page_title: Lockheed AH-56 Cheyenne
section_title: Production contract canceled
hierarchical_section_title: Lockheed AH-56 Cheyenne / Operational history / Production contract canceled
context_page_description: The Lockheed AH-56 Cheyenne was an attack helicopter developed by Lockheed for the United States Army. It rose from the Army's Advanced Aerial Fire Support System program to field the service's first dedicated attack helicopter. Lockheed designed the Cheyenne using a four-blade rigid-rotor system and configured the aircraft as a compound helicopter with low-mounted wings and a tail-mounted thrusting propeller driven by a General Electric T64 turboshaft engine. The Cheyenne was to have a high-speed dash capability to provide armed escort for the Army's transport helicopters, such as the Bell UH-1 Iroquois.
  In 1966, the Army awarded Lockheed a contract for ten AH-56 prototypes, but as a stopgap also ordered the less complex Bell AH-1G Cobra as an interim attack aircraft for combat in Vietnam War. The AH-56's maiden flight took place on 21 September 1967. In January 1968, the Army awarded Lockheed a production contract, based on flight testing progress. A fatal crash and technical problems affecting performance put Cheyenne development behind schedule, resulting in the cancellation of the production contract on 19 May 1969.
context_section_description: The Army issued a cure-notice to Lockheed on 10 April 1969, citing 11 technical problems, and unsatisfactory progress on the program. The main issues were the half-P hop vibration issue, and the aircraft gross weight exceeding program requirements. In response, Lockheed proposed an "improved flight control system" (ICS) to reduce rotor oscillations, and steps for removing excess weight and addressing other minor issues in production helicopters. The Army felt Lockheed's solutions to the cure-notice issues would delay the program and increase costs. Citing Lockheed's inability to meet the production timeline, the Army canceled the AH-56 production contract on 19 May 1969, but retained the development contract in hopes that the issues could be resolved.
  In September 1969, Cheyenne prototype #10 (s/n 66-8835) underwent wind tunnel testing at NASA Ames Research Center, to research the half-P hop and drag issues. The engineers did not realize that the fixed mounts used to secure the aircraft in the wind tunnel would not allow the helicopter to move relative to the rotor, as it did in flight. As a result, there was no natural damping of the rotor pitching motion. The remote controllers' lack of sensory feedback from helicopter compounded the situation. During high speed testing to replicate the half-P hop vibration, the rotor oscillations quickly accelerated out of control and struck the tail boom, resulting in the destructive breakup of the helicopter.
  Lockheed worked on modifying the AH-56 design to address the vibration and other issues. As a precaution, Cheyenne #9 (s/n 66-8834) was fitted with an ejection seat for the pilot after the March accident. The downward firing ejection seat was placed in the forward seat in place of the gunner's station. This prototype would be used for all remaining envelope expansion flights. Prototype #9 also received an upgraded transmission and drivetrain, and a hinged rear canopy in place of the original sliding canopy around 1970. The new transmission allowed the T64-GE-16 turboshaft engine output to be increased from a derated 3,435 horsepower (2,561 kW) to 3,925 horsepower (2,927 kW). The new canopy eliminated the canopy vibrations.
  Cheyenne prototype #6 (s/n 66-8831) began conducting weapons testing at Yuma Proving Ground, Arizona, demonstrating the ability for the gunner and pilot to accurately fire on separate targets on each side on the helicopter. Towards the end of 1970, the Army funded work on TOW missile guidance and night sighting systems. Prototypes #6 and #9 were also tested and evaluated at Yuma Proving Ground from 30 January to 23 December 1971, to determine if stability and control systems were sufficient. Deficiencies were identified in lateral directional stability, uncommanded motion during maneuvering, high vibration, and poor directional control during sidewards flying.
  Following the testing at Yuma, prototype #9 received the improved T64-GE-716 engine producing 4,275 shp (3,188 kW) and the planned production version of the ICS system. With these upgrades, the helicopter surpassed its performance requirements. However, under certain conditions stability and control did not completely satisfy the test pilots. Lockheed had studied ways to prevent unstable feedback from the gyro. The solution was to relocate the gyro from the top of the rotor head to below the transmission with flexible connections to the rotor. The pilot's controls were connected to hydraulic servomotors then connected through springs to the gyro. This system prevented rotor vibration forces from transmitting back into the flight controls. It was called the "advanced mechanical control system" (AMCS) and was installed on Cheyenne #7 in 1972 to improve handling and rotor stability.
caption: An AH-56 hovering over a helipad
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