Lockheed AH-56 Cheyenne
The Lockheed AH-56 Cheyenne is 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 the helicopter's development behind schedule, resulting in the cancellation of the production contract on 19 May 1969. Development of the Cheyenne continued in the hope that the helicopter would eventually enter service.
As American involvement in the Vietnam War was winding down, the Army canceled the Cheyenne program on 9 August 1972. By this time, the AH-1 Cobra was widely deployed by the Army in South Vietnam and equipped with the TOW anti-tank missile. Controversy with the United States Air Force over the Cheyenne's role in combat as well as the political climate regarding military acquisition programs had caused the Army to amend the service's attack helicopter requirements in favor of a twin-engine conventional helicopter, viewed as less technical and more survivable. The Army announced a new program for an Advanced Attack Helicopter on 17 August 1972, which led to the development of the Hughes AH-64 Apache.
Development
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 drafted a Qualitative Material Requirement for an interim, commercial off-the-shelf 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.Based on the guidance from the Secretary of the Army, CDC established Qualitative Material Development Objectives for a rotary-wing aircraft with cruise speed, dash speed, and the capability to hover out-of-ground-effect at on a day. The speed requirements were derived from the speed of aircraft the helicopter would escort. The Director of Defense Research and Engineering conditionally approved the changes to the development objectives, pending his review of the proposed program. He also directed the Army to determine whether or not any other helicopter could offer an improvement in performance over the UH-1B in the meantime.
As a result, the Army Material Command conducted a study to determine if the development objectives were feasible and also established a program office for the Fire-support Aerial System. 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.
AAFSS competition
On 26 March 1964, the Army Chief of Staff redesignated the FAS program as the Advanced Aerial Fire Support System. The development objectives document for the AAFSS was approved in April 1964, and on 1 August 1964, the Transportation Research and Engineering Command contacted 148 prospective contractors with a request for proposals. 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. Meanwhile, the Army also continued to pursue an interim aircraft for combat in Vietnam until the AAFSS could be fielded, resulting in development of the Bell AH-1 Cobra which became the backbone of the Army's attack helicopter fleet during and after the Vietnam War.
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
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 grenade launcher, or a 7.62 mm minigun. The belly turret included a 30 mm automatic cannon with 360° of rotation. Mechanical stops prevented the belly turret from aiming at any part of the helicopter.
Six external hardpoints were located along the bottom of the helicopter, with two under each wing and two on the fuselage under the sponsons. The two inner wing hardpoints could carry pods of three BGM-71 TOW anti-tank missiles. 2.75-inch rockets in 7-rocket or 19-rocket launchers could be carried on the four wing hardpoints. The two fuselage mounts were dedicated to carrying external fuel tanks. The wing hardpoints were also plumbed to allow the carriage of additional fuel tanks if required.
Operational history
Flight testing
Flight testing began with the first flight of the second AH-56 in September 1967. During early flight tests, a rotor instability issue was discovered when the aircraft was flying at low altitude in ground effect. 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.
The project suffered a setback on 12 March 1969, when the rotor on prototype #3 struck the fuselage and caused the aircraft to crash, killing the pilot, David A. Beil. The accident occurred on a test flight where the pilot was to manipulate the controls to excite 0.5P oscillations in the rotor; 0.5P is a vibration that happens once per two main rotor revolutions, where P is the rotor's rotational speed. The accident investigation noted that safety mechanisms on the controls had apparently been disabled for the flight. The investigation concluded that the pilot-induced oscillations had set up a resonant vibration that exceeded the rotor system's ability to compensate. After the investigation, the rotor and control systems were modified to prevent the same problem from occurring again.