CH-47D Chinook Helicopter: Development, Design & Future

The Development Process

This paper examines the CH-47D helicopter, focusing on five aspects of its development process through to the implementation stage. It looks at how the helicopter was developed, the reasons behind its creation, its future, its significance, and the major issues experienced during development and how they were overcome.

The Department of Defense announced in 1956 plans for the replacement of the CH-37 helicopters with an improved CH-47D. This decision was reached after several Army aviation corps voted in favor of creating helicopters powered by turbine engines. The Army also recommended that the new helicopters be large enough to airlift heavy military equipment (Spenser, 1998).

Following the Army's decision to use larger and more powerful helicopters, Vertol was contracted to begin designing and building a new tandem-rotor helicopter known as the V-107 in 1957. The following year, Vertol was further contracted to design the YHC-1A — the V-107's successor — with a capacity to carry up to 20 troops. The YHC-1A appeared better enhanced than the V-107; however, it was considered too heavy for air assaults and too light for transport. This led the Army to procure a heavier transport helicopter. Consequently, modification and improvement of the YHC-1A commenced, paving the way for the CH-46 Sea Knight in 1962, which was adopted by the U.S. Marines.

During the same period, the U.S. Army ordered a large helicopter, Model 114, referred to as the HC-1B. The HC-1B was later redesignated the CH-47A in 1962 in accordance with the United States Tri-Service Aircraft Designation System. The CH-47A helicopter was propelled by twin turboshaft engines attached at the helicopter's rear and connected by drive shafts to the rotors. These helicopters were fitted with engines producing 2,200 horsepower (Anderton & Miller, 1989).

The U.S. Army subsequently opted to upgrade the CH-47A, leading to the creation of the more powerful CH-47C. The CH-47C featured upgraded engines, composite rotor blades, a redesigned cockpit to reduce pilot workload, improved and redundant electrical systems, an advanced flight control system, and improved avionics. This overhaul of the CH-47C ultimately led to the creation of the CH-47D. The CH-47D was further refined to reduce maintenance costs and to digitalize flight controls.

The CH-47D upgrade program was initiated in 1976 to bring the Army's fleet of CH-47A/B/C helicopters to a common standard. The first flight of a prototype rebuilt CH-47D occurred in 1979, and after evaluations and testing, the Army placed its first production CH-47D order in 1980, with the first production aircraft flying in 1982. The CH-47D officially entered service with the U.S. Army in 1984.

During the upgrade process, the aircraft were stripped down to the airframe and rebuilt from scratch. Upgrades included Honeywell T55-L-712 turboshaft engines, improved and uprated transmissions, improved avionics and flight control systems, composite rotor blades, and a triple external cargo hook system. Equipment reliability was also improved by simplifying the transmission lubrication system and rerouting hydraulic lines to reduce potential leakages.

Although originally fitted with twin T55-L-712 engines, current CH-47D models are equipped with T55-GA-714A engines. With the three-hook cargo system, the CH-47D can carry heavy internal loads of more than 25,000 pounds at speeds exceeding 150 mph. The D-model serves as the principal mover of the 155mm M198 howitzer — which fires 30 rounds per minute — during air assault operations. Like most U.S. Army helicopters, the CH-47D carries advanced avionics and electronics, including the Global Positioning System (GPS).

Reasons for the Development

The first in the long line of CH-47D predecessors was the YHC-1B tandem-rotor transport helicopter, which entered service in 1961. It was designed to serve the Army and Air Force as a medium-lift helicopter and evolved into several variants. Immediately after the Gulf War, Boeing and the U.S. Army developed a plan for a major fleet upgrade that led to the CH-47D. During the upgrade period, more than 500 early models underwent an extensive modernization process in Philadelphia, producing an entirely new CH-47 fleet (Dunstan, 2003).

The CH-47D was developed because predecessors such as the YHC-1A had tandem-rotor configurations that were not fully effective in flight. Earlier models also suffered from poor avionics and communication systems and low-power engines producing only 2,200 horsepower — insufficient to meet Army requirements. Furthermore, former helicopter engines could not handle a 25,000-pound load during troop transport missions (Combet, 2010).

The YHC-1A and its predecessors were also considered too heavy for assaults and too light for transport, leading to the procurement of a heavier helicopter suitable for both roles (Frawley, 2002). This led to modification and improvement of the YHC-1A into the CH-46 Sea Knight in 1962 for the U.S. Marines, and these improvements ultimately contributed to the creation of the CH-47D.

The CH-47D was created in part because it could be readily upgraded to meet evolving Army and Air Force specifications. It was designed to provide increased engine power, improved and uprated transmissions, and better avionics and flight control systems. Reliability was also a key driver: by rerouting hydraulic lines and simplifying the transmission lubrication system, leakages were reduced, lowering both operational and maintenance costs.