-Pac powerplant has resulted in many sales to operators who provide support to offshore gas/oil prospecting and production companies, as well as to air taxi organisations. Eight Model 212s were delivered to the Civil Air Authority of China during 1979, and were the first helicopters supplied to the People's Republic of China by a US manufacturer.
The Model 212 is also built under licence by Agusta in Italy as the Agusta-Bell AB.212, with initial deliveries made during 1971. These are generally similar to the American-built aircraft, but Agusta has developed an ASW version designated AB.212ASW. Structural strengthening took place, plus the provision of deck restraint gear, and the PWAC PT6T-6 Twin-Pac turboshaft rated at 1398 kW for take-off.
D.Donald "The Complete Encyclopedia of World Aircraft", 1997
Canadian government approval to develop twin-engined UH-1 with P&WC PT6T-3 Twin-Pac announced 1 May 1968; more powerful PT6T-3B introduced June 1980; manufacture transferred to Canada August 1988.
CH-135: Canadian version, originally CUH-1N. Total 50 delivered 1971/72.
UH-1N: US Air Force, Navy and Marines version. Total 320 delivered.
UH-1Y: Dynamic and avionics upgrade.
Twin Two-Twelve: Civil version; first flight 16 April 1969; FAA certification 30 October 1970; FAA Category A transport certification 30 June 1971; IFR certification granted by FAA, UK CAA, Norwegian DCA and Canadian DoT; first-ever single-pilot IFR certification with fixed floats granted June 1977.
AB212: Agusta-built version.
AB212ASW: Agusta-built version for anti-submarine warfare.
Description refers to 212 version.
DESIGN FEATURES: All-metal two-blade semi-rigid teetering main rotor with interchangeable blades; underslung feathering axis head; rotor brake optional.
FLYING CONTROLS: Fully powered hydraulic controls; gyroscopic stabiliser bar above main rotor; automatic variable incidence tailplane; IFR versions have large fin above cabin to improve roll-yaw responses during manual instrument flying.
STRUCTURE: Metal main rotor blades have extruded aluminium nose sections and laminates; glass fibre safety straps provide redundant load path; fuselage conventional light metal.
LANDING GEAR: Tubular skid type. Lock-on ground handling wheels, high skid gear, fixed floats and emergency pop-out nylon float bags optional.
POWER PLANT: Pratt & Whitney Canada PT6T-3B Turbo Twin-Pac, comprising two PT6 turboshafts coupled to combining gearbox with single output shaft. Engine rating 1,342kW for T-O, 1,193kW maximum continuous; OEI rating 764kW for 2.5 minutes, 723kW for 30 minutes. Transmission rating 962kW for T-O, 846kW maximum continuous and 764kW OEI. Five interconnected rubber fuel cells, total usable capacity 818 litres. Two 76 or 341 litre auxiliary fuel tanks optional, to provide maximum possible capacity of 1,499 litres. Single-point refuelling on starboard side of cabin. Oil capacity 11.5 litres for engines, 8.5 litres for transmission.
ACCOMMODATION: Pilot and up to 14 passengers. Dual controls optional. In cargo configuration, has total internal volume of 7.02m3, including baggage space in tailboom, capacity 181kg. Forward-opening crew door each side of fuselage. Two doors each side of cabin; forward (jettisonable) door hinged to open forward, rear door sliding aft. Accommodation heated and ventilated.
SYSTEMS: Dual hydraulic systems, pressure 69 bars each, maximum flow rate 22.7 litres/min. Open reservoir. 28V DC electrical system supplied by two completely independent 30V 200A (derated to 150A) starter/generators. Secondary AC power supplied by two independent 250VA single-phase solid-state inverters. A third inverter can acquire automatically the load of a failed inverter. 40Ah Ni/Cd battery. AiResearch air cycle environmental control unit optional.
AVIONICS: Comms: Optional IFR avionics include dual AlliedSignal KTR 908 720-channel VHF com transceivers.
EQUIPMENT: Optional equipment includes a stretcher kit, cargo hook, cargo sling and rescue hoist.
Jane's Helicopter Markets and Systems
Technical data for Bell Model 212
Engine: 1 x PWAC PT6T-3 Turbo Twin-Pac turboshaft, rated at 962kW, main rotor diameter: 14.69m, length with rotors turning: 17.46m, height: 4.53m, take-off weight: 5080kg, empty weight: 2786kg, cruising speed: 230km/h, service ceiling: 4330m, range: 420km