The Bristol Aeroplane Company 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 rotor was mounted above the cockpit with 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 which resulted in it being flown in three distinct configurations. The butterfly tail initially fitted provided excessive roll-yaw coupling and this 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 experienced within the fuselage by what is described as 'a primary bending mode' which coincided with the frequency created by the rotor blades. As a result, new four blade rotors had to be fitted although this hugely delayed its first forward 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.
The initial two prototypes were followed by three further evaluation prototypes of the 173 Mk 3, fitted with more powerful Leonides engines, four blade rotors and a taller vertical fin.
The first of these was hovered on 9th November 1956 although development problems and severe overheating resulted in the cancellation of the project in favour of the Bristol 192 Belvedere and no further test flying took place.
For subsequent developments, please refer to the Bristol Type 191 / 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 48 ft 9in||Twin four blade 48 ft 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|
|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 www.aerospacebristol.org.uk|