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YHC-1A / HC-1B Chinook

Development of the medium lift Boeing Vertol (models 114 and 414) CH-47 Series Chinook began in late 1956, when the Department of the Army announced plans to replace the H-37 helicopter, which was powered by piston-driven engines, with a new, turbine-powered aircraft. A design competition was held and, in September 1958, a joint Army-Air Force source selection board recommended that the Army procure the Boeing Vertol medium transport helicopter. However, the necessary funds to proceed with full-scale development were not available and the Army vacillated in its design requirements. There were those in the Army who felt that this new helicopter should be a light tactical transport aimed at the mission of the old H-21's and H-34's and, consequently, sized for approximately fifteen troops. Another faction believed that the new transport should be much larger to serve as an artillery prime mover and have minimum interior dimensions compatible with the Pershing Missile system. This "sizing" problem was a critical decision.

On June 25, 1958 the Army issued an invitation for a General Management Proposal for the US Army Medium Transport Helicopter. Five aircraft manufacturers submitted proposals for the project. Vertol was selected to produce the YCH-1B as the Army's new medium transport helicopter. The first Vertol prototype, called the YHC-1A, was tested by the Army to derive engineering and operational data. Three aircraft were built with a maximum troop capacity of twenty. This model eventually became Vertol's commercial 107 and the Marine Sea Knight.

The YHC-1A was considered by most of the Army users to be too heavy for the assault role and too light for the transport role. The decision was made to procure a heavier transport helicopter and at the same time upgrade the Huey as a tactical troop transport. This decision was to determine the pattern of airmobile operations for the next decade. As a consequence, the Army concept of air assault operations differed from the Marines because, among many reasons, the very nature of the equipment demanded different methods of employment.

The "sizing" of the Chinook was directly related to the growth of the Huey and the Army's tacticians' insistance that initial air assaults be built around the squad. There was a critical stage in the Huey program when the technicians insisted that the Army should not go beyond the UH-1B model with Bell; that there should be a new tactical transport "between" the Huey and medium transport helicopter. The early Bell XH-40 had been standardized as the HU-1 and was envisioned then as the replacement for the L-20 utility airplane and the H-19 utility helicopter. Further growth versions of the Bell machine were planned to replace the bulk of the missions then performed by the Sikorsky H-84 and the Vertol H-21.

When it was decided to go to the UH-1D (after an awkward pause on the original "C" design), the proper Chinook size became apparent. By resolutely pushing for the Huey and the Chinook, the Army accelerated its airmobility program by years. The Army finally settled on the larger Chinook as its standard medium transport helicopter. The Vertol HC-1B Chinook put on the drawing boards to replace the piston powered Sikorsky H-87.

The YHC-1A is a twin turbine, tandem rotor, tactical transport helicopter powered by two General Electric T58-GE-6 free turbine engines. It represents an advance in the art of helicopter design due to its excellent handling characteristics, positive dynamic stability, and low vibration levels at high speed. The major undesirable system (SAS) for adequate stability and control, inability to utilize full engine power due to transmission torque limits, excessive rpm droop during power application which can lead to the loss of generator output and a dual SAS failure, and slow beep trim rate. Many of the problem areas existing in the YHC-1A were eliminated by design changes for the Vertol 107 Model II which was in production.

A preliminary design study was conducted by Boeing Vertol in 1961 to establish a tandem helicopter configuration capable of the following minimum performance: (1) 1600 nautical miles ferry range at zero headwind with 1 hour fuel reserve, (2) 200 mph speed with a minimum payload of 800 pounds, and (3) flying qualities to meet MIL-H-8501. Aerodynamic parametric studies and basic design layouts indicated that configurations derived from either the Boeing-Vertol 107 series or YHC-1B (Chinook) series could meet or exceed these performance requirements. Both blade radius and chord must be increased slightly, for either series, to insure that the ratio of payload to empty weight is maintained in going to the high performance configuration. Drag reduction and reduced blade loading were required to meet the range and speed requirements, but the installed power was not increased. It was concluded that either the advanced 107 or advanced YHC-1B were capable of the performance requirements and could be used to demonstrate the feasibility of high performance.

By 1962 the HC-1B Chinook helicopter, planned replacement for the H-37A Mojave medium transport helicopter, was being tested at the Vertol Division of Boeing Airplane Company. Chinook was built specifically to carry the Pershing. Its lifting capability was three tons and it would transport a crew of three plus 33 fully-equipped troops, or 24 litters. The Chinook is powered by two turbine engines, making the helicopter capable of a speed of 130 knots. Deliveries under the initial contract began in December 1962.

In July 1962 DoD redesignated all U.S. military aircraft and the HC-1B was redesignated the as the CH-47A.



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