|Bristol Type 173|
The Belvedere general purpose transport helicopter, which entered service with the RAF in the autumn of 1961, had behind it a 14-year development period during which, at different times, it had been considered as a commercial passenger carrier, and a naval antisubmarine helicopter. It originated as the Bristol Type 173, two Mk.1 prototypes of which were begun in 1948 to Ministry of Supply Specification E.4/47. The first of these machines, G-ALBN, made its maiden flight on 3 January 1952, after some eight months of ground trials and tethered flights. It was Britain's first tandem-rotor helicopter design, and in this early form was powered by two 575hp Alvis Leonides 73 piston engines and utilised two 3-blade main rotors, with their control systems, of the type fitted to the single-engined Bristol Type 171 Sycamore. In 1953 this machine was handed over to the Admiralty for Naval trials. On 31 August 1953 a prototype 173 Mk.2 (G-AMJI) was flown, differing from the first machine in having two pairs of stub wings to off-load the rotors, the rear pair carrying upright fins at their extremities. These features were later removed, G-AMJI reverting to the Vee-tailed configuration of the Mk.1 and joining its stablemate for Naval trials as XH379. It subsequently returned to the civil register for a spell in BEA colours before being written off in a landing accident in 1956.
Meanwhile three more prototypes had been ordered, with 850hp Leonides Majors and metal instead of wooden rotor blades. In the event, only one of these (XE286) was flown, the other two being utilised for ground testing. In 1956 the Royal Navy decided to adopt the Bristol machine for the antisubmarine role, placing an order for sixty-eight aircraft. The production version, to be known as the Bristol 191, was to have folding rotor blades and a shorter fuselage, to enable it to use existing carrier deck-lifts, and the rear legs of the quadricycle undercarriage shortened to facilitate loading of an external torpedo. At about the same time the RAF ordered twenty-six of the standard model as the Bristol 192.
K.Munson "Helicopters And Other Rotorcraft Since 1907", 1968
The 173 was the first British two-engined helicopter to be developed. Its development and life span are spread over 19 years and though like many heavy helicopters it was a largely military machine, it was also intended as the first helicopter airliner for BEA service.
The 173 appeared on the drawing board in 1948 and ground tests started three years later. It was interesting not only because of its two three-blade counter-rotating rotors, but also because it could fly on one Alvis Leonides 73 engine and the centre of gravity could be displaced. The two rotors were synchronized by a shaft in conjunction with a gearbox. In the event of a breakdown the shaft could transmit power from the working engine. The rear rotor was carried on a pylon which was part of the vertical fin structure. Two tailplanes were set at a sharp angle to improve longitudinal and lateral stability.
Ground resonance was cured by linking the right and left oleo struts of the undercarriage with small-bore hydraulic piping. As G-ALBN the first 173 flew on August 24, 1952.
A Mk.2 followed with stub wings and an improved undercarriage. The Mk.3 however, with more powerful engines, (two Alvis Leonides Majors rated at 850shp each) as well as four-bladed rotors, marked an even greater advance. Seating too, was up from 14 in the Mk.1 and 2 to 16 in the Mk.3. Unfortunately however, the Mk.3 suffered from cooling problems and its service trials in 1956 were not entirely successful.
In July 1958 the Bristol 192 made its maiden flight and this marked the successful climax to the development of the 173.
Bill Gunston "The Illustrated Encyclopedia of Commercial Aircraft", 1980
This, the first British two-engined twin-rotor helicopter, first went on to the drawing-board in 1948 and started its ground tests in May 1951. Besides the two three-bladed contra-rotating rotors set in tandem, this rotorcraft had two interesting features: firstly, its centre of gravity could be appreciably displaced; secondly, it could continue to fly on one engine only. The 173 had virtually the 171's transmission system, rotor assemblies and engine installation, except that one engine rotates in the opposite direction to the other.
The two rotors could be synchronized by a shaft in conjunction with a gearbox. This shaft was also used in the event of breakdown in one engine to transmit power to the affected rotor from the engine still working. The rear rotor was carried on a pylon forming part of the fixed vertical fin structure. On each side there was also a tail-plane structure set at a marked dihedral angle to increase both longitudinal and lateral stability.
Ground resonance originally caused some trouble. The problem was overcome by linking the right- and left-hand oleo-struts of the undercarriage by small-bore hydraulic piping. The latter formed the two arcs of a circle seen above the upper part of the legs.
P.Lambermont "Helicopters and Autogyros of the World", 1958