The long history of co-operation between Sikorsky and Westland began in 1947 with the Model S-51 which was built in Britian until the end of 1955, when the Westland Widgeon appeared. This had a redesigned cabin for four passengers plus the pilot, and a three-blade rotor driven by a 500hp Alvis Leonides engine. About 15 were built alongside production of the larger Whirlwind, the British version of the Sikorsky S-55.

A few passenger transport versions of the Widgeon were produced, some ambulance versions with room for two stretchers and a medical attendant inside the cabin, and at least one model equipped for rescue operations with an external hoist capable of lifting up to 455kg. A few Widgeons were also exported to Brazil and Jordan, but one of the chief operators of this helicopter was the Hong Kong police force, which used it for port surveillance.

G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984

Negotiations in 1946 between Westland Aircraft of Yeovil, Somerset, and Sikorsky in the USA led to a licence for construction of the Sikorsky S-51 in the UK. Basically similar to the US-built aircraft, the type was assembled from British-built components and in all but one variant was powered by the Alvis Leonides engine. The first civil Westland/ Sikorsky WS-51 was flown on 5 October 1948 and on 24 July 1951 the type became the first British-built helicopter to gain a certificate of airworthiness. Before that, in 1950, a version designated Dragonfly HR.Mk 1 equipped the Royal Navy's first helicopter squadron, No. 705 formed at RNAS Gosport. Built to a total of 133, the Dragonfly was followed by a developed version, the Westland Widgeon, the prototype of which was a conversion of a Dragonfly to provide five-seat capacity and which benefited from the introduction of the improved rotor of the Sikorsky S-55. Widgeon production totalled 14, several of them conversions of Dragonflies.

Variants

Dragonfly HR.Mk 1: initial air/sea rescue (ASR) version for Royal Navy with the Alvis Leonides 50 radial engine

Dragonfly HC.Mk 2: similar to Dragonfly HR.Mk 1 but equipped as casualty evacuation aircraft for the RAF

Dragonfly HR.Mk 3: major production ASR version for Royal Navy (58 built); generally as Dragonfly HR.Mk 1 but introduced all-metal rotor

Dragonfly HC.Mk 4: casualty evacuation version for RAF, similar to Dragonfly HR.Mk 3

Dragonfly HR.Mk 5: final ASR version for Royal Navy, similar to Dragonfly HR.Mk 3

Westland/Sikorsky Mk 1A: civil version with 388kW Alvis Leonides 521/1 engine; most used as civil transports, but small numbers to Japan for rescue, and to Italian and Thai air forces

Westland/Sikorsky Mk 1B: civil version similar to Mk 1A but with 336kW Pratt & Whitney R-985-B4 Wasp Junior engine

D.Donald "The Complete Encyclopedia of World Aircraft", 1997

Technical data for Westland "Widgeon" Engine: 1 x Alvis Leonides 521/2 radial pistone engine, rated at 388kW, main rotor diameter: 14.99m, cruising speed: 142km/h