Military


CH-46 Sea Knight [H-49] "Phrog"

The H-46 is a twin-turbine powered dual-piloted tandem-rotor helicopter. H-46 aircraft are powered by two General Electric T-58 Series engines. The aircraft is 16 feet 8 inches tall. There are six rotor blades on the aircraft, each measuring 25 feet 6 inches. With blades spread, the aircraft is 84 feet 4 inches long. The average weight of the H-46 is 18,000 pounds, with a maximum lift capability of 6,000 pounds. It can carry 25 combat-loaded troops, or can be outfitted to carry medical evacuation litters in case of disaster. It has the fuel endurance to stay airborne for approximately two hours, or up to three hours with an extra internal tank. The cabin contains provisions for accommodating 25 troops and crew members. The cabin also contains an integral cargo and rescue system.

Readily identified among current Navy and Marine Corps helicopters are the H-46 series Sea Knights, with their tandem rotor configuration setting them apart from the single rotor design of other Navy/Marine helos. Tandem rotors have been a feature of all production helos built by Boeing/Vertol, and its original predecessor company, Piasecki.

The H-46 "Sea Knight" helicopter is one of the largest helicopters in the US Navy inventory. The helicopter has the ability to land and taxi in the water in case of emergency, and is able to stay afloat for up to two hours in two-foot seas. Because of its tandem rotor design, the "Sea Knight" is an extremely versatile aircraft. It is able to excel in various flight maneuvers, such as rearward and sideward flight, while other helicopters are extremely limited. This makes the helicopter ideal for its primary Navy mission of vertical replenishment.

The Sea Knight was originally designated the H-49. The Boeing CH-46 Sea Knight helicopter has served the US Navy and Marine Corps faithfully since the early 1960s. The Boeing Company Vertol Division, Philadelphia, Pennsylvania, built H-46 aircraft in the 1960s and 1970s. It has had several major upgrades including: airframe conversions of H-46D and H-46F to CH-46E in the late 1970s, and the Safety, Reliability, and Maintainability Program which started in the late 1980s and completed in the early 1990s.

As a Marine Corps platform, the H-46E is used primarily during cargo and troop transport. The United States Marine Corps has been flying the CH-46 Sea knight helicopter, commonly known as the "Phrog," since 1962. The Phrog is a descendant of Boeing's first turbine-powered helicopter -- the Boeing 107 -- which first appeared in 1958, and which is still flying various missions throughout the world. In 1961, the USMC picked the CH-46 as its new medium-lift troop transport, replacing the ageing UH-34. Over 600 Phrogs were delivered to the USMC between 1962 and 1971. By the early 1990s fewer than 250 airframes were still flying in the Corps. USMC CH-46Es are scheduled to be replaced by the MV-22. The Marine's CH-46Es will gradually be reduced from the current 226 to 5 by 2015, the last year the H-46 will be in the rotorcraft inventory. Some sources mention an "RH-46" minesweeper and a "VH-46F" VIP transport, but these are very poorly attested and specifics are lacking. The UH-46A was a CH-46A modified for Navy vertical replenishment program, and the UH-46D was a CH-46D modified for Navy vertical replenishment program.

This venerable aircraft's primary mission areas in the Navy (as the H-46D) include Combat Logistics Support and Vertical Replenishment (VERTREP), Search and Rescue, and Special Operations. The unique tandem-rotor design of the Sea Knight permits increased agility and superior handling qualities in strong relative winds from all directions, allowing, in particular, rapid direction changes during low airspeed maneuvering. This capability has resulted in the safe, efficient and graceful transfer of many millions of tons of cargo and many thousands of passengers over the years.

In 1958, Vertol completed a company sponsored prototype of a new helicopter design, powered by two Lycoming T-53 turbine engines for potential military or civilian use. The 107 first flew in April, exhibiting most of the basic configuration characteristics to be found in all of its 107-series successors. The Army ordered three YHC-1As which were developed as GE-T-58-powered military evaluation vehicles under a Bureau of Aeronautics contract. First flying in August 1959, the YHC-1As were followed by an improved commercial/export model, the 107-11.

During 1960, the Marines evolved a requirement for a twin-turbine troop/cargo assault helicopter to replace the piston engine types then in use. Following a design competition with the HR3S , Boeing/Vertol was selected to build its model 107M as the HRB-1, early in 1961. The official military designation of HRB-1 (H-Helicopter, R-Transport, B-Boeing) was given the 107 along with the nickname of Sea Knight. The HRB-1 followed the typical Vertol design having two rotors in tandem. Two General Electric T-58 shaft turbine engines, exactly the same as those in the HSS-2, were mounted in the rear and on top of the 46-foot-long fuselage and powered the 51-foot diameter rotors. For the primary assault mission, the empty weight was listed as 11,641 pounds and a maximum gross weight limited to 8,621 pounds. The cabin section of the fuselage measured approximately 24 feet long, 6 feet high, and 6 1/2 feet wide allowing for 17 combat-equipped troops or 15 litter patients. The helicopter was manned by a crew of three with the maximum sea level airspeed limited to 137 knots. The overall length of the prospective assault aircraft was quite long, 84 feet. A hydraulically operated ramp was incorporated in the rear of the cabin in order to facilitate loading and unloading of troops and large pieces of cargo.

It retained the general configuration of its predecessors, including the aft sponsons carrying the fixed main gear, a fixed nose gear and built-in emergency flotation provisions so it could land and take off from the water in light seas. Special features included power-operated blade folding, integral cargo handling provisions, a rear loading ramp that could be left open in flight, personnel recovery and rescue equipment, and provisions for hoisting 10,000 pounds externally. These and other features marked a significant step forward in helicopter capability in the time period.

The drive system of the CH-46 helicopters includes two highspeed engine drive shafts. One of these shafts is connected between each engine and the combining transmission or mixbox. The General Electric T58 engine has tho output shaft connected to the engine coupling adapter. A set of Thomas coupling plates, whose function is to correct for misalignment, joins the engine coupling adapter to the splined adapter. The main shaft body has a male spline at one end which is inserted in the female counterpart of the splined adapter and terminates with a curvic coupling. The use of the spline provides for the freedom of longitudinal motion. A curvic coupling adapter joins the shaft through another set of Thomas coupling plates to the mixbox input pinion adapter. The curvic coupling is provided to facilitate engine or transmission changes. The "balanced shaft assembly" consists of the engine coupling adapter through and including the Thomas coupling plates at the mixbox end of the shaft.

First flight in August 1962 was followed by a change in designation to CH-46A, development flight testing, (including the first NPE in January 1963), and BIS trials beginning in March 1964. Fleet introduction of CH-46As with the Marines and UH-46As with the Navy took place in November 1964. The latter were modified for use in the vertical replenishment role.

The CH-46 Sea Knight was first procured in 1964 to meet the medium-lift requirements of the Marine Corps in Viet Nam with a program buy of 600 aircraft. The aircraft has served the Marine Corps in all combat and peacetime environments. However, normal airframe operational and attrition rates have taken the assets to the point where a medium lift replacement is required. The safety and capability upgrades are interim measures to allow continued safe and effective operation of the Sea Knight fleet until a suitable replacement is fielded.

In late 1966 and early 1967, Vertol conducted a series of experiments with an H-46 which had been converted into a compound helicopter similar to that which had created so much interest prior to the design of the CH-53. Short "stub wings" were mounted directly behind the cockpit and also on the rear tail pylon as part of the company's effort to improve speed and payload. The concept, as in all compound helicopters, was that in forward flight the wings would provide some of the lift necessary, allowing the rotor blades to move faster and give the aircraft a higher speed. Also the aircraft's rear rotor pylon has been moved aft and the forward one streamlined. Provisions were made for fuel tanks carried on the outside of the aircraft, and the entire fuel system was adaptable for inflight refueling. The helicopter was also used as a flying guinea pig to try out new ideas. After a number of successful flights, the aircraft crashed and was destroyed, but Vertol continued to experiment with ways to improve the CH-46 series.

Production continued in subsequent years, along with modifications to improve some of the H-46's characteristics. With service in Southeast Asia came installation of guns and armor. Increased power requirements were met by installation of higher powered T-58-GE-10s in the CH/UH-46D models, which also featured new cambered (droop snoot) rotor blades. The final CH-46E, with further increased power, was preceded by the last production version, the CH-46F, before production was completed with delivery of the 524th H-46 in February 1971.

The early A models later served as search and rescue HH-46As. CH-46s equip Marine Reserve squadrons, and conversion of earlier aircraft to the new CH-46E version was completed with fiberglass blades slated added to its other improvements.

The CH-60 Fleet Combat Support Helicopter will complement and eventually replace the Navy's aging fleet of H-46 helicopters. As a result of the advanced airframe life of the H-46 fleet, the Navy's logistics helicopter force experienced a near-term inventory shortfall.



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