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Bristol 173

The experimental Bristol 173 led to production of the Belvedere for the RAF
Bristol 173 G-ALBN fly-past 16X9 Fly-past by the prototype Bristol 173 G-ALBN


The Bristol 173 was a tandem rotor commercial helicopter design with a new passenger fuselage joining a pair of three-bladed Bristol Type 171 Sycamore rotor systems, the forward one mounted above the cockpit and the rear one atop the rear fin structure.


The aircraft encountered problems in development with stability and control and both ground resonance and structural resonance; as a result, it was flown in three distinct configurations. The butterfly tail initially fitted provided excessive roll-yaw coupling and was later replaced by a horizontal surface with tip fins.


The prototype 173 Mk 1 (G-ALBN) made its first hovering flight on 3rd January 1952, when the aircraft refused to transition to forward flight and sat backwards onto the runway. This problem was caused by an inadequate collective pitch range on the rear rotor. On its next flight, the aircraft encountered ground resonance on landing.


High vibration was also encountered within the fuselage due to a primary bending mode coinciding with the rotor blade passing frequency. As a result, new four blade rotors were fitted although this hugely delayed its first flight out of the hover.  During 1953, it carried out evaluation trials with the RAF (using the designation XF785) as well as sea trials with the Royal Navy on board HMS Eagle.


Bristol 173 XH379 ground running The second prototype Bristol 173 XH379 ground running
The second prototype Type 173 Mk 2 (G-AMJI / XH379) was flown with a pair of tandem stub wings fore and aft to improve the cruise speed as well as a revised undercarriage.  The stub wings were later removed during naval trials and replaced by an unswept tailplane.
The aircraft was then leased to British European Airways for eveluation but the airline never exercised their option to buy the aircraft.  Sadly the Mk 2 was destroyed in an air crash at Filton Air Show in 1956 although thankfully all the crew members survived.
Bristol 173 winged 16X9 Second prototype Bristol 173 G-AMJI in winged configuration


The initial two prototypes were followed by three further evaluation prototypes of the 173 Mk 3 with more powerful Leonides engines, four blade rotors and a taller vertical fin.


The first of these was hovered on 9th November 1956 but development problems and overheating resulted in the cancellation of the project in favour of the Bristol Belvedere and no further test flying took place.


For subsequent developments, refer to the Type 191 and 192 Belvedere.


  Type 173 Mk 1 Type 173 Mk 3
Powerplant Two 550 hp Alvis Leonides Two 850 hp Alvis Leonides Major
Rotor diameter Twin three blade 48 ft 7 in, later twin four blade 48ft 9in Twin four blade 48ft 9in
Max Weight 10,600 lb 13500 ib
Capacity  2 crew and 13 passengers 2 crew and 14 passengers
Cruise Speed 115 mph 115 mph
Range                               185 miles 300 miles

Number built

              5                   1 x Mk 1 - 1 x Mk 2 - 3 x Mk 3 (only one of which was flown)


G-ALBN / XF785                  Held in store by Bristol Aero Collection for display at Filton

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