Four bladed success
The Bell 412
One of the design elements that has made the earlier Huey models so useful to land and ship based armed forces was the two bladed main rotor. This allowed the helicopter to be carried in transport aircraft without removing the blades and minimised the deck and hangar space on carriers and other ships that the Huey required without the need to include a blade folding mechanism, common on larger helicopters. This meant that Hueys on deployment arrived almost ready to go, speeding their entry into service in any overseas theatre. The Huey’s two-bladed main rotor had steadily grown in chord and in length, from 44ft (13.41m) span on the XH40 to 52ft (15.85m) on the 214ST. This had been for two main reasons, to absorb the increasing power of the developing turbine engines fitted to the helicopter, and in pursuit of the holy grail of aircraft design, to increase the performance to meet customer demands. The development of the two bladed rotor had reached its limits, particularly in terms of reducing noise and vibration, problems long associated with it, which meant that on September 8, 1978, Bell announced the development of a new four bladed design based on the Bell 212.
Two Bell 212 airframes were taken from the production line and modified to become the prototypes of the 412, retaining the 1800hp Pratt and Whitney PT6T-3B Turbo Twin Pac powerplant driving the new lightweight composite rotor. The rotor head had elastomeric bearings like those of the late model two bladed rotors, these removing the need for mechanical hinges and dampers, so greatly simplifying the system as well as reducing the weight. The fact that this was a relatively straightforward modification of the existing design meant that the first flights of the two prototypes occurred less than a year later, in August and December of 1979, immediately showing tremendous improvements in the noise and vibration levels experienced in the cabin. Basing the design on an existing helicopter also meant that Federal Aviation Administration (FAA) certification was swift, Type Certification being approved on January 9, 1981, the first deliveries of production helicopters occurring only nine days later to Alaska based ERA Helicopter Inc. Aside from the noise and vibration improvements, the Bell 412 proved to have a better climb performance and a higher service ceiling than the two bladed 212, as well as a much improved maximum cruising speed of 143mph (230kph) over the original model’s 115mph (186kph). The sheer utility of the Huey design remained in the 220cu ft cabin with its large two piece doors enabling easy access, this still being a massive internal space for a helicopter of this class which could be configured with up to 13 passenger seats or any combination of seats and equipment depending on the customers needs. During the early 1980s, Bell responded to customer feedback by introducing the Model 412SP or Special Performance. This was fitted with the PT6T-3BF powerplant with its improved single engined performance and was modified to provide an increased maximum take off weight and featured new internal layouts and seating options. The biggest change was the increase in internal
The Bell 412 was the first version of the Huey to replace the traditional two bladed main rotor with a four bladed one. Although Bell had produced experimental variants with more than two blades, this was the company’s first production helicopter with this feature.
fuel capacity to extend the range and loiter time, which increased military interest in the new version, attracting orders from Honduras and Norway among many others. In 1984, airframe vibration was reduced still further with the introduction of a pendulum damper system on the rotor head of aircraft on the production line, which was also offered as an upgrade kit to existing helicopters. In June 1986, an armed version of the 412SP was proposed, a single example, N412AH, being produced as a demonstrator. Known as the 412AH for Attack Helicopter, it could carry a 19 round rocket pod on either side of the cabin and was fitted with a Lucas Aerospace turret under the nose. This housed a .50 cal machine gun with 875 rounds and was slaved to a helmet mounted Sperry Head Tracker sight from the AH-1S Cobra which enabled the pilot to ‘look and shoot’. Despite some interest in the concept, no orders for the 412AH were forthcoming. As with other Bell types, production of the 412 was transferred to the Canadian factories in January 1989 to free up space at Fort Worth. The development of the PT6T engines and their improved transmission led to the 412SP being replaced on the production lines by the 412HP (High Performance) in 1991, the new version being fitted with the -3BG or 1920hp -3D versions of the Pratt and Whitney powerplant. Currently, there are two versions of the Bell 412 in production, the first being the 412EP (Enhanced Performance), introduced in 1993 and fitted with the -3DF version of the engines. The new model was also fitted with a dual digital automatic flight control system which included both an automated approach to hover and automated hover facilities. This model was the basis of a customised development for the Royal Canadian Air Force known as the 412CF, designated the CH-146 Griffon in service. Ordered in 1992, 100 of this version were delivered to the RCAF between 1995 and 1997, used in the search and rescue, combat support and tactical transport roles with 10 RCAF Squadrons and not expected to be retired until at least 2021. The Bell 412EP has also been the basis of two versions acquired by the Royal Air Force and various versions of the aircraft have been produced under licence in Indonesia and Italy, all of which will be covered later in this issue. The other version still in production is the 412EPI, fitted with the 2143hp PT6T-9 powerplant which features a Full Authority Digital Engine Control (FADEC) system. The other avionics and electronics have been similarly upgraded, the 412EPI featuring the Bell Basix Pro Integrated Avionics System in a completely modernised glass cockpit. The airframe has also been modified in association with BLR Aerospace, the fin changing shape to a much narrower chord with a curved trailing edge known as the ‘Fast Fin’, which, when coupled with the dual strakes fitted along the lower faces of the tailboom offer improved fuel efficiency, handling, tail rotor effectiveness and crosswind tolerance by altering the airflow around the rear of the helicopter. This aerodynamic improvement has also been introduced on to the 412EP and is available as an upgrade to earlier versions as it significantly improves the hot and high performance of the helicopter and increases the useful load in these conditions. The latest versions of the design are incredibly advanced aircraft and are a fitting continuance of the Huey family line.
The Bell 412EP sold worldwide, this being a Nigerian registered example equipped with the emergency flotation devices on the landing skids.
Left: The first customer for the Bell 412 was Era Helicopter Inc, operating in Alaska among other states. The expanded performance of the Bell 412EP made it popular with fire and rescue departments throughout the US and Canada, such as the San Diego Fire Department as seen in action here.
An interesting comparison between the early instrumentation, in this case as fitted to an RAF Griffin HT.1 of the Search and Rescue Training Unit, a type based on the 412EP, and the latest advanced glass cockpit of the Bell 412EPI.
Eighteen Bell 412SPS were purchased by the Royal Norwegian Air Force, all of which are still in service in 2014. Here a flight inserts a team of US Marines at the port of Orkanger during NATO exercise Strong Resolve 2002.
The Royal Canadian Air Force CH-146 Griffon was based on the Bell 412SP and is used in the combat support, transport and search and rescue roles.