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Aviation History
1953
1953 - 0106.PDF
104 FLIGHT BRITISH PORTFOLIO the drive is transmitted to the tail rotor via a universal joint and a bevel gearbox. The main rotor blades are of wood, with leading edges of improved wood, and can be folded for parking; metal blades will be available later. Rotor speed is high, and this has the dual advantage of per mitting high cruising speed and at the same time providing a large reserve of kinetic energy which makes for safe landing under autorotative conditions following engine failure at any height and speed. In spite of this high rotor speed, aircraft life is estimated at 7,500 hr, and the whole rotor assembly is strong enough to survive in the unlikely event of failure of one blade. Fire pre cautions are comprehensive and the flight instruments are driven off the main rotor so that they continue to function in the event of engine failure. The Bristol 173 is a very significant step forward in helicopter design, being one of the very first twin-engined machines to be produced in the world. Most other existing multi-rotor machines, such as the Cierva Airhorse and the current Piaseckis, are single- cngined. All-up weight of the 173 is 10,600 lb, and at this figure not only THE BRISTOL AEROPLANE CO., LTD., who have produced over 170 prototype aircraft, entered the helicopter field shortly after the last war. Having to design their machines from scratch, as it were, they started at a disadvantage relative to Westlands in regard to production; but two fine helicopters—Types 171 and 173— are now flying, and the 171 is already in service in some numbers. The Bristol 171 is a general-purpose, 4-5 seater of conventional layout. Two side-by-side pilots' seats are provided, with a bench-type seat for two or three in rear of this across the fuselage. Three prototypes were built; the present civil version is the Mk 3, and various military models (British designation Sycamore) have since been derived from this. The Sycamore H.C. 10 ambulance has provision for two stretchers, secured across the fuselage in place of the rear seat, the ends of the stretchers being covered by detachable transparent blisters and the noimal sliding doors being replaced by hinged, jettisonable doors. The H.C. 11 is an army co-operation version, and has a rope ladder for dropping or picking up personnel without landing, and strong points under the fuselage for attachment of heavy loads. The Mk 50 is being produced for the Royal Australian Navy, and incor porates a power-operated winch for rescue operations. A further version (H.R.12) has been under test for Coastal Command, and successful deck-landing trials have been carried out with the civil Type 171. Structurally, the centre fuselage is of tubular steel, covered with light-alloy inspection panels. The cabin and tail assemblies are of monocoque construction. The Alvis Leonides is mounted hori zontally and drives the main rotor through centrifugal clutch, free-wheel and gearbox: Bristol 173 Mk 3 Bristol Type 173
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