Bristol Sycamore

During the Second World War, new methods of aircraft propulsion were devised and experimented with; in particular, breakthroughs in rotary aircraft, such as gyrocopters and helicopters, were making such aircraft more practical.[2] In 1944, Bristol established a specialised helicopter division shortly after the Allied invasion of Europe, when engineers from the Airborne Forces Experimental Establishment (AFEE) at Beaulieu became available. The AFEE had been conducting its own work on the development of rotorcraft designs under the noted helicopter pioneer Raoul Hafner; however, the successful use of Horsa and Hamilcar gliders during Operation Overlord had led to helicopter development being recognised as a priority. Hafner, whose company had been acquired by Bristol was promptly appointed by the company as the head of Bristol's new helicopter division.[3][1][2]

In June 1944, work commenced on the development on a four-seat helicopter intended for both civil and military use; it was out of this programme that the Sycamore would emerge.[1] During development, particular emphasis was assigned to the producing the necessary level of endurance of the rotorcraft's mechanical components. On 25 July 1947, the first prototype, VL958, which was powered by a 450-horsepower (340 kW) Pratt & Whitney Wasp Junior (there being no suitable engine in the Bristol range), performed the type's maiden flight.[1][2] In mid-1948, the third prototype, which had been built to the improved Sycamore Mk.2 standard, was completed; this model had been fitted with a 550-horsepower (410 kW) Alvis Leonides engine, the Leonides engine would become the standard powerplant for all subsequent Sycamore production. On 25 April 1949, a certificate of airworthiness was granted for the Sycamore, the first such to be granted to a British helicopter.[1][2]

During the flight test programme, Bristol's key development pilots for the Type 171 included Charles "Sox" Hosegood and Col. Robert "Bob" Smith. In 1951, a Bristol-owned Sycamore Mk.2 was used during a series of deck landing trials performed on board the Royal Navy aircraft carrier HMS Triumph.[4] An improved model of the helicopter, designated as the Sycamore Mk.3, was rapidly developed; it featured an increased capacity for five occupants, a wider fuselage and a shortened nose.[2] A total of 23 Sycamore Mk.3s were produced, 15 of these were principally used for joint evaluation purposes by the Royal Air Force (RAF), Army Air Corps (AAC), and British European Airways (BEA).[1][2]

Versions of the Sycamore up to and including the Mk.3A retained the standard two-seat cockpit layout, placing the pilot in the left-hand seat and the co-pilot in the right.[2] However, on the main production model, designated as the Sycamore Mk.4, this seating arrangement had been switched to the standard American practice of positioning the pilot's seat on the right. There were also a number of other developments that had featured upon the earlier versions, such as a four-door design, which had been standardised upon the Sycamore Mk.4. This version entered RAF service, receiving the military designation of HR14.[2]

Civil versions were not marketed under the Sycamore name, they were instead known simply as the Bristol Type 171.[2] By May 1958, over 150 Sycamores had been manufactured and four units per month were being built.[5]

A sectioned Sycamore main gearbox, displayed in the Deutsches Museum, Munich, Germany

The Bristol Sycamore was one of the first production helicopters to be developed.[5] Each Sycamore was manufactured with all of the necessary fixed fittings to enable it to be quickly adapted for any of six major roles: search and rescue, air ambulance, passenger transport, freight transport, aerial crane and dual instruction; it was also used for other specialised roles.[5] The Sycamore seated four-to-five occupants, depending on the model; it was usually fitted with three folding canvas seats as well as a single rotating seat besides the pilot. In addition to the passenger cabin, it had a separate luggage compartment.[5]

A specialised air ambulance model of the Sycamore was developed during the early 1950s. In this configuration, up to two patients were carried inside the cabin on stretchers stacked one above the other; this was different to the usual arrangement of the era of using externally-mounted "pods" for carrying patients.[6] To provide the extra width necessary in the cabin, detachable Perspex blisters were fitted on each side of the cabin. The stretcher racks could be folded into the sides of the cabin, providing room for up to three sitting casualties instead; an electrical supply outlet was available for connecting electric blankets. Next to the pilot was a swivelling seat for a medical attendant.[6]

The blades of the three-bladed main rotor were attached to the rotor head with lightweight interleaving steel plates while tie-rods carried the centrifugal tension loads.[5] The blade levers were connected using ball joints to the arms of a control spider, the cone of which was actuated up and down by the collective pitch lever which changed the pitch of all the blades; an irreversible mechanism was used to prevent blade loads being transferred back to the control stick. The blades were supported when stationary or turning slowly by droop stops, which maintained a minimum clearance between the blade tips and the tailboom even in high winds; these stops were withdrawn above 100 RPM so that full freedom of movement for effective flight was possible.[7]

The Sycamore was powered by a single Alvis Leonides piston engine of 550 hp.[8] The engine was mounted below and to the rear of the main rotor on a flexible mounting to reduce vibrations transmitted to the helicopter structure. It was isolated in a fireproof enclosure which was fitted with fire detection and extinguishing equipment to meet certification requirements.[8] Air was drawn through a forward-facing grill to cool the gearbox before passing through the engine cowling and leaving the fuselage. The engine power was controlled by the collective pitch lever. In order to maintain the rotor speed at its required setting fuel to the engine had to be automatically varied as the rotor pitch setting commanded by the pilot changed the load on the engine; fine adjustment of engine power was achieved by twisting the pitch lever.[8] The Sycamore had a relatively high rotor speed for the era, which was claimed to give a smoother ride and be safer in the event of engine failure.[8]

A West German air force Sycamore Mk.14 at the 1958 Farnborough Air Show

From 1952 to 1955 the Sycamore was used for various trials by the Air Sea Warfare Development Unit[9] which was stationed at RAF St. Mawgan. In April 1953, the Sycamore HR14 entered service with 275 Squadron of the RAF and went on to serve with nine squadrons in total. Various marks of Sycamore served with the RAF; they would primarily be used as air ambulances (Sycamore HC.10), for Army communications (Sycamore HC.11) and for search and rescue operations (Sycamore HR.12 through the Sycamore HR.14).[2] In 1953, No. 275 Squadron, equipped with Sycamores, became the RAF's first helicopter search and rescue squadron in Great Britain.[10] The type was used by the Royal Air Force Central Flying School for pilot training purposes.[10]

The Sycamore was heavily used during the Malayan Emergency (1948–1960), typically deploying Army foot patrols into the jungle. Numerous Sycamores were transferred to the Far East Air Force to participate in the conflict; however, the type was involved in a number of crashes in the region which had occurred as a result of tail or main rotor blade problems.[10][11] In response, a series of blade trials were conducted prior to a modified blade design being adopted and Sycamore operations in the theatre being resumed. Following the end of most combat operations in August 1960, Sycamores remained in the region, including a detachment in Brunei, to support British forces stationed there to deter further aggression by Malayan communist guerrillas.[10]

Sycamore of 103 Squadron RAF in Cyprus, 1961

The type also saw combat service with the RAF during the Cyprus Emergency and the Aden Emergency, in addition to other operations. In December 1971, the last of the RAF's Sycamores were officially retired; this had been due to critical parts having reached the end of their fatigue life. However 32 Squadron continued to operate two Sycamores until August 1972.[10][2]

A total of 50 Sycamores were delivered to the German Federal Government.[2] Three helicopters were produced for the Belgian Government for use in the Belgian Congo; it had been picked due to the type's good capabilities in tropical environments, as proven during its combat use in Malaysia.[12] The Sycamore also has the distinction of being the second helicopter type to be used by the Australian Defence Force; a total of 10 were delivered to the Royal Australian Navy.[2]

The Sycamore was also used in a variety of civilian roles. A single example was used during the construction of the M1 motorway between London and Leeds; the type provided support in various roles, including to perform aerial surveying, communication across various sites, the carriage of both personnel and equipment, and the mitigation of flooding on the project.[13] Sycamores operated by Australian National Airways were routinely available for charter, performing tasks such as the aerial surveying of mining claims, supply missions, and the transporting of equipment across the remote Outback areas of the nation.[14]

Forward section of the Bristol Sycamore

The interior of a Sycamore

Data from [30][2]

General characteristics

• Crew: two
• Capacity: accommodation for three passengers
  • Payload: 1,000 lb (454 kg)
• Length: 61 ft 1.3 in (18.62 m)
• Height: 13 ft 10.6 in (4.23 m)
• Empty weight: 3,810 lb (1,728 kg)
• Gross weight: 5,600 lb (2,540 kg)
• Powerplant: 1 × Alvis Leonides piston engine, 550 hp (410 kW)
• Main rotor diameter: 48 ft 7 in (14.8 m)

Performance

• Maximum speed: 132 mph (212 km/h, 115 kn)
• Range: 330 mi (531 km, 290 nmi)
• Endurance: 3½ hours

Aircraft of comparable role, configuration, and era

• H-5 Dragonfly
• Westland Dragonfly
• Mil Mi-1

Related lists

• List of aircraft of the Royal Air Force

• RAF Museum page on the Bristol Sycamore
• helis.com pages on the Bristol Sycamore
• 171 Production list