Development of the 5F.1 Dolphin fighter was a private venture by the Sopwith Aviation Company during the 1st World War. Designed by chief engineer, Herbert Smith, the prototype first flew on 23rd May 1917 and, after trials at Martlesham Heath and operational assessment in France, an order for 500 machines was placed in June of that year.
The aircraft was armed with two synchronised Vickers .303 in machine guns firing forward and either one or two .303 in Lewis guns flexibly mounted to provide a limited arc of upward and lateral forward fire. It was also capable of carrying up to 4 x 25 lb bombs. The Dolphin Mk I was fitted with a 200 hp Hispano-Suiza 8 engine that initially suffered from inadequate lubrication and reduction gearing failure – problems that slowed deliveries.
The Dolphin Mk I entered RAF service towards the end of 1917 with the first squadrons becoming operational in February 1918. Its initial few weeks in France were characterised by teething problems but it soon proved successful and well liked by its pilots. It was fast, manoeuvrable and relatively easy to fly. When functioning properly, the engine permitted excellent performance at high altitude and the aircraft was often deployed against German reconnaissance machines operating above 20,000 ft – where the warm and comfortable Dolphin cockpit came into its own!
Four RAF squadrons operated the Dolphin with several other units using it in small numbers. After the Armistice, production of two improved versions, the Mk II and Mk III, was abandoned and the Mk I was rapidly phased out with the last squadron relinquishing its aircraft in July 1919. One Canadian squadron was equipped with the Dolphin shortly before hostilities ended and a dozen machines were operated by Poland in the Polish-Soviet War in 1920.
A total of 2,072 Dolphin Mk Is were produced. Some 1,500 airframes stored whilst awaiting engines at the time of the Armistice were scrapped. There are no known survivors but a reproduction that crashed in 1990 is being restored to airworthiness at Old Rhinebeck Aerodrome, New York State, whilst another, incorporating some original parts, is being restored for static display at the RAF Museum, Cosford.
On 4th July 1918, No 91 Squadron based at RAF Tangmere was designated a fighter squadron and began re-equipping with Dolphins. The unit remained at Tangmere until the end of August at which time it re-deployed to Kenley.