AH-56 Cheyenne
Encyclopedia
The Lockheed AH-56 Cheyenne was a single-engine attack helicopter
developed by Lockheed
for the United States Army
's Advanced Aerial Fire Support System (AAFSS) program to produce the Army's first dedicated attack helicopter. Lockheed designed the AH-56 utilizing a four-bladed rigid-rotor and configured the aircraft as a compound helicopter; with low-mounted wings and a tail-mounted thrusting propeller. The Cheyenne was powered by a GE T64
turboshaft engine and was intended to provide a 212-knot (244 mph, 393 km/h) dash capability in order to serve as an armed escort to the Army's transport helicopters, such as the UH-1 Iroquois
.
In 1966, the Army awarded Lockheed a contract to develop 10 prototypes of the AH-56. The first flight of an AH-56 occurred 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 production contract being canceled on 19 May 1969. Cheyenne development 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
.
convened the Howze Board
to review Army aviation requirements. The results of the board envisioned an airmobile division that was supported by 90 armed aircraft. 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
modified with armament systems capable of mounting machine guns and rockets.
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-armor missiles.
(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 Manager's 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 UH-1B aircraft until development of the FAS could proceed.
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 a solution for 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 selection. 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 funds for pre-production activities to support a production order for an initial 375 aircraft on 8 January 1968. Manufacture of the 10 Cheyenne prototypes was completed by 1969.
The Cheyenne's compound helicopter design included 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 and thrust from the pusher prop, relieved the aerodynamic requirements 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.
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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 had a +/- 100° of rotation from 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 sponson
s. 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 could also be used to carry additional fuel tanks if needed.
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) hit the fuselage and killed the pilot. 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 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.The rotor blades and control system were stiffened, the mass of the gyro was increased, and the geometry of the rotor was adjusted.
In September 1969, Cheyenne prototype #10 (s/n 66-8835) underwent wind tunnel
testing at NASA Ames Research Center, to research the half-P, 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 causing the helicopter to breakup and be destroyed.
Lockheed worked on modifying the AH-56 design to address the vibration and other issues.The collective boost system and gyro-to-rotor connection were changed, eliminating the half-P oscillations. Other vibrations were solved by removing weight from rotor head leading and trailing edges, and the rotation of the tail rotor was reversed to improve sidewards flight to the left below 30 knots (35 mph, 56 km/h). 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 Grounds, Arizona, demonstrating the ability for the gunner and pilot to accurately fire on separate targets on each side on the helicopter. By 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 Grounds 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, the 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.
of 1948. The Department of Defense (DOD) conducted a study that concluded that Air Force's A-X program, the Navy's proposed 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 Navy. The report never referred to the Cheyenne by name and only offered a lukewarm recommendation for the Army to continue to seek to procure 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.U.S. Army reports state projected unit costs in $3.2–3.8 million range. Landis and Jenkins (2000) states a $3 million unit cost in 1972.
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 lead 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 knots (282 mph, 454 km/h) and 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 and the firm did not develop another helicopter.
Attack helicopter
An attack helicopter is a military helicopter with the primary role of an attack aircraft, with the capability of engaging targets on the ground, such as enemy infantry and armored vehicles...
developed by Lockheed
Lockheed Corporation
The Lockheed Corporation was an American aerospace company. Lockheed was founded in 1912 and later merged with Martin Marietta to form Lockheed Martin in 1995.-Origins:...
for the United States Army
United States Army
The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...
's Advanced Aerial Fire Support System (AAFSS) program to produce the Army's first dedicated attack helicopter. Lockheed designed the AH-56 utilizing a four-bladed rigid-rotor and configured the aircraft as a compound helicopter; with low-mounted wings and a tail-mounted thrusting propeller. The Cheyenne was powered by a GE T64
General Electric T64
-External links:* *...
turboshaft engine and was intended to provide a 212-knot (244 mph, 393 km/h) dash capability in order to serve as an armed escort to the Army's transport helicopters, such as the UH-1 Iroquois
UH-1 Iroquois
The Bell UH-1 Iroquois is a military helicopter powered by a single, turboshaft engine, with a two-bladed main rotor and tail rotor. The helicopter was developed by Bell Helicopter to meet the United States Army's requirement for a medical evacuation and utility helicopter in 1952, and first flew...
.
In 1966, the Army awarded Lockheed a contract to develop 10 prototypes of the AH-56. The first flight of an AH-56 occurred 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 production contract being canceled on 19 May 1969. Cheyenne development 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
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...
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
Advanced Attack Helicopter
The Advanced Attack Helicopter was a United States Army program to develop an advanced ground attack helicopter beginning in 1972. The Advanced Attack Helicopter program followed cancellation of the Lockheed AH-56 Cheyenne. After evaluating industry proposals, the AAH competition was reduced to...
(AAH) on 17 August 1972, which led to the development of the AH-64 Apache
AH-64 Apache
The Boeing AH-64 Apache is a four-blade, twin-engine attack helicopter with a tailwheel-type landing gear arrangement, and a tandem cockpit for a two-man crew. The Apache was developed as Model 77 by Hughes Helicopters for the United States Army's Advanced Attack Helicopter program to replace the...
.
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 Secretary of Defense McNamaraRobert McNamara
Robert Strange McNamara was an American business executive and the eighth Secretary of Defense, serving under Presidents John F. Kennedy and Lyndon B. Johnson from 1961 to 1968, during which time he played a large role in escalating the United States involvement in the Vietnam War...
convened the Howze Board
Howze Board
Howze Board is the informal name of the Tactical Mobility Requirements Board that was created at the request of Secretary of Defense Robert McNamara to review and test new concepts integrating helicopters into the United States Army. It gave birth to idea of airmobility. It was named after its...
to review Army aviation requirements. The results of the board envisioned an airmobile division that was supported by 90 armed aircraft. 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
UH-1 Iroquois
The Bell UH-1 Iroquois is a military helicopter powered by a single, turboshaft engine, with a two-bladed main rotor and tail rotor. The helicopter was developed by Bell Helicopter to meet the United States Army's requirement for a medical evacuation and utility helicopter in 1952, and first flew...
modified with armament systems capable of mounting machine guns and rockets.
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-armor missiles.
Attack helicopter requirements
In December 1962, Combat Development Command (CDC) drafted a Qualitative Material Requirement (QMR) for an interim, commercial off-the-shelfCommercial off-the-shelf
In the United States, Commercially available Off-The-Shelf is a Federal Acquisition Regulation term defining a nondevelopmental item of supply that is both commercial and sold in substantial quantities in the commercial marketplace, and that can be procured or utilized under government contract...
(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
United States Army Materiel Command
The U.S. Army Materiel Command is the primary provider of materiel to the United States Army.The Command's mission includes the research & development of weapons systems as well as maintenance and parts distribution....
(AMC) conducted a study to determine if the development objectives were feasible and also established a Program Manager's 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 UH-1B aircraft 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 CommandAviation Applied Technology Directorate
The United States Army Aviation Applied Technology Directorate is a tenant activity located at Fort Eustis, Virginia. It is a directorate of the Aviation and Missile Research, Development, and Engineering Center , a part of the Research, Development and Engineering Command .-History:The...
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 a solution for 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 selection. 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 funds for pre-production activities to support a production order for an initial 375 aircraft on 8 January 1968. Manufacture of the 10 Cheyenne prototypes was completed by 1969.
Design
General Electric T64
-External links:* *...
turboshaft engine. Thrust was provided by a pusher propeller
Pusher configuration
In a craft with a pusher configuration the propeller are mounted behind their respective engine. According to Bill Gunston, a "pusher propeller" is one mounted behind engine so that drive shaft is in compression...
at the rear of the aircraft. At high speeds, the amount of lift provided by the wings and thrust from the pusher prop, relieved the aerodynamic requirements 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.
Helmet mounted display
A helmet mounted display is a device used in some modern aircraft, especially combat aircraft. HMDs project information similar to that of head-up displays on an aircrew’s visor or reticle, thereby allowing him to obtain situational awareness and/or cue weapons systems to the direction his head...
for aiming weapons.
Weapon turrets were mounted at the nose and the middle of aircraft underbelly. The nose turret had a +/- 100° of rotation from 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 sponson
Sponson
Sponsons are projections from the sides of a watercraft, for protection, stability, or the mounting of equipment such as armaments or lifeboats, etc...
s. The two inner wing hardpoints could carry pods of three TOW
BGM-71 TOW
The BGM-71 TOW is an anti-tank missile. "BGM" is a weapon classification that stands for "Multiple Environment , Surface-Attack , Missile ". "TOW" is an acronym that stands for "Tube-launched, Optically-tracked, Wire command data link, guided missile"...
missiles. 2.75-inch (70 mm) rockets
Mk 4/Mk 40 Folding-Fin Aerial Rocket
The Mk 4 Folding-Fin Aerial Rocket , sometimes called the Mighty Mouse, was a 2.75 in diameter unguided rocket weapon commonly used by U.S. military aircraft. It was intended as an air-to-air weapon to allow interceptor aircraft to shoot down enemy bombers with greater range and effectiveness than...
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 could also be used to carry additional fuel tanks if needed.
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.Changes included a stiffer pusher propeller mount, adjustment of the pusher propeller blade natural frequency, and stiffening of the rear cockpit canopy door.Lockheed and the Army held a 13-minute demonstration "first flight" for the public at the Van Nuys Airport
Van Nuys Airport
Van Nuys Airport is a public airport located in Van Nuys in the San Fernando Valley section of the city limits of Los Angeles, California, United States. No major commercial airlines fly into this airport; it is used by private, chartered, and small commercial aircraft...
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
Flight envelope
In aerodynamics, the flight envelope or performance envelope of an aircraft refers to the capabilities of a design in terms of airspeed and load factor or altitude. The term is somewhat loosely applied, and can also refer to other measurements such as maneuverability...
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) hit the fuselage and killed the pilot. 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 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
Pilot-induced oscillation
Pilot-induced oscillations, as defined by MIL-HDBK-1797A, are sustained or uncontrollable oscillations resulting from efforts of the pilot to control the aircraft and occurs when the pilot of an aircraft inadvertently commands an often increasing series of corrections in opposite directions, each...
s had set-up a resonant vibration
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
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.The rotor blades and control system were stiffened, the mass of the gyro was increased, and the geometry of the rotor was adjusted.
Production contract canceled
The Army issued a cure-noticeA list of problems that are required to be addressed prior to production. 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
Wind tunnel
A wind tunnel is a research tool used in aerodynamic research to study the effects of air moving past solid objects.-Theory of operation:Wind tunnels were first proposed as a means of studying vehicles in free flight...
testing at NASA Ames Research Center, to research the half-P, 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 causing the helicopter to breakup and be destroyed.
Lockheed worked on modifying the AH-56 design to address the vibration and other issues.The collective boost system and gyro-to-rotor connection were changed, eliminating the half-P oscillations. Other vibrations were solved by removing weight from rotor head leading and trailing edges, and the rotation of the tail rotor was reversed to improve sidewards flight to the left below 30 knots (35 mph, 56 km/h). 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 Grounds, Arizona, demonstrating the ability for the gunner and pilot to accurately fire on separate targets on each side on the helicopter. By 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 Grounds 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, the 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
Servomechanism
thumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...
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 AgreementKey West Agreement
The Key West Agreement is the colloquial name for the policy paper Function of the Armed Forces and the Joint Chiefs of Staff drafted by James V. Forrestal, the first United States Secretary of Defense...
of 1948. The Department of Defense (DOD) conducted a study that concluded that Air Force's A-X program, the Navy's proposed 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
William W. Momyer
General William Wallace Momyer was commander of the U.S. Air Force Tactical Air Command.-Early life:Momyer was born in 1916, in Muskogee, Oklahoma, attended Broadway High School in Seattle, Washington, and graduated with a bachelor of arts degree from the University of Washington in 1937...
, who cited helicopter casualty statistics of Operation Lam Son 719
Operation Lam Son 719
Operation Lam Son 719 was a limited-objective offensive campaign conducted in southeastern portion of the Kingdom of Laos by the armed forces of the Republic of Vietnam between 8 February and 25 March 1971, during the Vietnam War...
.
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
Bell 309
|-See also:-References:* Verier, Mike. Bell AH-1 Cobra. Osprey Publishing, 1990. ISBN 0-85045-934-6.-External links:*...
and Sikorsky S-67 Blackhawk
Sikorsky S-67 Blackhawk
The Sikorsky S-67 Blackhawk was a private-venture, prototype attack helicopter built in 1970 with Sikorsky Aircraft R&D funds. A tandem, two-seat aircraft designed around the dynamic drive and rotor systems of the Sikorsky S-61, it was designed to serve as an attack helicopter or to transport up to...
. 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 Navy. The report never referred to the Cheyenne by name and only offered a lukewarm recommendation for the Army to continue to seek to procure 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.U.S. Army reports state projected unit costs in $3.2–3.8 million range. Landis and Jenkins (2000) states a $3 million unit cost in 1972.
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 lead to the AH-64 Apache
AH-64 Apache
The Boeing AH-64 Apache is a four-blade, twin-engine attack helicopter with a tailwheel-type landing gear arrangement, and a tandem cockpit for a two-man crew. The Apache was developed as Model 77 by Hughes Helicopters for the United States Army's Advanced Attack Helicopter program to replace the...
, 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 knots (282 mph, 454 km/h) and 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 and the firm did not develop another helicopter.
Survivors
- No. 2 66-8827 is on display at Fort PolkFort PolkFort Polk is a United States Army installation located in Vernon Parish, approximately 7 miles east of Leesville, Louisiana and 20 miles north of DeRidder, Louisiana....
, Louisiana. - No. 5 66-8830 is stored at Army Aviation MuseumUnited States Army Aviation MuseumThe United States Army Aviation Museum is an aviation museum located on Fort Rucker near Ozark, Alabama. It has the largest collection of helicopters held by a museum in the world. The museum features some 50 aircraft on public display with aviation artifacts ranging from a replica of the Wright...
, Fort Rucker, Alabama. - No. 6 66-8831 is on display at Fort CampbellFort CampbellFort Campbell is a United States Army installation located astraddle the Kentucky-Tennessee border between Hopkinsville, Kentucky, and Clarksville, Tennessee...
. - No. 7 66-8832 is on display at Army Aviation Museum, Fort Rucker.
Specifications (AH-56A)
See also
External links
- Assault Helicopters. U.S. Centennial of Flight Commission web site
- Shrader, Cecil L., COL (Ret.), USA. "Attack Helicopter Transformation". Letter to the editor. ARMY Magazine. January 2003. Association of the United States Army.
- Attack Helicopter Evaluation, AH-56A Cheyenne Compound Helicopter. U.S. Army, June 1972.
- AH-56 page on GlobalSecurity.org
- AH-56A pictures and specs