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CH-46A Sea Knight

Both the YHC-lA and the Model 107 were based on earlier designs by Frank Piasecki. He had considerable success utilizing two main rotors mounted in a tandem (one on each end of the aircraft) configuration. Since the rotors turned in opposite directions, lift was partially equalized on each side of the aircraft and there was no need for an anti-torque rotor.

The redesign of the 107 into what was originally called the HRB-1 (Helicopter, Transport, Boeing) for the Marine Corps required major modifications. The most pressing one was to install a rotor blade folding mechanism. Without it the helicopter could not operate from amphibious assault ships. This modification was not an easy task, for the basic 107 design had fully-articulated rotor heads. Thus any addition of weight for a blade fold system would require major revisions of the entire rotor. These modifications in turn would make it necessary to strengthen the transmissions and those parts of the fuselage to which they were attached. Vertol, however, was successful in designing an electrically operated system in which the blades from both the forward and aft rotor heads folded inward and were stored above the center of the aircraft.

The second problem revolved around what had been noted on the initial orientation flights of the YHC-lA at Quantico. The rear ramp and doors had to be increased in size to permit entry of a jeep. Such change required careful engineering, for the fuselage of an aircraft is much like the shell of an egg. As long as the shell is fully intact, it retains a remarkable amount of strength for its weight. But if a hole is cut into the shell, the strength is quickly lost. Any widening of the rear door would have to be compensated for by greatly increasing the strength of the surrounding fuselage.

The final problem was that new models of the T-58 free turbine were to be installed which could produce more power than the ones in the 107. The greater power was certainly desirable, but it required even more redesign. Most critical were the drive shafts from the two jet engines to the main transmission. These "high speed" shafts had to be balanced precisely. At the speed they were turning, the slightest vibration would create massive strain on the aircraft. All helicopters were subjected to vibration, particularly from a fully articulated rotor head, but the large and relatively slow bumps and thumps from such a source while uncomfortable, did not seriously affect the aircraft. High frequency vibration was another matter for the stress produced was determined by the square of the vibration.

The engineers at Vertol had their work cut out for them. What finally emerged on 30 April 1962, when the Navy accepted the first aircraft for testing, superficially resembled the YHC-lA and the 107 but was basically an entirely new helicopter.

The CH-46, as the HRB-1 was known under the unified designation system, had two 50-foot, contra-rotating rotors mounted on pylons, directly over the cockpit and the extreme rear of the aircraft. The rotors overlapped each other at the center of the aircraft for a distance of 16 feet. To prevent the blades from striking each other in this overlap area, the two rotors were interconnected by a carefully geared drive shaft.

With the blades folded for movement on the deck of an LPH, the aircraft measured slightly less than 45 feet long and 15 feet wide . With them extended, the aircraft was 83 feet long. The cargo compartment had no obstructions throughout its 24-foot length to hinder the entry of vehicles and troops. It was almost perfectly six feet square. This clean cabin was made possible by the use of small stub wings or sponsons attached to the outside of the fuselage. They doubled as fuel tanks and mounting points for the main landing gear. The sponsons also added stability if the aircraft were landed in the water, for which provisions had been incorporated.

When viewed from the side the CH-46 had two very distinct features. The nose landing gear was much longer than the main ones and gave the aircraft the appearance of squatting down to the rear with the rear tail pylon towering over the rest of the aircraft. In the aft pylon were both General Electric T-58-8B free-turbine engines and the main transmission. Each engine was connected to the transmission through other gear boxes by individual high-speed drive shafts. Another shaft was placed outside, along the top of the fuselage, and connected the front transmission to the one in the rear. Also in the pylon were the auxiliary power unit (a small jet engine which provided electrical and hydraulic power when the rotors were not turning) and other accessories required by the aircraft.

To solve the problem of the bulk of the basic machinery of the aircraft being located directly above the enlarged hole in the egg shell created by expanding the opening for the ramp, the Vertol engineers designed what was essentially a shelf extending rearward from the back of the cabin over the ramp doors. The engines, main transmission, and other equipment were mounted on this platform .

Empty, the CH 46 weighed 11,641 pounds and with 2,400 pounds of fuel and a crew of three was designed to carry either 4,000 pounds of cargo or 17 combat-equipped Marines. Under emergency overload condition, the cargo capacity could be increased to almost 7,000 pounds. Its top speed was 137 knots.

A helicopter which had undergone such an extensive redesign of almost all critical parts as had the 107 to create the CH-46 would require exhaustive testing. Any new aircraft normally encountered areas which would need further refinement and the CH-46 was to be no exception . The initial flight, which had been scheduled in June 1962, was delayed four months and was not completed until 16 October 1962. The first eight aircraft all were scheduled for the test program. The next six were to be delivered to operating units for initial training of crews.

The first phase of the Navy Preliminary Evaluation (NPE) for the new helicopter was conducted by Patuxent River personnel at the Vertol plant in Morton during the period 14 through 30 January 1963. The changes from the 107 had created new factors in the CH-46. The 107 prototype helicopter was very smooth. It was known prior to testing that the additional mass distribution to the rotor heads for the . automatic blade folding system would cause vibrations. These were excessively high in all flight regimes. The vibrations from the blade fold system, however, were of low frequency. They made for an uncomfortable ride but imposed little stress on the aircraft.

More serious were other vibrations. There had been difficulty in assuring proper alignment in the high speed engine shafts. During the testing the aircraft involved was realigned each night to insure proper balance. This problem was related to the [other] vibration problem. Improved methods for realigning are also under study. Any misalignment of the shafts could create extremely high frequency vibrations which could impose serious stress on the aircraft.

The problems were neither unusual nor unexpected. In spite of the apparent seriousness of some of the items listed above, the inspection team summarized that the evaluation was successful and the momentum generated toward correction of the deficiencies was "outstanding."

As the design and testing of the CH-46 continued, the Marine Corps made final plans for the introduction of the new helicopter. Starting in FY 1963, each year a new CH-46 squadron was to be commissioned until four were formed. This was the same expansion which had caused General Shoup to be wary of the introduction of Marine helicopters into Vietnam. In addition during the same period of time, each year one UH-34 squadron would be equipped with the CH-46. The conversion would be complete by FY 1970. At that time, all the UH-34s would have been taken out of service and each of the 15 medium transport squadrons would be operating 24 CH-46s. The goal for the end of FY 68 was 10 CH-46 squadrons with five other units operating at reduced strength of UH-34s. Procurement of the first 14 CH-46s was now scheduled to be completed in November 1963. Starting the next month, aircraft were to be produced at an initial rate of one per month and increase to five per month in December 1964. By 1967 it was estimated that the manufacturer could produce 96 helicopters per year until conversion was complete.

While the build up of the CH-46s was underway, the venerable UH-34 would continue to he purchased until sufficient numbers of the new helicopter could be produced. Not until January 1964 was the Marine Corps to stop receiving the "Huss".

The original schedule required that four CH-46s be delivered in September 1963 for the Fleet Introduction Program (FIP). Additional helicopters were to be available in January 1964. Almost as soon as testing of the aircraft had begun, there was a revision in the time table. In January 1963 BuWeps concluded that the target date a year hence might have to be changed to May, although production was expected to catch up a few months later. The new design of the CH-46 continued to plague the engineers. The fifth test aircraft was four months late in being delivered and the sixth was provisionally accepted on 24 July, six months behind the original schedule.

The delays centered around the vibration caused by the blade-fold mechanism and the high-speed shafts. At the end of December, NATC reported that the helicopter had successfully passed all portions of phase three of the preliminary evaluation, but it considered improved vibration levels mandatory for Bureau of Inspection and Survey trials. Vertol had, however, on a high priority basis made progress. It was a vexing problem. Several different modifications were attempted. Finally, the last week in August 1964, a solution was found and it was concluded that NATC flights indicate satisfactory vibration levels for unrestricted Fleet Release. The engineers had reduced the cockpit vibration to acceptable limits by the installation of three absorbers. The absorbers constitute a weight reduction in payload of approximately 355 pounds. The loss in lift capability was unfortunate but it represented another example of the difficulties in designing a helicopter.

Even before the absorbers had been agreed on as the solution, on 30 June, the first three CH-46s were delivered to HMM-265 at New River. During Operation STEELPIKE in the fall of 1964, this squadron remained at New River converting to the new medium helicopter. The day after HMM-265 received its first CH-46s, on 1 July, a second squadron of the aircraft was activated at Santa Ana - HMM-164. The continued difficulties with vibration and delays in production at Vertol, however, held up delivery of CH-46s. By mid-1965, HMM-164 had received 23 CH-46s and was engaged in intensive training. Over six years after General Pate first had recommended a replacement for the interim HUS, the Marine Corps had a medium helicopter that increased the total lift capability without reducing seriously the numbers of other aircraft. The wait was worth it.


The US Navy also acquired the Sea Knight, though in smaller numbers, beginning with 24 "UH-46A" shipboard resupply ("vertical replenishment / vertrep") helicopters. Initial delivery of the UH-46A was in July 1965.

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Page last modified: 07-07-2011 02:29:02 ZULU