Staff Writer (Updated: 4/25/2016):
The story of the CH-47 began in the mid-1950s with the U.S. Army looking to replace its stock of Sikorsky CH-37 "Mojave" transport helicopters. Production of this system - which was not an outright success due to operating expenses, size, and reliance on piston-driven engines - was limited to just 154 units. Service began in July of 1956 and ended relatively quickly in the late 1960s. The turbine engine soon arrived and replaced piston engines in helicopters through such successful products as the Bell UH-1 "Huey" medium-lift transport helicopter so thought was no being given to an all-new, medium-lift system based on the more powerful and capable drive units.
Various contractors were in play early on to deliver a solution to the ongoing Army requirement. To help sell the idea of a new turbine -powered platform, Vertol engineers fleshed out their "V-107" of 1957 and this was enough to prove the investment as a formal contract followed the following year to be based on the developmental "YHC-1A" designation. Three prototypes would be featured for evaluation.
However, the YHC-1A failed to sell itself to the Army after testing and was instead taken on by the USMC as the CH-46 "Sea Knight" tandem rotor transport. The decision was then made to simply develop a dimensionally larger version of that same helicopter and this formally became company Model 114 and assigned the Army designation of "HC-1B". A first flight (a hovering action) followed on September 21st, 1961.
Due to the 1962 redesignation initiative covering all aircraft then in American service, the HC-1B was redesignated to the more common "CH-47" with its initial production model now recognized as the CH-47A. It was granted the name of "Chinook" in keeping with Army tradition of naming its helicopters after Native American tribes. The CH-47A entered service in August of 1962 to begin its long and storied career.
As completed, the CH-47 was an easily identifiable aircraft with its twin main rotor design. Each rotor utilized a three-bladed assembly seated atop mast mountings, the rotors arranged as counter-rotating units which eliminated the need for a tail rotor unit (the natural occurring torque of a spinning rotor was offset by another rotor spinning in the opposite direction). The high placement of the main rotors also cleared all areas immediately around the aircraft for ground personnel - no immediate danger was to be had from a spinning tail rotor so common to other helicopter transports. The cockpit was fitted well-forward in the design with excellent vision for the two pilots. The fuselage was rectangular in its general shape as seen from the side profile and dotted with vision ports. At the rear of the fuselage was a large, powered cargo door that lowered to double as the loading ramp to provide unfettered access to the cargo hold within. The engines were fitted outboard of each rear fuselage side to provide the needed forward thrust while also driving the two main rotors. The undercarriage relied a four-legged system, fixed during flight, and wheeled. Internally, the helicopter could seat up to 55 troops, 24 medical litters with medical crew, or an equal amount of cargo. An external cargo hook allowed the helicopter to sling loads under the fuselage to double its carrying capability in-the-field. The original engines were Lycoming T55-L-7 turboshaft types of 2,650 horsepower (each) with two being fitted.
The United States Army took on a total of 354 CH-47A models and these were then followed into service by the improved CH-47B. The B-model would become a "bridge" offering of sorts between the original A-models and the much improved C-model form still to come. The CH-47B fitted 2 x Lycoming T55-L-7C engines of 2,850 horsepower each as well as new, asymmetrical rotor blades. The rear rotor mounting was redesigned and there was inherent support for the M60 General Purpose Machine Gun (GPMG) mounted at the side doors for local defense/suppression. The cargo ramp at rear was also modified to support the M60D/M41 weapon station - the CH-47B flying with its cargo ramp down to allow this gunner to engage trailing threats. Its greater overall power made it a better medium-lift performer.