Blackburn Dart

During late 1919, Blackburn commenced design work on what would become the Dart; it was initially pursued as a private venture. Around this time, the Air Ministry had reissued its requirement for a new carrier-based torpedo-bomber that would replace the existing Sopwith Cuckoo, which was the standard torpedo bomber of the era being flown from the aircraft carriers of the Royal Navy.[1] Blackburn's efforts were led by their chief designer, Major Frank Arnold Bumpus; the design process was undertaken in close collaboration with the British engineering company D. Napier & Son. This new aircraft was originally named the T.1 Swift.[2][1]

It was a relatively large single-seat biplane, which was in several respects considered to be a conventional design for the era.[1] It was built around a central nucleus in which the centre fuselage, top center section, lower wing roots, and undercarriage attachments were all integrated into a single rigid structure entirely composed of steel tubing. This structure provided sufficient strength as to not only withstand the high stresses of carrier operations, but also carry a heavy concentrated payload, while also being relatively easy to maintain and repair.[3] A similar tubular structure was also used for the rear fuselage. The fuselage had an unusual humped appearance due to the decking forward of the pilot sloping sharply downwards to maximise forward visibility over the engine, which was particularly useful during landings.[3]

The Swift was outfitted with staggered two-bay equal-span wings that could be folded for storage aboard ship; this arrangement meant Blackburn was the first British company to address the issue of a foldable staggered wing cellule.[4] The centre section of the wing structure was steel, while the outer sections were composed of wood, all of which were covered by fabric. Ailerons were present on all four wings.[4] The tail unit had a braced tailplane and fin with a balanced rudder. The divided landing gear had mainwheels on oleo legs that allowed the fitting of a standard torpedo below the fuselage.[5][6]

One innovation of the design was the detachable mounting for the engine, which facilitated its replacement as a complete power unit and enabled its swapping in a matter of hours.[3] Another advanced feature was the presence of a fireproof bulkhead between the engine bay and the self-sealing fuel tank in the fuselage, which could accommodate up to 66 gallons of fuel; an additional gravity-fed tank housing up to 15 gallons was present in the centre-top section. While it was not necessary to jettison the undercarriage wheels prior to releasing the torpedo, unlike some contemporary aircraft, they were releasable as to minimise the tendency for the aircraft to nose over during a forced water landing, for which floatation bags were present in the fuselage.[3] The undercarriage incorporated a triangular support structure, a lengthy support axle, and a rubber compression shock absorber leg. Slings fitted to the top centre section were used to hoist the aircraft aboard ship.[3]

By July 1920, the prototype Swift was sufficiently complete as to permit it to be statically displayed to the public at the Olympia Aero Show, but was fitted with a non-functional radiator and without any controls.[1] At the time, the aircraft had already been placed on the British Government's secret list, which meant that it could only be displayed without specialised naval equipment, such as its torpedo release apparatus. Thus, when exhibited, the torpedo had to be placed on the ground between the undercarriage rather than being placed in an operational position.[1]

A Dart at RAF Martlesham Heath, circa 1925

During September 1920, the prototype Swift conducted its maiden flight; it reportedly almost crashed during the flight on account of its miscalculated centre of gravity.[7][3] This issue was successfully resolved by sweeping back the wings. Following these modification, the Swift proved largely satisfactory in flight; on 23 December 1920, it was brought to RAF Martlesham Heath for full performance trials by the Aeroplane and Armament Experimental Establishment.[8] One requested change, made in January 1921, was the fitting of an enlarged rudder to improve directional control. By 20 April, all trials had been completed.[8]

On 9 May 1921, following the installation of a new engine, a stiffened cowling, and arrestor claws on the stub axles, the prototype was dispatched to Gosport.[8] Shortly thereafter, the type's first deck landing was performed by the Canadian pilot Gerald Boyce onboard HMS Argus.[8] Having suitably impressed officials, Blackburn received an order for a further three aircraft to conduct service trials as per Air Ministry Specification 3/20. These aircraft, which were equipped to Admiralty requirements, received the name Dart.[9][10]

The Dart had its wingspan reduced by 2 ft 11in, which brought the twin tips closer to the outboard interplane struts, was the most visually distinctive change.[10] Other modifications included the adoption of more powerful engines, either a Napier Lion IIB or V engine that was mounted with a thrust line angled upward. The claw arrestor gear that had been fitted experimentally to the Swift prototype was also fitted as standard.[10]

During October 1921, the prototype Dart performed its first flight; on 24 October, it was flown from Brough to RAF Martlesham for full performance testing.[10] It reportedly handled well, in spite of its size, and even exhibited a then-remarkable stalling speed of 43 mph (69 km/h). The first prototype also participated in deck handling trials aboard Argus. The second and third prototypes underwent various evaluations, most of which were performed at Martlesham.[11] A series of competitive trials against the Handley Page Hanley were conducted at Gosport, during which the Dart emerged as the victor and thus became the new standard torpedo bomber of the Fleet Air Arm. An initial production contract for 26 aircraft was issued to Blackburn.[12]

During March 1922, deliveries commenced, having been built at Blackburn's Olympia Works and tested at Brough beforehand.[12] Three additional batches of ten Darts each were produced between 1923 and 1924; a larger contract for 32 aircraft was issued in August 1924. As late as November 1926, more small orders for replacement aircraft were also received. Production to the Dart came to an close during 1928, by which point 117 aircraft had been produced for the FAA.[13]

An export model of the Dart retained the Swift name. This was powered by the Napier Lion engine, capable of producing up to 450 hp (340 kW). The aircraft's ability to carry up to 1,500 lb of either bombs or a single 18-inch torpedo was attractive to international operators.[14] Seven aircraft were built as the Swift Mk II, two for the Japanese Navy, three for the Spanish Navy, and two for the United States Navy. The U.S. Navy aircraft were designated Swift F by Blackburn and Blackburn BST-1 by the U.S. Navy, however, following competitive trials held during 1921, the U.S. Navy decided not to proceed with an order, opting to purchase the Douglas DT-2 instead. The aircraft themselves were retained in San Diego for a time, being used for experimental purposes.[15]