The Bristol Type 171 Sycamore is an early helicopter developed and built by the helicopter division of the Bristol Aeroplane Company. The name refers to the seeds of the sycamore tree, Acer pseudoplatanus, which fall with a rotating motion.[2] It has the distinction of being the first British helicopter to receive a certificate of airworthiness, as well as being the first British-designed helicopter to be introduced by and to serve with the Royal Air Force (RAF).[2]
Typically capable of seating up to three passengers, the type was often used as a transport for both passengers and cargo alike. In RAF service, the Sycamore was normally used in the search and rescue and casualty evacuation roles. The type proved the value of rotorcraft to easily traverse inhospitable or otherwise inaccessible terrain; the Sycamore made valuable contributions to British military activities during the Malayan Emergency, the Cyprus Emergency, and the Aden Emergency, in addition to other operations.
In addition to its British military service, various models of the Sycamore were produced and operated by a number of users, including overseas military operations and civil customers. Civilian operations typically involved transportation, mountain rescue, and aerial survey work. In 1959, production of the Sycamore ended after 180 rotorcraft had been completed.[1]
Development
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 Airspeed Horsa and General Aircraft Hamilcargliders 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 (340kW)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 (410kW)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 Sycamore Mk.4, this seating arrangement was switched to the 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 HR.14.[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]
Design
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 550hp (410kW).[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]
Operational history
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 RAF[9] which was stationed at RAF St. Mawgan. In April 1953, the Sycamore HR14 entered service with No. 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 to 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 RAF Central Flying School for pilot training purposes.[10] In June 1953, a Sycamore towing a large RAF Ensign led a flypast by 640 British and Commonwealth aircraft at the Coronation Review of the RAF.[11]
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][12] 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]
Fifty 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.[13] The Sycamore was the second helicopter type to be used by the Australian Defence Force; ten 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.[14] 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.[15]
Royal Australian Air Force – Only two Sycamores were in service with the RAAF from 1951 to 1965. The two helicopters were used for general support duties at the Woomera Rocket Range in South Australia.
Belgian Air Force: Belgium used three Mk.14B's (registered as B1/OT-ZKA, B2/OT-ZKB & B3/OT-ZKC) to equip the metropolitan power in the Congo with a rescue flight based at Kamina Air Base,[17]Katanga Province. Used between 1954 and 1960.
Sycamore HR.14 XJ918 – RAF Museum Cosford, Cosford.[2] This aircraft saw active service during the Malayan Emergency and was one of the last four Sycamores to be officially retired from RAF service in 1971. XJ918 was relocated to the RAF Museum Cosford in 1983. Relocated to Ulster Aviation Society in late 2022. [10][30]
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Bridgman, Leonard, ed. Jane's All The World's Aircraft 1951–1952. London: Samson Low, Marston & Company, Ltd 1951.
Jefford, C G (1988). RAF Squadrons. A comprehensive record of the movement and equipment of all RAF squadrons and their antecedents since 1912. Shrewsbury: Airlife. ISBN1-85310-053-6.
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