Lockheed AH - 56 Cheyenne 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 ( AAFSS ) 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 . It also featured low - mounted wings , a tail - mounted thrusting propeller , and a single 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 . 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 . Development of the Cheyenne continued in the hope that the helicopter would eventually enter service . On 9 August 1972 , the Army canceled the Cheyenne program . 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 - engined conventional helicopter , viewed as less technical and more survivable . The Army announced a new program for an Advanced Attack Helicopter ( AAH ) on 17 August 1972 , which led to the development of the 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 missile s . 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 140 – knot ( 161 mph , 259 km / h ) cruise speed and a 1 , 500 - pound ( 680 kg ) 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 ( QMDO ) for a rotary - wing aircraft with 195 - knot ( 224 mph , 361 km / h ) cruise speed , 220 - knot ( 253 mph , 407 km / h ) dash speed , and the capability to hover out - of - ground - effect ( OGE ) at 6 , 000 feet ( 1 , 830 m ) on a 95 °F ( 35 °C ) day . The speed requirements were derived from the speed of aircraft the helicopter would escort . The Director of Defense Research and Engineering ( DDRE ) 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 ( 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 helicopter s , 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 ( AAFSS ) . The development objectives document ( QMDO ) 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 ( RFP ) . 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 . 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 . 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 . The Cheyenne ' s design was that of a compound helicopter , which 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 produces 20 % of the lift and could be adjusted by collective pitch control changes . The Cheyenne achieved speeds over 200 knots ( 230 mph , 370 km / h ) , 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 . 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 TOW missiles . 2 . 75 - inch ( 70 mm ) 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 ( s / n 66 - 8827 ) 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 30 - knot ( 35 mph , 56 km / h ) 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 170 knots ( 196 mph , 315 km / h ) in forward flight , 25 knots ( 29 mph , 46 km / h ) sidewards , and 20 knots ( 23 mph , 37 km / h ) rearwards . The project suffered a setback on 12 March 1969 , when the rotor on prototype # 3 ( s / n 66 - 8828 ) 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 ( or half - P hop ) 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 oscillation s had set up a resonant vibration that exceeded the rotor system ' s ability to compensate . After the investigation , the rotor and control systems would be modified to prevent the same problem from occurring again . 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 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 ) , and 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 . 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 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 . On 22 October 1971 , the Senate Armed Services subcommittee on Tactical Air Power conducted hearings to evaluate the CAS mission and the pending programs . The most damaging testimony for the Army ' s program came from the commander of the Air Force ' s Tactical Air Command , General William W . Momyer , who cited helicopter casualty statistics of Operation Lam Son 719 . 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 task force conducted flight evaluations of the AH - 56 , along with two industry alternatives for comparison : the Bell 309 King Cobra and Sikorsky S - 67 Blackhawk . Analysis of the three helicopters determined that the Bell and Sikorsky helicopters could not fulfill the Army ' s requirements . 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 Cheyenne program was canceled by the Secretary of the Army on 9 August 1972 . 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 . On 17 August 1972 , the Army initiated the Advanced Attack Helicopter ( AAH ) program . AAH sought an attack helicopter based on combat experience in Vietnam , with a lower top speed of 145 kn ( 167 mph , 269 km / h ) 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 215 kn ( 247 mph , 398 km / h ) in level flight and in a dive achieved 245 kn ( 282 mph , 454 km / h ) ; it also demonstrated improved maneuverability at high speeds . Prototype # 7 was the last Cheyenne to fly . Lockheed had counted on the Cheyenne to establish itself in the helicopter market with its rigid rotor technology , but the ambitious project was unsuccessful . The firm did not pursue development of another helicopter .