Bristol Type 173

Bristol Type 173

Company type	Helicopter
Industry	Aerospace
Predecessor	Bristol Type 171 Sycamore
Founder	Raoul Hafner
Fate	Evolved into Bristol Belvedere
Successor	Bristol Belvedere
Headquarters	United Kingdom
Key people	Raoul Hafner (designer)
Products	Twin-engine tandem rotor transport helicopter prototypes
Owner	Bristol Aeroplane Company
Parent	Bristol Aeroplane Company

The Bristol Type 173 was a British-designed twin-engine, tandem rotor transport helicopter constructed by the Bristol Aeroplane Company. Conceived by Raoul Hafner primarily as a civilian transport, it attracted military interest as well. Although it never reached mass production, the design evolved into the Bristol Belvedere, which served with the Royal Air Force from 1961 until 1969.

Design and development

File:XF785-Bristol173-2006.jpg

The initial Bristol 173 prototype preserved in the Bristol Aero Collection at Kemble airfield in 2006

The Type 173 was developed as a tandem rotor variant of the earlier single-rotor Type 171 Sycamore. It incorporated two Sycamore Leonides Major engines and rotor assemblies combined with an entirely new fuselage structure. The rotor gearboxes were interconnected via a driveshaft, allowing one engine to power both rotors in case of an engine failure.

The first Type 173 Mk 1, registered as G-ALBN, completed its initial hover on 3 January 1952 but displayed a tendency to move backwards. On its second hover attempt, ground resonance issues occurred upon landing. Rectifying these problems delayed its first transition to forward flight until 24 August 1952.

In 1953, the Royal Air Force conducted evaluations of the aircraft, designated with serial number XF785. The helicopter also underwent sea trials aboard the Royal Navy’s aircraft carrier HMS Eagle. To address fuselage resonance, four-bladed rotors were later installed.

The second aircraft, known as Type 173 Mk 2 and assigned military serial XH379, featured updated landing gear with castoring front wheels and fixed rear wheels. It was equipped with small stub wings at both front and rear to enhance cruise speed; however, these were removed after several flights during Naval trials, and a horizontal tailplane was added. These tests prompted Royal Navy interest for potential carrier-based operations.

In August 1956, British European Airways leased the second prototype for evaluation purposes. Unfortunately, during an air show at Filton in September that year, the Mk 2 crashed nose-first while transitioning from hover to forward flight. Though the crew escaped injury, the helicopter was destroyed. Following this accident, the tailplane from Mk 2 was salvaged and attached to Mk 1 to explore optimal stability configurations.

Both Mk 1 and Mk 2 prototypes were powered by two nine-cylinder Alvis Leonides 73 engines, each producing approximately 550 horsepower (410 kW).

Subsequently, Bristol secured a contract from the Ministry of Supply for three enhanced evaluation helicopters designated Type 173 Mk 3. These featured four-bladed rotors, a taller rear pylon to improve performance in turbulent conditions, reinforced airframes, and were powered by more powerful fourteen-cylinder Alvis Leonides Major engines delivering around 850 hp. These models were intended for assessment by both Royal Air Force and Royal Navy units, which had issued tandem helicopter specifications in 1953.

The Royal Navy’s specification HR.146 called for a shipborne helicopter powered by Leonides Major engines capable of anti-submarine warfare, search and rescue, and transport roles; Specification HR.149 outlined similar requirements for the Royal Canadian Navy. Meanwhile, the Royal Air Force’s specification H.150 sought a versatile transport helicopter able to carry bulky external loads.

In response, Bristol developed three variants: Type 191 for the Royal Navy requirement, Type 192 tailored to the Royal Air Force’s needs, and Type 193 intended for Canadian production to fulfill their naval specifications. By April 1956, orders were placed for a total of ninety-four helicopters across these three types—initially prototypes fitted with Leonides Major engines and production models planned to utilize Napier Gazelle turboshaft engines.

The third Type 173 achieved its first hover on 9 November 1956; however, subsequent flights were halted due to issues with engine overheating and transmission failures encountered during ground testing. Although these problems were relatively minor, they contributed to the Admiralty’s decision to cancel their British naval order in favour of procuring the Westland-built version of the American Sikorsky S-58.

Shortly after this cancellation, Canada also withdrew from the Type 193 project as part of budget reductions. Consequently, Bristol concentrated efforts on developing the Gazelle-powered Type 192 variant for the Royal Air Force. Two of the cancelled Type 191 units were repurposed as Gazelle ground test rigs while another was utilized as a control fatigue rig. The original RAF order increased from twenty-two to twenty-six aircraft; the first Type 192—later named Belvedere—completed its maiden flight on 5 July 1958.

Variants

Type 173 Mk 1

The initial prototype model.

Type 173 Mk 2

The second iteration prototype.

Type 173 Mk 3

An additional three prototypes developed for military testing, featuring four-bladed rotors.

Type 191

A planned naval adaptation that never achieved flight; the first two units were repurposed as Gazelle ground test rigs supporting the Type 192 project.

Type 192

A Royal Air Force military transport helicopter, designated Belvedere.

Type 193

A proposed version of the Type 191 intended for the Royal Canadian Navy, which was never constructed.

Operators

Template:UK

• British European Airways (evaluation use)
• Royal Air Force (evaluation use)
• Royal Navy (evaluation use)

Aircraft on display

The prototype Type 173, bearing the registration XF785, was housed at the RAF Museum Cosford until 2002. It was then relocated to Kemble to be exhibited at the Bristol Aero Collection until that facility closed in May 2012. Afterwards, it was transferred to Aerospace Bristol for public display.