The Sopwith 5F.1 Dolphin was a British fighter aircraft manufactured by the Sopwith Aviation Company. It was used by the Royal Flying Corps and its successor, the Royal Air Force, during the First World War.
Design and development
In early 1917, Sopwith's chief engineer Herbert Smith began designing a new fighter (internal Sopwith designation 5F.1) powered by the 200 hp Hispano-Suiza 8B. The resulting Dolphin was a two-bay, single-seat biplane. The upper wings were attached to an open steel cabane frame above the cockpit. To maintain the correct center of gravity, the lower wings were positioned 13 inches forward of the upper wings, creating the Dolphin’s distinctive negative wing stagger. The pilot sat with his head raised through the frame, where he had an excellent field of view. This configuration sometimes caused difficulty for novice pilots, who found it difficult to keep the aircraft pointed at the horizon because the nose was not visible from the cockpit.
The Dolphin was armed with two fixed, synchronised Vickers machine guns and one or two unsynchronized Lewis guns flexibly mounted to the forward cabane crossbar. The mounting provided three positions in elevation and limited sideways movement.
The first Dolphin prototype was powered by a geared 150 hp Hispano-Suiza 8 and featured a deep chin radiator. Three subsequent prototypes incorporated modifications to the radiator, upper fuselage decking and vertical stabilizer. The Dolphin first flew on 23 May 1917. In June, Dolphin prototypes were sent to France for operational assessment and Martlesham Heath for official trials. The Ministry of Munitions placed an order for 500 aircraft on 29 June. Production commenced in October 1917, with 121 Dolphins delivered by the end of the year.
The Dolphin Mk I became operational with Nos. 19 and 79 Squadrons in February 1918. Nos. 87 and 23 Squadrons followed in March. The Dolphin’s debut was marred by several incidents in which British and Belgian pilots attacked the new aircraft, mistaking it for a German type. For the next few weeks, Dolphin pilots accordingly exercised caution near other Allied aircraft.
New pilots also voiced concern over the Dolphin’s wing arrangement, fearing serious injury to the head and neck in the event of a crash. Early aircraft were often fitted with improvised crash pylons over the cockpit to protect the pilot's head. Operational usage eventually showed that fears of pilot injury from overturning were largely unfounded. Crash pylons thereafter disappeared from frontline aircraft, though they were often retained on training aircraft. Night-flying Dolphins of No. 141 Squadron, a Home Defence unit, had metal loops fitted above the inner set of interplane struts.
Despite early problems, the Dolphin proved successful and generally popular with pilots. The aircraft was fast, maneuverable, and easy to fly, though a sharp stall was noted. The cockpit was warm and comfortable, in part because the radiator pipes ran alongside the cockpit walls.
Four Royal Air Force squadrons operated the Dolphin as their primary equipment, while other squadrons used it in small numbers. One Canadian Air Force squadron equipped with Dolphins but did not become operational before the Armistice.
The scarcity and unreliability of the French-built Hispano-Suiza 8B proved to be the Dolphin's most serious shortcoming. Use of insufficiently hardened metal in the pinion gears led to numerous failures of the reduction gearing, particularly in engines built by the French firm Brasier. The engine also suffered persistent lubrication problems. Limited production capacity for the Hispano-Suiza engine, and the priority afforded to French aircraft, slowed Dolphin deliveries. Availability of the Hispano-Suiza improved in early 1918, as the French firm Emile Mayen began deliveries on an order placed by the British Admiralty.
Use of the Lewis guns
When functioning properly, the Hispano-Suiza afforded the Dolphin excellent performance at high altitude. Accordingly, the Dolphin was often deployed against German reconnaissance aircraft such as the Rumpler C.VII, which routinely operated at altitudes above 20,000 ft. The Dolphin could attack with its two fixed Vickers guns, or from below with the upward firing Lewis guns. The Lewis guns proved unpopular in service, however, as they were difficult to aim and tended to swing into the pilot's face. Pilots also feared that the gun butts would inflict serious head injuries in the event of a crash. Most pilots therefore discarded the Lewis guns, though a minority retained one or both guns specifically for use against reconnaissance aircraft.
Pilots of No. 87 Squadron experimentally fitted some aircraft with forward firing Lewis guns on top of the lower wing, outboard of the propeller arc. These guns could fire incendiary ammunition, which could not be used with the synchronized Vickers guns. However, the 97-round ammunition drums could not be changed once empty, nor could the pilot clear gun jams. This field modification did not become standard. No. 87 Squadron also explored the use of equipment to supply pilots with oxygen at high altitude. The experiment was abandoned after trials showed that the oxygen tanks exploded when struck by gunfire.
Dolphins were rapidly phased out of the postwar inventory. Nos. 19 and 87 Squadrons demobilized in February 1919, followed by No. 23 Squadron in March. The last RAF unit to operate Dolphins was No. 79 Squadron, based at Bickendorf, Germany, as part of the Army of Occupation. The squadron demobilized in July 1919.
The last Dolphins to see combat service were 10 examples used by Polish forces in the Polish-Soviet War. From August 1920, these aircraft were primarily used for ground attack duties in the Battle of Warsaw and other actions. They were soon grounded due to lack of spare parts.
In October 1920, two Polish Dolphins were loaned to the Ukrainian Air Force (1. Zaporoska Eskadra Ukraińska) for use against the Soviets. Both aircraft were returned to the Poles in February 1921.
Production and planned developments
A total of 2,072 Dolphin Mk I aircraft were produced by Sopwith, Darracq Motor Engineering Co. and Hooper & Co. Approximately 1,500 Dolphins were stored awaiting engines at the time of the Armistice. These airframes were eventually scrapped incomplete.
Two developments of the Dolphin were planned. The French firm SACA (Société Anonyme des Constructions Aéronautiques) commenced licensed production of the Dolphin Mk II in 1918. The RAF expressed no interest in this variant, which was intended for the French Aéronautique Militaire and the US Army Air Service. The Mk II's 300 hp direct-drive Hispano-Suiza 8F gave a maximum speed of 225 km/h (140 mph) and a ceiling of 8,047 m (24,600 ft). The new engine was considerably larger than the 200 hp version and required an enlarged, bulbous cowling that fully enclosed the guns. The Mk II also featured an additional fuel tank, a variable incidence tailplane, and longer exhaust pipes. The Air Service anticipated delivery of over 2,000 Mk II aircraft by the summer of 1919, but only a few were delivered before the Armistice.
Meanwhile, persistent difficulties with the geared 200 hp Hispano-Suiza 8B prompted development of the Dolphin Mk III, which used a direct-drive version of the 200 hp engine. The Mk III first flew in October 1918 and went into production just as hostilities ended. Many existing Dolphins were also converted to Mk III standard at aircraft repair depots by removing the reduction gearing and fitting a modified cowling.
There are no known complete surviving Dolphin airframes, but at least two reproductions exist. A Dolphin is under construction at the Royal Air Force Museum in Cosford, England, which incorporates some original parts from serial nos. D5329 and C3988.
Another accurate Dolphin reproduction was built by Cole Palen for Old Rhinebeck Aerodrome in upstate New York. Powered by a vintage direct-drive Hispano-Suiza engine, this aircraft regularly flew at Palen's weekend air shows from 1980 onward. The Dolphin was placed on static display after suffering minor damage in a September 1990 crash landing. In November 2007, Old Rhinebeck Aerodrome began restoring the aircraft to flying condition. When completed, it will be painted in the markings of No. 19 Squadron.
Specifications (Dolphin Mk I)
Published - July 2009
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