Lockheed AH-56 Cheyenne

Background Prior to the development of the AH-56, all armed helicopters had been modifications to existing aircraft designed for unarmed uses. In 1962, then U.S. Secretary of Defense Robert McNamara convened the Howze Board to review Army aviation requirements. The board recommended an airmobile division supported by 90 armed helicopters. The recommendation of the Howze Board came at the same time the Army was preparing to deploy its first armed escort helicopters to Vietnam; 15 UH-1A Iroquois were modified with systems for mounting machine guns, grenade launchers, and rocket pods. In June 1962, Bell Helicopter presented a new helicopter design to Army officials, in the hopes of soliciting funding for further development. The D-255 Iroquois Warrior was envisioned as a purpose-built attack aircraft based on the UH-1B airframe and dynamic components, with a nose-mounted ball turret, a belly-mounted gun pod, and stub wings for mounting rockets or SS.10 anti-tank missiles. Attack helicopter requirements In December 1962, Combat Development Command (CDC) drafted a Qualitative Material Requirement (QMR) for an interim, commercial off-the-shelf (COTS) aircraft, with a cruise speed and a payload. This was seen as an attempt by Army officials, anticipating the potential of the D-255, to acquire an interim aircraft to fill the escort role until the Army could determine the requirements for a dedicated armed helicopter. However, the Secretary of the Army disapproved the interim approach and directed that the Army look for a more advanced system that would dramatically improve over current helicopter designs. As a result, the Army Material Command (AMC) conducted a study to determine if the development objectives were feasible and also established a program office for the Fire-support Aerial System (FAS). AMC recommended to narrow the competition to compound helicopters, as they were considered the only helicopter configuration at the time capable of being developed to meet the objectives. In March 1964, the Secretary of the Army advised DDRE that modification of existing aircraft would not approach the required performance of the FAS program; the Army would continue using the armed UH-1B until development of the FAS could proceed. Bell submitted the D-262, a modification of the D-255, but still a conventional helicopter design. Sikorsky submitted the S-66, which featured a "Rotorprop" that would serve as a tail rotor but as speeds increased would rotate 90° to act as pusher propeller. Convair submitted their Model 49, a tail sitting coleopter. Lockheed submitted the CL-840 design, a rigid-rotor compound helicopter with both a pushing propeller and a conventional tail rotor mounted at the end of the tail. The Army announced Lockheed and Sikorsky as winners of Project Definition Phase contracts on 19 February 1965. Lockheed and Sikorsky developed proposals for their respective designs, establishing three configurations to satisfy both the development objectives and a revised RFP based on a draft requirements document. An evaluation board studied each company's proposal and then submitted its recommendation to a selection authority council on 6 October 1965. On 3 November 1965, the Army announced Lockheed as the winner of the AAFSS program. The Army perceived Lockheed's design as less expensive, able to be delivered sooner, and a lower technical risk than Sikorsky's Rotorprop. On 17 December 1965, the Army released the final requirements document. The document added fourteen requirements that were not previously addressed by Lockheed's proposal, including the addition of an aerial rocket armament subsystem. On 23 March 1966, the Army awarded Lockheed an engineering and development contract for 10 prototypes, designating the aircraft AH-56A. Initial operating capability was planned for 1972 with an optimistic target of late 1970. Lockheed began construction of the aircraft at its Van Nuys, California facility, and on 3 May 1967, Lockheed held a roll-out ceremony for the AH-56A. The aircraft was christened Cheyenne by the Army. The first flight of the AH-56 occurred on 21 September 1967. The Secretary of Defense approved pre-production funding to support an initial production order for 375 aircraft on 8 January 1968. Manufacture of the 10 Cheyenne prototypes was completed by 1969. ==Design==

, Alabama|alt=A three-quarter rear view of AH-56 #7 on display outside of the U.S. Army Aviation Museum at Fort Rucker, Alabama. Lockheed designed the Cheyenne as a compound helicopter, which combines a helicopter with fixed-wing features for increased performance, usually speed. The design included features such as a rigid main rotor, low-mounted wings, and a pusher propeller. The Cheyenne was powered by a General Electric T64 turboshaft engine. Thrust was provided by a pusher propeller at the rear of the aircraft. At high speeds, the amount of lift provided by the wings, along with thrust from the pusher prop, reduced the aerodynamic loading of the rotor. At such speeds, the rotor produced up to 20% of the lift, which could be adjusted by collective pitch control changes. Rotor tilt was controlled through gyroscopic precession. The Cheyenne achieved speeds over , but as a compound helicopter was unable to qualify for speed records in helicopter categories. The Cheyenne had a two-seat tandem cockpit featuring an advanced navigation and fire control suite. The tandem seating placed the pilot in the rear seat, and the gunner in the front seat. An unusual feature of the gunner's station was that the entire seat, sighting system, and firing controls rotated to keep the gunner facing the same direction as the gun turret being controlled. The gun-sight afforded the gunner direct viewing from the turret by way of a periscope sight. The pilot had a helmet mounted sight system for aiming weapons. Weapon turrets were mounted at the nose and the middle of aircraft underbelly. The nose turret could rotate +/- 100° from the aircraft's centerline and could mount either a 40 mm (1.57 in) grenade launcher, or a 7.62 mm (0.308 in) minigun. The belly turret included a 30 mm (1.18 in) automatic cannon with 360° of rotation. Mechanical stops prevented the belly turret from aiming at any part of the helicopter. ==Operational history==

Flight testing Flight testing began with the first flight of the second AH-56 (s/n 66-8827) in September 1967. As the flight envelope was expanded, this instability and other minor problems were discovered and quickly addressed. Lockheed and the Army held a 13-minute demonstration "first flight" for the public at the Van Nuys Airport on 12 December 1967. During the flight, the Cheyenne demonstrated some of the new capabilities brought about by the thrusting propeller; the helicopter could slow down or accelerate without pitching the nose up or down, as well as being able to pitch the nose down or up at a hover, without causing the aircraft to accelerate forwards or backwards. The Cheyenne demonstrated a stationary hover in a crosswind, and at the end of the flight landed on its two forward landing gear, "bowed" to the audience and then gently set the tail landing gear down as it taxied to parking. By March 1968, the AH-56 had established a flight envelope of in forward flight, sidewards, and rearwards. Production contract canceled The Army issued a cure-notice to Lockheed on 10 April 1969, citing 11 technical problems, and unsatisfactory progress on the program. The Army felt Lockheed's solutions to the cure-notice issues would delay the program and increase costs. but retained the development contract in hopes that the issues could be resolved. 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. 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. 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. Program demise In 1971, political friction increased between the Army and the Air Force over the close air support (CAS) mission. The Air Force asserted that the Cheyenne would infringe on the Air Force's CAS mission in support of the Army, which had been mandated with the Key West Agreement of 1948. The Department of Defense (DOD) conducted a study that concluded that the Air Force's A-X program, the Marine Corps' Harrier, and the Cheyenne were significantly different that they did not constitute a duplication of capabilities. The Army convened a special task force under General Marks in January 1972, to reevaluate the requirements for an attack helicopter. The purpose of the Marks Board was to develop an "updated and defensible" material needs document. The Army also conducted a weapons demonstration for the Senate Armed Services Committee in early 1972, to show off the Cheyenne's firepower and garner support for attack helicopter development. The first TOW missile that was fired in the demonstration failed and went into the ground. The second missile was fired and hit the target. Previously, 130 TOW missiles had been fired without failure, but the failure of the first missile was now linked to perception of the aircraft. In April 1972, the Senate published its report on CAS. The report recommended funding of the Air Force's A-X program, which would become the A-10 Thunderbolt II and limited procurement of the Harrier for the Marine Corps. The report never referred to the Cheyenne by name and only offered a lukewarm recommendation for the Army to continue procurement of attack helicopters, so long as their survivability could be improved. The helicopter's large size and inadequate night/all-weather capability were reasons stated by the Army for the cancellation. The Cheyenne's analog and mechanical weapons systems were becoming out of date as new digital systems that were more accurate, faster, and lighter were being developed. The Cheyenne's unit cost had increased and was likely to increase further if new avionics were incorporated. AAH sought an attack helicopter based on combat experience in Vietnam, with a lower top speed of and twin engines for improved survivability. Lockheed offered the CL-1700, a modified version of the Cheyenne with two engines and omitted the pusher propeller, without success. The AAH program led to the AH-64 Apache, which entered service in the mid-1980s. After the cancellation, the Army conducted an evaluation of the seventh Cheyenne equipped with the AMCS flight control system. The testing showed the AMCS removed most of the remaining control problems, improved stability, improved handling, and decreased the pilot workload. With the AMCS, the Cheyenne reached a speed of in level flight and in a dive achieved ; it also demonstrated improved maneuverability at high speeds. Prototype #7 was the last Cheyenne to fly. == Civilian variants ==

Lockheed designed three civilian passenger variants of the Cheyenne layout, the CL-1026, CL-1060, and CL-1090. The last two digits represented the number of passenger seats for each design. The 1026 was to seat 30-35 passengers, the 1060 seat sixty passengers, and the 1090 seat ninety. These variants did not progress beyond the initial design stage. ==Surviving aircraft==

• No. 2 66-8827 is on display at Fort Johnson, Louisiana. • No. 5 66-8830 is stored at the Army Aviation Museum, Fort Rucker, Alabama. • No. 6 66-8831 is on display at Fort Campbell, Kentucky/Tennessee • No. 7 66-8832 is on display at the Army Aviation Museum, Fort Rucker. No longer on outdoor display. ==Specifications (AH-56A)==