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Lockheed AH-56 Cheyenne

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Template:Infobox Aircraft The Lockheed AH-56 Cheyenne was a two-seat, single-engine, rigid-rotor, compound helicopter with low-mounted, fixed wings and retractable main landing gear. It was armed with a 30 mm cannon in a belly turret and either a 7.62 mm minigun or a 40 mm grenade launcher in a nose turret. It was also capable of being armed with 2.75 inch (70 mm) rockets and TOW missiles on wing hardpoints.

The AH-56A was the design winner of the United States Army's Advanced Aerial Fire Support System program in 1965 to establish a technologically advanced, dedicated attack helicopter. The Cheyenne's compound helicopter design was intended to provide a 212-knot dash capability in order to serve as an armed escort to the Army's transport helicopters, such as the UH-1 Iroquois. Lockheed was awarded a production contract in 1967 but technical problems and a fatal crash during development resulted in the production contract being canceled in 1969.[1]

Development continued in the hopes that eventually the Army would be able to field the Cheyenne. The program was canceled by the Army on 9 August 1972.[2][3]

Development

Advanced Aerial Fire Support System

In the mid-1960s the U.S. Army concluded that a purpose-built gunship with more speed and firepower was required in the face of increasingly intense ground fire (often using heavy machine guns and anti-tank rockets) from Viet Cong and NVA troops. Based on this realization, and with the growing involvement in Vietnam, the U.S. Army developed the requirements for a dedicated attack helicopter, the "Advanced Aerial Fire Support System" (AAFSS). The aircraft would be able to cruise at 195 knots with dash speed of 220 kn. It would be able hover-out-of-ground effect (OGE) at 6,000 feet (PA) and 95°F (35°C), and carry a much larger payload of weapons.[4]

In August 1964, the Army released its request for proposals for the AAFSS. In February 1965, Lockheed and Sikorsky were selected as finalists. In the end, Lockheed's design was announced the winner in November 1965 on the basis of the design being less expensive, available earlier, and having less technical risk. The quantitative requirements for the AAFSS were finally released by the Army on 17 December 1965, which included several changes including adding an aerial rocket subsystem. In all, fourteen requirements were added to what was in Lockheed's proposal.[5]

AH-56 design and testing

On 23 March 1966 the Army awarded Lockheed an engineering and development (ED) contract for ten prototypes, designating the aircraft AH-56A. Lockheed began construction of the aircraft at its Van Nuys, California facility. On 3 May 1967, Lockheed held a roll-out ceremony for the AH-56A, and the aircraft was christened Cheyenne by the Army.[6] Later in 1967, options for production were finalized and added to the contract.[5] A rigid rotor helicopter had been built by Lockheed as the XH-51 and the AH-56 followed in its path.

The first flight of the AH-56 was accomplished on 21 September 1967 with the second prototype aircraft (s/n 66-8827). Lockheed and the Army later held a 13-minute demonstration "first flight" for the public at the Van Nuys Airport on 12 December 1967. During the flight, the aircraft demonstrated the new capabilities brought about by the thrusting propeller. For instance, the Cheyenne could slow down or accelerate without pitching the nose up or down. It was able to land on its two forward landing gear, taxi then gently set the tail landing gear down. The helicopter also held a stationary hover in a 30-knot crosswind.[7]

During early flight tests, a rotor instability was discovered when the aircraft was flying in ground effect.[8] As the flight envelope was expanded, this instability and other minor problems were discovered and quickly addressed.[9][10] On 8 January 1968, the Secretary of Defense approved funds for pre-production activities to support a production order for an initial 375 aircraft.[11][12] By March 1968, the AH-56 had established a flight envelope of 170 knots (196 mph, 315 km/h) in forward flight, 25 knots (28.8 mph, 46.3 km/h) sidewards, and 20 knots (23 mph, 37 km/h) rearwards.[10] Ten Cheyenne prototypes were built by 1969.[13]

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 (half-P hop) in the rotor. 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 would be modified to prevent the same problem from occurring again.[14][15]

Cheyenne prototype #10 (s/n 66-8835) was destroyed in September 1969, during wind tunnel testing at NASA Ames Research Center as Lockheed researched half-P vibration and drag issues.[16]

Production contract canceled

The Army issued a cure-notice[17] to Lockheed on 10 April 1969, citing 11 technical problems and unsatisfactory progress on the program.[1] 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. On 19 May 1969, citing Lockheed's inability to meet the production timeline, the Army canceled the AH-56 production contract [18] but retained the development contract in hopes that the issues could be resolved.[19]

As development continued, prototype #9 (s/n 66-8834) was fitted with an ejection seat for the pilot. The downward firing ejection seat was placed in the forward seat in place of the gunner's station. With this configuration, prototype #9 would be used for all remaining envelope expansion flights.[20] Lockheed worked on modifying the AH-56 design to address the vibration and other issues.[21] Cheyenne prototype #6 (s/n 66-8831) conducted 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.[22]

Cheyenne prototype #9 also received an upgraded transmission and drivetrain, which allowed the T64-GE-16 turboshaft engine output to be increased from a derated 3,435 shp (2,561 kW) to 3,925 shp (2,927 kW). This prototype also received a hinged rear canopy in place of the original sliding canopy which eliminated canopy vibration. By the end of 1970, testing was progressing satisfactory and the Army funded work on TOW guidance and night sighting systems.[22]

Throughout 1971, the AH-56 was tested and evaluated at Yuma Proving Grounds to determine if its stability and control were sufficient. The sixth and ninth Cheyennes took part in this testing. The evaluation reported deficiencies included lateral directional stability, uncommanded motion during maneuvering, high vibration, and poor directional control during sidewards flying. Afterwards, the ninth AH-56 received the improved T64-GE-716 engine producing 4,275 shp (3,188 kW) and a production version of the ICS system. With these upgrades, the helicopter surpassed its requirements for performance. However, under certain conditions stability and control did not completely satisfy its pilots.[23]

Program demise

A 1972 congressional report from the Senate which recommended funding of the United States Air Force's A-10 and the United States Navy's Harrier programs brought the end to the Cheyenne, and the program was canceled by the Secretary of the Army in August 1972.[2] The program was halted in August 1972.[24]

The development of the new technologies on the AH-56 led to many cost and time overruns. This, coupled with Vietnam war experience and an adjustment in the Army's specification,[25] along with heavy campaigning by the Air Force which was in the midst of trying to get the A-10 authorized,[26] led to the Cheyenne's demise in favor of the AH-64 Apache, and continued production of AH-1 Cobra, which was quite successful in Vietnam, and would serve in USMC use beyond 2000. The Army would trade speed for survivability in the specification which would lead to the AH-64.

Design

AH-56A Cheyenne at U.S. Army Aviation Museum, Ft. Rucker, AL, displaying the stub-wing, and both tail rotors.

The AH-56 Cheyenne was a compound helicopter with a two-seat tandem cockpit, fixed low wings and retractable landing gear. The cockpit seating places the pilot in the rear seat and the gunner in the front seat.[27]

The Cheyenne had a short but substantial wing on each side of the airframe and a rigid main rotor. Thrust was provided by a pusher propeller at the rear of the aircraft. Since the main rotor is not relied on for the full amount of lift (provided by the wings) or thrust (from the pusher prop), the Cheyenne was able to reach high speeds - in excess of 200 knots. The design is classed as a compound helicopter, or gyrodyne, and was unable to qualify for speed records in helicopter categories. The Cheyenne also featured an advanced navigation and fire control suite which became the basis for the fire control suite in the AH-64 Apache.

File:Ah-56 side.jpg
Front/Side view of AH-56

Turrets were mounted at the nose and at the middle of aircraft underbelly. The nose turret included either a 40 mm grenade launcher or a 7.62 mm minigun and had a +/- 100° of rotation from centerline. The belly turret included a 30 mm automatic cannon with 360° of rotation. An unusual feature of the AH-56 was the gunner's stabilized gun station. The AH-56 gunner's station including seat and consoles 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 via a periscope sight. Limits prevented the belly turret from aiming at any part of the helicopter. The pilot had a helmet mounted sighting system for aiming weapons.[28]

The AH-56 had six external hardpoints with two under each wing and two on the fuselage. The two inner wing hardpoints could carry pods of 3 TOW missiles. 2.75 in (70 mm) rockets in 7 rocket or 19 rocket launchers could carried on the four wing hardpoints. The two fuselage mounts were dedicated to carrying fuel tanks. The wing hardpoints could carry fuel tanks if needed.[29]

Specifications (AH-56A)

Data from Jane's Aircraft[30]

General characteristics

  • Crew: 2 pilots: 1 pilot, 1 copilot/gunner (front seat)

Performance Armament

  • Guns:
1x nose turret with either an M129 40 mm grenade launcher or an XM196 7.62x51 mm machine gun and
1x belly turret with an XM140 30 mm cannon

See also

Related development

Aircraft of comparable role, configuration, and era

Related lists

References

Notes
  1. ^ a b U.S. Army 1973, p. 7.
  2. ^ a b U.S. Army 1973, p. 9.
  3. ^ Landis and Jenkins 2000, pp. 79-82.
  4. ^ U.S. Army 1973, pp. 1-2.
  5. ^ a b U.S. Army 1973, pp. 4-5.
  6. ^ Landis and Jenkins 2000, pp. 35-36.
  7. ^ Landis and Jenkins 2000, p. 45.
  8. ^ Robb 2006, p. 46.
  9. ^ Changes included a stiffer pusher propeller mount, adjustment of the pusher propeller blade natural frequency, and stiffening of the rear cockpit canopy door.
  10. ^ a b Landis and Jenkins 2000, p. 53.
  11. ^ U.S. Army 1973, p. 6.
  12. ^ Landis and Jenkins 2000, p. 48.
  13. ^ Landis and Jenkins 2000, p. 69.
  14. ^ The rotor blades and control system were stiffened, the mass of the gyro was increased, and the geometry of the rotor was adjusted.
  15. ^ Landis and Jenkins 2000, pp. 69-70.
  16. ^ Landis and Jenkins 2000, pp. 73-75.
  17. ^ A list of problems that are required to be addressed prior to production.
  18. ^ Center of Military History. Department of Army Historical Summary Chapter 3. Department of the Army. 1969. Washington D.C. Accessed on 29 September 2008.
  19. ^ Landis and Jenkins 2000, pp. 70-72.
  20. ^ Landis and Jenkins 2000, pp. 69-70.
  21. ^ 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 (55.6 km/h).
  22. ^ a b Landis and Jenkins 2000, pp. 76-77.
  23. ^ Landis and Jenkins 2000, pp. 77-78.
  24. ^ Landis and Jenkins 2000, pp. 79-82.
  25. ^ TOW MISSILE SYSTEM CHRONOLOGY 1961-2000, U.S. Army, accessed 1 October 2007.
  26. ^ Landis and Jenkins 2000, p. 81.
  27. ^ Landis and Jenkins 2000, p. 85-93.
  28. ^ Landis and Jenkins 2000, pp. 54-58, 91-93.
  29. ^ Landis and Jenkins 2000, pp. 58, 93-95.
  30. ^ John W. R. Taylor, ed. (1969). Jane's All the World's Aircraft, 1969-70. Sampson, Low, Marston & Co.
Bibliography
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