Bristol Sycamore

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. 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).

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.

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. 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 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.

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. 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. 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.

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. 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. 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).

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. 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.

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