The first VTOL concepts to be tried were three "tail sitting" airplanes, the Lockheed XFV-1, the Convair XFY-1, and the Ryan X-13 Vertijet. The next concept tried was to keep the main body of the aircraft in a conventional sense but tilt the wing and engines from the vertical to the horizontal. The LTV-Hiller-Ryan XC-142A, a large four-engine, four-propeller transport, was such an aircraft.
In 1959 the Army, Navy and Air Force began work on the development of a prototype V/STOL airplane that could augment helicopters in transport-type missions. If this prototype program were successful, an airplane based on the prototype experience could be developed. By 27 January 1961, the Air Force and Army had shown a renewed interest in a VTOL aircraft and through a series of DOD actions an agreement had been reached wherein all services consented to participate in the development of a prototype VTOL transport. BuWeps, the DOD-appointed manager for the tri-service aircraft, then issued a revised statement of requirements which specified the same payload but extended the aircraft's radius to approximately 250 miles and increased the cruising airspeed to 250-300 knots and the maximum airspeed to 300-400 knots. However, for the Marine Corps mission, the requirement stated that the fuel load could be reduced so that the maximum gross weight would not exceed 35,000 pounds so long as a 100-mile nautical radius of action could be flown.
Inherent in the design of any advanced VTOL aircraft was the problem of "downwash," the hypervelocity winds directed at the ground during landing, take-off, and hovering flight. Conventional helicopters generated high velocity downwash winds, often uncomfortable and a nuisance, but still tolerable. As an example, the most severe downwash generated from a helicopter came from the HR2S which had a "disc-loading" of 7.5 lbs/sq. ft. The downwash of the HR2S helicopter was strong enough to blow men and equipment about a ship's deck or create clouds of sand and dirt at unprepared landing sites. The convertiplane type VTOL "disc-loading" would probably have been on the order of 10 lbs/sq ft, while those of more sophisticated designs as high as 35-50 lbs/sq ft, making well-prepared landing sites a prerequisite for useful employment. It appeared that for the typical Marine short range / low altitude troop support mission there was little requirement for VTOL aircraft of advanced design. Their productivity could not compete with helicopters and the enhanced combat survival potential offered by speeds in excess of 200 knots was offset by poorer productivity and logistic support problems.
By August 1961, the Navy recognized that the four-engine tilt-wing aircraft, the design which had now been selected for the tri-service evaluation instead of the compound helicopter, would be unsuitable for Navy or Marine Corps use and withdrew from the program. Long before this time, however, the CMC and CNO had recognized that any production aircraft resulting from the high-speed VTOL program would not reach the fleet in time to replace the HR2S. In view of this, and at the Commandant's urging, the CNO issued on 27 March 1961 a revised developmental characteristic (AO-17501-3) for a medium assault transport helicopter with essentially the same requirements as the convertiplane (AO-17501-2) but with a cruising airspeed of only 150 knots. The gross weight was also to be limited to a maximum of 35,000 pounds. On 24 August 1962 BuWeps announced the Sikorsky Aircraft CH-53A Sea Stallion helicopter as the winner.
The Chance-Vought Hiller-Ryan VHR-447 design won a competition in September 1961 for a VTOL transport for the US Armed Forces. As a result of the Tri-Service evaluation of all proposals, the design development contract for five airplanes was awarded to Vought, and five prototypes were ordered. The contract was signed in early 1962 with first flight specified for July 1964.
Built by an industrial team of LTV, Ryan and Hiller, the XC-142A was powered by four T-64 turboshaft engines built by General Electric and driving four 15.5-foot Hamilton Standard fiberglass propellers. The US Air Force was the developing agency for the Department of Defense, with the joint sponsorship of the Navy and Army.
The XC-142A was the largest and fastest VTOL transport airplane flying at that time. It achieved more than 400 miles per hour at cruise altitude during flight testing. The payload was carried in a cargo compartment that was 30 feet long, 7.5 feet wide and 7 feet high. Two pilots and one crewman/load master made up the flight crew. To accommodate adverse winds, the wing tilted upward more than 90 degrees which allowed the airplane to fly backward at 20 knots. By banking left or right, the airplane could fly sideways at 40 knots. The four T64 turboprop engines propelled four conventional airscrews and a horizontally mounted three-blade variable-pitch tail rotor through a system of cross-shafting and gear trains, making it possible to maintain flight on any two engines in an emergency. The wing was able to rotate through an angle of 100 degrees, giving the XC-142A the ability to hover in a tail wind. During VTOL flight, roll control was achieved by means of differential collective propeller pitch, yaw control by means of ailerons working in the propeller slipstream, and pitch control by means of the variable-pitch tail rotor. During transition, a mechanical mixing linkage integrated the VTOL control system with conventional ailerons and tail control surfaces in correct proportions as a function of the wing tilt angle. In normal cruising flight, control was achieved by conventional control surfaces, with the tail rotor locked. A dual four-function stabilizer system ensured stability during IFR flight, hovering, and transition.
The flight of the XC-142A was successfully completed on 29 September 1964. The first XC-142A (No. 2) was delivered to the test team in July 1965. During the XC-142A program, a total of 420 hours were flown in 488 flights. The five XC-142A's were flown by 39 different military and civilian pilots. In 1966, while operational tests were being performed, the Air Force requested Vought to submit a proposal for a production C-142B. A major improvement was proposed to eliminate the Navy carrier compatibility requirement. After reviewing the C-142B proposal, the tri-services management team could not develop a requirement for a V/STOL transport. XC-142A testing was terminated and one flying airplane was turned over to the National Aeronautics and Space Administration (NASA) for research testing from May 1966 to May 1970.
After the ensuing flight test program, it appeared that production orders would follow, but due to changes in requirements, and political and fiscal aspects, the program never materialized. Performance was not as good as had been hoped for, partly due to the USAF waiving its weight-improvement program, and partly due to the propellors not delivering the expected thrust. The program called for the building of five prototypes, but cross-shaft problems, along with operator errors, resulted in a number of hard landings causing damage. One of the limitations found in the plane was an instability between wing angles of 35 and 80 degrees, encountered at extremely low altitudes. There were also high side forces which resulted from yaw and weak propeller blade pitch angle controls.
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