| Ling-Temco-Vought XC-142A "Tri Service" 1493 |  |
 |
|
| Ling-Temco-Vought XC-142A "Tri Service" 1493 | |
|
|
The XC-142 Tilt-Wing V/STOL program had its roots from the recommendations from a government advisory group in 1959. The group recommended that a full-size aircraft was required, with specific requirements for the Navy and Army. Beyond the obvious military applications of such a system, there were also many that felt that the plane could also have considerable civilian applications. The group noted that all previous VTOL programs at the time had been built to illustrate a particular principle, and the fact that it could be accomplished. Few of these concepts had any operational military capabilities. Consequently, with the XC-142, it was decided that this system would be tested in an operational environment. With that goal in mind, it was decided that the system would fulfill requirements for all three military services. Thus was born the first tri-service VTOL. In 1961, a Request for Proposal was released, and in September, the proposal from Vought-Hiller-Ryan was announced as the winner. It was also announced that the Air Force would manage the program with the cost of the program to be shared equally by each of the services. Vought Aeronautics Division of Ling-Temco-Vought was the prime contractor, with Hiller and Ryan serving as the major subcontractors. Vought subcontracted the design and fabrication of the empennage, aft section, engine nacelles, and wing to Ryan. The overall transmission system and selected components were subcontracted to Hiller, which was also responsible for the flap and aileron fabrication. Compared to previous test bed experiences, the new V/STOL was a large aircraft with significantly optimistic performance goals, similar to those of the Army's DHC-4 "Caribou" transport. The XC-142 grossed out at about 16900kg loaded with an empty weight of about 10780kg. The plane had a fuselage length just exceeding 17.7m, with a maximum height of 7.9m and a sizable wing span of 20.6m. The model carried a single tall vertical tail that provided 12m2 of area. The wings carried large trailing double-slotted flaps the entire length of each wing and were mounted high on the fuselage. The bulky fuselage was designed to carry significant cargo, with the cargo compartment being 9.15m in length with a 2.1m height and width. That volume equated to about 32 full-loaded troops and gear, or four tons of cargo. In addition to that capability, there was also the ability to carry 370 litres of fuel. There was also a planned capability for auxiliary tanks which would greatly add to the range. Power consisted of four 3080hp General Electric T64-GE-1 engines, mounted in nacelles on the wings, which were all cross-linked together. Each drove a giant four-bladed 4.7m Hamilton-Standard fiberglass propeller, the tips of each practically overlapping each other. Later in the program, Hamilton Standard would provide an improved version of the propeller using the 2FF blade design, which featured a wider planform, rounded tips, and a more pronounced twist than the earlier 2EF blades. The goal of the new design was to improve aerodynamic load distribution and overcoming a static load problem. The four engines also drove a fifth propeller, a three-bladed fiberglass type, in the tail through an interconnected gear and shaft train. Therefore, power was available to turn all five propellers when one, two, or three engines were shut down. The tail propeller rotated in a horizontal plane and was declutched and braked for cruise flight. Tying all this power together was obviously an intricate and complicated setup. Through cross-shafting gearboxes, the rotation from each engine was brought together at the top of the fuselage. The power was then sent back to the tail rotor through a tail propeller shaft, into the tail propeller gearbox, and on to the variable pitch tail propeller. The propulsion system of the XC-142 was definitely an over-powered situation. For example, the plane could lose an engine on take-off and still clear a 15.25m barrier in 122m carrying a 4500kg payload. Also, with all engines operating, the plane had a rate of climb at sea level of 34.5m/s. On a hot day, even with an engine out, the XC-142 showed a climb rate of 17.8m/s. Control of the craft during the ascent stage was intricate to say the least with roll controlled by differential propeller pitch. Pitch control was accomplished by the eight-foot, three-bladed variable pitch tail rotor. Yaw control was provided by ailerons powered by propeller slipstream deflection, actually a second VTOL concept being employed in the XC-142. The craft had a unique capability with the main lift system in that the wing was capable of rotating through 98 degrees instead of the expected straight-vertical position. The wing tilt mechanism consisted of two screw-jack actuators driven by a centrally-located hydraulic motor. The tilt was controlled by a variable rate switch on each collective lever, or by a constant rate switch. This allowed the plane to hover in a stationary mode in a tailwind condition. The trailing edge of the wings carried three-section, double-slotted flaps in three sections, with the center and outboard sections operated also as ailerons. The flaps were programmed automatically with changing wing tilt, although the pilot had an override capability. Leading edge slats were used for stall suppression, and were mounted outboard of each engine nacelle and operated automatically as a function of flap position. The vertical tail was operated as a standard rudder-and-fin set-up, which supported the slab-type unit horizontal tail assembly. The magic in the design was probably in the intricate control system, a fully-powered irreversible type with artificial feel forces and powered by dual independent hydraulic systems. Dual cockpit controls, consisting of conventional rudder pedals, control sticks, and collective levers for all take-offs and landings, provided the highest technology of the system. The XC-142 design also considered logistics implications, in addition to the VTOL design goals, with the tail rotor rigged to fold to the port side to reduce the storage length and protect against damage during a loading operation. The first XC-142 was rolled out in early 1964 with its first conventional flight being made in September 1964, its first hover three months later, and first transition two months later than that. The Air Force extensively tested the XC-142's capabilities with cargo flights, cargo, and paratrooper drops, along with desert, mountain, rescue, and carrier operations. In 1966, one of the XC-142s passed operational tests to prove the model in carrier operations. In quick succession, the plane accomplished 44 short take-offs and landings, along with six vertical take-offs and landings from the USS Bennington. The carrier trails were accomplished using the number five prototype, which was crewed by both USMC, Navy, and Army pilots. The flight regime covered VTOL operations at a variety of speeds, which occurred at wind conditions from 10 to 55km/h. A large variety of wings and flap tilt angles were used during the testing. Also, there were landings accomplished with three and six degree glide slopes. In an amazing demonstration, the plane negotiated a 360-degree turn within the width of the flight deck. That same year, one of the prototypes was also tested in an overwater pickup operation. The plane lifted a man from a life raft to determine its capability for rescue and recovery. A standard Navy horse collar sling was attached to 38m of cable and then lowered through a floor hatch just aft of the cockpit. The tests proved that there were no problems with effects of the propeller downwash or slipstream turbulence. The program called for the building of five prototypes, but cross-shaft problems, along with some operator errors, resulted in a number of hard landings causing damage to the complete fleet. The most serious of the mishaps, resulting from a tail rotor driveshaft failure, caused three fatalities. The May 1967 accident took place near the Dallas, Texas, LTV plant and occurred in a heavily-wooded area where fire started after the impact. The flight plan for the ill-fated prototype included a rapid decrease in altitude from 2440m to 915m, effectively simulating a pilot rescue under combat conditions. A nose-over at low altitude followed, from which the crew could not recover. The crash aircraft was XC-142 #1 which had flown 148 times at the time of the crash. Other incidents included the following:
The final decision on the disposition of the aircraft occurred during the Category II Operational Suitability Program, which was conducted at the Air Force Flight Test Center. The testing consisted of 113 flights, totaling 163.9 hours, which was accomplished between July 1965 and August 1967. Three of the XC-142s also participated in a major operational test demonstration during the program, where the planes participated in demonstrations of VTOL, STOL, and movement of Jeep-mounted 106mm recoilless rifles, unloading of three-quarter ton trucks with towed 105mm Howitzers, dump trucks, and 450-kg A-22 containers. For a typical XC-142 design mission, the plane could operate with a gross weight of 16900kg, including a four-ton payload. At that weight condition, the plane could take off vertically, cruise 370km near 480km/h, hover for ten minutes, and then land. One of the limitations found in the plane, even though the overall test results were very positive, was an instability between wing angles of 35 and 80 degrees which was encountered at extremely low altitudes. There were also high side forces which resulted from yaw and weak propeller blade pitch angle controls. Another XC-142 complaint was the excessive vibration and noise in the cockpit, when coupled with an excessively high pilot workload, and which presented a considerable challenge in the cockpit. The program was a considerable effort, with 39 different pilots flying the prototypes for a total of 420 hours. The greatest national exposure the XC-142 received during its flight test program occurred when the #4 prototype participated in the 1967 Paris Air Show. The technology contributions which were derived from the program were felt to have made the program worth its effort. In retrospect, it has to be assumed that if the mechanical problems experienced with the XC-142 could have been solved, the plane could well have achieved operational status. The only remaining XC-142, #2, currently is on display at the Air Force Museum at Wright-Patterson Air Force Base near Dayton, Ohio. S.Markman & B.Holder "Straight Up: A History of Vertical Flight", 2000
|
