In 1956, the Vertol Aircraft Corporation began developing a turbine-powered member of the "flying banana" family pioneered by Frank Piasecki. The result was a new design which was more compact than the previous angular-fuselage type, with a watertight belly to permit ditching and the powerplant installed at the base of the tail pylon.

The new Model 107 prototype with two 877shp Lycoming T53 turbines flew on 12 August 1958. In July of that year, the US Army ordered 10 Model 107s, designated YHC-1A, with the uprated 1065shp General Electric YT58 turbine and a rotor diameter increased by 0.6m. The first YHC-1A flew on 27 August 1959, but in the meantime, the US Army had ordered five YHC-1Bs (Model 114), a scaled-up variant which was better suited to meet its need for a tactical transport helicopter, and consequently the order for the Model 107 was reduced to only three machines. The third of these was later returned to the company, which converted it into the Model 107-11, prototype of the civil version.

However, when the US Navy set up a new design competition for a medium-lift transport helicopter in 1960, this was won by the Boeing-Vertol 107M, a modified version of the YHC-1A. A batch of 50 was initially ordered, the first of which was tested in October 1962. Designated CH-46A Sea Knight, the 107M was used for troop transport. During the Vietnam War the Marines also installed a 7.62mm machine gun, which was fired through the cabin door. A total of 498 have been ordered by the Marine Corps and 24 by the US Navy. Several variants have been produced including the CH-46A for the Marines (160); the UH-46A Sea Knight for the US Navy (24); the CH-46D with an uprated engine for the Marines (266); the UH-46D for the US Navy (10); the UH-46B for evaluation by the USAF; the RH-46E minehunters for the US Navy, and the CH-46F for the Marines (174), which is similar to the CH-46D but with improved electronics. Seven civil aircraft were used by New York Airways from 1962, while 18, designated CH-113, were ordered by the Canadian Air Force and 14, designated HPK-4, by Sweden.

The Model 107 has also been built under license in Japan by Kawasaki Heavy Industries in civil and military versions: the KV-107/11-2 commercial version for passenger transport adopted by Kawasaki, the Thai government and New York Airways; the KV-107/11-3 minehunters; the KV-107/11-4 for tactical transport, 59 of which have been built for the Japanese Ground Self-Defense Force; the KV-107/11-5 rescue version for the Japanese Air Self-Defense Force and the Swedish Navy (38 built); the KV-107/11-7 six-eleven-seat VIP transport version, only one of which has been built for the Thai government; and the KV-107/IIA version for hot climates and high altitudes.

G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984

In May 1957 Frank Piasecki and his Vertol design team began work on a new company-funded twin-engined, tandem rotor cargo helicopter designated the Model 107. The aircraft was essentially a turbine-powered update of Piasecki's proven CH-21 and was intended to fill an anticipated Army requirement for a medium assault transport helicopter capable of lifting an entire infantry platoon and all its associated equipment.

The first Model 107 prototype made its maiden flight on 22 April 1958 and three months later the Army ordered ten examples, designated YHC-1A, for service test and evaluation. However, prior to the delivery of the first article the Army decided Vertol's larger and more capable Model 114 (later better known as the CH-47 Chinook) better fulfilled the revised medium assault transport requirement, and consequently reduced the YHC-1A order to just three aircraft. These machines (serials 58-5514 through -5516) were used primarily to familiarize Army flight crews with the capabilities of turbine-powered helicopters, and all three were eventually returned to the manufacturer. Vertol continued development of the Model 107, which later served in large numbers with the U.S. Navy and Marine Corps as the CH-46 Sea Knight.

S.Harding "U.S.Army Aircraft since 1947", 1990

Shortly after the formation of Vertol Aircraft Corporation in March 1956, the company initiated a design study for a twin-turbine commercial transport aircraft. In formulating the design, special attention was given to ensure that it would be suitable also for military use if the armed forces showed an interest in its procurement. As a result, the tandem rotor layout, which had been developed fully by Vertol, and by the Piasecki Helicopter Corporation before it, was adopted because of its known performance and reliability. Twin turbines were chosen to power this new helicopter for, despite the fact that they had not then acquired a long history of reliability and economy, there was no doubt that these engines offered a superb power/weight ratio, and were improving progressively all the time. To limit noise and provide maximum cabin space the engines were mounted above the fuselage, at the aft end of the cabin. To speed loading/unloading operations a large ramp formed the undersurface of the upswept rear fuselage, and this was sufficiently robust to allow straight-on loading of vehicles and/or bulky freight. A sealed and compartmented fuselage made it possible for this new helicopter to be operated from water, as well as land surfaces.

Allocated the designation Vertol Model 107, a prototype entered construction in May 1957, and the first flight of this aircraft was recorded on 22 April 1958. Company testing and development progressed well, and an extensive demonstration tour aroused considerable interest. First of the armed forces wishing to evaluate this new helicopter was the US Army which, in July 1958, ordered 10 slightly modified aircraft under the designation YHC-1A; the first of these flew for the first time on 27 August 1959. By that time the US Army had become more interested in a larger, more powerful helicopter which Vertol had developed from the Model 107 and, in consequence, reduced its order to only three YCH-1As. Subsequently, the company equipped the third of these with 783kW General Electric T58-GE-6 turboshaft engines and rotors of increased diameter, and this derivative was fitted out with a commercial interior as the Model 107-II prototype, which first flew on 25 October 1960. By that time Vertol had become a division of The Boeing Company.

When the US Marine Corps showed an interest in this aircraft, one was modified as the Boeing Vertol Model 107M, powered by T58-GE-8 engines, and this was successful in.winning the USMC's design competition in February 1961, being ordered into production under the designation HRB-1 (changed to CH-46A in 1962), and the name Sea Knight. Since that time Sea Knights have been used extensively by both the USMC and the US Navy. The former uses these helicopters for troop transport, the latter mainly in the vertical replenishment (VERTREP) role, carrying stores, ammunition and personnel from logistic support ships to combat ships at sea.

The first of the CH-46As flew on 16 October 1962, and testing continued into late 1964, with the first US Marine squadrons taking these aircraft into service in early 1965. Since then a number of versions have been built, these including the CH-46D for the USMC, generally similar to the CH-46A, but with 1044kW T58-GE-10 turboshaft engines; theCH-46F for the USMC, generally similar to the CH-46D, but with additional avionics; the UH-46A Sea Knight, similar to CH-46A, procured by the US Navy with first deliveries to Utility Helicopter Squadron 1 in July 1964; and the UH-46D for the US Navy, virtually the same as the CH-46D. The US Marine Corps has updated 273 of its Sea Knights to CH-46E standard, with 1394kW General Electric T58-GE-16 turboshafts and other improvements. Six utility models, almost identical to the CH-46A, were delivered to the RCAF in 1963-4 under the designation CH-113 Labrador, and 12 similar aircraft were built for the Canadian Army during 1964-5, these being designated CH-113A Voyageur. Under a Canadian Armed Forces' Search And Rescue Capability Upgrade Project (SARCUP), Boeing of Canada was contracted to modify six CH-113s and five CH-113As to an improved SAR standard by mid-1984. In 1962-3 Boeing Vertol supplied Model 107-IIs to Sweden for service with the air force in the search and rescue role, and with the navy for ASW and minesweeping duties: both of these versions have the designation HKP-4

In 1965, Kawasaki in Japan acquired from Boeing Vertol the worldwide sales rights for the Model 107-II, and in 1981 continued to produce these helicopters under the designation KV-107/IIA. A number of versions have been built and remain in production, and these are listed below.

VARIANTS

KV-107/II-2: airline version, with accommodation for two flight crew, a stewardess and 25 passengers; 11 built; improved KV-107/IIA-2 available currently

KV-107/11-3: mine counter-measures (MCM) version for JMSDF (two), plus seven of the uprated KV-107/11A-3 model

KV-107/II-4: tactical cargo/troop transport for JGSDF with strengthened cabin flooring; 42 supplied as such, with the last of 18 uprated KV-107/ IIA-4 versions delivered in late 1981

KV-107/II-5: designation of 13 long-range SAR helicopters for JASDF; 19 uprated, but otherwise similar aircraft, are designated KV-107/IIA-5, the last three being delivered during 1981; eight KV-107/II-5s supplied to Swedish navy without powerplant, these having Rolls-Royce Gnome H.1200 turboshafts installed in Sweden; Swedish navy designation HKP-4C

KV-107/II-7: designation of one six/eleven-seat VIP transport

KV-107/11A-17: designation of single long-range transport for Tokyo Metropolitan Police Department; has a forward passenger compartment and aft cargo hold

KV-107/II-SM-1: designation of four helicopters equipped as firefighters

KV-107/IIA-SM-2: aeromedical and rescue version

D.Donald "The Complete Encyclopedia of World Aircraft", 1997

In 1956, Vertol began preliminary design and engineering of a twin-turbine transport helicopter for commercial and military applications. The main objective was to take full advantage of the high power, small size and light weight of the shaft-turbine engines then becoming available. To achieve the best possible hovering performance, the traditional Vertol tandem-rotor layout was retained, and the turbines were mounted above the rear of the cabin, on each side of the aft rotor pylon. This results in maximum unobstructed cabin area and permits the use of a large rear ramp for straight-in loading of vehicles and bulky freight.

Construction of a prototype, designated Model 107, was started in May 1957, and this aircraft flew for the first time on 22 April 1958, powered by two 641kW Lycoming T53 turboshaft engines. It was designed for water landing capability, without the addition of special flotation gear or boat hull design, and was intended to carry 23 to 25 passengers in normal airline standard accommodation.

Standard commercial 107 Model II, modified from one of the three YHC-1A (CH-46A) helicopters built for evaluation by the US Army, flew for the first time 25 October 1960, followed by the first production model on 19 May 1961. FAA certification received 26 January 1962; entered scheduled service with New York Airways 1 July that year.

The first order for the CH-46A assault transport version for the US Marine Corps was placed in February 1991, with first flight on 16 October 1962. Four squadrons were operating CH-46As by June 1965 and the type entered service in Vietnam in March 1966. Kawasaki Heavy Industries obtained a licence in December 1965 to build the Model 107 in Japan. First KV107II obtained Japanese and US type approval in Spring 1968.

VERSIONS

107 Model II: Standard commercial version, with two 932kW (1,250 shp) General Electric CT58 turboshaft. Available as an airliner with roll-out rear baggage container or utility model with rear-loading ramp.

CH-46A (formerly HRB-1) Sea Knight: US Marine Corps assault transport version of the 107 Model II powered by two 932kW General Electric T58-GE-8B turboshafts. Withdrawn from service.

CH-46D Sea Knight: Generally similar to CH-46A, but with 1,044kW General Electric T58-GE-10 turboshaft and cambered rotor blades. Withdrawn from service.

CH-46E Sea Knight: Upgraded CH-46A with 1,394kW General Electric T58-GE-16 turboshafts and other modifications including provision of crash attenuating seats for pilot and co-pilot, a crash and combat resistant fuel system and improved rescue system. Initial fleet modifications began in 1977, and the first CH-46E modified at Cherry Point NV, Naval Rework Facility was rolled out 3 August 1977.

CH-46F Sea Knight: Generally similar to CH-46D, with the same engines and rotor blades. Contains additional electronics equipment. All CH-46s delivered since July 1968 were of this version. Withdrawn from service.

UH-46A Sea Knight: Similar to CH-46A. Ordered by US Navy for operation from AFS or AOE combat supply ships to transport supplies, ammunition and missiles as well as aviation spares to combatant vessels under way at sea. Secondary tasks include transfer of personnel and SAR. First deliveries in July 1964. Withdrawn from service.

UH-46D Sea Knight: Generally similar to UH-46A, but with 1,044kW General Electric T58-GE-10 shaft turbine engines and cambered rotor blades. UH-46s delivered since September 1966 were of this version.

CH-113 Labrador: Six utility models delivered to RCAF in 1963-64 for SAR duties. Generally similar to CH-46A. Two 932kW General Electric T58-GE-8B turboshafts. Larger capacity fuel tanks (total 3,408 litres) giving a range of over 1,050km. All six upgraded under Search and Rescue Capability Upgrading Programme (SARCUP).

CH-113A Voyageur: Twelve aircraft in a similar configuration to that of CH-46A, delivered to Canadian Army in 1964-65 as troop and cargo carriers in logistical and tactical missions. Eight upgraded to SARCUP configuration.

Hkp 4C: Built for Royal Swedish Navy (45) and Air Force (10) in 1962-63, with Bristol Siddeley Gnome H.1200 turboshaft engines and fuel tanks of 3,786 litres capacity. Naval version has equipment for anti-submarine and mine countermeasures operations. Since upgraded with Gnome H.1400 turboshafts and new avionics.

HH-46D: Rescue version in service with the US Navy.

UH-46D: Base utility and rescue helicopter. In service with the US Navy.

KV-107II/IIA: Kawasaki-built versions of the Model 107 manufactured in Japan.

DESIGN FEATURES: Two three-blade rotors in tandem, rotating in opposite directions. The CH/UH-46 has power-operated blade folding. Power is transmitted from each engine through individually overrunning clutches into the aft transmission, which combines the engine outputs, thereby providing a single power output to the interconnecting shaft which enables both rotors to be driven by either engine.

STRUCTURE: Square-section stressed-skin semi-monocoque structure built primarily of high-strength bare and alclad aluminium alloy. Transverse bulkheads and built-up frames support transmission, power plant and landing gear. Loading ramp forms undersurface of upswept rear fuselage on utility and military models. Baggage container replaces ramp on airliner version. Fuselage is sealed to permit operation from water.

LANDING GEAR: Non-retractable tricycle type, with twin-wheels on all three units. Oleo-pneumatic shock-absorbers manufactured by Loud (main gear) and Jarry (nose gear). Goodyear tubeless tyres size 8 x 5.5, pressure 10.55kg/cm², on all wheels. Goodyear disc brakes.

POWER PLANT: See Versions.

ACCOMMODATION: 107 Model II: Standard accommodation for two pilots, stewardess and 25 passengers in airliner. Seats in eight rows, in pairs on port side and single seats on starboard side (two pairs at rear of cabin) with central aisle. Airliner fitted with parcel rack and a roll-out baggage container, with capacity of approximately 680kg, located in underside of rear fuselage. Ramp of utility model is power-operated on the ground or in flight and can be removed or left open to permit carriage of extra long cargo.

CH/UH-46: Crew of three, 25 troops and troop commander. Door at front of troop compartment on starboard side. Door is split type; upper half rolls on tracks to stowed position in fuselage crown, lower half is hinged at the bottom and opens outward, with built-in steps. Loading ramp and hatch at rear of fuselage can be opened in flight or on the water. Floor has centre panel stressed for 1,464kg/m². A row of rollers on each side for handling standard military pallets or wire baskets. Outer portion of floor is vehicle treadway stressed for 454kg rubber-tyred wheel loads. Cargo and personnel hoist system includes a variable-speed winch capable of 907kg cable pull at 9m/min for cargo loading or 272kg cable pull at 30m/min for personnel hoisting; it can be operated by one man. A 4,535kg capacity hook for external loads is installed in a cargo hatch in the floor.

SYSTEMS: CH/UH-46: Cabin heated by Janitrol combustion heater. Hydraulic system provides 105kg/cm² pressure for flying control boost, 210kg/cm² for other services. Electrical system includes two 40kVA AC generators and a Leland 200A DC generator. Solar APU provides power for starting and systems check-out.

ELECTRONICS AND EQUIPMENT: Blind-flying instrumentation standard. CH-46 has dual stability augmentation systems and automatic trim system.

Jane's Helicopter Markets and Systems

- The prototype flew on 22 April 1958, with the first production CH-46 following on 16 October 1962.

- The US Army tested a version of the CH-46 but decided not to operate it.

- In 1965 the Sea Knight replaced the Sikorsky H-34 with Marine units in Vietnam.

- Some 669 Sea Knights were built; US Navy and Marine Corps models served in Operation Desert Storm.

- Other military versions of this helicopter are employed in Canada, Japan and Sweden.