Lockheed AH-56 Cheyenne

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.

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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 [Vous devez être inscrit et connecté pour voir ce lien], [Vous devez être inscrit et connecté pour voir ce lien] facility, and on 3 May 1967, Lockheed held a roll-out ceremony for the AH-56A. The aircraft was christened Cheyenne by the Army.


^{[Vous devez être inscrit et connecté pour voir ce lien]} The first flight of the AH-56 occurred on 21 September 1967.

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The Secretary of Defense approved funds for pre-production activities to support a production order for an initial 375 aircraft on 8 January 1968.

^{[Vous devez être inscrit et connecté pour voir ce lien][Vous devez être inscrit et connecté pour voir ce lien]} Manufacture of the 10 Cheyenne prototypes was completed by 1969.[Vous devez être inscrit et connecté pour voir ce lien]

Design

The Cheyenne's compound helicopter design included a rigid main rotor, low-mounted wings, and a pusher propeller.

The Cheyenne was powered by a[Vous devez être inscrit et connecté pour voir ce lien] turboshaft engine.

Thrust was provided by a [Vous devez être inscrit et connecté pour voir ce lien] 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.

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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 [Vous devez être inscrit et connecté pour voir ce lien] for aiming weapons.^{[Vous devez être inscrit et connecté pour voir ce lien]
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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.^{[Vous devez être inscrit et connecté pour voir ce lien]}

Six external hardpoints were located along the bottom of the helicopter, with two under each wing and two on the fuselage under the [Vous devez être inscrit et connecté pour voir ce lien].

The two inner wing hardpoints could carry pods of three [Vous devez être inscrit et connecté pour voir ce lien] missiles. [Vous devez être inscrit et connecté pour voir ce lien] 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.

Operational history

Flight testing

Flight testing began with the first flight of the second AH-56 (s/n 66-8827) in September 1967.^{[Vous devez être inscrit et connecté pour voir ce lien][Vous devez être inscrit et connecté pour voir ce lien]}

During early flight tests, a rotor instability issue was discovered when the aircraft was flying at low altitude [Vous devez être inscrit et connecté pour voir ce lien].

As the flight envelope was expanded, this instability and other minor problems were discovered and quickly addressed.^{[Vous devez être inscrit et connecté pour voir ce lien]
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Lockheed and the Army held a 13-minute demonstration "first flight" for the public at the [Vous devez être inscrit et connecté pour voir ce lien]
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 [Vous devez être inscrit et connecté pour voir ce lien] 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 [Vous devez être inscrit et connecté pour voir ce lien] had set-up a [Vous devez être inscrit et connecté pour voir ce lien] 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.^{[Vous devez être inscrit et connecté pour voir ce lien]}

Production contract canceled


The Army issued a cure-notice^{[Vous devez être inscrit et connecté pour voir ce lien]} to Lockheed on 10 April 1969, citing 11 technical problems, and unsatisfactory progress on the program^{[Vous devez être inscrit et connecté pour voir ce lien]}
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,^{[Vous devez être inscrit et connecté pour voir ce lien]}but retained the development contract in hopes that the issues could be resolved.^{[Vous devez être inscrit et connecté pour voir ce lien]
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In September 1969, Cheyenne prototype #10 (s/n 66-8835) underwent [Vous devez être inscrit et connecté pour voir ce lien] 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.^{[Vous devez être inscrit et connecté pour voir ce lien]}

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 [Vous devez être inscrit et connecté pour voir ce lien], Arizona, demonstrating the ability for the gunner and pilot to accurately fire on separate targets on each side on the helicopter.

^{[Vous devez être inscrit et connecté pour voir ce lien]} By the end of 1970, the Army funded work on TOW missile guidance and night sighting systems.

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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.

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With these upgrades, the helicopter surpassed its performance requirements. However, under certain conditions stability and control did not completely satisfy the test pilots.

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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 [Vous devez être inscrit et connecté pour voir ce lien] 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 [Vous devez être inscrit et connecté pour voir ce lien] of 1948.

The Department of Defense (DOD) conducted a study that concluded that Air Force's A-X program, the Navy's proposed [Vous devez être inscrit et connecté pour voir ce lien], 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 [Vous devez être inscrit et connecté pour voir ce lien], who cited helicopter casualty statistics of [Vous devez être inscrit et connecté pour voir ce lien].
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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 [Vous devez être inscrit et connecté pour voir ce lien] and [Vous devez être inscrit et connecté pour voir ce lien].

Analysis of the three helicopters determined that the Bell and Sikorsky helicopters could not fulfill the Army's requirements.